Sterically hindered iminophosphorane complexes of vanadium, iron, cobalt and nickel: a synthetic, structural and catalytic study

Abstract

Bis(aryliminophosphoranyl)alkanes L¹ = (CH₂)_n(R¹₂PNR²)₂ (n = 1 or 2, R¹ = Ph or Me, R² = Ph, C₆H₂Me₃-2,4,6 or C₆H₃Prⁱ₂-2,6) reacted with cobalt and nickel dihalides to give chelate complexes of the type MX₂(L¹). The reaction of Li[HC(Ph₂PNR²)₂] with MX₂ afforded the compounds MX{HC(Ph₂PNR²)₂} (M = Co, X = Cl; M = Ni, X = Br). The crystal and molecular structures of CoCl₂{CH₂(Ph₂PNPh)₂} and NiBr{HC(Ph₂P NC₆H₃Prⁱ₂)₂} were determined. While the former has the expected tetrahedral structure, the latter is square planar. Ab initio calculations on the latter show that the bonding of the bis(iminophosphoranyl)methanide ligand to nickel involves π delocalisation over the N₂P₂C framework as well as σ bonding between nickel and the P₂CH carbon atom. Bis(iminophosphoranyl)pyridines C₅H₃N(R₂PNR′)₂-2,6 (R = Ph or cyclohexyl, R′ = C₆H₂Me₃-2,4,6, C₆H₃Prⁱ₂-2,6 or SiMe₃) reacted with VCl₃(thf)₃, FeX₂ (X = Cl or Br) or CoCl₂ to give the corresponding octahedral (V) or five-co-ordinate (Fe, Co) complexes. The crystal structure of FeBr₂{C₅H₃N(Ph₂PNSiMe₃)₂-2,6} has been determined; the complex is trigonal bipyramidal. In the presence of methylaluminoxane [MeAlO]_n, cobalt complexes of bis(aryliminophosphoranyl)methane are moderately active in ethene polymerisation, while bis(iminophosphoranyl)pyridine iron complexes show only low activity. The vanadium complexes polymerise ethene under ambient conditions and give polymers of very high molecular weight.