Reactions of co-ordinated ligands. Part 40. Synthesis, structure, and reactivity of dirhenium µ-allylidene complexes; crystal structures of[Re2(µ-σ:η3-CHCHCMe2)(CO)8] and [Re2(µ-σ:η3-CHCHCMe2)(CO)9]

Abstract

Reaction of [Re₂(µ-H)(µ-CHCHEt)(CO)₈] with 3,3-dimethylcyclopropene at room temperature leads to the formation of the µ-allylidene complex [Re₂(µ-σ:η³-CHCHCMe₂)(CO)₈](1), which has been structurally characterised by X-ray crystallography. The molecule contains two Re(CO)₄ fragments bridged by a µ-CHCHCMe₂ ligand where the Re–Re separation is 3.058(1)Å. The µ-allylidene fragment adopts a transoid geometry, and possible reasons for this preference are discussed. A plausible reaction pathway for the formation of (1) is described. Reaction of (1) with carbon monoxide leads to metal–metal bond cleavage and the formation of [Re₂(µ-σ:η³-CHCHCMe₂)(CO)₉](2), which has also been characterised by X-ray crystallography. The reaction has resulted in the transformation of (1) into a Re(CO)₅ substituted η³-ally1 moiety, which is η³-bonded to a Re(CO)₄ unit. The Re–Re separation in (2) is 4.190( 1)Å. Complex (2) is thermally stable, however u.v. photolysis leads to the formation of a µ-butadienyl complex.