Hydridovinyl complexes

Jimenez-Catano R, Niu S, Hall MB. Theoretical studies of inorganic and organometallic reaction mechanisms. 10. Reversal in stability of rhodium and iridium j2-ethene and hydridovinyl complexes. Organometallics 1997 16(9) 1962-1968. 

The reaction chemistry of the hydridovinyl complexes has, however, often proven unexpectedly complicated, owing to their propensity to effect intra and zfzf rmolecular C-H activations. For instance, under an atmosphere of ethylene, benzene solutions of 202 undergo a thermal (60 °C) reaction to afford exclusively 210. This reaction is presumed to involve the intermediacy of Tp Ir(C6H5)2 (f/ -C2H4), an unobserved product of the C—H activation of benzene. This proposal is supported by the fact that Tp Ir(C6H5)2(N2) (211) can be isolated (Section II-C.2), and converts cleanly to 210 under comparable conditions. 

Hydridovinyl complexes of the type discussed have been widely implicated as the active species in a variety of C-H activation reactions observed for simple bis(7t-alkene) complexes. However, since these reactions typically commence from the bis(7t-alkene) complexes, a summary of these processes is warranted here, though many are more appropriately discussed elsewhere. 

Early investigations of the stability of these Ti-alkene and hydridovinyl complexes, and in particular the influence of the Tp ligand, concluded that (i) methyl substituents (e.g. in Tp, are essential to the thermal stability of... 

Several trifluoromethylpyrazoles, for instance 3(5)-trifluoromethyl-5(3)-methylpyrazole (44) were used to prepare platinum complexes and the x-ray structure of the resulting complex reported <83 C774>. The ligand tris[3-trifluoromethyl-5-methyl(pyrazol-l-yl)]borate (82) yields a hydridovinyl iridium complex more stable than its j/ -ethylene isomer <89JA5480>. The x-ray structure of the related platinum complex (83) has been described <831C774>. 

Similar mixtures of hydrido-allyl compounds are obtained from complexes of 1-propene and 1-butene (Scheme 28), the difference being that discrete Tp Ir (alkene)2 complexes have not been isolated, only their hydridovinyl isomers. It is interesting to note that the analogous reaction of Tp IrH(CH = CH2) (rj - 2 -4) (203) affords (kinetically) the tethered butene complex Tp IrH ((7-7T-CH2CH2CH = CH2) (216, 80%, vide supra), and a small quantity of the hydrido-crotyl complex Tp IrH(f/ -C3H4Me) (217), which becomes the sole product upon prolonged heating (100 °C, 20 h). A series of related hydrido-allyl complexes (328-331) are obtained from ambient temperature photolysis of the 1,3-butadiene complexes 232-234 (Section III-B.2). 

Metal alkenyls have received relatively little attention beyond the ubiquitous vinyl complexes that are obtained either thermally or photochemically from simple alkene complexes of the type Tp M(t/ -C2H4)(L) (L = C2H4, PR3, NCMe, CO, Scheme 49, see also Section III-B.l). The reactivity of these species largely mirrors that of their 71-alkene precursors, in which their intermediacy has often been implicated. Most of this chemistry, and studies establishing the relative stability and accessibility of the Ti-alkene and hydridovinyl forms, are discussed in detail in Section III-B.l however, some merit further attention. 

The involvement of alkene complexes in C H activation reactions, both thermal and photochemieal, is widespread and well doeumented. However, given the diversity of reagents and outcomes encoimtered, mechanistic studies have necessarily focussed on individual reaction schemes, with emphasis upon optimising and accounting for the observed outcomes. One exeeption, that has been explored in some detail, is the intramolecular C-H activation of olefin eomplexes (typieally of ethylene) giving rise to hydridovinyl species. This reaction is, in fact, of fundamental importance, since it has been directly implicated in many inter-molecular activations, the hydridovinyl being often considered the active species. 

The seminal report in this area documented the thermally-induced isomerisation of Tp Ir(CO)(>/ -C2H4) (229) to Tp = TrH(CH=CH2)(CO) (289), which proceeds cleanly at 60 °C. This thermodynamic preference is the reverse of that observed for the Cp and Cp analogues, which favour the j/ -alkene complex, as does the photochemically generated Tp RhH(CH=CH2)(CO) (290), which rapidly reverts to Tp Rh( -C2H4)(CO) (148) under ambient conditions. This disparity between the rhodium and iridium 71-alkene complexes has been widely observed. However, while the rhodium hydridovinyls are rarely isolated, the kinetic accessibility of Tp RhH(CH = CH2)(f/ -C2H4) (294) has been established. It was found that on treatment with NCMe or pyridine, Tp Rh( -C2H4)2 (118) cleanly converts to Tp Rh(CH2CH3)(CH = CH2)(L) (L = NCMe (156), py (293)),... 

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