Reactions of co-ordinated ligands. Part 37. Synthesis and structure of the dimolybdenum µ-allylidene complex [Mo2(µ-σ:η3-CHCHCMe2)(CO)4(η-C5H5)2]; formation and crystal structure of the oxo-µ-allylidene complex [Mo2O(µ-σ:η3-CHCHCMe2)(µ-CO)(CO)(η-C5H5)2]. Thermal rearrangement to alkyne and 1,3-diene dimolybdenum species and structure of [Mo3(µ3-η4-CCHCMeCH)(CO)4(η-C5H5)3]

Abstract

Reaction of the unsaturated dinuclear complex [Mo₂(CO)₄(η-C₅H₅)₂] with 3,3-dimethylcyclopropene leads to ring opening and formation of the µ-allylidene complex [Mo₂(µ-σ,η³-CHCHCMe₂)(CO)_4–(η-C₅H₅)₂](1), which has been structurally identified by X-ray crystallography. The molecule contains two molybdenum atoms singly bonded [Mo(1)–Mo(2) 3.1 057(9)Å]. A CHCHCMe₂ fragment bonds σ to Mo(1) and as an allyl, η³, to Mo(2), i.e. as a dimethyl substituted µ-allylidene where the two olefinic protons of the original cyclopropene retain a relative cis configuration. The mechanism of formation of (1) is discussed in terms of a reaction at a dinuclear centre. In refluxing wet acetonitrile (1) is transformed into the mixed-valence µ-allylidene complex [Mo₂O(µ-σ,η³-CHCHCMe₂)(µ-CO)(CO)(η-C₅H₅)₂](2). An X-ray crystal-structure determination shows that the oxygen ligand is co-ordinated to Mo(1) which is σ bonded to the µ-allylidene fragment; the Mo–Mo bond is reduced to 2.881(1)Å. In refluxing toluene–hexane (1) rearranges to form the alkyne complex [Mo₂(µ-PrⁱC₂H)(CO)₄(η-C₅H₅)₂](3), an isoprene complex [Mo₂{CH₂CHC(Me)CH₂}(CO)₂(η-C₅H₅)₂](4), and a tinuclear complex [Mo₃(µ₃-η⁴-CCHCMeCH)(CO)₄(η-C₅H₅)₃](5), the last being identified by X-ray crystallography. The molecule is a triangular array of three molybdenum atoms, which show typical Mo–Mo single bond separations. The µ-allylidene fragment present in (1) loses three hydrogen atoms in the formation of (5), generating a methyl substituted molybdacyclopentadiene. The mechanism of formation of (3), (4), and (5) is discussed.