Journal of the American Chemical Society, Vol.134, No.17, 7301-7304, 2012
Groups 5 and 6 Terminal Hydrazido(2-) Complexes: N-beta Substituent Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation States
Brightly colored terminal hydrazido(2-) (dme)MCl3(NNR2) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl4(NNR2) complexes have been synthesized and characterized. Perturbing the electronic environment of the beta (NR2) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N-beta lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium diallcylhydrazides, which pyramidalize N-beta in order to reduce the overlap between the Nb=N-alpha pi bond and the N-beta lone pair. For W complexes, increasing the energy of N-beta eventually leads to reduction from formally [W-VI N-NR2] with a hydraziclo(2-) ligand to [W-IV=N=NR2] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives.