Hexacoordinate Silacyclobutane Dichelate Complexes: Structure

Jan 10, 2011 - Department of Chemistry, Technion−Israel Institute of Technology, Haifa 32000, Israel. † Institute of Petrochemical Synthesis, RAS,...
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Organometallics 2011, 30, 405–413 DOI: 10.1021/om101081u

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Hexacoordinate Silacyclobutane Dichelate Complexes: Structure, Properties, and Ligand Crossover )

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Shiri Yakubovich,‡ Boris Gostevskii,‡ Inna Kalikhman,‡ Mark Botoshansky,§ Leonid E. Gusel’nikov,†, Vadim A. Pestunovich,z, and Daniel Kost*,‡ Department of Chemistry, Ben-Gurion University, Beer Sheva 84105, Israel, §Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, †Institute of Petrochemical Synthesis, RAS, Moscow, Russia, and zIrkutsk Institute of Organic Chemistry, RAS, Siberian Division, Russia. Deceased. )

‡

Received November 16, 2010

Hexacoordinate dichelate silacyclobutane complexes have been synthesized from dichlorosilacyclobutane and O-trimethylsilylated hydrazides by transsilylation. Like previously reported hexacoordinate silicon complexes, they readily and quantitatively undergo ligand exchange with other silicon compounds (XSiCl3 and differently substituted O-trimethylsilylated hydrazides), evidence that ionic dissociation does not play a significant role in the exchange mechanism. Germanium tetrachloride causes central-element exchange and formation of analogous hexacoordinate germanium complexes. Likewise, silicon tetrachloride replaces germanium from its hexacoordinate complexes, obeying certain selectivity constraints. When silicon complexes have strongly electron-withdrawing chelate-ring substituents (CF3 or CH2CN), GeCl4 causes, in addition to central-element exchange, also oxidative opening of the four-membered ring and addition of two chlorine atoms. Both chelate exchange and central-element exchange are shown to be dominated by monodentate ligand priorities.

Introduction The chemistry of silacyclobutane has attracted considerable attention because of its unique reactivity1 and potential as a reactant in polymer formation.2 Neutral hexacoordinate silicon dichelates based on hydrazide ligands3 have previously been reported to readily undergo complete exchange of ligands with their differently substituted precursors (eq 1), between two differently substituted complexes (eq 2)4 and with germanium analogues.5 Since bidentate ligands rapidly and quantitatively migrate from one silicon atom to another and to germanium (eq 3), it seemed plausible that the welldocumented reversible ionic dissociation of these complexes in chloroform and dichloromethane solutions might be the

trigger for the ligand exchange reactions.6 To test this hypothesis, a series of silacyclobutane complexes have been synthesized, in which ionic dissociation is effectively prevented by the presence of two nonionizable carbon ligands in the fourmembered silacyclobutane ring.

*To whom correspondence should be addressed. E-mail: kostd@ bgu.ac.il. (1) For recent studies on silacyclobutanes see: (a) Gusel’nikov, L. E. Coord. Chem. Rev. 2003, 244, 149–240. (b) Gusel'nikov, L. E.; Avakyan, V. G.; Guselnikov, S. L. J. Am. Chem. Soc. 2002, 124, 662–671. (c) Pestunovich, V. A.; Lazareva, N. F.; Albanov, A. I.; Kozyreva, O. B.; Volkova, V. V.; Gusel'nikov, L. E. ARKIVOC 2006, 116–125. (d) Troegel, D.; Lippert, W. P.; M€ oller, F.; Burschka, C.; Tacke, R. J. Organomet. Chem. 2010, 695, 1700– 1707. (2) Gallei, M.; Klein, R.; Rehahn, M. Macromolecules 2010, 43, 1844–1854, and references therein. (3) For reviews see: (a) Kost, D.; Kalikhman, I. Adv. Organomet. Chem. 2004, 50, 1–106. (b) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314. (4) Sergani, S.; Kalikhman, I.; Yakubovich, S.; Kost, D. Organometallics 2007, 26, 5799–5802. (5) Yakubovich, S.; Kalikhman, I.; Kost, D. J. Chem. Soc., Dalton Trans. 2010, 39, 9241–9244. (6) Kost, D.; Kingston, V.; Gostevskii, B.; Ellern, A.; Stalke, D.; Walfort, B.; Kalikhman, I. Organometallics 2002, 21, 2293–2305, and other papers in the series “Donor Stabilized Silyl Cations”. r 2011 American Chemical Society

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Results and Discussion Synthesis and Structure. Like other polychlorosilanes,3,7 dichlorosilacyclobutane (1) reacts smoothly with silylated hydrazides (2) to form neutral hexacoordinate dichelate complexes (3) in high yields (eq 4). When only one molar equivalent of 2 is used, the major isolated product is the pentacoordinate monochelate (4). The only previously reported analogues of 3 are dichelates with N-alkylideneimino donor groups (5, eq 5), which spontaneously rearrange to the tricyclic complexes (6),8 rather than the present dimethylamino donor groups.

Figure 1. ORTEP representation of the molecular structure of 3a in the crystal, depicted at the 30% probability level and omitting hydrogen atoms. Crystallographic disorder in the CF3 groups is removed for clarity.

The silacyclobutane complexes 3 and 4 were characterized by their various solution NMR spectra and elemental analyses, and the structures of five of them were also determined by single-crystal X-ray diffraction analysis. The molecular structures in the solid state of compounds 3a-d and the monochelate 4c are depicted in Figures 1-5, respectively, and selected bond lengths and angles are listed in Table 1. Compounds 3a-d feature distorted octahedral geometries, with the nitrogen ligands in trans position relative to each other and the oxygen atoms cis, as is commonly found in similar dichelate silicon complexes.3 (7) Kalikhman, I. D.; Gostevskii, B. A.; Bannikova, O. B.; Voronkov, M. G.; Pestunovich, V. A. J. Organomet. Chem. 1989, 376, 249. (8) Gostevskii, B.; Kalikhman, I.; Tessier, C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics 2005, 24, 5786–5788.

Figure 2. ORTEP representation of the molecular structure of 3b in the crystal, depicted at the 30% probability level and omitting hydrogen atoms. Only symmetry-unique atoms have been labeled.

It may be noteworthy that the N-methyl signals in the 1H and 13C NMR spectra of compounds 3 appear as one singlet at ambient temperature in CDCl3 solution (see Experimental Section), despite the C2 molecular symmetry, which requires two singlets. Only when cooled to lower temperature (260 K) do the singlets in 3b and 3d split into two signals in each case.

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Figure 3. ORTEP representation of the molecular structure of 3c in the crystal, depicted at the 30% probability level and omitting hydrogen atoms. Only symmetry-unique atoms have been labeled.

Figure 4. ORTEP representation of the molecular structure of 3d in the crystal, depicted at the 30% probability level and omitting hydrogen atoms.

This indicates a dynamic process, which takes place on the “NMR time scale” and renders the two diastereotopic N-methyl groups equivalent. The only reasonable process that can invert the chirality about the silicon center and cause rapid equilibration of the N-methyl groups is an interchange of the oxygen ligands by twisting of the O-Si-O plane through a “bicapped tetrahedron” intermediate or transition state,9 such that the chelate rings reverse their helical spatial arrangement, causing enantiomerization of the complex. The fact that no splitting of methyl signals is observed down to 260 K in 3a and 3c is evidence that with the electron-withdrawing (9) For a discussion of rapid ligand exchange through a bicapped tetrahedron see: (a) Kost, D.; Kalikhman, I.; Raban, M. J. Am. Chem. Soc. 1995, 117, 11512–11522. (b) Kost, D.; Kalikhman, I.; Krivonos, S.; Stalke, D.; Kottke, T. J. Am. Chem. Soc. 1998, 120, 4209–4214.

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Figure 5. ORTEP representation of the molecular structure of 4c in the crystal, depicted at the 30% probability level and omitting hydrogen atoms.

substituents these complexes have lower barriers for enantiomerization compared to 3b and 3d, with the electron-releasing substituents. This is rationalized by noting that electronwithdrawing substituents weaken the coordination of the nitrogen donor to silicon.10 As a result, it is easier to extend this bond to facilitate the twist of the two chelate rings next to each other in the course of helicity reversal. The C-Si-C bond angles in the four-membered silacyclobutane rings are substantially smaller than 90°, ranging between 75.6° and 78.1° (see Table 1). In the pentacoordinate 4c, the four-membered ring occupies axial and equatorial positions about the silicon, presumably to be able to keep a near 90° bond angle; however, in this case the actual C-Si-C angle is also in the same range, 77.3°. This small bond angle allows for a much greater C-C-C angle in the ring, near 100°. Examination of other, literature reported,11 silacyclobutane crystal structures confirms that, in all cases, regardless of the silicon coordination number, the C-Si-C angles are generally near 80°, while the C-C-C bond angles are near 100°, with various degrees of silacyclobutane ringpuckering. These angles are geometrically required in order to accommodate the long Si-C and shorter C-C bonds in the four-membered ring. Interestingly, in the pentacoordinate monochelate 4c the chloro ligand occupies an equatorial position, while carbon takes the axial position. This uncommon arrangement enables the small C-Si-C angle and avoids excessive ring strain. (10) (a) Gostevskii, B.; Adear, K.; Sivaramakrishna, A.; Silbert, G.; Stalke, D.; Kocher, N.; Kalikhman, I.; Kost, D. Chem. Commun. 2004, 1644–1645. (b) Gostevskii, B.; Silbert, G.; Adear, K.; Sivaramakrishna, A.; Stalke, D.; Deuerlein, S.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.; Kost, D. Organometallics 2005, 24, 2913–2920. (11) (a) C-Si-C angle in tetracoordinate silacyclobutane, 79°: Daiss, J. O.; Penka, M.; Burschka, C.; Tacke, R. Organometallics 2004, 23, 4987–4994. (b) C-Si-C angles in tetracoordinate silacyclobutanes, 79.7°, 80.7°: Peckham, T. J.; Nguyen, P.; Bourke, S. C.; Wang, Q.; Harrison, D. G.; Zoricak, P.; Russell, C.; Liable-Sands, L. M.; Rheingold, A. L.; Lough, A. J.; Manners, I. Organometallics 2001, 20, 3035–3043. (c) C-Si-C angle in tetracoordinate silacyclobutane, 82.6°: Jain, R.; Brunskill, A. P. J.; Sheridan, J. B.; Lalancette, R. A. J. Organomet. Chem. 2005, 690, 2272–2277. (d) Diequatorial C-Si-C angle in pentacoordinate silacyclobutane, 82.4°: Gericke, R.; Gerlach, D.; Wagler, J. Organometallics 2009, 28, 6831–6834. (e) Axial, equatorial C-Si-C angles in pentacoordinate silacyclobutanes, 77.3°, 77.8°: Spiniello, M.; White, J. M. Organometallics 2000, 19, 1350–1354.

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O(1)-Si(1)-N(1) O(1)-Si(1)-Cl(1) C(3)-Si(1)-Cl(1) O(1)-Si(1)-C (3) O(1)-Si(1)-C(1) C(3)-Si(1)-N(1) C(3)-Si(1)-C(1) C(1)-C(2)-C(3) 80.63(5) 80.82(5) 89.13(5) 160.79(5) 97.56(6) 171.39(6) 171.63(6) 97.12(6) 76.78(6) 100.95(11) O(1)-Si(1)-N(1) O(1)-Si(1)-N(1)# O(1)#-Si(1)-O(1) N(1)#-Si(1)-N(1) O(1)-Si(1)-C(1) O(1)-Si(1)-C(1)# C(1)#-Si(1)-C(1) C(1)-C(2)-C(1)# 80.36(7) 86.75(7) 88.78(13) 161.95(10) 171.14(11) 98.11(10) 75.61(15) 100.3(3) O(1)-Si(1)-N(1) O(1)-Si(1)-N(1)# O(1)#-Si(1)-O(1) N(1)#-Si(1)-N(1) O(1)-Si(1)-C(8) O(1)-Si(1)-C(8)# C(8)#-Si(1)-C(8) C(8)-C(9)-C(8)# 80.52(5) 80.15(5) 87.11(7) 162.51(6) 173.37(7) 97.75(7) 97.44(8) 173.51(7) 78.10(8) 101.06(14) O(1)-Si(1)-N(2) O(2)-Si(1)-N(4) O(2)-Si(1)-O(1) N(4)-Si(1)-N(2) O(1)-Si(1)-C(3) O(2)-Si(1)-C(3) O(1)-Si(1)-C(1) O(2)-Si(1)-C(1) C(3)-Si(1)-C(1) C(1)-C(2)-C(3)

Bond Angles (deg)

1.8069(12) 1.8107(12) 2.1000(14) 1.0937(14) 1.9027(18) 1.551(3) Si(1)-O(1) Si(1)-O(2) Si(1)-N(2) Si(1)-N(4) Si(1)-C(1) C(1)-C(2)

Si(1)-O(1) Si(1)-N(1) Si(1)-C(8) O(1)-C(1) N(2)-C(1) C(8)-C(9)

80.55(4) 85.75(4) 88.17(6) 160.91(5) 97.82(5) 171.78(4) 76.80(7) 100.81(12)

O(1)-Si(1)-N(2) O(2)-Si(1)-N(5) O(2)-Si(1)-O(1) N(5)-Si(1)-N(2) O(1)-Si(1)-C(13) O(2)-Si(1)-C(13) O(1)-Si(1)-C(11) O(2)-Si(1)-C(11) C(13)-Si(1)-C(11) C(13)-C(12)-C(11)

Si(1)-O(1) Si(1)-N(1) Si(1)-C(3) O(1)-C(4) N(2)-C(1) Si(1)-Cl(1) C(1)-C(2) 1.7935(10) 1.7919(11) 2.0888(12) 2.0845(12) 1.9268(14) 1.2992(18) 1.558(2) 1.7998(8) 2.0852(10) 1.9248(11) 1.3091(12) 1.2877(14) 1.5517(15) Si(1)-O(1) Si(1)-N(1) Si(1)-C(1) O(1)-C(10) N(2)-C(10) C(1)-C(2) 1.7789(16) 2.0982(19) 1.925(2) 1.303(3) 1.278(3) 1.537(3)

Bond Lengths (A˚)

Si(1)-O(1) Si(1)-O(2) Si(1)-N(2) Si(1)-N(5) Si(1)-C(13) N(4)-C(6) C(11)-C(12)

3d 3c 3b 3a

Table 1. Selected Crystallographic Bond Lengths and Angles for 3a, 3b, 3c, 3d, and 4c

4c

78.38(5) 105.81(5) 112.94(6) 140.72(7) 100.31(7) 94.05(7) 77.29(7) 99.07(13)

1.7183(11) 2.1245(15) 1.8930(16) 1.3466(18) 1.277(2) 2.1121(6) 1.555(2)

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The sum of equatorial bond angles amounts to 359.47°, i.e., a nearly perfect silicon-centered equatorial plane. Ligand Exchange Reactions. Compounds 3a-e were allowed to react with ZSiCl3 (7a-e) in CDCl3 solutions in 5 mm NMR tubes (eq 6). With Z = Ph, Cl, and H (7c-e), the exchange takes place at ambient temperature and is essentially complete within minutes. With Z = alkyl groups (7a, 7b), the exchange is slower and generally requires immersion in an 80 °C oil bath for 1 h.

The outcome of the ligand exchange reactions was determined by their various NMR spectra: the disappearance of signals due to starting materials and parallel growth of product signals, in particular formation of 1. It turns out that all of the reactants (7) caused complete ligand exchange and the reactions proceeded smoothly with formation of 1. This is not too surprising, in view of the previously established “ligand priority list”:4 since silacyclobutane has two alkyl ligands, which have the lowest ligand priority, they are readily replaced by the two incoming ligands of overall higher priority (a chloro and either a carbon, hydrido, or another chloro ligand). These results clearly indicate that ionic dissociation is not an essential feature of the ligand-exchange reactions. In fact, ionic dissociation is irrelevant, since the present silacyclobutane complexes undergo ligand exchange just as easily and rapidly as the previously reported chloro complexes.4 In addition, the results strongly suggest that no Si-C bond cleavage takes place during ligand exchange: the fact that in all of the reactions dichlorosilacyclobutane (1) is formed as the only product of the silacyclobutane moiety during ligand exchange rules out ring-opening, since it would be highly unlikely to expect that, after opening, the four-membered ring would spontaneously and quantitatively close again to form 1. A possible mechanistic insight into the exchange described in eq 6 is gained from the reaction of 3b (R = t-Bu) and 7e (Z = H), as demonstrated in Figure 6. The 29Si NMR spectra of the reaction products are shown. 7e was chosen because it allows immediate recognition of compounds with Si-H bonds, apparent by their characteristic doublet resulting from large one-bond Si-H coupling. The reaction was carried out in two fashions, first (Figure 6A) with excess (more than 2 molar equiv) of 3b. This caused exhaustive transfer of chelate rings (i.e., bidentate ligands) to 7e, resulting in exclusive appearance of the Si-H doublet in the hexacoordinate dichelate 8 (R = t-Bu, Z = H). The excess 3b is manifest in the formation of the pentacoordinate monochelate 4b (see eq 4), an intermediate formed from 3b after the donation of a single chelate ring, which does not react further to transfer another chelate ring, because there are no more 7e or 9 (see Figure 6B) molecules available to accept the transfer. Conversely, when the reaction is carried out applying an excess of 7e, a different pentacoordinate monochelate intermediate (9, Figure 6B) is observed: the one resulting from transfer of one chelate ring to 7e. In this case, the hydrogen atom remains attached to silicon, as is evident from the doublet at ca. -74 ppm, typical of pentacoordination. In

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Figure 6. 29Si NMR spectra of the reaction of 3b with 7e in CDCl3 solution: (A) with an excess of 3b, the pentacoordinate intermediate 4b is formed, along with hexacoordinate 8 (R = t-Bu, Z = H); (B) with an excess of 7e, the pentacoordinate intermediate 9 accumulates; and (C) same solution after 24 h; 9 has disappeared possibly by disproportionation to 7e and 8.

addition, some of the hexacoordinate exchange product (8) and a large excess of 7e are found in the spectrum. Interestingly, after 24 h at ambient temperature, the intermediate has completely disappeared (Figure 6C), probably through disproportionation to 7e and 8, suggesting that the hexacoordinate 8 is thermodynamically more favorable than the intermediate. These results indicate that exchange occurs by transfer of whole chelate rings: either one or both rings, from one silicon atom to another. This must be accompanied by simultaneous back-transfer of chloro ligands from the accepting silicon atom to the one donating the chelate ring. In other words, it may be concluded again that exchange does not involve silicon-carbon bond cleavage. A similar ligand-exchange reaction is also found between hexacoordinate complexes 3a-c and silylated hydrazides 2 (eq 7). The net effect of this exchange is the replacement of the ring-substituents R in complexes 3. Substituent priorities in this reaction are similar to those reported previously for regular complexes 8;4 namely, the more electron-withdrawing substituents R (R = CF3) are replaced from their complexes (3a) by less electron-withdrawing groups (t-Bu, in 2b). This apparently opposite priority of ring substituents in comparison with monodentate ligands leads to the thermodynamically most stable hexacoordinate complexes.

The stepwise transfer of bidentate ligands from one silicon compound to another is demonstrated again in Figure 7 by the reaction of a silacyclobutane complex, 3a, with the silylated hydrazide, 2b. Figure 7A shows the 29Si NMR spectrum of the reaction mixture immediately after dissolution. The major components are the starting materials 2b and 3a, with traces of the exchange products (2a and 3b) visible. In Figure 7B the same reaction is carried out with an excess of 3a. The major product is the mixed complex 10, with one CF3 and one t-Bu ring substituent, and only a trace of the doubly exchanged di-tert-butyl complex 3b. This may be an indication of the instability of the trifluoromethyl-substituted 3a (as previously reported for trifluoromethyl complexes):4,10 when both 3a and the mixed 10 are available, during the course of the reaction, for further chelate exchange with 2b, 3a is more reactive and hence exchanges with 2b preferentially over 10, causing accumulation of 10. In other words, the complex with two CF3 groups wants to get rid of its substituted chelate ring more strongly than 10, which has already been partly stabilized by exchange of one of the chelate rings. However, when the reaction is run under an excess of 2b (Figure 7C), there is no competition for the substituted chelate rings, and all of the trifluoromethylsubstituted material (3a and 10) is quantitatively converted to the tert-butyl-substituted complex 3b. The preference of tert-butyl as substituent in hexacoordinate complexes 3 over trifluoromethyl is seen also in the reverse reaction: when 3b reacts with 2a, even when the latter is present in great excess, only the mixed complex 10 (R1 = CF3, R2 = t-Bu) is formed. No trace of the doubly exchanged complex, 3a, is found. Similar results were obtained also in the exchange reactions of 3a and 2c: either all of the CF3-substituted complex (3a) was converted to the phenyl analogue 3c, or, in the presence of excess 3a, the major product was a mixed complex (10, R1 = CF3, R2 = Ph). In the reverse reaction, 3c with 2a, the only product observed was the mixed complex 10. CF3 was unable to replace both of the phenyl substituents in 3c, even after hours of reflux and excess of 2a.

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Figure 7. 29Si NMR spectra of the exchange reaction of 3a with 2b in CDCl3 solution: (A) immediately after dissolution; (B) with an excess of 3a, featuring accumulation of the mixed complex 10; (C) with an excess of 2b; only the completely exchanged product (3b) is observed.

Another ligand exchange reaction (eq 8) was attempted and is described below. Diphenyldichlorosilane (11) has been known to form relatively weak hexacoordinate complexes.12 When subjected to the exchange reaction with hexacoordinate complexes 3a-c, ligand transfer proceeded only to the first stage, transfer of one of the chelate rings to 11 to form 12, and modifying the reactant 3, after loss of one of the chelate rings, to 4. Even in the presence of two molar equivalents of 3 relative to 11 and after 2 h at chloroform reflux temperature, the monochelates 12b and 12c did not take up a second chelate moiety to form the expected dichelate hexacoordinate complexes. No exchange reaction was observed in a solution of 3a with 11.

The pentacoordinate products 4b, 4c, 12b, and 12c were identified by their typical 29Si NMR chemical shifts at -41.7, -43.7, -49.1, and -50.9 ppm, respectively, and by comparison with authentic samples and with the crystallographically characterized 4c. Ligand Interchange between Two Complexes. Ligand exchange takes place also when two hexacoordinate complexes are mixed in solution.4 When two differently substituted silacyclobutane complexes 3 are mixed in CDCl3 solution, after a short while at ambient temperature a third signal develops in the 29Si NMR spectrum, corresponding to the mixed dichelate (eq 9). The product distribution depends on the nature of the ring substituents R: when these do not differ substantially in electron-withdrawing power, such as two alkyl substituents, the product distribution is roughly the statistical 1:2:1, with 10 being the major product. However, there appears to be a driving force favoring the mixed complex even beyond the statistical bias: a symmetrically substituted complex with electron-withdrawing substituents, such as 3a, is less stable (12) Kalikhman, I.; Gostevskii, B.; Botoshansky, M.; Kaftory, M.; Tessier, C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics 2006, 25, 1252–1258.

than other complexes and tends to have at least one of the chelate rings replaced with a more favorable chelate. Thus, in the case of 3a reacting with 3b, the symmetrical reactants almost completely disappear to form the mixed complex (10). If one of the reactants is present in excess, the other one completely disappears, within NMR detection limits, to give way to the mixed complex and to the excess reactant, presumably to allow for maximum stabilization of the less stable complex (i.e., 3a) by formation of the mixed complex 10.

Central-Element Exchange. Compounds 3b-d react readily with GeCl4, with replacement of the central silicon by germanium to form 13b-d (eq 10). The germanium compounds were characterized by comparison with authentic samples or by analogy of their 1H and 13C NMR spectra with those of differently substituted authentic compounds.5 Thus, when 3b reacts with GeCl4 in CDCl3 solution for 20 min at ambient temperature, the high-field 29Si NMR signal characteristic of the hexacoordinate reactant 3b disappears, and instead the signal of dichlorosilacyclobutane is observed. The structure is further confirmed by the 1H and 13C NMR signals, which have shifted relative to those in 3b, but remain within the range expected for hexacoordinate complexes (see Experimental Section). Similar exchange takes place also with 3c and 3d. One may safely assume that, like in the other exchange reactions, the process involves consecutive transfer of chelating (bidentate) ligand units from silicon to germanium.

The reaction of 3a and 3e, with the electron-withdrawing ring substituents, is quite different. In these two reactions the central-element exchange is accompanied by a simultaneous

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Table 2. Crystallographic Data and Experimental Parameters for the Structure Analyses of 3a-d and 4c

CCDC no. empirical formula form mass, g mol-1 collection T, K cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z Fcalcd, Mg/m3 F(000) θ range, deg no. of coll reflns no. of indep reflns Rint no. of reflns used no. of params Goof R1 wR2 [I > 2σI)] R1 wR2 (all data) max./min. res electron dens, e A˚-3

3a

3b

3c

3d

4c

795674 C11H18F6N4O2Si 380.38 200(2) triclinic P1 7.7482(7) 9.5333(9) 12.3898(12) 75.284(2) 79.816(2) 72.996(2) 841.25(14) 2 1.502 392 2.29-28.50 8738 4158 0.0198 4158 273 0.996 0.0409 0.0889 0.0519 0.0967 0.388/-0.291

795675 C17H35N4O2Si 355.58 293(2) monoclinic C2/c 15.667(3) 15.259(3) 8.712(2) 90 90.70(2) 90 2082.6(7) 4 1.134 780 1.86-25.41 7468 1887 0.0610 1887 110 1.025 0.0564 0.1568 0.0767 0.1705 0.423/-0.375

795676 C21H27N4O2Si 395.56 120(2) monoclinic C2/c 19.877(6) 8.133(2) 13.351(4) 90 114.675(6) 90 1961.4(10) 4 1.340 844 2.26-30.13 8117 2858 0.0185 2858 130 1.009 0.0397 0.1146 0.0436 0.1181 0.358/-0.387

795677 C13H30N6O2Si 330.52 120(2) triclinic P1 8.9359(10) 9.4458(10) 11.0692(12) 80.800(2) 84.202(2) 70.193(2) 866.63(16) 2 1.267 360 1.87-29.05 6702 4534 0.0167 4534 207 1.008 0.0441 0.1065 0.0527 0.1136 0.688/-0.348

795678 C12H17ClN2OSi 268.82 120(2) monoclinic P21/n 7.4164(15) 11.718(2) 15.959(3) 90 99.620(4) 90 1367.4(5) 4 1.306 568 2.17-27.10 5832 2930 0.0187 2930 156 1.037 0.0361 0.0996 0.0441 0.1060 0.673/-0.380

oxidation and ring-opening reaction, as shown in eq 11. In this reaction germanium oxidizes the four-membered ring by addition of two chlorine atoms and ring-opening. The products 14a and 14e were identified by the various NMR spectra, although not isolated. In particular, the appearance of three new signals in the 1H and 13C NMR spectra, shown (by 1H-decoupling and DEPT experiments) to belong to three CH2 groups, suggested the structure assigned to 14.

resonances of 15 and 16 are shifted substantially to lower field, relative to the silacyclobutane compounds (3) and other hexacoordinate silicon compounds. It is concluded that these compounds are in dynamic equilibrium in solution with lower coordination species, as represented in eq 13. This is in agreement with a recent report by Wagler and co-workers,13 in which similar low-field 29Si NMR resonances had indicated similar equilibria. The low coordination may be attributed to the low electron-withdrawing power of the cycloalkyl ligands, which is insufficient to render silicon electrophilic enough to bind to the donor atoms. The silacyclobutane complexes (3) are exceptional in their relatively high-field 29Si NMR chemical shifts, probably due to their “ring strain release Lewis acidity”,11d,14 which prefers the smaller bond angles associated with higher (five or six) coordination numbers.

To study the generality of this ring-opening reaction, analogous complexes were synthesized with silacyclopentane (15ac) and silacyclohexane (16a-c) ring systems. In neither of these complexes was ring-opening and oxidation observed, and exchange of the central element proceeded smoothly (eq 12). It may be concluded that both ring strain and the presence of electronwithdrawing susbstituents contribute to the tendency of complexes 3a and 3e to undergo the oxidation reaction.

Examination of the 29Si NMR spectra of the silacycloalkane compounds (3, 15, and 16, see Table 3) shows that the

(13) Brendler, E.; W€achtler, E.; Wagler, J. Organometallics 2009, 28, 5459–5465. (14) (a) Denmark, S. E. J. Org. Chem. 2009, 74, 2915–2927. (b) Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008, 41, 1486– 1499.

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Table 3. Comparison of 29Si NMR Chemical Shifts (ppm) of Silacycloalkane Dichelate Complexes 3a-c, 15a-c, and 16a-c compd indexa

ring substituenta

silacyclobutane (3)

silacyclopentane (15)

silacyclohexane (16)

a c b

CF3 phenyl t-Bu

-105.5 -111.2 -114.1

-33.3 -44.7 -14.3

-8.7 -16.1 -25.6

a

Listed in order of decreasing electron-withdrawing power.

Within the families of silacyclobutane (3a-c) and silacyclohexane (16a-c) complexes in Table 3 the trend in each group makes good sense: the stronger the electron-withdrawing power of the ring substituent, the weaker the donor property of the corresponding NMe2 group, resulting in weaker NfSi coordination and hence lower field average chemical shift. In contrast, the trend fails for the silacyclopentane compounds, in which 15b stands out with its unexpected relatively low field resonance. This may be due to some direct steric repulsion between the bulky tert-butyl groups and the silacyclopentane ring, which may lack the flexibility attributed to cyclohexane rings, interfering with proper chelate ring closure. The change in chemical shift is much greater in 16a-c than it is in 3a-c, probably because the latter are essentially hexacoordinate, and hence do not change substantially with changing substituents. In 16, however, the change of substituents causes much greater changes in the equilibrium position of eq 13, resulting in greater changes in 29Si chemical shifts.

Experimental Section The reactions were carried out under dry argon using Schlenk techniques. Solvents were dried and purified by standard methods. NMR spectra were recorded on a Bruker Avance DMX500 spectrometer operating at 500.13, 125.76, and 99.36 MHz, respectively, for 1H, 13C, and 29Si spectra. Spectra are reported in δ (ppm) relative to TMS, as determined from standard residual solvent proton (or carbon) signals for 1H and 13C and directly from TMS for 29Si. NMR spectra were measured at 297 K, unless otherwise reported. Melting points were measured in sealed capillaries using a Buchi melting point instrument and are uncorrected. Elemental analyses were performed by MEDAC Ltd., Chobham, Surrey, UK. Single-crystal X-ray diffraction measurements were performed on a Bruker Smart Apex on a D8-goniometer. Crystallographic details are listed in Table 2. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC). The CCDC numbers are listed in Table 2. Product 13a has been reported previously.5 Bis(N-(dimethylamino)trifluoroacetimidato-N0 ,O)silacyclobutane (3a). A mixture of 1.32 g (5.8 mmol) of 2a and 0.41 g (0.3 mmol) of 1 in 5 mL of chloroform was stirred at room temperature for 2 h. The volatiles were removed under reduced pressure (0.1 mmHg), and the residue was washed with 5 mL of n-hexane. A single crystal for X-ray analysis was grown from n-hexane. Yield: 1.00 g (92%), mp 105-106 °C. 1H NMR (CDCl3): δ 1.40 (m, 2H, CH2), 1.79 (m, 4H, CH2), 2.83 (s, 12H, NCH3). 13C NMR (CDCl3): δ 12.23, 32.45 (CH2), 49.22 (NCH3), 117.29 (q, 1J(F-C) = 277 Hz, CF3), 156.99 (q, 2J(F-C) = 37 Hz, CdN). 29Si NMR (CDCl3): δ -105.5. Anal. Calcd for C11H18F6N4O2Si: C, 34.73; H, 4.77; N, 14.73. Found: C, 34.68; H, 5.21; N, 14.99. Bis(N-(dimethylamino)pivaloimidato-N0 ,O)silacyclobutane (3b). 3b was prepared as described for 3a, from 2b (1.23 g, 5.7 mmol) and 0.38 g (2.8 mmol) of 1. Yield: 0.98 g (98%), mp 133-135 °C. 1 H NMR (CDCl3): δ 1.04 (s, 18H, (CH3)3), 1.28 (m, 2H, CH2), 1.61 (m, 4H, CH2), 2.74 (s, 12H, NCH3). 13C NMR (CDCl3): δ 12.64, 32.55 (CH 2), 27.43 (C(CH 3)3), 35.13 (C(CH 3)3), 49.46, 49.64 (NCH 3), 174.08 (CdN). 29Si NMR (CDCl 3): δ -114.1. Anal. Calcd for C17H36N4O2Si: C, 57.26; H, 10.18; N, 15.71. Found: C, 56.95; H, 10.45; N, 16.16.

Bis(N-(dimethylamino)benzimidato-N0 ,O)silacyclobutane (3c). 3c was prepared as described for 3a, from 2c (1.26 g, 5.3 mmol) and 0.37 g (2.6 mmol) of 1. Yield: 0.99 g (95%), mp 159-161 °C. 1 H NMR (CDCl3): δ 1.50 (m, 2H, CH2), 1.90 (m, 4H, CH2), 3.02 (s, 12H, NCH3), 7.28-7.84 (m, 10H, Ph). 13C NMR (CDCl3): δ 12.80, 32.74 (CH2), 49.80 (NCH3), 127.34, 127.91, 130.54, 131.99 (Ph), 164.59 (CdN). 29Si NMR (CDCl3): δ -111.2. Anal. Calcd for C21H28N4O2Si: C, 63.60; H, 7.12; N, 14.12. Found: C, 63.13; H, 6.99; N, 13.69. Bis(N-(dimethylamino)N0 -(dimethyl)glicineimidato-N00 ,O)silacyclobutane (3d). 3d was prepared as described for 3a, from 2d (1.31 g, 6.5 mmol) and 0.45 g (3.2 mmol) of 1. Yield: 1.01 g (95%), mp 120-122 °C. 1H NMR (CDCl3): δ 1.25 (m, 2H, CH2), 1.66 (m, 4H, CH2), 2.66 (s, 12H, NCH3), 2.69 (s, 12H, NCH3). 13C NMR (CDCl3): δ 12.72, 32.95 (CH2), 36.32 (CNCH3), 50.55 (NNCH3), 162.62 (CdN). 29Si NMR (CDCl3): δ -112.7. Anal. Calcd for C13H30N6O2Si: C, 47.24; H, 9.15; N, 25.43. Found: C, 47.41; H, 9.43; N, 26.04. Bis(N-(dimethylamino)cyanoacetimidato-N0 ,O)silacyclobutane (3e). 3e was prepared as described for 3a, from 2e (1.18 g, 5.9 mmol) and 0.42 g (2.9 mmol) of 1. Yield: 0.78 g (82%), mp 128-130 °C. 1H NMR (CDCl3): δ 1.31 (m, 2H, CH2), 1.67 (m, 4H, CH2), 2.72 (s, 12H, NCH3), 3.16 (s, 4H, NCCH2). 13C NMR (CDCl3): δ 12.20, 32.29 (CH2), 21.55 (NCCH2), 49.27 (NCH3), 114.36 (CtN), 160.17 (CdN). 29Si NMR (CDCl3): δ -109.4. Anal. Calcd for C13H22N6O2Si: C, 48.42; H, 6.88; N, 26.06. Found: C, 48.43; H, 7.04; N, 26.19. Chloro(N-(dimethylamino)pivaloimidato-N0 ,O)silacyclobutane (4b). 4b was prepared as described for 3a, from 2b (0.66 g, 3.0 mmol) and 0.43 g (3.1 mmol) of 1. Yield: 0.72 g (96%). 1H NMR (CDCl3): δ 1.21 (s, 9H, (CH3)3), 1.71 (m, 2H, CH2), 1.90 (m, 4H, CH2), 2.54 (s, 6H, NCH3). 13C NMR (CDCl3): δ 11.40, 33.51 (CH2), 26.56 (C(CH3)3), 35.00 (C(CH3)3), 46.99 (NCH3), 174.04 (CdN). 29Si NMR (CDCl3): δ -41.2. Chloro(N-(dimethylamino)benzimidato-N0 ,O)silacyclobutane (4c). 4c was prepared as described for 3a, from 2c (1.27 g, 7.4 mmol) and 1.04 g (7.4 mmol) of 1. A single crystal for X-ray analysis was grown from diethyl ether. Yield of crystals: 0.72 g (50%), mp 102-104 °C. 1H NMR (CDCl3): δ 1.39-1.78 (m, 6H, CH2), 2.62 (s, 6H, NCH3), 7.34-7.96 (m, 5H, Ph). 13C NMR (CDCl3): δ 11.52, 33.64 (CH2), 47.55 (NCH3), 127.51, 128.65, 129.44, 131.70 (Ph), 163.52 (CdN). 29Si NMR (CDCl3): δ -40.2. Anal. Calcd for C12H17ClN2OSi: C, 53.62; H, 6.37; N, 10.42. Found: C, 53.20; H, 6.45; N, 10.36. Chloro(N-(dimethylamino)pivaloimidato-N0 ,O)diphenylsilane (12b). 12b was prepared as described for 3a, from 2b (0.28 g, 1.3 mmol) and 0.54 g (2.1 mmol) of 11. Yield: 0.42 g (90%). 1 H NMR (CDCl3): δ 1.34 (s, 9H, (CH3)3), 1.90 (s, 6H, NCH3), 7.42-7.80 (m, 10H, Ph). 13C NMR (CDCl3): δ 27.05 (C(CH3)3), 35.80 (C(CH3)3), 48.44 (NCH3), 171.00 (CdN), 127.75, 128.20, 129.56, 131.62, 131.81, 132.68, 133.92, 136.28 (Ph). 29Si NMR (CDCl3): δ -49.0. Chloro(N-(dimethylamino)benzimidato-N0 ,O)diphenylsilane (12c). 12c was prepared as described for 3a, from 2c (0.46 g, 2.0 mmol) and 0.52 g (2.1 mmol) of 11. Yield: 0.64 g (89%). 1H NMR (CDCl3): δ 1.93 (s, 6H, NCH3), 7.26-8.02 (m, 15H, Ph). 13 C NMR (CDCl3): δ 48.21 (NCH3), 164.61 (CdN), 127.79, 127.94, 128.03, 128.37, 129.85, 131.04, 131.97, 131.64 (Ph). 29Si NMR (CDCl3): δ -51.0. Bis(N-(dimethylamino)pivaloimidato-N0 ,O)dichlorogermanium (13b). 13b was prepared as described for 3a, from 2b (0.61 g,

Article 2.9 mmol) and 0.32 g (1.5 mmol) of GeCl4. Yield: 0.61 g (94%). 1 H NMR (CDCl3): δ 1.08 (s, 18H, (CH3)3), 2.98, 3.07 (2s, 12H, NCH3). 13C NMR (CDCl3): δ 26.99 (C(CH3)3), 35.98 (C(CH3)3) 50.64, 50.68 (NCH3), 173.64 (CdN). Bis(N-(dimethylamino)benzimidato-N0 ,O)dichlorogermanium (13c). 13c was prepared as described for 3a, from 2c (0.52 g, 2.2 mmol) and 0.24 g (1.1 mmol) of GeCl4. Yield: 0.47 g (91%). 1 H NMR (CDCl3): 3.34 (s, 12H, NCH3), 7.37-7.92 (m, 10H, Ph). 13C NMR (CDCl3): δ 51.06, 51.12 (NCH3), 127.47, 128.20, 130.95, 131.48 (Ph), 163.93 (CdN). Bis(N-(dimethylamino)N0 -(dimethyl)glicineimidato-N00 ,O)dichlorogermanium (13d). 13d was prepared as described for 3a, from 2d (0.41 g, 2.0 mmol) and 0.23 g (1.1 mmol) of GeCl4. Yield: 0.38 g (89%). 1H NMR (CDCl3): δ 2.72 (s, 12H, CNCH3), 2.93, 3.05 (2s, 12H, NNCH3). 13C NMR (CDCl3): δ 35.64 (CNCH3), 52.07, 52.23 (NNCH3), 161.13 (CdN). Bis(N-(dimethylamino)cyanoacetimidato-N0 ,O)dichlorogermanium (13e). 13e was prepared as described for 3a, from 2e (0.40 g, 2.0 mmol) and 0.24 g (1.1 mmol) of GeCl4. Yield: 0.38 g (87%). 1 H NMR (CDCl3): δ 3.04, 3.10 (2s, 12H, NCH3), 3.32 (s, 4H, NCCH2). 13C NMR (CDCl3): δ 22.42 (NCCH2), 50.61, 50.75 (NCH3), 113.47 (CtN), 160.46 (CdN). Bis(N-(dimethylamino)trifluoroacetimidato-N0 ,O)-3-(chloro)propylchlorosilicon(IV) (14a). A mixture of 0.32 g (0.8 mmol) of 3a and 0.28 g (1.3 mmol) of GeCl4 in 5 mL of chloroform was stirred at room temperature for 1 h. The volatiles were removed under reduced pressure (0.1 mmHg), and the residue was washed with 5 mL of n-hexane. Two products, 14a and 13a, were obtained and identified by their NMR spectra (13a by analogy with an authentic sample) in the mixture. An excess of GeCl4 was added, resulting in quantitative conversion of 14a to 13a. 14a: 1H NMR (CDCl3): δ 1.19-1.28 (m, 2H, CH2), 1.90 (t, 3J(C-H) = 10 Hz, 2H, CH2), 2.04 (t, 3J(C-H) = 10 Hz, 2H, CH2), 2.99 (b, 12H, NCH3). 13C NMR (CDCl3): δ 22.20, 29.87, 35.62 (CH2), 51.03 (NCH3), 116.93 (q, 1J(F-C) = 277 Hz, CF3), 156.42 (q, 2J(F-C) = 37 Hz, CdN). 29Si NMR: δ -126.7. Bis(N-(dimethylamino)cyanoacetimidato-N0 ,O)-3-(chloro)propylchlorosilicon(IV) (14e). 14e was prepared as described for 14a, from 3e (0.51 g, 1.6 mmol) and 1.70 g (7.9 mmol) of GeCl4. 14e: 1H NMR (CDCl3): δ 1.19-1.29 (m, 2H, CH2), 1.89 (t, 3J(C-H) = 10 Hz, 2H, CH2), 2.04 (t, 3J(C-H) = 10 Hz, 2H, CH2), 2.91 (b, 12H, CH3), 3.26 (s, 4H, NCCH2). 13C NMR (CDCl3): δ 22.31, 30.05, 35.78 (CH2), 21.33 (NCCH2), 52.40 (NCH3), 113.42 (CtN), 159.88 (CdN). 29Si NMR (CDCl3): δ -128.16. Bis(N-(dimethylamino)trifluoroacetimidato-N0 ,O)silacyclopentane (15a). 15a was prepared as described for 3a, from 2a (0.74 g, 3.3 mmol) and 0.25 g (1.6 mmol) of dichlorosilacyclopentane. The volatiles were removed under reduced pressure (0.1 mmHg), and the oily gray residue was washed with 5 mL of n-hexane. Yield: 0.59 g (91%). 1H NMR (CDCl3): δ 0.58-0.62, 1.55-1.58 (2 m, 8H, CH2), 2.60 (s, 12H, NCH3). 13C NMR (CDCl3): δ 11.28, 23.17 (CH2), 48.40 (NCH3), 117.35 (q, 1J(F-C) = 277 Hz, CF3), 150.09 (q, 2J(F-C) = 37.4 Hz, CdN). 29Si NMR (CDCl3): δ -33.3. Anal. Calcd for C12H20F6N4O2Si: C, 36.54; H, 5.11; N, 14.21. Found: C, 36.06; H, 5.50; N, 14.12.

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Bis(N-(dimethylamino)pivaloimidato-N0 ,O)silacyclopentane (15b). 15b was prepared as described for 3a, from 2b (0.67 g, 3.1 mmol) and 0.24 g (1.6 mmol) of dichlorosilacyclopentane. The volatiles were removed under reduced pressure (0.1 mmHg), and the oily colorless residue was washed with 5 mL of n-hexane. Yield: 0.50 g (86%). 1H NMR (CDCl3): δ 0.26-0.37, 1.42-1.55 (2 m, 8H, CH2), 1.06 (s, 18H, (CH3)3), 2.56 (s, 12H, NCH3). 13C NMR (CDCl3): δ 11.77, 22.83 (CH2), 27.46 ((CH3)3), 35.51 (C(CH3)3), 50.22 (NCH3), 172.67 (CdN). 29Si NMR (CDCl3): δ -14.3. Bis(N-(dimethylamino)benzimidato-N0 ,O)silacyclopentane (15c). 15c was prepared as described for 3a, from 2c (0.60 g, 2.5 mmol) and 0.20 g (1.3 mmol) of dichlorosilacyclopentane. The volatiles were removed under reduced pressure (0.1 mmHg), and the white solid residue was washed with 5 mL of n-hexane. Yield: 0.45 g (84%), mp 80-83 °C. 1H NMR (CDCl3): δ 0.61-0.69, 1.53-1.63 (2 m, 8H, CH2), 2.69 (s, 12H, CH3), 7.27-7.92 (m, 10H, Ph). 13C NMR (CDCl3): δ 11.60, 23.36 (CH2), 49.25 (NCH3), 127.21, 127.97, 130.43, 132.59 (Ph), 160.66 (CdN). 29Si NMR (CDCl3): δ -44.7. Anal. Calcd for C22H30N4O2Si: C, 64.36; H, 7.36; N, 13.65. Found: C, 64.00; H, 7.38; N, 13.66. Bis(N-(dimethylamino)trifluoroacetimidato-N0 ,O)silacyclohexane (16a). 16a was prepared as described for 3a, from 2a (0.87 g, 3.8 mmol) and 0.32 g (1.9 mmol) of dichlorosilacyclohexane. The volatiles were removed under reduced pressure (0.1 mmHg), and the oily colorless residue was washed with 5 mL of n-hexane. Yield: 0.66 g (84%). 1H NMR (CDCl3): δ 1.02-1.80 (m, 10H, CH2), 2.60 (s, 12H, CH3). 13C NMR (CDCl3): δ 15.31, 24.47, 29.47 (CH2), 46.35 (CH3), 117.46 (q, 1J(F-C) = 277 Hz, CF3), 140.81 (q, 2J(F-C) = 37.4 Hz, CdN). 29Si NMR (CDCl3): δ -8.7. Bis(N-(dimethylamino)pivaloimidato-N0 ,O)silacyclohexane (16b). 16b was prepared as described for 3a, from 2b (0.67 g, 3.1 mmol) and 0.24 g (1.6 mmol) of dichlorosilacyclohexane. The volatiles were removed under reduced pressure (0.1 mmHg), and the oily colorless residue was washed with 5 mL of n-hexane. Yield: 0.28 g (82%). 1H NMR (CDCl3): δ 1.33-1.76 (m, 10H, CH2), 1.10 (s, 18H, (CH3)3), 2.30 (s, 12H, NCH3). 13C NMR (CDCl3): δ 14.78, 25.11, 29.10 (CH2), 27.70 ((CH3)3), 36.35 (C(CH3)3), 47.19 (NCH3), 166.86 (CdN). 29Si NMR (CDCl3): δ -25.6. Bis(N-(dimethylamino)benzimidato-N0 ,O)silacyclohexane (16c). 16c was prepared as described for 3a, from 2c (0.52 g, 2.2 mmol) and 0.20 g (1.2 mmol) of dichlorosilacyclohexane. The volatiles were removed under reduced pressure (0.1 mmHg), and the oily colorless residue was washed with 5 mL of n-hexane. Yield: 0.43 g (86%). 1H NMR (CDCl3): δ 1.01-1.88 (m, 10H, CH2), 2.52 (s, 12H, CH3), 7.28-8.02 (m, 10H, Ph). 13C NMR (CDCl3): δ 14.47, 24.84, 29.08 (CH2), 46.99 (CH3), 127.09, 127.96, 130.05, 133.42 (Ph), 155.47 (CdN). 29Si NMR (CDCl3): δ -16.1.

Acknowledgment. Financial support by the Israel Science Foundation, grant No. ISF-242/09, is gratefully acknowledged. Supporting Information Available: Crystallographic data for 3a-d and 4c in the form of CIF files are available free of charge via the Internet at http://pubs.acs.org.