Synthesis and reactivity of an N-triphos Mo(0) dinitrogen complex

Abstract

The preparation and reactivity of a novel molybdenum dinitrogen complex supported by a nitrogen-centred tripodal phosphine ligand (N-triphos, N(CH₂PPh₂)₃, NP₃^Ph) are reported. Reaction of N-triphos with [MoX₃(THF)₃] (X = Cl, Br, I) gave the Mo(III) complex [MoX₃(κ²-NP₃^Ph)(THF)] (1), where bidentate N-triphos coordination was observed. Reduction of this complex in the presence of dppm (bis(diphenylphosphino)methane) gave the dinitrogen complex [Mo(N₂)(dppm)(κ³-NP₃^Ph)] (2), which exhibits moderate dinitrogen activation. An additional hydride complex, [Mo(H)₂(dppm)(κ³-NP₃^Ph)] (4), was produced either as a minor side product during the reduction step, or as a major product by direct hydrogenation of the dinitrogen complex 2. The reactivity of the dinitrogen complex 2 with a range of Lewis acids was also investigated. At low temperatures, protic or borane Lewis acids (H⁺, BBr₃ and tris(pentafluorophenyl)borane (BCF)) were found to coordinate to the apical nitrogen atom of the N-triphos ligand, with no conclusive evidence of any functionalisation of the dinitrogen ligand. Alkali metal Lewis acid addition to 2 resulted in the unexpected rearrangement of the N-triphos ligand to form [Mo(dppm)(PMePh₂)(PCP)][B(C₆F₅)₄] (7), where PCP, [Ph₂PCNHCH₂PPh₂] is the carbenic ligand formed upon rearrangement from the reaction of 2 with M[B(C₆F₅)₄] (M = Li, Na or K). Single crystal X-ray diffraction of complexes 1, 2, 4 and 7 provided structural confirmation of the N-triphos molybdenum complexes described.