Reactive Intermediate Chemistry

the solid state, bis[2-pyridyl-2,2-bis(trimethylsilyl)methyl]stannylene (163)177 and bis[2,4,6-tris(trifluoromethyl)phenyl]stannylene (164)178 were synthesized and characterized by X-ray crystallography. The stabilization of these stannylenes is the result of intramolecular contacts between the tin and neighboring nitrogen or fluorine atoms (Scheme 14.71). The chemical shift in 119Sn NMR spectrum for

Scheme 14.71

163 was strongly temperature dependent, varying in the range of 120–150 ppm. The 119Sn NMR resonance of 164 is split into 13 lines by coupling with the fluorine atoms of the trifluoromethyl groups in the ortho positions [J(119Sn F) ¼ 240 Hz],

clearly indicating the existence of the fluorine–tin contacts in solution as well. In 1976, Lappert and co-workers142c,157,179 reported the first stable dialkylstan-

nylene (154, Scheme 14.71) in solution. They found that 154 exists as a monomer in the gas phase and as a dimer 153 in the solid state, whereas it exists as a monomer–dimer equilibrium mixture in solution. Extensive studies on the reactions of 154 were carried out, especially on oxidative addition and insertion reactions

leading to a variety of new organotin compounds such as 165 and 166 (Scheme 14.72).142c,157,179

Scheme 14.72

On the other hand, the first stable dialkylstannylene (136) that is monomeric in the solid state was synthesized and crystallographically characterized by Kira et al. in 1991140 (Scheme 14.71). Their helmet-like bidentate ligand was useful for the stabilization of the central tin atom as in the cases of its silicon and germanium analogues (see above), and the protecting ability is much higher than the two

bis(trimethylsilyl)methyl groups in 154. The signal for the central tin atom in the 119Sn NMR spectrum of 136 appears at 2323 ppm as a sharp singlet that is the

Soon after the isolation of 136, Tokitoh et al.181 described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn: (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 C in solution, and it showed a deep purple color (lmax ¼ 561 nm) in hexane. The 119Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between 30 and 60 C, indicating the absence of a monomer–dimer equilibrium.

Tokitoh and co-workers182 further succeeded in the synthesis of overcrowded diarylstannylenes, Tbt(Tcp)Sn: (170; Tcp ¼ 2,4,6-tricyclohexylphenyl) and Tbt (Tpp)Sn: [171; Tpp ¼ 2,4,6-tris(1-ethylpropyl)phenyl], by the exhaustive desulfurization of the corresponding tetrathiastannolanes with a trivalent phosphine reagent (Scheme 14.74). Since only a few convenient precursors have been available for the generation of stannylenes, this new method should provide us with a useful synthetic route for a variety of overcrowded stannylenes. The successful synthesis of a series of Tbt-substituted diarylstannylenes enabled the systematic comparison of their electronic absorptions with those of the previously reported overcrowded diarylstannylenes, which led to the elucidation of the substituent effect on the n!p transition of stannylenes.

Power and co-workers161 reported the synthesis and characterization of a new type of sterically crowded diarylstannylene (172), that bears two bulky m-terphenyl type ligands (2,6-Mes2C6H3) and exists as a monomer even in the solid state (Scheme 14.75).

Scheme 14.75

8. SYNTHESIS AND REACTIONS OF STABILIZED PLUMBYLENES

In contrast to the remarkable progress in the chemistry of divalent organic compounds of silicon, germanium, and tin, the heaviest congener of this series, that is, divalent organolead compounds (plumbylenes), are less well known. They usually occur as reactive intermediates in the preparation of plumbanes (R4Pb) and undergo polymerization and/or disproportionation in the absence of suitable stabilizing groups on the lead atom.183

Although dicyclopentadienyllead(II) compounds, formally called plumbylenes, have been known since 1956,184 they are not the congeners of carbenes since they are stabilized by Z5-coordination of cyclopentadienyl ligands. In 1974, the first stable diaminoplumbylene [(Me3Si)2N]2Pb: (173) was synthesized by Lappert and

702 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Although these two plumbylenes 183 and 184 were found to have close intramolecular contacts and exist as a dimer in the solid state, the lead–lead distances in their dimeric form are still much longer than the theoretically predicted values

(186, Scheme 14.81). Compound 186 showed a shorter Pb–Pb length [3.0515(3) A]

and much larger trans-bent angles (43.9 and 51.2 ) than those observed for 1830 and 1840. These data strongly indicate that 186 is the first molecule with a lead–lead double bond in the solid state, although 186 was found to dissociate into the monomeric plumbylene (185) in solution. Furthermore, they examined the synthesis of a heteroleptic plumbylene, Tip[(Me3Si)3Si]Pb: (187), by the treatment of diarylplumbylene Tip2Pb: (185) and disilylplumbylene [(Me3Si)3Si]2Pb:, and found that the product exists as the diplumbene Tip[[(Me3Si)3Si]Pb Pb[Si(SiMe3)3]Tip (188) in the solid state (Scheme 14.81).199 The X-ray structure analysis of 188 reveals

the centrosymmetrical diplumbene structure with a trans-bent angle of 42.7 and

˚ 186 a Pb Pb bond length of 2.9899(5) A. This bond is even shorter than that of

and very close to the theoretically predicted value for the parent diplumbene.

Scheme 14.81

In order to elucidate the relationship between the structure of plumbylene dimers and the bulkiness of substituents, Weidenbruch and co-workers199 synthesized and characterized much less hindered diarylplumbylene Mes2Pb: (189), which was iso-

and trans-bent angle (71.2 ) of 190 suggest that the character of the lead–lead bonding interaction in plumbylene dimers is delicate and changeable.

Scheme 14.82

The progress in the chemistry of plumbylenes is far behind those of its lighter congeners (silylenes, germylenes, and stannylenes). Most of the recent reports on plumbylenes are describe their synthesis and structural analysis; there have been

very few descriptions of their reactivities.

Recently, however, Tokitoh and co-workers186,193,194,200 reported a versatile reactivity of Tbt-substituted plumbylenes 182a–d, that is, insertion reactions with alkyl halides, diphenyl dichalcogenides, bromine abstraction from carbon tetrabromide, and sulfurization with elemental sulfur as shown in Scheme 14.83.

These results indicate that the reactivity of a plumbylene, the heaviest congener of a carbene, is essentially the same as those observed for its lighter relatives. The successful isolation of the reaction products in the case of plumbylenes 182 might be the result of effective steric protection not only for the starting plumbylenes but

S

Scheme 14.85

carbon disulfide. Compound 192 was also obtained by the reaction of bis(arylthio) plumbylene (193) with carbon disulfide, indicating the double insertion of carbon disulfide into the two Pb S bonds (Scheme 14.85). Although the migratory insertion of carbon disulfide into a metal–sulfur bond is known for some transition metal complexes, such a double insertion of carbon disulfide as suggested above has not been observed in the case of other divalent species of heavier group 14 (IVA) elements, that is, silylenes,149 germylenes,163 and stannylenes.206

9. CONCLUSION AND OUTLOOK

As can be seen in this chapter, the chemistry of divalent species of heavier group 14 (IVA) elements is still rapidly growing. The remarkable development of ligands for thermodynamic and kinetic stabilization has provided a number of stable examples of heavier homologues of carbenes, whose remarkable chemical and physical properties will stimulate the further progress in this field. The progress in the chemistry of stannylenes and plumbylenes is much slower than that in the areas of silylenes and germylenes. This relatively slow pace is probably the result of the weakness of bonds involving such heavy elements. A comparative study on the whole chemistry of divalent species of group 14 (IVA) elements including carbenes should be important to produce new concepts and applications for these low-coordinate species.

SUGGESTED READING

P.P. Gaspar, M. Xiao, D. H. Pae, D. J. Berger, T. Haile, T. Chen, D. Lei, W. R. Winchester, and P. Jiang, ‘‘The Quest for Triplet Ground State Silylenes,’’ J. Organometal. Chem. 2002, 646, 68.

706 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

N.Tokitoh and R. Okazaki, ‘‘Recent Topics in the Chemistry of Heavier Congeners of Carbenes,’’ Coord. Chem. Rev. 2000, 210, 251.

M. Haaf, T. A. Schmedake, and R. West, ‘‘Stable Silylenes,’’ Acc. Chem. Res. 2000, 33, 704.

M.Weidenbruch, ‘‘Some Silicon, Germanium, Tin and Lead Analogues of Carbenes, Alkenes, and Dienes,’’ Eur. J. Inorg. Chem. 1999, 373.

P.P. Gaspar and R. West, ‘‘Silylenes,’’ in The Chemistry of Organosilicon Compounds, Vol. 2, Z. Rappoport and Y. Apeloig, Eds., John Wiley & Sons, Inc., Chichester, UK, 1998, Part 3, p. 2463.

W. Ando and Y. Kabe, ‘‘Highly Reactive Small-ring Monosilacycles and Medium-ring Oligosilacycles,’’ in The Chemistry of Organosilicon Compounds, Vol. 2, Z. Rappoport and Y. Apeloig, Eds., John Wiley & Sons, Inc., Chichester, UK, 1998, Part 3, p. 2401.

W. P. Neumann, ‘‘Germylenes and Stannylenes,’’ Chem. Rev. 1991, 91, 311.

C.-H. Liu and T.-L. Hwang, ‘‘Inorganic Silylenes. Chemistry of Silylene, Dichlorosilylene and Difluorosilylene,’’ in Advances in Inorganic Chemistry and Radiochemistry, Vol. 28, H. J. Emeleus and A. G. Sharpe, Eds., Academic Press, Orlando, FL, 1985, p. 1.

REFERENCES

1.(a) P. P. Gaspar and B. J. Herold, in Carbene Chemistry, W. Kirmse, Ed., Academic Press, New York, 1971. (b) P. P. Gaspar and G. S. Hammond, in Carbenes, R. A. Moss and M. Jones, Jr., Eds., Wiley-Interscience, New York, 1975, 207. (c) P. P. Gaspar, in Reactive Intermediates, Vol. 1, M. Jones, Jr. and R. A. Moss, Eds., John Wiley & Sons, Inc., New York, 1978, p. 229. (d) P. P. Gaspar, in Reactive Intermediates, Vol. 2, M. Jones, Jr. and R. A. Moss, Eds., John Wiley & Sons, Inc., New York, 1981, p. 335.

2.P. P. Gaspar and R. West, in The Chemistry of Organosilicon Compound, Vol. 2,

Z.Rappoport and Y. Apeloig, Eds., John Wiley & Sons, Inc., UK. Chichester, 1998, Part 3, p. 2463.

3.(a) P. Rivie`re, M. Rivie`re-Baudet, J. Satge´, in Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. Stone, and E. W. Abel, Eds., Pergamon, Oxford, 1982, Chapter 10. (b) W. P. Neumann, Chem. Rev. 1991, 91, 311.

4.(a) C.-H. Liu and T.-L. Hwang, in Advances in Inorganic Chemistry and Radiochemistry, Vol. 28, H. J. Emeleus and A. G. Sharpe, Eds., Academic Press, Orlando, FL, 1985, p. 1.

(b) J. Satge´, Pure Appl. Chem. 1984, 56, 137.

5.For reviews on silylenes, see: (a) R. West, Pure Appl. Chem. 1984, 56, 1634. (b) G. Raabe and J. Michl, Chem. Rev. 1985, 85, 419. (c) R. West, Angew. Chem., Int. Ed. Engl.

1987, 26, 1201. (d) J. Barrau, J. Escudie´, and J. Satge´, Chem. Rev., 1990, 90, 283. (e) P. Jutzi, J. Organometal. Chem. 1990, 400, 1. (f) M. F. Lappert and R. S. Rowe, Coord. Chem. Rev. 1990, 100, 267. (g) J. Satge´, J. Organometal. Chem. 1990, 400, 121. (h) T. Tsumuraya, S. A. Batcheller, and S. Masamune, Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (i) M. Weidenbruch, Coord. Chem. Rev. 1994, 130, 275. (j) A. G. Brook and M. Brook, Adv. Organometal. Chem., 1996, 39, 71. (k) I. Hemme and U. Klingebiel, Adv. Organometal. Chem. 1996, 39, 159. (l) M. Driess, Adv. Organometal. Chem., 1996, 39, 193. (m) R. Okazaki and R. West, Adv. Organometal. Chem. 1996, 39, 232. (n) K. M. Baines and W. G. Stibbs, Adv. Organometal. Chem. 1996, 39, 275. (o) M. Driess and

H.Gru¨tzmacher, Angew. Chem., Int. Ed. Engl. 1996, 35, 827. (p) P. P. Power, J. Chem.