Cooperative double deprotonation of bis(2-picolyl)amine leading to unexpected bimetallic mixed valence (M-I, MI) rhodium and iridium complexes

Abstract

Cooperative reductive double deprotonation of the complex [Rh I(bpa)(cod)]+ ([4]+, bpa = PyCH 2NHCH2Py) with one molar equivalent of base produces the bimetallic species [(cod)Rh(bpa-2H)Rh(cod)] (7), which displays a large Rh -I,RhI contribution to its electronic structure. The doubly deprotonated ligand in 7 hosts the two >Rh(cod)> fragments in two distinct compartments: a >square planar compartment> consisting of one of the Py donors and the central nitrogen donor and a >tetrahedral π-imine compartment> consisting of the other pyridine and an >imine C=N> donor. The formation of an >imine donor> in this process is the result of substantial electron transfer from the {bpa-2H}2- ligand to one of the rhodium centers to form the neutral imine ligand bpi (bpi = PyCH2N=CHPy). Hence, deprotonation of [RhI(bpa)(cod)] + represents a reductive process, effectively leading to a reduction of the metal oxidation state from RhI to Rh-I. The dinuclear iridium counterpart, complex 8, can also be prepared, but it is unstable in the presence of 1 mol equiv of the free bpa ligand, leading to quantitative formation of the neutral amido mononuclear compound [Ir I(bpa-H)(cod)] (2). All attempts to prepare the rhodium analog of 2 failed and led to the spontaneous formation of 7. The thermodynamic differences are readily explained by a lower stability of the M-I oxidation state for iridium as compared to rhodium. The observed reductive double deprotonation leads to the formation of unusual structures and unexpected reactivity, which underlines the general importance of >redox noninnocent ligands> and their substantial effect on the electronic structure of transition metals. © 2011 American Chemical Society.