Tungsten and molybdenum dinitrogen complexes supported by a pentadentate tetrapodal phosphine ligand: comparative spectroscopic, electrochemical and reactivity studies

Abstract

The tungsten dinitrogen complex [W(N₂)(P^Me₂PP^Ph₂)] (2) (P^Me₂PP^Ph₂ = [2-({bis[3-(diphenylphosphino)propyl]-phosphino}methyl)-2-methylpropane-1,3-diyl]bis(dimethylphosphine)) is synthesized and characterized by X-ray diffraction as well as IR and NMR spectroscopies, showing strong analogies to its molybdenum analogue [Mo(N₂)(P^Me₂PP^Ph₂)] (1). Whereas cyclic voltammetry studies indicate very similar redox potentials, detailed electrochemical and IR-spectroelectrochemical investigations reveal characteristic differences between 1 and 2 upon electrochemical oxidation in THF. Protonation of 2 with HBAr^F (BAr^F = tetrakis(3,5-bis(trifluoromethyl)-phenyl)borate) leads to the hydrazido(2-) derivative 3 which is spectroscopically characterized as well. In the presence of SmI₂/H₂O slightly overstoichiometric conversion of N₂ to ammonia (2.75 equiv.) is observed. Although this is far below the activity of the Mo-complex 1, it renders 2 the first W complex to produce more than 2 equivalents of NH₃ from N₂ upon addition of protons and reductant.