Cyclopentadienone complexes of molybdenum: synthesis of carbonyl, acetonitrile, phosphine and phosphido-bridged derivatives

Abstract

Reaction of [Mo(CO)₆] with 2 equivalents of tetraphenylcyclopentadienone (tetracyclone) in refluxing toluene gives [Mo(CO)₂(η⁴-C₄Ph₄CO)₂]1a in ca. 95% yield. With 1 equivalent of tetracyclone in boiling heptane, [{Mo(CO)₃(σ:η⁴-C₄Ph₄CO)}₂]2 is produced as well as 1a. This dimeric complex has been crystallographically characterised [triclinic, space group P(no. 2), a= 8.980(3), b= 11.332(4), c= 13.526(5)Å; α= 77.55(3), β= 71.94(3), γ= 81.02(3)°, Z= 2 monomer units]; each molybdenum is in a distorted-octahedral environment, co-ordinated by the η⁴-diene ligand, three carbonyl ligands, and the ketonic carbonyl of the other centrosymmetrically related tetracyclone. Another convenient route into the chemistry of tetracyclone molybdenum complexes is provided by [Mo(CO)₃(NCMe)(η⁴-C₄Ph₄CO)]5 which is formed in high yield by treating [Mo(CO)₃(NCMe)₃] with tetracyclone. One (but only one) of the tetracyclone ligands of complex 1a can be displaced by chelating phosphines to give [Mo(CO)₂(L–L)(η⁴C₄Ph₄CO)](L–L = Ph₂PCH₂PPh₂ or Ph₂PCH₂CH₂PPh₂); the stereochemistry and fluxionality of these complexes is briefly discussed. Thermal reaction of 1a with monodentate phosphines (L = PPh₃ or PPh₂Me) yields [Mo(CO)₃L(η⁴-C₄Ph₄CO)]7 by a carbonyl redistribution process; the same compounds can be prepared from 5 by displacement of the labile acetonitrile ligand, as can an analogous species with L = PPh₂H. Thermolysis of [Mo(CO)₃(PPh₂H)(η⁴-C₄Ph₄CO)] or thermal reaction of 1a with PPh₂ H gives the phosphido-bridged complex [Mo₂(CO)₃(µ-PPh₂)(µ-σ:η⁵-C₄Ph₄COH)]8. The crystal structure of 8 has also been determined [triclinic, space group P(C_i′¹ no. 2), a= 12.806(10), b= 14.173(8), c= 17.958(7)Å, α= 83.75(4), β= 86.92(5), γ= 71.57(5)°, Z= 2] and reveals a formal MoMo double bond [2.923(2)Å] bridged by a diphenylphosphido group and a tetracyclone ligand bonded in a σ:η⁵ manner. The other tetracyclone is bonded as an η⁵-hydroxytetraphenylcyclopentadienyl ligand, having taken up the proton released by oxidative addition of the P–H bond.