DOI: 10.1002/chem.201404628
Full Paper
& Silicon Complexes
NHC!SiCl4 : An Ambivalent Carbene-Transfer Reagent** Tobias Bçttcher,*[a] Simon Steinhauer,[a] Lesley C. Lewis-Alleyne,[b] Beate Neumann,[a] HansGeorg Stammler,[a] Bassem S. Bassil,[b] Gerd-Volker Rçschenthaler,[b] and Berthold Hoge[a]
Abstract: The addition of BCl3 to the carbene-transfer reagent NHC!SiCl4 (NHC = 1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl3] + and [(NHC)2SiCl3] + with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC!SiCl4 served as a transfer reagent for the NHC ligand. The addition of BF3·OEt2, on the other hand, gave NHC!BF3 as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C2F5)2SiCl2 was treated with NHC!SiCl4. In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl3] + and [(NHC)2SiCl3] + were detected with [(C2F5)SiCl3] as counterion. A similar result
was already reported for the reaction of NHC!SiCl4 with (C2F5)2SiH2, which gave [(NHC)2SiCl2H][(C2F5)SiCl3]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC!Si(C2F5)2Cl2 and NHC!Si(C2F5)2ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC!SiCl4 towards phosphines is discussed. The carbene complex NHC!PCl3 shows similar reactivity to NHC!SiCl4, and may even serve as a carbene-transfer reagent as well.
Introduction
NHC is the smallest imidazolidine-based carbene which, unlike its unsaturated analogue, has not been isolated in its free form yet.[6] Addition of 1 to PCl3 quantitatively yielded NHC!PCl3 with liberation of SiCl4 and is the first example of selective NHC transfer between two p-block elements. The addition of two equivalents of 1 to the highly Lewis acidic (C2F5)2SiH2 (2), on the other hand, did not afford the expected carbene adduct of 2. Instead the salt [(NHC)2SiCl2H][(C2F5)2SiCl3] (E, Figure 1) with pentacoordinate silicon in both the anion and the cation was obtained.[7] Despite the large number of silicon(IV) complexes reported in the literature, and the ongoing and extensive research in this field, only a few cationic complexes of silicon(IV) halides and hydrides with a coordination number lower than six have been structurally characterized so far, including tricoordinate silylium ions.[8] The reported examples are all stabilized with the help of NHCs as donor ligands
In the so-called renaissance of main group chemistry over the past two decades, N-heterocyclic carbenes (NHCs) have evolved into an indispensable class of ligands for the stabilization of unusual oxidation states and coordination motifs of pblock compounds.[1] Access to these new complexes is almost exclusively by the addition of free and uncoordinated carbenes to stable p-block precursors.[2] This, however, excludes NHCs that are not stable and therefore not isolable in their free form. Moreover, the presence of free NHCs may lead to unwanted side reactions due to their high basicity, strong nucleophilicity, and reducing nature.[3] To avoid these synthetic obstacles, indirect procedures such as oxidative addition and carbene transfer are useful alternatives. For the synthesis of transition metal NHC complexes, both methods are already established protocols and have allowed the introduction of carbene ligands that are otherwise not isolable in their free form.[4] We recently introduced NHC!SiCl4 (1; NHC = 1,3-dimethylimidazolidin-2-ylidene) as a carbene-transfer reagent.[5] This
[a] Dr. T. Bçttcher, S. Steinhauer, B. Neumann, Dr. H.-G. Stammler, Prof. Dr. B. Hoge Fakultt fr Chemie, Universitt Bielefeld Anorganische Chemie II Universittsstrasse 25, 33615 Bielefeld (Germany) E-mail: [email protected] [b] Dr. L. C. Lewis-Alleyne, Dr. B. S. Bassil, Prof. Dr. G.-V. Rçschenthaler Jacobs University Bremen School of Engineering and Science Campus Ring 1, 28759 Bremen (Germany)
Figure 1. Structurally characterized tetra- and pentacoordinate cationic halo/ hydrido silicon(IV) complexes (Dipp = 2,6-diisopropylphenyl).
[**] NHC: 1,3-dimethylimidazolidin-2-ylidene. Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper (Figure 1). Complexes A and B were obtained by Driess et al. by addition of one and two equivalents, respectively, of free NHC to H3Si(OTf).[9] Filippou and co-workers prepared compounds C and D by addition of free NHC to the corresponding silicon tetrahalide.[10] The analogous salt with X = Cl has not been reported yet, and the addition of free NHC to SiCl4 affords 1:1 adducts, as first reported by Kuhn et al. in 1995.[11] Herein, we report on new results with the carbene transfer reagent NHC!SiCl4 (1) revealing its ambivalent character. Examples for both characteristics, that is the synthesis of novel carbene complexes by NHC transfer and 1 acting as a source of tetra- and pentacoordinate silicon(IV) cations, are presented.
Scheme 1. Stepwise formation of salts 4 and 5.
Lewis acidic compared to that in 1, and the carbene ligand is transferred from the remaining equivalent of 1 to yield 5 with liberation of SiCl4 (Scheme 1). Single crystals of both salts suitable for XRD were obtained from solutions in acetonitrile at 30 8C. Both compounds crystallize in the monoclinic space group P21/c. The silicon atom in 4 is in a tetrahedral environment (Figure 2). The CSi bond
Results and Discussion As recently reported, the addition of two equivalents of NHC!SiCl4 (1) to (C2F5)2SiH2 (2) yields the salt [(NHC)2SiCl2H] [(C2F5)2SiCl3] (E).[7] During the reaction, Cl/H metathesis occurs and NHC!SiCl3H was identified as an intermediate. The anion of E is formed by abstraction of chloride from NHC!SiCl3H, under the assumption that the tetracoordinate cation [(NHC)SiCl2H] + is an intermediate in the reaction. Such a cation has a higher Lewis acidity than the remaining equivalent of neutral and pentacoordinate NHC!SiCl3H, and thus initiates a carbene transfer to eventually yield E while silicochloroform is liberated. Attempts to synthesize the sought-after tetracoordinate cation by treating only one equivalent of 1 with 2 remained unsuccessful and only gave E and unconverted starting materials. However, to overcome the halide metathesis process, 1 was treated with (C2F5)2SiCl2 (3) in 1:1 molar ratio in deuterated acetonitrile instead, and the reaction was monitored by multinuclear NMR spectroscopy. According to the 19 F NMR spectrum, the reaction quantitatively yielded [(C2F5)2SiCl3] as the only species with C2F5 groups. In 1H,29Si 2D NMR correlation experiments, no cross-peaks for the corresponding cations could be detected at room temperature, although the 1H NMR spectroscopic data clearly indicated the presence of a coordinated NHC ligand. To slow down possible exchange equilibria, the reaction mixture was cooled to 40 8C. Then, two cross-peaks were detected, one at d = 110.5, which corresponds to pentacoordinate silicon and compares well with that of the cation in E (d = 125.4), and the other at d = 20.1, corresponding to the supposed tetracoordinate species [(NHC)SiCl3] + [Eq. (1)].
Figure 2. Crystal structure of 4. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 190.7(2), Si1Cl1 200.55(5), Si1Cl2 200.81(5), Si1Cl3 200.35(5), C1Si1Cl1 108.40(5); C1-Si1-Cl2 106.04(5), C1Si1-Cl3 112.92(5).
length (190.7(2) pm) compares well with that of 1 (192.8(3) pm). The mean SiCl bond length (200.57(5) pm) is slightly longer than that of SiCl4 in the gas phase (196.3 pm)[12] but considerably shorter than the mean axial (220.8(2) pm) and equatorial (207.3(2) pm) SiCl bond lengths in pentacoordinate 1. In both salts the anion and the cation are well separated from each other with interionic Si···ClBCl3 distances (4: 390.6(1) pm, 5: 468.6(1) pm) that exceed the sum of the van der Waals radii for Si and Cl (385 pm). The silicon atom in 5 is in a trigonal-bipyramidal environment with the NHC ligands in the equatorial positions (Figure 3). The mean SiC bond length (193.3(2) pm) compares well with that of pentacoordinate 1 (192.8(3) pm) and is only slightly longer than that of 4 (190.7(2) pm). The mean axial SiCl bond length (220.78(4) pm) and equatorial SiCl bond length (206.81(4) pm) compare well with those of 1 (220.8(2) and 207.3(2) pm). The equatorial sum of angles is 3608, and the axial Cl1-Si1-Cl3 bond angle (175.67(2)8) is almost linear. The cation of 5 is formally a {SiCl3} + moiety stabilized by two NHC ligands. In the previously reported isostructural cation E (Figure 1), the chloride anion at the equatorial position is replaced by a hydride. The derivative with two hydrides and one chloride has not been synthesized yet. However, Driess et al. reported the structure of the cation
Reaction of 1 with BCl3 To rule out possible involvement of chloride in the exchange process, a stronger acceptor for chloride was chosen. Addition of one equivalent of BCl3 to a solution of 1 in dichloromethane afforded [(NHC)SiCl3][BCl4] (4) in 98 % yield. Changing the 1:BCl3 molar ratio to 2:1 gave [(NHC)2SiCl3][BCl4] (5) in 96 % yield. In the first step, one equivalent of BCl3 abstracts a chloride ion from 1 to give 4. The silicon atom in 4 becomes more &
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Full Paper THF. For the reaction of 1 with 2 and 3, there is a competition between NHC transfer and chloride transfer yielding a neutral NHC adduct and a salt of a chlorosilicate, respectively. The ionic products are better stabilized by the more polar solvent (acetonitrile), and the less polar solvent (dichloromethane) affords the neutral carbene adducts. The addition reaction of 1 to 2 also proceeds quantitatively but is accompanied by H/Cl metathesis (Scheme 2). The carbene adduct NHC!Si(C2F5)2ClH (7) was identified as the only product containing C2F5 groups. The complex NHC!SiCl3H was obtained as the primary byproduct. As previously reported, NHC!SiCl3H redistributes in solution to give 1 and NHC!SiCl2H2.[7] The 29Si NMR resonances for 6 (d = 105.3) and 7 (d = 120.1) compare well with those of 1 (d = 103.9), NHC!SiCl3H (d = 104.5), and NHC!SiCl2H2 (d = 125.2).[5, 7] Isolation of pure 7 from the metathesis byproducts by crystallization turned out to be unsuccessful so far due to very similar solubilities of the involved species. However, a single crystal suitable for XRD could be isolated for its molecular structure determination from a dichloromethane solution of the reaction mixture kept at 30 8C. Compound 6 crystallizes in the triclinic space group P1¯, and compound 7 in the orthorhombic space group Pbca. In both compounds the silicon atom is in a trigonal-bipyramidal environment (Figures 4 and 5). The axial posi-
Figure 3. Crystal structure of 5. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 193.0(2), C6Si1 193.6(2), Si1Cl1 221.19(4), Si1Cl2 206.81(4), Si1Cl3 220.37(4); C1-Si1-C6 125.56(5), C1-Si1Cl2 116.59(4), C6-Si1-Cl2 117.79(4), Cl1-Si1-Cl3 175.67(2).
[(NHCDipp)2SiH3] + (B), in which both NHC ligands reside at the equatorial positions.
NHC transfer to (C2F5)2SiH2 (2) and (C2F5)2SiCl2 (3) As part of the above-described studies on the reaction of 1 with silanes 2 and 3, the solvent was changed from acetonitrile to dichloromethane. Surprisingly, on monitoring both reactions by 19F NMR spectroscopy, no signal for the expected chlorosilicate [(C2F5)2SiCl3] could be detected. Instead the products of an NHC transfer were obtained. Compound 1 forms a 1:1 adduct with 3 to give NHC!Si(C2F5)2Cl2 (6; Scheme 2). The reaction proceeds quantitatively according to
Scheme 2. Reaction of 1 with 2 and 3 in CH2Cl2.
Figure 4. Crystal structure of 6. All CH protons have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C5Si1 199.3(2), Si1Cl1 208.73(7), Si1Cl2 208.45(8), Si1C1 199.9(2), Si1C3 199.9(2), av CN 132.7(3); C5-Si1-Cl1 123.30(7), C5-Si1-Cl2 119.05(7), Cl1-Si1-Cl2 117.65(3), C1-Si1-C3 172.03(9).
the 19F NMR data, and single crystals of 6 were obtained after keeping a solution of the raw product in dichloromethane at 30 8C (63 % yield). This result was unexpected, as previous calculations concluded that the bond dissociation energy of the CSi bond in 1 decreases in coordinating solvents.[5] Initially, THF was reported as a good medium for carbene transfer reactions with 1. Here, the choice of solvents is restricted by the highly Lewis acidic silanes, which do not allow the use of
tions are occupied by the C2F5 groups, and the equatorial sum of angles is 3608 for both complexes. The most striking difference is the elongated carbene–silicon bonds in 6 (199.3(2) pm) and 7 (196.0(1) pm) compared with NHC!SiCl4 (192.8(3) pm), NHC!SiCl3H (191.4(2) pm), and NHC!SiCl2H2 (190.7(2) pm). The longer bonds also seem to be reflected in a much higher sensitivity of 6 and 7 towards even traces of moisture. Compound 1, on the other hand, can be handled without the use of a glovebox with short exposure to air.
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Figure 6. Crystal structure of 8. All CH protons have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1B1 165.6(2), C1N1 132.4(2), C1N2 132.5(2), B1F1 137.8(2), B1F2 139.4(2), B1F3 138.4(2); N1-C1-N2 110.1(1), C1-B1-F1 112.5(1), C1-B1F2 107.6(1), C1-B1-F3 108.2(1).
donor ligand results in a hexacoordinate complex with the NHC ligand at the axial position, and thus the CSi bond length is increased and the bond dissociation energy decreased. The THF molecule can weakly coordinate to the silicon atom and therefore activate the NHC dissociation. It was then interesting to test the reactivity of 1 towards stronger donor molecules. As previously reported, the addition of PCl3 to 1 quantitatively yields NHC!PCl3 (9).[5] No reaction was observed for 1 and PPh3. However, when PMe3 was condensed into a solution of 1, the reaction mixture changed color from colorless to dark yellow. The complex (PMe3)2SiCl4 was identified as the product according to its literature data (Scheme 4).[14] This result is consistent with the earlier calculations, but it is in contrast to transition metal complexes, in which phosphine ligands can be substituted for more strongly binding carbenes.
Figure 5. Crystal structure of 7. All CH protons and minor-occupancy disordered atoms have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 196.0(1), Si1 Cl1 210.46(5), Si1C6 201.0(1), Si1C8 201.3(1), av C1N 132.7(2); C1-Si1-Cl1 127.18(4), C6-Si1-C8 177.84(6).
Reaction of 1 with BF3·OEt2 As reported above, BCl3 was used for chloride abstraction from 1 to yield the salts 4 and 5. Attempts to transfer the carbene to boron by changing the reaction conditions remained unsuccessful and exclusively yielded the salt 4. Interestingly, when BCl3 was replaced by BF3·OEt2 the carbene-transfer product NHC!BF3 (8) was obtained as an air- and moisture insensitive colorless solid in 64 % yield after recrystallization (Scheme 3).
Scheme 3. NHC transfer to BF3.
Kuhn et al. and Arduengo et al. reported 1:1 addition reactions of free carbenes and BF3·OEt2 to give analogous compounds.[13] Single crystals suitable for XRD were obtained from a toluene solution stored at 30 8C. Compound 8 crystallizes in the monoclinic space group P21/n (Figure 6). The characteristic NMR spectroscopic data of 8 (11B: d = 0.8 ppm, 1JBF = 38 Hz) and the structural parameters are in good agreement with the previously reported complexes.
Scheme 4. Reactions of 1 with phosphines.
As the reaction of 1 with BCl3 readily yielded the carbenestabilized silicon(IV) cations, it was worth testing the carbene complex NHC!PCl3 (9) as a source for phosphorus(III) cations. By condensing 0.5 equivalents of BCl3 into a solution of 9 in dichloromethane, the salt [(NHC)2PCl2][BCl4] (10) was obtained in 99 % yield (Figure 7). Its formation seems to follow a similar reaction pathway as proposed for its silicon counterpart 5 (Scheme 1), which suggests that 9 may serve as a carbenetransfer reagent as well. An analogous compound to 10 was recently obtained by Weigand et al. by using sterically demanding free carbenes (NHCDipp).[15] The procedure now reported allows for an alternative access to this species without using free carbene.
Reactions of 1 with phosphines When compound 1 was first reported as an NHC-transfer reagent, it was experimentally found that the transfer process works best in THF as reaction medium. It was further concluded by theoretical calculations that the carbene–silicon bond becomes increasingly more labile in coordinating solvents.[5] The silicon atom in 1 is five-coordinate, and an additional &
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Full Paper periments. Elemental analyses were performed by the microanalytical laboratory of the department of Inorganic Chemistry II (ACII), University of Bielefeld, Bielefeld, Germany. Compounds NHC!SiCl4 (1) (C2F5)2SiH2 (2), and (C2F5)2SiCl2 (3), and were prepared according to procedures reported elsewhere.[5, 16]
X-ray crystallography X-ray diffraction data were collected on an Agilent Supernova diffractometer, except for those of 8, which were measured on a Bruker Nonius KappaCCD. Olex2[17] was used to solve the structures of 5 and 7 by charge flipping,[18] and all others were solved by direct methods and with the ShelX program package[19] by using least-squares minimization. All heavy atoms were refined anisotropically, except for a carbon atom in the disordered C2F5 group in 7. The hydrogen atoms were placed in calculated positions by using a riding model, except for the hydrogen atom bonded to silicon in compound 7, which was refined isotropically.
Figure 7. Crystal structure of 10. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1P1 186.5(1), C6P1 186.3(1), C1N1 132.3(1), C1N2 132.8(1), C6N3 132.3(2), C6N4 132.5(2), P1Cl1 229.73(4), P1Cl2 232.83(4); C1-P1-C6 111.88(5), Cl1-P1-Cl2 169.38(2), N1-C1-N2 111.7(1), N3-C6N4 112.3(1).
Data for X-ray structure determination of 8 were collected on a Bruker Nonius KappaCCD diffractometer at 100(2) K with graphite-monochromated MoKa radiation (l = 71.073 pm). Numerical absorption correction was done by using SADABS. The structures were solved by direct methods with SAINT and refined by fullmatrix least-squares cycles by using SHELX-97.[19] All heavy atoms were refined anisotropically, and the hydrogen atoms were either found directly and refined isotropically or placed in calculated positions. Crystallographic data and structure refinement for 4–8 and 10 are summarized in Table 1.
Conclusion New reactions with the carbene transfer reagent NHC!SiCl4 (1) were presented. The reaction of 1 with the highly acidic silanes (C2F5)2SiH2 (2) and (C2F5)2SiCl2 (3) in acetonitrile gave salts with carbene-stabilized silicon(IV) cations [(NHC)SiCl3] + , [(NHC)2SiCl3)] + , and [(NHC)2SiCl2H] + , respectively, with the chlorosilicate [(C2F5)2SiCl3] as the anion. By changing the solvent to dichloromethane the reactions exclusively yielded the products of carbene transfer NHC!Si(C2F5)2Cl2 (6) and NHC!Si(C2F5)2ClH (7) instead. This selectivity is attributed to the polarity of the solvent, whereby the ionic products are better stabilized in the more polar solvent, and the neutral adducts are obtained from the less polar solvent. Reaction of 1 with BCl3 exclusively yielded the salts [(NHC)SiCl3][BCl4] (4) and [(NHC)2SiCl3][BCl4] (5), according to the stoichiometry. However, when BCl3 was replaced by BF3·OEt2, the product of carbene transfer NHC!BF3 (8) was obtained. The reactivity of 1 towards different phosphines was also briefly discussed. Compounds 6 and 7 may serve as promising precursors for electron-poor silylenes. Furthermore, it was shown that the carbene complex NHC!PCl3 can also act as a carbene-transfer reagent and is a source of carbene-stabilized phosphorus(III) cations.
CCDC 965801 (4), 965802 (5), 1015845 (6), 1015846 (7), 1013798 (8) and 1015847 (10) 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.
Syntheses Compound 4: A solution of 1 (1.46 g, 5.45 mmol) in dichloromethane (40 mL) was placed in a 100 mL Schlenk flask. The solution was cooled to 196 8C and BCl3 (6.0 mmol, 1.1 equiv) was condensed into the flask. The reaction mixture was allowed to reach room temperature over 12 h with constant stirring. Removal of all volatile components gave compound 4 (2.05 g, 5.32 mmol). Single crystals suitable for XRD were obtained from a solution of 4 in acetonitrile at 30 8C. Yield: 98 %. Elemental analysis calcd (%) for C5H10BCl7N2Si (385.21): C 15.59, H 2.62, N 7.27; found: C 16.07, H 3.05, N 7.45; 1H NMR (CDCl3): d = 3.49 (s, 6 H, CH3), 4.23 ppm (s, 4 H, CH2); 11B{1H} NMR (CDCl3): d = 6.9 (s, [BCl4]); 1H,13C HMBC (CDCl3): d(1H)/d(13C) = 3.49/37.2 (d, CH3/CH3, 1J(C,H) = 141 Hz), 3.49/53.4 (s, CH3/CH2), 4.23/53.4 (d,m, CH2/CH2, 1J(C,H) = 155 Hz), 3.49/161.3 (s, CH2/Ccarbene), 4.23/161.3 ppm (s, CH3/Ccarbene); 1H,29Si HMBC NMR (CDCl3): d(1H)/d(29Si) = 3.49/20.9 (s, CH3/Si), 4.23/20.9 ppm (s, CH2/Si).
Experimental Section General remarks All reactions were carried out under an atmosphere of dry nitrogen by using standard Schlenk-line techniques unless mentioned otherwise. Solvents were dried with a Braun MB-SPS 800 system. Acetonitrile and dichloromethane were stored over 3 molecular sieves. CD3CN) and CDCl3 were distilled over CaH2. All other chemicals were purchased from commercial sources and were used as received. NMR spectra were recorded on a Bruker Avance III 300 spectrometer (operating frequencies: 29Si 59.63 MHz, 19F 282.40 MHz, 13C 75.47 MHz, 11B 96.29 MHz, 1H 300.13 MHz) with positive shifts downfield from the external standards [CCl3F (19F), Si(CH3)4 (29Si, 13C, 1H), BF3·Et2O (11B)]. The 13C and 29Si NMR shifts were acquired by 1H,13C, and 1H,29Si 2D HMBC NMR correlation exChem. Eur. J. 2014, 20, 1 – 8
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Compound 5: Compound 5 was synthesized by following the procedure for 4. Quantities used: 1 (2.33 g, 8.69 mmol) in dichloromethane (50 mL), BCl3 (4.3 mmol, 0.5 equiv). Yield: 96 %. Elemental analysis calcd (%) for C10H20BCl7N4Si (483.36): C 24.85, H 4.17, N 11.59; found: C 24.88, H 4.81, N 11.17; 1H NMR (CD3CN): d = 3.35 (s, 6 H, CH3), 3.85 ppm (s, 4 H, CH2); 11B{1H} NMR (CD3CN): d = 6.9 ppm (s, [BCl4]); 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.35/37.0 (d, 1 J(C,H) = 142 Hz, CH3/CH3), 3.35/51.8 (s, CH3/CH2), 3.85/51.8 (d,m, 1 J(C,H) = 152 Hz, CH2/CH2), 3.35/169.7 (s, CH3/Ccarbene), 3.85/ 169.7 ppm (s, CH2/Ccarbene); 1H,29Si HMBC NMR (CD3CN): d(1H)/ d(29Si) = 3.35/110.5 (s, CH3/Si), 3.85/110.5 ppm (s, CH2/Si).
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Full Paper Table 1. Crystallographic data and structure refinement for 4–8 and 10.
formula Mr T [K] radiation/l [pm] crystal system space group a [pm] b [pm] c [pm] a [8] b [8] g [8] V [106 pm3] Z 1calcd [Mg m3] m [mm1] F(000) crystal size [mm] 2 q range for data collection reflns collected independent reflns R(int) data/restraints/parameters GoF on F2 R1 [I > 2 s(I)][a] Rw [all data][b] largest difference peak/hole [e 3]
4
5
6
7
8
10
C5H10BCl7N2Si 385.20 100.0(1) CuKa/154.18 monoclinic P21/c 1317.811(13) 915.426(10) 1242.381(14) 90 98.8748(10) 90 1480.81(3) 4 1.728 12.829 768.0 0.21 0.17 0.16 6.8–144.08 31173 2916 0.0346 2916/0/147 1.073 0.0201 0.0524 0.38/0.30
C10H20BCl7N4Si 483.35 100.0(1) MoKa/71.073 monoclinic P21/c 1397.338(19) 1260.272(13) 1244.060(16) 90 112.3079(16) 90 2026.85(4) 4 1.584 1.040 984.0 0.32 0.28 0.28 6.3–65.88 116764 5914 0.0365 5914/0/212 1.057 0.0247 0.0631 1.16/0.96
C9H10Cl2F10N2Si 435.18 100.0(1) MoKa/71.073 triclinic P1¯ 867.52(3) 877.01(2) 1201.03(3) 85.1446(19) 76.864(2) 60.513(3) 774.19(4) 2 1.867 0.602 432.0 0.21 0.15 0.08 5.3–60.28 45422 4505 0.0312 4505/0/219 1.145 0.0388 0.1006 0.60/0.42
C9H11ClF10N2Si 400.74 100.0(1) MoKa/71.073 orthorhombic Pbca 941.983(7) 1417.157(9) 2232.361(18) 90 90 90 2980.06(4) 8 1.786 0.444 1600.0 0.31 0.27 0.20 9.4–60.08 163135 4325 0.0397 4325/37/224 1.042 0.0354 0.0892 0.68/0.45
C5H10BF3N2 165.96 100(2) MoKa/71.073 monoclinic P21/n 1074.37(2) 640.750(10) 1175.58(2) 90 113.4970(10) 90 742.17(2) 4 1.485 0.142 344 0.30 0.30 0.20 7.4–56.6 27504 1844 0.0498 1844/0/140 1.006 0.0374 0.1070 0.446/0.216
C10H20BCl6N4P 450.78 100.0(1) MoKa/71.073 monoclinic P21/c 1422.58(3) 1163.993(18) 1296.85(3) 90 115.464(3) 90 1938.80(7) 4 1.544 0.968 920.0 0.18 0.13 0.12 6.3–60.28 44283 5649 0.0294 5649/0/203 1.052 0.0241 0.0613 0.59/0.39
[a] R1 = S j j Fo j j Fc j j /S j Fo j . [b] Rw = {S[w(F 2oF 2c)2]/S[w(F 2o)2]}1/2.
2 F, CF2), 125.6 (br s, 2 F, CF2), 80.5 ppm (s, 6 F, CF3); 1H,29Si HMBC NMR (CD3CN): d(1H)/d(29Si) = 3.31/120.1 (s, CH3/Si), 5.10/ 120.1 ppm (d, SiH/Si, 1J(Si,H) = 261 Hz).
Compound 6: A solution of 1 (0.55 g, 2.04 mmol) in dichloromethane (30 mL) was cooled to 196 8C. Compound 3 (2.10 mmol) was condensed into the solution and the reaction mixture was allowed to reach room temperature. The reaction mixture was stirred for an additional 12 h. After removal of all volatile components under reduced pressure, the crude product was dissolved in the minimum amount of dichloromethane. The solution was kept at 80 8C and gave single crystals of 6 (0.56 g, 1.29 mmol). Yield: 63 %. Elemental analysis calcd (%) for C9H10Cl2F10N2Si (435.16): C 24.84, H 2.32, N 6.44; found: C 24.72, H 2.79, N 6.57; 1H NMR (CDCl3): d = 3.47 (s, 6 H, CH3), 3.79 ppm (s, 4 H, CH2); 1H,13C HMBC (CDCl3): d(1H)/d(13C) = 3.47/36.7 (d, 1J(C,H) = 141 Hz, CH3/CH3), 3.47/ 51.9 (s, CH3/CH2), 3.79/51.9 (d,m, 1J(C,H) = 152 Hz, CH2/CH2), 3.47/ 174.1 (s, CH3/Ccarbene), 3.79/174.1 ppm (s, CH2/Ccarbene); 19F NMR (CDCl3): d = 122.6 (s, 4 F, CF2), 77.5 ppm (s, 6 F, CF3); 1H,29Si HMBC NMR (CDCl3): d(1H)/d(29Si) = 3.47/105.3 (s, CH3/Si), 3.79/ 105.3 ppm (s, CH2/Si).
Compound 8: BF3·Et2O (16.6 g, 117 mmol) was added to a solution of 1 (14.2 g, 53.0 mmol) in THF (200 mL) with stirring at 0 8C. The reaction mixture was allowed to reach room temperature and was stirred for an additional 12 h. All volatile components were removed under reduced pressure and the raw product was dissolved in minimum amount of hot toluene. The solution was kept at 30 8C to give single crystals of 8 (5.63 g, 33.9 mmol). Yield: 64 %; 1 H NMR (CD3CN); d = 3.13 (s, 6 H, CH3), 3.63 ppm (s, 4 H, CH2); 11 1 B{ H} NMR (CDCl3): d = 0.8 ppm (q, 1J(B,F) = 38 Hz); 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.13/33.9 (d, CH3/CH3, 1J(C,H) = 140 Hz), 3.13/ 50.7 (s, CH3/CH2), 3.63/50.7 (d,m, CH2/CH2, 1J(C,H) = 151 Hz), 3.13/ 177.2 (s, CH3/Ccarbene), 3.63/177.2 ppm (s, CH2/Ccarbene); 19F NMR (CD3CN): d = 140.8 ppm (q, 1J(B,F) = 38 Hz).
Compound 7: A solution of 1 (0.50 g, 1.87 mmol) in dichloromethane (30 mL) was cooled to 196 8C. Compound 2 (0.93 mmol) was condensed into the solution and the reaction mixture was allowed to reach room temperature. The reaction mixture was stirred for an additional 12 h. After removal of all volatile components the reaction mixture was analyzed by multinuclear NMR spectroscopy, and compound 7 was identified alongside previously reported NHC!SiCl4, NHC!SiCl3H, and NHC!SiCl2H2.[5, 7] The reaction mixture was dissolved in the minimum amount of dichloromethane and stored at 80 8C to give crystals suitable for structure determination of 7. 1H NMR (CD3CN): d = 3.31 (s, 6 H, CH3), 3.78 (s, 4 H, CH2), 5.10 ppm (m, 1 H, Si-H).1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.31/36.0 (d, 1J(C,H) = 141 Hz, CH3/CH3), 3.31/51.7 (s, CH3/CH2), 3.78/ 51.7 (d,m, 1J(C,H) = 153 Hz, CH2/CH2), 3.31/172.2 (s, CH3/Ccarbene), 3.78/172.2 ppm (s, CH2/Ccarbene); 19F NMR (CD3CN): d = 128.0 (br s,
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Compound 10: A solution of 9 (0.53 g, 2.26 mmol) in dichloromethane (20 mL) was cooled to 196 8C. BCl3 (1.13 mmol, 0.5 equiv) was condensed into the flask. The reaction mixture was allowed to reach room temperature and was stirred for an additional 12 h. Removal of all volatile components gave 10 (0.50 g, 1.12 mmol). Single crystals suitable for XRD were obtained from a solution of 4 in acetonitrile at 30 8C. Yield: 99 %. Elemental analysis calcd (%) for C10H20BCl6N4P (450.79): C 26.64, H 4.47, N 12.43; found: C 26.60, H 4.52, N 12.34; 1H NMR (CD3CN): d = 3.35 (s, 6 H, CH3), 3.95 ppm (s, 4 H, CH2); 11B{1H} NMR (CD3CN): d = 6.9; 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.35/37.2 (d, CH3/CH3, 1J(C,H) 141 Hz), 3.35/51.7 (s, CH3/CH2), 3.95/51.7 (d,m, 1J(C,H) = 155 Hz, CH2/CH2), 3.35/169.1 (s, CH3/Ccarbene), 3.95/169.1 ppm (s, CH2/Ccarbene); 31P{1H} NMR (CD3CN): d = 108.6 ppm (s).
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Full Paper Acknowledgements T.B. acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen postdoctoral research fellowship. A postdoctoral fellowship to L.C.L.-A. from the Deutscher Akademischer Austausch Dienst is gratefully acknowledged.
[8]
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hauer, T. Bçttcher, N. Schwarze, B. Neumann, H.-G. Stammler, B. Hoge, Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201406311; Angew. Chem. 2014, DOI: 10.1002/ange.201406311. a) R. J. P. Corriu and J. C. Young, The Silicon-Heteroatom Bond (Chapter 1: Hypervalent Silicon Compounds), Wiley: Chichester, England, 1989; b) C. Chuit, R. J. P. Corriu, C. Reye, J. C. Young, Chem. Rev. 1993, 93, 1371 – 1448; c) R. R. Holmes, Chem. Rev. 1996, 96, 927 – 950; d) W. Levason, G. Reid, W. Zhang, Coord. Chem. Rev. 2011, 255, 1319 – 1341; e) J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1994, 13, 2430 – 2443; f) J. B. Lambert, L. Kania, S. Zhang, Chem. Rev. 1995, 95, 1191 – 1201; g) C.-H. Ottosson, D. Cremer, Organometallics 1996, 15, 5309 – 5320; h) Z. Xie, J. Manning, R. W. Reed, R. Mathur, P. D. W. Boyd, A. Benesi, C. A. Reed, J. Am. Chem. Soc. 1996, 118, 2922 – 2928; i) C. A. Reed, Acc. Chem. Res. 1998, 31, 325 – 332; j) P. P. Gaspar, Science 2002, 297, 785 – 786; k) K. C. Kim, C. A. Reed, D. W. Elliott, L. J. Mueller, F. Tham, L. Lin, J. B. Lambert, Science 2002, 297, 825 – 827; l) V. Y. Lee, A. Sekiguchi, Acc. Chem. Res. 2007, 40, 410 – 419; m) T. Kppers, E. Bernhardt, R. Eujen, H. Willner, C. W. Lehmann, Angew. Chem. Int. Ed. 2007, 46, 6346 – 6349; Angew. Chem. 2007, 119, 6462 – 6465. Y. Xiong, S. Yao, M. Driess, Z. Naturforsch. B 2013, 68, 445 – 452. a) A. C. Filippou, O. Chernov, G. Schnakenburg, Angew. Chem. Int. Ed. 2009, 48, 5687 – 5690; Angew. Chem. 2009, 121, 5797 – 5800; b) A. C. Filippou, Y. N. Lebedev, O. Chernov, M. Strassmann, G. Schnakenburg, Angew. Chem. Int. Ed. 2013, 52, 6974 – 6978; Angew. Chem. 2013, 125, 7112 – 7116. N. Kuhn, T. Kratz, D. Blser, R. Boese, Chem. Ber. 1995, 128, 245 – 250. L. Pauling, L. O. Brockway, J. Am. Chem. Soc. 1935, 57, 2684 – 2692. a) N. Kuhn, G. Henkel, T. Kratz, J. Kreutzberg, R. Boese, A. H. Maulitz, Chem. Ber. 1993, 126, 2041 – 2045; b) A. J. Arduengo III, F. Davidson, R. Krafczyk, W. J. Marshall, R. Schmutzler, Monatsh. Chem. 2000, 131, 0251 – 0265. a) G. A. Ozin, J. Chem. Soc. D 1969, 104; b) I. R. Beattie, G. A. Ozin, J. Chem. Soc. A 1970, 370; c) H. E. Blayden, M. Webster, Inorg. Mater. Inorg. Nucl. Chem. Letters 1970, 6, 703 – 705. K. Schwedtmann, M. H. Holthausen, K. O. Feldmann, J. J. Weigand, Angew. Chem. Int. Ed. 2013, 52, 14204 – 14208; Angew. Chem. 2013, 125, 14454 – 14458. a) B. Hoge, S. Steinhauer, J. Bader, N. Ignati’ev, 20th Symposium on Fluorine Chemistry, Kyoto, Japan 2012; b) B. Hoge, S. Steinhauer and N. Ignati’ev, 6th European Silicon Days, Lyon, France 2012. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Puschmann, J. Appl. Crystallogr. 2009, 42, 339 – 341. L. Palatinus, G. Chapuis, J. Appl. Crystallogr. 2007, 40, 786 – 790. G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112 – 122.
Received: July 28, 2014 Published online on && &&, 0000
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FULL PAPER & Silicon Complexes T. Bçttcher,* S. Steinhauer, L. C. Lewis-Alleyne, B. Neumann, H.-G. Stammler, B. S. Bassil, G.-V. Rçschenthaler, B. Hoge && – && NHC!SiCl4 : An Ambivalent CarbeneTransfer Reagent
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One way or another: NHC!SiCl4 is not only a carbene-transfer reagent that is useful for the synthesis of p-block complexes with N-heterocyclic carbene (NHC) ligands that are not available in their free form, but it also serves as a potent precursor for NHC-stabilized
silicon(IV) cations (see scheme). Selected examples are shown in which NHC!SiCl4 reacts as a source of carbene or silicon cations, depending on the reaction conditions. NHC: 1,3-dimethylimidazolidin-2-ylidene.
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& Silicon Complexes
NHC!SiCl4 : An Ambivalent Carbene-Transfer Reagent** Tobias Bçttcher,*[a] Simon Steinhauer,[a] Lesley C. Lewis-Alleyne,[b] Beate Neumann,[a] HansGeorg Stammler,[a] Bassem S. Bassil,[b] Gerd-Volker Rçschenthaler,[b] and Berthold Hoge[a]
Abstract: The addition of BCl3 to the carbene-transfer reagent NHC!SiCl4 (NHC = 1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl3] + and [(NHC)2SiCl3] + with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC!SiCl4 served as a transfer reagent for the NHC ligand. The addition of BF3·OEt2, on the other hand, gave NHC!BF3 as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C2F5)2SiCl2 was treated with NHC!SiCl4. In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl3] + and [(NHC)2SiCl3] + were detected with [(C2F5)SiCl3] as counterion. A similar result
was already reported for the reaction of NHC!SiCl4 with (C2F5)2SiH2, which gave [(NHC)2SiCl2H][(C2F5)SiCl3]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC!Si(C2F5)2Cl2 and NHC!Si(C2F5)2ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC!SiCl4 towards phosphines is discussed. The carbene complex NHC!PCl3 shows similar reactivity to NHC!SiCl4, and may even serve as a carbene-transfer reagent as well.
Introduction
NHC is the smallest imidazolidine-based carbene which, unlike its unsaturated analogue, has not been isolated in its free form yet.[6] Addition of 1 to PCl3 quantitatively yielded NHC!PCl3 with liberation of SiCl4 and is the first example of selective NHC transfer between two p-block elements. The addition of two equivalents of 1 to the highly Lewis acidic (C2F5)2SiH2 (2), on the other hand, did not afford the expected carbene adduct of 2. Instead the salt [(NHC)2SiCl2H][(C2F5)2SiCl3] (E, Figure 1) with pentacoordinate silicon in both the anion and the cation was obtained.[7] Despite the large number of silicon(IV) complexes reported in the literature, and the ongoing and extensive research in this field, only a few cationic complexes of silicon(IV) halides and hydrides with a coordination number lower than six have been structurally characterized so far, including tricoordinate silylium ions.[8] The reported examples are all stabilized with the help of NHCs as donor ligands
In the so-called renaissance of main group chemistry over the past two decades, N-heterocyclic carbenes (NHCs) have evolved into an indispensable class of ligands for the stabilization of unusual oxidation states and coordination motifs of pblock compounds.[1] Access to these new complexes is almost exclusively by the addition of free and uncoordinated carbenes to stable p-block precursors.[2] This, however, excludes NHCs that are not stable and therefore not isolable in their free form. Moreover, the presence of free NHCs may lead to unwanted side reactions due to their high basicity, strong nucleophilicity, and reducing nature.[3] To avoid these synthetic obstacles, indirect procedures such as oxidative addition and carbene transfer are useful alternatives. For the synthesis of transition metal NHC complexes, both methods are already established protocols and have allowed the introduction of carbene ligands that are otherwise not isolable in their free form.[4] We recently introduced NHC!SiCl4 (1; NHC = 1,3-dimethylimidazolidin-2-ylidene) as a carbene-transfer reagent.[5] This
[a] Dr. T. Bçttcher, S. Steinhauer, B. Neumann, Dr. H.-G. Stammler, Prof. Dr. B. Hoge Fakultt fr Chemie, Universitt Bielefeld Anorganische Chemie II Universittsstrasse 25, 33615 Bielefeld (Germany) E-mail: [email protected] [b] Dr. L. C. Lewis-Alleyne, Dr. B. S. Bassil, Prof. Dr. G.-V. Rçschenthaler Jacobs University Bremen School of Engineering and Science Campus Ring 1, 28759 Bremen (Germany)
Figure 1. Structurally characterized tetra- and pentacoordinate cationic halo/ hydrido silicon(IV) complexes (Dipp = 2,6-diisopropylphenyl).
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Full Paper (Figure 1). Complexes A and B were obtained by Driess et al. by addition of one and two equivalents, respectively, of free NHC to H3Si(OTf).[9] Filippou and co-workers prepared compounds C and D by addition of free NHC to the corresponding silicon tetrahalide.[10] The analogous salt with X = Cl has not been reported yet, and the addition of free NHC to SiCl4 affords 1:1 adducts, as first reported by Kuhn et al. in 1995.[11] Herein, we report on new results with the carbene transfer reagent NHC!SiCl4 (1) revealing its ambivalent character. Examples for both characteristics, that is the synthesis of novel carbene complexes by NHC transfer and 1 acting as a source of tetra- and pentacoordinate silicon(IV) cations, are presented.
Scheme 1. Stepwise formation of salts 4 and 5.
Lewis acidic compared to that in 1, and the carbene ligand is transferred from the remaining equivalent of 1 to yield 5 with liberation of SiCl4 (Scheme 1). Single crystals of both salts suitable for XRD were obtained from solutions in acetonitrile at 30 8C. Both compounds crystallize in the monoclinic space group P21/c. The silicon atom in 4 is in a tetrahedral environment (Figure 2). The CSi bond
Results and Discussion As recently reported, the addition of two equivalents of NHC!SiCl4 (1) to (C2F5)2SiH2 (2) yields the salt [(NHC)2SiCl2H] [(C2F5)2SiCl3] (E).[7] During the reaction, Cl/H metathesis occurs and NHC!SiCl3H was identified as an intermediate. The anion of E is formed by abstraction of chloride from NHC!SiCl3H, under the assumption that the tetracoordinate cation [(NHC)SiCl2H] + is an intermediate in the reaction. Such a cation has a higher Lewis acidity than the remaining equivalent of neutral and pentacoordinate NHC!SiCl3H, and thus initiates a carbene transfer to eventually yield E while silicochloroform is liberated. Attempts to synthesize the sought-after tetracoordinate cation by treating only one equivalent of 1 with 2 remained unsuccessful and only gave E and unconverted starting materials. However, to overcome the halide metathesis process, 1 was treated with (C2F5)2SiCl2 (3) in 1:1 molar ratio in deuterated acetonitrile instead, and the reaction was monitored by multinuclear NMR spectroscopy. According to the 19 F NMR spectrum, the reaction quantitatively yielded [(C2F5)2SiCl3] as the only species with C2F5 groups. In 1H,29Si 2D NMR correlation experiments, no cross-peaks for the corresponding cations could be detected at room temperature, although the 1H NMR spectroscopic data clearly indicated the presence of a coordinated NHC ligand. To slow down possible exchange equilibria, the reaction mixture was cooled to 40 8C. Then, two cross-peaks were detected, one at d = 110.5, which corresponds to pentacoordinate silicon and compares well with that of the cation in E (d = 125.4), and the other at d = 20.1, corresponding to the supposed tetracoordinate species [(NHC)SiCl3] + [Eq. (1)].
Figure 2. Crystal structure of 4. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 190.7(2), Si1Cl1 200.55(5), Si1Cl2 200.81(5), Si1Cl3 200.35(5), C1Si1Cl1 108.40(5); C1-Si1-Cl2 106.04(5), C1Si1-Cl3 112.92(5).
length (190.7(2) pm) compares well with that of 1 (192.8(3) pm). The mean SiCl bond length (200.57(5) pm) is slightly longer than that of SiCl4 in the gas phase (196.3 pm)[12] but considerably shorter than the mean axial (220.8(2) pm) and equatorial (207.3(2) pm) SiCl bond lengths in pentacoordinate 1. In both salts the anion and the cation are well separated from each other with interionic Si···ClBCl3 distances (4: 390.6(1) pm, 5: 468.6(1) pm) that exceed the sum of the van der Waals radii for Si and Cl (385 pm). The silicon atom in 5 is in a trigonal-bipyramidal environment with the NHC ligands in the equatorial positions (Figure 3). The mean SiC bond length (193.3(2) pm) compares well with that of pentacoordinate 1 (192.8(3) pm) and is only slightly longer than that of 4 (190.7(2) pm). The mean axial SiCl bond length (220.78(4) pm) and equatorial SiCl bond length (206.81(4) pm) compare well with those of 1 (220.8(2) and 207.3(2) pm). The equatorial sum of angles is 3608, and the axial Cl1-Si1-Cl3 bond angle (175.67(2)8) is almost linear. The cation of 5 is formally a {SiCl3} + moiety stabilized by two NHC ligands. In the previously reported isostructural cation E (Figure 1), the chloride anion at the equatorial position is replaced by a hydride. The derivative with two hydrides and one chloride has not been synthesized yet. However, Driess et al. reported the structure of the cation
Reaction of 1 with BCl3 To rule out possible involvement of chloride in the exchange process, a stronger acceptor for chloride was chosen. Addition of one equivalent of BCl3 to a solution of 1 in dichloromethane afforded [(NHC)SiCl3][BCl4] (4) in 98 % yield. Changing the 1:BCl3 molar ratio to 2:1 gave [(NHC)2SiCl3][BCl4] (5) in 96 % yield. In the first step, one equivalent of BCl3 abstracts a chloride ion from 1 to give 4. The silicon atom in 4 becomes more &
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Full Paper THF. For the reaction of 1 with 2 and 3, there is a competition between NHC transfer and chloride transfer yielding a neutral NHC adduct and a salt of a chlorosilicate, respectively. The ionic products are better stabilized by the more polar solvent (acetonitrile), and the less polar solvent (dichloromethane) affords the neutral carbene adducts. The addition reaction of 1 to 2 also proceeds quantitatively but is accompanied by H/Cl metathesis (Scheme 2). The carbene adduct NHC!Si(C2F5)2ClH (7) was identified as the only product containing C2F5 groups. The complex NHC!SiCl3H was obtained as the primary byproduct. As previously reported, NHC!SiCl3H redistributes in solution to give 1 and NHC!SiCl2H2.[7] The 29Si NMR resonances for 6 (d = 105.3) and 7 (d = 120.1) compare well with those of 1 (d = 103.9), NHC!SiCl3H (d = 104.5), and NHC!SiCl2H2 (d = 125.2).[5, 7] Isolation of pure 7 from the metathesis byproducts by crystallization turned out to be unsuccessful so far due to very similar solubilities of the involved species. However, a single crystal suitable for XRD could be isolated for its molecular structure determination from a dichloromethane solution of the reaction mixture kept at 30 8C. Compound 6 crystallizes in the triclinic space group P1¯, and compound 7 in the orthorhombic space group Pbca. In both compounds the silicon atom is in a trigonal-bipyramidal environment (Figures 4 and 5). The axial posi-
Figure 3. Crystal structure of 5. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 193.0(2), C6Si1 193.6(2), Si1Cl1 221.19(4), Si1Cl2 206.81(4), Si1Cl3 220.37(4); C1-Si1-C6 125.56(5), C1-Si1Cl2 116.59(4), C6-Si1-Cl2 117.79(4), Cl1-Si1-Cl3 175.67(2).
[(NHCDipp)2SiH3] + (B), in which both NHC ligands reside at the equatorial positions.
NHC transfer to (C2F5)2SiH2 (2) and (C2F5)2SiCl2 (3) As part of the above-described studies on the reaction of 1 with silanes 2 and 3, the solvent was changed from acetonitrile to dichloromethane. Surprisingly, on monitoring both reactions by 19F NMR spectroscopy, no signal for the expected chlorosilicate [(C2F5)2SiCl3] could be detected. Instead the products of an NHC transfer were obtained. Compound 1 forms a 1:1 adduct with 3 to give NHC!Si(C2F5)2Cl2 (6; Scheme 2). The reaction proceeds quantitatively according to
Scheme 2. Reaction of 1 with 2 and 3 in CH2Cl2.
Figure 4. Crystal structure of 6. All CH protons have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C5Si1 199.3(2), Si1Cl1 208.73(7), Si1Cl2 208.45(8), Si1C1 199.9(2), Si1C3 199.9(2), av CN 132.7(3); C5-Si1-Cl1 123.30(7), C5-Si1-Cl2 119.05(7), Cl1-Si1-Cl2 117.65(3), C1-Si1-C3 172.03(9).
the 19F NMR data, and single crystals of 6 were obtained after keeping a solution of the raw product in dichloromethane at 30 8C (63 % yield). This result was unexpected, as previous calculations concluded that the bond dissociation energy of the CSi bond in 1 decreases in coordinating solvents.[5] Initially, THF was reported as a good medium for carbene transfer reactions with 1. Here, the choice of solvents is restricted by the highly Lewis acidic silanes, which do not allow the use of
tions are occupied by the C2F5 groups, and the equatorial sum of angles is 3608 for both complexes. The most striking difference is the elongated carbene–silicon bonds in 6 (199.3(2) pm) and 7 (196.0(1) pm) compared with NHC!SiCl4 (192.8(3) pm), NHC!SiCl3H (191.4(2) pm), and NHC!SiCl2H2 (190.7(2) pm). The longer bonds also seem to be reflected in a much higher sensitivity of 6 and 7 towards even traces of moisture. Compound 1, on the other hand, can be handled without the use of a glovebox with short exposure to air.
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Figure 6. Crystal structure of 8. All CH protons have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1B1 165.6(2), C1N1 132.4(2), C1N2 132.5(2), B1F1 137.8(2), B1F2 139.4(2), B1F3 138.4(2); N1-C1-N2 110.1(1), C1-B1-F1 112.5(1), C1-B1F2 107.6(1), C1-B1-F3 108.2(1).
donor ligand results in a hexacoordinate complex with the NHC ligand at the axial position, and thus the CSi bond length is increased and the bond dissociation energy decreased. The THF molecule can weakly coordinate to the silicon atom and therefore activate the NHC dissociation. It was then interesting to test the reactivity of 1 towards stronger donor molecules. As previously reported, the addition of PCl3 to 1 quantitatively yields NHC!PCl3 (9).[5] No reaction was observed for 1 and PPh3. However, when PMe3 was condensed into a solution of 1, the reaction mixture changed color from colorless to dark yellow. The complex (PMe3)2SiCl4 was identified as the product according to its literature data (Scheme 4).[14] This result is consistent with the earlier calculations, but it is in contrast to transition metal complexes, in which phosphine ligands can be substituted for more strongly binding carbenes.
Figure 5. Crystal structure of 7. All CH protons and minor-occupancy disordered atoms have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1Si1 196.0(1), Si1 Cl1 210.46(5), Si1C6 201.0(1), Si1C8 201.3(1), av C1N 132.7(2); C1-Si1-Cl1 127.18(4), C6-Si1-C8 177.84(6).
Reaction of 1 with BF3·OEt2 As reported above, BCl3 was used for chloride abstraction from 1 to yield the salts 4 and 5. Attempts to transfer the carbene to boron by changing the reaction conditions remained unsuccessful and exclusively yielded the salt 4. Interestingly, when BCl3 was replaced by BF3·OEt2 the carbene-transfer product NHC!BF3 (8) was obtained as an air- and moisture insensitive colorless solid in 64 % yield after recrystallization (Scheme 3).
Scheme 3. NHC transfer to BF3.
Kuhn et al. and Arduengo et al. reported 1:1 addition reactions of free carbenes and BF3·OEt2 to give analogous compounds.[13] Single crystals suitable for XRD were obtained from a toluene solution stored at 30 8C. Compound 8 crystallizes in the monoclinic space group P21/n (Figure 6). The characteristic NMR spectroscopic data of 8 (11B: d = 0.8 ppm, 1JBF = 38 Hz) and the structural parameters are in good agreement with the previously reported complexes.
Scheme 4. Reactions of 1 with phosphines.
As the reaction of 1 with BCl3 readily yielded the carbenestabilized silicon(IV) cations, it was worth testing the carbene complex NHC!PCl3 (9) as a source for phosphorus(III) cations. By condensing 0.5 equivalents of BCl3 into a solution of 9 in dichloromethane, the salt [(NHC)2PCl2][BCl4] (10) was obtained in 99 % yield (Figure 7). Its formation seems to follow a similar reaction pathway as proposed for its silicon counterpart 5 (Scheme 1), which suggests that 9 may serve as a carbenetransfer reagent as well. An analogous compound to 10 was recently obtained by Weigand et al. by using sterically demanding free carbenes (NHCDipp).[15] The procedure now reported allows for an alternative access to this species without using free carbene.
Reactions of 1 with phosphines When compound 1 was first reported as an NHC-transfer reagent, it was experimentally found that the transfer process works best in THF as reaction medium. It was further concluded by theoretical calculations that the carbene–silicon bond becomes increasingly more labile in coordinating solvents.[5] The silicon atom in 1 is five-coordinate, and an additional &
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Full Paper periments. Elemental analyses were performed by the microanalytical laboratory of the department of Inorganic Chemistry II (ACII), University of Bielefeld, Bielefeld, Germany. Compounds NHC!SiCl4 (1) (C2F5)2SiH2 (2), and (C2F5)2SiCl2 (3), and were prepared according to procedures reported elsewhere.[5, 16]
X-ray crystallography X-ray diffraction data were collected on an Agilent Supernova diffractometer, except for those of 8, which were measured on a Bruker Nonius KappaCCD. Olex2[17] was used to solve the structures of 5 and 7 by charge flipping,[18] and all others were solved by direct methods and with the ShelX program package[19] by using least-squares minimization. All heavy atoms were refined anisotropically, except for a carbon atom in the disordered C2F5 group in 7. The hydrogen atoms were placed in calculated positions by using a riding model, except for the hydrogen atom bonded to silicon in compound 7, which was refined isotropically.
Figure 7. Crystal structure of 10. All CH protons and the anion have been omitted for clarity. Thermal ellipsoids set at 50 % probability. Selected bond lengths [pm] and angles [8]: C1P1 186.5(1), C6P1 186.3(1), C1N1 132.3(1), C1N2 132.8(1), C6N3 132.3(2), C6N4 132.5(2), P1Cl1 229.73(4), P1Cl2 232.83(4); C1-P1-C6 111.88(5), Cl1-P1-Cl2 169.38(2), N1-C1-N2 111.7(1), N3-C6N4 112.3(1).
Data for X-ray structure determination of 8 were collected on a Bruker Nonius KappaCCD diffractometer at 100(2) K with graphite-monochromated MoKa radiation (l = 71.073 pm). Numerical absorption correction was done by using SADABS. The structures were solved by direct methods with SAINT and refined by fullmatrix least-squares cycles by using SHELX-97.[19] All heavy atoms were refined anisotropically, and the hydrogen atoms were either found directly and refined isotropically or placed in calculated positions. Crystallographic data and structure refinement for 4–8 and 10 are summarized in Table 1.
Conclusion New reactions with the carbene transfer reagent NHC!SiCl4 (1) were presented. The reaction of 1 with the highly acidic silanes (C2F5)2SiH2 (2) and (C2F5)2SiCl2 (3) in acetonitrile gave salts with carbene-stabilized silicon(IV) cations [(NHC)SiCl3] + , [(NHC)2SiCl3)] + , and [(NHC)2SiCl2H] + , respectively, with the chlorosilicate [(C2F5)2SiCl3] as the anion. By changing the solvent to dichloromethane the reactions exclusively yielded the products of carbene transfer NHC!Si(C2F5)2Cl2 (6) and NHC!Si(C2F5)2ClH (7) instead. This selectivity is attributed to the polarity of the solvent, whereby the ionic products are better stabilized in the more polar solvent, and the neutral adducts are obtained from the less polar solvent. Reaction of 1 with BCl3 exclusively yielded the salts [(NHC)SiCl3][BCl4] (4) and [(NHC)2SiCl3][BCl4] (5), according to the stoichiometry. However, when BCl3 was replaced by BF3·OEt2, the product of carbene transfer NHC!BF3 (8) was obtained. The reactivity of 1 towards different phosphines was also briefly discussed. Compounds 6 and 7 may serve as promising precursors for electron-poor silylenes. Furthermore, it was shown that the carbene complex NHC!PCl3 can also act as a carbene-transfer reagent and is a source of carbene-stabilized phosphorus(III) cations.
CCDC 965801 (4), 965802 (5), 1015845 (6), 1015846 (7), 1013798 (8) and 1015847 (10) 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.
Syntheses Compound 4: A solution of 1 (1.46 g, 5.45 mmol) in dichloromethane (40 mL) was placed in a 100 mL Schlenk flask. The solution was cooled to 196 8C and BCl3 (6.0 mmol, 1.1 equiv) was condensed into the flask. The reaction mixture was allowed to reach room temperature over 12 h with constant stirring. Removal of all volatile components gave compound 4 (2.05 g, 5.32 mmol). Single crystals suitable for XRD were obtained from a solution of 4 in acetonitrile at 30 8C. Yield: 98 %. Elemental analysis calcd (%) for C5H10BCl7N2Si (385.21): C 15.59, H 2.62, N 7.27; found: C 16.07, H 3.05, N 7.45; 1H NMR (CDCl3): d = 3.49 (s, 6 H, CH3), 4.23 ppm (s, 4 H, CH2); 11B{1H} NMR (CDCl3): d = 6.9 (s, [BCl4]); 1H,13C HMBC (CDCl3): d(1H)/d(13C) = 3.49/37.2 (d, CH3/CH3, 1J(C,H) = 141 Hz), 3.49/53.4 (s, CH3/CH2), 4.23/53.4 (d,m, CH2/CH2, 1J(C,H) = 155 Hz), 3.49/161.3 (s, CH2/Ccarbene), 4.23/161.3 ppm (s, CH3/Ccarbene); 1H,29Si HMBC NMR (CDCl3): d(1H)/d(29Si) = 3.49/20.9 (s, CH3/Si), 4.23/20.9 ppm (s, CH2/Si).
Experimental Section General remarks All reactions were carried out under an atmosphere of dry nitrogen by using standard Schlenk-line techniques unless mentioned otherwise. Solvents were dried with a Braun MB-SPS 800 system. Acetonitrile and dichloromethane were stored over 3 molecular sieves. CD3CN) and CDCl3 were distilled over CaH2. All other chemicals were purchased from commercial sources and were used as received. NMR spectra were recorded on a Bruker Avance III 300 spectrometer (operating frequencies: 29Si 59.63 MHz, 19F 282.40 MHz, 13C 75.47 MHz, 11B 96.29 MHz, 1H 300.13 MHz) with positive shifts downfield from the external standards [CCl3F (19F), Si(CH3)4 (29Si, 13C, 1H), BF3·Et2O (11B)]. The 13C and 29Si NMR shifts were acquired by 1H,13C, and 1H,29Si 2D HMBC NMR correlation exChem. Eur. J. 2014, 20, 1 – 8
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Compound 5: Compound 5 was synthesized by following the procedure for 4. Quantities used: 1 (2.33 g, 8.69 mmol) in dichloromethane (50 mL), BCl3 (4.3 mmol, 0.5 equiv). Yield: 96 %. Elemental analysis calcd (%) for C10H20BCl7N4Si (483.36): C 24.85, H 4.17, N 11.59; found: C 24.88, H 4.81, N 11.17; 1H NMR (CD3CN): d = 3.35 (s, 6 H, CH3), 3.85 ppm (s, 4 H, CH2); 11B{1H} NMR (CD3CN): d = 6.9 ppm (s, [BCl4]); 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.35/37.0 (d, 1 J(C,H) = 142 Hz, CH3/CH3), 3.35/51.8 (s, CH3/CH2), 3.85/51.8 (d,m, 1 J(C,H) = 152 Hz, CH2/CH2), 3.35/169.7 (s, CH3/Ccarbene), 3.85/ 169.7 ppm (s, CH2/Ccarbene); 1H,29Si HMBC NMR (CD3CN): d(1H)/ d(29Si) = 3.35/110.5 (s, CH3/Si), 3.85/110.5 ppm (s, CH2/Si).
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formula Mr T [K] radiation/l [pm] crystal system space group a [pm] b [pm] c [pm] a [8] b [8] g [8] V [106 pm3] Z 1calcd [Mg m3] m [mm1] F(000) crystal size [mm] 2 q range for data collection reflns collected independent reflns R(int) data/restraints/parameters GoF on F2 R1 [I > 2 s(I)][a] Rw [all data][b] largest difference peak/hole [e 3]
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C5H10BCl7N2Si 385.20 100.0(1) CuKa/154.18 monoclinic P21/c 1317.811(13) 915.426(10) 1242.381(14) 90 98.8748(10) 90 1480.81(3) 4 1.728 12.829 768.0 0.21 0.17 0.16 6.8–144.08 31173 2916 0.0346 2916/0/147 1.073 0.0201 0.0524 0.38/0.30
C10H20BCl7N4Si 483.35 100.0(1) MoKa/71.073 monoclinic P21/c 1397.338(19) 1260.272(13) 1244.060(16) 90 112.3079(16) 90 2026.85(4) 4 1.584 1.040 984.0 0.32 0.28 0.28 6.3–65.88 116764 5914 0.0365 5914/0/212 1.057 0.0247 0.0631 1.16/0.96
C9H10Cl2F10N2Si 435.18 100.0(1) MoKa/71.073 triclinic P1¯ 867.52(3) 877.01(2) 1201.03(3) 85.1446(19) 76.864(2) 60.513(3) 774.19(4) 2 1.867 0.602 432.0 0.21 0.15 0.08 5.3–60.28 45422 4505 0.0312 4505/0/219 1.145 0.0388 0.1006 0.60/0.42
C9H11ClF10N2Si 400.74 100.0(1) MoKa/71.073 orthorhombic Pbca 941.983(7) 1417.157(9) 2232.361(18) 90 90 90 2980.06(4) 8 1.786 0.444 1600.0 0.31 0.27 0.20 9.4–60.08 163135 4325 0.0397 4325/37/224 1.042 0.0354 0.0892 0.68/0.45
C5H10BF3N2 165.96 100(2) MoKa/71.073 monoclinic P21/n 1074.37(2) 640.750(10) 1175.58(2) 90 113.4970(10) 90 742.17(2) 4 1.485 0.142 344 0.30 0.30 0.20 7.4–56.6 27504 1844 0.0498 1844/0/140 1.006 0.0374 0.1070 0.446/0.216
C10H20BCl6N4P 450.78 100.0(1) MoKa/71.073 monoclinic P21/c 1422.58(3) 1163.993(18) 1296.85(3) 90 115.464(3) 90 1938.80(7) 4 1.544 0.968 920.0 0.18 0.13 0.12 6.3–60.28 44283 5649 0.0294 5649/0/203 1.052 0.0241 0.0613 0.59/0.39
[a] R1 = S j j Fo j j Fc j j /S j Fo j . [b] Rw = {S[w(F 2oF 2c)2]/S[w(F 2o)2]}1/2.
2 F, CF2), 125.6 (br s, 2 F, CF2), 80.5 ppm (s, 6 F, CF3); 1H,29Si HMBC NMR (CD3CN): d(1H)/d(29Si) = 3.31/120.1 (s, CH3/Si), 5.10/ 120.1 ppm (d, SiH/Si, 1J(Si,H) = 261 Hz).
Compound 6: A solution of 1 (0.55 g, 2.04 mmol) in dichloromethane (30 mL) was cooled to 196 8C. Compound 3 (2.10 mmol) was condensed into the solution and the reaction mixture was allowed to reach room temperature. The reaction mixture was stirred for an additional 12 h. After removal of all volatile components under reduced pressure, the crude product was dissolved in the minimum amount of dichloromethane. The solution was kept at 80 8C and gave single crystals of 6 (0.56 g, 1.29 mmol). Yield: 63 %. Elemental analysis calcd (%) for C9H10Cl2F10N2Si (435.16): C 24.84, H 2.32, N 6.44; found: C 24.72, H 2.79, N 6.57; 1H NMR (CDCl3): d = 3.47 (s, 6 H, CH3), 3.79 ppm (s, 4 H, CH2); 1H,13C HMBC (CDCl3): d(1H)/d(13C) = 3.47/36.7 (d, 1J(C,H) = 141 Hz, CH3/CH3), 3.47/ 51.9 (s, CH3/CH2), 3.79/51.9 (d,m, 1J(C,H) = 152 Hz, CH2/CH2), 3.47/ 174.1 (s, CH3/Ccarbene), 3.79/174.1 ppm (s, CH2/Ccarbene); 19F NMR (CDCl3): d = 122.6 (s, 4 F, CF2), 77.5 ppm (s, 6 F, CF3); 1H,29Si HMBC NMR (CDCl3): d(1H)/d(29Si) = 3.47/105.3 (s, CH3/Si), 3.79/ 105.3 ppm (s, CH2/Si).
Compound 8: BF3·Et2O (16.6 g, 117 mmol) was added to a solution of 1 (14.2 g, 53.0 mmol) in THF (200 mL) with stirring at 0 8C. The reaction mixture was allowed to reach room temperature and was stirred for an additional 12 h. All volatile components were removed under reduced pressure and the raw product was dissolved in minimum amount of hot toluene. The solution was kept at 30 8C to give single crystals of 8 (5.63 g, 33.9 mmol). Yield: 64 %; 1 H NMR (CD3CN); d = 3.13 (s, 6 H, CH3), 3.63 ppm (s, 4 H, CH2); 11 1 B{ H} NMR (CDCl3): d = 0.8 ppm (q, 1J(B,F) = 38 Hz); 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.13/33.9 (d, CH3/CH3, 1J(C,H) = 140 Hz), 3.13/ 50.7 (s, CH3/CH2), 3.63/50.7 (d,m, CH2/CH2, 1J(C,H) = 151 Hz), 3.13/ 177.2 (s, CH3/Ccarbene), 3.63/177.2 ppm (s, CH2/Ccarbene); 19F NMR (CD3CN): d = 140.8 ppm (q, 1J(B,F) = 38 Hz).
Compound 7: A solution of 1 (0.50 g, 1.87 mmol) in dichloromethane (30 mL) was cooled to 196 8C. Compound 2 (0.93 mmol) was condensed into the solution and the reaction mixture was allowed to reach room temperature. The reaction mixture was stirred for an additional 12 h. After removal of all volatile components the reaction mixture was analyzed by multinuclear NMR spectroscopy, and compound 7 was identified alongside previously reported NHC!SiCl4, NHC!SiCl3H, and NHC!SiCl2H2.[5, 7] The reaction mixture was dissolved in the minimum amount of dichloromethane and stored at 80 8C to give crystals suitable for structure determination of 7. 1H NMR (CD3CN): d = 3.31 (s, 6 H, CH3), 3.78 (s, 4 H, CH2), 5.10 ppm (m, 1 H, Si-H).1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.31/36.0 (d, 1J(C,H) = 141 Hz, CH3/CH3), 3.31/51.7 (s, CH3/CH2), 3.78/ 51.7 (d,m, 1J(C,H) = 153 Hz, CH2/CH2), 3.31/172.2 (s, CH3/Ccarbene), 3.78/172.2 ppm (s, CH2/Ccarbene); 19F NMR (CD3CN): d = 128.0 (br s,
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Compound 10: A solution of 9 (0.53 g, 2.26 mmol) in dichloromethane (20 mL) was cooled to 196 8C. BCl3 (1.13 mmol, 0.5 equiv) was condensed into the flask. The reaction mixture was allowed to reach room temperature and was stirred for an additional 12 h. Removal of all volatile components gave 10 (0.50 g, 1.12 mmol). Single crystals suitable for XRD were obtained from a solution of 4 in acetonitrile at 30 8C. Yield: 99 %. Elemental analysis calcd (%) for C10H20BCl6N4P (450.79): C 26.64, H 4.47, N 12.43; found: C 26.60, H 4.52, N 12.34; 1H NMR (CD3CN): d = 3.35 (s, 6 H, CH3), 3.95 ppm (s, 4 H, CH2); 11B{1H} NMR (CD3CN): d = 6.9; 1H,13C HMBC (CD3CN): d(1H)/d(13C) = 3.35/37.2 (d, CH3/CH3, 1J(C,H) 141 Hz), 3.35/51.7 (s, CH3/CH2), 3.95/51.7 (d,m, 1J(C,H) = 155 Hz, CH2/CH2), 3.35/169.1 (s, CH3/Ccarbene), 3.95/169.1 ppm (s, CH2/Ccarbene); 31P{1H} NMR (CD3CN): d = 108.6 ppm (s).
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Full Paper Acknowledgements T.B. acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen postdoctoral research fellowship. A postdoctoral fellowship to L.C.L.-A. from the Deutscher Akademischer Austausch Dienst is gratefully acknowledged.
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Received: July 28, 2014 Published online on && &&, 0000
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
FULL PAPER & Silicon Complexes T. Bçttcher,* S. Steinhauer, L. C. Lewis-Alleyne, B. Neumann, H.-G. Stammler, B. S. Bassil, G.-V. Rçschenthaler, B. Hoge && – && NHC!SiCl4 : An Ambivalent CarbeneTransfer Reagent
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Chem. Eur. J. 2014, 20, 1 – 8
www.chemeurj.org
One way or another: NHC!SiCl4 is not only a carbene-transfer reagent that is useful for the synthesis of p-block complexes with N-heterocyclic carbene (NHC) ligands that are not available in their free form, but it also serves as a potent precursor for NHC-stabilized
silicon(IV) cations (see scheme). Selected examples are shown in which NHC!SiCl4 reacts as a source of carbene or silicon cations, depending on the reaction conditions. NHC: 1,3-dimethylimidazolidin-2-ylidene.
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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