Organometallics 2009, 28, 6831–6834 DOI: 10.1021/om900741a
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Ring-Strain-Formation Lewis Acidity? A Pentacoordinate Silacyclobutane Comprising Exclusively Equatorial Si-C Bonds Robert Gericke, Daniela Gerlach, and J€ org Wagler* Institut f€ ur Anorganische Chemie, Technische Universit€ at Bergakademie Freiberg, D-09596 Freiberg, Germany Received August 25, 2009 Summary: Although silacyclobutanes appear susceptible to penta- and hexacoordination of their Si atoms because of their inherent so-called ring-strain-release Lewis acidity, we succeeded in synthesizing a pentacoordinate (trigonal-bipyramidal) silacyclobutane with equatorial Si-C bonds. A related silacyclobutane with an axial-equatorial Si-C situation was found by quantum-chemical calculations to have a low energy barrier (ca. 3 kcal/mol) for its transformation into an isomer with exclusively equatorial Si-C bonds.
From the synthesis point of view, silacyclobutanes attracted attention for at least two reasons: One is their activated Si-C bonds, which allowed for intramolecular rearrangement reactions1 and for ring-insertion and ringopening addition reactions.2 Another interesting point is their inherent so-called ring-strain-release Lewis acidity (Scheme 1).3 That is, the narrow C-Si-C angle (e.g., 84° in 1,1-dichlorosilacyclobutane4), creating a great discrepancy to a tetrahedral arrangement of the Si-bound atoms, renders both an axial-equatorial (ax-eq) location of the four-membered ring in a trigonal-bipyramidal (TBP) Sicoordination sphere and cis sites in an octahedral coordination sphere more favorable, whereas this concept would predict the location of two silacyclobutane Si-C bonds on eq TBP sites less favorable because of an increased discrepancy between the real (20° for 2), whereas φ decreases notably (to 5.8° for 4 and 4.3° for 2) upon transformation into the Cs-symmetric conformers, thus comprising the silacyclobutane Si-C bonds in eq positions. As indicated above, a PES scan was performed for complex 4 on the same level of theory as that for compound 2 [B3LYP/6-31G(d); Figure 2]. In sharp contrast to the latter, the Cs-symmetric arrangement of which represents a (10) Crystal structure analysis of 4: C14H15N3O2Si, CCDC 744879, T = 100(2) K; monoclinic, P21/c; a = 7.0863(2), b = 10.2087(3), c = 17.7160(5) A˚, β = 94.384(2)°; V = 1277.86(6) A˚-3; Z = 4; μ(Mo KR) = 0.189 mm-1; θmax = 27°; 13 020 reflections (2761 unique, Rint = 0.0592), 184 parameters, GOF = 1.059, R1/wR2 [I > 2σ(I)] = 0.0432/0.0964, R1/wR2 (all data) = 0.0678/0.1043, residual electron density (highest peak, deepest hole) = 0.310, -0.347 e/A˚3. (11) Novikov, V. P.; Dakkouri, M.; Vilkov, L. V. J. Mol. Struct. 2006, 800, 146.
Note
Figure 2. Relative energies of 2 (diamonds) and 4 (squares) depending on the Ncentral-Si-Ceq angle as the PES scan [determined with B3LYP/6-31G(d)].
maximum along the PES, the Cs-symmetric configuration of 4 is a minimum and no local minimum was found for a configuration of 4 with ax-eq-situated Si-C bonds. Again, SPECs were performed using B3LYP/6-311þG(2d,p) and MP2/6-31G(d,p), which proved the Cs-symmetric conformer of 4 to be more stable by 3.8 and 4.5 kcal/mol, respectively, with reference to the conformers, the Ncentral-Si-Ceq angle of which was contracted by 30° (B3LYP) or 35° (MP2), the contraction necessary to furnish the minimum energy conformation for 2. In order to separate the contributions of Si-located ring strain of silacyclobutane and rigidity of the ONN0 -chelating ligand, the following analyses were performed: (i) For each step along the PES scans of Figure 2, the silacyclobutane fragment Si(CH2)32þ was removed from the respective molecular conformation and the energy of the remaining dianionic ligand was calculated [SPECs using B3LYP/6-311þG(2d,p) and MP2/6-31G(d,p)]. The results are summarized in Figure 3. As one would expect, both ligands achieve an energetic minimum upon planarization, i.e., at the stages of Cs-symmetric arrangement of the complexes 2 and 4. Whereas π conjugation throughout the ligand backbone can be considered to be responsible for this general trend, the carbamide moiety of 4 obviously renders the ligand backbone 32- less rigid. (ii) In order to probe the Si-located ring strain along the PES scans in Figure 2, the energy of C-Si-C angle relaxation was considered a representative measure. Thus, for each step along the scans, the central CH2 group of silacyclobutane was replaced by two H atoms, followed by optimization of the H-atom positions using B3LYP/6-31G(d). The H3C-Si-CH3 angle, still comprising the Si-located ring strain, was then allowed to relax, still using B3LYP/6-31G(d). Finally, the energies of the strained and relaxed states were determined as SPECs using B3LYP/ 6-311þG(2d,p) and MP2/6-31G(d,p) (Figure 4). Surprisingly, the H3C-Si-CH3 angle relaxation energies of the different conformers of 2 and 4 do not show any extraordinary dependence on the location of the former silacyclobutane within the TBP geometry. A slight preference of the ax-eq arrangement becomes obvious (with 3.4 and 1.9 kcal/mol for 2 and 4, respectively), which does not appear to be sufficient to overcompensate for the rigidity of the dianionic ligand backbones used (ca. 9.3 and 6.4 kcal/mol preference for the Cs-symmetric conformation with respect to the slightly folded ligands necessary to accommodate the ax-eq-situated silacyclobutane, as found for 2 in the solid
Organometallics, Vol. 28, No. 23, 2009
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Figure 3. Relative relaxation energies [MP2/6-31G(d,p)] of the dianionic ligands 12- (diamonds) and 32- (squares) depending on the Ncentral-Si-Ceq angle in 2 and 4, respectively (starting configurations taken from the PES scan of Figure 2).
Figure 4. Relative C-Si-C angle relaxation energies [MP2/ 6-31G(d,p)] of the complexes 1-SiMe2 (diamonds) and 3-SiMe2 (squares) depending on the Ncentral-Si-Ceq angle in 2 and 4, respectively [starting configurations obtained by replacing silacyclobutanes taken from the PES scan of Figure 2 by SiMe2 groups, retaining the Si and C atomic coordinates and only optimizing methyl H-atom positions using B3LYP/6-31G(d) prior to relaxation].
state). Further contributions of the overall bonding situation, which cannot as easily be separated from the molecular systems for computational analyses, may dominate the conformational preferences of 2 and 4. One can consider that charge distribution in the ligands and charge compensation upon complex formation is such an influence: Whereas the ligand acid 1-H2 comprises its N-H bond at the pyrrole system, compound 3-H2 bears an N-H bond at the amide N atom. Upon deprotonation and complex formation, the same preferences might still hold for the resulting silicon compounds, thus supporting a short eq Si-N bond to the amide N atom in compound 4 and a short eq Si-N bond to the pyrrole N atom in compound 2. In conclusion, we can say that the molecular conformation of compound 4, as found in the solid state and by computational analyses, is not at all based on such things as “ringstrain-formation Lewis acidity”. The model of “ring-strainrelease Lewis acidity” still holds for this compound with the silacyclobutane Si-C bonds in an eq-eq location, which was shown to lower Si-located ring strain when transformed into a conformer with ax-eq-located silacyclobutane Si-C bonds. The effect of Si-located ring strain on the resulting molecular conformation, however, was found to be a less pronounced factor. Instead, rigidity of the tridentate chelating ligand applied proved sufficient to overcome the rather small energetic barrier associated with transferring the
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ax-eq-located silacyclobutane into the equatorial plane. Although 4 represents, to our best knowledge, the first example of a crystallographically evidenced pentacoordinate silacyclobutane with two silacyclobutane eq Si-C bonds, examples of phospha-,12 tantala-13 and tungstacyclobutane14 structures exhibiting this feature were already reported. Furthermore, the latter two even lack a rigid chelating ligand. Hence, we can conclude that “ring-strainrelease Lewis acidity” only plays a limited role in the (12) Aly, H. A. E.; Barlow, J. H.; Russell, D. R.; Smith, D. J. H.; Swindles, M.; Trippett, S. J. Chem. Soc., Chem. Commun. 1976, 449. (13) Wallace, K. C.; Liu, A. H.; Dewan, J. C.; Schrock, R. R. J. Am. Chem. Soc. 1988, 110, 4964. (14) (a) Bochkarev, L. N.; Begantsova, Y. E.; Bochkarev, A. L.; Stolyarova, N. E.; Grigorieva, I. K.; Malysheva, I. P.; Basova, G. V.; Platonova, E. O.; Fukin, G. K.; Baranov, E. V.; Kurskii, Y. A.; Abakumov, G. A. J. Organomet. Chem. 2006, 691, 5240. (b) Schrock, R. R.; DePue, R. T.; Feldman, J.; Shaverien, C. J.; Dewan, J. C.; Liu, A. H. J. Am. Chem. Soc. 1988, 110, 1423. (c) Shaverien, C. J.; Dewan, J. C.; Schrock, R. R. J. Am. Chem. Soc. 1986, 108, 2771.
Gericke et al.
coordination behavior of heterocyclobutanes (including silacyclobutanes), thus rendering the lowered steric demand of the -(CH2)3- moiety (with reference to two methyl groups), another important factor that enhances the susceptibility of silacyclobutanes toward Si hypercoordination.
Acknowledgment. Technische Universit€at Bergakademie Freiberg is acknowledged for financial support. D.G. is grateful to Deutscher Akademischer AustauschDienst for a scholarship and to Prof. Dr. Thomas Heine and Dr. Andreas W. Ehlers for helpful advice on the quantum-chemical calculations. Supporting Information Available: X-ray crystallographic data in CIF format for the structure determination of compound 4 (CCDC 744879) and details about the quantumchemical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
