Reactions of co-ordinated ligands. Part 30. The transformation of methylenecyclopropanes into cationic η4-trimethylenemethanemolybdenum complexes, reactions with nucleophilic reagents, and the molecular structure of [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4

Abstract

Reaction of [Mo2(CO)6(η-C5Me5)2] with methylenecyclopropane and AgBF4 in CH2Cl2 affords the cationic trimethylenemethane complex [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4]. Methylenecyclopropane and 2,2-dimethylmethylenecyclopropane react with [Mo(NCMe)2(CO)2(η-C5H5)][BF4] to give [Mo{η4-C(CH2)3}(CO)2(η-C5H5)][BF4] and [Mo{η4-C(CH2)2CMe2}(CO)2(η-C5H5)][BF4] respectively. A single-crystal X-ray diffraction study of [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4] confirmed that ring opening of methylenecyclopropane had occurred. The complex crystallises in the centrosymmetric orthorhombic space group Pbca with a= 12.822(2), b= 12.311 (3), c= 22.660(4)A, and Z= 8 ion pairs. The structure has been solved by conventional methods and refined by full-matrix least squares to R= 0.0674 for 2 747 observed reflections at 268 ± 1 K. In the cation the trimethylenemethane ligand adopts an orientation that is syn with respect to the OC–Mo–CO angle. It is pyramidal with the CH2 groups bent towards the molybdenum atom by an average of 12.4°. There is intramolecular congestion involving the C5Me5 and C(CH2)3 ligands that may contribute towards the observed asymmetric bonding of the former to the metal atom. Extended Huckel molecular-orbital calculations suggest, that the observed syn stereochemistry is electronically preferred, and that the barrier to rotation of the η4-C(CH2)3 ligand relative to a Mo(CO)2(η-C5H5)+ fragment is high. The stereochemistry of the ring-opening reaction is disrotatory-out as exemplified by the conversion of cis-and trans-2,3-dimethylmethylenecyclopropane into syn,syn-dimethyl and syn,anti-dimethyl-trimethylene-methane complexes. The reaction of these cationic η4-trimethylenemethane complexes with the nucleophiles BH4–, OH–, CuMe2–, and SPh– affords η3-allylic complexes derived from attack on the peripheral carbons.