6262
J. Phys. Chem. 1996, 100, 6262-6265
Toward Stable Silylenes Tama´ s Veszpre´ mi,* La´ szlo´ Nyula´ szi, and Tama´ s Ka´ rpa´ ti Department of Inorganic Chemistry, Technical UniVersity of Budapest, 1521 Budapest, Gelle´ rt te´ r 4, Hungary ReceiVed: September 20, 1995; In Final Form: January 31, 1996X
2-Silapyridine and its divalent silicon isomer 2-silapyridine-2-ylidene have been investigated by ab initio calculations using HF, MP2, and DFT levels and standard 6-31G* and 6-311G(2d) basis sets. It was shown that both molecules are stable minima on the potential energy surface. The silylene ring is more stable by 8.8-11.5 kcal/mol than the silane. The two structures are separated by 56-77 kcal/mol energy barrier (depending on the level of theory). Both the geometry of the rings and isodesmic reaction energies indicate a significant aromatic stabilization. No stable SidSi bonded dimer of the silylene have been found at the HF/6-31G* level. The head-to tail Si-N bonded dimer was found to be less stable than the two monomers by 18.39 kcal/mol at the HF/6-31G* level. On the basis of the present calculations a substituted 2-silapyridine2-ylidene is a likely candidate for the synthesis of a new stable silylene.
Introduction Until recently, divalent silicon species (silylenes) were generally considered to be reactive and unstable molecules. It has been reported1 that electronegative substituents, donating their lone electron pairs to the empty orbital of silicon, are able to stabilize the singlet state of silylene. The maximum stabilization could be achieved with an -NH2 substituent.2 According to an appropriate isodesmic reaction aminosilylene has a considerable 22.0 kcal/mol stabilization energy3 relative to SiH2. HSi-NH2 is by 14 kcal/mol more stable4 than its tetravalent isomer, H2SidNH. Further stabilization is exerted by a second -NH2 group, although its effect is smaller than that of the first amino moiety. Nevertheless, NH2-Si-NH2 is still not stable enough to be synthesized, although a fourmembered ring 1 containing N-Si(II)-N unit was observed by low-temperature matrix isolation spectroscopy.5 2 is a stable compound; however, it dimerizes within a few days.6
Si-NH2 is about 100°, while it is 84.7° in 3 and 110.4° in 4.3,8 This effect brings about 8 and 10 kcal/mol destabilization in 3 and 4, respectively.8,3 The systematic study of the two opposing effects, aromaticity and ring strain, opens the possibility of designing other potentially synthesizable silylenes. The aim of this work is to study the geometry, electronic structure and stability of the sixmembered ring 5, a molecule which is formally a superposition of a Si-N and a cis-butadiene unit. Although the expected stabilization with only one nitrogen is smaller than in 3 or 4, the six π-electrons make the aromatic stabilization possible while the strain in this ring is thought to be smaller than in 3 or 4. The structure and stability of 5 can be compared to its “classical” isomer 6, which contains tetravalent silicon. Similar isomer pairs, phosphinine-2-ylidene (7) and phosphinine (8) were investigated in a recent work.9 Both isomers were stable minima on the potential surface. Although the carbene derivative, 7, was less stable by 75 kcal/mol than 8, the 28 kcal/mol barrier between the two isomers indicated that this carbene is a possible target for synthetic chemists. Calculations
The first stable silylene was synthesized by Denk and his co-workers in 1994.7 In this molecule the N-Si-N unit was further stabilized by forming a five-membered ring with a CdC unit (3). The six π-electrons are able to form an aromatic system in the resulting molecule which seems to be the main factor of the heretofore missing stabilization energy.6,8 Another example for the stabilization of the N-Si-N unit was shown in our previous work.3 In the six-membered ring 4 the stabilization of one silylene unit is 37.9 kcal/mol, which is slightly larger than that achieved by two amino groups on silylene (37.2 kcal/mol). While the cyclic delocalization in 3 and 4 increases the stability, significant ring strain destabilizes these molecules. The N-Si-N angle at the MP2/6-31G* optimum in the free NH2X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-6262$12.00/0
Quantum chemical calculations were carried out with the GAUSSIAN 92/DFT program package.10 The structure of the investigated compounds was fully optimized at the HF and MP2 levels using standard 6-31G* and 6-311G(2d) basis sets. Geometry optimizations with the density functional method were also performed using Becke’s correlated fuctional11 (BLYP) and the 6-31G* basis set (hereafter DFT/6-31G*). Second-derivative calculations at the MP2/6-31G* and the DFT/6-31G* levels revealed that the structures obtained are real minima on the potential energy surface. Since no essential difference could be found in the results at the various levels used (see Table 1), only the results from MP2/6-31G* calculations will be dealt with in the following discussion. Results and Discussion The calculated geometries, energies, lowest harmonic frequencies, and zero-point energies (ZPE) are collected in Table © 1996 American Chemical Society
Toward Stable Silylenes
J. Phys. Chem., Vol. 100, No. 15, 1996 6263
TABLE 1: Total Energies (Etot in au), Relative Energies (∆E in kcal/mol), Lowest Calculated Harmonic Frequencies (ν7 cm-1), and Selected Geometrical Data (in Å and deg) at Different Levels of Theory
TABLE 2: Bond Distances (r, in angstrom Å), Wiberg Indexes (I), Electron Densities (G), and Ellipticities (E) at the Bond Critical Points in Different Molecules Containing cis-Butadiene Units at the MP2/6-31G* Level of Theory
1 at different levels of theory. As a comparison, 3- and 4-silapyridineylidines and their silicon isomers (9-12) have also been calculated and are shown in Table 1.
The most stable compoundssas expectedsare the 2-silapyridine derivatives, 5 and 6. The silylene ring, 5, is more stable by 8.8-11.5 kcal/mol than the silane, 6, depending on the level of theory used. In a comparison of the analogous compounds, 7 and 8, the carbene derivative 7 is about 75 kcal/mol less stable than 8. Since aminosilylene is by 14 kcal/mol more stable than silaneimine,4 it might be concluded that the aromatic stabilization of the silylene ring is somewhat less than that of the silane ring. The relative stability of 5 and 6 depends on the substituents of the ring. For example, 1-methyl-2-silapyridine-2-ylidene is 5.6 kcal/mol less stable than the respective silane isomer, 2-methyl-2-silapyridine (at the MP2/6-31G* level), reflecting the different relative strengths of the Si-C and N-C bonds. The C-C bond lengths in 5 show some alternation between C3, C4, C5, and C6. No alternation can be found in 6.
Comparing the cis-butadiene moiety to other cis-butadiene units (Table 2) the alternation of the bond lengths in 5 is much less than in butadiene and is similar to that of pyrrole12 which indicates a sizeable delocalization. Similar conclusion can be drawn from the comparison of the Wiberg indexes and the electron densities at the bond critical points, which were determined by the Extreme program of Bader.13 The delocalization is also supported by the calculated N-C bond length (1.362 Å) which is somewhat shorter than in pyrrole (1.372 Å). The Si(II)-N distance in 5 is 1.754 Å, longer than in the monomer aminosilylene (1.719 Å)3 where the lone-pair interaction between the Si and N atoms is strong or diaminosilylene (1.727 Å)8 where it is moderate. Similarly, the Wiberg index of the Si-N bond in 5 is 0.63, while it is 0.83 (smaller than 1.0) in HSi-NH2. The Si(II)-N “single bond” length can be estimated using the bond length of a HSi-NH2 molecule in which the NH2 group is rotated by 90° relative to the optimum position, thus switching off the stabilizing interaction between the nitrogen lone electron pair and the empty out-of-plane orbital of silicon (1.783 Å). The Si-N bond shortening relative to this longest bond may be a measure of this “π-interaction”. Considering the shortest and longest distances as “double” and “single” bonds, respectively, the Si-N bond length in the ring 5 indicates a 45% “double-bond” character. The Si(II)-C bond length in 5 is 1.840 Å. This value is significantly shorter than in HSi-CH3 (1.904 Å) or in CH2dCH-SiH (1.879 Å). The Wiberg indexes of the corresponding SiC bonds again show a
6264 J. Phys. Chem., Vol. 100, No. 15, 1996 similar behavior (0.90 and 0.77 in 5 and in CH2dCH-SiH, respectively). The lengthening of Si(II)-N and shortening of Si(II)-C bonds seems to be a phenomenon similar to the bondlength equalization in the aromatic molecules. The Si-N and Si-C distances are considerably longer in the silylene ring than in the silane. It should be emphasized, however, that the Si-X bond lengths in silylenes are generally longer than in silanes as a consequence of the greater p character of the bond14 (cf. H2SidNH and HSi-NH2).1 To study the stabilization of rings 5 and 6 and elucidate the contribution of the aromatic character and the ring strain in the stabilization, the usual homodesmic reactions were investigated (I, II). Similar reactions were considered for 3 as a comparison (III). All possible conformers of the fragments in reactions I-III
5 + HSi-NH2 + 2CH2dCH2 f CH2dCH-Si-NH2 + CH2dCH-NH-SiH + CH2dCH-CHdCH2 (I) ∆E ) 14.7 kcal/mol
RS ) 5.5 kcal/mol
6 + H2SidNH + 2CH2dCH2 f CH2dCH-SiHdNH + CH2dCH-NdSiH2 + CH2dCH-CHdCH2 (II) ∆E ) 19.4 kcal/mol
RS ) 14.1 kcal/mol
3 + 2HSi-NH2 + CH2dCH2 f H2N-Si-NH2 + 2CH2dCH-NH-SiH (III) ∆E ) 6.9 kcal/mol
RS ) 11.4 kcal/mol
were optimized at the HF/6-31G* level of theory and those having the lowest energy were reoptimized at the MP2/6-31G* level. Second derivatives were also calculated at this level. For the reactions the stabilization energy values (∆E) of the MP2/ 6-31G*//MP2/6-31G*+ZPE are shown. To estimate and compare the ring strain, two sets of calculations were carried out for all the related fragments. First, all the possible fourmembered chain molecules at the right-hand side of the eqs I-III were optimized in cis form (the global minima of the fourmembered chains are generally trans forms). The optimization was than repeated, with bond angles fixed at the values found in the respective ring, resulting in new reaction energies. Ring strain (RS) is defined as the difference between the two reaction energies calculated above. It should be noted that the above ring strain is somewhat overestimated, due to the repulsing effect of the terminal hydrogens, which is the largest at the “fixed angle geometries”. From a comparison of the reaction energies the largest aromatic stabilization is found in 6. The stabilization of the five-membered ring, 3, is about half of that in 5. The resulting 14.7 kcal/mol in 5 is comparable to that in the corresponding reaction of the carbene derivative 7 (16.5 kcal/mol). Similar homodesmic reaction energies of benzene, phosphabenzene, pyridine, and silabenzene at the same level of theory were found to be 28.2,15 27.1,15 28.0, and 23.5 kcal/mol, respectively, indicating that the aromatic stabilization in the “classical” aromatic molecules is significantly larger than in cyclic 6-πelectron carbenes or silylenes. The homodesmic reaction, however, measures the superimposed effects of aromatic stabilization and ring strain. Considering the rather large ring strain for 3, the small homodesmic reaction energy becomes understandable. It is worth to note that a different homodesmic reaction (IV) was given for the same molecule by Denk et al.7
Veszpre´mi et al.
In this reaction the stabilization energy was 13.9 kcal/mol, by 7 kcal/mol larger than in (III). Apparently, in reaction IV the similar rings suffer from similar ring strain, thus resulting in larger net stabilization than in (III). Aromatic stabilization refers to imaginary reference structures and gives therefore little direct information about the reactivity of the molecule. In the case of silylenes the most reactive center of the molecule is the divalent silicon; thus from the point of view of the reactivity, the stabilization on that center seems to be more informative than the aromatic stabilization. Reactions V and VI measure the stabilization of the Si(II)-N bonded
3 + 2NH3 + SiH2 + CH4 f 2HSi-NH2 + NH2-CH3 + H2CdCH-NH2 (V) ∆E ) 15.5 kcal/mol
RS ) 15.3 kcal/mol
5 + NH3 + SiH2 + 2CH4 f HSi-NH2 + NH2-CH3 + HSi-CH3 + 2CH2dCH-CHdCH2 (VI) ∆E ) 36.1 kcal/mol
RS ) 6.9 kcal/mol
structures upon ring formation with respect to silylenes H2NSiH and CH3-SiH at the right-hand side. Therefore, the final stabilization of the silylene unit in the rings is the sum of the reaction energies plus the stabilization already present in HSiNH2 (22.0 kcal/mol3) and HSiCH3 (0 kcal/mol1). Thus, for 3 and 5 15.5 + (2 × 22.0 (HSiNH2)) ) 59.5 kcal/mol and 36.1 + 22.0 (HSiNH2) + 0 (HSiCH3) ) 58.1 kcal/mol stabilization is obtained for the silylene center, respectively. The present conclusion that the final stabilization in 3 and 5 are similar, can be rationalized considering that the ring strain in 5 is smaller than in 3, while the effect of two nitrogen atoms in 3 is larger than the stabilizing effect of one nitrogen plus one vinyl groups. To get information about the kinetic stability of 5, two possible chemical reactions were investigated, the [1,2]sigmatropic hydrogen shift and the propensity for dimerization. Since in an attempted synthesis bulky protecting groups are likely to be used about the silylene center, the bimolecular pathway of the [1,2]sigmatropic hydrogen shift is not dealt with here. To study the possible mechanism of the hydrogen shift from nitrogen to silicon, calculations for the transition state between 5 and 6 and for the energy barrier between the two isomers were carried out (Figure 1). The first-order saddle point for the motion of the hydrogen atom was found to be 77.6, 61.0, and 56.6 kcal/mol (HF/6-31G*+ZPE, MP2/6-31G*+ZPE, and DFT/6-31G*+ZPE levels of theory, respectively) higher in energy than 5. Such a reaction thus is improbable, and both isomers may exist under unimolecular conditions. The ring in the transition state is almost planar with the exception of the reacting hydrogen atom. This hydrogen is situated nearly at the middle of the Si-N bond and the Si-H-N plane is at 106° relative to the ring plane. The Si-H and H-N distances are 1.616 and 1.492 Å, respectively. The bond lengths in the ring do not change significantly during the process, and generally they are between the lengths in 5 and 6. Presumably the aromatic structure of the molecule is subject of small changes during the motion of the hydrogen. The dimerization of 5, a characteristic reaction of silylenes, has been investigated at the HF/6-31G* level. During the
Toward Stable Silylenes
Figure 1. Energies of 6 and the transition structure relative to 5 at the MP2/6-31G* level of theory and some important geometrical parameters of the transition structure.
SCHEME 1
SCHEME 2
J. Phys. Chem., Vol. 100, No. 15, 1996 6265 theory) more stable than the Si(IV) derivative. Changing the H on N and Si atoms on 5 and 6, respectively, to an alkyl group, the relative stability is reversed. The N-methyl-substituted 5 is less stable by 5.6 kcal/mol than the Si-methyl substituted 6. The alternation of the C-C bond lengths indicates that the delocalization in 6 is larger than in 5. Nevertheless, the distribution of the C-C bond lengths in 5 is similar to that in pyrrole. Similar conclusion can be drawn from the study of the Wiberg bond orders. According to the isodesmic reaction energies, both compounds are aromatic, the stabilization being larger by 4.7 kcal/mol in 6 than in 5. Comparing the synthesizeable five-membered molecule 3 with the six membered silylene derivative 5, the stabilization of the silylene units are very close in value. The ring strain in 3 is larger than in 5. To investigate the kinetic stability of 5, the dimerization and the unimolecular reaction to 6 were considered. The barrier leading to isomerization to 6 is prohibitively high (about 60 kcal/mol). During the HF/6-31G* calculations no stable Si)Si bonded dimer of 2-silapyridine-2-ylidene has been found. The head-to tail bonded Si-N dimer was by 18.39 kcal/mol less stable than the two monomers. Since in the case of the slowly dimerizing 2 (with R ) H) it was found that the dimer is more stable than two monomers by 7.8 kcal/mol, the present result indicates that 5 is a likely target to synthetic chemists. Acknowledgment. Financial support from OTKA T014339 and 014955 is gratefully acknowledged. The authors are indebted to one of the referees for drawing their attention to the Si-N head-to tail dimerization. References and Notes
dimerization (Scheme 1) a double-bond formation is assumed between the two rings. For this reason the two monomer molecules were positioned in the same plane, and their distance was decreased. Within this constraint no energy minimum (only a first-order saddle point) was found. The energy of this firstorder saddle point at the HF-6-31G*//HF/6-31G* level was by 26.11 kcal/mol larger than that of the two isolated monomers. Further investigations allowing an out-of-plane tilt of the rings also failed to give a stable minimum: the optimization ended in two separated monomeric units. A similar dimerization scheme of 7 indicated that the dimer is more stable than the monomer at the HF/6-31G* level by 105.9 kcal/mol.9 It has recently been shown16 that an energetically favorable path for dimerization of aminosilylenes is resulting in the headto tail Si-N dimer (Scheme 2). Such a dimer, with the two rings in trans form (as in Scheme 2) is indeed a minimum on the HF/6-31G* surface. The cis form is less stable by a further 2 kcal/mol. The stability of the trans dimer is less by 18.39 kcal/mol than that of two monomers even without considering the effect of BSSE. For silylene 2, which dimerized upon staying, the dimer was more stable than two monomers by 7.8 kcal/mol16 at the HF/6-31G** level. Conclusions The investigations of the two 2-silapyridine isomers 5 and 6 show that both molecules exist as separate minima on the potential energy surface. Among the two isomers the silylene derivative is 8.8-11.5 kcal/mol (depending on the level of
(1) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Chandrasekar, J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 260. Apeloig, Y.; Karni, M. J. Chem. Soc., Chem. Commun. 1985, 1048. Ikura, K. K.; Goddard, W. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1992, 114, 48. (2) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karni, M.; Chandrasekar, J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 270. (3) Nyula´szi, L.; Ka´rpa´ti, T.; Veszpre´mi, T. J. Am. Chem. Soc. 1994, 116, 7239. (4) Heinemann, C.; Herrmann, W. A.; Thiel, W. J. Organomet. Chem. 1994, 475, 73. (5) Veith, M.; Werle, E.; Lisowsky, R.; Ko¨ppe, R.; Schno¨ckel, II. Chem. Ber. 1992, 125, 1375. (6) Denk M.; Green J. C.; Metzler N.; Wagner M. J. Chem. Soc., Dalton Trans. 1994, 2405. (7) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691. (8) Blakeman, P.; Gehrhus B.; Green J. C.; Heinicke, J.; Lappert, M.; Kindermann, M.; Veszpre´mi, T. J. Chem. Soc., Dalton Trans., in press. (9) Nyula´szi, L.; Szieberth, D.; Veszpre´mi, T. J. Org. Chem. 1995, 60, 1647. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gil, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92, Revision F, Gaussian Inc.: Pittsburgh, PA, 1993. (11) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (12) Nyula´szi, L.; Va´rnai, P.; Veszpre´mi, T. THEOCHEM 1995, 358, 55. (13) The electron density distribution was analyzed at the MP2/6-31G* geometries by the extreme program of Bader (Biegler-Konig, F. W.; Bader, R. W. F.; Tang, T. H. J. Comput. Chem. 1982, 3, 946). (14) Apeloig, Y. Theoretical Aspects of Organosilicon Compounds. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; John Wiley: Chichester, UK, 1989. (15) Nyula´szi, L.; Veszpre´mi, T. J. Phys. Chem., in press. (16) Apeloig, Y.; Mu¨ller, T. J. Am. Chem. Soc. 1995, 117, 5363.
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