Reactions of co-ordinated ligands. Part 38. The reactions of 1,2-dienes,1,3-dienes, and alkynes with the µ-allylidene complex [Mo2(µ-σ:η3-CHCHCMe2)-(Co)4(η-C5H5)2]; molecular structures of [Mo(Co)2(η-C5H5){µ-η3:η3-CH2C-(CH2)CHCHCMe2}Mo(CO)2(η-C5H5)], [Mo2{µ-η4-CMe2CHCHCHC(BUt)CO}-(CO)2(η-C5H5)2] and [Mo(CO)2(η-C5H5){µ-η2:η3-C(Me)C(Me)C(O)CHCHC-Me2}Mo(CO)2(η-C5H5)]

Abstract

Reaction of the µ-allylidene complex [Mo₂(µσ:η³-CHCHCMe₂)(CO)₄(η-C₅H₅)₂] with propa-1,2-diene leads to cleavage of the metal–metal bond and formation of the bis-allylic complex [Mo(CO)₂(η-C₅H₅){µ-η³:η³-CH₂C(CH₂)CHCHCMe₂}Mo(CO)₂(η-C₅H₅)] identified by X-ray crystallography. In contrast, trans-penta-1,3-diene reacts via an alternative reaction path involving carbonyl ‘insertion’ to form [Mo(CO)₂{η³-MeCHCHCHCHC(O)CHCHCMe₂}-(η-C₅H₅)]. The corresponding reactions of alkynes have also been studied. The electronegatively substituted alkyne MeO₂CCCCO₂Me forms [Mo₂{µ-σ:η³-C(CO₂Me)C(CO₂Me)CHCHCMe₂-}(CO)₄(η-C₅H₅)₂], whereas but-2-yne leads to the crystallographically identified dinuclear species [Mo(CO)₂(η-C₅H₅){µ-η²:η³-C(Me)C(Me)C(O)CHCHCMe₂}Mo(CO)₂(η-C₅H₅)]. Reaction with Bu^tC₂H or PrⁱC₂H leads to an unusual cyclisation reaction and the formation of [Mo₂{µ-η⁴-[graphics omitted]O}(CO)₂(η-C₅H₅)₂](R = Bu^t or Prⁱ). An X-ray crystallographic study of the Bu^t substituted complex established a Binuclear structure containing a Mo₂(η-C₅H₅)₂ core asymmetrically bridged by two carbonyl ligands and 2-t-butyl-6,6-dimethylcyclohexa-2,4-dienone. The Mo(1)–Mo(2) internuclear separation [2.524(1)Å] is consistent with the presence of a MoMo triple bond. The cyclohexa-2,4-dienone ligand is bonded asymmetrically to the Mo₂ core, and this is related to non-bonding interactions. The structures, bonding, and mechanisms of formation of these molecules are discussed.