Redox Activity of Noninnocent 2,2'-Bipyridine in Zinc Complexes: An Experimental and Theoretical Study

We report on a systematical reactivity study of β-diketiminate zinc complexes with redox-active 2,2'-bipyridine (bpy). The reaction of LZnI (L = HC[C(Me)N(2,6-iPr₂C₆H₃)]₂) with NaB(C₆F₅)₄ in the presence of bpy yielded [LZn(bpy)][B(C₆F₅)₄] (1), with bpy serving as a neutral ligand, whereas reduction reactions of LZnI with 1 or 2 equiv of KC₈ in the presence of bpy gave the radical complex LZn(bpy) (2) and [2.2.2-Cryptand-K][LZn(bpy)] (3), in which bpy either acts as a π-radical anion or a diamagnetic dianion, respectively. The paramagnetic nature of 2 was confirmed via solution magnetic susceptibility measurements, and UV-vis spectroscopy shows that 2 exhibits absorption bands typical for bpy radical species. The EPR spectra of 2 and its deuterated analog 2- demonstrate that the spin density is localized to the bpy ligand. Density functional theoretical calculations and natural bond orbital analysis were employed to elucidate the electronic structure of complexes 1-3 and accurately reproduced the structural experimental data. It is shown that reduction of the bpy moiety results in a decrease in the β-diketiminate co-ligand bite angle and elongation of the Zn-N(β-diketiminate) bonds, which act cooperatively and in synergy with the bpy ligand by decreasing Zn-N(bpy) bond lengths to stabilize the energy of the LUMO.