Activation of P4 by Singlet Silylene (SiH2): A Computational Study

Feb 11, 2009 - Synopsis. Ab initio explorations of the interactions of singlet silylene (SiH2) with tetrahedral P4 (white phosphorus) are reported tha...
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Organometallics 2009, 28, 1289–1294

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Articles Activation of P4 by Singlet Silylene (SiH2): A Computational Study Robert Damrauer* and Sally E. Pusede Chemistry Department, Downtown Campus; UniVersity of Colorado DenVer Campus Box 194, P.O. Box 173364; DenVer, Colorado 80217-3364 ReceiVed October 5, 2008

Ab initio explorations of the interactions of singlet silylene (SiH2) with tetrahedral P4 (white phosphorus) are reported that lead to six stable one-to-one species. Five of these adducts have bonds between silicon and phosphorus: (1) a P-P insertion adduct A, (2) two cyclotriphosphirene rotational isomers Ccis and Ctrans, (3) a structure labeled D, where a hydrogen atom has migrated to phosphorus, and (4) an apical adduct labeled E. Another adduct (B) that has been characterized has no covalent bond between silicon and phosphorus, but SiH2 is bound by ∼5.5 kcal/mol. Characterization of the transition states connecting several of these species leads to a partial understanding of the potential energy surface. The lowest energy species, the C2V insertion adduct A, is substantially more stable than the other adducts. The structures found computationally are compared with the one-to-one insertion adduct of P4 and a nucleophilic (stable) silylene reported by Driess and co-workers. The computational results suggest that non-nucleophilic silylene reactions with P4 might be profitably explored experimentally. Introduction Considerable recent activity has focused on the activation/ functionalization of white phosphorus (P4), particularly in addressing interesting new phosphorus-containing structures and potential catalytic processes.1-4 Whereas most P4 activation studies have explored transition metal territory, some have addressed interactions with main-group metals and nonmetals. Such recent examples demonstrate interesting reactivity of white phosphorus with gallium,5,6 aluminum,7,8 and thallium9 as well as carbon10,11 and silicon12-14 species. Particularly important to this study are the white phosphorus reactions of carbenes10,11 and silylenes.12 Bertrand and co-workers have studied the reactions of stable, nucleophilic carbenes that lead to various phosphorus-containing products.10,11 Some incorporate P4 moieties yielding tetraphosphatrienes;10 others lead to more complex species such as P12 clusters.11 Our recent computational study of the interaction of * [email protected]. (1) Lynam, J. M. Angew. Chem., Int. Ed. 2008, 47, 831–833. (2) Peruzzini, M.; Gonsalvi, L.; Romerosa, A. Chem. Soc. ReV. 2005, 34, 1038–1047. (3) Barbaro, P.; Di Vaira, M.; Peruzzini, M.; Costantini, S. S.; Stoppioni, P. Angew. Chem., Int. Ed. 2008, 47, 4425–4427. (4) Ehses, M.; Romerosa, A.; Peruzzini, M. Top. Curr. Chem. 2002, 220, 107–140. (5) Power, M. B.; Barron, A. R. Angew. Chem., Int. Ed. 1991, 30, 1353– 1354. (6) Uhl, W.; Benter, M. Chem. Commun. 1999, 1999, 771–772. (7) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs, R. Angew. Chem., Int. Ed. 1994, 33, 199–200. (8) Peng, Y.; Fan, H.; Zhu, H.; Roesky, H. W.; Magull, J.; Hughes, C. E. Angew. Chem., Int. Ed. 2004, 43, 3443–3445. (9) Fox, A. R.; Wright, R. J.; Rivard, E.; Power, P. P. Angew. Chem., Int. Ed. 2005, 44, 7729–7733. (10) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2007, 46, 7052–7055.

singlet and triplet methylene (CH2) with P4 predicted that simple singlet carbene reactivity is different from that observed with more complex, stable, nucleophilic carbenes, particularly in the insertion of singlet CH2 into a P-P bond.15 Related to the carbene studies are the reactions reported by Driess and co-workers of the zwitterionic, stabilized silylene 1 with P4.12 Both single P-P and double P-P insertion products (2 and 3) have been isolated and characterized. The single insertion product 2 suggests a different reactivity for silylene 1 than for the stabilized carbenes 4 and 5 studied by Bertrand and co-workers.10,11 Given that Group 14 divalent carbon and silicon analogs display contrasting experimental behavior, a computational study similar to our methylene study15 has been carried out on singlet silylene (SiH2) and P4 to shed further light on these differences. It is reported here.

Computational Methods Preliminary geometry optimizations based on a previously used protocol15 were carried out without symmetry constraints (C1 point group) using density functional theory (DFT) with the B3LYP exchange-correlation functional16-18 and the 6-311++G(3df,3p)19-23 basis. These optimizations located several structures that were clearly of higher symmetry than C1. These were studied at higher symmetry with the results indicated in the body of the paper. Zero

10.1021/om800956h CCC: $40.75  2009 American Chemical Society Publication on Web 02/11/2009

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point energy corrections have been applied to all reported structures. Energies relative to the C2V insertion product A are reported throughout the body of the paper. Explicit energies as well as optimized Cartesian coordinates of all reported structures and tables of important normal modes assigned to irreducible representations can be found in Supporting Information. Frequency calculations that were carried out by standard techniques on the optimized structures indicate that products A-E are local minima (no “imaginary” frequency). The transition states found were connected with their corresponding stable species using intrinsic reaction coordinate (IRC) methodology.24 All computations were carried out using the GAMESS25 suite of programs with MacMolPlot26 visualization of the molecular structures. Because several structures in preliminary studies suggested weak bonding between the SiH2 moiety and P4, the convergence tolerance in GAMESS was increased 10-fold beyond the default (to OPTTOL ) 10-5) for all the reported species. A new feature of the latest version of MacMolPlot (v7.2.1) (http://www.scl.ameslab.gov/∼brett/MacMolPlt/#Downloading) simplifies coordinate transformation from lower to higher symmetry point groups when appropriate. This feature allowed transformation to either Cs or C2V for various optimized structures that initially were obtained without symmetry constraints (C1).

Results and Discussion Silylenes have been investigated thoroughly for many years; their formation, reactivity, isolation, and properties having been studied by many organosilicon workers.27,28 The parent species, SiH2, has offered a particular challenge in terms of its singlettriplet splitting, both in experimental and computational studies. While no controversy has surrounded the assignment of its singlet (1A1) groundstate, considerable time elapsed before general agreement was reached that the energy gap to triplet (3B1) SiH2 is some 21 kcal/mol.29 The corresponding singlettriplet splitting for methylene (CH2) has the triplet state more stable by ∼9 kcal/mol.29 Computational work indicates that the silylene singlet-triplet gap is influenced both by the geometry of the H-Si-H angle and the electronegativity of substituents attached to divalent silicon in substituted silylenes. The singlet and triplet potential curves are affected differently by altering the H-Si-H angle in SiH2, with a singlet-triplet curve crossing at ∼130°. Triplet silylenes tend to be stabilized by strongly electropositive groups on silicon. Considerable effort has focused on preparing substituted silylenes, where the angle between the substituents is expanded.27-31 These efforts culminated in the preparation of the first groundstate triplet silylene [(t-Bu)3Si]2Si by Sekiguchi and co-workers.30 Continuing efforts along related lines recently afforded the alkali metal-substituted silylenes M(t-Bu3Si)2Si (M ) Li and K).31 DFT computations on the lithium-substituted silylene predict that the triplet species is the groundstate for all angles considered.31 Because singlet SiH2 is so much more stable than its triplet counterpart, this computational study exclusively addresses SiH2 interactions with P4 in the singlet manifold. Geometry optimizations in which singlet silylene has been placed in various initial

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orientations and at various distances from P4 have been conducted without symmetry constraints (C1). Six resulting stable structures have been located, of which five (A, Ctrans, Ccis, D, and E) have bonds between silicon and phosphorus. Isomer B has the SiH2 moiety close, but not bonded, to the P4 tetrahedron. These structures were found at B3LYP/6311++G(3df,3p)//B3LYP/6-311++G(3df,3p). Examination of their structural features indicated that several have higher symmetry than C1: A (C2V), Ccis (Cs), D (Cs), and E (Cs). Their relative energies (zero point energy corrected) and structural characteristics are shown in the accompanying representations. Structural Results. Structures A (insertion) (0.0 kcal/mol), Ccis32 (cyclotriphosphirene) (38.7 kcal/mol) (all energies are relative to A), Ctrans (cyclotriphosphirene) (35.8 kcal/mol), and E (SiH2-to-apex) (33.0 kcal/mol) have structural characteristics similar to those of products 1-3 found in the computational study of singlet CH2 and P4, although the relative energies of the analogs are dramatically different.15 The insertion product is the lowest energy structure by a considerable measure in both studies, but in contrast to the CH2 work, where the cyclotriphosphirene (∼22 kcal/mol) and the apical isomer (∼42 kcal/mol) are distinctly different in energy, the analogous structures Ccis, Ctrans, and E are of similar energy with a spread of only ∼6 kcal/mol. Perhaps more to the point, A is separated by at least 33 kcal/mol from these other stable isomers, although only by ∼22 kcal/mol from D. Structure A has C2V symmetry and is quite similar to the analogous CH2 insertion product.15 Its Si-P-P angles (∼85°) and “adjacent” P-P bonds forming the “bicyclic” P4 base (2.26 Å) are nearly equal. These P-P bonds and the P-P joining the phosphorus base (2.18 Å) are the same length as those in the CH2 insertion structure (2.25 and 2.20 Å).15 The Si-P bonds are longer (2.26 Å) as expected, but the H-Si-H angle (∼107°) and the H-C-H angle in the CH2 insertion structure (∼109°) are similar. These values compare favorably with the P-P bond distance in P4 (2.21Å),33 typical Si-P distances (∼2.25 Å),34 and the H-Si-H angle in singlet SiH2 (∼90°).35 Assignment of frequencies (normal modes) to irreducible representations for this C2V structure are tabulated in Supporting Information as are the other higher symmetry species discussed below. Stable structure (B) in which SiH2 is not covalently linked to the P4 cluster has SiH2 bound by ∼5.5 kcal/mol. The distance of the silicon atom to the proximate phosphorus atoms is ∼2.6-2.9 Å, the H-Si-H angle is 95.9°, and the P4 cluster is slightly distorted. The P-P bond lengths range between 2.20 and 2.32 Å with the P-P bond closest to the SiH2 having the longer 2.32 Å bond while the other P-P bonds are close to 2.20 Å. “Non-bonded” complex B, not surprisingly, has a number of low frequency modes, the first few of which are ∼60, 112, and 138 cm-1. The cis and trans C isomers are similar, differing only in the juxtaposition of the SiH2 moiety to the P3 ring. Structure Ccis has a H-Si-H bond angle of ∼112°, Si-P and Pexocyclic-Pcyclic bond distances of 2.08 and 2.26 Å, and ring P-P bond distances of 2.23, 2.23, and 2.02 Å. The corresponding Ctrans has nearly identical bonding characteristics: a H-Si-H bond angle of

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∼112°, Si-P and Pexocyclic-Pcyclic bond distances of 2.08 and 2.24 Å, and ring P-P bond distances of 2.24, 2.24, and 2.01 Å. In both isomers, the shortened Si-P and ring P-P bonds suggest the SidP bond and the PdP of a triphosphirene ring (no isolated triphosphirenes are known).11 A typical SidP bond is 2.07 Å,34 and a PdP is 2.02 Å.33

Structures D and D′, in which a hydrogen atom has migrated from SiH2 to phosphorus, have been found, as described in the next section. The structures might appear to have arisen from the insertion product A by a Si-P bond scission and hydrogen atom transfer, but there is no evidence that D and A are related. Structure D has Si-P and P-P bond lengths ranging from 2.24 to 2.27 Å and 2.25 to 2.25 Å, both fairly typical of such single bonds.

The apical Cs structures E and E′ have been found as described in the next section, with a H-Si-H bond angle of ∼95 °, an Si-P bond distance of 2.38 Å, and ring P-P bond distances ranging between 2.17 and 2.19 (Ptriangular base-Papex) and 2.22 and 2.24 (Ptriangular base-Ptriangular base) Å. The H-Si-H bond angle is similar to that of singlet SiH2, while the Si-P distance is similar to the H2SiPH3 ylid (∼2.33 Å) and longer than

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the SidP bond in H2SiPH (∼2.07 Å).34 The P-P bonds and the slight distortion of the P4 cluster are very similar to those reported in the apical CH2 isomer.15

Transition State Results. Detailed exploration of the potential energy surface has been undertaken to investigate possible connections between the stable species A-E. These results are presented in Figures 1-3. The most extensive reaction coordinate examined (Figure 1) connects the most stable species A with B over an ∼39 kcal/mol (TSAB) and with Ccis over an ∼44 kcal/mol barrier (TSACcis). Structure B is obtained through transition state TSAB by two bond scissions, both of Si-P bonds. To approach TSACcis, two bonds undergo significant change: a Si-P bond and a P-P bond are broken as electrons are redistributed to form the two multiple bonds in Ccis, one in the ring, the other forming the SidP bond. Ccis and Ctrans, in turn, are connected over a very small (∼0.1 kcal/mol) barrier (TScis-trans), which simply involves rotation about a P-P single bond. Not surprisingly, the relief of steric interaction makes Ctrans the more stable isomer by ∼3 kcal/mol.32 Figure 2 illustrates the connection between D and D′ over TSDD′. These species (D, D′, and TSDD’) have Cs, Cs, and C2V symmetry. The 11.4 kcal/mol barrier for the transfer of a hydrogen atom from silicon-to-phosphorus and a Si-P bond scission is difficult to estimate, given that typical Si-H, Si-P, and P-H bond energies are ∼90, ∼70, and ∼81 kcal/mol.34 Figure 3 illustrates the interesting dynamic process of SiH2 hopping from apex-to-apex via a transition state (TSEE′) (∼5

kcal/mol relative to E). This transition state has the silicon atom juxtaposed at the midpoint of a P-P edge, equidistant from the phosphorus atoms on a P-P edge (η2-like) (Cs). The apical attachments of SiH2 to the two phosphorus atoms shown in E and E′ in Figure 3 are attachments to the phosphorusphosphorus bond termini. We are aware of no processes of this type for P4,4 but such fluxional behavior is a common feature of many molecules ranging from ammonia (inversion) to nonrigid transition metal complexes of varying coordination number.36 Fluxional processes (carbonyl and hydride isomerization) of transition metal clusters similarly are well-known.36 (11) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2007, 129, 14180–14181. (12) Xiong, Y.; Yao, S.; Brym, M.; Driess, M. Angew. Chem., Int. Ed. 2007, 46, 4511–4513.

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Figure 1. Species related to insertion structure A (energies in kcal/ mol, zero point energy corrected, relative to A, not drawn to scale).

Figure 3. Reaction coordinate for SiH2 movement from apex-toapex (E to E′) (energies in kcal/mol, zero point energy corrected, relative to A, not drawn to scale).

Figure 2. Reaction coordinate for movement between D to D′ (energies in kcal/mol, zero point energy corrected, relative to A, not drawn to scale).

Conclusions Computational studies examining the interaction of singlet SiH2 and tetrahedral P4 have led to stable adducts that range from the most stable insertion product A to an adduct B in which SiH2 and P4 are bound by ∼5.5 kcal/mol but have no covalent linkage. Structural comparisons reveal similarities among the species A, Ccis, Ctrans, and E and analogous adducts found in the singlet CH2 and P4 study.15 Structures analogous to B and D were not found in the previously reported CH2 and P4 study.15 In the case of B, this may be because carbon is less electropositive than silicon. A series of transition states linking the (13) Lerner, H.-W.; Bolte, M.; Karaghiosoff, K.; Wagner, M. Organometallics 2004, 23, 6073–6076. (14) Chan, W. T. K.; Garcia, F.; Hopkins, A. D.; Martin, L. C.; McPartlin, M.; Wright, D. S. Angew. Chem., Int. Ed. 2007, 46, 3084–3086. (15) Damrauer, R.; Pusede, S. E.; Staton, G. M. Organometallics 2008, 27, 3399–3402. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (17) Stephens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627. (18) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345–351. (19) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650–654. (20) Takahashi, M.; Kawazoe, Y. Organometallics 2005, 24, 2433–2440.

covalently bound adducts A, Ccis, Ctrans, D and D′, and E and E′ help glimpse the possible dynamic processes of such species. A computational study as this suggests the possibility/necessity of further experimental work in this realm. (21) Bauschlicher, C. W., Jr.; Partridge, H. Chem. Phys. Lett. 1995, 240, 533–540. (22) Martin, J. M. L. J. Chem. Phys. 1998, 108, 2791–2800. (23) Damrauer, R.; Noble, A. L. Organometallics 2008, 27, 1707–1715. (24) Jensen, F. Introduction to Computational Chemistry, 2nd ed.; John Wiley & Sons: Chichester, 2007.

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Acknowledgment. R.D. thanks the University of Colorado Denver for providing sabbatical support for this work. (25) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; et al. J. Comput. Chem. 1993, 14, 1347–1363. (26) Bode, B. M.; Gordon, M. S. J. Mol. Graphics Modell. 1998, 16, 133–138. (27) Karni, M.; Kapp, J.; Schleyer, P. v. R.; Apeloig, Y. In The Chemistry of Organic Silicon Compounds (The Chemistry of Functional Groups); Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons, LTD: Chichester, 2001; Vol. 3, pp 1-163. (28) Gaspar, P. P.; West, R. In The Chemistry of Organic Silicon Compounds (The Chemistry of Functional Groups); Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons, LTD: Chichester, 2001;Vol. 3, pp 24632568. (29) Tokitoh, N.; Ando, W. In ReactiVe Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.; Wiley-Interscience: Hoboken, NJ, 2004; pp 651-715. (30) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Tero-Kubota, S. J. Am. Chem. Soc. 2003, 125, 4962–4963. (31) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Gaspar, P. P. J. Am. Chem. Soc. 2008, 130, 426–427. (32) The Ccis and Ctrans designations used in the text are meant to allow the reader to readily keep track of these stable, isomeric conformers. They are not intended to be proper names.

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Various people have helped with technical aspects and are warmly thanked: Professor Niels Damrauer, Professor Mark Gordon, Dr. Mike Schmidt, and Dr. Brett Bode. Supporting Information Available: Tables containing (1) explicit and relative energies for the species reported, (2) Cartesian coordinates of the optimized structures for the species reported, and (3) important normal modes assigned to irreducible representations. This material is available free of charge via the Internet at http://pubs.acs.org. OM800956H

(33) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Pergamon Press: Oxford, 1997. (34) Baboul, A. G.; Schlegel, H. B. J. Am. Chem. Soc. 1996, 118, 8444– 8451. (35) Pak, C.; Rienstra-Kiracofe, C.; Schaefer, H. F., III J. Phys. Chem. 2000, 104, 11232–11242. (36) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999.