Influence of Ligand Geometry in Bimetallic Ytterbocene Complexes of Bridging Bis(bipyridyl) Ligands

Abstract

Two new bimetallic complexes, [Cp*2Yb]2(μ-1,3-(2,2‘-bipyridyl)-5-tBu-C6H3) (1) and [Cp*2Yb]2(μ-1,4-(2,2‘-bipyridyl)-C6H4) (2), and their corresponding two-electron oxidation products [1]2+ and [2]2+ have been synthesized with the aim of determining the impact of the bridging ligand geometry on the electronic and magnetic properties of these materials. Electrochemistry, optical spectroscopy, and bulk susceptibility measurements all support a ground-state electronic configuration of the type [(f)13-(πa*)1-(πb*)1-(f)13]. Density functional theory calculations on the uncomplexed bridging ligands as doubly reduced species also indicate that the diradical electronic configuration is the lowest lying for both meta- and para-bis(bipyridyl) systems. The electrochemical and optical spectroscopic data indicate that the electronic coupling between the metal centers mediated by the diradical bridges is weak, as evidenced by the small separation of the metal-based redox couples and the similarity of the f−f transitions of the associated dicationic complexes ([1]2+ and [2]2+) relative to those of the monometallic [Cp*2Yb(bpy)]+ analogue. The magnetic susceptibility data show no evidence for exchange coupling between the paramagnetic metal centers in the neutral complexes, but do indicate weak exchange coupling between YbIII and ligand radical spins on each of the effectively independent halves of the bimetallic complexes. These findings are in contrast to those reported recently for CoIII/II dioxolene bimetallic complexes bridged by these same bis(bipyridyl) ligands. The difference is attributed in part to the dominant singlet diradical character of the bridging ligands in the ytterbocene complexes. These experimental and theoretical results are consistent with expectations for organic diradical spin orientations for meta versus para substituents across a phenylene linker, but this effect does not induce significant longer-range superexchange or electronic interactions between the metal centers in these systems.