Seven-Membered Cyclic Dialkylstannylene and - ACS Publications

Mar 16, 2009 - Keith Izod,* Corinne Wills, William Clegg, and Ross W. Harrington. Main Group Chemistry Laboratories, School of Chemistry, Bedson Build...
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Seven-Membered Cyclic Dialkylstannylene and -Plumbylene Compounds Stabilized by Agostic-type B-H · · · E Interactions [E ) Sn, Pb] Keith Izod,* Corinne Wills, William Clegg, and Ross W. Harrington Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, UniVersity of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K. ReceiVed December 16, 2008

The reaction between [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 and either Cp2Sn or Cp2Pb in toluene cleanly gives the compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E in moderate yield [E ) Sn (10), Pb (11)]. NMR spectra of crude samples indicate that 10 is predominantly formed as the rac isomer; for 11 there is no evidence for the formation of the meso diastereomer at all. Crystallization from diethyl ether yields the solvates 10 · Et2O and 11 · Et2O; X-ray crystallography reveals that these compounds crystallize as discrete rac-dialkylstannylene and -plumbylene species in which there are two short agostic-type B-H · · · E contacts. A DFT study suggests that these agostic-type interactions stabilize 10 and 11 by 47.7 and 42.7 kcal mol-1, respectively. Calculations on the corresponding meso diastereomers of 10 and 11 suggest that the tin and lead centers in these compounds have close contacts to just one hydrogen atom of a BH3 group, although a second, weaker B-H · · · E contact is observed in each case. These contacts afford an overall stabilization of 41.3 and 32.6 kcal mol-1, respectively, for the mesostannylene and -plumbylene. Introduction Since Arduengo and co-worker’s landmark report of the synthesis of the first stable N-heterocyclic carbene (NHC), the chemistry of carbene compounds has seen a dramatic rise in popularity; these compounds have now become an essential addition to the armory of ligands in transition metal chemistry and catalysis.1 This, in turn, has sparked renewed interest in the heavier group 14 analogues of these species (R2E, E ) Si, Ge, Sn, Pb), the first examples of which were isolated over three decades ago.2,3 There are three principal methods by which such compounds may be stabilized: (i) steric protection by bulky substituents, preventing attack at the electron-deficient group 14 center; (ii) intra- or intermolecular coordination by a Lewis base, mitigating the electron deficiency at the group 14 center; and (iii) the presence of heteroatoms such as N or O directly adjacent to the group 14 center, which both increases the singlet-triplet energy separation through stabilization of the Si, Ge, Sn, or Pb lone pair and mitigates the electron-deficiency at the group 14 center through efficient pπ-pπ overlap between the heteroatom lone pairs and the vacant p-orbital on the group 14 atom. * Corresponding author. E-mail: [email protected]. (1) For a recent review of NHCs and their metal complexes see: (a) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (2) For reviews of the chemistry of heavier group 14 carbene analogues see: (a) Barrau, J.; Rima, G. Coord. Chem. ReV. 1998, 178-180, 593. (b) Tokitoh, N.; Okazaki, R. Coord. Chem. ReV. 2000, 210, 251. (c) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (d) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (e) Klinkhammer, K. W. In Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; Wiley: New York, 2002; Vol. 2, pp 283-357. (f) Veith, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 1. (g) Ku¨hl, O. Coord. Chem. ReV. 2004, 248, 411. (h) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (i) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209. (3) (a) Davidson, P. J.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1973, 317. (b) Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. J. Chem. Soc., Chem. Commun. 1976, 261.

Although significant progress has been made in the synthesis of heteroatom-substituted heavier group 14 carbene analogues, such as the diamides (R2N)2E and their cyclic counterparts (direct analogues of NHCs), far less is known about hydrocarbylsubstituted species.2 In particular, while Power and co-workers, and others, have made substantial headway in the synthesis of stable diaryl-substituted compounds Ar2E,4 few examples are known of the corresponding dialkyl-substituted compounds (R3C)2E; such compounds typically rely solely on steric protection for their stability. Although Lappert and co-workers reported the first example of a dialkylstannylene, {(Me3Si)2CH}2Sn (1), in the early 1970s,3 this compound dimerizes to the corresponding tetraalkyldistannene (1a) in the solid state and is observed in its monomeric form only in the gas phase (Chart 1); in solution compound 1 is subject to a dynamic equilibrium (4) (a) Yang, X.-J.; Wang, Y.; Wei, P.; Quillan, B.; Robinson, G. H. Chem. Commun. 2006, 403. (b) Phillips, A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 7520. (c) Pu, L.; Olmstead, M. M.; Power, P. P.; Schiemenz, B. Organometallics 1998, 17, 5602. (d) Stu¨rmann, M.; Weidenbruch, M.; Klinkhammer, K. W.; Lissner, F.; Marsmann, H. Organometallics 1998, 17, 4425. (e) Lay, U.; Pritzkow, H.; Gru¨tzmacher, H. Chem. Commun. 1992, 260. (f) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics 1997, 16, 1920. (g) Gru¨tzmacher, H.; Pritzkow, H.; Edelmann, F. T. Organometallics 1991, 10, 23. (h) Brooker, S.; Buijink, J.-K.; Edelmann, F. T. Organometallics 1991, 10, 25. (i) Tajima, T.; Takeda, N.; Sasamori, T.; Tokitoh, N. Organometallics 2006, 25, 3552. (j) Spikes, G. H.; Peng, Y.; Fettinger, J. C.; Power, P. P. Z. Anorg. Allg. Chem. 2006, 632, 1005. (k) Wegner, G. L.; Berger, R. J. F.; Schier, A.; Schmidbaur, H. Organometallics 2001, 20, 418. (l) Weidenbruch, M.; Schlaefke, J.; Schafer, A.; Peters, K.; von Schnering, H. G.; Marsmann, H. Angew. Chem., Int. Ed. 1994, 33, 1846. (m) Braunschweig, H.; Drost, C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J.-M. Angew. Chem., Int. Ed. 1997, 36, 261. (n) Bender, J. E.; Banaszak Holl, M. K.; Kampf, J. W. Organometallics 1997, 16, 2743. (o) Kano, N.; Shibita, K.; Tokitoh, N.; Okazaki, R. Organometallics 1999, 18, 2999. (p) Jutzi, P.; Schmidt, H.; Neumann, B.; Stammler, H.-G. Organometallics 1996, 15, 741. (q) Pu, L.; Twamley, B.; Power, P. P. Organometallics 2000, 19, 2874. (r) Peng, Y.; Ellis, B. D.; Wang, X. P.; Power, P. P. J. Am. Chem. Soc. 2008, 130, 12268.

10.1021/om801189h CCC: $40.75  2009 American Chemical Society Publication on Web 03/16/2009

2212 Organometallics, Vol. 28, No. 7, 2009

Izod et al. Chart 1

between the stannylene and distannene forms.5 The first truly monomeric dialkylstannylene and -germylene were isolated in 1991 by Kira, Sakurai, and co-workers (2)6 and by Jutzi and co-workers (3),7 respectively. Subsequently, until our recent reports, only four further dialkyl-substituted heavier group 14 carbene analogues (4-7)8-11 had been structurally characterized, although several intramolecularly base-stabilized derivatives are known.12 In this regard, we recently reported the synthesis of two new examples of this class of compound, [{nPr2P(BH3)}(Me3Si)C(CH2)]2E [E ) Sn (8), Pb (9)].13,14 These compounds are stabilized both by the presence of sterically demanding groups adjacent to the tin and lead centers and by short B-H · · · E agostic-type interactions, which help to reduce the electrondeficiency of these atoms. Such agostic-type interactions appear to be a new method by which heavier group 14 carbene analogues may be stabilized, and so we were keen to explore the generality of this principle. This paper describes the extension of this agostic-stabilization method to a new dialkylstannylene and -plumbylene, rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E [E ) Sn (10), Pb (11)], and reports the synthesis, structural characterization, and spectroscopic properties of these compounds, along with a theoretical exploration of their bonding. (5) Zilm, K. W.; Lawless, G. A.; Merrill, R. M.; Millar, J. M.; Webb, G. G. J. Am. Chem. Soc. 1987, 109, 7236. (6) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1991, 113, 7785. (7) Jutzi, P.; Becker, A.; Stammler, H. G.; Neumann, B. Organometallics 1991, 10, 1647. (8) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9722. (9) Kira, M.; Ishida, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C.; Ignatovich, L.; Sakurai, H. Chem. Lett. 1999, 263. (10) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D.; Smith, J. D.; Zhang, S. Organometallics 2000, 19, 49. (11) Eaborn, C.; Ganicz, T.; Hitchcock, P. B.; Smith, J. D.; Sozerli, S. E. Organometallics 1997, 16, 5621. (12) For examples see: (a) Drost, C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J.-M. Chem. Commun. 1997, 1141. (b) Drost, C.; Hitchcock, P. B.; Lappert, M. F. Organometallics 1998, 17, 3838. (c) Wingerter, S.; Gornitzka, H.; Bertermann, R.; Pandey, S. K.; Rocha, J.; Stalke, D. Organometallics 2000, 19, 3890. (d) Engelhardt, L. M.; Jolly, B. S.; Lappert, M. F.; Raston, C. L.; White, A. H. J. Chem. Soc., Chem. Commun. 1988, 336. (e) Jolly, B. S.; Lappert, M. F.; Engelhardt, L. M.; White, A. H.; Raston, C. L. J. Chem. Soc., Dalton Trans. 1993, 2653. (f) Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Drew, M. G. B.; Rashid, H.; Convery, M. A.; Fenske, D. J. Chem. Soc., Dalton Trans. 1998, 2749. (g) Benet, S.; Cardin, C. J.; Cardin, D. J.; Constantine, S. P.; Heath, P.; Rashid, H.; Teixeira, S.; Thorpe, J. H.; Todd, A. K. Organometallics 1999, 18, 389. (13) Izod, K.; McFarlane, W.; Tyson, B. V.; Carr, I.; Clegg, W.; Harrington, R. W. Organometallics 2006, 25, 1135. (14) Izod, K.; McFarlane, W.; Wills, C.; Clegg, W.; Harrington, R. W. Organometallics 2008, 27, 4386.

Results and Discussion Synthesis and Characterization. The reaction between [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]215 and 1 equiv of either Cp2Sn16 or Cp2Pb14,17 in toluene at room temperature cleanly gives the compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E [E ) Sn (10), Pb (11)] (Scheme 1). Unexpectedly, compounds 10 and 11 differ significantly in their solubilities; whereas 11 is soluble in diethyl ether and may be crystallized from this solvent, compound 10 is much less soluble and is most conveniently purified by Soxhlet extraction into this solvent. Although compounds 10 and 11 are potentially diastereomeric, we have been able to isolate only the rac isomers of both compounds. For the dialkylstannylene 10 31P{1H} NMR spectra of the crude reaction mixtures from several experiments exhibit, in addition to signals for the dominant rac isomer [1.6 ppm (q, JPB ) 81 Hz, 2JSnP ≈ 360 Hz)], a broad, featureless signal due to a second species [ca. 3.5 ppm], which we attribute to the meso diastereomer; this species is formed in proportions varying from zero to approximately 25%. For the lead compound 11 there is no evidence for the formation of a second stereoisomer, and only signals for rac-11 are observed in 31P{1H} spectra of the crude reaction mixtures. In common with the vast majority of alkali metal derivatives of chiral carbanions, the dilithio precursor [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 racemizes rapidly in solution, as indicated by NMR spectroscopy.15 We therefore attribute the dominance of the rac isomer in both cases to the increased stabilization associated with two agostic-type interactions in this form, compared to the single agostic-type interaction predicted for the meso form (see DFT calculations below). Crystallization of 10 and 11 from cold diethyl ether yields the solvates 10 · Et2O and 11 · Et2O as yellow or orange needles, respectively, suitable for X-ray crystallography. Compounds 10 · Et2O and 11 · Et2O are essentially isostructural and are almost isomorphous: compound 10 · Et2O crystallizes in the space group Ibca with a crystallographic C2 axis bisecting the C-Sn-C angle, whereas 11 · Et2O crystallizes in the space group Pbca and has only approximate C2 symmetry; the latter structure approximates Ibca, but reflections that would be absent for this space group have significant intensities, averaging about (15) Izod, K.; Bowman, L. J.; Wills, C.; Clegg, W.; Harrington, R. W. Dalton Trans., in press. (16) Fischer, E. O.; Grubert, H. Z. Naturforsch. B 1956, 11, 423. (17) (a) Fischer, E. O.; Grubert, H. Z. Anorg. Allg. Chem. 1956, 286, 237. (b) Armstrong, D. R.; Davidson, M. G.; Moncrieff, D.; Russell, C. A.; Stourton, C.; Steiner, A.; Stalke, D.; Wright, D. S. Organometallics 1997, 16, 3340.

Dialkylstannylene and -Plumbylene Compounds

Organometallics, Vol. 28, No. 7, 2009 2213 Scheme 1

20% of the other reflections. Diethyl ether solvent molecules are highly disordered in 10 · Et2O, but ordered in 11 · Et2O. The molecular structures of 10 · Et2O and 11 · Et2O are shown in Figure 1, along with selected bond lengths and angles. Both 10 · Et2O and 11 · Et2O crystallize as discrete molecular species; the shortest Sn · · · Sn and Pb · · · Pb distances are 8.110 and 8.084 Å, respectively. The tin and lead atoms are coordinated by the two carbanion centers of the ligands, generating puckered seven-membered chelate rings, with C-Sn-C and C-Pb-C bite angles of 119.60(15)° and 119.04(15)°, respectively. The wide nature of the ligand bite angles and the similarity in the bite angles for these two compounds may be attributed to a combination of the large steric bulk of the ligand and the formation of a somewhat disfavored seven-membered heterocycle [cf. C-Sn-C and C-Pb-C bite angles of 85.32(5)°

and 83.16(15)° for rac-8 and rac-9, respectively]. These bite angles compare with C-Sn-C and C-Pb-C bite angles of 117.6(1)° and 117.1(2)°, respectively, in the closely related seven-membered cyclic dialkylstannylene 6 and -plumbylene 7.10,11 The Sn-C distance of 2.389(3) Å in 10 · Et2O is considerably longer than the Sn-C distances in rac-8 [2.2984(14) and 2.3046(14) Å]13 and in 6 [2.284(3) and 2.286(3) Å].10 The Pb-C distances in 11 · Et2O of 2.492(5) and 2.480(5) Å are also substantially longer than the Pb-C distances in rac-9 [2.390(4) and 2.402(4) Å]14 and in 7 [2.397(6) and 2.411(5) Å].11 In view of the closely related steric properties of the ligands in 10/11 and 6/7, the substantially longer Sn-C and Pb-C distances in the former may be attributed to the presence of additional Sn · · · H and Pb · · · H contacts in these compounds,

Figure 1. Molecular structures of (a) 10 · Et2O and (b) 11 · Et2O (two views) with 40% probability ellipsoids. H atoms bonded to carbon and solvent of crystallization are omitted for clarity. Selected bond lengths (Å) and angles (deg) for 10 · Et2O: Sn-C(1) 2.389(3), Sn · · · H(2) 2.30(4), C(1)-P 1.818(4), C(1)-Si(1) 1.917(4), C(1)-Si(2) 1.903(3), P-B 1.971(5), B-H(1) 1.16(4), B-H(2) 1.30(4), B-H(3) 1.16(3), C(1)-Sn-C(1A) 119.60(15); for 11 · Et2O: Pb-C(1) 2.492(5), Pb-C(18) 2.480(5), Pb · · · H(1C) 2.59(6), Pb · · · H(2C) 2.58(5), C(1)-P(1) 1.812(5), C(18)-P(2) 1.816(5), C(1)-Si(1) 1.892(6), C(1)-Si(2) 1.906(5), C(18)-Si(3) 1.900(5), C(18)-Si(4) 1.901(6), P(1)-B(1) 1.971(8), P(2)-B(2) 1.945(7), B(1)-H(1A) 1.09(6), B(1)-H(1B) 1.14(5), B(1)-H(1C) 1.12(6), B(2)-H(2A) 1.10(6), B(2)-H(2B) 1.05(6), B(2)-H(2C) 1.03(5), C(1)-Pb-C(18) 119.04(15).

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which effectively increase the coordination numbers of the tin and lead centers. The structures of both 10 · Et2O and 11 · Et2O exhibit two short E · · · H contacts, one contact to a hydrogen atom of each BH3 group in each case. The Sn · · · H distance of 2.30(4) Å in 10 · Et2O is similar to the Sn · · · H distances in rac-8 [2.32(2) Å for the two Sn · · · H contacts],13 while the Pb · · · H distances of 2.58(5) and 2.59(6) Å in 11 · Et2O are similar to the Pb · · · H distances in rac-9 [2.43(6) and 2.46(6) Å];14 these distances lie well within the sum of the van der Waals radii of H and either Sn or Pb [3.37 and 3.22 Å, respectively]. There are very few reports of compounds in which there are short B-H · · · Sn or B-H · · · Pb contacts; to the best of our knowledge, outside of our own studies, the only example of the former is found in the anion of [10-endo-(SnPh3)-10-µ-H-7,8-nido-C2B9H10][transIr(CO)(PPh3)2(MeCN)], in which the tin(IV) center is directly bonded to the boron atom of a carborane cage [Sn · · · H 2.349 Å].18 Similarly, outside of our own studies, only one other B-H · · · Pb contact has been reported: the tris(2-mercaptoimidazolyl)borate complex (TmPh)2Pb [TmPh ) HB(2-S,3PhC3N2)3], in which one of the TmPh ligands adopts an inverted η4-coordination mode, binding the lead atom through its three S-donors and the central B-H group, and has a Pb · · · H distance of 2.39 Å.19 In this latter case the bonding between the lead(II) center and the borate ligand has been described as predominantly ionic in nature, and it has been suggested that the Pb · · · H contact should not be considered a primary bonding interaction. In contrast, DFT calculations on 11 indicate that the Pb · · · H bond involves significant delocalization of electron density from the B-H σ-bond onto the lead atom and, therefore, that the Pb · · · H interaction has significant covalent character (see below). The 1H, 13C{1H}, 11B{1H}, and 31P{1H} spectra of both 10 and 11 are as expected and exhibit no unusual signals or dynamic behavior (cf. 8 and 9). The solvent of crystallization in each case is only very weakly held and is rapidly lost under vacuum, as demonstrated by 1H NMR spectroscopy; however, we were able to obtain satisfactory elemental analyses of these solvates. The 31P{1H} NMR spectrum of 10 consists of a broad quartet, due to coupling to 11B, at 1.6 ppm, exhibiting poorly resolved satellites due to coupling to 117/119Sn as a pair of shoulders on the main signal; the 117/119Sn-31P coupling constant of approximately 360 Hz was estimated by simulation of this signal. Similarly, the 31P{1H} spectrum of 11 consists of a broad quartet at 0.3 ppm, on which satellites due to coupling to 207Pb are only poorly resolved (simulation of this spectrum suggests a 207Pb-31P coupling constant of approximately 450 Hz). The 119Sn{1H} spectrum of 10 exhibits a very broad singlet at 320 ppm (∆ν1/2 ) 740 Hz), on which coupling to 31P is not resolved. This signal lies to substantially higher field than the corresponding signals for the majority of diaryl- and dialkylstannylenes; for example, the 119Sn chemical shifts of the dialkylstannylenes 2 and 6 are 2323 and 2299 ppm,6,10 respectively, whereas the 119Sn chemical shifts of the diarylstannylenes {2,6-(2,6-iPr2C6H3)2C6H3}2Sn and {2,6-(2,4,6Me3C6H2)2C6H3}2Sn are 2235 and 1971 ppm, respectively.4f,4j The 119Sn chemical shift of 10 is, however, similar to those of the diarylstannylenes {2,4,6-(CF3)3C6H2}2Sn (723 ppm)4g and {2,6-(Me2N)2C6H3}2Sn (442 ppm),12a in which direct Sn · · · F or Sn-N contacts mitigate the electron-deficiency of the tin(II) (18) Kim, J.; Kim, S.; Do, Y. J. Chem. Soc., Chem. Commun. 1992, 938. (19) Bridgewater, B. M.; Parkin, G. Inorg. Chem. Commun. 2000, 3, 534.

Izod et al.

center. The higher field chemical shift observed for 10 may thus be attributed to the decrease in electron-deficiency of the tin center in this compound due to the two agostic-type Sn · · · H interactions, clearly implying that these contacts are maintained in solution. Correspondingly high-field 119Sn chemical shifts were observed for rac- and meso-8 (578 and 787 ppm, respectively), in which agostic-type Sn · · · H interactions are also thought to persist in solution.13 Unfortunately, we were unable to observe by NMR spectroscopy the unique protons of the BH3 groups that are involved in the agostic-type interactions; we were similarly unable to distinguish these protons in the variabletemperature 1H and 1H{11B} NMR spectra of 8 and 9.13,14 It is likely that dynamic exchange among the hydrogen atoms of each BH3 group is a very low-energy process and, therefore, that such exchange will be fast on the NMR time-scale, resulting in only a time-averaged signal for these protons. In this regard, Shimoi and co-workers have suggested an upper limit of just 30 kJ mol-1 at 193 K for the free energy of activation for exchange between the terminal and bridging H atoms in the transition metal complexes M(CO)5{η1-H3BPMe3} [M ) Cr, Mo, W].20 A similarly high-field chemical shift is observed for the lead analogue 11: the 207Pb{1H} NMR spectrum of 11 consists of a broad, featureless singlet at 5920 ppm (∆ν1/2 ) 780 Hz). This compares with a 207Pb chemical shift of 10050 ppm for the dialkylplumbylene 711 and of 9430 and 8844 ppm for the diarylplumbylenes {2,6-(2,6-iPr2C6H3)2C6H3}2Pb and {2,6(2,4,6-Me3C6H2)2C6H3}2Pb, respectively.4f,4j The 207Pb chemical shift of 11 more closely resembles those of the diarylplumbylene {2,4,6-(CF3)3C6H2}2Pb (4878 ppm)4h and the mixed alkyl/ arylplumbylene (2,4,6-tBu3C6H2)(3,5-tBu2C6H3CMe2CH2)Pb (5067 ppm), in which the electron-deficiency of the lead(II) center is mitigated by either Pb · · · F or putative Pb · · · H-C contacts; for comparison, the 207Pb chemical shifts of rac- and meso-9 are 4580 and 5430 ppm, respectively.14 The solid-state infrared spectra of 10 and 11 exhibit lowfrequency absorptions at 2022 and 2033 cm-1, respectively, which may be attributed to the B-H stretching vibrations associated with the agostic-type B-H · · · E contacts. The remaining B-H stretching vibrations lie in the ranges 2097-2393 cm-1 for 10 and 2163-2351 cm-1 for 11. We were unable to obtain a UV-visible spectrum of 10 due to its poor solubility in nonpolar solvents; however, the UV-visible spectrum of 11 in methylcyclohexane exhibits a moderate absorption at 343 nm (ε ) 1270 dm3 mol-1 cm-1), which may be assigned to a transition from the lone pair on the lead atom to the vacant 6p-orbital (see TD-DFT calculations below). DFT Calculations. In order to gain greater insight into the nature of the bonding in 10 and 11, we have undertaken a DFT study of these compounds (10a and 11a, respectively) and of their meso diastereomers (10b and 11b for the Sn and Pb compounds, respectively). Geometries were optimized using the B3LYP hybrid functional21 with a Lanl2dz basis set22 on Sn (20) (a) Shimoi, M.; Nagai, S.-I.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704. (b) Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20, 3211. (c) Shimoi, M.; Atoh, K.; Kawano, Y.; Kodama, G.; Ogino, H. J. Organomet. Chem. 2002, 659, 102. (21) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345. (22) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.

Dialkylstannylene and -Plumbylene Compounds

Organometallics, Vol. 28, No. 7, 2009 2215

Table 1. Comparison of Crystallographically Determined and Calculated Bond Lengths (Å) and Angles (deg) for 10 and 11 10 · Et2O

10a

E-Ca

2.389(3)

2.424

C-P

1.818(4)

1.842

C-Si

1.917(4) 1.903(3)

1.930 1.928

P-B

1.971(5)

1.967

B-H( · · · E)

1.30(4)

1.24

B-H

1.16(3) 1.16(4)

1.21 1.20

E···H

2.30(4)

2.36

C-E-C a

119.60(15) 119.27

10b

11 · Et2O

11a

Table 2. Frontier Orbital Composition and Selected NBO Energies for 8, 9, 10, and 11 rac-8a meso-8a rac-9b meso-9b

11b

2.403 2.492(5) 2.524 2.503 2.518 2.480(5) 2.617 1.853 1.812(5) 1.832 1.842 1.824 1.816(5) 1.816 1.948 1.892(6) 1.923 1.932 1.923 1.906(5) 1.918 1.917 1.920 1.900(5) 1.913 1.914 1.901(6) 1.904 1.964 1.971(8) 1.969 1.960 1.962 1.945(7) 1.964 1.22 1.12(6) 1.24 1.22 1.27 1.03(5) 1.26 1.21 1.14(5) 1.21 1.21 1.21 1.09(6) 1.21 1.21 1.20 1.10(6) 1.20 1.20 1.05(6) 1.20 2.11 2.59(6) 2.41 2.20 2.71 2.58(5) 2.61 116.64 119.04(15) 118.62 116.19

10 E ) Sn, 11 E ) Pb.

and Pb and a 6-31G(d,p) basis set23 on all other atoms. The location of minima was confirmed by the absence of imaginary vibrational frequencies. There is an excellent correspondence between the calculated structures of 10a and 11a and the structures of 10 · Et2O and 11 · Et2O obtained by X-ray crystallography (Table 1). Bond lengths are typically overestimated in the calculated structures by approximately 0.02-0.10 Å; however, the C-E-C angles in the calculated structures [119.27° and 118.62° for 10a and 11a, respectively] are close to the corresponding angles obtained by X-ray crystallography [119.60(15)° and 119.04(15)° for 10 · Et2O and 11 · Et2O, respectively]. For both 10a and 11a calculations predict two short E · · · H contacts [10a: Sn · · · H 2.36 Å; 11a: Pb · · · H 2.41 Å]; these compare well with the corresponding distances in 10 · Et2O [2.30(4) Å] and 11 · Et2O [2.58(5) and 2.59(6) Å]. The calculated structures also suggest a slight lengthening of the B-H bonds that are involved in agostictype interactions compared to the remaining B-H bonds [B-H( · · · Sn) 1.24, remaining B-H 1.21 Å; B-H( · · · Pb) 1.24, remaining B-H 1.21 Å]. In the meso diastereomers 10b and 11b (for which we do not have experimental data) DFT calculations predict two E · · · H contacts in each case, although one of these contacts is significantly shorter than the other: for 10b the two Sn · · · H distances are 2.11 and 2.71 Å, whereas for 11b the two Pb · · · H distances are 2.20 and 2.61 Å. In each case the short E · · · H contact is significantly shorter than the corresponding distances in 10a and 11a, suggesting that the single interaction in the meso diastereomers is stronger than either of the two such interactions in the corresponding rac isomers. Somewhat surprisingly, the two Sn-C and the two Pb-C distances in the calculated structures of 10b and 11b differ significantly from one another; the shorter distance in each case is associated with the side of the ligand involved in the weaker E · · · H interaction [10b: Sn-C 2.403 and 2.518 Å; 11b: Pb-C 2.503 and 2.617 Å]. This is not the case for 10a and 11a, in which the two Sn-C and the two Pb-C distances are essentially identical. Calculations indicate that 10a is more stable than 10b by 5.7 kcal mol-1, consistent with the presence of two significant agostic-type interactions in the former, compared to one strong (23) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.

HOMO (% s) (% p) LUMO (% s) (% p) E(2)d ∆Ee

86.7 13.3

86.7 13.3

92.1 7.9

92.2 7.8

0.0 100.0 20.0 19.9 45.1

1.0 99.0 31.0 2.6 33.8

0.0 100.0 18.8 16.9 40.6

0.5 99.5 24.0 3.4 30.3

10ac

10bc

88.3 89.0 11.7 11.0

11ac

11bc

94.1 94.5 5.9 5.5

0.0 1.4 0.0 0.8 100.0 98.6 100.0 99.2 21.6 38.9 19.1 29.9 21.6 4.7 19.0 5.7 47.7 41.3 42.7 32.6

a Ref 13. b Ref 14. c This work. d E(2) energy (kcal mol-1) from NBO calculations for the elements corresponding to B-H · · · E delocalizations. e Energy difference (kcal mol-1) between the ground-state structure and the structure in which the elements responsible for the principal B-H · · · E delocalizations have been deleted (see text).

and one weak agostic-type interaction in the latter; similarly, 11a is calculated to be 5.2 kcal mol-1 more stable than 11b. Natural bond orbital (NBO)24 analyses of 10a, 10b, 11a, and 11b reveal that the HOMO in each case is a lone pair on the tin or lead atom of essentially s-character (see Table 2), whereas the LUMO in each compound is comprised of an essentially pure, vacant p-orbital on the tin or lead atom, which lies perpendicular to the plane of the heterocycle. Although there is a significant difference between the C-Sn-C angle in 10 [experimental: 119.6(15)°, calculated for 10a: 119.27°] and the C-Sn-C angle in the analogous five-membered heterocycle rac-8 [experimental: 85.32(5)°, calculated for rac-8: 85.25°], there appears to be little difference in the nature of the HOMO and LUMO in each case.25 The s-character of the HOMO increases from 86.7 to 88.3% on increasing the heterocycle ring size from the five-membered ring in rac-8 to the sevenmembered ring in 10a; the LUMO in both cases has 100% p-character. Similarly, although the C-Pb-C angles in rac-9 and 11 are 83.16(15)° and 119.04(15)°, respectively [calculated for rac-9: 83.58°, for 11a: 118.62°], the percentage s-character of the HOMO increases only very slightly from 92.1% in the former to 94.1% in the latter; the LUMO has 100% p-character in both rac-9 and 11a.25 Thus it appears that there is no change in hybridization of the group 14 center on going from a five- to a seven-membered heterocycle, in spite of the large increase in C-E-C angles. Analysis of the donor-acceptor interactions in these compounds confirms that there is significant delocalization of B-H σ-bonding electron density into the vacant p-orbital on the tin or lead atom in each case. The E(2) energy, which may be obtained from NBO calculations, provides an estimate of the strength of these interactions: for 10a the E(2) energy for each of the two B-H · · · Sn interactions is 21.6 kcal mol-1, whereas for the lead analogue 11a the E(2) energies for the two B-H · · · Pb interactions are 19.0 and 19.1 kcal mol-1. The lower E(2) energies in the latter compound are consistent with a weaker interaction due to the large size and diffuse nature of the vacant 6p-orbital on the lead atom compared to the 5p-orbital (24) (a) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (Theochem) 1988, 169, 41. (b) Carpenter, J. E. Ph.D. thesis, University of Wisconsin, Madison, WI, 1987. (c) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (d) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066. (e) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 1736. (f) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (g) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (25) Incorrect values for the HOMO/LUMO composition of rac- and meso-9 were given in ref 14; the correct values are provided in Table 2 and in the text.

2216 Organometallics, Vol. 28, No. 7, 2009

on the tin center. For the meso diastereomer 10b two B-H · · · Sn interactions are present, one strong interaction, with an E(2) energy of 38.9 kcal mol-1, and one weaker interaction, with an E(2) energy of 4.7 kcal mol-1. The lead analogue 11b also exhibits one strong and one weak B-H · · · Pb interaction, with E(2) energies of 29.9 and 5.7 kcal mol-1, respectively. Selective deletion of the agostic-type interactions (using the NBODel routine in NBO 3.0)26 suggests that these interactions afford an overall stabilization to 10a and 11a of 47.7 and 42.7 kcal mol-1, respectively, whereas the combined effect of the strong and weak agostic-type interactions in 10b and 11b affords an overall stabilization energy of 41.3 and 32.6 kcal mol-1, respectively, for these compounds. Comparison of the stabilization energies of the sevenmembered heterocycles 10 and 11 with those of the fivemembered heterocycles 8 and 9 reveals a small, but consistent increase in the stabilization afforded by the B-H · · · E contacts in the former compounds compared to the latter (Table 2). This may possibly be attributed to the greater flexibility of the ligand in 10 and 11, which permits a closer approach of the B-H hydrogen atoms to the metal centers and thus affords a stronger agostic-type interaction for these compounds compared to 8 and 9. TD-DFT calculations indicate that the lowest energy allowed electronic transition in both 10a and 11a is from the lone pair at the tin or lead atom to the vacant p-orbital and that this transition occurs at 345 and 330 nm for 10a and 11a, respectively; this latter value compares well with the experimentally determined value of 343 nm for 11. For the meso diastereomers 10b and 11b this transition is calculated to occur at 374 and 366 nm, respectively.

Conclusions Compounds 10 and 11 represent two new examples of monomeric heavier group 14 carbene analogues, somewhat rare species; the dialkylplumbylene 11 is only the third compound of this type to be structurally characterized. In both 10 and 11 there are two short, agostic-type B-H · · · E interactions, which DFT studies suggest are significantly stabilizing. The isolation of 10 and 11 and the confirmation that agostic-type interactions contribute significantly to their stabilization suggests that such interactions may be used more generally to stabilize heavier group 14 carbene analogues and related electron-poor species.

Experimental Section All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen or in a nitrogenfilled glovebox. Toluene and diethyl ether were distilled under nitrogen from sodium or sodium/potassium alloy, respectively, and were stored over a potassium film. Deuterated toluene was distilled from potassium, deoxygenated by three freeze-pump-thaw cycles, and stored over activated 4A molecular sieves. The compounds [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2,15 SnCp2,16 and PbCp217 were prepared by previously published procedures. 1 H, 11B{1H}, 13C{1H}, 31P{1H}, 119Sn{1H}, and 207Pb{1H} NMR spectra were recorded on a JEOL Lambda500 spectrometer operating at 500.16, 160.35, 125.65, 202.35, 186.50, and 104.32 MHz, respectively; chemical shifts are quoted in ppm relative to tetramethylsilane (1H and 13C), external BF3(OEt2) (11B), external 85% H3PO4 (31P), external Me4Sn (119Sn), or external Me4Pb (207Pb), as appropriate. The positions of the BH3 signals in the 1H NMR spectra and JPH for these signals were determined using a selective 1H{11B} (26) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.

Izod et al. experiment. Infrared spectra were recorded as neat powders on a Varian 800 FTIR spectrometer; UV-visible spectra were recorded as 1.0 mM solutions in methylcyclohexane in matched quartz cells on a Hitachi F4500 spectrophotometer. Elemental analyses were obtained by the Elemental Analysis Service of London Metropolitan University. [{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2Sn (10). To a solution of freshly sublimed SnCp2 (0.23 g, 0.92 mmol) in toluene (20 mL) was added a solution of [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 (0.64 g, 0.92 mmol) in toluene (20 mL). The resulting solution was stirred for 1 h, then filtered. The solvent was removed in Vacuo from the filtrate, and the product was extracted into refluxing diethyl ether (50 mL) using a Soxhlet apparatus over a period of 3 h. The resulting yellow solution was cooled to -30 °C for 16 h to give yellow needles of 10 · Et2O, which were isolated by filtration. Isolated yield: 0.27 g, 45%. Anal. Calcd for C22H62B2OP2Si4Sn: C, 40.20; H, 9.51. Found: C, 40.06; H, 9.41. 1 H{11B} NMR (d8-toluene, 20 °C): δ 0.31 (s, 6H, SiMe2), 0.37 (s, 18H, SiMe3), 0.38 (s, 6H, SiMe2), 0.88 (m, 2H, CH2CH2), 1.03 (m, 2H, CH2CH2), 1.12 (t, 3H, Et2O), 1.23 (d, JPH ) 9.6 Hz, 6H, PMe), 1.31 (d, JPH ) 9.2 Hz, 6H, PMe), 1.46 (d, JPH ) 6.9 Hz, 6H, BH3), 3.27 (q, 2H, Et2O). 13C{1H} NMR (d8-toluene, 20 °C): δ 4.52 (d, JPC ) 25.9 Hz, SiMe3), 7.24 (d, JPC ) 1.9 Hz, SiMe2), 15.28 (Et2O), 15.71 (d, JPC ) 4.8 Hz, CH2), 18.07 (d, JPC ) 34.6 Hz, PMe), 18.71 (d, JPC ) 35.5 Hz, PMe), 65.60 (Et2O). 11B{1H} NMR (d8-toluene, 20 °C): δ -28.7 (d, JPB ) 81 Hz). 31P{1H} NMR (d8-toluene, 20 °C): δ 1.6 (q, JPB ) 81 Hz; 2JSnP ca. 360 Hz). 119 Sn{1H} NMR (d8-toluene, 20 °C): δ 320 (br). IR (solid, cm-1): 2963 (w), 2904 (w), 2733 (w), 2393 (m), 2166 (m), 2097 (m), 2022 (m), 1698 (w), 1414 (w), 1255 (m), 1083 (m), 1023 (m), 924 (m), 809 (s), 681 (m), 525 (m). [{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2Pb (11). To a solution of freshly sublimed PbCp2 (0.39 g, 1.16 mmol) in toluene (20 mL) was added a solution of [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 (0.81 g, 1.16 mmol) in toluene (20 mL), excluding light as much as possible. The resulting solution was stirred for 1 h, then filtered. The solvent was removed in Vacuo from the filtrate, and the product was extracted into diethyl ether (30 mL). The orange solution was cooled to -30 °C for 16 h to give orange needles of 11 · Et2O, which were isolated by filtration. Isolated yield: 0.39 g, 45%. Anal. Calcd for C22H62B2OP2PbSi4: C, 35.43; H, 8.34. Found: C, 35.36; H, 8.29. 1H{11B} NMR (d8-toluene, 20 °C): δ 0.30 (s, 18H, SiMe3), 0.33 (s, 6H, SiMe2), 0.39 (s, 6H, SiMe2), 0.85 (m, 2H, CH2CH2), 1.00 (m, 2H, CH2CH2), 1.12 (t, 6H, Et2O) 1.19 (d, JPH ) 9.2 Hz, 6H, PMe), 1.22 (d, JPH ) 9.7 Hz, 6H, PMe), 1.26 (d, JPH ) 13.3 Hz, 6H, BH3), 3.27 (q, 4H, Et2O). 13 C{1H} NMR (d8-toluene, 20 °C): δ 5.21 (SiMe3), 6.49 (SiMe2), 9.45 (d, JPC ) 11.5 Hz, SiMe2), 15.27 (Et2O), 16.76 (d, JPC ) 22.9 Hz, CH2CH2), 21.53 (d, JPC ) 34.5 Hz, PMe), 22.32 (d, JPC ) 33.6 Hz, PMe), 65.60 (Et2O). 11B{1H} NMR (d8-toluene, 20 °C): δ -36.5 (d, JPB ) 77 Hz). 31P{1H} NMR (d8-toluene, 20 °C): δ 0.3 (q, JPB ) 77 Hz; JPbP ca. 450 Hz). 207Pb NMR (d8-toluene, 20 °C): δ 5920 (br s). IR (solid, cm-1): 2941 (m), 2351 (s), 2234 (m), 2163 (w), 2033 (m), 1250 (m), 1116 (m), 1005 (m), 839 (m), 734 (s). UV-visible (1.0 mM, methylcyclohexane): λmax 343 nm (ε ) 1270 dm3 mol-1 cm-1). Crystal Structure Determinations of 10 · Et2O and 11 · Et2O. Measurements were made at 150 K on a Nonius KappaCCD diffractometer using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Cell parameters were refined from the observed positions of all strong reflections. Intensities were corrected semiempirically for absorption, based on symmetryequivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. Table 3 gives further details. All non-hydrogen atoms were refined anisotropically, and C-bound H atoms were constrained with a riding model, while B-bound H atoms were freely refined; U(H)

Dialkylstannylene and -Plumbylene Compounds

Organometallics, Vol. 28, No. 7, 2009 2217

Table 3. Crystallographic Data for 10 · Et2O and 11 · Et2O formula fw cryst size/mm cryst syst space group a/Å b/Å c/Å V/Å3 Z µ/mm-1 transmn coeff range reflns measd unique reflns Rint reflns with F2 > 2σ parameters R (on F, F2 > 2σ)a Rw (on F2, all data)a goodness of fita min., max. electrondensity/e Å-3

10 · Et2O

11 · Et2O

C18H52B2P2Si4Sn.C4H10O 657.3 0.23 × 0.08 × 0.02 orthorhombic Ibca 16.220(4) 20.048(7) 21.707(6) 7059(4) 8 0.964 0.809-0.981 23 985 4031 0.066 2563 142 0.040 0.102 1.029 0.76, -0.65

C18H52B2P2PbSi4.C4H10O 745.8 0.34 × 0.30 × 0.10 orthorhombic Pbca 16.168(3) 20.182(7) 21.656(9) 7066(4) 8 5.015 0.280-0.634 44 824 6188 0.050 3770 329 0.030 0.062 1.053 1.18, -0.79

a Conventional R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(Fo2 - Fc2)2/ ∑w(Fo2)2]1/2; S ) [∑w(Fo2 - Fc2)2/(no. data - no. params)]1/2 for all data.

was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. The highly disordered ether solvent in 10 · Et2O was treated by the SQUEEZE procedure.27 Programs were Nonius COLLECT and associated programs, and SIR97 and SHELXTL for structure solution, refinement, and molecular graphics.28 DFT Calculations. Geometry optimizations on the gas-phase molecules were performed with the Gaussian03 suite of programs (revision C.02)29 on a 224-core Silicon Graphics Altix 4700 computer with 1.6 GHz Montecito Itanium2 processors and 896 (27) Speck, A. L. J. Appl. Crystallogr. 2003, 36, 7. (28) (a) Nonius BV COLLECT; Delft: The Netherlands, 1997; p 2000. (b) Duisberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220. (c) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidon, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (d) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.

Gb of memory, via the EPSRC National Service for Computational Chemistry Software (http://www.nsccs.ac.uk). Optimizations were performed using the B3LYP hybrid functional21 with a Lanl2dz effective core potential basis set22 for Sn and Pb and a 6-31G(d,p) all-electron basis set on the remaining atoms23 [B3LYP/Lanl2dz,631G(d,p); default parameters were used throughout]. Minima were confirmed by the absence of imaginary vibrational frequencies. Natural bond orbital analyses were performed using the NBO 3.0 module of Gaussian03;25 the stabilization energy associated with the B-H · · · E interactions was calculated using the NBODel routine in which the elements affording this interaction were selectively deleted.26 TD-DFT studies were carried out at the B3LYP/ Lanl2dz,6-31G(d,p) level of theory on the gas-phase molecules.

Acknowledgment. The authors are grateful to the EPSRC for support. Supporting Information Available: For 10 · Et2O and 11 · Et2O details of structure determination, atomic coordinates, bond lengths and angles, and displacement parameters in CIF format. For 10a, 10b, 11a, and 11b details of DFT calculations, final atomic coordinates, and energies. This material is available free of charge via the Internet at http://pubs.acs.org. Observed and calculated structure factor details are available from the authors upon request. OM801189H (29) 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.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, J.; Stratmann, R.; Yazyev, R. E.; Austin, O.; Cammi, A. J; Pomelli, R.; Ochterski, C.; Ayala, J. W.; Morokuma, P. Y.; Voth, K; Salvador, G. A.; Dannenberg, P.; Zakrzewski, J. J.; Dapprich, V. G.; Daniels, S.; Strain, A. D.; Farkas, M. C.; Malick, O.; Rabuck, D. K.; Raghavachari, A. D.; Foresman, K.; Ortiz, J. B.; Cui, J. V.; Baboul, Q.; Clifford, A. G.; Cioslowski, S.; Stefanov, J.; Liu, B. B.; Liashenko, G.; Piskorz, A.; Komaromi, P.; Martin, I.; Fox, R. L.; Keith, D. J.; Al-Laham, M. A.; 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.