DOI: 10.1002/chem.201304478
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& C C Coupling | Very Important Paper |
Unusual Nitrile–Nitrile and Nitrile–Alkyne Coupling of Fc CN and Fc CC CN Lisanne Becker,[a] Frank Strehler,[b] Marcus Korb,[b] Perdita Arndt,[a] Anke Spannenberg,[a] Wolfgang Baumann,[a] Heinrich Lang,*[b] and Uwe Rosenthal*[a]
Abstract: The reactions of the Group 4 metallocene alkyne complexes, [Cp*2M(h2-Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr, Cp* = h5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc CN and Fc CC CN (Fc = Fe(h5-C5H5)(h5-C5H4)), is described. In case of Fc CN an unusual nitrile–nitrile C C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were obtained. As the first step of the reaction with 1 b, the nitrile was coordinated to give [Cp*2Zr(h2-Me3SiC2SiMe3)(NC-Fc)] (2 b). The reactions with
the 3-ferrocenyl-2-propyne-nitrile Fc CC CN lead to an alkyne–nitrile C C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4 a, b). For M = Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3 a, b and 4 a, b was investigated, showing decomposition after oxidation, leading to different redox-active products.
Introduction During the last decades, the formation and the reactivity of small metallacycles was investigated, especially that of the three- and five-membered “all-C” cycles like metallacyclopropenes, 1-metallacyclopent-3-ynes (metallacyclopentynes), 1metalla-cyclopenta-2,3,4-trienes (metallacyclocumulenes), and 1-metalla-cyclopenta-2,3-dienes (metallacycloallenes).[1] The metallacyclopropenes can be obtained by a reduction of the metallocene dichlorides in the presence of an alkyne. An excess of the alkyne leads in most cases to alkyne–alkyne coupling and the formation of the 1-metalla-cyclopenta-2,4-dienes (A, Scheme 1).[2] These compounds are well studied and described as intermediates in the formation of different organic compounds, such as benzene or pyridine.[3] In contrast to the above transformations, the formation of the analogous heterometallacycles,[4] B and C, by the heterocoupling of a nitrile and an alkyne or a the homocoupling of two nitriles, has been insufficiently investigated. Owing to this
[a] L. Becker, Dr. P. Arndt, Dr. A. Spannenberg, Dr. W. Baumann, Prof. Dr. U. Rosenthal Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert-Einstein-Straße 29a, 18059 Rostock (Germany) Fax: (+ 49) 381-1281-51176 E-mail: [email protected] [b] F. Strehler, M. Korb, Prof. Dr. H. Lang Technische Universitt Chemnitz, Faculty of Natural Sciences Institute of Chemistry, Inorganic Chemistry 09107 Chemnitz (Germany) Fax: (+ 49) 371-531-21219 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304478. Chem. Eur. J. 2014, 20, 3061 – 3068
Scheme 1. Coupling reactions of CC and CN bonds.
lack of knowledge, we were interested in studying the reactions of nitriles with Group 4 metallocenes. The reaction of zirconocene compounds with diynes and different nitriles (for example, Ph CN, tBu CN) was already described in detail by Xi and co-workers.[5] The chosen nitrile influences significantly the outcome of the reaction with Group 4 metallocenes, thus indicating a broad reaction pattern. The 1-metalla-2-aza-cyclopenta-2,4-dienes (B) established by nitrile–alkyne C C coupling are also known as intermediates in the formation of pyridines.[6] This first step of the reaction supports the preparation of pyridines with a certain substitution pattern in high yields. Although these reactions are known and have been applied for many years, the 1-metalla-2-aza-cyclopenta-2,4-dienes (B) were not often isolated and characterized. To the best of our knowledge, just one aluminum complex consisting of this structural motif was obtained by such a coupling.[7] In the other cases of alkyne–nitrile coupling to give comparable five-membered cycles, the established compounds were stabilized to give B1 (Scheme 2). The formation of B1
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Full Paper Results and Discussion We were interested in the reactions of [Cp*2M(h2Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr) with the ferrocenyl nitriles, Fc CN and Fc CC CN, respectively. C C coupling to give 1-metalla-2,5-diaza-cyclopenta-2,4dienes (C, Scheme 1) was observed in the case of Fc CN. At ambient temperature, 2 b is formed by the coordination of Fc CN to 1 b[19] (Scheme 4). This complex was isolated in high yields as a purple solid. The alkyne signals in the 1H and the 13C{1H} NMR spectra are shifted and broadened in contrast to those of 1 b (2 b: 1H: 0.57 ppm, 13C: 4.9 ppm; 1 b: 1H:
Scheme 2. Stabilization possibilities of B.
was observed for zirconium,[8] tungsten,[9] iridium,[10] osmium,[11] and ruthenium.[12] As the first step, the metallacyclopropenes of these elements reacted with nitriles to give 1-metalla-2-azacyclopenta-2,4-dienes. Subsequently, an additional ligand L (e.g., nitrile, phosphine) coordinates and a hydrogen atom is bonded to the nitrogen atom. The origin of this proton can be an additional proton source or ligand L itself.[13] The other possible and comparable products of alkyne–nitrile coupling are B2 and B3 (Scheme 2). Compounds of type Scheme 4. Reactions of 1 a and 1 b with Fc CN. B2 were established in the reaction of bimetallic complexes, 0.19 ppm, 13C: 4.0 ppm).[19] These data are in good agreement which consist either of a titanium and a boron or a zirconium [14] with those of analogue complexes, [Cp*2Zr(h2-Me3SiC2SiMe3)and an aluminum center, with nitriles. The NC unit inserts into the B Cplanar or Al Cplanar bond under formation of the (NC-R)] (1H: 0.54 (R = Ph), 0.56 ppm (R = pTol); 13C: 4.9 (R = complexes B2. The compounds of type B3 were obtained by Ph), 4.9 ppm (R = pTol)).[18] The typical resonances of the coordi[15] a nitrile–alkyne coupling at ditungsten complexes. nated Fc CN are slightly shifted as well compared to the free nitrile (2 b: 1H: 4.07 ppm (Cp); 13C: 70.7 ppm (Cp); Fc CN: 1H: In contrast to alkyne–alkyne and alkyne–nitrile C C coupling, reactions that form complexes of structural types A and 3.87 ppm (Cp); 13C: 70.6 ppm (Cp)). Owing to the broadening B, nitrile–nitrile C C coupling leading to 1-metalla-2,5-diaza-cyof the signals of the two quaternary C atoms (Calkyne) of 2 b, clopenta-2,4-dienes (C, Scheme 1) was very rarely observed. It these signals could not be recognized in the 13C NMR spec[7] [16] was described for aluminum and silylenes, but there were trum.[20] In the IR spectra, the typical resonances of the CN only two hints for this reaction with the early transition bond is likewise shifted from 2222 to 2150 cm 1 in 2 b. [17] The reaction of 1 b with two equivalents of Fc CN at metals and their molecular structure was determined by Xhigher temperature leads to nitrile–nitrile C C coupling and ray crystal structure analysis.[18] Notably, these ring-strained heterometallacycles, C, differ from the well-known 1,4-diaza1-zircona-2,5-diaza-cyclopenta-2,4-diene (3 b) was obtained in buta-1,4-diene complexes (1-metalla-2,5-diaza-cyclopent-3high yield (Scheme 4). As mentioned above, such a remarkable enes). In spite of the similarity to alkyne–alkyne coupling, this C C coupling is uncommon.[7, 16–18] selective reaction is not predictable owing to the differences The NMR and IR spectra of this new complex are comparabetween nitriles and alkynes. Moreover, an N N or an N C ble to those of the analogous compounds with Ph, pTol, or coupling to other products (Scheme 3, C1 and C2) is in princioTol as substituents instead of Fc (Table 1).[18] As a result of the ple possible as well and the route favored might depend on formation of the C=N bond, the signal of the carbon atom in the metal, the ligand, and the structure of the nitrile. the 13C NMR spectrum is shifted from 119.6 to 165.3 ppm and the IR adsorption bands of this bond are identified at 1580 and 1618 cm 1. Compound 3 b was isolated as yellow crystals, which were suitable for X-ray crystal-structure analysis (Figure 1). The C=N bonds (C1 N1 1.279(2), C2 N2 1.272(2) ) and the C C bond (C1 C2 1.5514(16) ) correspond to those of [Cp*2Zr(-N=C(Ph)-C(Ph) = N-)][18] (Table 1). For the formation of 3 b from 1 b, an associative or a dissociative reaction path are possible. It was often observed that zirconium complexes prefer the associative one[1d] and the formation of 2 b at room temperature might be a hint for this. To Scheme 3. Possible products of nitrile–nitrile couplings. Chem. Eur. J. 2014, 20, 3061 – 3068
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Full Paper Table 1. Comparison of 3 a, 3 b, and [Cp*2M(-N=C(Ph)-C(Ph) = N-)] (3 aPh: M = Ti, 3 b-Ph: M = Zr).[18, 23] 3a
3 a-Ph
3b
3 b-Ph
156.4
156.4
165.3
167.4
IR (C=N) [cm 1]
1525 1573
1528 1591
1580 1618
1575 1642
Bond lengths (C N) []
1.276(2) 1.274(2)
1.274(2) 1.276(2)
1.279(2) 1.272(2)
1.267(2) 1.268(2)
13
C NMR (C=N) [ppm]
Scheme 5. Formation of 3 a’.
Figure 1. Molecular structure of 3 b in the solid state. Hydrogen atoms are omitted for clarity. The thermal ellipsoids correspond to 30 % probability. Selected bond lengths [] and angles [deg]: C1 C2 1.5514(16), C1 N1 1.279(2), C2 N2 1.272(2), Zr1 N1 2.1086(10), Zr1 N2 2.1206(10), C1-N1-Zr1 116.40(8), C2-N2-Zr1 115.71(8), N1-C1-C2 114.74(10), N2-C2-C1 115.78(10).
67.3, 69.0 (C5H4), 69.4 ppm (C5H5)) are in the same range as those for 3 b (1H: 4.02, 4.34 (C5H4), 4.31 ppm (C5H5); 13C: 67.5, 69.9 (C5H4), 69.5 ppm (C5H5)). Complex 3 a, which is isostructural to 3 b, was isolated as red crystals in high yields. Recently, the structural, energetic, and chemical properties of the 1-metalla-2,5-diaza-cyclopenta-2,4-dienes were reported.[18] These compounds may possess interesting electrochemical properties when ferrocenyl substituents are introduced. Therefore, 3 a, 3 b, and [Cp*2M(-N=C(R)-C(R) = N-)] (3 a-Ph: M = Ti, R = Ph; 3 b-pTol: M = Zr, R = pTol) have been studied by cyclic voltammetry (CV) using a dry tetrahydrofuran solution containing 0.1 mol L 1 [NnBu4][B(C6F5)4] as the supporting electrolyte. The measurements were carried out at a scan rate of 100 mV s 1, if not otherwise mentioned, and the potentials are referenced to the FcH/FcH + redox couple, as recommended by IUPAC.[24] Compounds 3 a and 3 a-Ph show irreversible redox processes of the starting material in any potential range (Figure 2, Table 2). Two oxidation processes that lead to complete decomposition in multicyclic experiments are detected for both
Table 2. Electrochemical data of 3 a, 3 b, and [Cp*2M(-N=C(R)-C(R) = N-)] (3 a-Ph: M = Ti, R = Ph, 3 b-pTol: M = Zr, R = pTol).[23]
corroborate this assumption, a C6D6 solution of 2 b was warmed to 60 8C. 1H and 13C NMR spectra were recorded before the heating and after 12 days at 60 8C. The result of this experiment was a complete conversion of 2 b into 3 b, 1 b and free Me3SiCCSiMe3. The titanium complex 1 a[21] did not react with Fc CN at ambient temperature to give a coordination product, which is a hint for a dissociative reaction path. This different behavior of 1 a and 1 b was already described in the reaction with other aryl nitriles[18] and is caused by the size of the metals.[1d] Warming a solution containing 1 a and Fc CN to 55 8C resulted in the formation of 3 a. By exceeding this temperature, the corresponding 1,4-diaza-buta-1,3-diene complex (3 a’) is obtained (Scheme 5). The supposed origin of the hydrogen atoms for its formation is either the solvent or a C H activation of the [Cp*2Ti] fragment leading to the tucked-in complex.[22] Concerning the typical signals of the five-membered cycle of 3 a in the 13C NMR and IR spectra, this compound is, as 3 b, comparable to its analogue, 1-titana-2,5-diaza-cyclopenta-2,4dienes, [Cp*2Ti(-N=C(R)-C(R) = N-)] (R = Ph, pTol, oTol) (Table 1).[18] The signals of the ferrocenyl units in the 1H and the 13C NMR spectra (1H: 3.98, 4.22 (C5H4), 4.26 ppm (C5H5); 13C: Chem. Eur. J. 2014, 20, 3061 – 3068
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3a 0 1
E ’ [mV] (DEp [mV]) E 02’ [mV] E 03’ [mV] (DEp [mV]) E 04’ [mV] (DEp [mV]) E 05’ [mV]
275
3 a-Ph [a]
115[a] 226[b] (101) 373[b] (73) 721[b]
205
[a]
3b
3 b-pTol [c]
1166[c] (118) > 50 (broad)[a] –
355[a] –
763 (74) 342[a] 532
–
642
–
–
–
–
[a] Irreversible process; Epc ; [b] Decomposition product; [c] Decomposition product, occurs after first cycle.
complexes, with significantly higher potentials for 3 a-Ph (3 a: 275, 115 mV; 3 a-Ph: 205, 355 mV). Owing to the irreversibility of the first events, the second ones cannot be assigned unequivocally to 3 a or 3 a-Ph and may belong to decomposition products. Because of the dissimilarity of the potential range of the oxidations, two different multistate decomposition mechanisms are presumed. The oxidation of the titanium center might be the first step in the case of 3 a-Ph owing to the absence of further redox-active units, which initiate decomposition by Ti N bond cleavage. For 3 a, the first step is predicted
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Full Paper substituent stabilizes the compound against oxidation and leads to the detection of three redox events for 3 b, all events being at higher potentials (342, 532, 642 mV). Assignment of the first one to the zirconocene or the ferrocenyl unit is not possible and, owing to its irreversibility, the following ones cannot unambiguously be assigned to the starting complex. By using the cathodic direction for the initial step of the cyclic voltammogram, no redox processes were observed for 3 b and 3 b-pTol below 200 mV in the first cycle. After measuring in the anodic direction, quasireversible/reversible redox events occurred (3 b: 763 mV (DEp = 74 mV), 3 b-pTol: 1166 mV (DEp = 118 mV)). These events are assigned to the reduction of a decomposition fragment, which consists of a zirconocene fragment that might be different for 3 b and 3 b-pTol because to the large difference between the two values. Interestingly, an elimination of Fc CN was not observed during the electrochemical decomposition of 3 b, a result that differs from 3 a. The 3-ferrocenyl-2-propyne-nitrile, Fc CC CN, reacted with 1 a and 1 b differently than Fc CN. The conversion of 1 b with two equivalents of the nitrile to 4 b occurred within 15 min at ambient temperature even if a 1:1 ratio was applied (Scheme 6). Established complex 4 b corresponds to the compounds of the structural type B (Scheme 1). To the best of our knowledge, this is the first time that a nitrile–alkyne C C coupling of two identical substrates was observed.
Figure 2. Cyclic voltammograms at 25 8C; supporting electrolyte: 0.1 mol l 1 [NnBu4][B(C6F5)4] in dry tetrahydrofuran, glassy carbon working electrode (surface area 0.031 cm2), scan rate 100 mV s 1. All potentials measured versus FcH/FcH + : (A) First and second cycle of 3 a (solid) and Fc CN as product after multicyclic measurement (dashed). (B) 3 b (solid) and 3 b-pTol (dashed).
to take place at the ferrocenyl unit. A subsequent transfer of an electron from the titanocene fragment to the ferrocenyl groups, a process that has already been described for [Cp2Ti(h4-FcC4Fc)],[25] might initiate a decomposition pathway similar to that of 3 a-Ph. Two additional redox events were detected for 3 a at 226 and 373 mV, events that showed a (quasi)reversible behavior (DEp is 101 or 73 mV, respectively) during the first five cycles, followed by an irreversible oxidation at 721 mV. After multicyclic measurements, only the event at 373 mV remained, which was assigned to free Fc CN (370 mV in tetrahydrofuran versus FcH/FcH + ). This leads to the conclusion that the disappearing event at 226 mV might belong to an intermediate that consists of a ferrocenyl unit and decomposes slowly under the measurement conditions forming Fc CN. Such an elimination of nitrile was already described for the treatment of 3 a-Ph with, for example, H2 or CO2.[18] In accordance to 3 a and 3 a-Ph, the zirconium compounds, 3 b and 3 b-pTol, show irreversible redox behavior during oxidation, leading to complete decomposition (Figure 2). For 3 bpTol, only one process above 50 mV was observed, a process that was attributed to the zirconocene fragment. The ferrocene Chem. Eur. J. 2014, 20, 3061 – 3068
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Scheme 6. Reactions of 1 a and 1 b with Fc CC CN.
Compound 4 b was isolated as red crystals, which were suitable for X-ray crystal-structure analysis (Figure 3). The fivemembered metallacycle consists of two double (C3 N1 1.273(2), C4 C5 1.370(2) ) and one single bond (C3 C4 1.506(2) ). The analysis shows that this complex stabilizes itself by dimerization. Therefore the free nitrile function coordinates to the metal center of another molecule under formation of an eight-membered dimetallacycle. The detailed characterization by NMR spectroscopy is impeded by the formation of an equilibrium between 4 b and its corresponding monomer. The main component of this equilibrium might be 4 b but a clear assignment is not possible. In the IR spectra, the absorption bands at 1550, 1492, and 1433 cm 1 are assigned to the C=N and C=C bonds. As a result of the formation of 4 b, as a dimer, the typical signals of the CN and CC bonds changed their order and were shifted in comparison to those
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Full Paper 175.9 ppm (C=C) and 141.4 ppm (C=N). The signals of the stillintact alkyne and nitrile unit (84.4, 87.4 (CC), 123.6 ppm (CN)) are shifted as well, owing to the large modification of the p-electron delocalization. Additionally, we investigated the electrochemical behavior of 4 a and 4 b. For both complexes, irreversible redox events of the starting material were detected at any potential range (Figure 4, Table 3). Starting the measurement in the cathodic
Figure 3. Molecular structure of 4 b in the solid state. Hydrogen atoms and the solvent molecule are omitted for clarity. The Cp* ligands are depicted by their centroids. The thermal ellipsoids correspond to 30 % probability. Selected bond lengths [] and angles [deg]: Zr1 N1 2.1256(12), C5 Zr1 2.4445(15), C3 N1 1.273(2), C3 C4 1.506(2), C4 C5 1.370(2), C6 N2 1.1611(19), C3-N1-Zr1 125.08(10), C4-C5-Zr1 114.02(10), N1-C3-C4 118.91(13), C5-C4-C3 111.30(13).
of free Fc CC CN (4 b: 2114 (CN), 2196 cm 1 (CC); Fc CC CN: 2249 (CN), 2142 cm 1 (CC)[26]). The conversion of 1 a into 4 a with two equivalents of Fc CC CN requires higher temperature and longer reaction times than the analogous conversion of 1 b into 4 b. This observation can be explained by the different size of the metal, as already mentioned in case of the reactions with Fc CN. As the first step in the formation of 4 b, we propose a coordination of one molecule of Fc CC CN to 1 b through its CN bond, similar to that shown in Scheme 4 for 2 b. This might promote the elimination of Me3SiCCSiMe3 and the formation of the alkyne complex [Cp*2Zr(h2-Fc C2-CN)] as an intermediate of the coupling reaction. In case of 1 a, a dissociative reaction path seems to be favored with the first step being the elimination of the alkyne. For Fc CC CN, the selective formation of compound 4 and the preference for alkyne–nitrile C C coupling over alkyne–alkyne or nitrile–nitrile coupling are in both cases remarkable. In contrast to 4 b, compound 4 a seems not to be stabilized by dimerization. The coordination of the NC moiety to the metal is probably inhibited by the coordination sphere and the size of the metal. Hints for this assumption could be the good solubility and the smaller shift of the resonance of the CN bond in the IR spectra (2140 cm 1). The advantage of the good solubility is the possibility to characterize the nitrile–alkyne C C coupling product, 4 a, by NMR spectroscopy. As a consequence of coupling, the two ferrocenyl units are different, compared to 3 a and 3 b, as is evident by the duplication of the signals in the 1H NMR spectrum. Two singlets are assigned to the C5H5 units (4.27, 4.31 ppm) and 4 multiplets to the C5H4 groups (4.30, 4.31, 4.58, 5.04 ppm). In the 13C NMR spectrum, six signals for the quaternary carbon atoms were identified. Owing to the formation of the five-membered metallacycle, the resonances of the former alkyne (86.0, 59.8 ppm) and nitrile unit (106.1 ppm) are significantly shifted to 159.2, Chem. Eur. J. 2014, 20, 3061 – 3068
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Figure 4. Cyclic voltammograms at 25 8C; supporting electrolyte: 0.1 mol l 1 [NnBu4][B(C6F5)4] in dry tetrahydrofuran, glassy carbon working electrode (surface area 0.031 cm2). All potentials measured vs. FcH/FcH + : (A) First and second cycle of 4 a, scan rate 100 mV s 1. (B) First cycle of 4 b (solid) and fifth cycle of 4 b (dashed), scan rate 300 mV s 1.
direction, no redox processes in the initial step were observed for the titanium complex 4 a. The first cycle shows three irreversible oxidations at 369, 257, and 25 mV, events that lead to the decomposition of the starting material, and two quasireversible redox events at 225 (DEp = 119 mV) and 523 mV (DEp = 159 mV). After multicyclic measurements, the last two events remain, while the first three processes disappeared. The quasireversible redox events may be assigned to a ferrocenylcontaining compound, which is formed by electrochemically induced decomposition of 4 a. The release of Fc CC CN by C C bond cleavage can be excluded because the redox potential of Fc CC CN (333 mV in tetrahydrofuran versus FcH/
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Full Paper Table 3. Electrochemical data of 4 a and 4 b. 4a E 01’ [mV] (DEp [mV]) E 02’ [mV] E 03’ [mV] E 04’ [mV] E 05’ [mV] (DEp [mV]) E 06’ [mV] (DEp [mV])
4b
1163[c] (96) 369[a] 257[a] 25[a] 225[b] (119) 523[b] (159)
1347[d] 897[c] 432[a] 205[a] 15[a] 141[a]
[a] Irreversible process; Epc; [b] Decomposition product; [c] Decomposition product, occurs after first cycle, Epa ; [d] Decomposition product, occurs after second cycle, Epa.
FcH + ) could not be observed during the study. In the second cycle, a quasireversible event at 1163 mV (DEp = 96 mV) appeared, an event that may be assigned to a titanocene-containing decomposition product, as already described for 3 b and 3 b-pTol. Interestingly, this event only occurs when the potential is raised above 100 mV first. This shows that the decomposition follows a multistep mechanism. The cyclic voltammogram of 4 b shows four irreversible redox events at 423, 205, 15, and 141 mV in the first cycle, event that lead to decomposition. These events are followed by several probably (quasi)reversible redox processes between 100 and 650 mV. It was not possible to obtain these scans in good resolution, neither by a variation of the scan rate, nor by square-wave voltammetry. It was necessary to measure the cyclic voltammogram of 4 b with a faster scan rate of 300 mV s 1, otherwise even the processes between 500 and 100 mV were not separated due to the fast decomposition of 4 b. After increasing the maximum potential above 650 mV, an additional irreversible reduction occurred at 897 mV. In the second cycle, this reduction was still observable, but a further irreversible reduction event appeared at 1347 mV. This also indicates a multistep decomposition mechanism for 4 b during multicyclic measurements.
Conclusion We investigated the reactions of [Cp*2M(h2-Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr) with the ferrocenyl nitriles, Fc CN and Fc CC CN, respectively. In the case of Fc CN, an unusual nitrile–nitrile C C homocoupling was observed and two 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were isolated. In contrast to this, the reaction with Fc CC CN yielded the 1metalla-2-aza-cyclopenta-2,4-dienes (4 a, b) by nitrile–alkyne heterocoupling. All appropriate complexes, 3 a, 3 b, 4 a, and 4 b, were obtained in high yields. Remarkably, both types of coupling reactions were highly selective. For Fc CN, the two nitriles might be coupled in three different ways: C C, C N, or N N (Scheme 3). The application of Cp* ligands and Fc CN affects very selectively the preference of C C coupling, giving compounds of type C (Scheme 1). Additionally, for Fc CC CN, which has a C C Chem. Eur. J. 2014, 20, 3061 – 3068
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and a C N triple bond, only heterocoupling of the nitrile and the alkyne to give compound 4 of type B (Scheme 1) was found from all the three possible types of C C coupling reactions. This isolation of just one product, as the other aspect of selectivity, is remarkable. The electrochemical measurements conducted for 3 a, 3 aPh, 3 b, 3 b-pTol, 4 a, and 4 b showed irreversible oxidations for all compounds, events that initiate complete decomposition. In the case of 3 a, this includes the formation of an unstable intermediate that eliminates Fc CN. The different influence of the ferrocenyl substituent on the 1-metalla-2,5-diaza-cyclopenta-2,4-diene is notable. The zirconium compound, 3 b, is more stable against oxidation than its analogue, 3 b-pTol, whereas it is exactly the opposite for the titanium complexes. For 4 a and 4 b, the formation of redox-active products and several redox processes were observed. Owing to the different potentials of these intermediates, different decomposition mechanisms are predicted.
Experimental Section General information All manipulations were carried out in an oxygen- and moisture-free argon atmosphere by using standard Schlenk and drybox techniques. Non-halogenated solvents were dried over sodium/benzophenone and freshly distilled prior to use. The compounds [Cp*2Ti(h2-Me3SiC2SiMe3)][21] and [Cp*2Zr(h2-Me3SiC2SiMe3)],[19] [27] Fc CN were synthesized as previously described in literature. The following instruments were used: NMR spectra: Bruker AV300 and AV400 (1H and 13C chemical shifts were referenced to the solvent signals: [D6]benzene (dH 7.16, dC 128.0), [D8]THF (dH 1.73, dC 25.2), and [D8]toluene (dH 2.08, dC 20.4)); IR: Bruker Alpha FT-IR spectrometer; MS: Finnigan MAT 95-XP from Thermo-Electron; elemental analysis: Leco Tru Spec elemental analyzer; melting points: METTLER-TOLEDO MP 70 (melting points are uncorrected and were measured in sealed capillaries). Electrochemical measurements were conducted at 25 8C under argon on 1.0 mmol L 1 solutions in dry, air-free tetrahydrofuran solutions containing 0.1 mol l 1 of [N(nBu)4][B(C6F5)4] as supporting electrolyte by using a Voltalap PGZ 100 radiometer electrochemical workstation interfaced with a personal computer. A three-electrode cell using a Pt auxiliary electrode, a glassy carbon working electrode (surface area 0.031 cm2), and an Ag/Ag + (0.01 mol l 1 [AgNO3]) reference electrode, mounted on a Luggin capillary, was used. The working electrode was pretreated by polishing on a Buehler microcloth first with 1 mm and then 1=4 mm diamond paste. The reference electrode was constructed from a silver wire inserted into a solution of 0.01 mol l 1 [AgNO3] and 0.1 mol l 1 [N(nBu)4][B(C6F5)4] in acetonitrile, in a Luggin capillary with a porous Vycor tip. This Luggin capillary was inserted into a second Luggin capillary with porous Vycor tip filled with a 0.1 mol l 1 [N(nBu)4][B(C6F5)4] solution in tetrahydrofuran. Successive experiments under the same experimental conditions showed that all formal reduction and oxidation potentials were reproducible within 5 mV. Experimental potentials were referenced against an Ag/Ag + reference electrode but results are presented referenced against ferrocene as an internal standard, as required by IUPAC.[22] To achieve this, because the ferrocene/ferrocenium couple FcH/FcH + interferes with the ferrocenyl signals, each experiment was first performed in the absence of any internal standard
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Full Paper and then repeated in the presence of 1 mmol L 1 ferrocene (FcH) or decamethylferrocene (Cp*2Fe). Data were then manipulated on a Microsoft Excel worksheet to set the formal reduction potentials of the FcH/FcH + couple to 0.0 V, while the FcH/FcH + couple itself was at 220 mV versus Ag/Ag + , DEp = 61 mV.[28] When Cp*2Fe was used as an internal standard, the experimentally measured potential was converted into E versus FcH/FcH + by addition of 0.53 V. Diffraction data for 3 a, 3 b, and 4 a were collected on a Bruker Kappa APEX II Duo diffractometer using graphite-monochromated Mo-Ka radiation and, for Fc CC CN, on an Oxford Gemini S diffractometer with graphite-monochromated Cu Ka radiation. The structures were solved by direct methods and refined by fullmatrix least-squares procedures on F2 with the SHELXTL software package.[29] Diamond was used for graphical representations.[30] CCDC-972479 (3 a), 972478 (3 b), 972477 (4 b) and 969828 (Fc CC CN) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Preparation of 2b To a solution of [Cp*2Zr(h2-Me3SiC2SiMe3)] (1 b) (0.267 g, 0.5 mmol) in toluene (10 mL), a solution of Fc CN (0.105 g, 0.5 mmol) in toluene (10 mL) was added at ambient temperature under stirring. The color of the mixture turned immediately purple. After 3 h of stirring at ambient temperature, all volatiles were removed in vacuum. Yield: 301 mg (0.40 mmol, 80 %). M.p: 161 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 0.57 (s, 18 H, SiMe3), 1.84 (s, 30 H, C5Me5), 3.85 (m, 2 H, C5H4), 4.07 (s, 5 H, C5H5), 4.32 ppm (m, 2 H, C5H4); 13C NMR (75 MHz, C6D6, 297 K): d = 4.9 (SiMe3), 12.4 (C5Me5), 53.9 (iC (C5H4)), 70.7 (C5H5), 71.1, 71.4 (aCH/bCH (C5H4), 112.4 ppm (C5Me5); IR (Nujol): n˜ = 2150 (w), 1550 (w), 1459 (s), 1028(w), 823 cm 1 (m); MS (EI): m/z (%): 360 (100) [Cp*2Zr] + , 211 (45) [Fc CN] + , 170 (6) [Me3SiC2SiMe3] + , 135 (4) [Cp*] + ; elemental analysis calcd (%) for C39H57FeNSi2Zr: C 63.03, H 7.73, N 1.88; found: C 63.10, H 7.60, N 1.73.
Preparation of 3a Fc CN (0.105 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added to a solution of [Cp*2Ti(h2-Me3SiC2SiMe3)] (1 a) (0.122 g, 0.25 mmol) in toluene (10 mL). This solution was warmed to 55 8C whereby the color changed from orange to red. After 4 d, at 55 8C all volatiles were removed in vacuum and the red residue was dissolved in tetrahydrofuran. The red solution was allowed to stand at 78 8C and red crystals formed within 1 d. These were separated by decanting and dried in vacuum. Yield: 163 mg (0.22 mmol, 89 %). M.p: 169–170 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.85 (s, 30 H, C5Me5), 3.98 (m, 4 H, C5H4), 4.22 (m, 4 H, C5H4), 4.26 ppm (s, 10 H, C5H5); 13C NMR (75 MHz, C6D6, 297 K): d = 11.8 (C5Me5), 67.3, 69.0 (C4-C7/C14-C17), 69.4 (C5H5), 90.9 (iC (C5H4)), 120.1 (C5Me5), 156.4 ppm (C=N); IR (ATR): n˜ = 3083 (vw), 2901 (w), 1573 (vw), 1525 (w), 1375 (w), 1018 (m), 806 (s), 622 (m), 475 cm 1 (vs); MS (CI): m/z (%): 740 (100) [M + ], 606 (10) [M-Cp*] + , 528 (40) [M-FcCNH] + , 317 (4) [Cp*2Ti] + , 212 (13) [FcCN] + ; elemental analysis calcd (%) for C42H48Fe2N2Ti: C 68.13, H 6.53, N 3.78; found: C 68.05, H 6.71, N 3.55.
Analytical data of the side component 3a’ Obtained after applying higher reaction temperature of 80 8C: M.p. 344 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.87 (s, 30 H, C5Me5), 4.00–4.08 (m, 8 H, C5H4), 4.15 (s, 10 H, C5H5), 6.64 ppm Chem. Eur. J. 2014, 20, 3061 – 3068
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(s, 2 H, NH); 13C NMR (75 MHz, C6D6, 297 K): d = 12.2 (C5Me5), 67.2 (C5H4), 69.0 (C5H5), 70.6 (iC), 115.9 (C5Me5); MS (CI): m/z (%): 742 (17) [M + ], 426 (100) [HN-C(Fc) = C(Fc)-NH + 2H] + , 212 ppm (67) [FcCN] + .
Preparation of 3b Fc CN (0.105 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added under stirring to a solution of [Cp*2Zr(h2Me3SiC2SiMe3)] (1 b) (0.133 g, 0.25 mmol) in toluene (10 mL). The reaction solution was stirred at 80 8C for 11 days and the color turned from purple to yellow. All volatiles were removed under vacuum and the remaining yellow residue dissolved in toluene. At 78 8C, yellow crystals formed and were separated by decanting and dried under vacuum. Yield: 166 mg (0.22 mmol, 85 %). M.p: 210–211 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.87 (s, 30 H, C5Me5), 4.02 (m, 4 H, C5H4), 4.31 (s, 10 H, C5H5), 4.34 ppm (m, 4 H, C5H4); 13C NMR (75 MHz, C6D6, 297 K): d = 11.1 (C5Me5), 67.5, 69.9 (C4-C7/C14-C17), 69.5 (C5H5), 91.7 (iC (C5H4)), 118.5 (C5Me5), 165.3 ppm (C=N); IR (ATR): n˜ = 3094 (vw), 2899 (w), 1618 (w), 1580 (w), 1105 (m), 807 (s), 466 cm 1 (vs); MS (CI): m/z (%): 782 (83) [M] + , 571 (16) [M-(FcCN)] + , 360 (7) [Cp*2Zr] + , 211 (24) [FcCN] + ; elemental analysis calcd (%) for C42H48Fe2N2Zr: C 64.36, H 6.17, N 3.57; found: C 64.34, H 6.44, N 3.34.
Conversion of 2b to 3b Compound 2 b was synthesized according to the procedure mentioned above. A small amount of 2 b was dissolved in C6D6 and filled into a J Young NMR tube. Subsequently it was warmed to 60 8C for 12 days. 1H and 13C NMR spectra were recorded before and after the heating.
New procedure for the synthesis of Fc CC CN (2-cyano-1-chlorovinyl)ferrocene (1.0 g, 3.7 mmol) (for synthetic procedure, see the Supporting Information) and KOH (0.215 g, 3.8 mmol) were dissolved in tert-butanol (25 mL) and stirred for 2 h at 60 8C. Subsequently, water (50 mL) was added and the mixture was extracted three times with dichloromethane (30 mL each). The combined organic layers were dried over MgSO4 and all volatiles were removed in vacuum. The crude product was purified by flash chromatography using silica and n-hexane/ethyl acetate (4:1, v/v) as eluent. Yield: 865 mg (3.7 mmol, 100 %) Orange prisms suitable for single-crystal X-ray diffraction analysis could be isolated by slow evaporation of a chloroform solution of Fc CC CN at ambient temperature. Analytical data are according to the literature.[26]
Preparation of complex 4a Fc CC CN (0.118 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added to a solution of [Cp*2Ti(h2-Me3SiC2SiMe3)] (1 a) (0.122 g, 0.25 mmol) in toluene (10 mL). The solution was warmed to 60 8C for 6 d and the color changed from orange to red-brown. All volatiles were removed in vacuum and the remaining red residue analyzed. Yield: 173 mg (0.22 mmol, 88 %). M.p: 118 8C (dec.) under Ar; 1H NMR (300 MHz, [D8]THF, 297 K): d = 1.93 (s, 30 H, C5Me5), 4.27 (s, 5 H, C5H5 (Fc-A)), 4.30 (m, 1 H, bCH (Fc-B)), 4.31 (m, bCH (Fc-A)), 4.31 (s, 5 H, C5H5 (Fc-B)), 4.58 (m, 1 H, aCH (FcA)), 5.04 ppm (m, 1 H, aCH (Fc-A)); 13C NMR (75 MHz, [D8]THF, 297 K): d = 11.9 (C5Me5), 66.2 (iC (Fc-A)), 68.5 (bCH (Fc-B)), 69.3 (aCH (Fc-B)), 69.8 (bCH (Fc-A)), 70.3 (C5H5 (Fc-B)), 70.6 (C5H5 (Fc-A)), 72.2 (aCH (Fc-A)), 84.4 (C6), 85.0 (iC (Fc-B)), 87.4 (C5), 122.1 (C5Me5),
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Full Paper 123.6 (C1), 141.4 (C4), 159.2 (C3), 175.9 ppm (C2); IR (ATR): n˜ = 2901 (w), 2186(w), 2140 (w), 1526 (w), 1491 (w), 1431(w), 1375 (m), 1006 (m), 815 (vs), 728 (vs), 694 (m), 496 cm 1 (vs); MS (EI): m/z (%): 788 (1) [M] + , 603 (1) [M-Fc] + , 577 (21) [M-(Fc CN)] + , 318 (6) [Cp*2Ti] + , 235 (10) [Fc C2-CN] + ; elemental analysis calcd (%) for C46H48Fe2N2Ti: C 70.07, H 6.14, N 3.55; found: C 70.04, H 6.25, N 3.36.
[6]
[7]
Preparation of 4b [8]
To [Cp*2Zr(h2-Me3SiC2SiMe3)] (1 b) (0.209 g, 0.4 mmol) in toluene (10 mL), a solution of Fc CC CN (0.185 g, 0.8 mmol) in toluene (10 mL) was added at ambient temperature under stirring. The color of the mixture turned immediately blue and after 15 min it was red-brown. All volatiles were removed in vacuum and the remaining red solid was dissolved in warm toluene (50 8C). By cooling to ambient temperature, red crystals of 4 b formed. Yield (with one molecule of toluene): 198 mg (0.12 mmol, 88 %). M.p: 282 8C (dec.) under Ar; IR (ATR) n˜ = 2898 (w), 2196 (vw), 2114(m), 1550 (w), 1492 (m), 1433(w), 689 (vs), 500 cm 1 (vs); MS (CI): m/z (%): 499 (2) [Cp*Zr(Fc C2-CN)(CCN)] + , 399 (1) [Cp*2Zr(CCN)] + , 387 (1) [Cp*2Zr(CN)] + , 210 (1) [Fc C2] + , 185 (1) [Fc] + , 135 (59) [Cp*] + ; elemental analysis calcd (%) for C92H96Fe4N4Zr2 + C7H8 calcd: C 67.72, H 5.97, N 3.19; found: 67.71, H 5.96, N 3.19. The characterization by NMR spectroscopy is difficult owing to the formation of an equilibrium between 4 b and its monomer. The signals observed in the 1H and 13C NMR spectra, assigned to the main and the side component, are listed in the Supporting Information.
[9] [10] [11] [12] [13]
[14] [15] [16]
Acknowledgements We thank the staff of LIKAT for the technical and analytical support. Financial support by the Deutsche Forschungsgemeinschaft (RO 1269/8–1) and the Fonds der Chemischen Industrie is acknowledged.
[17]
[18] [19]
Keywords: alkynes · coupling reactions · electrochemistry · metallacycles · nitriles [1] Selected examples and citations therein: a) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 2005, 24, 456 – 471; b) U. Rosenthal, V. V. Burlakov, M. A. Bach, T. Beweries, Chem. Soc. Rev. 2007, 36, 719 – 728; c) N. Suzuki, D. Hashizume, Coord. Chem. Rev. 2010, 254, 1307 – 1326; d) T. Beweries, U. Rosenthal, Science of Synthesis Knowledge Updates 2011/4; e) D. S. K. Podiyanachari, R. Frçhlich, C. G. Daniliuc, J. L. Petersen, G. Kehr, G. Erker, N. Suzuki, S. Yuasa, K. Hagimori, S. Inoue, T. Asada, T. Takemoto, Y. Masuyama, Dalton Trans. 2012, 41, 10811 – 10816; f) J. Ugolotti, G. Kehr, R. Frçhlich, S. Grimme, G. Erker, J. Am. Chem. Soc. 2009, 131, 1996 – 2007; g) N. Suzuki, T. Shimura, Y. Sakaguchi, Y. Masuyama, Pure Appl. Chem. 2011, 83, 1781 – 1788. [2] U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 2003, 22, 884 – 900. [3] Recent examples and citations therein: a) T. Araki, K. Noguchi, K. Tanaka, Angew. Chem. 2013, 125, 5727 – 5731; Angew. Chem. Int. Ed. 2013, 52, 5617 – 5621; b) I. Thiel, H. Jiao, A. Spannenberg, M. Hapke, Chem. Eur. J. 2013, 19, 2548 – 2554; c) S. Okamoto, Y.-k. Sugiyama, Synlett 2013, 1044 – 1060; d) Y. Satoh, Y. Obora, J. Org. Chem. 2013, 78, 7771 – 7776. [4] a) T. Beweries, M. Haehnel, U. Rosenthal, Catal. Sci. Technol. 2013, 3, 18 – 28; b) T. Beweries, U. Rosenthal, Nat. Chem. 2013, 5, 649 – 650. [5] Recent examples: a) W.-X. Zhang, S. Zhang, X. Sun, M. Nishiura, Z. Hou, Z. Xi, Angew. Chem. 2009, 121, 7363 – 7367; Angew. Chem. Int. Ed. 2009, 48, 7227 – 7231; b) S. Zhang, J. Zhao, W.-X. Zhang, Z. Xi, Org. Lett. 2011,
Chem. Eur. J. 2014, 20, 3061 – 3068
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13, 1626 – 1629; c) W.-X. Zhang, S. Zhang, Z. Xi, Acc. Chem. Res. 2011, 44, 541 – 551; d) J. Zhao, S. Zhang, W.-X. Zhang, Z. Xi, Coord. Chem. Rev. 2013, DOI:10.1016/j.ccr.2013.08.032. a) T. Takahashi, F.-Y. Tsai, M. Kotora, J. Am. Chem. Soc. 2000, 122, 4994 – 4995; b) T. Takahashi, F.-Y. Tsai, Y. Li, H. Wang, Y. Kondo, M. Yamanaka, K. Nakajima, M. Kotora, J. Am. Chem. Soc. 2002, 124, 5059 – 5067; c) D. Suzuki, Y. Nobe, Y. Watai, R. Tanaka, Y. Takayama, F. Sato, H. Urabe, J. Am. Chem. Soc. 2005, 127, 7474 – 7479. C. Cui, S. Kçpke, R. Herbst-Irmer, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt, B. Wrackmeyer, J. Am. Chem. Soc. 2001, 123, 9091 – 9098. a) G. Erker, R. Zwettler, J. Organomet. Chem. 1991, 409, 179 – 188; b) J. Zhao, S. Zhang, W.-X. Zhang, Z. Xi, Organometallics 2011, 30, 3464 – 3467. a) P. Legzdins, S. A. Lumb, Organometallics 1997, 16, 1825 – 1827; b) P. Legzdins, S. A. Lumb, V. G. Young, Organometallics 1998, 17, 854 – 871. M. Paneque, C. M. Posadas, M. L. Poveda, N. Rendn, K. Mereiter, Organometallics 2007, 26, 3120 – 3129. M. Baya, M. A. Esteruelas, A. I. Gonzlez, A. M. Lpez, E. OÇate, Organometallics 2005, 24, 1225 – 1232. T.-C. Hu, P.-I. Hsiao, T.-H. Wang, Y.-W. Yang, C.-Y. Chang, Y.-H. Wu, W.-C. Sun, M.-S. Yu, C.-Y. Lee, Y.-H. Lo, Dalton Trans. 2011, 40, 12663 – 12666. For a niobium alkyne complex, a similar reaction with a nitrile was observed, but instead of the coordination of an additional ligand, the bonding situation in the five-membered cycle was changed and a Nb C double bond was formed (P. Lorente, C. Carfagna, M. Etienne, B. Donnadieu, Organometallics 1996, 15, 1090 – 1092). P. Binger, F. Sandmeyer, C. Krueger, Organometallics 1995, 14, 2969 – 2976. M. H. Chisholm, D. M. Hoffman, J. C. Huffman, J. Am. Chem. Soc. 1984, 106, 6815 – 6826. a) M. Weidenbruch, P. Will, K. Peters, Z. Anorg. Allg. Chem. 1996, 622, 1811 – 1813; b) P. Jutzi, D. Eikenberg, B. Neumann, H.-G. Stammler, Organometallics 1996, 15, 3659 – 3663; c) L. Wang, W. Chen, Z. Li, X.-Q. Xiao, G. Lai, X. Liu, Z. Xu, M. Kira, Chem. Commun. 2013, 49, 9776 – 9778; and references therein. a) P. T. Wolczanski, Ph. D. Thesis, California Institute of Technology, 1981; b) J. E. Bercaw, D. L. Davies, P. T. Wolczanski, Organometallics 1986, 5, 443 – 450. L. Becker, P. Arndt, H. Jiao, A. Spannenberg, U. Rosenthal, Angew. Chem. Int. Ed. 2013, 52, 11396 – 11400; Angew. Chem. 2013, 125, 11607 – 11611. J. Hiller, U. Thewalt, M. Polasek, L. Petrusova, V. Varga, P. Sedmwra, K. Mach, Organometallics 1996, 15, 3752 – 3759. U. Rosenthal, A. Ohff, M. Michalik, H. Gçrls, V. V. Burlakov, V. B. Shur, Angew. Chem. 1993, 105, 1228 – 1230; Angew. Chem. Int. Ed. Engl. 1993, 32, 1193 – 1195. V. V. Burlakov, A. V. Polyakov, A. I. Yanovsky, Yu. T. Struchkov, V. B. Shur, M. E. Vol’pin, U. Rosenthal, H. Gçrls, J. Organomet. Chem. 1994, 476, 197 – 206. W. W. Lukens, P. T. Matsunaga, R. A. Andersen, Organometallics 1998, 17, 5240 – 5247. The compounds with R = Ph and R = pTol were resynthesized according to the published procedures to obtain these values of the resonances in the IR spectra. G. Gritzner, J. Kuta, Pure Appl. Chem. 1984, 56, 461 – 466. K. Kaleta, A. Hildebrandt, F. Strehler, P. Arndt, H. Jiao, A. Spannenberg, H. Lang, U. Rosenthal, Angew. Chem. 2011, 123, 11444 – 11448; Angew. Chem. Int. Ed. 2011, 50, 11248 – 11252. S. Kato, M. T. R. Beels, P. La Porta, W. B. Schweizer, C. Boudon, J.-P. Gisselbrecht, I. Biaggio, F. Diederich, Angew. Chem. 2010, 122, 6343 – 6347; Angew. Chem. Int. Ed. 2010, 49, 6207 – 6211. A. Kivrak, M. Zora, J. Organomet. Chem. 2007, 692, 2346 – 2349. J. R. Aranzaes, M.-C. Daniel, D. Astruc, Can. J. Chem. 2006, 84, 288 – 299. G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112 – 122. K. Brandenburg, DIAMOND. Version 3.1e. Crystal Impact Gbr, 2007, Bonn, Germany.
Received: November 15, 2013
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Full Paper
& C C Coupling | Very Important Paper |
Unusual Nitrile–Nitrile and Nitrile–Alkyne Coupling of Fc CN and Fc CC CN Lisanne Becker,[a] Frank Strehler,[b] Marcus Korb,[b] Perdita Arndt,[a] Anke Spannenberg,[a] Wolfgang Baumann,[a] Heinrich Lang,*[b] and Uwe Rosenthal*[a]
Abstract: The reactions of the Group 4 metallocene alkyne complexes, [Cp*2M(h2-Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr, Cp* = h5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc CN and Fc CC CN (Fc = Fe(h5-C5H5)(h5-C5H4)), is described. In case of Fc CN an unusual nitrile–nitrile C C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were obtained. As the first step of the reaction with 1 b, the nitrile was coordinated to give [Cp*2Zr(h2-Me3SiC2SiMe3)(NC-Fc)] (2 b). The reactions with
the 3-ferrocenyl-2-propyne-nitrile Fc CC CN lead to an alkyne–nitrile C C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4 a, b). For M = Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3 a, b and 4 a, b was investigated, showing decomposition after oxidation, leading to different redox-active products.
Introduction During the last decades, the formation and the reactivity of small metallacycles was investigated, especially that of the three- and five-membered “all-C” cycles like metallacyclopropenes, 1-metallacyclopent-3-ynes (metallacyclopentynes), 1metalla-cyclopenta-2,3,4-trienes (metallacyclocumulenes), and 1-metalla-cyclopenta-2,3-dienes (metallacycloallenes).[1] The metallacyclopropenes can be obtained by a reduction of the metallocene dichlorides in the presence of an alkyne. An excess of the alkyne leads in most cases to alkyne–alkyne coupling and the formation of the 1-metalla-cyclopenta-2,4-dienes (A, Scheme 1).[2] These compounds are well studied and described as intermediates in the formation of different organic compounds, such as benzene or pyridine.[3] In contrast to the above transformations, the formation of the analogous heterometallacycles,[4] B and C, by the heterocoupling of a nitrile and an alkyne or a the homocoupling of two nitriles, has been insufficiently investigated. Owing to this
[a] L. Becker, Dr. P. Arndt, Dr. A. Spannenberg, Dr. W. Baumann, Prof. Dr. U. Rosenthal Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert-Einstein-Straße 29a, 18059 Rostock (Germany) Fax: (+ 49) 381-1281-51176 E-mail: [email protected] [b] F. Strehler, M. Korb, Prof. Dr. H. Lang Technische Universitt Chemnitz, Faculty of Natural Sciences Institute of Chemistry, Inorganic Chemistry 09107 Chemnitz (Germany) Fax: (+ 49) 371-531-21219 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304478. Chem. Eur. J. 2014, 20, 3061 – 3068
Scheme 1. Coupling reactions of CC and CN bonds.
lack of knowledge, we were interested in studying the reactions of nitriles with Group 4 metallocenes. The reaction of zirconocene compounds with diynes and different nitriles (for example, Ph CN, tBu CN) was already described in detail by Xi and co-workers.[5] The chosen nitrile influences significantly the outcome of the reaction with Group 4 metallocenes, thus indicating a broad reaction pattern. The 1-metalla-2-aza-cyclopenta-2,4-dienes (B) established by nitrile–alkyne C C coupling are also known as intermediates in the formation of pyridines.[6] This first step of the reaction supports the preparation of pyridines with a certain substitution pattern in high yields. Although these reactions are known and have been applied for many years, the 1-metalla-2-aza-cyclopenta-2,4-dienes (B) were not often isolated and characterized. To the best of our knowledge, just one aluminum complex consisting of this structural motif was obtained by such a coupling.[7] In the other cases of alkyne–nitrile coupling to give comparable five-membered cycles, the established compounds were stabilized to give B1 (Scheme 2). The formation of B1
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Full Paper Results and Discussion We were interested in the reactions of [Cp*2M(h2Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr) with the ferrocenyl nitriles, Fc CN and Fc CC CN, respectively. C C coupling to give 1-metalla-2,5-diaza-cyclopenta-2,4dienes (C, Scheme 1) was observed in the case of Fc CN. At ambient temperature, 2 b is formed by the coordination of Fc CN to 1 b[19] (Scheme 4). This complex was isolated in high yields as a purple solid. The alkyne signals in the 1H and the 13C{1H} NMR spectra are shifted and broadened in contrast to those of 1 b (2 b: 1H: 0.57 ppm, 13C: 4.9 ppm; 1 b: 1H:
Scheme 2. Stabilization possibilities of B.
was observed for zirconium,[8] tungsten,[9] iridium,[10] osmium,[11] and ruthenium.[12] As the first step, the metallacyclopropenes of these elements reacted with nitriles to give 1-metalla-2-azacyclopenta-2,4-dienes. Subsequently, an additional ligand L (e.g., nitrile, phosphine) coordinates and a hydrogen atom is bonded to the nitrogen atom. The origin of this proton can be an additional proton source or ligand L itself.[13] The other possible and comparable products of alkyne–nitrile coupling are B2 and B3 (Scheme 2). Compounds of type Scheme 4. Reactions of 1 a and 1 b with Fc CN. B2 were established in the reaction of bimetallic complexes, 0.19 ppm, 13C: 4.0 ppm).[19] These data are in good agreement which consist either of a titanium and a boron or a zirconium [14] with those of analogue complexes, [Cp*2Zr(h2-Me3SiC2SiMe3)and an aluminum center, with nitriles. The NC unit inserts into the B Cplanar or Al Cplanar bond under formation of the (NC-R)] (1H: 0.54 (R = Ph), 0.56 ppm (R = pTol); 13C: 4.9 (R = complexes B2. The compounds of type B3 were obtained by Ph), 4.9 ppm (R = pTol)).[18] The typical resonances of the coordi[15] a nitrile–alkyne coupling at ditungsten complexes. nated Fc CN are slightly shifted as well compared to the free nitrile (2 b: 1H: 4.07 ppm (Cp); 13C: 70.7 ppm (Cp); Fc CN: 1H: In contrast to alkyne–alkyne and alkyne–nitrile C C coupling, reactions that form complexes of structural types A and 3.87 ppm (Cp); 13C: 70.6 ppm (Cp)). Owing to the broadening B, nitrile–nitrile C C coupling leading to 1-metalla-2,5-diaza-cyof the signals of the two quaternary C atoms (Calkyne) of 2 b, clopenta-2,4-dienes (C, Scheme 1) was very rarely observed. It these signals could not be recognized in the 13C NMR spec[7] [16] was described for aluminum and silylenes, but there were trum.[20] In the IR spectra, the typical resonances of the CN only two hints for this reaction with the early transition bond is likewise shifted from 2222 to 2150 cm 1 in 2 b. [17] The reaction of 1 b with two equivalents of Fc CN at metals and their molecular structure was determined by Xhigher temperature leads to nitrile–nitrile C C coupling and ray crystal structure analysis.[18] Notably, these ring-strained heterometallacycles, C, differ from the well-known 1,4-diaza1-zircona-2,5-diaza-cyclopenta-2,4-diene (3 b) was obtained in buta-1,4-diene complexes (1-metalla-2,5-diaza-cyclopent-3high yield (Scheme 4). As mentioned above, such a remarkable enes). In spite of the similarity to alkyne–alkyne coupling, this C C coupling is uncommon.[7, 16–18] selective reaction is not predictable owing to the differences The NMR and IR spectra of this new complex are comparabetween nitriles and alkynes. Moreover, an N N or an N C ble to those of the analogous compounds with Ph, pTol, or coupling to other products (Scheme 3, C1 and C2) is in princioTol as substituents instead of Fc (Table 1).[18] As a result of the ple possible as well and the route favored might depend on formation of the C=N bond, the signal of the carbon atom in the metal, the ligand, and the structure of the nitrile. the 13C NMR spectrum is shifted from 119.6 to 165.3 ppm and the IR adsorption bands of this bond are identified at 1580 and 1618 cm 1. Compound 3 b was isolated as yellow crystals, which were suitable for X-ray crystal-structure analysis (Figure 1). The C=N bonds (C1 N1 1.279(2), C2 N2 1.272(2) ) and the C C bond (C1 C2 1.5514(16) ) correspond to those of [Cp*2Zr(-N=C(Ph)-C(Ph) = N-)][18] (Table 1). For the formation of 3 b from 1 b, an associative or a dissociative reaction path are possible. It was often observed that zirconium complexes prefer the associative one[1d] and the formation of 2 b at room temperature might be a hint for this. To Scheme 3. Possible products of nitrile–nitrile couplings. Chem. Eur. J. 2014, 20, 3061 – 3068
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Full Paper Table 1. Comparison of 3 a, 3 b, and [Cp*2M(-N=C(Ph)-C(Ph) = N-)] (3 aPh: M = Ti, 3 b-Ph: M = Zr).[18, 23] 3a
3 a-Ph
3b
3 b-Ph
156.4
156.4
165.3
167.4
IR (C=N) [cm 1]
1525 1573
1528 1591
1580 1618
1575 1642
Bond lengths (C N) []
1.276(2) 1.274(2)
1.274(2) 1.276(2)
1.279(2) 1.272(2)
1.267(2) 1.268(2)
13
C NMR (C=N) [ppm]
Scheme 5. Formation of 3 a’.
Figure 1. Molecular structure of 3 b in the solid state. Hydrogen atoms are omitted for clarity. The thermal ellipsoids correspond to 30 % probability. Selected bond lengths [] and angles [deg]: C1 C2 1.5514(16), C1 N1 1.279(2), C2 N2 1.272(2), Zr1 N1 2.1086(10), Zr1 N2 2.1206(10), C1-N1-Zr1 116.40(8), C2-N2-Zr1 115.71(8), N1-C1-C2 114.74(10), N2-C2-C1 115.78(10).
67.3, 69.0 (C5H4), 69.4 ppm (C5H5)) are in the same range as those for 3 b (1H: 4.02, 4.34 (C5H4), 4.31 ppm (C5H5); 13C: 67.5, 69.9 (C5H4), 69.5 ppm (C5H5)). Complex 3 a, which is isostructural to 3 b, was isolated as red crystals in high yields. Recently, the structural, energetic, and chemical properties of the 1-metalla-2,5-diaza-cyclopenta-2,4-dienes were reported.[18] These compounds may possess interesting electrochemical properties when ferrocenyl substituents are introduced. Therefore, 3 a, 3 b, and [Cp*2M(-N=C(R)-C(R) = N-)] (3 a-Ph: M = Ti, R = Ph; 3 b-pTol: M = Zr, R = pTol) have been studied by cyclic voltammetry (CV) using a dry tetrahydrofuran solution containing 0.1 mol L 1 [NnBu4][B(C6F5)4] as the supporting electrolyte. The measurements were carried out at a scan rate of 100 mV s 1, if not otherwise mentioned, and the potentials are referenced to the FcH/FcH + redox couple, as recommended by IUPAC.[24] Compounds 3 a and 3 a-Ph show irreversible redox processes of the starting material in any potential range (Figure 2, Table 2). Two oxidation processes that lead to complete decomposition in multicyclic experiments are detected for both
Table 2. Electrochemical data of 3 a, 3 b, and [Cp*2M(-N=C(R)-C(R) = N-)] (3 a-Ph: M = Ti, R = Ph, 3 b-pTol: M = Zr, R = pTol).[23]
corroborate this assumption, a C6D6 solution of 2 b was warmed to 60 8C. 1H and 13C NMR spectra were recorded before the heating and after 12 days at 60 8C. The result of this experiment was a complete conversion of 2 b into 3 b, 1 b and free Me3SiCCSiMe3. The titanium complex 1 a[21] did not react with Fc CN at ambient temperature to give a coordination product, which is a hint for a dissociative reaction path. This different behavior of 1 a and 1 b was already described in the reaction with other aryl nitriles[18] and is caused by the size of the metals.[1d] Warming a solution containing 1 a and Fc CN to 55 8C resulted in the formation of 3 a. By exceeding this temperature, the corresponding 1,4-diaza-buta-1,3-diene complex (3 a’) is obtained (Scheme 5). The supposed origin of the hydrogen atoms for its formation is either the solvent or a C H activation of the [Cp*2Ti] fragment leading to the tucked-in complex.[22] Concerning the typical signals of the five-membered cycle of 3 a in the 13C NMR and IR spectra, this compound is, as 3 b, comparable to its analogue, 1-titana-2,5-diaza-cyclopenta-2,4dienes, [Cp*2Ti(-N=C(R)-C(R) = N-)] (R = Ph, pTol, oTol) (Table 1).[18] The signals of the ferrocenyl units in the 1H and the 13C NMR spectra (1H: 3.98, 4.22 (C5H4), 4.26 ppm (C5H5); 13C: Chem. Eur. J. 2014, 20, 3061 – 3068
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3a 0 1
E ’ [mV] (DEp [mV]) E 02’ [mV] E 03’ [mV] (DEp [mV]) E 04’ [mV] (DEp [mV]) E 05’ [mV]
275
3 a-Ph [a]
115[a] 226[b] (101) 373[b] (73) 721[b]
205
[a]
3b
3 b-pTol [c]
1166[c] (118) > 50 (broad)[a] –
355[a] –
763 (74) 342[a] 532
–
642
–
–
–
–
[a] Irreversible process; Epc ; [b] Decomposition product; [c] Decomposition product, occurs after first cycle.
complexes, with significantly higher potentials for 3 a-Ph (3 a: 275, 115 mV; 3 a-Ph: 205, 355 mV). Owing to the irreversibility of the first events, the second ones cannot be assigned unequivocally to 3 a or 3 a-Ph and may belong to decomposition products. Because of the dissimilarity of the potential range of the oxidations, two different multistate decomposition mechanisms are presumed. The oxidation of the titanium center might be the first step in the case of 3 a-Ph owing to the absence of further redox-active units, which initiate decomposition by Ti N bond cleavage. For 3 a, the first step is predicted
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Full Paper substituent stabilizes the compound against oxidation and leads to the detection of three redox events for 3 b, all events being at higher potentials (342, 532, 642 mV). Assignment of the first one to the zirconocene or the ferrocenyl unit is not possible and, owing to its irreversibility, the following ones cannot unambiguously be assigned to the starting complex. By using the cathodic direction for the initial step of the cyclic voltammogram, no redox processes were observed for 3 b and 3 b-pTol below 200 mV in the first cycle. After measuring in the anodic direction, quasireversible/reversible redox events occurred (3 b: 763 mV (DEp = 74 mV), 3 b-pTol: 1166 mV (DEp = 118 mV)). These events are assigned to the reduction of a decomposition fragment, which consists of a zirconocene fragment that might be different for 3 b and 3 b-pTol because to the large difference between the two values. Interestingly, an elimination of Fc CN was not observed during the electrochemical decomposition of 3 b, a result that differs from 3 a. The 3-ferrocenyl-2-propyne-nitrile, Fc CC CN, reacted with 1 a and 1 b differently than Fc CN. The conversion of 1 b with two equivalents of the nitrile to 4 b occurred within 15 min at ambient temperature even if a 1:1 ratio was applied (Scheme 6). Established complex 4 b corresponds to the compounds of the structural type B (Scheme 1). To the best of our knowledge, this is the first time that a nitrile–alkyne C C coupling of two identical substrates was observed.
Figure 2. Cyclic voltammograms at 25 8C; supporting electrolyte: 0.1 mol l 1 [NnBu4][B(C6F5)4] in dry tetrahydrofuran, glassy carbon working electrode (surface area 0.031 cm2), scan rate 100 mV s 1. All potentials measured versus FcH/FcH + : (A) First and second cycle of 3 a (solid) and Fc CN as product after multicyclic measurement (dashed). (B) 3 b (solid) and 3 b-pTol (dashed).
to take place at the ferrocenyl unit. A subsequent transfer of an electron from the titanocene fragment to the ferrocenyl groups, a process that has already been described for [Cp2Ti(h4-FcC4Fc)],[25] might initiate a decomposition pathway similar to that of 3 a-Ph. Two additional redox events were detected for 3 a at 226 and 373 mV, events that showed a (quasi)reversible behavior (DEp is 101 or 73 mV, respectively) during the first five cycles, followed by an irreversible oxidation at 721 mV. After multicyclic measurements, only the event at 373 mV remained, which was assigned to free Fc CN (370 mV in tetrahydrofuran versus FcH/FcH + ). This leads to the conclusion that the disappearing event at 226 mV might belong to an intermediate that consists of a ferrocenyl unit and decomposes slowly under the measurement conditions forming Fc CN. Such an elimination of nitrile was already described for the treatment of 3 a-Ph with, for example, H2 or CO2.[18] In accordance to 3 a and 3 a-Ph, the zirconium compounds, 3 b and 3 b-pTol, show irreversible redox behavior during oxidation, leading to complete decomposition (Figure 2). For 3 bpTol, only one process above 50 mV was observed, a process that was attributed to the zirconocene fragment. The ferrocene Chem. Eur. J. 2014, 20, 3061 – 3068
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Scheme 6. Reactions of 1 a and 1 b with Fc CC CN.
Compound 4 b was isolated as red crystals, which were suitable for X-ray crystal-structure analysis (Figure 3). The fivemembered metallacycle consists of two double (C3 N1 1.273(2), C4 C5 1.370(2) ) and one single bond (C3 C4 1.506(2) ). The analysis shows that this complex stabilizes itself by dimerization. Therefore the free nitrile function coordinates to the metal center of another molecule under formation of an eight-membered dimetallacycle. The detailed characterization by NMR spectroscopy is impeded by the formation of an equilibrium between 4 b and its corresponding monomer. The main component of this equilibrium might be 4 b but a clear assignment is not possible. In the IR spectra, the absorption bands at 1550, 1492, and 1433 cm 1 are assigned to the C=N and C=C bonds. As a result of the formation of 4 b, as a dimer, the typical signals of the CN and CC bonds changed their order and were shifted in comparison to those
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Full Paper 175.9 ppm (C=C) and 141.4 ppm (C=N). The signals of the stillintact alkyne and nitrile unit (84.4, 87.4 (CC), 123.6 ppm (CN)) are shifted as well, owing to the large modification of the p-electron delocalization. Additionally, we investigated the electrochemical behavior of 4 a and 4 b. For both complexes, irreversible redox events of the starting material were detected at any potential range (Figure 4, Table 3). Starting the measurement in the cathodic
Figure 3. Molecular structure of 4 b in the solid state. Hydrogen atoms and the solvent molecule are omitted for clarity. The Cp* ligands are depicted by their centroids. The thermal ellipsoids correspond to 30 % probability. Selected bond lengths [] and angles [deg]: Zr1 N1 2.1256(12), C5 Zr1 2.4445(15), C3 N1 1.273(2), C3 C4 1.506(2), C4 C5 1.370(2), C6 N2 1.1611(19), C3-N1-Zr1 125.08(10), C4-C5-Zr1 114.02(10), N1-C3-C4 118.91(13), C5-C4-C3 111.30(13).
of free Fc CC CN (4 b: 2114 (CN), 2196 cm 1 (CC); Fc CC CN: 2249 (CN), 2142 cm 1 (CC)[26]). The conversion of 1 a into 4 a with two equivalents of Fc CC CN requires higher temperature and longer reaction times than the analogous conversion of 1 b into 4 b. This observation can be explained by the different size of the metal, as already mentioned in case of the reactions with Fc CN. As the first step in the formation of 4 b, we propose a coordination of one molecule of Fc CC CN to 1 b through its CN bond, similar to that shown in Scheme 4 for 2 b. This might promote the elimination of Me3SiCCSiMe3 and the formation of the alkyne complex [Cp*2Zr(h2-Fc C2-CN)] as an intermediate of the coupling reaction. In case of 1 a, a dissociative reaction path seems to be favored with the first step being the elimination of the alkyne. For Fc CC CN, the selective formation of compound 4 and the preference for alkyne–nitrile C C coupling over alkyne–alkyne or nitrile–nitrile coupling are in both cases remarkable. In contrast to 4 b, compound 4 a seems not to be stabilized by dimerization. The coordination of the NC moiety to the metal is probably inhibited by the coordination sphere and the size of the metal. Hints for this assumption could be the good solubility and the smaller shift of the resonance of the CN bond in the IR spectra (2140 cm 1). The advantage of the good solubility is the possibility to characterize the nitrile–alkyne C C coupling product, 4 a, by NMR spectroscopy. As a consequence of coupling, the two ferrocenyl units are different, compared to 3 a and 3 b, as is evident by the duplication of the signals in the 1H NMR spectrum. Two singlets are assigned to the C5H5 units (4.27, 4.31 ppm) and 4 multiplets to the C5H4 groups (4.30, 4.31, 4.58, 5.04 ppm). In the 13C NMR spectrum, six signals for the quaternary carbon atoms were identified. Owing to the formation of the five-membered metallacycle, the resonances of the former alkyne (86.0, 59.8 ppm) and nitrile unit (106.1 ppm) are significantly shifted to 159.2, Chem. Eur. J. 2014, 20, 3061 – 3068
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Figure 4. Cyclic voltammograms at 25 8C; supporting electrolyte: 0.1 mol l 1 [NnBu4][B(C6F5)4] in dry tetrahydrofuran, glassy carbon working electrode (surface area 0.031 cm2). All potentials measured vs. FcH/FcH + : (A) First and second cycle of 4 a, scan rate 100 mV s 1. (B) First cycle of 4 b (solid) and fifth cycle of 4 b (dashed), scan rate 300 mV s 1.
direction, no redox processes in the initial step were observed for the titanium complex 4 a. The first cycle shows three irreversible oxidations at 369, 257, and 25 mV, events that lead to the decomposition of the starting material, and two quasireversible redox events at 225 (DEp = 119 mV) and 523 mV (DEp = 159 mV). After multicyclic measurements, the last two events remain, while the first three processes disappeared. The quasireversible redox events may be assigned to a ferrocenylcontaining compound, which is formed by electrochemically induced decomposition of 4 a. The release of Fc CC CN by C C bond cleavage can be excluded because the redox potential of Fc CC CN (333 mV in tetrahydrofuran versus FcH/
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Full Paper Table 3. Electrochemical data of 4 a and 4 b. 4a E 01’ [mV] (DEp [mV]) E 02’ [mV] E 03’ [mV] E 04’ [mV] E 05’ [mV] (DEp [mV]) E 06’ [mV] (DEp [mV])
4b
1163[c] (96) 369[a] 257[a] 25[a] 225[b] (119) 523[b] (159)
1347[d] 897[c] 432[a] 205[a] 15[a] 141[a]
[a] Irreversible process; Epc; [b] Decomposition product; [c] Decomposition product, occurs after first cycle, Epa ; [d] Decomposition product, occurs after second cycle, Epa.
FcH + ) could not be observed during the study. In the second cycle, a quasireversible event at 1163 mV (DEp = 96 mV) appeared, an event that may be assigned to a titanocene-containing decomposition product, as already described for 3 b and 3 b-pTol. Interestingly, this event only occurs when the potential is raised above 100 mV first. This shows that the decomposition follows a multistep mechanism. The cyclic voltammogram of 4 b shows four irreversible redox events at 423, 205, 15, and 141 mV in the first cycle, event that lead to decomposition. These events are followed by several probably (quasi)reversible redox processes between 100 and 650 mV. It was not possible to obtain these scans in good resolution, neither by a variation of the scan rate, nor by square-wave voltammetry. It was necessary to measure the cyclic voltammogram of 4 b with a faster scan rate of 300 mV s 1, otherwise even the processes between 500 and 100 mV were not separated due to the fast decomposition of 4 b. After increasing the maximum potential above 650 mV, an additional irreversible reduction occurred at 897 mV. In the second cycle, this reduction was still observable, but a further irreversible reduction event appeared at 1347 mV. This also indicates a multistep decomposition mechanism for 4 b during multicyclic measurements.
Conclusion We investigated the reactions of [Cp*2M(h2-Me3SiC2SiMe3)] (1 a: M = Ti, 1 b: M = Zr) with the ferrocenyl nitriles, Fc CN and Fc CC CN, respectively. In the case of Fc CN, an unusual nitrile–nitrile C C homocoupling was observed and two 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were isolated. In contrast to this, the reaction with Fc CC CN yielded the 1metalla-2-aza-cyclopenta-2,4-dienes (4 a, b) by nitrile–alkyne heterocoupling. All appropriate complexes, 3 a, 3 b, 4 a, and 4 b, were obtained in high yields. Remarkably, both types of coupling reactions were highly selective. For Fc CN, the two nitriles might be coupled in three different ways: C C, C N, or N N (Scheme 3). The application of Cp* ligands and Fc CN affects very selectively the preference of C C coupling, giving compounds of type C (Scheme 1). Additionally, for Fc CC CN, which has a C C Chem. Eur. J. 2014, 20, 3061 – 3068
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and a C N triple bond, only heterocoupling of the nitrile and the alkyne to give compound 4 of type B (Scheme 1) was found from all the three possible types of C C coupling reactions. This isolation of just one product, as the other aspect of selectivity, is remarkable. The electrochemical measurements conducted for 3 a, 3 aPh, 3 b, 3 b-pTol, 4 a, and 4 b showed irreversible oxidations for all compounds, events that initiate complete decomposition. In the case of 3 a, this includes the formation of an unstable intermediate that eliminates Fc CN. The different influence of the ferrocenyl substituent on the 1-metalla-2,5-diaza-cyclopenta-2,4-diene is notable. The zirconium compound, 3 b, is more stable against oxidation than its analogue, 3 b-pTol, whereas it is exactly the opposite for the titanium complexes. For 4 a and 4 b, the formation of redox-active products and several redox processes were observed. Owing to the different potentials of these intermediates, different decomposition mechanisms are predicted.
Experimental Section General information All manipulations were carried out in an oxygen- and moisture-free argon atmosphere by using standard Schlenk and drybox techniques. Non-halogenated solvents were dried over sodium/benzophenone and freshly distilled prior to use. The compounds [Cp*2Ti(h2-Me3SiC2SiMe3)][21] and [Cp*2Zr(h2-Me3SiC2SiMe3)],[19] [27] Fc CN were synthesized as previously described in literature. The following instruments were used: NMR spectra: Bruker AV300 and AV400 (1H and 13C chemical shifts were referenced to the solvent signals: [D6]benzene (dH 7.16, dC 128.0), [D8]THF (dH 1.73, dC 25.2), and [D8]toluene (dH 2.08, dC 20.4)); IR: Bruker Alpha FT-IR spectrometer; MS: Finnigan MAT 95-XP from Thermo-Electron; elemental analysis: Leco Tru Spec elemental analyzer; melting points: METTLER-TOLEDO MP 70 (melting points are uncorrected and were measured in sealed capillaries). Electrochemical measurements were conducted at 25 8C under argon on 1.0 mmol L 1 solutions in dry, air-free tetrahydrofuran solutions containing 0.1 mol l 1 of [N(nBu)4][B(C6F5)4] as supporting electrolyte by using a Voltalap PGZ 100 radiometer electrochemical workstation interfaced with a personal computer. A three-electrode cell using a Pt auxiliary electrode, a glassy carbon working electrode (surface area 0.031 cm2), and an Ag/Ag + (0.01 mol l 1 [AgNO3]) reference electrode, mounted on a Luggin capillary, was used. The working electrode was pretreated by polishing on a Buehler microcloth first with 1 mm and then 1=4 mm diamond paste. The reference electrode was constructed from a silver wire inserted into a solution of 0.01 mol l 1 [AgNO3] and 0.1 mol l 1 [N(nBu)4][B(C6F5)4] in acetonitrile, in a Luggin capillary with a porous Vycor tip. This Luggin capillary was inserted into a second Luggin capillary with porous Vycor tip filled with a 0.1 mol l 1 [N(nBu)4][B(C6F5)4] solution in tetrahydrofuran. Successive experiments under the same experimental conditions showed that all formal reduction and oxidation potentials were reproducible within 5 mV. Experimental potentials were referenced against an Ag/Ag + reference electrode but results are presented referenced against ferrocene as an internal standard, as required by IUPAC.[22] To achieve this, because the ferrocene/ferrocenium couple FcH/FcH + interferes with the ferrocenyl signals, each experiment was first performed in the absence of any internal standard
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Full Paper and then repeated in the presence of 1 mmol L 1 ferrocene (FcH) or decamethylferrocene (Cp*2Fe). Data were then manipulated on a Microsoft Excel worksheet to set the formal reduction potentials of the FcH/FcH + couple to 0.0 V, while the FcH/FcH + couple itself was at 220 mV versus Ag/Ag + , DEp = 61 mV.[28] When Cp*2Fe was used as an internal standard, the experimentally measured potential was converted into E versus FcH/FcH + by addition of 0.53 V. Diffraction data for 3 a, 3 b, and 4 a were collected on a Bruker Kappa APEX II Duo diffractometer using graphite-monochromated Mo-Ka radiation and, for Fc CC CN, on an Oxford Gemini S diffractometer with graphite-monochromated Cu Ka radiation. The structures were solved by direct methods and refined by fullmatrix least-squares procedures on F2 with the SHELXTL software package.[29] Diamond was used for graphical representations.[30] CCDC-972479 (3 a), 972478 (3 b), 972477 (4 b) and 969828 (Fc CC CN) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Preparation of 2b To a solution of [Cp*2Zr(h2-Me3SiC2SiMe3)] (1 b) (0.267 g, 0.5 mmol) in toluene (10 mL), a solution of Fc CN (0.105 g, 0.5 mmol) in toluene (10 mL) was added at ambient temperature under stirring. The color of the mixture turned immediately purple. After 3 h of stirring at ambient temperature, all volatiles were removed in vacuum. Yield: 301 mg (0.40 mmol, 80 %). M.p: 161 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 0.57 (s, 18 H, SiMe3), 1.84 (s, 30 H, C5Me5), 3.85 (m, 2 H, C5H4), 4.07 (s, 5 H, C5H5), 4.32 ppm (m, 2 H, C5H4); 13C NMR (75 MHz, C6D6, 297 K): d = 4.9 (SiMe3), 12.4 (C5Me5), 53.9 (iC (C5H4)), 70.7 (C5H5), 71.1, 71.4 (aCH/bCH (C5H4), 112.4 ppm (C5Me5); IR (Nujol): n˜ = 2150 (w), 1550 (w), 1459 (s), 1028(w), 823 cm 1 (m); MS (EI): m/z (%): 360 (100) [Cp*2Zr] + , 211 (45) [Fc CN] + , 170 (6) [Me3SiC2SiMe3] + , 135 (4) [Cp*] + ; elemental analysis calcd (%) for C39H57FeNSi2Zr: C 63.03, H 7.73, N 1.88; found: C 63.10, H 7.60, N 1.73.
Preparation of 3a Fc CN (0.105 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added to a solution of [Cp*2Ti(h2-Me3SiC2SiMe3)] (1 a) (0.122 g, 0.25 mmol) in toluene (10 mL). This solution was warmed to 55 8C whereby the color changed from orange to red. After 4 d, at 55 8C all volatiles were removed in vacuum and the red residue was dissolved in tetrahydrofuran. The red solution was allowed to stand at 78 8C and red crystals formed within 1 d. These were separated by decanting and dried in vacuum. Yield: 163 mg (0.22 mmol, 89 %). M.p: 169–170 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.85 (s, 30 H, C5Me5), 3.98 (m, 4 H, C5H4), 4.22 (m, 4 H, C5H4), 4.26 ppm (s, 10 H, C5H5); 13C NMR (75 MHz, C6D6, 297 K): d = 11.8 (C5Me5), 67.3, 69.0 (C4-C7/C14-C17), 69.4 (C5H5), 90.9 (iC (C5H4)), 120.1 (C5Me5), 156.4 ppm (C=N); IR (ATR): n˜ = 3083 (vw), 2901 (w), 1573 (vw), 1525 (w), 1375 (w), 1018 (m), 806 (s), 622 (m), 475 cm 1 (vs); MS (CI): m/z (%): 740 (100) [M + ], 606 (10) [M-Cp*] + , 528 (40) [M-FcCNH] + , 317 (4) [Cp*2Ti] + , 212 (13) [FcCN] + ; elemental analysis calcd (%) for C42H48Fe2N2Ti: C 68.13, H 6.53, N 3.78; found: C 68.05, H 6.71, N 3.55.
Analytical data of the side component 3a’ Obtained after applying higher reaction temperature of 80 8C: M.p. 344 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.87 (s, 30 H, C5Me5), 4.00–4.08 (m, 8 H, C5H4), 4.15 (s, 10 H, C5H5), 6.64 ppm Chem. Eur. J. 2014, 20, 3061 – 3068
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(s, 2 H, NH); 13C NMR (75 MHz, C6D6, 297 K): d = 12.2 (C5Me5), 67.2 (C5H4), 69.0 (C5H5), 70.6 (iC), 115.9 (C5Me5); MS (CI): m/z (%): 742 (17) [M + ], 426 (100) [HN-C(Fc) = C(Fc)-NH + 2H] + , 212 ppm (67) [FcCN] + .
Preparation of 3b Fc CN (0.105 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added under stirring to a solution of [Cp*2Zr(h2Me3SiC2SiMe3)] (1 b) (0.133 g, 0.25 mmol) in toluene (10 mL). The reaction solution was stirred at 80 8C for 11 days and the color turned from purple to yellow. All volatiles were removed under vacuum and the remaining yellow residue dissolved in toluene. At 78 8C, yellow crystals formed and were separated by decanting and dried under vacuum. Yield: 166 mg (0.22 mmol, 85 %). M.p: 210–211 8C (dec.) under Ar; 1H NMR (300 MHz, C6D6, 297 K): d = 1.87 (s, 30 H, C5Me5), 4.02 (m, 4 H, C5H4), 4.31 (s, 10 H, C5H5), 4.34 ppm (m, 4 H, C5H4); 13C NMR (75 MHz, C6D6, 297 K): d = 11.1 (C5Me5), 67.5, 69.9 (C4-C7/C14-C17), 69.5 (C5H5), 91.7 (iC (C5H4)), 118.5 (C5Me5), 165.3 ppm (C=N); IR (ATR): n˜ = 3094 (vw), 2899 (w), 1618 (w), 1580 (w), 1105 (m), 807 (s), 466 cm 1 (vs); MS (CI): m/z (%): 782 (83) [M] + , 571 (16) [M-(FcCN)] + , 360 (7) [Cp*2Zr] + , 211 (24) [FcCN] + ; elemental analysis calcd (%) for C42H48Fe2N2Zr: C 64.36, H 6.17, N 3.57; found: C 64.34, H 6.44, N 3.34.
Conversion of 2b to 3b Compound 2 b was synthesized according to the procedure mentioned above. A small amount of 2 b was dissolved in C6D6 and filled into a J Young NMR tube. Subsequently it was warmed to 60 8C for 12 days. 1H and 13C NMR spectra were recorded before and after the heating.
New procedure for the synthesis of Fc CC CN (2-cyano-1-chlorovinyl)ferrocene (1.0 g, 3.7 mmol) (for synthetic procedure, see the Supporting Information) and KOH (0.215 g, 3.8 mmol) were dissolved in tert-butanol (25 mL) and stirred for 2 h at 60 8C. Subsequently, water (50 mL) was added and the mixture was extracted three times with dichloromethane (30 mL each). The combined organic layers were dried over MgSO4 and all volatiles were removed in vacuum. The crude product was purified by flash chromatography using silica and n-hexane/ethyl acetate (4:1, v/v) as eluent. Yield: 865 mg (3.7 mmol, 100 %) Orange prisms suitable for single-crystal X-ray diffraction analysis could be isolated by slow evaporation of a chloroform solution of Fc CC CN at ambient temperature. Analytical data are according to the literature.[26]
Preparation of complex 4a Fc CC CN (0.118 g, 0.5 mmol) was dissolved in toluene (10 mL) and subsequently added to a solution of [Cp*2Ti(h2-Me3SiC2SiMe3)] (1 a) (0.122 g, 0.25 mmol) in toluene (10 mL). The solution was warmed to 60 8C for 6 d and the color changed from orange to red-brown. All volatiles were removed in vacuum and the remaining red residue analyzed. Yield: 173 mg (0.22 mmol, 88 %). M.p: 118 8C (dec.) under Ar; 1H NMR (300 MHz, [D8]THF, 297 K): d = 1.93 (s, 30 H, C5Me5), 4.27 (s, 5 H, C5H5 (Fc-A)), 4.30 (m, 1 H, bCH (Fc-B)), 4.31 (m, bCH (Fc-A)), 4.31 (s, 5 H, C5H5 (Fc-B)), 4.58 (m, 1 H, aCH (FcA)), 5.04 ppm (m, 1 H, aCH (Fc-A)); 13C NMR (75 MHz, [D8]THF, 297 K): d = 11.9 (C5Me5), 66.2 (iC (Fc-A)), 68.5 (bCH (Fc-B)), 69.3 (aCH (Fc-B)), 69.8 (bCH (Fc-A)), 70.3 (C5H5 (Fc-B)), 70.6 (C5H5 (Fc-A)), 72.2 (aCH (Fc-A)), 84.4 (C6), 85.0 (iC (Fc-B)), 87.4 (C5), 122.1 (C5Me5),
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Full Paper 123.6 (C1), 141.4 (C4), 159.2 (C3), 175.9 ppm (C2); IR (ATR): n˜ = 2901 (w), 2186(w), 2140 (w), 1526 (w), 1491 (w), 1431(w), 1375 (m), 1006 (m), 815 (vs), 728 (vs), 694 (m), 496 cm 1 (vs); MS (EI): m/z (%): 788 (1) [M] + , 603 (1) [M-Fc] + , 577 (21) [M-(Fc CN)] + , 318 (6) [Cp*2Ti] + , 235 (10) [Fc C2-CN] + ; elemental analysis calcd (%) for C46H48Fe2N2Ti: C 70.07, H 6.14, N 3.55; found: C 70.04, H 6.25, N 3.36.
[6]
[7]
Preparation of 4b [8]
To [Cp*2Zr(h2-Me3SiC2SiMe3)] (1 b) (0.209 g, 0.4 mmol) in toluene (10 mL), a solution of Fc CC CN (0.185 g, 0.8 mmol) in toluene (10 mL) was added at ambient temperature under stirring. The color of the mixture turned immediately blue and after 15 min it was red-brown. All volatiles were removed in vacuum and the remaining red solid was dissolved in warm toluene (50 8C). By cooling to ambient temperature, red crystals of 4 b formed. Yield (with one molecule of toluene): 198 mg (0.12 mmol, 88 %). M.p: 282 8C (dec.) under Ar; IR (ATR) n˜ = 2898 (w), 2196 (vw), 2114(m), 1550 (w), 1492 (m), 1433(w), 689 (vs), 500 cm 1 (vs); MS (CI): m/z (%): 499 (2) [Cp*Zr(Fc C2-CN)(CCN)] + , 399 (1) [Cp*2Zr(CCN)] + , 387 (1) [Cp*2Zr(CN)] + , 210 (1) [Fc C2] + , 185 (1) [Fc] + , 135 (59) [Cp*] + ; elemental analysis calcd (%) for C92H96Fe4N4Zr2 + C7H8 calcd: C 67.72, H 5.97, N 3.19; found: 67.71, H 5.96, N 3.19. The characterization by NMR spectroscopy is difficult owing to the formation of an equilibrium between 4 b and its monomer. The signals observed in the 1H and 13C NMR spectra, assigned to the main and the side component, are listed in the Supporting Information.
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Acknowledgements We thank the staff of LIKAT for the technical and analytical support. Financial support by the Deutsche Forschungsgemeinschaft (RO 1269/8–1) and the Fonds der Chemischen Industrie is acknowledged.
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Received: November 15, 2013
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