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.