Synthesis and reactivity of η3-γ-lactonyl complexes of molybdenum; crystal structures of [Mo{η3-OC(O)CHCHCH}(CO)2(η-C5Me5)], [Mo{η-2PhCH2NHCHCHCH(CO2H)}(CO)2(η5-C9H7)] and [Mo{η3-OC(O)CHCHCHCO}(NCMe)(CO)(η-C5H5)]

Abstract

Reaction of 2-(trimethylsilyloxy)furan with the compounds cis-[Mo(NCMe)₂(CO)₂L][BF₄](L =η-C₅H₅, η-C₅Me₅ or η⁵-C₉H₇) afforded the η³-γ-lactonyl complexes [Mo{η³-[graphic omitted]H}(CO)₂L]. The structure of one of these species, [Mo{η³-[graphic omitted]H}(CO)₂(η-C₅Me₅)], has been established by a single-crystal X-ray diffraction study, which confirmed that the γ-lactonyl moiety is bound to the molybdenum via three carbon atoms as an η³-allyl. Treatment of these lactonyl complexes with nucleophilic reagents (amines, methoxide) resulted in lactone ring opening and overall addition of the nucleophile to the γ-carbon of the lactone ring, rather than at the lactonyl carbonyl carbon atom as might have been expected. The product of the reaction between [Mo{η³-[graphic omitted]H}(CO)₂(η⁵-C₉H₇)] and PhCH₂NH₂ has been structurally characterised by an X-ray diffraction study as the zwitterionic, η²-alkene complex [Mo{η²-PhCH₂NHCHCHCH(CO₂H}(CO)₂(η⁵-C₉H₇)]. Similarly, [Mo{η³-[graphic omitted]H}(CO)₂(η-C₅H₅)] and methoxide anion gives, after acidification, [Mo{anti-η³-(MeO)CHCHCH(CO₂H)}(CO)₂(η-C₅H₅)], in which the lactone ring has been cleaved to give an η³-allyl moiety ligating the metal centre. An extended Hückel molecular orbital calculation on [Mo{η³-[graphic omitted]H}(CO)₂(η-C₅Me₅)] suggests that these reactions proceed via initial attack at the metal centre, followed by a rearrangement which effectively transfers the nucleophilic moiety to the γ-carbon of the lactone ring. Reaction of the η³-lactonyl complex [Mo{η³-[graphic omitted]H}(CO)₂(η-C₅H₅)] with HBF₄·Et₂O resulted in a remarkable ring-enlargement reaction, in which a co-ordinated carbon monoxide formally inserts into the lactone carbon–oxygen bond to form the crystallographically characterised complex [Mo{η³-[graphic omitted]O}(NCMe)(CO)(η-C₅H₅)].