Preparation and some reactions of Group VI metal monodentate bis-phosphine carbonyl complexes. Mechanistic aspects of chelate-ring formation

Abstract

A series of complexes LM(CO)₅[M = Cr, Mo, or W; L = Me₂PCH₂CH₂PMe₂(dmpe), Ph₂PCH₂PPh₂(dpm), Ph₂PCH₂CH₂PPh₂(dpe), Ph₂PCH₂CH₂CH₂PPh₂(dpp), or Ph₂PCH₂CH₂AsPh₂(ape)] has been prepared and characterised by analysis, and i.r., mass, and n.m.r. spectroscopy. The complexes can be methylated with Me₃OBF₄ to give [(MeL)M(CO)₅]BF₄. The acid-assisted nucleophilic substitution reaction used in the formation of the complexs can also be applied to their conversion into bridged complexes, L[M(CO)₅]₂. Variations in geminal complexes coupling constants ²J_PCH and aromatic solvent induced shifts for the series of complexes(dmpe)M(CO)_n(M = Cr, Mo, or W; n= 4 or 5) are discussed and compared with those in (Me₂PXCH₂)₂(X = O or S) and [(Me₃PCH₂)₂](BF₄)₂, whose syntheses are also reported. Changes in the ³¹P chemical shift are used to demonstrate that the chelation shift is comparable to the co-ordination shift. Kinetic studies of the rate of the chelation reaction, LM(CO)₅→ LM(CO)₄+ CO, have been used to elucidate details of its mechanism. The reaction follows first-order kinetics, as required for the intramolecular process. The magnitude of the enthalpy of activation (ca. 140 kJ mol^–1) and the similarity in rate between phosphorus and arsenic nucleophiles, suggest a large dissociative component in the activation. The positive entropy of activation and particularly its large variation (+6 to +71 J K^–1 mol^–1) suggest a concerted process in the transition state. The smaller the potential chelate ring, the faster the reaction in the series L = Ph₂P(CH₂)_nPPh₂(n= 1, 2 or 3); this appears to be largely an entropy effect.