Assessing the Metal–Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties

A one-pot synthetic procedure for a series of bimetallic Nb/M complexes, Cl-Nb( i PrNPPh 2 ) 3 M-X (M = Fe (2), Ni (4), Cu (5)), is described. A similar procedure aimed at synthesizing a Nb/Co analogue instead affords i PrN═Nb( i PrNPPh 2 ) 2 (μ-PPh 2 )Co-I (3) through cleavage of one phosphinoamide P-N bond under reducing conditions. Complexes 4 and 5 are found to have short Nb-M distances, corresponding to unusual metal-metal bonds between Nb and these first row transition metals. For comparison, a series of heterobimetallic O≡Nb( i PrNPPh 2 ) 3 M-X complexes (M = Fe (7), Co (8), Ni (9), Cu (10)) was synthesized. In these complexes, the Nb V center is engaged in sufficient π-bonding to the terminal oxo ligand to remove the driving force for direct metal-metal interactions. A comparison of the cyclic voltammograms of 2 and 4-10 reveals that the presence of a second metal shifts the redox potentials of both Nb and the late metal center anodically, even when direct metal-metal interactions are not present.