η7→η3Hapticity interconversion in cycloheptatrienyl complexes of molybdenum and tungsten. Crystal structures of [Mo(NCS)(CO)2(C10H8N2)(η3-C7H7)] and [MoCl(CO)2(Ph2PCH2CH2PPh2)(η3-C7H6C6H4F-4)

Abstract

A series of reactions of the general type [MX(CO)2(η7-C7H6R)]n++ 2L′→[MX(CO)2L′2(η3-C7H6R)]n+ involving η7→η3 hapticity conversion at a molybdenum or tungsten co-ordinated cycloheptatrienyl ring have been examined with the objective of determining the effects of variation of X, M and R on the reactivity of [MX(CO)2(η7-C7H6R)]n+ towards adduct formation with ligands L′= NCMe, L′2= Ph2PCH2CH2PPh2(dppe) or 2,2′-bipyridyl (bipy). The complexes [MoX(CO)2(η7-C7H7)] exhibit a markedly X-dependent reactivity. The derivatives with X = NCO or NCS form adducts [MoX(CO)2(dppe)(η3-C7H7)]1 and 2, [MoX(CO)2(bipy)(η3-C7H7)]3 and 4 and [MoX(CO)2(NCMe)2(η3-C7H7)]{as an equilibrium mixture with [MoX(CO)2(η7-C7H7)] in NCMe}. By contrast [MoX(CO)2(η7-C7H7)](X = CCPh, C6F5 or SnPh3) do not react with dppe, bipy or NCMe at room temperature. Tungsten complexes [WX(CO)2(η7-C7H7)]n+ are activated towards adduct formation with L′= NCMe by comparison with the molybdenum analogues. Thus [WX(CO)2(η7-C7H7)](X = NCO or Br) react with NCMe to form [WX(CO)2(NCMe)2(η3-C7H7)](X = NCO 5 or Br 6). The composition of equilibrium mixtures of [Mo(CO)2(NCMe)(η7-C7H6R)]+ and [Mo(CO)2(NCMe)3(η3-C7H6R)]+ in NCMe is dependent upon R and temperature; the % composition of the η7-C7H6R component is enhanced by electron-donating ring substituents (R = Me) and by an increase in temperature. The crystal structures of 4 and [MoCl(CO)2(dppe)(η3-C7H6C6H4F-4)] have been determined. Both exhibit a pseudo-octahedral ligand arrangement at the Mo but, whilst 4 possesses a symmetrical structure with NCS located trans to the η3-C7H7 ring, the latter complex adopts an asymmetric ligand arrangement with one phosphorus of the dppe ligand positioned trans to the cycloheptatrienyl ring.