Organometallics 2009, 28, 5909–5914 DOI: 10.1021/om900587m
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On the Stability of Six-Membered-Ring Carbenes and Silylenes Kinga Nyiri and Tamas Veszpremi* Inorganic and Analytical Chemistry Department, Budapest University of Technology and Economics (BUTE), Szent Gell ert t er 4, 1521 Budapest, Hungary Received July 8, 2009
The stability of the recently synthesized six-membered-ring silylene (1) and its eight structural isomers was investigated by quantum chemical methods. The structure and stability of the three most stable isomers (a, b, c) were compared to the well-known stable Denk-silylene (3). The related analogous carbenes and germylenes were also studied. According to the calculated isodesmic reaction energies and NICS(0) values, 1 is nonarmatic, but since the missing aromaticity is compensated by the reduced ring strain, it is nearly as stable as 3. The more stable five-membered-ring isomer (b) shows considerable aromatic character. The analysis of the molecular orbitals indicate that 1 is less reactive in both nucleophilic and electrophilic reactions than 3. The high kinetic stability found in studying the dimerization and the complex formation with NH3 and the 1,2H-migration reaction suggest the synthesizability of b and c. On the other hand, the analogous carbene derivatives seem to be less stable in both thermodynamic and kinetic aspects.
Introduction Research on divalent carbon and silicon species has developed rapidly since the synthesis of the first isolable carbene,1 2, and silylene,2 3. These and the subsequent bottleable compounds3 usually contain a five-membered ring with two nitrogen atoms adjacent to the divalent carbon or silicon (e.g., 5, 6). The conditions of the stability of these molecules, the effect of the proper substituents, the aromaticity, and the small ring strain have been well investigated.4 The most effective stabilizing substituent is nitrogen in the R-position with its electron-withdrawing ability in the σ-system and
donation in the π-system.5 The majority of the isolated molecules were stabilized by aromatic conjugation of the ring.6 Nevertheless, the synthesis of some nonarmoatic carbenes and silylenes suggests that aromaticity is an important but not necessary condition of the stability.3a,b,7,8 Naturally, the ring strain caused by the disortion of the N-Si-N angle has an opposite effect, which should be small in these structures. Up to the last couple of years only the five-membered-ring molecules and some less stable open-chain structures8 were isolated. In 2006 however, the synthesis of bis(diisopropylamino)cyclopropenylidene, a three-membered-ring carbene,9 a structural isomer of the Arduengo carbene, broke the ice. The high stability of this compound could be explained by its substantial kinetic stability supported by the π-electron shift from the β-amino substituents via a nonaromatic ylidic structure.10 Also, in 2006 the isolation of the first stable six-membered-ring germylene11 and silylene,12 a didehydro-1,
*Corresponding author. E-mail:
[email protected]. (1) Arduengo, A. J.III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (2) Denk, M.; Lennon, J. R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691. (3) (a) Arduengo, A. J.III; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027. (b) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (c) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Bl€aser, D. J. Chem. Soc., Chem. Commun. 1995, 193, 1932. (d) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Bl€aser, D. J. Organomet. Chem. 1996, 521, 211. (e) Heinicke, J.; Oprea, A.; Kindermann, M. K.; Karpati, T.; Nyulaszi, L.; Veszpremi, T. Chem.;Eur. J. 1998, 4, 541. (f) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 2005, 631, 1383. (g) Arduengo, A. J.III; Goerlich, J. R.; Marshall, W. J. Liebigs Ann. 1997, 365. (h) Hahn, F. E.; Wittenbecher, L; Boese, R.; Blaser, D. Chem.;Eur. J. 1999, 5, 1931. (4) (a) Kirmse, W. Angew. Chem., Int. Ed. 2004, 43, 1767. (b) Veszpremi, T. How to design stable silylenes. In Advances in Molecular Structure Research; JAI Press Inc.: Stamford, CT, 2000; Vol. 6, p 267. (c) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (5) (a) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, I.; Karni, M.; Chandrasekhar, J.; Schleyer, P. R. J. Am. Chem. Soc. 1986, 108, 270. (b) Nyulaszi, L.; Belghazi, A.; Kis-Szetsi, S.; Veszpremi, T.; Heinicke J. J. Mol. Struct. (THEOCHEM) 1994, 313, 73. (c) Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989. (d) Walsh, R. Acc. Chem. Res. 1981, 14, 246. (e) Wilt, J. W.; Lusztyk, J.; Peeran, M.; Ingold, K. U. J. Am. Chem. Soc. 1988, 110, 281. (f) Gordon, M. S.; Truong, T. N.; Bonderson, E. K. J. Am. Chem. Soc. 1986, 108, 1421.
(6) (a) Heinemann, C.; M€ uller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023. (b) Blakeman, P.; Gehrhus, B.; Green, J. C.; Heinicke, J.; Lappert, M. F.; Kindermann, M.; Veszpremi, T. J. Chem. Soc., Dalton Trans. 1996, 26, 1475. (c) Veszpremi, T.; Nyulaszi, L.; Hajgato, B.; Heinicke, J. J. Mol. Struct. (THEOCHEM) 1998, 431, 1. (d) B€ohme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039. (7) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9722. (8) (a) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew. Chem. 1996, 108, 1211. (b) Lavallo, V.; Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. J. Am. Chem. Soc. 2004, 126 (28), 8670. (c) Lee, G.-H.; West, R.; M€uller, T. J. Am. Chem. Soc. 2003, 125, 8114. (d) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288, 834. (e) Vignolle, J.; Catto€en, X.; Bourissou, D. Chem. Rev., in press (9) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, B. Science 2006, 312, 722. (10) Pinter, B.; Veszpremi, T. Organometallics 2008, 27, 5571. (11) Driess, M.; Yao, S.; Brym, M.; van W€ ullen, C. Angew. Chem., Int. Ed. 2006, 45, 4349. (12) Driess, M.; Yao, S.; Brym, M.; van W€ ullen, C.; Lentz, D. J. Am. Chem. Soc. 2006, 128, 9628.
r 2009 American Chemical Society
Published on Web 09/23/2009
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a
Bond lengths (pm) and bond angles (deg) at the B3LYP/cc-pVTZ level. *Calculated bond lengths of a and D.
3-silapiperazine derivative (1 in Scheme 1), and some saturated six-membered-ring N-heterocyclic carbenes13 (7, 8 in Scheme 1) was reported. Six-membered-ring silylenes and carbenes were investigated earlier with theoretical methods.14 The authors tried to develop aromatic structures modifying the number of nitrogens and divalent carbon or silicon atoms in the ring. The synthesis of 1 (and 1-Ge), a cyclic compound with seven π-electrons, was, however, unexpected. The aim of this work was to study the electronic structure and the stability of 1. We discuss the geometry, the unusual MO structure, the type and magnitude of electronic stabilization, and the possible reactivity. As a comparison, we investigate some related silylenes, the isomers of 1 and also the highly stable diazasilole D (Scheme 1). The structure, stability, and synthesizability of the analogous carbenes and germylenes are also investigated. The observed carbenes and germylenes are marked with -C or -Ge after the symbol of the analogus silylene. All the studied structures are collected in Scheme 1.
Calculations All calculations were carried out using the Gaussian0315 suite of programs. Geometry optimization and frequency analysis (13) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2008, 27 (13), 3279. (14) (a) Nyul aszi, L.; Karpati, T.; Veszpremi, T. J. Am. Chem. Soc. 1994, 116, 7239. (b) Veszpremi, T.; Nyulaszi, L.; Karpati, T. J. Phys. Chem. 1996, 100, 6262. (15) Frisch, M. J.; et al. Gaussian 03, Revision C. 02; Gaussian, Inc.: Wallingford, CT, 2004.
were performed at the B3LYP/cc-pVTZ16 level. To test the reliability of the computations, we compared the B3LYP/ccpVTZ results of a to MP2/cc-pVTZ,17 MP3/6-311þþG**, and CCSD/6-311þþG**18 calculations and found no essential difference between the DFT and the higher level methods in the calculated geometry and charge distribution. Second-derivative and harmonic frequency calculations verified that the structures obtained are real minima on the potential energy surface. Isodesmic reaction energies, Kohn-Sham orbitals,19 and NICS(0)20 values were also calculated at the B3LYP/cc-pVTZ level. To investigate the kinetic stability of the divalent species, characteristic reactions of silylenes and carbenes were studied. Isomerization, dimerization, and complex formation with ammonia of the most stable molecules were calculated using the B3LYP/cc-pVTZ level.
Results and Discussion Geometry. The calculated geometrical parameters of a were in good agreement with the observed values (see Supporting Information) with the exception of the butadiene moiety of the molecule. The almost balanced CdC and C-C (16) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (d) Dunning, T. H. Jr. J. Chem. Phys. 1989, 90, 1007. (17) Head-Gordon, M.; Pople, A. J.; Frisch, J. M. Chem. Phys. Lett. 1988, 153, 503. (18) Scuseria, E. G.; Schaefer, F. H.III. J. Chem. Phys. 1989, 90, 3700. (19) Stowasser, R.; Hoffmann, R. J. Am. Chem. Soc. 1999, 121, 3414. (20) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. J. Am. Chem. Soc. 1996, 118, 6317.
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distances of the butadiene moiety in the observed X-ray structure (CdCendo 139.0 pm; C-Cendo 140.2; CdCexo 141.2 pm) may suggest a conspicuous extra conjugation. On the other hand, in the calculated (gas-phase) structure the single-bond-double-bond alternation (CdCendo 134.3 pm; C-Cendo 138.9 pm; CdCexo 134.6 pm) is only slightly smaller than in the free butadiene (CdC 133.4 pm; C-C 145.3 pm). From this a separated butadiene unit and no extra π-conjugation in the gas-phase structure can be concluded. Since the calculated geometry is very similar at the B3LYP, MP2, and CCSD levels, we have to assume that the origin of the discrepancy is the difference between the (calculated) gasphase and the (observed) solid-phase structure. It has been shown, however, that the energy difference between the fully optimized structure and the one computed at the experimental geometry is only 5.2 kcal/mol.11 A fairly large difference can be found in the five-membered-ring isomer, b, in the exo- and the endocyclic double bond (CdCendo 136.4 pm; CdCexo 133.6 pm). This indicates the conjugation of the endo CdC bond in the ring and the fading of the original conjugation in the butadiene unit. Nevertheless, the short C-C bond (144.6 pm) also assumes some conjugation between the exo CdC bond and the aromatic π-system of the ring. On the other hand, the bond lengths in c (C-C 149.1 pm; CdC 133.9 pm) show no conjugation in the ring. The geometry of the related carbon and germanium derivatives suggests the same conclusions (see Supporting Information). Relative Energies. The computed relative energies and Gibbs free energies for the observed isomers of the [EN2C4H6] potential energy hypersurface (E= C, Si, Ge) are summarized in Table 1. The global minimum of the Si and Ge derivatives is the five-membered ring b, while the sixmembered-ring isomer a is only the second most stable isomer (Table 1). It is easy to recognize that b is a substituted derivative of the well-known Denk-silylene D. On replacing the hydrogens on the nitrogen atoms with methyl or phenyl groups, the stability order of the isomers does not change. b-Me and b-Ph are ΔG = 2.1 and 4.3 kcal/mol more stable than the similarly substituted isomer a (at the B3LYP/6311þþG** level). The third most stable isomer, c, is only 2.3 kcal/mol less stable than a. In general aminosilylenes and -germylenes are thermodynamically more stable than their tetravalent isomers.21 Indeed, the tetravalent isomers f-i were found to be 25-30 kcal/mol less stable than the divalent species. As a consequence, 1,2H-isomerization of a-c is not a thermodynamically favorable process. The more energetic isomers d and e are 11.4 and 38.6 kcal/mol less stable than a, respectively. The stability order of germanium compounds is very similar. The divalent carbene isomers also show the same stability order (b-C > a-C > c-C); however, because of the very stable CdN double bond, the tetravalent carbon isomers are 25-35 kcal/mol more stable than the divalent species. The different relative energy order of the carbene and silylene isomers could also be explained with the large difference in the divalent state stabilization energies (DSSE) of the singlet carbene and silylene.22 (21) Heinemann, C.; Herrmann, W. A.; Thiel, W. J. J. Organomet. Chem. 1994, 475, 73. (22) (a) Horner, D. A.; Grev, R. S.; Schaefer, H. F.III J. Am. Chem. Soc. 1992, 114, 2093. (b) Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 5.
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Table 1. Calculated Relative Energies and Relative Gibbs Free Energies of Isomers a-i at the B3LYP/cc-pVTZ Level (in kcal/mol) E = Si
a b c d e f g h i
E=C
E = Ge
ΔE
ΔG
ΔE
ΔG
ΔE
ΔG
3.40 0.00 6.84 15.03 42.84 31.48 34.62 31.28 32.78
3.81 0.00 6.10 15.16 42.38 29.21 32.10 28.07 29.68
30.25 26.42 34.48 47.59 67.93 5.72 8.50 0.00 0.11
31.03 26.58 33.96 47.97 67.97 6.37 8.61 0.00 0.18
6.34 0.00 9.40 18.44 38.76 56.46 60.06 38.81 39.96
6.34 0.00 8.40 18.15 38.48 53.57 56.59 34.99 36.38
Table 2. Calculated Isodesmic Reaction Energies (in kcal/mol) and NICS(0) Indices at the B3LYP/cc-pVTZ Level ΔEtot ΔEsub ΔEstrain ΔEring1 ΔEring2 NICS(0) E = Si
a b c D Ds E=C a-C b-C c-C A u E = Ge a-Ge b-Ge c-Ge H
55.7 59.1 52.2 58.7 33.6 118.9 122.7 114.6 123.4 101.6 61.1 67.4 58.0 67.0
30.3 30.3 30.3 30.3 30.3 92.2 92.2 92.2 92.2 92.2 33.7 33.7 33.7 33.7
3.6 14.5 10.9 14.7 9.7 16.6 31.8 22.7 40.8 15.0 2.3 12.5 9.7 12.5
29.0 43.3 32.8 43.0 13.1 43.2 62.3 45.2 72.0 24.4 29.6 46.2 34.0 45.8
5.8 16.4 7.3 18.2 7.1 2.2 15.0 2.5 18.7 2.8 7.1 19.0 8.9 20.7
3.9 -8.6 -2.2 -10.3 1.4 -10.9 -2.5 -13.3 0.0 4.6 -8.9 -1.8 -10.5
Orbital Correlations. To investigate the stability of a, b, and c, the isomers were compared to the highly stable silylene D. It is easy to imagine that the molecular orbitals of a, b, c, and D can be formally built up as a combination of a diamino-silylene and a double-bonded hydrocarbon unit (butadiene in a, b, and c and ethylene in D). Figure 1 shows the Kohn-Sham orbital-correlation diagram of a and D, indicating the MO formation from their parent molecules. Although a and b are asymmetric molecules, the shape and the origin of the orbitals show a relationship to the corresponding MOs of D; therefore we used the same notation for these molecules. It can be easily seen that the HOMOs of D and a are different. It is a b1 symmetry orbital in D, which incorporates the empty out-of-plane p-orbital of the divalent silicon, while it has a nodal plane at the silicon (a2 symmetry) in a. This may indicate that a is even less sensitive to nucleophilic attacks than the well-known inactive silylene D. The lone electron pair of both silylenes can be assigned to the second orbital. Comparing the two molecules, a definite stabilization of about 0.2 eV can be seen in a because of the smaller disortion of the N-Si-N angle. This fact also suggests an enhanced stability of a against electrophilic attacks. Since the HOMO of D is 0.2 eV more stable than that of a, the total energy gained from these two orbitals is almost the same in the two molecules. The HOMO and HOMO-1 of the Arduengo carbene (A) are 0.3 and 0.6 eV lower in energy than the corresponding orbitals of a-C, which may indicates that the related sixmembered-ring carbene is more reactive than A. In structure b the butadiene unit is connected to the diamino-silylene in the 1,2- position, while in a it is connected in the 1,3-position. This causes a large difference between the electronic structure of a and b. The orbital sequence of b is the
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Figure 1. MO correlation diagram of a and D.
Figure 2. MO correlation diagram of b and D.
same as in D, and the analogous MOs are close in energy. Obviously, b has an additional π-orbital compared to D, and the excess orbital is situated at almost the same energy (-7.56 eV) as the ethylene π-orbital. (Since this MO belongs almost excusively to the vinyl substituent of the molecule, we did not assign a symmetry symbol to this orbital.) From this, it can be concluded that the conjugation between the two double bonds of b is diminished; the endocylic double bond is combined in the ring π-system, while the other CdC bond behaves as a weakly conjugated vinyl group. The same tendency was found in the MO correlation of the analogous carbenes (b-C vs A), but the separation of the butadiene part was not as significant as in b. The HOMO of the five-membered-ring silylene and carbene isomers c and c-C are also of a2 symmetry, and the lone pair orbitals are found to be lower in energy than those in the corresponding synthesized molecules A and D, indicating a reduced nucleophilic and electrophilic reactivity. The orbital correlations of the germylene isomers are very similar to the analogous silylenes. Thermodynamic Stability. To get more information about the stabilization of a and its isomers, the different stabilization effects should be separately studied. For this we quantified the three most important factors of stability using appropriate homodesmic reactions (Scheme 2). Equation 1 is a typical bond separation reaction that measures the total stability of the molecule (ΔE1 = ΔEtot) compared to the simple SiH2 molecule. As in eq 2, the cyclic silylene and Si(NH2)2 are compared; the difference between eq 1 and eq 2 measures the effect of the substituents:
It follows from this definition that because the same Rsubstituents belong to all the investigated molecules, the same substituent effect has been calculated for all the different silylenes. The substituent effect may be distinguished considering the difference in the N-Si-N angles. In our model, however, this disortion was counted in the ring strain (ΔEstrain), which is defined with the reaction energy of eq 3:
ΔE sub ¼ ΔE 1 - ΔE 2
ΔE strain ¼ ΔE3 In eq 3 the molecules on the right side were optimized with bond angles and dihedral angles fixed at the values found in the respective ring, while on the left side they were fully optimized (in cis form). The stabilizing effect of the delocalization in the ring (ΔEring1) may be considered as the rest of the total stabilization energy:
ΔEring1 ¼ ΔEtot - ΔEsub þ ΔEstrain However, in this way fairly large delocalization energies were found even for the nonaromatic derivatives; therefore in order to obtain more realistic values (ΔEring2) we used eq 4.
ΔE ring2 ¼ ΔE4 - ΔE sub The calculated data are summarized in Table 2. The total stabilization energies show very similar values at the silylene isomers a-c and D, while the saturated ring Ds is about 20 kcal/mol less stable. For the isomeric molecules in accordance with its definition, ΔEtot shows the same relative energies as the energy order in Table 1.
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Scheme 2
As expected, the strain in the six-membered-ring molecule a is smaller than in the five-membered rings. The two different estimations of delocalization give very different energy values but similar tendencies (see Supporting Information). In both ways comparable aromatic stabilization can be found in b and D and much less (or no) aromaticity in a and c. The same conclusion could be drawn from the NICS(0) indices (Table 2). The analogous carbenes and germylenes do not show very different behavior. The larger substituent effect of the carbenes derives from the fact that the C-N bonds are stronger than the Si-N (or Ge-N) bonds.23 Studying the geometry, the MO correlation, the energy of isodesmic reactions, and NICS(0) values, it has been verified that a is not aromatic. It is obvious that the 6 πelectron aromatic structure from a could only be formed if one of the π-electrons is shifted toward the substituent of the ring. However, changing the CH2 group to electronwithrawing (dO, dS, dNH, dpentafulvenyl) and electron-donating (dtriafulvenyl, dSiMe2, dPMe) groups did not cause a large difference in the NICS(0) values (see Supporting Information). We could not find an
appropriate substituent that could cause an even moderate aromaticity. Kinetic Stability. To compare the kinetic stability of the studied molecules, isomerization to double-bonded silanes by one hydrogen migration (e.g., a f f or a f g) and dimerization were investigated. It has been shown that the 1,2H-isomerization of D is not a thermodynamically favorable process and the activation barrier of the reaction is high.21 As seen from Table 3, the same conclusion can be drawn from the isomerization of a and b. The isomerization energies and the activation barriers of D, a, and b were found to be very similar (Table 3). On the other hand, the isomerization of a-C is a thermodynamically favorable process with negative ΔG and about 10 kcal/mol less activation barrier than that in the analogous silylene. Nevertheless, we have to note that the stable Arduengo carbene shows very similar data. The relationship between the dimerization ability and stability of silylenes is well investigated.24 Two different dimerization models are known, a disilene formation and a four-membered-ring formation.25 We tried both paths of dimerization for a and b, but no stable dimer was found. It
(23) Schleyer, P. v. R.; Stout, P. D. J. Chem. Soc., Chem. Commun. 1986, 1373.
(24) Olah, J.; Veszpremi, T. J. Organomet. Chem. 2003, 686, 112. (25) Apeloig, Y.; M€ uller, T. J. Am. Chem. Soc. 1995, 117, 5363.
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Table 3. Gibbs Free Energies and Activation Gibbs Free Energies for the 1,2H-Migration Reactions (in kcal/mol)a ΔG ΔG#
aff
afg
bfh
bfi
D
25.4 61.5
28.3 63.0
28.1 49.2
29.7 49.7
28.6 48.2
a-C f g-C
b-C f h-C
a-C f f-C
b-C f i-C
ΔG -24.7 -22.4 -26.4 45.9 47.4 42.0 ΔG# a Calculated at the B3LYP/cc-pVTZ level.
A
-26.6 41.7
-26.4 41.6
Table 4. Dimerization Energies and Gibbs Free Energiesa a-C
cis trans a
b-C
c-C
A
ΔE
ΔG
ΔE
ΔG
ΔE
ΔG
ΔE
ΔG
-40.0 -38.1
-26.5 -24.7
-16.3 -16.8
-3.1 -3.4
-42.0
-27.6
-10.7
1.2
Calculated at the B3LYP/cc-pVTZ level.
has been shown that the dimerization energy of carbenes shows an excellent linear correlation with their stability.26 The highly stable Arduengo carbene (A) dimerizes in an exothermic reaction, but the reaction Gibbs free energy is positive (see Table 4). On the other hand, the dimerization of a-C and its divalent isomers is a thermodynamically favorable process (ΔG < 0). In addition, the dimerization Gibbs free energy of the six-membered-ring carbene a-C is about 9 kcal/mol lower than that of the synthesized saturated derivative u (ΔG = -17.3 kcal/mol). This suggests only a moderate stability of a-C. The immunity of silylenes a, b, c, and D against nucleophilic attack was studied via their ability to form complexes with ammonia. For a and c the complex formation is a slightly exothermic process (ΔE= -4.9 and -4.2 kcal/mol, respectively), and the calculated Gibbs free energies are positive (ΔG = 6.4 and 5.7 kcal/mol). We could not find (26) Nyul aszi, L.; Veszpremi, T.; Forr o, A. Phys. Chem. Chem. Phys. 2000, 2, 3127. (27) (a) Ol ah, J.; De Proft, F.; Veszpremi, T.; Geerlings, P. J. Phys. Chem. A 2005, 109, 1608. (b) Olah, J.; Veszpremi, T.; De Proft, F.; Geerlings, P. J. Phys. Chem. A 2007, 111, 10815. (28) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562.
a stable adduct between ammonia and D27 or b at the B3LYP/cc-pVTZ and the MP2/cc-pVTZ levels (see Supporting Information). Next we plan to study the adduct formation of a, b, c, and D with carbene.28
Conclusion We investigated the kinetic and thermodynamic stability of the recently isolated six-membered-ring silylene (a) and its isomers. The global minimum of the [SiN2C4H6] potential energy hypersurface corresponds to the five-membered-ring structure b, while a is only the second most stable isomer. Another five-membered-ring isomer, c, is only ΔG = 2.3 kcal/mol less stable than a. In the case of the analogous carbon derivatives the tetravalent isomers are more stable than the respective carbenes. From the structure and energy of the molecular orbitals it can be concluded that a is less reactive in both nucleophilic and electrophilic reactions than the Denk-silylene. The bond distances of the butadiene moiety suggest that while b is an aromatic molecule, in a and c there is no extended delocalization. The isodesmic reaction energies also indicate no aromatic stabilization in a and c. Therefore the diminishing ring strain makes the stability of these structures competitive to D. On the other hand, b is as aromatic as D. The MO correlation suggests that the CdC bonds in the butadiene unit separate and the endocyclic double bond is incorporated into the aromatic system. The high kinetic stability found studying the dimerization and 1,2H-migration reaction of a, b, and c explains the surprising isolability of a and suggests the possible synthesis of b and c. The investigation of the related carbenes indicates the reduced kinetic stability of these structures, which makes the synthesis of the appropriate carbene derivatives more complicated.
Acknowledgment. The authors thank the Hungarian Scientific Research Foundation (OTKA K76806) for financial support. Supporting Information Available: Optimized geometries, the computed energies, NICS(0) values, and the correlation between ΔEring1 and ΔEring2 are available free of charge via the Internet at http://pubs.acs.org.
