Cage Silaphosphanes with a PâSi Dative Bond - Organometallics

Organometallics 2009, 28, 5305–5315 DOI: 10.1021/om900250f

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Cage Silaphosphanes with a PfSi Dative Bond Valery F. Sidorkin* and Evgeniya P. Doronina A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, Favorsky, 1, Irkutsk 664033, Russia Received April 2, 2009

The design of the cage silaphosphanes XSi(-L-)3P (X = Me (a), F (b); L = YCH2Z (4-19), YC6H4 (20-23), C6H4Z (24-27), C10H6 (28), YCH2CH2Z (29-37), YC6H4Z (38-46); Y, Z = O, NH, CH2, S) has been carried out by MP2 and B3LYP methods using 6-31G(d) and 6-311þG(d) basis sets. These species can exist in two forms, namely, with the phosphorus lone electron pair (LEP) directed inside the cage (endo), enabling PfSi coordination, or outside the cage (exo), excluding it. The relative stabilities of the two isomers depend on the properties of the Si and P bridgehead atom surroundings as well as on the size and nature of the side chains (L). Among the 50 studied cage species XSi(-L-)3P possessing the bridgehead Si and P atoms in the 1- and 5-positions, only 3 structures (L=SC6H4 (23a,b), C10H6 (28b)) exist exclusively in the endo form with the pentacoordinate silicon atom. On going from the systems XSi(-L-)3P with the 1,5bridgehead Si and P atoms to the similar 1,6-bridgehead systems, the number of stable endo isomers increases to 16. Remarkable among the latter is the molecule FSi(NHC6H4NH)3P (42b), with a record short PfSi dative bond of covalent character, as follows from the AIM and ELF analyses. The configuration of the bonds around the SiV atom in 42b corresponds to practically an ideal trigonal bipyramid (ηe =100%). Introduction

tions; see the reviews in ref 1 and the references cited therein).

The peculiar structure, unusual spectral characteristics, and reactivity of the atrane species A (substituted 5-azabicyclo[3.3.3]undecanes) have been attributed to the binding interaction of the bridge atoms M and N.1 By the Verkade terminology,1f they do not include pro(tetrahedral M, externally pyramidal N, no MN interaction) and quasi-atrane (tetrahedral M, practically planar N, very weak MN interaction) systems. In molecules A, which can be considered as intramolecular complexes, the chelate effect (steric assistance to the NfM coordination) is most strongly expressed, so that they are more stable than their acyclic, monocyclic, and bicyclic analogues, all other things being equal.1e With all this in mind, these compounds are of growing interest for a wide community of researchers (proved by a huge number of publica*To whom correspondence should be addressed. Tel: þ7-3952426545. Fax: þ7-3952-419346. E-mail: [email protected]. (1) (a) Voronkov, M. G. Pure Appl. Chem. 1969, 19, 399. (b) Sidorkin, V. F.; Pestunovich, V. A.; Voronkov, M. G. Usp. Khim. 1980, 49, 789. Russ. Chem. Rev. (Engl. Transl.) 1980, 49, 414. (c) Voronkov, M. G.; Baryshok, V. P. J. Organomet. Chem. 1982, 239, 199. (d) Hencsei, P.; Parkanyi, L. In Reviews on Silicon, Germanium, Tin and Lead Compounds; Gielen, M., Ed.; Freund: Tel Aviv, Israel, 1985; Vol. 8, p 191. (e) Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; Vol. 1, p 227. (f) Verkade, J. G. Coord. Chem. Rev. 1994, 137, 233. (g) Lukevics, E.; Pudova, O. A. Teor. Eksp. Khim. Khim. Geterotsikl. Soedin. 1996, 1605; Chem. Heterocycl. Compd. (Engl. Transl.) 1996, 32, 1381. (h) Pestunovich, V.; Kirpichenko, S.; Voronkov, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, p 1447. (i) Baukov, Y. I.; Tandura, S. N. The Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; Wiley: Chichester, U.K., 2002; Vol. 2, p 963. r 2009 American Chemical Society

Among atranes A, silatranes (M=Si) have been the most studied by various physical and chemical methods,1a,1c,1d and thanks to those studies, the myth about the biological inertness of silicon compounds was shattered.1a,2 It could be anticipated that substitution of the donor amine fragment (NC3) in A by its phosphine analogue (PC3) would give structures B with no less interesting properties. However, to the best of our knowledge, there have been no data on their synthesis; numerous attempts to synthesize compounds XSi(OCH2CH2)3P with PfSi coordination failed.38 (2) (a) Voronkov, M. G.; Zeltschan, G. I.; Lukevics, E. Silicium and Life; Plenum: New York, 1971; p 330 (Silicium und Leben. Academic-Verlag: Berlin, 1975; p 370). (b) Voronkov, M. G.; Baryshok, V. P. Use of Silatranes for Medicine and Agriculture; Tolstikov, G. A., Ed.; Publishing House of the Siberian Branch of Russian Academy of Sciences: Novosibirsk, Russia, 2005; p 258. (c) Tacke, R.; Wannagat, U. Top. Curr. Chem. 1979, 84, 1. Published on Web 08/26/2009

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Note that there have been very few data on organic derivatives of SiV with a PfSi dative bond and practically no information on their electronic structure. The formation of 1:1 and 1:2 acyclic complexes of tetrahalosilanes SiHal4 (Hal = F, Cl, Br) with Me3P was experimentally proved.3a The authors’ opinion on the relative weakness of these complexes is supported by the calculated energy of formation of the systems SiF4 3 PH3 (2.4 kcal/mol, CCSD(t)/ 6-311þG**//MP2(fc)/6-31G**)3b and SiF4 3 PMe3 (4.9 kcal/ mol, B3LYP/6-311þG**).3c Only two monochelates, 14 and 2,5 have been structurally characterized. In the recently synthesized 1-phospha-4-silabicyclo[2.2.2]octanes 6 of type 3, Si 3 3 3 P coordination is impossible due to the unfavorable spatial configuration of the phosphorus bonds. Is it possible to synthesize tricyclic compounds of pentacoordinate silicon with a PfSi dative bond? What factors favor it? If such compounds do exist, what is the nature of the attractive PfSi interaction? To answer these questions, we have analyzed the structure of a large series of cage compounds with 1,5 (4-28) and 1,6 (29-46) Si and P bridgehead atoms. These species could have a geometry with the phosphorus LEP directed inward the cage (endo), favorable for its donor-acceptor (DA) interaction with the orbitals of the XSiY3 moiety, or with the LEP directed outward the cage (exo), which rules out the possibility of PfSi coordination (pro-atrane).1f

Computational Methods Computations of silaphosphane molecules 4-46 have been performed at the MP2 and B3LYP levels of theory using the 6-31G(d) and 6-311þG(d) basis sets. The correspondence of the (3) (a) Beattie, I. R.; Ozin, G. A. J. Chem. Soc. A 1969, 2267. (b) Schoeller, W. W.; Rozhenko, A. Eur. J. Inorg. Chem. 2000, 375. (c) Holloczki, O.; Nyulaszi, L. Organometallics 2009, 28, 4159. (4) Karch, H. H.; Richter, R.; Witt, E. J. Organomet. Chem. 1996, 521, 185. (5) Toshimitsu, A.; Saeki, T.; Tamao, K. J. Am. Chem. Soc. 2001, 123, 9210. (6) (a) Ochida, A.; Hara, K.; Ito, H.; Sawamura, M. Org. Lett. 2003, 5, 2671. (b) Ochida, A.; Ito, H.; Miyahara, T.; Sawamura, M. Chem. Lett. 2006, 35, 294. (c) Hamasaka, G.; Ochida, A.; Hara, K.; Sawamura, M. Angew. Chem., Int. Ed. 2007, 46, 5381. (d) Tsuji, H.; Inoue, T.; Kaneta, Ya.; Sase, S.; Kawachi, A.; Tamao, K. Organometallics 2006, 25, 6142. (7) (a) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (b) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; 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 03W; Gaussian Inc., Pittsburgh, PA, 2003.

Sidorkin and Doronina structures to minima on the PES was confirmed by positive eigenvalues of the MP2 (for 4-12) and B3LYP Hessians. Relative stabilities (ΔE) of the endo and exo isomers 4-46 were estimated as the difference in their electronic energies (ΔEel) corrected for the zero-point vibrational energies (ΔZPE): ΔE= ΔEel - ΔZPE. The polar solvent effect on the structures of 4-46 was estimated using the polarized continuum model IEF-PCM.7 The calculations were performed using the GAUSSIAN program package.8 The GAMESS electronic structure code9 was employed for performing the resource-consuming harmonic frequency calculations with the same exponents and number of d polarization functions as had been used for the geometry optimizations. The pentacoordinate character of the silicon atom, ηe, in species 28-46 was estimated by the equation10 ηe ¼

P3

120 -1 =3 θ n ¼1 n 1 - 120 -109:5

100% where θn values are the equatorial

angles at the silicon atom. Analysis of the MP2(Full)/6-31G(d) electron distribution in species 4-46 in terms of the atoms in molecules (AIM) theory was performed using the MORPHY 1.0 program.11 The electron localization functions (ELF) of Bekke and Edgecombe12 were calculated at the HF/6-31G(d)//MP2/6-31G(d) level of theory using the TopMod program set13 and visualized with the gOpenMol program.14

Results and Discussion According to the MP2 and B3LYP calculations there are two minima corresponding to the endo and exo isomers on the PES of all the cage silaphosphanes XSi(-L-)3P (4-46). 1. Silaphosphanes Containing Si and P 1,5-Bridgehead Atoms (Derivatives of 1-Sila-5-phosphabicyclo[3.3.3]undecanes). Replacement of the nitrogen atom in silatranes XSi(YCH2CH2)3N by the larger phosphorus atom causes drastic changes in the corresponding potential function: the onewell potential15 becomes a two-well potential.16 Its typical shape for silaphosphanes 4-12 is shown in Figure 1, using (9) 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.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347. (10) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11, 2099. (11) (a) Popelier, P. L. A. Comput. Phys. Commun. 1996, 93, 212. (b) Popelier, P. L. A. Chem. Phys. Lett. 1994, 228, 160. (12) (a) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397. (b) Silvi, B.; Savin, A. Nature 1994, 371, 683. (c) See additional information and the literature on the ELF analysis at a legal part of the Internet website of the Max Planck Institute for the Chemical Physics of Solids (http:// www.cpfs.mpg.de/ELF/). (13) (a) Noury, S.; Krokidis, X.; Fuster, F.; Silvi, B. TopMod package; Universite Pierre et Marie Curie, France, 1997. (b) Noury, S.; Krokidis, X.; Fuster, F.; Silvi, B. Comput. Chem. 1999, 23, 597. (14) (a) Laaksonen, L. J. Mol. Graphics 1992, 10, 33. (b) Bergman, D. L.; Laaksonen, L.; Laaksonen, A. J. Mol. Graphics Modell. 1997, 15, 301. (15) (a) Csonka, G. I.; Hencsei, P. J. Comput. Chem. 1994, 15, 385. (b) Csonka, G. I.; Hencsei, P. J. Comput. Chem. 1996, 17, 767. (c) Schmidt, M. W.; Windus, T. L.; Gordon, M. S. J. Am. Chem. Soc. 1995, 117, 7480. (d) Boggs, J. E.; Chunyang, P.; Pestunovich, V. A.; Sidorkin, V. F. J. Mol. Struct. (THEOCHEM) 1995, 357, 67. (e) Galasso, V. J. Phys. Chem. A 2004, 108, 4497. (f) Sidorkin, V. F.; Belogolova, E. F.; Gordon, M. S.; Lazarevich, M. I.; Lazareva, N. F. Organometallics 2006, 26, 4568. (16) The two-well potential in silaphosphanes 4-12 allows one to consider the structures with short and long Si-P distances as “bond stretching isomers”. See, for example: Rohmer, M.-M.; Benard, M. Chem. Soc. Rev., 2001, 30, 340. Earlier, an analogous fact was mentioned for the series of atranes A by the example of the radical cation of the phosphatrane P(NHCH2CH2)3N. See: Windus, T. L.; Schmidt, M. W.; Gordon, M. S. J. Am. Chem. Soc. 1994, 116, 11449. Nyulaszi, L.; Veszpremi, T.; D’Sa, B. A.; Verkade, J. Inorg. Chem. 1996, 35, 6102. Karpati, T.; Veszpremi, T.; Thirupathi, N.; Liu, X.; Wang, Z.; Ellern, A.; Nyulaszi; Verkade, J. G. J. Am. Chem. Soc. 2006, 128, 1500.

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geometrical bond angles Re in the heterocyclic skeletons of the endo isomers of 4-12 exceeds the exo forms. For Pthat for i Rei)|=89° for the example, in FSi(OCH2CH2)3P i|(R b P structure with the PfSi bond and i|(Rbi - Rei)|=79° for the exo form. The atomization energy difference (ΔEat) for silaphosphane XSi(L)3P isomers in general and for 4-12 in particular is defined at the model level19 by the Si-P bond interaction energy (ESiP) and the difference in the strain energy (Estrain) of the corresponding heterocycles:

ΔEat ≈ -ESiP þ ðEstrain endo - Estrain exo Þ

Figure 1. Dependence of MP2/6-31G(d) relative energies (ΔE) on Si 3 3 3 D distances (dSiD) for the species FSi(OCH2CH2)3D with D= N, P. The zero energy corresponds to the most stable structure.

the example of the molecule FSi(OCH2CH2)3P, a structural analogue of the fluorosilatrane FSi(OCH2CH2)3N. Regardless of the nature of the Si and P bridgehead atom surroundings, silaphosphanes XSi(YCH2Z)3P (4-12) exist exclusively in the exo form (Table 1). According to the MP2 and B3LYP methods the exo isomers are >8 kcal/mol more stable than the corresponding endo isomers in the gas phase and in a polar medium. Note that B3LYP and MP2 methods give ΔE values which are in quantitative agreement with each other. Thus, the reason for failures in the synthesis of the atrane-like compounds 4-12 potentially possessing a pentacoordinate Si atom becomes clear. At first sight, the situation looks rather surprising. Indeed, the Si 3 3 3 P internuclear distance in the endo isomers of 4-12 (see Table 1) is close to the sum of the Si and P covalent radii (∼2.27 A˚),17 which suggests the presence of a PfSi dative bond. The above assumption is confirmed by the AIM analysis of the electron distribution F(r) of the endo isomers of silaphosphanes 4-12, which revealed three ring critical points (RCP’s) (3, þ1) corresponding to the SiYCH2ZP five-membered rings and a bond critical point (BCP) (3, -1) in the Si 3 3 3 P region (Figure 2). Similarly, the exo forms of 4-12 are characterized by three RCP’s (3, þ1) and one cage critical point (CCP) (3, þ3). Molecular graphs for compounds 4-12 are exemplified by that for the molecule FSi(CH2CH2CH2)3P (12b), with the longest Si 3 3 3 P contact. They are presented in Figure 2 only for the SiCCCPCCC moiety (in the endo form it is obviously bicyclic, while in the exo form it is monocyclic). The cage structure of silaphosphanes 4-12 does not allow us to present their entire molecular graph in a pictorial form. The presence of a short Si-P bonding contact in the endo forms of 4-12 cannot make them more stable than the alternative exo structures. One possible reason is unfavorable steric factors for the former. This is proved by an AIM18 analysis, by comparing the sums of deviations of the geometrical bond angles XYZ (Re) from thePangles between the bond paths XY and YZ (Rb) (modulo) i|(Rbi - Rei)| for the isomers under investigation. The total deformation of the (17) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: New York, 1960. (18) (a) Bader, R. F. W. Atoms in Molecules: a Quantum Theory; Clarendon Press: Oxford, U.K., 1990. (b) Howard, S. T.; Platts, J. A.; Alder, R. W. J. Org. Chem. 1995, 60, 6085.

ð1Þ

The values of ΔEat are positive (that is, the exo forms of 4-12 are more stable than the endo forms), when the condition (Estrainendo - Estrainexo) > |ESiP| is fulfilled. The latter inequality can be treated quantitatively to estimate the value of ESiP (different coordination numbers of silicon in the endo and exo forms make it incorrect to use the standard molecular mechanics programs for Estrain calculations). To do so, we employed a well-known procedure,20 namely, the approximation of the calculated dependence of the relative energies ΔE of 4-12 on the Si 3 3 3 P internuclear distance dSiP by the Morse potential V(dSiP) (Figure 3)

!!2 rffiffiffiffiffiffiffi k ðdSiP - dSiP 0 Þ VðdSiP Þ ¼ D 1 - exp 2D

ð2Þ

where D is the dissociation energy of the Si-P bond (neglecting the zero point vibrations), dSiP0 is its equilibrium length, and k is the force constant. Parameters of eq 2 for the molecule FSi(OCH2CH2)3P have been calculated by the nonlinear model of √ the least-squares method to give D = 44.0 kcal/mol and (k/2D)=1.42 A˚-1. These values give an adequate description of the typical plot ΔE=f(dSiP - dSiP0) in the region of the potential well20b,20c of the endo isomer calculated at the MP2/6-31G(d) level (Figure 3). The value of D(SiP) for the polyatomic molecule 6b found by the use of the Morse potential is, for obvious reasons, not very reliable. Nevertheless, it can be assumed to be large enough. Indeed, the value of dSiP for 6b (Table 1) by only ∼0.07 A˚ exceeds the mean length of the ordinary bond SiIV-PIII (dSiP ∼ 2.3 A˚), whose energy is ∼72 kcal/mol.21 In the AIM analysis, to estimate the energy of an A-B contact, eq 3 is used:

EAB ¼ -DðABÞ ¼ Ve =2,

ð3Þ

where Ve is the potential energy density in BCP located in the internuclear region A-B.22 The value of ESiP calculated from (19) Equation 1 has been created from two presuppositions. (1) The deviationPof atomization energy of 4-46, Eat, from their sum Pof bond energies Ebond is caused by the strain energy Estrain (Eat=- Ebond þ Estrain) See, for example: Daschevskii, V. G. Zh. Strukt. Khim. 1968, 9, 289; J. Struct. Chem. (Engl. Transl.) 1968, 9, 226. (2) The difference in P Ebond of the endo and exo forms of silaphosphanes is conditioned by ESiP. See: Voronkov, M. G.; Sidorkin, V. F.; Pestunovich, V. A.; Zelchan, G. I. Khim. Geterotsikl. Soedin. 1975, 5, 715. (20) (a) Rioux, F.; Schmidt, M. W.; Gordon, M. S. Organometallics 1997, 16, 158. (b) Lewin, J. L.; Cramer, C. J. Mol. Pharm. 2004, 1, 128. (c) Olsen, L.; Rydberg, P.; Rod, T. H.; Ryde, U. J. Med. Chem. 2006, 49, 6489. (21) Zachariah, M. R.; Melius, C. F. J. Phys. Chem. A 1997, 101, 913. (22) (a) Espinosa, E.; Molins, E. J. Chem. Phys. 2000, 113, 5686. (b) Zhurova, E. A.; Stash, A. I.; Tsirelson, V. G.; Zhurov, V. V.; Bartashevich, E. V.; Potemkin, V. A.; Pinkerton, A. A. J. Am. Chem. Soc. 2006, 128, 14728. (c) Zhurova, E. A.; Zhurov, V. V.; Pinkerton, A. A. J. Am. Chem. Soc. 2007, 129, 13887. (d) Sidorkin, V. F.; Doronina, E. P.; Chipanina, N. N.; Aksamentova, T. N.; Shainyan, B. A. J. Phys. Chem. A 2008, 112, 6227.

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Table 1. B3LYP/6-31G(d), MP2/6-31G(d) (in Italics), and PCMa B3LYP/6-31G(d) (in Parentheses) Parameters for Si 3 3 3 P Internuclear Distance (dSiP, A˚) and the Relative Stabilitiesb (ΔE, kcal/mol) of the Endo and Exo Isomers of XSi(YCH2Z)3P (4-12) compd

dSiP

no.

X

Y

Z

endo

exo

ΔE

4a 5a 6a 7a 8a 9a 10a 11a 12a 4b 5b 6b 7b 8b 9b 10b 11b 12b

Me Me Me Me Me Me Me Me Me F F F F F F F F F

O O O NH NH NH CH2 CH2 CH2 O O O NH NH NH CH2 CH2 CH2

O NH CH2 O NH CH2 O NH CH2 O NH CH2 O NH CH2 O NH CH2

2.202 2.285 2.410 2.268 2.302 2.389 2.351 2.426 2.536 2.187; 2.189; (2.177) 2.266 2.384; 2.369; (2.325) 2.119 2.266 2.341 2.297 2.358 2.448; 2.419; (2.412)

3.669 3.778 3.764 3.776 3.812 3.879 3.751 3.812 3.895 3.618; 3.621; (3.602) 3.725 3.693; 3.669; (3.658) 3.719 3.756 3.804 3.702 3.764 3.828; 3.787; (3.791)

45.7 42.7 30.5 48.5 37.5 23.3 40.7 29.9 20.8 42.9; 44.0; (39.4) 39.4 26.4; 24.8; (17.1) 42.3 31.3 16.8 30.6 20.8 12.2; 8.1; (10.4)

a

PCM calculations were performed for ε = 47. b The energy of the exo form was taken as the zero point.

Figure 2. Molecular graphs of the endo and exo isomers of silaphosphane FSi(CH2CH2CH2)3P. Bond critical points (BCP’s, (3, -1)) are denoted by solid squares, ring critical points (RCP’s, (3, þ1)) are designated by large open circles, and cage criticalpoints (CCP, (3, þ3)) are marked by asterisks.

eq 3 for 6b (30.8 kcal/mol) is substantial, although it is ∼13 kcal/mol less than that determined by the use of the Morse potential. In the literature, to estimate the sum of the M-N interaction energy and the steric strain in atranes A, the corresponding isodesmic reactions are used.15e,20a For the species FSi(OCH2CH2)3D (D=N, P), it can be written as follows:

FSi½-OðCH2 Þ2 -3 Dþ 3MeOH f FSi½-OMe3 þ D½-ðCH2 Þ2 OH3 From the MP2 calculations (with ZPE taken into account) the silaphosphane FSi(OCH2CH2)3P is unstable with respect to methanolysis: ΔEisodesmic = -ESiP þ Estrain = 38.8 kcal/ mol.23 In contrast, the fluorosilatrane FSi(OCH2CH2)3N is stable, but ΔEisodesmic is not as great: ΔEisodesmic =-ESiN þ Estrain =-1.7 kcal/mol. These estimates of the ESiP value of 6b(endo) of ∼44 kcal/mol (from the Morse potential) and ∼31 kcal/mol (from the AIM analysis) substantially exceed those known from the literature estimates of the ESiN energy (23) Obviously, the MP2 value of ΔEisodesmic=15 kcal/mol for the exo form of 6b determines the value of its steric strain Estrain.

Figure 3. Morse fit of the MP2/6-31G(d) Si-P stretching potential of the endo isomer of FSi(OCH2CH2)3P in the neighborhood of the minimum.

of silatranes varying within 13-22 kcal/mol1h (the value obtained from the Morse potential was ∼19 kcal/mol).20a Therefore, the values found for ΔEisodesmic for the species FSi(OCH2CH2)3D point to a much higher strain of the silaphosphane backbone (D = P) in comparison with silatrane (D=N). Note that according to the MP2 calculations the intermolecular complex FSi(OMe)3 3 PMe3 (as well as SiF4 3 PH33b and SiF4 3 PMe3)3c is weak. The energy of formation of 6b which, according to ref 20a can be considered as an estimate of the dative SiP interaction in the molecule in the absence of ring strains, is as low as 2.2 kcal/mol: that is, substantially less than the above values of 44 kcal/mol (from the Morse potential) and 31 kcal/mol (from the AIM analysis).

FSiðOMeÞ3 þPMe3 f FSiðOMeÞ3 3 PMe3 This is not surprising, since the internuclear distance Si-P in FSi(OMe)3 3 PMe3 at 1.51 A˚ exceeds that in the endo form of the intramolecular complex 6b, FSi(OCH2CH2)3P (Table 1), in which the P donor and the Si acceptor centers are sterically forced to stay close together.

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Figure 4. B3LYP/6-31G(d) (values in Roman type) and MP2/6-31G(d) (values in boldface type) optimized geometry of the endo isomers of FSi(C10H6)3P and XSi(SC6H4)3P. Bond distances are given in A˚ and bond angles in deg.

Figure 5. Map of the Laplacian of the electron density and properties of the Si-P bond critical points of the simplest silaphosphane H3SiPH2 and of the endo isomers of the molecules XSi(SC6H4)3P and FSi(C10H6)3P. The cross sections in the SiPH and the YSiP planes are given. The diagrams are superimposed with the selected bonds. Bond critical points (BCP, (3, -1)) are denoted by solid squares, and ring critical points (RCP, (3, þ1)) are designated by large open cycles. Dashed lines correspond to r2F(r) > 0 (regions of charge depletion) and solid lines to r2F(r) < 0 (regions of charge concentration). The contour values in e/a05 are (0.002, (0.004, and (0.008.

Hence, in the endo isomers of silaphosphanes XSi(YCH2Z)3P a short PfSi dative contact is formed, but, at the same time, they are highly strained and, thus, thermodynamically less stable than the alternative exo isomers of 4-12 isomers. Then, a key question arises: could it become an insurmountable obstacle for the cage compounds XSi(-L-)3P with the Si and P 1,5-bridgehead atoms to exist exclusively in the endo form with the pentacoordinate silicon atom? From the results presented in Table 1 it is easy to follow the trend of a decrease in the ΔE value with lengthening of the equatorial Si-Y or P-Z bonds in 4-12 at given Z and Y, respectively. On this basis one should expect that the use of the sulfur atom as Y and/or Z would be sterically favorable for stabilization of the endo isomers of silaphosphanes XSi(SCH2Z)3P (Z = O, 13; Z = NH, 14; Z = CH2, 15), XSi(YCH2S)3P (Y = O, 16; Y = NH, 17; Y = CH2, 18), and XSi(SCH2S)3P (19). What actually occurs is that only the endo form of the species FSi(SCH2CH2)3P (15b) is 1 kcal/ mol more advantageous than its exo form at the B3LYP level of theory. For all the remaining species 13, 14, and 16-19 there is a significant energetic advantage (>5 kcal/mol) of the exo form possessing the tetracoordinate Si atom (see the Supporting Information).

A possible approach to a fundamental change in the value of the difference Estrainendo - Estrainexo, and thereby in the relative stabilities of the isomers (see eq 1) of the species XSi(-L-)3P with 1,5-bridgehead Si and P atoms, is an increase of the steric rigidity of the L side chains. In practice, this may be achieved by incorporating a benzene ring into L=YCH2CH2, CH2CH2Z or by using a 1,8-naphthylidene fragment, L=C10H6. Benzoannelation of 10 structures of 6 and 9-12 with the S and P atoms surrounded by second-period atoms results in a decrease in the ΔE value by 2-21 kcal/mol (see Table 1 and the Supporting Information). However, only one of them, namely FSi(NHC6H4)3P (21b), is relatively stable in the endo form. In this species |E endo| exceeds |E exo| by 4.6 kcal/mol at the B3LYP level of theory but only by 0.4 kcal/mol at the MP2 level. The benzoannelation effect is more strongly expressed when a benzene ring is incorporated into the chain SCH2CH2 (see Figure 4). Indeed, the species XSi(SC6H4)3P (X = Me, 23a; X = F, 23b) exist exclusively in the form with the orientation of the phosphorus atom LEP inside their cavity (23a, ΔE=Eendo - Eexo=-6.7 (B3LYP) kcal/mol; 23b, ΔE= -14.0 (B3LYP) and -9.0 (MP2) kcal/mol). The transition from XSi(SCH2CH2)3P to XSi(SC6H4)3P results in a marked shortening of the Si-P contact (for example, by 0.06

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Table 2. B3LYP/6-31G(d) (in Roman Type), MP2/6-31G(d) (in Italic Type), and PCMa B3LYP/6-31G(d) (in Parentheses) Structural Parameters of the XSiY3P Coordination Knot and the Degree of Pentacoordination of the Silicon Atom (ηe, %) for Stable Endo Isomers of Silaphosphanes XSi(Y-CH2CH2-Z)3P and Their Relative Stabilitiesb (ΔE, kcal/mol) bond length (A˚)

compd

valence angle (deg)

no.

X

Y

Z

Si-P

Si-X

Si-Y

X-Si-Y

Y-Si-Y

ηe

-ΔE

31a 33a 34a 36a 37a 30b 31b

Me Me Me Me Me F F

O NH NH CH2 CH2 O O

CH2 NH CH2 NH CH2 NH CH2

NH

NH

NH

CH2

92; 91; (97)

7.3; 12.2; (5.5)

36b 37b

F F

CH2 CH2

NH CH2

113.8 116.4 115.5 116.4 116.3 116.7 116.3; 116.0; (119.3) 119.3; 119.2; (119.6) 119.2; 119.1; (119.7) 118.5 118.4

7.8; 8.8; (8.6)

F

104.6 101.2 102.5 101.0 101.2 100.7 101.3; 101.8; (94.3) 94.8; 95.2; (92.6) 95.2; 95.5; (93.6) 97.2 97.3

93; 92; (96)

34b

1.670 1.765 1.761 1.933 1.936 1.659 1.656; 1.663; (1.678) 1.762; 1.762; (1.767) 1.765; 1.767; (1.768) 1.918 1.923

4.2 3.1 3.0 8.9 12.9 8.7 7.3; 10.0;c (11.7)

F

1.873 1.916 1.913 1.924 1.927 1.614 1.617; 1.623; (1.660) 1.668; 1.676; (1.695) 1.673; 1.681; (1.700) 1.669 1.671

41 66 57 66 65 69 65; 62; (93)

33b

3.006 2.849 2.937 2.973 2.950 2.799 2.842; 2.822; (2.536) 2.563; 2.529; (2.514) 2.548; 2.488; (2.510) 2.780 2.733

86 85

13.3 17.6

PCM calculations were performed for ε = 47. b The energy of the exo form was taken as zero. c ΔE values were calculated at the MP2/6-31G(d) þ ZPE(B3LYP/6-31G(d)) level of theory. a

(B3LYP) and 0.08 A˚ (MP2) for X=F). In contrast, for the silatranes XSi(OCH2CH2)3N the benzoannelation effect causes Si-N bond lengthening.24 According to the X-ray data, the Si-N contact in PhSi(OC6H4)3N is greater by 0.15 A˚ than that in PhSi(OCH2CH2)3N. The presence of the three 1,8-naphthylidene C10H6 moieties as L side chains, as opposed to silaphosphanes XSi(SC6H4)3P, may have different effects. Whereas FSi(C10H6)3P (28b) exists in the endo form in the isolated state and in polar medium (ΔE= -8.9 kcal/mol by B3LYP/6-31G(d), -9.6 kcal/mol by MP2/631G(d)//B3LYP/6-31G(d), -10.5 kcal/mol by PCM B3LYP/ 6-31G(d), ε=47), MeSi(C10H6)3P (28a), in contrast, exists in the exo form (ΔE=2.2 kcal/mol by B3LYP/6-31G(d)). The degree of pentacoordination of the silicon atom, ηe, in the molecules FSi(C10H6)3P and XSi(SC6H4)3P is very hig, and the SiP internuclear distances only slightly exceed the sum of the Si and P covalent radii of 2.27 A˚ (Figure 4). It is not surprising, then, that an AIM analysis (see Figure 5) of the electronic distribution F(r) revealed a (3, -1) bond critical point in the Si 3 3 3 P region and three (3, þ1) ring critical points in addition to those of the 1,8-naphthylidene fragment in FSi(C10H6)3P25 or benzyl fragment in XSi(SC6H4)3P. Judged from the values of F(rc), the sign and the value of the Laplacian r2F(rc), and the density of the electron energy E(rc) in BCP(SiP) as well as their location at the border of the region of charge concentration, the SiP contact in molecules 23a,b as well as in the simplest silaphosphane H3SiPH2 can be definitely described as covalent, while that in 28b can be described as polar covalent (for a more detailed discussion of the AIM classification of the bonds see part 2 below). The ordinary SiIV-PIII bond in the molecule H3SiPH2 (dSiP = 2.279 A˚ (B3LYP/6-31G(d)) and 2.257 A˚ (MP2/ 6-31G(d))) is ∼0.1 A˚ shorter than the dative bonds SiV-PIV (24) Boer, P.; Turley, J. W.; Flynn, J. J. J. Am. Chem. Soc. 1968, 90, 5102. (25) Note the possible formation of a CH 3 3 3 F hydrogen bond in endo and exo isomers of 28b (Figure 5), as indicated by the presence of (3, -1) critical point properties typical for weak (1-4 kcal/mol) H bonds in the H 3 3 3 F internuclear region for 28b(endo) (F(rc)=0.014 au, r2F(rc)=0.064 au, E(rc)=0.002 au) and for 28b(exo) (F(rc)=0.015 au, r2F(rc)=0.068 au, E(rc)=0.002 au). See: Koch, U.; Popelier, P. L. A. J. Phys. Chem. 1995, 39, 9747. Mallinson, P. R.; Smith, G. T.; Wilson, C. C.; Grech, E.; Wozniak, K. J. Am. Chem. Soc. 2003, 125, 4259. Grabovski, S. J. J. Phys. Org. Chem. 2004, 17, 18.

Figure 6. B3LYP/6-31G(d) (Roman type), MP2/6-31G(d) (boldface type), and B3LYP/6-311þG(d) (italic type) optimized geometries of compounds with the longest (FSi(OCH2CH2CH2)3P (31b)) and shortest (FSi(NHCH2CH2CH2)3P (34b)) Si-P internuclear distances. Bond distances are given in A˚ and bond angles in deg.

Figure 7. Molecular graph for the endo isomer FSi(OCH2CH2CH2)3P (31b). Bond critical points (BCP(3, -1)) are denoted by solid squares, and ring critical points RCP (3, þ1) are designated by large open circles.

in structures 23a,b and 28b. Therefore, it is not surprising (see Figure 5) that in BCP(SiIVPIII) the values of F(r) and E(rc) are notably higher than in BCP(SiVPIV). The energies of interaction SiP (ESiP) for compounds given in Figure 5, calculated from eq 3 , decrease in the order: H3SiPH2 (37.5 kcal/mol)>23b

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Table 3. Electron Density (G(rc), e/A˚3), Laplacian (r2G(rc), e/A˚5), and Electron Energy Density (E(rc), hartree/A˚3) at Selected (3, -1) Bond Critical Points of Endo Isomers of the Silaphosphanes XSi(YCH2CH2Z)3P compd

SiP

SiX

SiY

no.

X

Y

Z

F(rc)

r F(rc)

E(rc)

F(rc)

r F(rc)

E(rc)

F(rc)

r F(rc)

E(rc)

31a 33a 34a 36a 37a 30b 31b 33b 34b 36b 37b

Me Me Me Me Me F F F F F F

O NH NH CH2 CH2 O O NH NH CH2 CH2

CH2 NH CH2 NH CH2 NH CH2 NH CH2 NH CH2

0.148 0.187 0.160 0.162 0.168 0.214 0.198 0.328 0.330 0.221 0.239

1.012 0.689 0.790 0.806 0.785 0.735 0.797 0.091 0.251 0.569 0.507

-0.01 -0.04 -0.02 -0.02 -0.02 -0.05 -0.04 -0.16 -0.16 -0.06 -0.07

0.821 0.746 0.749 0.727 0.724 0.832 0.825 0.726 0.717 0.720 0.716

7.436 6.686 6.771 6.511 6.461 26.136 25.764 20.518 20.025 20.523 20.264

-0.48 -0.42 -0.42 -0.40 -0.39 -0.07 -0.07 -0.08 -0.09 -0.08 -0.08

0.812 0.781 0.787 0.727 0.725 0.846 0.853 0.798 0.798 0.757 0.753

20.909 14.306 14.552 5.426 5.346 21.853 22.132 14.443 14.246 5.635 5.518

-0.17 -0.28 -0.28 -0.41 -0.41 -0.19 -0.19 -0.30 -0.30 -0.44 -0.44

2

(32.7 kcal/mol)>23a (31 kcal/mol)>28b (29.2 kcal/mol). Note that the values of ESiP determined from the AIM analysis for molecules H3SiPH2, 23b, 23a, 28b, and 6b are of the same order.26 Therefore, the question of the possibility of existence of the cage compounds XSi(-L-)3P with the 1,5-bridgehead Si and P atoms exclusively in the form with a pentacoordinate silicon atom (vide supra) can be answered positively. 2. Silaphosphanes Containing Si and P 1,6-Bridgehead Atoms (Derivatives of 1-Sila-6-phosphabicyclo[4.4.4]tetradecanes). The presence of an additional methylene group (as compared with 4-12) in the side L-chains of compounds 29-37 dramatically changes the ratio of the strain energies in their endo exo forms. This follows from comparison of the P and i i i|(Rb - Re )| values which characterize the angle strain of the isomers 4-12 and 29-37 with the same surroundings of P the Si and P atoms: for FSi(OCH2CH2)3P (6b) i|(Rbi - Rei)| =89° (endo) and 79° (exo); for FSi(OCH2CH2CH2)3P (31b) P i i i|(Rb - Re )|= 72° (endo) and 88° (exo). This example shows that steric factors can generally be either favorable for stabilization of the endo isomers of the cage silaphosphanes (Estrainendo < Estrainexo) or unfavorable (Estrainendo > Estrainexo). So, taking into account the approximate eq 1, one could expect that silaphosphanes XSi(Y-CH2CH2-Z)3P with the phosphorus LEP directed inward the cavity would be thermodynamically preferable over isomers with the LEP directed outward. Indeed, according to the B3LYP and MP2 data this is true for the majority of compounds of the series 29-37 (Table 2). Among the 18 molecules studied, only 7 molecules are exceptions to this rule. Six of them (isomers of 29, 32, and 35) possess donor P centers surrounded by oxygen atoms that are the most electronegative among all the Z substituents used, and the seventh one, 30a, has Z=NH and Y=O. The energy advantages of the exo isomers of these molecules over their endo analogues are ∼5-14 kcal/mol, depending on the Si and P surroundings. It should be emphasized that the relative stabilities of the endo isomers of compounds 29-37 may either increase or slightly decrease on going from the isolated state to solutions in highly polar media (Table 2). The SiP contacts are much shorter (by 0.3-0.6 A˚ depending on the nature of groups (atoms) X, Y, Z (Tables 1 and 2)) in the highly strained structures 4(endo)-12(endo) than those for the low strained silaphosphanes 29(endo)37(endo). In contrast, the Si-Y and P-Z equatorial bonds in the endo isomers of XSi(Y-CH2-Z)3P and (26) The AIM estimate of ESiP for the molecule H3SiPH2 is ∼34 kcal/ mol lower than that known from the literature21 mean energy of the ordinary bond SiIV-PIII.

2

2

XSi(Y-CH2CH2-Z)3P do not differ as much. The former are shortened by 0.07 A˚ (in average), and the latter are lengthened by 0.04 A˚ with the size of the L chains approaching the standard values (Table 2): Si-O=1.62 A˚; Si-N=1.74 A˚; Si-C=1.91 A˚;27a P-O=1.66 A˚;27b P-N=1.73 A˚;27c P-C= 1.86 A˚.27c The dSiP values for compounds 29(endo)-37(endo) are intermediate between the sum of van der Waals radii for Si and P (rvdw(Si) þ rvdw(P) = 3.9 A˚)28 and the sum of their covalent radii (rcov(Si) þ rcov(P)=2.27 A˚)17 (Table 2) and fall in the range defined by the following equations: 3.9 - dSiP ≈ 1-1.5 A˚ and dSiP - 2.27 ≈ 0.2-0.8 A˚. In magnitude, such differences are typical for the classic Si-O coordination contacts in the O-Si monochelates29 and for the Si-N contacts in oxysilatranes30 (with regard to the van der Waals and covalent radii of O and N atoms). Note that there is a substantial effect of the σ-donating power of the axial substituent X at Si atom on the Si 3 3 3 P internuclear distance in molecules 29(endo)-37(endo). Indeed, the substitution of fluorine for the methyl group in these species results in a decrease in their dSiP values by ∼0.2-0.4 A˚ depending on the Si and P surroundings (Table 2). The values of the Si-Y and P-Z equatorial bonds therewith are changed only by ∼0.01 A˚. The same trans effect of X substituent is well-known in structural chemistry of hypervalent silicon compounds.31 Formally, an increase of electronegativity of Y and its decrease of Z in silaphosphanes XSi(Y-CH2CH2-Z)3P should favor the increase of the acceptor properties of Si atom and the donor properties of (27) (a) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, p 181. (b) Vijjulatha, M.; Swamy, K. C. K.; Vittal, J. J.; Koh, L. L. Polyhedron 1999, 18, 2249. (c) Zubiri, M. R. I.; Milton, H. L.; Cole-Hamilton, D. J.; Slawin, A. M. Z.; Woollins, J. D. Polyhedron 2004, 23, 693. (28) Bondi, A. J. Phys. Chem. 1964, 68, 441. (29) (a) Kost, D.; Kalikhman, I. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, p 1339. (b) Belogolova, E. F.; Sidorkin, V. F. J. Mol. Struct. (THEOCHEM) 2004, 668, 139. (30) (a) Shishkov, I. F.; Khristenko, L. V.; Rudakov, F. M.; Golubinskii, A. B.; Vilkov, L. V.; Karlov, S. S.; Zaitseva, G. S.; Samdal, S. Struct. Chem. 2004, 15, 11. (b) Shen, Q.; Hilderbrandt, R. L. J. Mol. Struct. 1980, 64, 257. (c) Forga0 cs, G.; Kolonits, M.; Hargittai, I. Struct. Chem. 1990, 1, 245. (d) Yoshikawa, A.; Gordon, M. S.; Sidorkin, V. F.; Pestunovich, V. A. Organometallics 2001, 20, 927. (e) Kobayashi, J.; Goto, K.; Kawashima, T.; Schmidt, M. W.; Nagase, S. J. Am. Chem. Soc. 2002, 124, 3703. (f) Trofimov, A. B.; Zakrzewski, V. G.; Dolgounitcheva, O.; Ortiz, J. V.; Sidorkin, V. F.; Belogolova, E. F.; Belogolov, M.; Pestunovich, V. A. J. Am. Chem. Soc. 2005, 127, 986. (31) (a) Voronkov, M. G.; Pestunovich, V. A.; Baukov, Yu. I. Metalloorg. Khim. 1991, 4, 1210; J. Organomet. Chem. USSR (Engl. Transl.) 1991, 4, 593. (b) Pestunovich, V. A.; Sidorkin, V. F.; Voronkov, M. G. In Progress in Organosilicon Chemistry; Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach Science: New York, 1995, 69.

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Figure 8. Maps of the Laplacian of the electron density (top) and ELF (bottom) for the axial XSiP moiety for endo isomers of silaphosphanes MeSi(NHCH2CH2NH)3P (33a) and FSi(OCH2CH2CH2)3P (31b) with mainly ionic and FSi(NHCH2CH2CH2)3P (34b) with mainly covalent nature of the SiP contact. For AIM, the cross-section in the YSiP plane is given. The diagrams are superimposed with the selected bond paths. Critical points (3, -1) are denoted by solid squares. Dashed lines correspond to r2F(r) > 0 (regions of charge depletion) and solid lines to r2F(r) < 0 (regions of charge concentration). The contour values in e/a05 are (0.002, (0.004, and ( 0.008. Legend for ELF = 0.83: (red) core domains; (green) disynaptic bonding domains; (blue) monosynaptic domains; (light green) disynaptic CH bonding domains. Since hydrogen atoms have no core electrons, CH disynaptic basins surround the hydrogen nuclei.

P atom. Therefore, the shortest SiP contact should be expected for Y = O and Z = CH2. In fact, however, it is realized for Y=NH and Z=NH, CH2 (Table 2). This may be due to a strong but hardly predictable steric effect on the geometrical parameters of the cage structures. Also noteworthy is a considerable difference in the width of intervals of the Si-P internuclear distance variation, ΔdSiP, in silaphosphanes 4-12 and 29-37 (see Tables 1 and 2) at the uniform variation of the nearest surroundings of the bridgehead atoms Si and P (for example: ΔdSiP = dSiP[FSi(CH2CH2CH2)3P] dSiP[FSi(OCH2CH2)3P] = 0.26 A˚, whereas ΔdSiP = dSiP[FSi(CH 2 CH 2 CH 2 CH 2 )3 P] - d SiP [FSi(OCH 2 CH 2 CH 2 )3 P] = 0.04 A˚). The difference in the values of ΔdSiP in 4-12 and 29-37 would not be as great if the electronic effects of the equatorial Y, Z substituents on ΔdSiP were dominating. The spatial structure of the XSiY3P coordination knot in compounds 29(endo)-37(endo) corresponds to a distorted trigonal bipyramid (TBP) (Figure 6 and Table 2). Indeed, the values of the X-Si-Y (∼95-104°) and YSi-Y (∼114-119°) bond angles in it are intermediate between those in the ideal TBP (90, 120°) and the tetrahedron (109.5°). This can be reasonably explained by the assumption of the existence of 1,6 attractive interaction between the Si and P bridgehead atoms in the endo isomers of molecules 29-37. Such an assumption is supported by the data of the

Table 4. B3LYP/6-31G(d) Structural Parameters for the XSiY3P Coordination Knot, Degree of Pentacoordination of the Silicon Atom (ηe, %) in Stable Endo Isomers of Silaphosphanes XSi(YC6H4Z)3P, and Their Relative Stabilitya (ΔE, kcal/mol) bond length (A˚)

compd no. X 42a 42b 43b 45b 46b a

Me F F F F

Y

Z

NH NH NH CH2 CH2

NH NH CH2 NH CH2

valence angle (deg)

Si-P Si-X Si-Y X-Si-Y Y-Si-Y ηe -ΔE 2.357 2.312 2.351 2.648 2.642

1.939 1.664 1.672 1.657 1.666

1.803 93.6 1.781 91.0 1.787 90.5 1.926 100.5 1.935 98.4

119.6 120.0 120.0 116.7 117.9

96 100 100 69 80

5.3 12.7 7.1 6.0 4.2

The energy of the exo form is taken to be zero.

AIM analysis of their electron distribution F(r), namely, by location of the bond critical points BCP (3, -1) in the Si 3 3 3 P internuclear region. The tricyclic structure of molecules 29-37 possessing the SiP coordination contact is unambiguously confirmed by the three ring critical points RCP (3, þ1) for the six-membered SiYCCZP moieties having conformations close to a chair type. In their structures the four atoms Si, Y, Z, Cβ (relative to P) lie almost in one plane while P and Cγ atoms deviate from it by ∼0.2-0.7 A˚. A typical molecular graph for structures 29(endo)-37(endo) is presented in Figure 7 (it is truncated for the reasons pointed out in part 1 above).

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Figure 9. B3LYP/6-31G(d) (Roman type), MP2/6-31G(d) (boldface type), and B3LYP/6-311þG(d) (italic type) optimized geometries of silaphosphanes MeSi(NHC6H4NH)3P, FSi(NHC6H4NH)3P, FSi(CH2C6H4NH)3P, and FSi[CH2(ButC6H3)NH]3P. Bond distances are given in A˚ and bond angles in deg.

The XSi, SiY, and SiP silicon bonds in all the considered structures 29(endo)-37(endo) correspond formally to intermediate types of interatomic interactions, as judged from the properties of their BCP’s (3, -1) (Table 3). According to the sign of the Laplacian of the electron density r2F(rc), they may be considered as ionic, whereas by the sign of the electron energy density E(rc), they are covalent.32 Using the quantitative Cremer-Kraka criterion,33 the classification of the bonds at the silicon atom becomes more definite. According to it, the axial XSi and equatorial SiY bonds are polar covalent in nature in all the compounds 29(endo)-37(endo). As to the nature of the SiP coordination contact, it is determined by the properties of the Si and P bridgehead atom surroundings (Figure 8). For all the stable endo isomers of the methyl derivatives MeSi(YCH2CH2Z)3P and for the (32) Analogous AIM characteristics (r2F(rc)>0; E(rc)0.2 e/A˚3; |E(rc)|>0.04 hartree/ A˚3. See: (a) Olsson, L.; Ottosson, C.-H.; Cremer, D. J. Am. Chem. Soc. 1995, 117, 7460. (b) Ottosson, C.-H.; Cremer, D. Organometallics 1996, 15, 5309.

molecule FSi(OCH2CH2CH2)3P (31b), the SiP contact can be described as predominantly ionic. This is confirmed by the F(rc) and |E(rc)| values (Table 3) at the corresponding BCP(SiP) and by their considerable remoteness from the charge concentration region (Figure 8). The compounds containing an ionic Si-P bond in the series 29(endo)-37(endo) are characterized by relatively long Si 3 3 3 P internuclear distances and relatively low ηe values (Table 2). Judging by the properties of BCP(SiP) and their positions at the boundary of the charge concentration region (Figure 8), the SiP coordination in the remaining five silaphosphanes 30b(endo), 33b(endo), 34b(endo), 36b(endo), and 37b(endo) is polar covalent. With this, the bonds with maximum covalent contributions are, apparently, the SiP dative bonds in the two molecules 33b and 34b with shorter SiP contacts (as compared to the other three structures) and a greater degree of pentacoordination ηe of the silicon atom (see F(rc) and |E(rc)| in Table 3). The question about the nature of the Si-F bond remains unclear in the AIM analysis. Indeed, the values of F(rc) > 0.2 e/A˚3 and |E(rc)| > 0.04 hartree/A˚3 at the BCP(SiF) of the Si-F bonds in the fluorine derivatives FSi(YCH2CH2Z)3P (see Table 3) suggest that, according to the Cremer-Kraka criterion,33 they are covalent. At the same time, judging from the positions of the BCP(SiF), which is located rather far from the negative charge concentration region (Figure 8), it should be described as ionic. This is a principal distinction of this bond from the Si-C, Si-N, and Si-O contacts considered above. In this connection it is pertinent to mention about the continuous discussion concerning the problem of

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use of the AIM analysis for elucidating fine details of the bonding in molecules.34 The above conclusions on the bond nature in the XSiY3P coordination knot in the stable endo isomers of 30b, 31, 33, 34, 36, and 37 made from the AIM analysis are, in general, in line with the data of the ELF analysis. A strong point of the latter analysis is the explicit and unambiguous interpretation of bonding, especially of the intermediate type (r2F(rc) > 0; E(rc) < 0).35 In the ELF approach, to all the bonds, defined by the AIM analysis as polar covalent ones, there correspond disynaptic basins V(X,Si), V(Si,Y), and V(Si,P) (Figure 8) with the values of population (N) falling into the interval 1.8-2.3 e (except FSi). This is an unambiguous indication of their predominant covalent character. The N value for the valence basin V(F,Si) is ∼0.9 e (less than 1); therefore, the dominance of the ionic component is predicted by ELF analysis35c for the FSi bonds in structures 29b-37b. The Si-P contacts in the methyl derivatives MeSi(YCH2CH2Z)3P and in the molecule FSi(OCH2CH2CH2)3P (31b) are described by monosynaptic valence domains V(P) with the values of N ≈ 2 e and, thus, have ionic character, as in the AIM analysis (Figure 8). The ELF basins of X-Si, Si-Y, and Si-P bonds have small fluctuations of population, δ (δ= σ2/N < 1, where σ2 is mean square deviation); thus, they are rather well localized.35c Structural parameters of silaphosphanes XSi(YCH2CH2Z)3P, calculated by B3LYP and by laborious MP2 methods, agree closely with each other (see Table 2 and Figure 6). The main discrepancy of ∼0.06 A˚ is observed for the Si-P coordination contact. As for the rest of the bond distances, the discrepancy does not exceed 0.02 A˚. The maximum disagreement for bond angles is ∼3°. The geometry of compounds 29-37 is slightly affected by the extension of the basis set from 6-31G(d) to 6-311þG(d) at the B3LYP level of theory (Figure 6). Actually, the changes in the bond distances and bond angles do not exceed ∼0.03 A˚ and ∼4°, respectively. It is known from numerous experimental and theoretical studies1c,15,29,36 that geometrical (as well as spectral) characteristics of the axial DfSiX 3c-4e bond (D=O, N) in the intramolecular trigonal-bipyramidal SiV complexes are hypersensitive toward medium effects. This is largely true also for silaphosphanes 29-37. For instance, when the isolated molecule 31b, having a rather long (weak) Si-P coordination contact, is placed into a polar solvent, its Si-P contact becomes essentially shorter (by ∼0.31 A˚!). In contrast, the relatively short Si-P distance in the gas phase for 34b is shortened only by ∼0.04 A˚ under the influence of a polar medium (Table 2). 3. Benzo Analogues of Silaphosphanes XSi(YCH2CH2Z)3P. On the basis of the works by Holmes et al. on silatranes (34) (a) Cioslowski, J.; Mixon, S. T. J. Am. Chem. Soc. 1992, 114, 4382. (b) Matta, C. F.; Hernandez-Trujillo, J.; Tang, T.-H.; Bader, R. F. W. Chem. Eur. J. 2003, 9, 1940. (c) Haaland, A.; Shorokhov, D. J.; Tverdova, N. V. Chem. Eur. J. 2004, 10, 4416. (d) Poater, J.; Sola, M.; Bickelhaupt, F. M. Chem. Eur. J. 2006, 12, 2889. (e) Bader, R. F. W. Chem. Eur. J. 2006, 12, 2896. (f) Poater, J.; Sola, M.; Bickelhaupt, F. M. Chem. Eur. J. 2006, 12, 2902. (g) Farrugia, L. J.; Evans, C.; Tegel, M. J. Phys. Chem. A 2006, 110, 7952. (35) (a) Grutzmacher, H.; Fassler, T. F. Chem. Eur. J. 2000, 6, 2317. (b) Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D.; Hiberty, P. C. Chem. Eur. J. 2005, 11, 6358. (36) (a) Sidorkin, V. F.; Belogolova, E. F.; Pestunovich, V. A. J. Mol. Struct. (THEOCHEM) 2001, 538, 59. (b) Belogolova, E. F.; Sidorkin, V. F. Izv. Akad. Nauk, Ser. Khim. 2003, 1392; Russ. Chem. Bull. Int. Ed. 2003, 52, 1472.


Figure 10. Maps of the Laplacian of the electron density (top) and ELF (bottom) for axial moieties MeSiP of MeSi(NHC6H4NH)3P and FSiP of FSi(NHC6H4NH)3P. For AIM, the cross-section in the NSiP plane is given. The diagrams are superimposed with the selected bond paths. Critical points (3, -1) are denoted by solid squares, and ring critical points (3, þ1) are designated by large open cycles. Dashed lines correspond to r2F(r) > 0 (regions of charge depletion) and solid lines to r2F(r) < 0 (regions of charge concentration). The contour values in e/ a05 are (0.002, (0.004, and (0.008. Legend for ELF = 0.83: (red) core domains; (green) bonding disynaptic domains; (blue) monosynaptic domains; (light green) disynaptic CH bonding domains. Since hydrogen atoms have no core electrons, CH disynaptic basins surround the hydrogen nuclei.

containing all six-membered rings, XSi[O(RMeC6H2)CH2]3N,37 we expected interesting structural effects upon benzoannelation of silaphosphanes 29-37. First of all, the decrease in number of thermodynamically stable endo isomers from 11 for XSi(Y-CH2CH2-Z)3P (see Table 2) to 5 for XSi(Y-C6H4-Z)3P (see Table 4) is surprising. Introduction of the benzene ring into the each side chain of the cage compounds 29(endo)-37(endo) leads to essential shortening of the Si-P (ΔdSiP) contact and a slight change in the X-Si bond distance (Tables 2 and 4). The maximum strengthening effect of the PfSi interaction is found for the endo isomer 33a (ΔdSiP = 0.49 A˚!). However, the shortest (Table 4) Si-P internuclear distances, close to the standard SiIV-PIII bond length of 2.26 A˚,27a is observed for two silaphosphanes 42b and 43b, which contain a highly electronegative fluorine atom as the X substituent at Si instead of a methyl group. With this, the configuration of the bonds at the (37) (a) Chandrasekaran, A.; Day, R. O.; Holmes, R. R. J. Am. Chem. Soc. 2000, 122, 1066. (b) Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes, R. R. Organometallics 2001, 20, 2331.

Article


pentacoordinate silicon atom in the last two structures is virtually an ideal trigonal bipyramid (ηe=100%) (Table 4 and Figure 9). The Si-P contact in the rest two stable endo isomers 45b and 46b was expected to be essentially shortened by using the steric effect of the t-Bu group.37b Indeed, its introduction into the benzene ring of the latter isomer decreases the dSiP value by 0.15 A˚ (Figure 9). It is also noteworthy that the data presented in Figure 9 are indicative of a low sensitivity of the geometry of benzo analogues 38-46 (like that of compounds 29-37, see part 2 above) to the method of calculation (B3LYP or MP2) or to the basis set size. On the basis of the comparison of the corresponding values of dXSi, dSiY, and dSiP made above, one could expect for retention of the nature of the X-Si and Si-Y bonds and an increase of the covalent contribution into the Si-P contact on going from silaphosphanes 29-37 to their benzo analogues 38-46, and this is exactly what happened (see Figures 8 and 10). The values of F(rc) and |E(rc)| characterizing the ratio of ionic and covalent contributions to the bonding by the AIM criterion of Cremer and Kraka33 are notably lower at the BCP(SiP) of compounds 29-37 than of the corresponding compounds 38-46. For example, for 33b F(rc)=0.328 e/A˚3 and |E(rc)|= 0.16 hartree/A˚3, and for 42b F(rc)=0.526 e/A˚3 and |E(rc)|= 0.32 hartree/A˚3. An impressive variation of the nature of the Si-P coordination contact from ionic to covalent occurring upon benzoannelation of molecule 33a can be seen in Figures 8 and 10.

Conclusion Cage silaphosphanes of the general structure XSi(-L-)3P (X = Me (a), F (b); L = YCH2Z (4-19), YC6H4 (20-23), C6H4Z (24-27), C10H6 (28), YCH2CH2Z (29-37), YC6H4Z (38-46); Y, Z=O, NH, CH2, S) can exist in the form with the phosphorus LEP directed into the cavity (endo) favorable for the formation of a PfSi dative bond or away from the cavity (exo), which rules out the possibility of PfSi coordination. This is the principal difference from their silatrane analogues XSi(-L-)3N that have no exo isomers. Accord(38) Voronkov, M. G. Private communication.

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ing to the MP2 and B3LYP methods among the 50 studied cage species XSi(-L-)3P possessing 1,5-bridgehead Si and P, only 3 structures (L = SC6H4 (23a,b), C10H6 (28b)) exist exclusively in the endo form with the pentacoordinate silicon atom having a very high degree of trigonal bipyramidalization (ηe from 89 up to 100%). On passing from XSi(-L-)3P with 1,5-bridgehead Si and P atoms to XSi(-L-)3P with 1,6bridgehead Si and P atoms, the number of stable endo isomers increases to 16. Insertion of the benzene ring into the Y-CH2CH2-Z chain results in a decrease in the number of stable endo isomers from 11 to 5 and to a substantial contraction of the internuclear distance Si-P. The most remarkable among the latter is the compound FSi(NHC6H4NH)3P (42b), having the a record short PfSi dative bond of covalent nature (from the AIM and ELF analyses). The bond configuration at its SiV atom presents a nearly ideal (ηe =100%) trigonal bipyramid. The results of the AIM analysis suggest that, in the general case, steric factors may have an unpredictable effect, either favoring or disfavoring the stabilization of the endo isomers of the studied cage silaphosphanes. The nature of the Si-P bond in the endo isomers of compounds XSi(-L-)3P varies from ionic to covalent (from the AIM and ELF analyses of the electron distribution), depending on the properties of the surroundings of the bridgehead Si and P atoms and the size of the side chains L.

Acknowledgment. We thank Dr. E. F. Belogolova, Prof. B. A. Shainyan, and Prof. M. S. Gordon for valuable discussions. We are grateful to Dr. P. L. A. Popelier for a copy of the MORPHY1.0 program and Prof. B. Silvi for a copy of the TopMod package of programs. Financial support of our work by a Russian Federation President Grant (No. NS-255.2008.3) is gratefully acknowledged. Supporting Information Available: Tables giving Cartesian coordinates of all investigated structures, parameters for Si 3 3 3 P internuclear distances (dSiP, A˚), and relative stabilities (ΔE, kcal/ mol) of the endo and exo isomers for XSi(YCH2Z)3P, XSi(YC6H4)3P, XSi(C6H4Z)3P, and XSi(C10H6)3P. This material is available free of charge via the Internet at http://pubs.acs.org.

Cage Silaphosphanes with a PâSi Dative Bond - Organometallics

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