Synthesis and Properties of η⁶-Silabenzene−M(CO)₃Complexes (M = Cr, Mo)

Abstract

Synthesis of the first neutral η 6 -silabenzene complexes [M(η 6 -C 5 H 5 SiTbt)(CO) 3 ] (M ) Cr (2), Mo (3); Tbt ) 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) was achieved by the ligand exchange reactions of [M(CH 3 CN) 3 (CO) 3 ] (M ) Cr, Mo) with the kinetically stabilized silabenzene 1. X-ray crystallographic analysis of 2 revealed that the η 6 -coordinated silabenzene rings of the complexes have almost planar geometries with delocalized π-electron structures. The structures of η 6 -silabenzene complexes 2 and 3 were fully characterized by NMR, UV/vis, and IR spectroscopic analysis. All the 1 H, 13 C, and 29 Si NMR chemical shifts of the SiC 5 H 5 rings of 2 and 3 were shifted to upper field relative to those for silabenzene 1, due to the π-coordination of the silabenzene ring to the metal center. The carbonyl stretching frequencies of 2 and 3 were observed in a region of lower wavenumber as compared to those of the corresponding group 6 metal complexes of benzene. In the UV/vis spectra of 2 and 3, the absorption maxima were slightly red-shifted compared to those for the corresponding benzene complexes. The formation of η 6 -silabenzene complexes 2 and 3 is very important from the viewpoints of the novel chemical reactivity of silaaromatics. Silabenzene complexes 2 and 3 were thermally stable under an inert atmosphere but air and moisture sensitive. These compounds underwent ready addition reactions with water exclusively at their 1,2-positions to give the corresponding hydroxysilane 8a. The high regioselectivity observed for the addition reactions to 2 and 3 is in a sharp contrast to the competitive 1,2-and 1,4-addition reactions of free silabenzene 1 with water.

Recent Advances in Metallaaromatic Chemistry

“…Analogues of benzene where one CH group is formally replacedb yamain group element are wellk nown and in many cases have been shown to retain as ignificant degree of aromatic character. [24][25][26][27] Some notable examples from recent years include bisma-, [28] alumina-, [29] sila-, [30][31][32][33] germa- [34] and stannabenzenes. [35] The prospect of p-delocalization in analogueso fb enzene that contain at ransitionm etal wasf irst considered theoretically by Thorn and Hoffmann.…”

Aromaticity of Group 14 Organometallics: Experimental Aspects

“…The neutral heavy aromatic compounds synthesized by Tokitoh and co‐workers (Section 3.1) have recently found a promising application as η 6 ‐C 5 ER 6 (E=Si, Ge) and η 6 ‐C 9 ER 8 π‐arene ligands (E=Sn) for complexes of Group 6 metals (Cr, Mo) 39. 120, 121 The complexation smoothly proceeded upon the ligand‐exchange reaction of stable silabenzene 6 ,33 germabenzene 9 ,36 and 2‐stannanaphthalene 13 39 with the transition‐metal complexes [M(CH 3 CN) 3 (CO) 3 ] (M=Cr, Mo) in either benzene or THF solvents to produce the corresponding [(η 6 ‐silabenzene)M(CO) 3 ] ( 44 a,b ),120 [(η 6 ‐germabenzene)M(CO) 3 ] ( 45 a,b ),121 and [(η 6 ‐2‐stannanaphthalene)Cr(CO) 3 ] ( 46 )39 complexes in good yields (Scheme ). As further examples of heavy aromatic compounds of this type, the remarkable sila‐ and 1,4‐disilabenzene complexes of ruthenium recently synthesized by Tilley and co‐workers should also be mentioned 122…”

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η6-Diborabenzene Ligand

“…Pyridines, typically η 1 donor ligands, can be coerced into η 6 coordination by use of bulky substituents in the 2 and 6 positions, [3][4] while halfsandwich complexes of η 6 phosphabenzene, [5][6][7] arsabenzene, [8][9] and stibabenzene 9 ligands have also been reported. Silabenzenes 10 and germabenzenes, 11 kinetically stabilised by large substituents, have also been coordinated to group 6 tricarbonyl fragments. Of the boron derivatives, 12 neutral borabenzenes, [13][14] in which a CH fragment of benzene is replaced by B, require stabilisation from a Lewis base for their isolation.…”