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Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies† Anastas Sidiropoulos, Brooke Osborne, Alexandr N. Simonov, Deepak Dange, Alan M. Bond, Andreas Stasch* and Cameron Jones* Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2]+, Mes = mesityl) with KC8, leads to reductive coupling of
Received 15th July 2014, Accepted 21st August 2014
the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C–C bond, while an electro-
DOI: 10.1039/c4dt02074j
chemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane
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solutions.
Introduction N-heterocyclic carbenes (NHCs) have proved of enormous importance to coordination chemistry since the first example was isolated in 1991.1 These species have been used as ligands in the formation of complexes involving nearly every nonradioactive metal in the periodic table. Most studied have been transition metal-NHC complexes, many examples of which have found application as homogeneous catalysts in numerous synthetic transformations.2 Although NHC complexes of the main group elements have not been as widely investigated,3 NHCs, and their more π-acidic relatives, cyclic alkyl(amino)carbenes (CAACs) (Fig. 1), are increasingly employed as highly nucleophilic ligands for the stabilisation of thermally labile and/or low oxidation state p-block fragments. Most recent interest in this domain has centred on the synthesis of an ever increasing number of carbene adducts of mono- and diatomic p-block element fragments, L→E←L and L→EE←L (L = NHC or CAAC, E = group 13–15 element), complexes which have
School of Chemistry, PO Box 23, Monash University, VIC 3800, Australia. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Crystal data, details of data collections and refinements for all crystal structures; ORTEP diagrams for 1, [6-DipCl]Cl and [(6-Dip)GeCl2]; and full details of the electrochemical studies. Crystallographic data (excluding structure factors) for all structures. CCDC 1012341–1012346. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02074j
14858 | Dalton Trans., 2014, 43, 14858–14864
Fig. 1 Examples of five- and six-membered singlet carbenes used in p-block coordination chemistry (Mes = mesityl, Dip = 2,6diisopropylphenyl).
been described as soluble allotropes of the coordinated element.4 The nature of the L–E and E–E bonding in these, and related, systems has been widely explored in computational studies,5 and has been the subject of a vigorous debate in the literature.6 Of most relevance to the current study are carbene complexes of low oxidation state group 15 element fragments. Important compound types that have been accessed in this area include NHC or CAAC complexes of P2,7 P4,8 P12,9 As2,10 Sb2 11 and PN.12 Equally fascinating, is the reactivity of these remarkable species. Recent examples that demonstrate this include the controlled one, two and four electron oxidations of [(IPr)EE(IPr)] (E = P or As; IPr = :C{N(Dip)CH}2, Dip = 2,6-diisopropylphenyl), which have yielded the products, [(IPr)EE(IPr)]•+ or 2+ and [(IPr)P(O)2P(O)2(IPr)],13–15 respectively. It is noteworthy that the latter compound represents the first stable complex of diphosphorus tetroxide. The development of much of this chemistry has relied on the accessibility of
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carbene adducts of the group 15 element trichlorides, [L→ECl3], as these complexes are typically (though not always) utilised as precursors to lower oxidation state species, via reduction methodologies. Although examples are known for all of the trichlorides, (E = P, As, Sb or Bi), only a handful of structurally characterised complexes have yet been reported, and the coordinating carbenes in these are invariably poorly π-acidic five-membered NHCs, e.g. IPr, or more nucleophilic and π-acidic CAACs.10,11,16–18 In order to increase the range of carbene-ECl3 adducts available to the synthetic chemist, we have investigated the preparation of such complexes incorporating the saturated expanded ring NHCs, 6-Dip and 6-Mes (Fig. 1).19,20 Our reasoning for this was that such carbenes are known to be more nucleophilic and sterically shielding than either unsaturated or saturated five-membered NHCs,19 while at the same time they should be less π-acidic than CAACs. Accordingly, if ECl3 adducts are accessible with these carbenes, their subsequent reductions might well yield products distinct to those previously reported from the reduction of NHC and CAAC-ECl3 complexes. The results of our efforts in this direction are reported herein.
Results and discussion (i)
Expanded ring NHC complexes of ECl3 (E = P, As or Sb)
The expanded ring NHCs, 6-Mes and 6-Dip, were chosen for this study because of their ease of preparation,19,20 and because they have previously proved their worth as ligands in p-block element coordination chemistry.21 Initially, 1 : 1 reactions between 6-Mes and ECl3 (E = P, As, Sb or Bi) in toluene were carried out. However, these generated mixtures of several products, as determined by NMR spectroscopic analyses, which proved difficult to separate by fractional crystallisation. It is worthy of mention that the predominant signal in the 31 1 P{ H} NMR spectrum of the 6-Mes/PCl3 reaction mixture appeared at δ 3.8 ppm, and this was assigned to the complex [(6-Mes)PCl3]. This signal is upfield of that for [(IPr)PCl3], δ 16.9 ppm,16 as might be expected given the greater nucleophilicity of 6-Mes over IPr. Attention then turned to the bulkier NHC, 6-Dip, which was similarly reacted with ECl3 (E = P, As or Sb) in 1 : 1 stoichiometries. These reactions proceeded cleanly when they were carried out at −78 °C, with subsequent warming to ambient temperature, to afford moderate to good yields of the adduct complexes, 1–3 (Scheme 1).22 In contrast, the reaction of 6-Dip with BiCl3 gave an intractable mixture of products. Furthermore, when PCl3 was treated with 6-Dip at room temperature,
Scheme 1
(i) ECl3 (E = P, As or Sb).
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an analysis of the reaction mixture by 31P NMR spectroscopy revealed a number of phosphorus containing products, of which only 1 and P2Cl4 (δ 156.1 ppm) were identifiable. The generation of P2Cl4 in this reaction suggests that 6-Dip can act as a reducing agent towards PCl3, as has previously been shown for the preparation of phosphorus(I) cations from fivemembered NHCs and PCl3.23,24 It is of note that the diphosphine, P2Cl4, has also previously been reported to be produced in the reaction of PCl3 with AsPh3, the latter of which acts as a Lewis basic reducing agent in the presence of AlCl3.25 Compounds 1–3 are thermally stable solids. Their solution 1 H and 13C{1H} NMR spectra all revealed one methine and two methyl signals corresponding to their Dip substituents. This implies the complexes have symmetrical structures in solution, which is at odds with the solid state structures determined for 1 and 3 (vide infra). The 31P{1H} NMR spectrum of 1 exhibits a singlet resonance at δ −10.9 ppm, which is ca. 16 ppm upfield of that for [(6-Mes)PCl3], as might be expected given the likely greater nucleophilicity of 6-Dip over 6-Mes. Interestingly, heating solutions of 1 in d6-benzene at 60 °C led to the decomposition of the compound over 1 hour, with the generation of a mixture of phosphorus containing products. The EI mass spectra of all three compounds exhibited molecular ion peaks. Compounds 1 and 3 were crystallographically characterised and found to be essentially isostructural. As a result, only the molecular structure of 3 is depicted in Fig. 2, while that for 1 can be found in the ESI.† The group 15 centres of both have saw-horse geometries with Cl(1) and Cl(3) in axial positions, and C(1), Cl(2) and a stereochemically active lone-pair of
Fig. 2 Molecular structure of 3 (25% thermal ellipsoids are shown; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°) for 3: Sb(1)–C(1) 2.288(2), Sb(1)–Cl(2) 2.3692(6), Sb(1)–Cl(1) 2.5260(7), Sb(1)– Cl(3) 2.5760(7), N(1)–C(1) 1.334(3), C(1)–N(2) 1.331(3), C(1)–Sb(1)–Cl(2) 102.61(6), C(1)–Sb(1)–Cl(1) 94.75(6), Cl(2)–Sb(1)–Cl(1) 87.59(2), C(1)– Sb(1)–Cl(3) 81.40(6), Cl(2)–Sb(1)–Cl(3) 85.35(2), Cl(1)–Sb(1)–Cl(3) 171.01(2), N(2)–C(1)–N(1) 120.3(2). Selected bond lengths (Å) and angles (°) for 1: Cl(1)–P(1) 2.2209(7), P(1)–C(1) 1.9234(14), P(1)–Cl(2) 2.0642(8), P(1)–Cl(3) 2.4406(7), N(1)–C(1) 1.3409(17), C(1)–N(2) 1.3336(18), C(1)– P(1)–Cl(2) 106.68(5), C(1)–P(1)–Cl(1) 92.71(5), Cl(2)–P(1)–Cl(1) 89.48(2), C(1)–P(1)–Cl(3) 83.78(5), Cl(2)–P(1)–Cl(3) 87.26(2), Cl(1)–P(1)–Cl(3) 174.31(2), N(2)–C(1)–N(1) 119.60(12).
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electrons in the equatorial positions. This is very similar to the geometries of all structurally characterised carbene adducts of group 15 trichlorides. The fact that the C(1)–E(1)–Cl(2) angle is more acute for 3 (102.61(6)°) than 1 (106.68(5)°), reflects greater s-character for the lone pair of the antimony adduct. The C–E bond lengths in the compounds are slightly longer than in previously reported carbene-ECl3 adducts,10,11,16–18,26 which might result from the more sterically hindered nature of 6-Dip. Not surprisingly, the axial Sb–Cl bonds in 3 (2.551 Å mean) are considerably longer than the equatorial Sb(1)–Cl(2) bond (2.3692(6) Å). This contrasts to the situation in 1, the equatorial (P(1)–Cl(2) 2.0642(8) Å) and one axial (P(1)–Cl(1) 1.923(1) Å) bonds of which are of a similar length, whereas the other axial bond (P(1)–Cl(3) 2.4406(7) Å) is ca. 0.4 Å longer than the other two P–Cl interactions. In this respect, Cl(3) can be viewed as being partially dissociated from the P-centre, and therefore, compound 1 could be considered as a contact ion pair, viz. [(6-Dip)Cl2P+⋯Cl−] (cf. [(IPr)PCl2][triflate]27). It is of note that the P–Cl distances in [(IPr)PCl3] follow a very similar pattern to those in 1.16 As discussed above, the solid state structures of 1 and 3 are not, at first glance, consistent with their solution NMR spectra. However, it is well known that saw-horse coordination complexes (including carbene-ECl3 adducts) readily undergo Berry-pseudorotation in solution, which accounts for this observation. (ii)
Reductions of carbene-ECl3 complexes
With 1–3 in hand, attempts to reduce the compounds with either KC8 or the magnesium(I) reagent, [{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH]−, Mes = mesityl),28 were carried out. In almost all cases, however, these reactions only returned a mixture of products, including significant amounts of the free carbene, 6-Dip. The one exception was the reduction of 1 with 3 equiv. of KC8 in THF. This afforded a good yield of the dicationic P4 complex, 4 (Scheme 2), as a lime-green crystalline solid. The synthesis of 4 can be compared with the KC8 (3 equiv.) reduction of [(IPr)PCl3], which gave neutral dimeric [(IPr)PP(IPr)];7b and the reaction of [(IPr)PCl2][triflate] with 1 equiv. of KC8, which gave the P2 cationic product, [{(IPr)ClP}2(μ-Cl)][triflate].27 It cannot be sure what the mechanism of reduction of 4 is at this stage, but it is possible that it leads to a P4 product, instead of a P2 dimer, because the greater steric hindrance of the carbene centre of 6-Dip, relative to that of IPr, precludes the latter outcome. Indeed the high steric loading of 6-Dip has previously been invoked to explain its inability to form 2 : 1 complexes with d-block metals.29 Considering the significant recent interest in cationic polyphosphorus systems,30 we were keen to explore alternative
Scheme 2
(i) KC8, THF.
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synthetic routes to 4. As previously noted, it was observed that the 1 : 1 reaction of 6-Dip with PCl3 can lead to reduction products, e.g. P2Cl4, under certain conditions. Given this, and the fact that NHCs are well known to reduce phosphorus halides,23,24 we carried out the reaction of 6-Dip with PCl3 in a 1 : 2 stoichiometry, and followed its course by 31P{1H} NMR spectroscopy. Although the reaction was not clean, it did generate significant amounts of 4, in addition to 1, P2Cl4, and a number of unidentified phosphorus containing products. Compound 4 could be crystallised from the product mixture, but in a lower yield than that obtained from the KC8 reduction of 1. It is assumed that the oxidised 6-Dip product from this reaction is the salt, [6-DipCl]Cl, which was, indeed, obtained from the reaction mixture in low yield as a crystalline solid (see ESI† for the X-ray crystal structure). The formation of 4 in this reaction is comparable to the preparation of the isostructural dicationic species, [(Ph3As)2(μ-P4)][AlCl4]2, from PCl3 and AsPh3/AlCl3.25 As previously mentioned, and depending on the stoichiometry employed, this reaction can also generate P2Cl4, as was seen in the preparation of 4. Of the spectroscopic data for 4, its 31P{1H} NMR is most informative. It exhibits an A2X2 spin system, with two triplet signals at high field (δA = −325.4 ppm, δX = −167.0 ppm, 1JAX = 184 Hz). This is remarkably similar to that reported for [(Ph3As)2(μ-P4)][AlCl4]2 (δA = −325.9 ppm, δX = −174.9 ppm, 1JAX = 160 Hz),25 which strongly suggests that 4 is also an ion separate salt. This was confirmed by an X-ray crystal structure analysis of the compound (see Fig. 3), which revealed it to consist of
Fig. 3 Structure of the dicationic component of 4 (25% thermal ellipsoids are shown; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): P(1)–C(1) 1.895(3), P(4)–C(29) 1.892(3), P(1)–P(3) 2.1952(14), P(1)–P(2) 2.2077(13), P(2)–P(4) 2.2050(15), P(2)–P(3) 2.2063(16), P(3)–P(4) 2.2056(13), N(1)–C(1) 1.326(4), C(1)–N(2) 1.330(4), N(3)–C(29) 1.322(4), N(4)–C(29) 1.328(4), C(1)–P(1)–P(3) 108.85(11), C(1)–P(1)–P(2) 106.13(11), C(29)–P(4)–P(2) 108.22(11), C(29)–P(4)–P(3) 105.29(10), P(3)–P(1)–P(2) 60.14(5), P(2)–P(4)–P(3) 60.03(5), P(4)–P(2)– P(1) 75.84(5), P(1)–P(3)–P(4) 76.09(5).
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a dicationic moiety, [(6-Dip)2(μ-P4)]2+, and two chloride anions that have no close contacts with any of the P-, N- or carbene centres. The core of the dication comprises a butterfly P4 fragment, the two P3 least squares planes of which intersect each other with a dihedral angle of 89.4°. All of the P–P distances in the compound are essentially identical, and the two 6-Dip fragments are arranged in an exo-, exo-fashion with respect to the P4 butterfly (cf. [(Ph3As)2(μ-P4)][AlCl4]2 25 and [(Mes*)2(μ-P4)],31 Mes* = C6H2But3-2,4,6). Both C–P distances (1.894 Å mean) are consistent with single bonds,32 and are close to those in related cationic phosphorus imidazoliumyl systems, e.g. 1.857 Å (mean) in [{(IPr)ClP-}2(μ-Cl)][triflate].27 (iii) Reductive coupling of the cyclic amidinium salt, [6-MesH]Br In an attempt to prepare a less hindered analogue of 4, an in situ generated toluene solution of [(6-Mes)PCl3] (see section (i)) was treated with an excess of KC8. As was the case with the reduction of [(6-Dip)PCl3], a number of phosphorus containing products were generated, though none exhibited an A2X2 pattern (cf. 4) in the 31P{1H} NMR spectrum of the reaction mixture. Despite this, work-up of the reaction was pursued, which, surprisingly, led to a low isolated yield of the bis(hexahydropyrimidine) 5. It is believed that this arose from the reduction of a small amount of [6-MesH][BF4], which is a precursor to 6-Mes, and possibly contaminated the sample of the carbene used in the preparation of [(6-Mes)PCl3]. Whatever the case, a rational moderate yielding synthesis of 5 was devised, whereby a THF solution of [6-MesH]Br was treated with 1.4 equiv. of KC8 (Scheme 3). We are not aware of any similar reductive couplings involving cyclic amidinium ions, or related imidazolium salts. There has, however, been one related report of the reduction of neutral 2-chloro-1,3-di(xylyl)imidazolidine4,5-dione, H(Cl)C{N(Xyl)C(vO)}2 (Xyl = 2,6-dimethylphenyl), which leads to a coupling reaction and formation of [–C(H){N(Xyl)C(vO)}2]2.33 It is noteworthy that attempts to reductively couple [6-DipH]Br via its treatment with KC8 met with failure, presumably because the more hindered analogue of 5, viz. (6-DipH)2 is not accessible on steric grounds. The NMR spectroscopic data for 5 are consistent with its proposed formulation, and imply that rotation about the central C–C bond of the compound in solution is heavily restricted. That is, both the 1H and 13C{1H} NMR spectra exhibit sharp signals for six chemically inequivalent methyl groups. The central methine protons resonate at δ 5.33 ppm, while the corresponding carbon resonance appears at δ
Scheme 3
(i) KC8, THF.
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76.9 ppm. An X-ray crystal structure determination of the compound (see Fig. 4) revealed it to sit on a 2-fold rotation axis, and confirmed the crowding about its central C–C bond. This crowding manifests itself by the length of that bond, 1.587(3) Å, which is significantly elongated with respect to normal C–C single bonds involving sp3-hybridised carbons, ca. 1.52 Å. For sake of comparison, the central C–C bond of the related compound, [–C(H){N(Xyl)C(vO)}2]2, is also longer than normal (1.579(2) Å).33 Considering the length of the N2(H)C–C(H)N2 bond in 5, and the fact that it bears four electron withdrawing N-substituents, it seemed likely that it would be quite susceptible to redox processes. To investigate this possibility, a d.c. and a.c. cyclic voltammetric study of dichloromethane solutions of 5 was carried out over the potential range, −2.6 to 1.0 V vs. Fc0/+ (supporting electrolyte: 0.40 M [NBun4][PF6]), using varying solution concentrations, electrode type (glassy carbon or gold), and other conditions (see ESI† for a detailed discussion). While the results varied substantially with the conditions employed, compound 5 was invariably very easy to oxidise and, for example, found to undergo an irreversible oxidation process over the potential range of −0.40 to −0.35 V vs. Fc0/+,
Fig. 4 Molecular structure of 5 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1) omitted). Selected bond lengths (Å) and angles (°): C(1)–C(1)’ 1.587(3), N(1)–C(1) 1.4652(17), C(1)–N(2) 1.4838(18), N(1)–C(1)–N(2) 108.33(11), N(1)–C(1)–C(1)’ 113.44(8), N(2)–C(1)–C(1)’ 108.10(13). Symmetry operation: ’ x, −y, −z + 1.
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when a glassy carbon electrode was used. In contrast, the compound could not be electrochemically reduced over the potential range investigated. Oxidation of 5 was found to involve two electrons, as testified by coulometric analysis of data derived from the exhaustive oxidative electrolysis at a platinum electrode in CH2Cl2 (0.20 M [NBun4][PF6]), at a controlled potential of ca. −0.10 V vs. Fc0/+. No definitive conclusions could be drawn regarding the mechanism of the inherently complicated oxidation process.
Conclusion In summary, reactions of the expanded ring N-heterocyclic carbene, 6-Dip, with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which have been crystallographically characterised and found to be monomeric in the solid state. Reductions of these complexes with KC8 or a magnesium(I) reagent typically afforded complex product mixtures, though the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, was isolated from the KC8 reduction of [(6-Dip)PCl3]. This was also found to result from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br, with KC8 leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C–C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.
Experimental section General considerations All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity dinitrogen. THF, toluene, benzene and hexane were distilled over molten potassium. Melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. Mass spectra were recorded at the EPSRC National Mass Spectrometric Service at Swansea University. Microanalyses were carried out at the Science Centre, London Metropolitan University. Reproducible microanalyses could not be obtained for 2 as all attempts to recrystallise the crude reaction product led only to amorphous or microcrystalline materials. That said, the purity of the compound was judged by NMR spectroscopy to be >95%. A reproducible microanalyses could not be obtained for 4 due to difficulties in completely removing the benzene of crystallisation by vacuum drying samples prior to analysis, and the extreme air and moisture sensitivity of the compound. IR spectra were recorded on solid samples using a Agilent Cary 630 attenuated total reflectance (ATR) spectrometer. 1H, 13C{1H} and 31P{1H} NMR spectra were recorded on either Bruker DPX300 or AvanceIII 400 spectrometers, and were referenced to the resonances of the solvent used or exter-
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nal 85% H3PO4 (31P{1H} NMR). 6-Dip and [6-MesH]Br were prepared by literature procedures,19,20 while all other reagents were used as received. Preparation of [(6-Dip)PCl3] 1. To a solution of PCl3 (0.137 g, 0.087 cm3, 1.00 mmol) in toluene (30 cm3) at −78 °C was added to a solution of 6-Dip (0.405 g, 1.00 mmol) in toluene (15 cm3). The mixture was warmed to ambient temperature, stirred for 30 minutes, then volatiles were removed in vacuo. The residue was extracted into benzene (40 cm3), the extract concentrated in vacuo to 15 cm3, then placed at 7 °C overnight to yield colourless crystals of 1 (0.46 g, 84%). M.p.: 145–147 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.11 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.57 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.87 (m, 2H, CH2CH2CH2), 3.60 (t, 3JHH = 5.2 Hz, 4H, NCH2CH2), 3.85 (sept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 7.07 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.14 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 20.8 (CH2CH2CH2), 24.7 (CH(CH3)2), 27.6 (CH(CH3)2), 30.0 (CH(CH3)2), 55.7 (NCH2), 125.9 (Ar-C), 131.6 (Ar-C), 137.4 (Ar-C), 148.2 (Ar-C), NCN signal not observed; 31P{1H} NMR (121 MHz, 298 K, C6D6): δ = −10.9; MS/EI m/z (%): 542.2 (M+, 1), 439.3 (6-DipCl+, 20), 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1652 (m), 1551 (w), 1317 (m), 1256 (w), 1100 (w), 1057 (w), 812 (m), 756 (m), 723 (m), 684 (m); anal. calcd for C28H40N2PCl3: C, 62.05%; H, 7.44%; N, 5.17%; found: C, 61.90%; H, 7.33%; N, 5.07%. Preparation of [(6-Dip)AsCl3] 2. To a solution of AsCl3 (0.181 g, 0.084 cm3, 1.00 mmol) in toluene (20 cm3) at −78 °C was added to a solution of 6-Dip (0.40 g, 0.99 mmol) in toluene (15 cm3). The mixture was warmed to ambient temperature, stirred for 30 minutes, before volatiles were removed in vacuo. The residue was extracted into benzene (30 cm3), the extract concentrated in vacuo to 10 cm3, then placed at 7 °C overnight to yield colourless crystals of 2 (0.23 g, 39%). M.p.: 148–150 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.17 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.74 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.86 (m, 2H, CH2CH2CH2), 3.44 (t, 3JHH = 5.2 Hz, 4H, NCH2CH2), 3.84 (sept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 7.18 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.24 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 20.4 (CH2CH2CH2), 24.8 (CH(CH3)2), 27.7 (CH(CH3)2), 30.1 (CH(CH3)2), 55.2 (NCH2), 125.9 (Ar-C), 131.9 (Ar-C), 136.8 (Ar-C), 148.3 (Ar-C), 180.8 (NCN); MS/EI m/z (%): 586.3 (M+, 1), 439.3 (6-DipCl+, 10), 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1654 (m), 1551 (w), 1324 (m), 1257 (w), 1098 (w), 1057 (w), 800 (m), 754 (m), 721 (m). Preparation of [(6-Dip)SbCl3] 3. To a solution of SbCl3 (0.230 g, 1.01 mmol) in toluene (25 cm3) at −78 °C was added to a solution of 6-Dip (0.41 g, 1.00 mmol) in toluene (10 cm3). The mixture was warmed to ambient temperature, stirred for 20 minutes, before volatiles were removed in vacuo. The residue was extracted into benzene (30 cm3), the extract concentrated in vacuo to 10 cm3, then placed at 7 °C overnight to yield colourless crystals of 3 (0.38 g, 61%). M.p.: 160–163 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.24 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.51 (d, 3JHH = 6.8 Hz, 12H,
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CH(CH3)2), 2.49 (br, 2H, CH2CH2CH2), 3.55 (br, 4H, NCH2CH2), 3.85 (br, 4H, CH(CH3)2), 7.29 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.41 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 19.7 (CH2CH2CH2), 24.2 (CH(CH3)2), 25.4 (CH(CH3)2), 29.8 (CH(CH3)2), 49.5 (NCH2), 125.8 (Ar-C), 131.9 (Ar-C), 137.2 (Ar-C), 146.0 (Ar-C), NCN signal not observed; MS/EI m/z (%): 632.1 (M+, 1), 404.3 (6-Dip+, 100); IR (ATR) ν/cm−1: 1655 (m), 1383 (m), 1256 (w), 1109 (m), 1057 (w), 800 (s), 755 (m), 711 (m); anal. calcd for C28H40N2SbCl3: C, 53.15%; H, 6.37%; N, 4.43%; found: C, 52.94%; H, 6.35%; N, 4.35%. Preparation of [(6-Dip)2(μ-P4)]Cl2 4. A solution of [(6-Dip)PCl3] (0.10 g, 0.18 mmol) in THF (20 cm3) was added to a slurry of KC8 (0.075 g, 0.55 mmol) in THF (20 cm3) at −80 °C. The mixture was warmed to room temperature and stirred for 4 hours. Volatiles were then removed in vacuo, and the residue was extracted into a hexane–benzene mixture (15/2 cm3). The extract was filtered and the filtrate placed at 7 °C overnight to yield lime-green crystals of 4. (0.04 g, 81%). M.p.: 155–157 °C (decomp.); 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 1.25 (d, 3JHH = 6.8 Hz, 24H, CH(CH3)2), 1.38 (d, 3JHH = 6.8 Hz, 24H, CH(CH3)2), 2.71 (br, 4H, CH2CH2CH2), 3.00 (sept, 3JHH = 6.8 Hz, 8H, CH(CH3)2), 4.13 (br, 8H, NCH2CH2), 7.33 (d, 3JHH = 7.4 Hz, 8H, m-Ar-H), 7.41 (t, 3JHH = 7.4 Hz, 4H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, CD2Cl2): δ = 19.1 (CH2CH2CH2), 23.8 (CH(CH3)2), 24.1 (CH(CH3)2), 28.2 (CH(CH3)2), 48.2 (NCH2), 124.4 (Ar-C), 130.4 (Ar-C), 135.1 (Ar-C), 144.1 (Ar-C), NCN signal not observed; 31P{1H} NMR (121 MHz, 298 K, CD2Cl2): δ = −167.0 (t, 1JPP = 184 Hz), −325.4 (t, 1JPP = 184 Hz); MS/EI m/z (%): 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1652 (m), 1542 (w), 1320 (m), 1277 (m), 1101 (m), 1003 (w), 806 (m), 757 (m), 723 (m). Preparation of (6-MesH)2 5. A solution of [6-MesH]Br (0.350 g, 0.87 mmol) in THF (20 cm3) was added to a slurry of KC8 (0.16 g, 1.22 mmol) in THF (20 cm3) at −80 °C. The mixture was warmed to room temperature and stirred for 1 h. Volatiles were removed in vacuo, the residue was extracted into benzene (40 cm3), and the extract was concentrated to 15 cm3. This was stored at 7 °C overnight to yield colourless crystals of 5. (0.09 g, 32%). M.p.: 168–172 °C; 1H NMR (400 MHz, 298 K, C6D6): δ = 1.46 (s, 6H, o-ArCH3), 1.49 (m, 2H, CH2C(H)(H)CH2), 1.52 (m, 2H, CH2C(H)(H)CH2), 1.97 (s, 6H, o-ArCH3), 2.21 (2× overlapping s, 12H, p-ArCH3), 2.30 (s, 6H, o-ArCH3), 2.70 (m, 2H, NC(H)(H)CH2), 2.80 (s, 6H, o-ArCH3), 2.99 (m, 2H, NC(H)(H)CH2), 3.64 (m, 2H, NC(H)(H)CH2), 4.51 (m, 2H, NC(H)(H)CH2), 5.33 (s, 2H, NC(H)N); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 16.9, 20.6, 20.9, 21.1, 21.8, 23.6 (Ar-CH3), 22.8 (CH2CH2CH2), 45.9, 47.6 (NCH2), 76.9 (NCN), 129.9, 130.2, 131.1, 131.6, 131.9, 132.8, 135.0, 135.9, 138.9, 140.6, 144.5, 146.5 (Ar-C); MS/EI m/z (%): 643.5 (M+, 15), 321.4 (M/2+, 100); IR (ATR) ν/cm−1: 1472 (m), 1400 (m), 1304 (m), 1209 (s), 1087 (m), 1032 (m), 846 (m), 793 (m); anal. calcd for C44H58N4: C, 82.18%; H, 9.09%; N, 8.71%; found: C, 82.17%; H, 9.14%; N, 8.24%. X-Ray crystallography Crystals of 1, 3–5, [6-DipCl]Cl and [(6-Dip)GeCl2] suitable for X-ray structural determination were mounted in silicone oil.
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Crystallographic measurements were carried using either an Oxford Gemini Ultra diffractometer using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å), or the MX1 beamline of the Australian Synchrotron (λ = 0.71090 Å for 4 and 5; 0.71080 Å for [6-DipCl]Cl). The software package Blu-Ice34 was used for synchrotron data acquisition, while the program XDS35 was employed for synchrotron data reduction. The structures were solved by direct methods and refined on F2 by full matrix least squares (SHELX97)36 using all unique data. Hydrogen atoms have been included in calculated positions (riding model) for all structures. Crystal data, details of data collections and refinement are given in Table S1 in the ESI.†
Acknowledgements CJ, AS, ANS and AMB thank the Australian Research Council. The EPSRC Mass Spectrometry Service at Swansea University is also thanked. Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.
References 1 A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1991, 113, 361. 2 Selected reviews: (a) M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485; (b) C. S. J. Cazin, N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, Springer, New York, 2011, vol. 32; (c) G. C. Fortman and S. P. Nolan, Chem. Soc. Rev., 2011, 40, 5151; (d) P. de Fremont, N. Marion and S. P. Nolan, Coord. Chem. Rev., 2009, 253, 862; (e) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122; (f ) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (g) D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev., 2000, 100, 39; and references therein. 3 Selected reviews: (a) N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005, 249, 829; (b) C. Jones, Chem. Commun., 2001, 2293; (c) C. J. Carmalt and A. H. Cowley, Adv. Inorg. Chem., 2000, 50, 1; and references therein. 4 Selected reviews: (a) D. J. D. Wilson and J. L. Dutton, Chem. – Eur. J., 2013, 19, 13626; (b) Y. Yang and G. H. Robinson, Dalton Trans., 2012, 41, 337; (c) D. Martin, M. Soleilhavoup and G. Bertrand, Chem. Sci., 2011, 2, 389; (d) Y. Yang and G. H. Robinson, Chem. Commun., 2009, 5201. 5 See for example: (a) G. Frenking, R. Tonner, S. Klein, N. Takagi, T. Shimizu, A. Krapp, K. K. Pandey and P. Parameswaran, Chem. Soc. Rev., 2014, 43, 5106; (b) N. Holzmann, A. Stasch, C. Jones and G. Frenking, Chem. – Eur. J., 2013, 19, 6467; (c) D. J. D. Wilson, S. Couchman and J. L. Dutton, Inorg. Chem., 2012, 51, 7657; (d) N. Holzmann, A. Stasch, C. Jones and G. Frenking, Chem. – Eur. J., 2011, 17, 13517.
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6 (a) D. Himmel, I. Krossing and A. Schnepf, Angew. Chem., Int. Ed., 2014, 53, 370; (b) G. Frenking, Angew. Chem., Int. Ed., 2014, 53, 6040; (c) D. Himmel, I. Krossing and A. Schnepf, Angew. Chem., Int. Ed., 2014, 53, 6047. 7 (a) O. Back, G. Kuchenbeiser, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2009, 48, 5530; (b) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2008, 130, 14970. 8 J. D. Masuda, W. W. Schoeller, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2007, 46, 7052. 9 J. D. Masuda, W. W. Schoeller, B. Donnadieu and G. Bertrand, J. Am. Chem. Soc., 2007, 129, 14180. 10 M. Y. Abraham, Y. Wang, Y. Xie, P. Wei, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, Chem. – Eur. J., 2010, 16, 432. 11 R. Kretschmer, D. A. Ruiz, C. E. Moore, A. L. Rheingold and G. Bertrand, Angew. Chem., Int. Ed., 2014, 53, 8427. 12 R. Kinjo, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2010, 49, 5930. 13 O. Black, B. Donnadieu, P. Parameswaran, G. Frenking and G. Bertrand, Nat. Chem., 2010, 2, 369. 14 M. Y. Abraham, Y. Wang, Y. Xie, R. J. Gillard Jr., P. Wei, B. J. Vaccaro, M. K. Johnson, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2013, 135, 2486. 15 Y. Wang, Y. Xie, P. Wei, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2013, 135, 19139. 16 Y. Wang, Y. Xie, M. Y. Abraham, R. J. Gilliard Jr., P. Wei, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, Organometallics, 2010, 29, 4778. 17 T. Böttcher, B. S. Bassil, L. Zhechkov, T. Heine and G. V. Röschenthaler, Chem. Sci., 2013, 4, 77. 18 A. Aprile, R. Corbo, K. V. Tan, D. J. D. Wilson and J. L. Dutton, Dalton Trans., 2014, 43, 764. 19 M. Iglesias, D. J. Beetstra, J. C. Knight, L.-L. Ooi, A. Stasch, S. Coles, L. Male, M. B. Hursthouse, K. J. Cavell, A. Dervisi and I. A. Fallis, Organometallics, 2008, 27, 3279. 20 E. L. Kolychev, I. A. Portnyagin, V. V. Shuntikov, V. N. Khrustalev and M. S. Nechaev, J. Organomet. Chem., 2009, 694, 2454. 21 (a) J. A. B. Abdalla, I. M. Riddlestone, R. Tirfoin, N. Phillips, J. I. Bates and S. Aldridge, Chem. Commun.,
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22
23 24 25
26
27
28
29 30 31 32 33
34
35 36
2013, 49, 5547; (b) H. B. Mansaray, A. D. L. Rowe, N. Phillips, J. Niemeyer, M. Kelly, D. A. Addy, J. I. Bates and S. Aldridge, Chem. Commun., 2011, 47, 12295. During the course of this study, attempts were made to prepare adducts of 6-Dip with group 14 element halides for sake of comparison with 1–3. However, only [(6-Dip)GeCl2] was obtained in this phase of the study, and only in very low yield. The X-ray crystal structure of this compound is described in the ESI.† B. D. Ellis, C. A. Dyker, A. Decken and C. L. B. Macdonald, Chem. Commun., 2005, 1965. K. Schwedtmann, M. H. Holthausen, K.-O. Feldmann and J. J. Weigand, Angew. Chem., Int. Ed., 2013, 52, 14204. M. Donath, E. Conrad, P. Jerabek, G. Frenking, R. Fröhlich, N. Burford and J. J. Weigand, Angew. Chem., Int. Ed., 2012, 51, 2964. For a six-membered carbene adduct of SbPhCl2, see: C. L. Dorsey, R. M. Mushinski and T. W. Hudnall, Chem. – Eur. J., 2014, 20, 8914. F. D. Henne, E.-M. Schnöckelborg, K.-O. Feldmann, J. Grunenberg, R. Wolf and J. J. Weigand, Organometallics, 2013, 32, 6674. (a) S. J. Bonyhady, C. Jones, S. Nembenna, A. Stasch, A. J. Edwards and G. J. McIntyre, Chem. – Eur. J., 2010, 16, 938; (b) C. Jones and A. Stasch, Top. Organomet. Chem., 2013, 45, 73. N. Philips, R. Tirfoin and S. Aldridge, Chem. – Eur. J., 2014, 20, 3825. A. P. M. Robertson, P. A. Gray and N. Burford, Angew. Chem., Int. Ed., 2014, 53, 6050. R. Riedel, H.-D. Hausen and E. Fluck, Angew. Chem., Int. Ed. Engl., 1985, 24, 1056. As determined from a survey of the Cambridge Crystallographic Database, July, 2014. M. G. Hobbs, T. D. Forster, J. Borau-Garcia, C. J. Knapp, H. M. Tuononen and R. Roesler, New J. Chem., 2010, 34, 1295. T. M. McPhillips, S. McPhillips, H. J. Chiu, A. E. Cohen, A. M. Deacon, P. J. Ellis, E. Garman, A. Gonzalez, N. K. Sauter, R. P. Phizackerley, S. M. Soltis and P. Kuhn, J. Synchrotron Rad., 2002, 9, 401. W. Kabsch, J. Appl. Crystallogr., 1993, 26, 795. G. M. Sheldrick, SHELX-97, University of Göttingen, 1997.
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Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies† Anastas Sidiropoulos, Brooke Osborne, Alexandr N. Simonov, Deepak Dange, Alan M. Bond, Andreas Stasch* and Cameron Jones* Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2]+, Mes = mesityl) with KC8, leads to reductive coupling of
Received 15th July 2014, Accepted 21st August 2014
the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C–C bond, while an electro-
DOI: 10.1039/c4dt02074j
chemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane
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solutions.
Introduction N-heterocyclic carbenes (NHCs) have proved of enormous importance to coordination chemistry since the first example was isolated in 1991.1 These species have been used as ligands in the formation of complexes involving nearly every nonradioactive metal in the periodic table. Most studied have been transition metal-NHC complexes, many examples of which have found application as homogeneous catalysts in numerous synthetic transformations.2 Although NHC complexes of the main group elements have not been as widely investigated,3 NHCs, and their more π-acidic relatives, cyclic alkyl(amino)carbenes (CAACs) (Fig. 1), are increasingly employed as highly nucleophilic ligands for the stabilisation of thermally labile and/or low oxidation state p-block fragments. Most recent interest in this domain has centred on the synthesis of an ever increasing number of carbene adducts of mono- and diatomic p-block element fragments, L→E←L and L→EE←L (L = NHC or CAAC, E = group 13–15 element), complexes which have
School of Chemistry, PO Box 23, Monash University, VIC 3800, Australia. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Crystal data, details of data collections and refinements for all crystal structures; ORTEP diagrams for 1, [6-DipCl]Cl and [(6-Dip)GeCl2]; and full details of the electrochemical studies. Crystallographic data (excluding structure factors) for all structures. CCDC 1012341–1012346. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02074j
14858 | Dalton Trans., 2014, 43, 14858–14864
Fig. 1 Examples of five- and six-membered singlet carbenes used in p-block coordination chemistry (Mes = mesityl, Dip = 2,6diisopropylphenyl).
been described as soluble allotropes of the coordinated element.4 The nature of the L–E and E–E bonding in these, and related, systems has been widely explored in computational studies,5 and has been the subject of a vigorous debate in the literature.6 Of most relevance to the current study are carbene complexes of low oxidation state group 15 element fragments. Important compound types that have been accessed in this area include NHC or CAAC complexes of P2,7 P4,8 P12,9 As2,10 Sb2 11 and PN.12 Equally fascinating, is the reactivity of these remarkable species. Recent examples that demonstrate this include the controlled one, two and four electron oxidations of [(IPr)EE(IPr)] (E = P or As; IPr = :C{N(Dip)CH}2, Dip = 2,6-diisopropylphenyl), which have yielded the products, [(IPr)EE(IPr)]•+ or 2+ and [(IPr)P(O)2P(O)2(IPr)],13–15 respectively. It is noteworthy that the latter compound represents the first stable complex of diphosphorus tetroxide. The development of much of this chemistry has relied on the accessibility of
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carbene adducts of the group 15 element trichlorides, [L→ECl3], as these complexes are typically (though not always) utilised as precursors to lower oxidation state species, via reduction methodologies. Although examples are known for all of the trichlorides, (E = P, As, Sb or Bi), only a handful of structurally characterised complexes have yet been reported, and the coordinating carbenes in these are invariably poorly π-acidic five-membered NHCs, e.g. IPr, or more nucleophilic and π-acidic CAACs.10,11,16–18 In order to increase the range of carbene-ECl3 adducts available to the synthetic chemist, we have investigated the preparation of such complexes incorporating the saturated expanded ring NHCs, 6-Dip and 6-Mes (Fig. 1).19,20 Our reasoning for this was that such carbenes are known to be more nucleophilic and sterically shielding than either unsaturated or saturated five-membered NHCs,19 while at the same time they should be less π-acidic than CAACs. Accordingly, if ECl3 adducts are accessible with these carbenes, their subsequent reductions might well yield products distinct to those previously reported from the reduction of NHC and CAAC-ECl3 complexes. The results of our efforts in this direction are reported herein.
Results and discussion (i)
Expanded ring NHC complexes of ECl3 (E = P, As or Sb)
The expanded ring NHCs, 6-Mes and 6-Dip, were chosen for this study because of their ease of preparation,19,20 and because they have previously proved their worth as ligands in p-block element coordination chemistry.21 Initially, 1 : 1 reactions between 6-Mes and ECl3 (E = P, As, Sb or Bi) in toluene were carried out. However, these generated mixtures of several products, as determined by NMR spectroscopic analyses, which proved difficult to separate by fractional crystallisation. It is worthy of mention that the predominant signal in the 31 1 P{ H} NMR spectrum of the 6-Mes/PCl3 reaction mixture appeared at δ 3.8 ppm, and this was assigned to the complex [(6-Mes)PCl3]. This signal is upfield of that for [(IPr)PCl3], δ 16.9 ppm,16 as might be expected given the greater nucleophilicity of 6-Mes over IPr. Attention then turned to the bulkier NHC, 6-Dip, which was similarly reacted with ECl3 (E = P, As or Sb) in 1 : 1 stoichiometries. These reactions proceeded cleanly when they were carried out at −78 °C, with subsequent warming to ambient temperature, to afford moderate to good yields of the adduct complexes, 1–3 (Scheme 1).22 In contrast, the reaction of 6-Dip with BiCl3 gave an intractable mixture of products. Furthermore, when PCl3 was treated with 6-Dip at room temperature,
Scheme 1
(i) ECl3 (E = P, As or Sb).
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an analysis of the reaction mixture by 31P NMR spectroscopy revealed a number of phosphorus containing products, of which only 1 and P2Cl4 (δ 156.1 ppm) were identifiable. The generation of P2Cl4 in this reaction suggests that 6-Dip can act as a reducing agent towards PCl3, as has previously been shown for the preparation of phosphorus(I) cations from fivemembered NHCs and PCl3.23,24 It is of note that the diphosphine, P2Cl4, has also previously been reported to be produced in the reaction of PCl3 with AsPh3, the latter of which acts as a Lewis basic reducing agent in the presence of AlCl3.25 Compounds 1–3 are thermally stable solids. Their solution 1 H and 13C{1H} NMR spectra all revealed one methine and two methyl signals corresponding to their Dip substituents. This implies the complexes have symmetrical structures in solution, which is at odds with the solid state structures determined for 1 and 3 (vide infra). The 31P{1H} NMR spectrum of 1 exhibits a singlet resonance at δ −10.9 ppm, which is ca. 16 ppm upfield of that for [(6-Mes)PCl3], as might be expected given the likely greater nucleophilicity of 6-Dip over 6-Mes. Interestingly, heating solutions of 1 in d6-benzene at 60 °C led to the decomposition of the compound over 1 hour, with the generation of a mixture of phosphorus containing products. The EI mass spectra of all three compounds exhibited molecular ion peaks. Compounds 1 and 3 were crystallographically characterised and found to be essentially isostructural. As a result, only the molecular structure of 3 is depicted in Fig. 2, while that for 1 can be found in the ESI.† The group 15 centres of both have saw-horse geometries with Cl(1) and Cl(3) in axial positions, and C(1), Cl(2) and a stereochemically active lone-pair of
Fig. 2 Molecular structure of 3 (25% thermal ellipsoids are shown; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°) for 3: Sb(1)–C(1) 2.288(2), Sb(1)–Cl(2) 2.3692(6), Sb(1)–Cl(1) 2.5260(7), Sb(1)– Cl(3) 2.5760(7), N(1)–C(1) 1.334(3), C(1)–N(2) 1.331(3), C(1)–Sb(1)–Cl(2) 102.61(6), C(1)–Sb(1)–Cl(1) 94.75(6), Cl(2)–Sb(1)–Cl(1) 87.59(2), C(1)– Sb(1)–Cl(3) 81.40(6), Cl(2)–Sb(1)–Cl(3) 85.35(2), Cl(1)–Sb(1)–Cl(3) 171.01(2), N(2)–C(1)–N(1) 120.3(2). Selected bond lengths (Å) and angles (°) for 1: Cl(1)–P(1) 2.2209(7), P(1)–C(1) 1.9234(14), P(1)–Cl(2) 2.0642(8), P(1)–Cl(3) 2.4406(7), N(1)–C(1) 1.3409(17), C(1)–N(2) 1.3336(18), C(1)– P(1)–Cl(2) 106.68(5), C(1)–P(1)–Cl(1) 92.71(5), Cl(2)–P(1)–Cl(1) 89.48(2), C(1)–P(1)–Cl(3) 83.78(5), Cl(2)–P(1)–Cl(3) 87.26(2), Cl(1)–P(1)–Cl(3) 174.31(2), N(2)–C(1)–N(1) 119.60(12).
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electrons in the equatorial positions. This is very similar to the geometries of all structurally characterised carbene adducts of group 15 trichlorides. The fact that the C(1)–E(1)–Cl(2) angle is more acute for 3 (102.61(6)°) than 1 (106.68(5)°), reflects greater s-character for the lone pair of the antimony adduct. The C–E bond lengths in the compounds are slightly longer than in previously reported carbene-ECl3 adducts,10,11,16–18,26 which might result from the more sterically hindered nature of 6-Dip. Not surprisingly, the axial Sb–Cl bonds in 3 (2.551 Å mean) are considerably longer than the equatorial Sb(1)–Cl(2) bond (2.3692(6) Å). This contrasts to the situation in 1, the equatorial (P(1)–Cl(2) 2.0642(8) Å) and one axial (P(1)–Cl(1) 1.923(1) Å) bonds of which are of a similar length, whereas the other axial bond (P(1)–Cl(3) 2.4406(7) Å) is ca. 0.4 Å longer than the other two P–Cl interactions. In this respect, Cl(3) can be viewed as being partially dissociated from the P-centre, and therefore, compound 1 could be considered as a contact ion pair, viz. [(6-Dip)Cl2P+⋯Cl−] (cf. [(IPr)PCl2][triflate]27). It is of note that the P–Cl distances in [(IPr)PCl3] follow a very similar pattern to those in 1.16 As discussed above, the solid state structures of 1 and 3 are not, at first glance, consistent with their solution NMR spectra. However, it is well known that saw-horse coordination complexes (including carbene-ECl3 adducts) readily undergo Berry-pseudorotation in solution, which accounts for this observation. (ii)
Reductions of carbene-ECl3 complexes
With 1–3 in hand, attempts to reduce the compounds with either KC8 or the magnesium(I) reagent, [{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH]−, Mes = mesityl),28 were carried out. In almost all cases, however, these reactions only returned a mixture of products, including significant amounts of the free carbene, 6-Dip. The one exception was the reduction of 1 with 3 equiv. of KC8 in THF. This afforded a good yield of the dicationic P4 complex, 4 (Scheme 2), as a lime-green crystalline solid. The synthesis of 4 can be compared with the KC8 (3 equiv.) reduction of [(IPr)PCl3], which gave neutral dimeric [(IPr)PP(IPr)];7b and the reaction of [(IPr)PCl2][triflate] with 1 equiv. of KC8, which gave the P2 cationic product, [{(IPr)ClP}2(μ-Cl)][triflate].27 It cannot be sure what the mechanism of reduction of 4 is at this stage, but it is possible that it leads to a P4 product, instead of a P2 dimer, because the greater steric hindrance of the carbene centre of 6-Dip, relative to that of IPr, precludes the latter outcome. Indeed the high steric loading of 6-Dip has previously been invoked to explain its inability to form 2 : 1 complexes with d-block metals.29 Considering the significant recent interest in cationic polyphosphorus systems,30 we were keen to explore alternative
Scheme 2
(i) KC8, THF.
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synthetic routes to 4. As previously noted, it was observed that the 1 : 1 reaction of 6-Dip with PCl3 can lead to reduction products, e.g. P2Cl4, under certain conditions. Given this, and the fact that NHCs are well known to reduce phosphorus halides,23,24 we carried out the reaction of 6-Dip with PCl3 in a 1 : 2 stoichiometry, and followed its course by 31P{1H} NMR spectroscopy. Although the reaction was not clean, it did generate significant amounts of 4, in addition to 1, P2Cl4, and a number of unidentified phosphorus containing products. Compound 4 could be crystallised from the product mixture, but in a lower yield than that obtained from the KC8 reduction of 1. It is assumed that the oxidised 6-Dip product from this reaction is the salt, [6-DipCl]Cl, which was, indeed, obtained from the reaction mixture in low yield as a crystalline solid (see ESI† for the X-ray crystal structure). The formation of 4 in this reaction is comparable to the preparation of the isostructural dicationic species, [(Ph3As)2(μ-P4)][AlCl4]2, from PCl3 and AsPh3/AlCl3.25 As previously mentioned, and depending on the stoichiometry employed, this reaction can also generate P2Cl4, as was seen in the preparation of 4. Of the spectroscopic data for 4, its 31P{1H} NMR is most informative. It exhibits an A2X2 spin system, with two triplet signals at high field (δA = −325.4 ppm, δX = −167.0 ppm, 1JAX = 184 Hz). This is remarkably similar to that reported for [(Ph3As)2(μ-P4)][AlCl4]2 (δA = −325.9 ppm, δX = −174.9 ppm, 1JAX = 160 Hz),25 which strongly suggests that 4 is also an ion separate salt. This was confirmed by an X-ray crystal structure analysis of the compound (see Fig. 3), which revealed it to consist of
Fig. 3 Structure of the dicationic component of 4 (25% thermal ellipsoids are shown; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): P(1)–C(1) 1.895(3), P(4)–C(29) 1.892(3), P(1)–P(3) 2.1952(14), P(1)–P(2) 2.2077(13), P(2)–P(4) 2.2050(15), P(2)–P(3) 2.2063(16), P(3)–P(4) 2.2056(13), N(1)–C(1) 1.326(4), C(1)–N(2) 1.330(4), N(3)–C(29) 1.322(4), N(4)–C(29) 1.328(4), C(1)–P(1)–P(3) 108.85(11), C(1)–P(1)–P(2) 106.13(11), C(29)–P(4)–P(2) 108.22(11), C(29)–P(4)–P(3) 105.29(10), P(3)–P(1)–P(2) 60.14(5), P(2)–P(4)–P(3) 60.03(5), P(4)–P(2)– P(1) 75.84(5), P(1)–P(3)–P(4) 76.09(5).
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a dicationic moiety, [(6-Dip)2(μ-P4)]2+, and two chloride anions that have no close contacts with any of the P-, N- or carbene centres. The core of the dication comprises a butterfly P4 fragment, the two P3 least squares planes of which intersect each other with a dihedral angle of 89.4°. All of the P–P distances in the compound are essentially identical, and the two 6-Dip fragments are arranged in an exo-, exo-fashion with respect to the P4 butterfly (cf. [(Ph3As)2(μ-P4)][AlCl4]2 25 and [(Mes*)2(μ-P4)],31 Mes* = C6H2But3-2,4,6). Both C–P distances (1.894 Å mean) are consistent with single bonds,32 and are close to those in related cationic phosphorus imidazoliumyl systems, e.g. 1.857 Å (mean) in [{(IPr)ClP-}2(μ-Cl)][triflate].27 (iii) Reductive coupling of the cyclic amidinium salt, [6-MesH]Br In an attempt to prepare a less hindered analogue of 4, an in situ generated toluene solution of [(6-Mes)PCl3] (see section (i)) was treated with an excess of KC8. As was the case with the reduction of [(6-Dip)PCl3], a number of phosphorus containing products were generated, though none exhibited an A2X2 pattern (cf. 4) in the 31P{1H} NMR spectrum of the reaction mixture. Despite this, work-up of the reaction was pursued, which, surprisingly, led to a low isolated yield of the bis(hexahydropyrimidine) 5. It is believed that this arose from the reduction of a small amount of [6-MesH][BF4], which is a precursor to 6-Mes, and possibly contaminated the sample of the carbene used in the preparation of [(6-Mes)PCl3]. Whatever the case, a rational moderate yielding synthesis of 5 was devised, whereby a THF solution of [6-MesH]Br was treated with 1.4 equiv. of KC8 (Scheme 3). We are not aware of any similar reductive couplings involving cyclic amidinium ions, or related imidazolium salts. There has, however, been one related report of the reduction of neutral 2-chloro-1,3-di(xylyl)imidazolidine4,5-dione, H(Cl)C{N(Xyl)C(vO)}2 (Xyl = 2,6-dimethylphenyl), which leads to a coupling reaction and formation of [–C(H){N(Xyl)C(vO)}2]2.33 It is noteworthy that attempts to reductively couple [6-DipH]Br via its treatment with KC8 met with failure, presumably because the more hindered analogue of 5, viz. (6-DipH)2 is not accessible on steric grounds. The NMR spectroscopic data for 5 are consistent with its proposed formulation, and imply that rotation about the central C–C bond of the compound in solution is heavily restricted. That is, both the 1H and 13C{1H} NMR spectra exhibit sharp signals for six chemically inequivalent methyl groups. The central methine protons resonate at δ 5.33 ppm, while the corresponding carbon resonance appears at δ
Scheme 3
(i) KC8, THF.
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76.9 ppm. An X-ray crystal structure determination of the compound (see Fig. 4) revealed it to sit on a 2-fold rotation axis, and confirmed the crowding about its central C–C bond. This crowding manifests itself by the length of that bond, 1.587(3) Å, which is significantly elongated with respect to normal C–C single bonds involving sp3-hybridised carbons, ca. 1.52 Å. For sake of comparison, the central C–C bond of the related compound, [–C(H){N(Xyl)C(vO)}2]2, is also longer than normal (1.579(2) Å).33 Considering the length of the N2(H)C–C(H)N2 bond in 5, and the fact that it bears four electron withdrawing N-substituents, it seemed likely that it would be quite susceptible to redox processes. To investigate this possibility, a d.c. and a.c. cyclic voltammetric study of dichloromethane solutions of 5 was carried out over the potential range, −2.6 to 1.0 V vs. Fc0/+ (supporting electrolyte: 0.40 M [NBun4][PF6]), using varying solution concentrations, electrode type (glassy carbon or gold), and other conditions (see ESI† for a detailed discussion). While the results varied substantially with the conditions employed, compound 5 was invariably very easy to oxidise and, for example, found to undergo an irreversible oxidation process over the potential range of −0.40 to −0.35 V vs. Fc0/+,
Fig. 4 Molecular structure of 5 (25% thermal ellipsoids are shown; hydrogen atoms, except H(1) omitted). Selected bond lengths (Å) and angles (°): C(1)–C(1)’ 1.587(3), N(1)–C(1) 1.4652(17), C(1)–N(2) 1.4838(18), N(1)–C(1)–N(2) 108.33(11), N(1)–C(1)–C(1)’ 113.44(8), N(2)–C(1)–C(1)’ 108.10(13). Symmetry operation: ’ x, −y, −z + 1.
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when a glassy carbon electrode was used. In contrast, the compound could not be electrochemically reduced over the potential range investigated. Oxidation of 5 was found to involve two electrons, as testified by coulometric analysis of data derived from the exhaustive oxidative electrolysis at a platinum electrode in CH2Cl2 (0.20 M [NBun4][PF6]), at a controlled potential of ca. −0.10 V vs. Fc0/+. No definitive conclusions could be drawn regarding the mechanism of the inherently complicated oxidation process.
Conclusion In summary, reactions of the expanded ring N-heterocyclic carbene, 6-Dip, with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which have been crystallographically characterised and found to be monomeric in the solid state. Reductions of these complexes with KC8 or a magnesium(I) reagent typically afforded complex product mixtures, though the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, was isolated from the KC8 reduction of [(6-Dip)PCl3]. This was also found to result from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br, with KC8 leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C–C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.
Experimental section General considerations All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity dinitrogen. THF, toluene, benzene and hexane were distilled over molten potassium. Melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. Mass spectra were recorded at the EPSRC National Mass Spectrometric Service at Swansea University. Microanalyses were carried out at the Science Centre, London Metropolitan University. Reproducible microanalyses could not be obtained for 2 as all attempts to recrystallise the crude reaction product led only to amorphous or microcrystalline materials. That said, the purity of the compound was judged by NMR spectroscopy to be >95%. A reproducible microanalyses could not be obtained for 4 due to difficulties in completely removing the benzene of crystallisation by vacuum drying samples prior to analysis, and the extreme air and moisture sensitivity of the compound. IR spectra were recorded on solid samples using a Agilent Cary 630 attenuated total reflectance (ATR) spectrometer. 1H, 13C{1H} and 31P{1H} NMR spectra were recorded on either Bruker DPX300 or AvanceIII 400 spectrometers, and were referenced to the resonances of the solvent used or exter-
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nal 85% H3PO4 (31P{1H} NMR). 6-Dip and [6-MesH]Br were prepared by literature procedures,19,20 while all other reagents were used as received. Preparation of [(6-Dip)PCl3] 1. To a solution of PCl3 (0.137 g, 0.087 cm3, 1.00 mmol) in toluene (30 cm3) at −78 °C was added to a solution of 6-Dip (0.405 g, 1.00 mmol) in toluene (15 cm3). The mixture was warmed to ambient temperature, stirred for 30 minutes, then volatiles were removed in vacuo. The residue was extracted into benzene (40 cm3), the extract concentrated in vacuo to 15 cm3, then placed at 7 °C overnight to yield colourless crystals of 1 (0.46 g, 84%). M.p.: 145–147 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.11 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.57 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.87 (m, 2H, CH2CH2CH2), 3.60 (t, 3JHH = 5.2 Hz, 4H, NCH2CH2), 3.85 (sept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 7.07 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.14 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 20.8 (CH2CH2CH2), 24.7 (CH(CH3)2), 27.6 (CH(CH3)2), 30.0 (CH(CH3)2), 55.7 (NCH2), 125.9 (Ar-C), 131.6 (Ar-C), 137.4 (Ar-C), 148.2 (Ar-C), NCN signal not observed; 31P{1H} NMR (121 MHz, 298 K, C6D6): δ = −10.9; MS/EI m/z (%): 542.2 (M+, 1), 439.3 (6-DipCl+, 20), 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1652 (m), 1551 (w), 1317 (m), 1256 (w), 1100 (w), 1057 (w), 812 (m), 756 (m), 723 (m), 684 (m); anal. calcd for C28H40N2PCl3: C, 62.05%; H, 7.44%; N, 5.17%; found: C, 61.90%; H, 7.33%; N, 5.07%. Preparation of [(6-Dip)AsCl3] 2. To a solution of AsCl3 (0.181 g, 0.084 cm3, 1.00 mmol) in toluene (20 cm3) at −78 °C was added to a solution of 6-Dip (0.40 g, 0.99 mmol) in toluene (15 cm3). The mixture was warmed to ambient temperature, stirred for 30 minutes, before volatiles were removed in vacuo. The residue was extracted into benzene (30 cm3), the extract concentrated in vacuo to 10 cm3, then placed at 7 °C overnight to yield colourless crystals of 2 (0.23 g, 39%). M.p.: 148–150 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.17 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.74 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.86 (m, 2H, CH2CH2CH2), 3.44 (t, 3JHH = 5.2 Hz, 4H, NCH2CH2), 3.84 (sept, 3JHH = 6.8 Hz, 4H, CH(CH3)2), 7.18 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.24 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 20.4 (CH2CH2CH2), 24.8 (CH(CH3)2), 27.7 (CH(CH3)2), 30.1 (CH(CH3)2), 55.2 (NCH2), 125.9 (Ar-C), 131.9 (Ar-C), 136.8 (Ar-C), 148.3 (Ar-C), 180.8 (NCN); MS/EI m/z (%): 586.3 (M+, 1), 439.3 (6-DipCl+, 10), 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1654 (m), 1551 (w), 1324 (m), 1257 (w), 1098 (w), 1057 (w), 800 (m), 754 (m), 721 (m). Preparation of [(6-Dip)SbCl3] 3. To a solution of SbCl3 (0.230 g, 1.01 mmol) in toluene (25 cm3) at −78 °C was added to a solution of 6-Dip (0.41 g, 1.00 mmol) in toluene (10 cm3). The mixture was warmed to ambient temperature, stirred for 20 minutes, before volatiles were removed in vacuo. The residue was extracted into benzene (30 cm3), the extract concentrated in vacuo to 10 cm3, then placed at 7 °C overnight to yield colourless crystals of 3 (0.38 g, 61%). M.p.: 160–163 °C (decomp.); 1H NMR (400 MHz, 298 K, C6D6): δ = 1.24 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.51 (d, 3JHH = 6.8 Hz, 12H,
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CH(CH3)2), 2.49 (br, 2H, CH2CH2CH2), 3.55 (br, 4H, NCH2CH2), 3.85 (br, 4H, CH(CH3)2), 7.29 (d, 3JHH = 7.4 Hz, 4H, m-Ar-H), 7.41 (t, 3JHH = 7.4 Hz, 2H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 19.7 (CH2CH2CH2), 24.2 (CH(CH3)2), 25.4 (CH(CH3)2), 29.8 (CH(CH3)2), 49.5 (NCH2), 125.8 (Ar-C), 131.9 (Ar-C), 137.2 (Ar-C), 146.0 (Ar-C), NCN signal not observed; MS/EI m/z (%): 632.1 (M+, 1), 404.3 (6-Dip+, 100); IR (ATR) ν/cm−1: 1655 (m), 1383 (m), 1256 (w), 1109 (m), 1057 (w), 800 (s), 755 (m), 711 (m); anal. calcd for C28H40N2SbCl3: C, 53.15%; H, 6.37%; N, 4.43%; found: C, 52.94%; H, 6.35%; N, 4.35%. Preparation of [(6-Dip)2(μ-P4)]Cl2 4. A solution of [(6-Dip)PCl3] (0.10 g, 0.18 mmol) in THF (20 cm3) was added to a slurry of KC8 (0.075 g, 0.55 mmol) in THF (20 cm3) at −80 °C. The mixture was warmed to room temperature and stirred for 4 hours. Volatiles were then removed in vacuo, and the residue was extracted into a hexane–benzene mixture (15/2 cm3). The extract was filtered and the filtrate placed at 7 °C overnight to yield lime-green crystals of 4. (0.04 g, 81%). M.p.: 155–157 °C (decomp.); 1H NMR (400 MHz, 298 K, CD2Cl2): δ = 1.25 (d, 3JHH = 6.8 Hz, 24H, CH(CH3)2), 1.38 (d, 3JHH = 6.8 Hz, 24H, CH(CH3)2), 2.71 (br, 4H, CH2CH2CH2), 3.00 (sept, 3JHH = 6.8 Hz, 8H, CH(CH3)2), 4.13 (br, 8H, NCH2CH2), 7.33 (d, 3JHH = 7.4 Hz, 8H, m-Ar-H), 7.41 (t, 3JHH = 7.4 Hz, 4H, p-Ar-H); 13C{1H} NMR (75 MHz, 298 K, CD2Cl2): δ = 19.1 (CH2CH2CH2), 23.8 (CH(CH3)2), 24.1 (CH(CH3)2), 28.2 (CH(CH3)2), 48.2 (NCH2), 124.4 (Ar-C), 130.4 (Ar-C), 135.1 (Ar-C), 144.1 (Ar-C), NCN signal not observed; 31P{1H} NMR (121 MHz, 298 K, CD2Cl2): δ = −167.0 (t, 1JPP = 184 Hz), −325.4 (t, 1JPP = 184 Hz); MS/EI m/z (%): 405.3 (6-DipH+, 100); IR (ATR) ν/cm−1: 1652 (m), 1542 (w), 1320 (m), 1277 (m), 1101 (m), 1003 (w), 806 (m), 757 (m), 723 (m). Preparation of (6-MesH)2 5. A solution of [6-MesH]Br (0.350 g, 0.87 mmol) in THF (20 cm3) was added to a slurry of KC8 (0.16 g, 1.22 mmol) in THF (20 cm3) at −80 °C. The mixture was warmed to room temperature and stirred for 1 h. Volatiles were removed in vacuo, the residue was extracted into benzene (40 cm3), and the extract was concentrated to 15 cm3. This was stored at 7 °C overnight to yield colourless crystals of 5. (0.09 g, 32%). M.p.: 168–172 °C; 1H NMR (400 MHz, 298 K, C6D6): δ = 1.46 (s, 6H, o-ArCH3), 1.49 (m, 2H, CH2C(H)(H)CH2), 1.52 (m, 2H, CH2C(H)(H)CH2), 1.97 (s, 6H, o-ArCH3), 2.21 (2× overlapping s, 12H, p-ArCH3), 2.30 (s, 6H, o-ArCH3), 2.70 (m, 2H, NC(H)(H)CH2), 2.80 (s, 6H, o-ArCH3), 2.99 (m, 2H, NC(H)(H)CH2), 3.64 (m, 2H, NC(H)(H)CH2), 4.51 (m, 2H, NC(H)(H)CH2), 5.33 (s, 2H, NC(H)N); 13C{1H} NMR (75 MHz, 298 K, C6D6): δ = 16.9, 20.6, 20.9, 21.1, 21.8, 23.6 (Ar-CH3), 22.8 (CH2CH2CH2), 45.9, 47.6 (NCH2), 76.9 (NCN), 129.9, 130.2, 131.1, 131.6, 131.9, 132.8, 135.0, 135.9, 138.9, 140.6, 144.5, 146.5 (Ar-C); MS/EI m/z (%): 643.5 (M+, 15), 321.4 (M/2+, 100); IR (ATR) ν/cm−1: 1472 (m), 1400 (m), 1304 (m), 1209 (s), 1087 (m), 1032 (m), 846 (m), 793 (m); anal. calcd for C44H58N4: C, 82.18%; H, 9.09%; N, 8.71%; found: C, 82.17%; H, 9.14%; N, 8.24%. X-Ray crystallography Crystals of 1, 3–5, [6-DipCl]Cl and [(6-Dip)GeCl2] suitable for X-ray structural determination were mounted in silicone oil.
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Crystallographic measurements were carried using either an Oxford Gemini Ultra diffractometer using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å), or the MX1 beamline of the Australian Synchrotron (λ = 0.71090 Å for 4 and 5; 0.71080 Å for [6-DipCl]Cl). The software package Blu-Ice34 was used for synchrotron data acquisition, while the program XDS35 was employed for synchrotron data reduction. The structures were solved by direct methods and refined on F2 by full matrix least squares (SHELX97)36 using all unique data. Hydrogen atoms have been included in calculated positions (riding model) for all structures. Crystal data, details of data collections and refinement are given in Table S1 in the ESI.†
Acknowledgements CJ, AS, ANS and AMB thank the Australian Research Council. The EPSRC Mass Spectrometry Service at Swansea University is also thanked. Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.
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