Dinuclear allyl and µ-vinyl complexes from the reactions of a µ-hydrido dimanganese and a µ-hydrido manganese–molybdenum complex with alkynes containing α-methyl or α-methylene substituents

Abstract

The photolytic reactions of [Mn₂(µ-H)(µ-PPh₂)(CO)₈] and of [(η-C₅H₅)(CO)₂Mo(µ-H)(µ-PPh₂)Mn(CO)₄] with alkynes containing α-methyl or α-methylene substituents have been studied. The dimanganese complex reacts with MeCCH to give, in comparable yields, the η³-allyl complex [Mn₂(µ-PPh₂)(η³-C₃H₅)(CO)₇] and an inseparable mixture of the two µ-vinyl complexes [Mn(µ-PPh₂)₂{µ-σ:η²-C(Me)CH₂}(CO)₇] and [Mn₂{µ-σ:η²-C(H)C(H)Me}(µ-PPh₂)(CO)₇]. With MeCCMe, [Mn₂(µ-PPh₂)(η³-syn-MeCHCHCH₂)(CO)₇] and [Mn₂{µ-σ:η²-C(Me)C(H)Me}(µ-PPh₂)(CO)₇] are obtained, again in comparable yields. Reactions of the dimanganese complex with EtCCH and PhCCMe proceed analogously. The molybdenum–manganese complex does not give µ-vinyl complexes on reaction with MeCCH, MeCCMe, or EtCCH. Instead MeCCH gives [(η-C₅H₅)(CO)₂Mo(µ-PPh₂)Mn(η³-C₃H₅)(CO)₃] as the sole product whereas MeCCMe and EtCCH both give the same inseparable isomeric mixture of [(η-C₅H₅)(CO)₂Mo(µ-PPh₂)Mn(η³-syn-MeCHCHCH₂)(CO)₃] and [η-C₅H₅(CO)₂Mo(µ-PPh₂)Mn(η³-anti-MeCHCHCH₂)(CO)₃]. Deuteriation studies show that the formation of the allyl complexes from the alkynes involves a 1,2 hydrogen shift within the ligand; a mechanistic scheme for the reactions of the dimanganese and molybdenum–manganese complexes with α-methyl- and α-methylene-substituted alkynes consistent with such a shift is proposed.