Molybdenum-based alkyne–isocyanide coupling reactions: synthesis of a reactive diiminometallacyclopentene complex

The complex [Mo(CO)(PhCCPh)₂(η-C₅Me₅)][BF₄] reacted with three equivalents of 2,6-dimethylphenyl isocyanide, CNxyl, to give the tris(isocyanide) complex [Mo(PhCCPh)(CNxyl)₃(η-C₅Me₅)][BF₄] 1. With four equivalents of CNxyl, alkyne–isocyanide coupling leads to the formation of the diiminometallacyclopentene [Mo{C(Nxyl)C(Ph)C(Ph)CNxyl}(CNxyl)₂(η-C₅Me₅)][BF₄] 2 which decomposes in thf at room temperature to give the tetrakis(isocyanide) complex [Mo(CNxyl)₄(η-C₅Me₅)][BF₄] 3. In dichloromethane, 2 gives the diiminocyclobutene xylNCC(Ph)C(Ph)CNxyl 4, the η²-iminoacyl complex [MoCl(CNxyl)₂(η²-xylNCCH₂Cl)(η-C₅Me₅)][BF₄] 5, formed by an addition reaction with dichloromethane, and the metallacyclobutene complex [Mo{C(Ph)C(Ph)CN(H)xyl}(CNxyl)₃(η-C₅Me₅)][BF₄]₂6. Complex 5 is more efficiently synthesized by photolysis of 3 in dichloromethane, but the same reaction in chloroform produces the dichloromethyl complex [MoCl(CHCl₂)(CNxyl)₃(η-C₅Me₅)][BF₄] 7 which thermally decomposes to [MoCl₂(CNxyl)₃(η-C₅Me₅)][BF₄] 8. Reaction of 2 with HCl in diethyl ether results in protonation of the metallacyclic ring and formation of the iminium metallacyclopentene [MoCl{C(Nxyl)C(Ph)C(Ph)CN(H)xyl}(CNxyl)₂(η-C₅Me₅)][BF₄] 9. Compound 3 is oxidised by AgBF₄ to give the molybdenum(IV) complex [MoF(CNxyl)₄(η-C₅Me₅)][BF₄]₂10. The molecular structures of 3–6, 7/8 and 10 have been determined by X-ray crystallography.