Transition-metal silene complexes

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Given the ability of transition-metal complexes to stabilize species via coordination, it is reasonable to expect that given the proper choice of transition metal fragment L M, silene complexes can be isolated and studied. As with analogous alkene coordination... 

When a methylene or dimethylgermyl group intervenes between the transition metal and the silyl group, as in 164, photolysis results in rearrangement to 165, which appears, for the methylene case, to involve the intermediacy of a metal-silene complex (Scheme 25). 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

In 1988, Tilley and coworkers first succeeded in the synthesis of a ruthenium-silene complex by the reaction of Cp Ru(PR3)Cl with a Grignard reagent, and the structure was confirmed by X-ray crystallography (Eq. 9) [10]. They later synthesized an iridium complex in a similar manner (Eq. 10) [11]. Berry synthesized a tungsten-silene complex by treatment of a tungsten-chloride complex with Mg (Eq. 11) [12]. Although reactions of transition metal-silene complexes with Mel, HX, and MeOH have been reported, little is known about their reactivities [11,12]. 

To examine the reactivity of a zirconium-silicon bond, Mori et al. synthesized a zirconium-silicon complex from Cp2ZrCl2 and Ph/BuSiLi (1 equiv.) and investigated the reactivity of this complex [ 14]. Furthermore, they found that the reaction of Cp2ZrCl2 with 2 equivalents of Me2PhSiLi gave disilylzirconocene, which was easily converted into a zirconium-silene complex via /J-hydrogen elimination (Scheme 1) [15]. This is the first example of the formation of an early transition metal-silene complex. In this chapter, the reactivity of the zirconium-silene complex is described. 

---