Arene–molybdenum chemistry: a novel redistribution reaction of alkenes on molybdenum forming cationic diene complexes and alkanes

Abstract

The dimeric complexes [(π-C₆H₆)Mo(R¹C₃H₃R²)Cl]₂, R¹C₃H₃R²= C₃H₅, MeC₃H₄, or MeC₃H₃Me, are formed from (π-C₆H₆)₂Mo and R¹C₃H₃R²Cl. They react with ethylaluminium dichloride in benzene giving violet solutions. With mono-olefins at 20° these give alkanes and, after hydrolysis and treatment with NH₄PF₆, cationic diene complexes of type [(π-C₆H₆)Mo(π-alkenyl)(diene)]⁺PF₆^–. Thus, propene undergoes oxidative dimerisation to the trans, trans-hexa-2,4-diene complex [(π-C₆H₆)Mo(π-C₃H₅)(π-C₆H₁₀)]⁺PF₆^– whereas cis-pent-2-ene yields the penta-1,3-diene complexes [(π-C₆H₆)Mo(π-MeC₃H₃Me)(C₅H₈)]⁺PF₆^– and [(π-C₆H₆)Mo(π-EtC₃H₄)(C₅H₈)]⁺PF₆^–. The diene compounds may also be synthesised from [(π-C₆H₆)Mo(π-R¹C₃H₃R²)Cl]₂ with the corresponding conjugated dienes. A possible mechanism is presented for the disproportionation of the mono-olefins into diene complexes and alkanes. The violet solutions also act as catalysts for the dimerisation of ethylene to butenes at 20° and for co-dimerisation of ethylene and propene to pentenes.