Stability and Structure of Carbene-Derived Neutral Penta- and

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics,. H-1111 Budapest, Szt. Gell´ert-t´er 4, Hungary. ...
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Organometallics 2009, 28, 4159–4164 DOI: 10.1021/om9002768

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Stability and Structure of Carbene-Derived Neutral Penta- and Hexacoordinate Silicon Complexes Oldamur Holl oczki and Laszl o Nyulaszi* Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt. Gell ert-t er 4, Hungary Received April 10, 2009

The stability and structure of the penta- and hexavalent complexes of SiF4 and SiCl4 with different carbenes have been investigated computationally. Apart from the known carbene complexes containing a pentavalent silicon center, stable neutral bis-carbene complexes with hexavalent silicon are also predicted to be stable compounds. While the largest binding energies of the pentavalent structures are somewhat higher in the SiF4 derivatives (21-30 kcal mol-1) than those for the analogous SiCl4 compounds (17-19 kcal mol-1), the hexacoordinated compounds exhibit almost the same stabilities (40-60 kcal mol-1) for both the chlorine and the fluorine derivatives. These stabilities are significantly higher than those for the hitherto synthesized amino and phosphino complexes. Not only the imidazol-2-ylidene-type carbene complexes but also the hexavalent bis(diisopropylamino) cyclopropylen-3-ylidene complex of SiCl4 or SiF4 are suggested to be an appropriate synthetic target.

Introduction Compounds of silicon with coordination numbers greater than four are of current interest due to their unusual bonding and their presence in nucleophilic reactions of silanes as relatively stable intermediates.1 The rather mild reaction conditions and/or high yield observed often in silicon chemistry can be related to the easy access to these hypervalent systems.2 Stable hypervalent silicone derivatives have been known since the beginning of the 19th century, when Gay-Lussac and J. Davy first observed, independently, the formation of the [SiF6]2- ion and the

adduct of SiF4 with ammonia.3 Since then, the number of known neutral “hypervalent” species remained low.1 Furthermore in most of these species the fifth (and sixth) bond is formed by a weak intramolecular donation. Silatranes,4a azasilatranes,4b and other compounds containing an amino or carbonyl group at the β- or γ-position from the silicon atom4 are typical examples of this group. As one of the first of these structures, the ammonia (and some analogous amine and phosphine) complexes of silicontetrahalides have thoroughly been examined. If an amine is treated with an excess of SiF4 or SiCl4, a complex with a 1:1 ratio of amine and silicon-tetrahalide is formed, containing a pentacoordinate silicon atom.5 According to their vibrational spectra, these compounds exhibit C3v symmetry, the amine occupying the axial position.5 If the amine (phosphine) is used in excess, hexacoordinate silicon compounds are formed, having a trans-octahedral molecular shape, as concluded from the vibrational spectra of several structures5 and by X-ray crystallography in case of the SiH2Cl2 3 2Py,6a,6b SiHCl3 3 2Py,6a SiCl4 3 2Py,6a and SiBr4 3 2Py6c complexes (Py: pyridine). The trichlorosilane adduct of pyridine dismutated into a mixture of the dichloro- and tetrachlorosilane derivatives.6a Interestingly, a [SiH2 3 4Py]2þ structure has also been reported, supposedly due to the mobility of the chlorine ions and the propensity of the highly coordinated silicon to be stabilized in charged form.6b According to tensiometric titration data, the stability of the amine complexes is highly dependent on the nature of the

*Corresponding author. E-mail: [email protected]. (1) (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; J. Young, C. Chem. Rev. 1993, 93, 1371. (b) Holmes, R. R. Chem. Rev. 1996, 96, 927. (c) Bento, A. B.; Bickelhaupt, F. M. J. Org. Chem. 2007, 72, 2201. (d) Pierrafixe, S. C. A. H.; Fonseca Guerra, C.; Bickelhaupt, F. M. Chem.;Eur. J. 2008, 14, 819. (2) Apart from ref 1 see also: Corriu, R. J. P.; Querin, C.; Henner, B. J. L.; Wong Chi Man, W. W. C. Organometallics 1988, 7, 237. (3) (a) Gay-Lussac, J. L.; Thenard, L. J. Mem. Phys. Chim. Soc. Arcueil 1809, 2, 317. (b) Davy, J. Phil. Trans. R. Soc. London 1812, 102, 352. (4) Apart from ref 1 see: (a) Frye, C. L.; Vogel, G. E.; Hall, J. A. J. Am. Chem. Soc. 1961, 83, 996. (b) Gudat, D.; Verkade, J. G. Organometallics 1989, 8, 2772. (c) Coriu, R. J. P.; Mazhar, M.; Poirier, M.; Royo, G. J. Organomet. Chem. 1986, 306, C5. (d) Voronkov, M. G.; Frolov, Yu. L.; D’yakov, V. M.; Chipanina, N. N.; Gubanova, L. I.; Gavrilova, G. A.; Klyba, L. V.; Aksamentova, T. N. J. Organomet. Chem. 1980, 201, 165. (e) Onan, K. D.; McPhail, A. T.; Ycder, C. H.; Hillyard, R. W. J. Chem. Soc., Chem. Commun. 1978, 209. (f ) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Yu. T.; Oleneva, G. I.; Kramarova, E. P.; Shipov, A. G.; Baukov, Yu. I. J. Chem. Soc., Chem. Commun. 1988, 683. (g) Coriu, R. J. P.; Royo, G.; de Saxce, A. J. Chem. Soc., Chem. Commun. 1980, 892. (h) Boyer, J.; Breiere, C.; Carre, F.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo, G.; Young, J. C. J. Chem. Soc., Dalton Trans. 1989, 43. (i) Klebe, G.; Hensen, K.; Fuess, H. Chem. Ber. 1983, 116, 3126. (j) Pongor, G.; Kolos, Zs.; Szalay, R.; Knausz, D. J. Mol. Struct. (THEOCHEM) 2005, 714, 87. (k) Szalay, R.; Pongor, G.; Harmat, V.; B€ ocskei, Zs.; Knausz, D. J. Organomet. Chem. 2005, 690, 1498.

(5) (a) Schnell, E. Monatsch. Chem. 1962, 93, 1136. (b) B€ urger, H.; Sawodny, W.; H€ ofler, F. Monatsch. Chem. 1965, 96, 1437. (c) Beattie, I. R. Q. Rev. 1963, 17, 382. (d) Beattie, I. R.; Ozin, G. A. J. Chem. Soc. (A) 1970, 370. (6) (a) Fester, G. W.; Wagler, J.; Brendler, E.; Kroke, E. Eur. J. Inorg. Chem. 2008, 5020. (b) Fester, G. W.; Wagler, J.; Brendler, E.; B€ ohme, U.; Roewer, G.; Kroke, E. Chem.;Eur. J. 2008, 14, 3164. (c) Bolte, M.; Hensen, K.; Spangenberg, B. J. Chem. Cryst. 2000, 30, 245.

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silicon-tetrahalide, showing decreasing stability toward the heavier halogens.7 Interestingly, however, in case of the analogous hexavalent phosphine complexes the stability trend is reversed, the SiBr4 and SiCl4 complexes being more stable than the SiF4 derivatives.7 Coordination adducts of silanes and bidantate ligands have enhanced stability, due to the chelate effect. In fact, most of the structurally characterized neutral hexacoordinate silicon compounds are silanes (having electronegative groups on the Si), complexed by 2,20 -bipyridyl8 or phenantroline.9 These compounds have been investigated by X-ray structural analysis and IR spectroscopy. Besides the preparative studies, the structure and the dissociation energies of the penta- and hexacoordinate SiF4 and SiCl4 complexes with certain Lewis bases have been studied by quantum chemical calculations.10a-10f Recently, Davydova et al. compared the dissociation energies gained by B3LYP and MP2 methods with DZP quality basis sets and at the CCSD(T)/ cc-pVTZ level.10e The small difference between the results of single-point calculations at B3LYP/DZP-optimized geometries and full optimization of the geometry at the B3LYP/TZ2P and MP2/TZ2P levels (performed for the axial isomer SiCl4.NH3) suggested the reliability of the B3LYP/DZP-optimized geometries. Compared to B3LYP, dissociation energies at the MP2 level were found to be higher by 5-6 and 10-12 kcal mol-1 for ammonia and pyridine complexes, respectively. The CCSD(T)/ cc-pVTZ level dissociation energies, however, compared more favorably to the B3LYP results than with MP2.10f,10g The calculated dissociation Gibbs free energies showed that the entropy contribution makes such silicon complexes unstable in the gas phase, although many of the examined structures have successfully been synthesized in solution (e.g., structures with ammonia, pyridine, bipyridyl ligands), indicating that the gas phase entropy calculations need to be treated with some reservation in the evaluation of complex stabilities. Stable carbenes are better nucleophiles than amines; thus they are suitable ligands for the formation of neutral hypervalent silicon species. Interestingly, however, only a single synthetic study is known on neutral pentacoordinate silicon compounds by Kuhn et al., namely, the coordination structures of N,N-dialkyl-4,5-dimethylimidazol-2-ylidenes, with SiCl4, SiCl2Me2, and SiCl2Ph2, by treating the carbene with an equimolar amount of the silicon derivative.11 The structure of these complexes has been confirmed by X-ray crystallography, having the carbene ligand at the equatorial position, which contrasts to the observed apicophilicity of the amines. Similar carbene-silicon complexes have been considered to explain the enhanced stability of imidazol-2-ylidenes in longchain silicone oils against hydrolysis and oxidation, without (7) Beattie, I. R.; Ozin, G. A. J. Chem. Soc. (A) 1969, 2267. (8) Kummer, D.; Chaudhry, S. C.; Debaerdemaeker, T.; Thewalt, U. Chem. Ber. 1990, 123, 945. (9) (a) Kummer, D.; K€ oster, H.; Speck, M. Angew. Chem., Int. Ed. Engl. 1969, 8, 699. (b) Kummer, D.; Balkir, A.; K€ oster, H. J. Organomet. Chem. 1979, 178, 29. (c) Farnham, W. B.; Whitney, J. F. J. Am. Chem. Soc. 1984, 106, 3992. (10) (a) Marsden, C. J. Inorg. Chem. 1983, 22, 3178. (b) Hu, J.; Schaad, L. J.; Hess, A.Jr. J. Am. Chem. Soc. 1991, 113, 1464. (c) Alkorta, I.; Rozas, I.; Elguero, J. J. Phys. Chem. A 2001, 105, 743. (d) Ignatyev, I. S.; Schaefer, H. F.III. J. Phys. Chem. A 2001, 105, 7665. (e) Davydova, E. I.; Timoshkin, A. Y.; Sevastianova, T. N.; Suvorov, A. V.; Frenking, G. THEOCHEM 2006, 767, 103. (f) Fleischer, H. Eur. J. Inorg. Chem. 2001, 393. (g) B3LYP method also provides a reasonably good description for SiH4Cl: Bento, A. P.; Sola, M.; Bickelhaupt, F. M. J. Comput. Chem. 2005, 26, 1497. (11) Kuhn, N.; Kratz, T.; Blaser, D.; Boese, R. Chem. Ber. 1995, 128, 245.

Holl oczki and Nyul aszi

modifying significantly their reactivity.12 While no spectroscopic evidence was found to support the formation of the pentavalent structure, B3LYP/6-31þG* optimization resulted in a complex between dimethylimidazol-2-ylidene and a shorter silicone chain. The energy of formation of a model complex was 6.0 and -6.9 kcal mol-1 at the B3LYP/6-31G*þZPE and MP2(fc)/6-31G*//B3LYP/6-31G*þZPE levels, respectively; thus in this case the MP2 method indicates slight stability, while B3LYP indicates slight instability.12 Since, however, neither the effect of BSSE nor that of the entropy contribution has been considered, and the bulky iPr and tBu groups on the nitrogen atoms were simplified by methyl in the calculations, to explain the observed air stability of the carbene in the siloxane matrix, explanations alternative to the formation of the pentavalent silicon complex might also be considered. Interestingly, while carbene complexes are known to behave similarly to the corresponding phosphine analogues, no carbene-derived hexacoordinate silicon compounds, analogous to SiX4 3 2PMe3, have been investigated. In this study, the stability of different 1:1 (pentavalent) and 2:1 (hexavalent) carbene complexes of silicon tetrahalides has been investigated computationally. Our main goal was to find trends of the stability of the different compounds, for a better understanding of their structure and stability, and to investigate the possibility of the synthesis of a novel series of carbene-substituted hexacoordinate silicon compounds.

Theoretical Calculations All calculations have been carried out with the Gaussian 03 program package.13 Full geometry optimizations were performed for all molecules at the B3LYP/6-311þG** level. For some of the smaller systems MP2/6-311þG** calculations were also carried out. At the optimized structures the eigenvalues of the Hessian have been checked to establish the nature of the stationary point obtained, and BSSE corrections were also performed by using the counterpoise method. For some of the smallest structures further CCSD(T)/cc-pVTZ//B3LYP/6-311þG level single-point calculations have been carried out to evaluate the accuracy of the B3LYP and MP2 results. In case of the 1,2-bis(diisopropylamino) cyclopropyl-3-ylidene complex of SiCl4 only B3LYP/6-31G* optimizations were performed, for economic reasons. Calculations of the analogous SiF4 complex with the smaller basis set resulted in small changes with respect to the 6-311þG** results only.

Results and Discussion Pentacoordinate Adducts of Carbenes and Silicon-Tetrahalides. Silicon tetrafluoride can form both pentavalent (equatorial and (12) Bonette, F.; Kato, T.; Destarac, M.; Mignani, G.; Cossı´ o, F. P.; Baceiredo, A. Angew. Chem., Int. Ed. 2007, 46, 8632. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

Article Scheme 1. Penta- and Hexavalent Silicon Derivatives Formed by Intermolecular Coordination of Monodentate Ligands

axial; see Scheme 2) and hexavalent complexes (trans and cis; see Scheme 3) with carbenes. The stability of these complexes (the energy of their formation from the corresponding carbene(s) and SiX4) has been evaluated at the B3LYP/ 6-311þG** level, and for some systems CCSD(T)/ccpVTZ//B3LYP/6-311þG** single-point energies have also been calculated (Table 1). The DFT stabilities were almost 10 kcal mol-1 lower than the CC results in the case of the hexacoordinate cyclopropylylidene-SiF4 adduct, while the difference was only a few kcal mol-1 in the case of the aminocarbenes. To further test the reliability of the DFT results, MP2/6-311þG** calculations were also carried out for diaminocarbene and cyclopropylidene-carbene. The MP2 stabilities differ from the B3LYP ones in the case of the equatorial substitution and the trans (disubstituted) structure by a few kcal mol-1, while for the axial substitution the stability difference between the two levels is even smaller. These results indicate that the B3LYP/6-311þG** calculations are reasonably accurate in searching synthetically accessible compounds. Accordingly, the complexes of SiCl4 have been investigated at the B3LYP/6-311þG** level only. Among the examined halogen- and/or hydrogen-substituted carbene ligands (CH2, CHF, CHCl, CF2, CCl2) only dichlorocarbene has formed an SiF4 complex with low stability. Also cyclopropylylidene forms a weak complex only, while the complexes with phosphinocarbene and thiolcarbene exhibit intermediate stability. The stability of the complex increases with the increasing stability of the carbene itself.14 Nevertheless, in the case of the most stable carbenes (e.g., diaminocyclopropylylidene, diaminocarbenes, imidazol-2-ylidenes) the complex stabilites are only slightly influenced by the nature of the carbene, except for pyridin2-ylidene,15 which is by far the most stable pentavalent structure. These stabilities are significantly larger than those for the corresponding ammonia, trimethylamine, pyridine, and trimethylphosphine complexes (-8.5, -8.2, -7.5, and -2.4 kcal mol-1, respectively, at the B3LYP/6-311þG** level). In the case of the most stable SiF4 complexes (ΔE > 20 kcal mol-1), the C-Si bond distance varies between 1.997 and 2.042 A˚ in the axial position and 1.933-2.003 A˚ in the equatorial position (for detailed data see the Supporting Information). The carbene ligand usually prefers the axial position, although the energy difference from the (14) The stability of a carbene (:CR2) can be defined as the energy of the :CR2 þ CH4 f H2CR2 þ :CH2 isodesmic reaction. The energy of this reaction was in excellent linear correlation with the dimerization energy of the carbene. See: Forr o, A.; Veszpremi, T.; Nyulaszi, L. Phys. Chem. Chem. Phys. 2000, 3127. (15) The possibilities for substituent stabilization of pyridin-2-ylidene as a free carbene were discussed recently: Holl oczki, O.; Nyulaszi J. Org. Chem. 2008, 73, 4794.

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Scheme 2. General Structure of Axial (1a) and Equatorial (1b) Pentavalent Structures

structure with the ligand in equatorial position is small. We have also calculated the barrier of the pseudorotation, which turned out to be low (e.g., 3.2 kcal mol-1 for cyclopropylylidene); consequently both of the isomers may be present in solution, resulting in dynamic NMR spectra. In the case of the amino, hydroxy, or thiolcarbenes the SiF4 complexes exhibited short hydrogen-fluorine distances, indicating hydrogen bonds. Accordingly, bond critical points16 were located between hydrogen and fluorine atoms; however, the corresponding stabilization effect is small.17 Interestingly, the C-N bond in the diaminocarbene-SiF4 complex is somewhat shorter (1.324 A˚ for the axial and 1.321 A˚ for the somewhat more stable equatorial isomer) than that in the free aminocarbene (1.339 A˚). The stability of SiCl4-carbene adducts is by ca. 5-10 kcal mol-1 (see Table 2) less than that of the corresponding SiF4 analogues.18 In these complexes the preference of the equatorial position is larger than for the SiF4 analogues, although the energy difference is rather small. The differences between the apicophilicity of the carbene in case of the fluorides and chlorides can be explained by the different steric repulsion and H-bonding ability of the two halogens. The steric encumbrance is apparently larger at the axial carbene substituent, and this effect destabilizes the chlorides with respect to their fluoride analogues. The HN disubstituted carbenes (imidazole-2-ylidene, diaminocarbene) are able to form (weak) hydrogen bonds preferably at the equatorial positions (see Scheme 2). This interaction is stronger with fluorides than with chlorides.17,18 (16) Bader, R. W. F. Acc. Chem. Res. 1985, 18, 9. (17) To evaluate the stabilization benefiting from the formation of the carbon-silicon bond only, the strength of the Si-F 3 3 3 H-Y (Y: N, O) interactions has also been investigated. Thus, the most stable H-bonded structures without carbene-silicon electron donation between these carbenes and SiF4 have been optimized, to get an estimate of the energy contribution of the hydrogen bond. The corresponding stabilities of these structures were -1.2 kcal mol-1 for aminocarbene, -2.5 kcal mol-1 for hydroxycarbene, and -1.0 kcal mol-1 for thiolcarbene, at the B3LYP/ 6-311þG** level. The weakness of the hydrogen bonds is understandable, since the F-H-Y angle is about 100-120°, while the strong hydrogen bonds require a 180° bond angle. Thus, the effect of the hydrogen bonds on the stability of the complexes is only minor. A further possible way to evaluate the strength of the hydrogen bonds is to calculate the rotational barrier of the carbene around the Si-C bond, which was found to be a higher value of 3.9 kcal mol-1 for the aminocarbene f SiF4 complex at the B3LYP/6-311þG** level. The stabilization gained by the H-bonds can also be overestimated by this method, because of the possible presence of an interaction between the vacant orbital of the carbene and the siliconfluorine bond, which also depends on the rotation of the carbene. Naturally, the replacement of the hydrogens interacting with the halogen, with a methyl group, could also be possible to evaluate this error, but the steric demand of these groups would presumably cause serious error. (18) Similarly to the SiF4 complexes, bond critical points between the hydrogen of the amino group and the chlorine atom have been found; hence the energy of these hydrogen bonds has also been investigated. As expected, the stabilization effect of the Y-H 3 3 3 Cl-Si interaction is even smaller (0.6, 1.4, and 0.6 kcal mol-1 for aminocarbene, hydroxycarbene, and thiolcarbene, respectively) than for the fluorine analogue.

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Scheme 3. General Structure of the trans (2a-c) and cis (3a-d) Hexavalent Species

Table 1. B3LYP/6-311þG** Energy of Formation of the Investigated Carbene-SiF4 Complexes from the Separated Carbene Ligands and SiF4 (in kcal mol-1)h pentacoordinate carbene

axial

CCl2 cyclopropylene-3-ylidenea,b 1,2-diaminocyclopropylene-3-ylidene 1,2-bis(diisopropylamino) cyclopropylene-3-ylidene hydroxycarbenec thiolcarbene aminocarbened phosphinocarbene diaminocarbenee,f amino-methylcarbene 1,3,3-trimethylpyrrolidine-2-ylidene thiazol-2-ylidene imidazol-2-ylidene 1,3-dimethylimidazol-2-ylidene pyridin-2-ylidene 1-methylpyridin-2-ylidene

-0.8 -11.5 -20.7 -23.3 -19.4 -14.1 -23.2 -12.3 -22.8 -24.7 -21.0 -16.8 -22.6 -20.0 -29.2 -25.7

equatorial -8.3 -19.0 -24.9 -14.4 -18.3 -6.9 -22.9 -20.5 -21.7 -16.0 -24.3 -19.2 -30.0 -19.0

hexacoordinate g

trans

cisg

-4.0 -24.7 -37.9 -42.1 -53.7 -34.7 -51.7 -26.2 -55.1 -55.1 -43.1 -39.6 -51.9 -46.0 -63.8 -51.6

-1.0 -18.9 -32.4 -45.0 -29.8 -46.0 -21.5 -47.9 -46.4 -34.0 -44.6 -33.7 -56.1 -42.9

MP2/6-311þG** stabilities are -12.6, -8.3, and -30.6 kcal mol-1 for the axial, equatorial, and trans structures, respectively. b CCSD(T)/cc-pVTZ// B3LYP/6-311þG** stabililities are -18.3 and -40.0 kcal mol-1 for the axial and trans structures, respectively. c CCSD(T)/cc-pVTZ//B3LYP/ 6-311þG** stabilility is -18.1 kcal mol-1 for the axial structure. dCCSD(T)/cc-pVTZ//B3LYP/6-311þG** stabililities are -22.0, -16.9, and -52.4 kcal mol-1 for the axial, equatorial, and trans structures, respectively. eMP2/6-311þG** are -26.4, -26.2, and -66.5 kcal mol-1 for the axial, equatorial, and trans structures, respectively. fCCSD(T)/cc-pVTZ//B3LYP/6-311þG** stabililities are -23.5, -22.6, and -59.8 kcal mol-1 for the axial, equatorial, and trans structures, respectively. gData for the most stable structure is presented. h Data in the table are corrected for BSSE, while data in the footnotes are BSSE uncorrected values. a

The 1,3-dimethylimidazol-2-ylidene-SiCl4 complex is of specific importance since it can be compared with the structurally characterized related equatorially bound 1,3,4,5-tetramethylimidazol-2-ylidene-SiCl4 complex.19 Our calculations indicate that the 1,3-dimethylimidazol-2-ylidene adduct exists with the carbene in equatorial position (we were not able to optimize the complex with axially bound carbene), in accordance with the experimental results. Interestingly, the analogous SiF4 complex was slightly more stable with the carbene in axial position (see Table 1). Despite the decrease in stability, the Si-C bond distances of the SiCl4 complexes (for detailed data see the Supporting Information) are in some cases shorter than those in the fluoride analogues. The stability of the different carbene complexes is somewhat related to the lone pair orbital energy of the carbene itself (see Table 3), although no direct (19) The experimentally examined tetramethylimidazol-2-ylidene forms complexes with silicon-tetrachloride with roughly the same stability (-14.2 and -29.3 kcal mol-1 for the equatorial and trans structures, respectively, while no axial isomer was found).

correlation could be found, presumably due to the aforementioned effect of steric factors and hydrogen bonding that influence the (otherwise very similar) stability of the complexes by a few kcal mol-1. Hexacoordinate Adducts of Carbenes and Silicon-Tetrahalides. The hexacoordinate silicon compounds, obtained by the addition of two carbenes to a silicon-tetrahalide, have numerous possible isomers, depending on the relative position of the ligands and their rotation (Scheme 3). Generally, the trans structures are more stable than cis by 5-10 kcal mol-1 (see Table 1).20 In most cases the 2a trans structure exhibits the largest stability. For the more bulky carbenes (1,3-dimethylimidazol-2-ylidene, 1-methylpyridin-2-ylidene), however, the alleviation of the steric repulsion makes 2c the most stable rotamer. Among the cis structures 3a isomers are the most stable in the case of the sterically least demanding carbenes (cyclopropylylidene, CHR-type carbenes), the energy difference (20) For the isomerization energies see the Supporting Information.

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Table 2. B3LYP/6-311þG** Energy of Formation of the Investigated Carbene-SiCl4 Complexes from the Separated Carbene Ligands and SiCl4 (in kcal mol-1 units)a

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Scheme 4. Examined Decomposition Reaction of Carbene-SiX4 Complexes

pentacoordinate hexacoordinate carbene

axial equatorial

CCl2 cyclopropylene-3-ylidene -3.9 1,2-diaminocyclopropylene-3-ylidene -13.5 1,2-bis(diisopropylamino) -14.2b cyclopropylene-3-ylidene aminocarbene -17.3 amino-methylcarbene -14.5 1,3,3-trimethylpyrrolidine-2-ylidene diaminocarbene -11.0 thiazole-2-ylidene -5.5 imidazol-2-ylidene -12.3 1,3-dimethylimidazol-2-ylidene pyridine-2-ylidene -19.0 1-methylpyridine-2-ylidene

trans

-4.5 -13.2 -15.1b

-23.8 -43.8 -28.6b

-14.2 -12.9 -7.6 -4.5 -13.7 -8.2 -17.9 -16.2

-50.9 -18.0 -46.6 -33.6 -48.9 -20.8 -60.2 -32.2

a Data in the table are corrected for BSSE. b B3LYP/6-311þG**// B3LYP/6-31G* level energies

Table 3. Energy of the Lone Pair of Some Free Carbenes (HF/ 6-311þG**//B3LYP/6-311þG**, in eV units) carbene hydroxycarbene aminocarbene diaminocarbene thiazol-2-ylidene imidazol-2-ylidene 1,3-dimethylimidazol-2-ylidene 1,3,3-trimethylpyrrolidin-2-ylidene aminomethylcarbene cyclopropylene-3-ylidene 1,2-diaminocyclopropylene-3-ylidene pyridin-2-ylidene 1-methylpyridin-2-ylidene

ELP -10.18 -9.53 -9.40 -10.19 -9.92 -9.67 -8.82 -9.07 -10.12 -9.22 -9.10 -8.90

between the possible rotamers being only 1-2 kcal mol-1. As the steric bulk of the carbene substituents increases, 3b (CCl2, diaminocyclopropylylidene), 3c (diaminocarbene, aminomethylcarbene, imidazol-2-ylidene, pyridin-2-ylidene), and 3d (1,3-dimethylimidazol-2-ylidene, 1-methylpyridin-2-ylidene) are obtained as the most stable cis isomer with the rotation of one or two of the carbene moieties. As seen in Table 1, the addition of the second carbene is thermodynamically favorable, and in most cases this process is somewhat more exothermic than the addition of the first carbene to silicon-tetrafluoride. In the case of the SiCl4 derivatives, only the isomers related structurally to the most stable SiF4 analogues have been examined, their stabilities being presented in Table 2. The addition of a second carbene to the carbeneSiCl4 adduct is more exothermic than in the case of the carbeneSiF4 complex; thus, the stability of the 2:1 carbene-SiCl4 adducts is comparable (or in some cases even higher than) to that for the corresponding SiF4 analogues. Again, these stabilities are significantly higher than those for the known ammonia, trimethylamine, pyridine, or trimethylphosphine complexes. (For comparison the stability of their SiF4 complexes is -21.6, -14.7, -18.6, and -4.9 kcal mol-1, respectively.) Pyridin-2-ylidene has significantly more stable complexes than any other investigated carbene; however, even a methyl substitution at the nitrogen reduces this stability significantly because of steric reasons. The aminocarbene-SiCl4 complex could not be found as a minimum; a 1,2-Cl-shift always

Table 4. B3LYP/6-311þG** Energy of the Insertion Reactions of Pentavalent SiF4 Derivatives (see Scheme 4) with Respect to the Most Stable Pentacoordinate Structure carbene CCl2 cyclopropylene-3-ylidene 1,2-diaminocyclopropylene-3-ylidene 1,2-bis(diisopropylamino) cyclopropylene-3-ylidene hydroxycarbene thiolcarbene aminocarbene phosphinocarbene diaminocarbene amino-methylcarbene 1,3,3-trimethylpyrrolidin-2-ylidene thiazol-2-ylidene imidazol-2-ylidene 1,3-dimethylimidazol-2-ylidene pyridin-2-ylidene 1-methylpyridin-2-ylidene

-18.5 2.7 28.4 36.4 -7.4 -10.6 6.0 -20.7 29.3 12.6 19.1 31.3 33.7

Table 5. B3LYP/6-311þG** Energy of the Insertion Reactions of Pentavalent SiCl4 Derivatives (see Scheme 4) with Respect to the Corresponding Most Stable Pentacoordinate Structure carbene cyclopropylene-3-ylidene 1,2-diaminocyclopropylene-3-ylidene 1,2-bis(diisopropylamino)cyclopropylene-3-ylidene aminocarbene amino-methylcarbene 1,3,3-trimethylpyrrolidin-2-ylidene diaminocarbene thiazole-2-ylidene imidazol-2-ylidene 1,3-dimethylimidazol-2-ylidene pyridin-2-ylidene 1-methylpyridin-2-ylidene

insertion -16.3 7.5 -12.7 -8.4 7.8 7.9 8.1

occurred during the optimization, and dichlorobis(aminochloromethyl)silane was obtained. The Si-C bond distances of the trans SiF4 structures are somewhat shorter than those of the corresponding cis isomers and the axial pentacoordinate structures. The same trends hold for the trans SiCl4 structures (for detailed data see the Supporting Information). Considerations Related to the Possible Synthesis of CarbeneDerived Hexacoordinate Silicon Compounds. The high complex stabilities are not necessarily enough to gain synthesizable compounds;21 hence the insertion of the carbene into the Si-X bond (Scheme 4) has also been investigated as a possible decomposition reaction. The reaction energies compared to the corresponding most stable pentacoordinate structures are presented in Tables 4 and 5. According to these values, the insertion into the Si-Cl bond is more exothermic than that into the Si-F bond, in agreement with the difference in the Si-X bond energies. Although in the case of the less stable carbenes this decomposition can be exothermic, in the case of (21) Hoffmann, R.; Schleyer, P. v. R.; Schaefer, H. F. Angew. Chem., Int. Ed. 2008, 47, 7164.

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Holl oczki and Nyul aszi

Table 6. B3LYP/6-311þG** Energy of Formation of the Pentaand Hexacoordinated Silicon Compounds Derived from Imidazol2-ylidene and Different Silanes (in kcal mol-1) pentacoordinate silane SiMe2Cl2 SiH4 SiMe4 Si(OMe)4 SiMe2(OMe)2

axial

hexacoordinate

equatorial

trans

0.3

-16.1

-4.4 3.2

-2.8 0.0

-22.3 8.9 33.0 -8.8 9.3

the most stable diamino or aromatic carbenes it is endothermic for both halides. Since silicon halides are very inconvenient to deal with, we also examined the corresponding imidazol-2-ylidene-derived hypercoordinate compounds of SiMe2Cl2, SiH4, SiMe4, Si(OMe)4, and SiMe2(OMe)2. Interestingly, SiMe2Cl2 shows similar ability to form equatorial pentacoordinate compounds to that of SiCl4. The significantly lower stability of the axial pentacoordinate and trans hexacoordinate structures can be attributed to the steric demand of the methyl groups. The stability of the other investigated silane adducts of imidazol2-ilidene was much lower12 (Table 6), and their formation was slightly exothermic only in the case of tetramethoxysilane, indicating the role of the presence of electronegative groups on the silicon atom. The calculations discussed above predict that the most stable pentavalent and hexavalent silicon complexes are formed with the most stable carbenes. A further apparent factor is that the hypervalent structures are more sensitive to the steric encumbrance than the tetravalent systems. Although the most stable complexes are formed with the parent pyridin-2-ylidene, this carbene is not isolable, and none of its derivatives have been synthesized yet. The complexes of the cyclic (alkyl)(amino)carbene (CAAC) 1,3,3trimethylpyrrolidin-2-ylidene exhibits also small stability (see Tables 1 and 2), despite the reported highly stable transition metal complexes of CAACs.22 Accordingly, 1,3,4,5-tetramethylimidazol-2-ylidene23 (modeled by 1,3-dimethylimidazol-2-ylidene) and 1,2-bis(diisopropylamino) cyclopropyl-3-ylidene24 have been considered. The energies of formation for the 1,3-dimethylimidazol-2-ylidene-derived penta- and hexavalent species are presented in Tables 1 and 2. The second coordination to both silicone halides is more exothermic than the addition of the first ligand; however, the entropy contribution disfavors the higher coordination. While the gas phase entropy is obtainable utilizing the computed vibrational frequencies, for the entropy contribution in solution estimates rather than accurate data can be obtained from computations.25 As we have noted above, the (22) Lavallo, V.; Canac, Y.; Praesang, C.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 5705. (23) Kuhn, N.; Kratz, T. Synthesis 1993, 561. (24) Lavallo, V.; Canac, Y.; Donnadieu, B.; Sch€ oller, W. W.; Bertrand, G. Science 2006, 312, 722.

use of computed gas phase entropies resulted in problems in estimating the stabilities of the existing amine-halogenosilane complexes.10e The fact that the pentacoordinate structure has been synthesized indicates that the entropy contribution in a 1:1 molar ratio was sufficient to balance the larger stability of the hexavalent structure. It is likely, however, that with the increasing carbene to silane ratio the hexacoordinate structure, which has a large stabilization energy, forms both in case of the NHCs and also for the cyclopropenylylidene, similarly to the analogous complexes known for pyridine and amines.

Summary and Conclusion Compared to the known penta- and hexavalent halogenosilane complexes with amines and phosphines, our calculations indicate that stable nucleophilic carbenes form even stronger complexes. Since energetically the formation of the hexavalent structure is more favorable than that of the pentavalent system, with an increasing concentration of the carbene the hitherto unknown hexavalent systems should also be synthesizable, extending the family of the neutral hexavalent silicon species. The most likely ligands to form hypervalent carbene complexes are 1,3,4,5-tetramethylimidazol-2ylidene or 1,2-bis(diisopropylamino)cyclopropylen-3-ylidene, and as Lewis acids, SiCl4 and SiF4 are most suitable.

Acknowledgment. Financial support from the Hungarian Scientific Research Fund (OTKA T049258) is gratefully acknowledged. Note Added after ASAP Publication. After this paper was published on the web on June 23, 2009, we were notified that the carbene SiX4 adducts suggested in this paper had been synthesized: Arduengo, A. J., III; Jones, P. G.; Krafczyk, R.; Marshall, W. J.; Schmutzler, R.; Th€ onnessen, H. Unpublished results, 1997. Jones, P. G. Private communication to the Cambridge Crystallographic Database, deposition number CCDC 611491. Supporting Information Available: Geometries (in Cartesian coordinates) and total energies of the optimized structures are available free of charge via the Internet at http://pubs.acs.org. (25) Although there are some empirical methods to correct the gas phase calculation results to evaluate the liquid phase values (e.g., the latter is about 50% of the former; for a detailed discussion see Lau, J. K.C.; Deubel, D. V. J. Chem. Theory Comput. 2006, 2, 103), we believe that the fitting of the energy values to experimental evidence for the formation of such species is more reliable. The effect of entropy on the formation of the complexes may be considered to be similar for all the ligands; therefore the trends in the energy of formation are correlated to those in the Gibbs free energy of complex formation. Since the experimentally characterized amine complexes exhibited significantly less energy of formation than the most stable carbene complexes examined in the present work, the latter structures are also potential synthetic targets. The evidence for the formation of the tetramethylimidazol-2-ylidene-chlorosilane complexes reported (see ref 11) also validates our conclusions.