Reactions of co-ordinated ligands. Part 53. Synthesis of dimolybdenum and ditungsten µ-σ,η2-(4e)-vinylidene and -allenylidene complexes; crystal structures of [N(PPh3)2][Mo2{µ-σ:η2-(3e)-C2Ph}(CO)4(η5-C9H7)2], [Mo2{µ-σ:η2-(4e)-CC(Ph)(CH2)4OMe}(CO)4(η-C5H5)2] and [Mo2{µ-σ:η2-(4e)-CCCMe2}(CO)4(η-C5H5)2]

Abstract

Addition of RCCLi to [Mo₂(CO)₄(η-C₅H₅)₂] affords the anionic 1 : 1 adducts Li[Mo₂{µ-σ:η²-(3e)-C₂R}(CO)₄(η-C₅H₅)₂], the structure of the analogous η⁵-indenyl analogue being confirmed by a single-crystal X-ray diffraction study with the relatively stable complex [N(PPh₃)₂][Mo₂{µ-σ:η²-(3e)-C₂Ph}(CO)₄(η⁵-C₉H₇)₂]. The same anionic species are also formed on deprotonation (LiBu^t) of the transversally bridged alkyne complexes [Mo₂(µ-RC₂H)(CO)₄(η-C₅H₅)₂]. Variable-temperature NMR studies suggest that the µ-acetylide ligand in these anions readily switches from one metal centre to the other. Protonation of Li[Mo₂{µ-σ:η²-(3e)-C₂Ph}(CO)₄(η-C₅H₅)₂] affords a relatively unstable ‘side-on’ bonded vinylidene [Mo₂{µ-σ:η²-(4e)-CCHPh}(CO)₄(η-C₅H₅)₂], which in solution at room temperature rearranges into [Mo₂(µ-PhC₂H)(CO)₄(η-C₅H₅)₂]. Reaction of Li[Mo₂{µ-σ:η²-(3e)-C₂Ph}(CO)₄(η-C₅H₅)₂] with MeOSO₂CF₃ in tetrahydrofuran (thf) affords via ring opening of the thf, the crystallographically identified stable complex [Mo₂{µ-σ:η²-(4e)-CC(Ph)(CH₂)₄OMe}(CO)₄(η-C₅H₅)₂]. This is the first structurally characterised ‘side-on’ bonded vinylidene. The vinylidene fragment donates four electrons to the dimolybdenum centre with Mo(1)–C(1) 1.909(5), Mo(2)–C(1) 2.179(5) and Mo(2)–C(2) 2.443(6)Å. The corresponding alkylation reaction in dichloromethane affords [Mo₂{µ-σ:η²-(4e)-CC(Ph)Me}(CO)₄(η-C₅H₅)₂], and in this way a wide range of ‘side-on’ bonded vinylidene complexes were synthesised, extension of the synthetic procedure affording the corresponding ditungsten systems. Variable-temperature NMR studies show that in solution some of these complexes exhibit interesting dynamic behaviour and these observations are discussed. In solution the compound [Mo₂{µ-σ:η²-(4e)-CC(Ph)Me}(CO)₄(η-C₅H₅)₂] undergoes an unusual thermal rearrangement into [Mo₂{µ-σ:η³-CHC(Ph)CH₂}(CO)₄(η-C₅H₅)₂], and it is shown that a variety of β-substituted vinylidenes undergo analogous rearrangement reactions to form µ-allylidene complexes. Protonation of the adduct Li[Mo{µ-σ:η²-(3e)-C₂C(Me)CH₂}(CO)₄(η-C₅H₅)₂] formed between CH₂C(Me)CCLi and [Mo₂(CO)₄(η-C₅H₅)₂] affords the first ‘side-on’ bonded allenylidene [Mo₂{µ-σ:η²-(4e)-CCCMe₂}(CO)₄(η-C₅H₅)₂], which was structurally characterised by X-ray crystallography. The molecule contains an asymmetrically bridged allenylidene, the first such molecule to be structurally identified. The Mo(1)–C(1) distance of 1.912(3)Å indicates the presence of a MoC double bond.