η7→η3 Hapticity 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.