Guanidinate Yttrium Complexes Containing Bipyridyl and Bis(benzimidazolyl) Radicals

Ancillary ligand scaffolds that sufficiently stabilize a metal ion to allow its coordination to an open-shell ligand are scarce, yet their development is essential for next-generation spin-based materials with topical applications in quantum information science. To this end, a synthetic challenge must be met: devising molecules that enable the binding of a redox-active ligand through facile displacement and clean removal of a weakly coordinating anion. Here, we probe the accessibility of unprecedented radical-containing rare-earth guanidinate complexes by combining our recently discovered yttrium tetraphenylborate complex [{(Me₃Si)₂NC(NⁱPr)₂}₂Y][(μ-η⁶-Ph)(BPh₃)] with the redox-active ligands 2,2'-bipyridine (bpy) and 2,2'-bis(benzimidazole) (Bbim), respectively, under reductive conditions. Our endeavor resulted in the first evidence of guanidinate complexes that contain radicals, namely, a mononuclear bipyridyl radical complex, {(Me₃Si)₂NC(NⁱPr)₂}₂Y(bpy^•) (1), and a dinuclear bis(benzimidazolyl) radical-bridged complex, [K(crypt-222)][{(Me₃Si)₂NC(NⁱPr)₂}₂Y]₂(μ-Bbim^•) (2'). The latter was achieved by an in situ reduction of [{(Me₃Si)₂NC(NⁱPr)₂}₂Y]₂(μ-Bbim) (2), which was isolated from a salt metathesis reaction. 1 and 2 were characterized by X-ray crystallography and IR and UV-vis spectroscopy. Variable-temperature electron paramagnetic resonance spectroscopy was applied to gain insight into the distribution of unpaired spin density on 1 and 2'. Density functional theory calculations were conducted on 1 and 2' to elucidate further their electronic structures. The redox activity of 1 and 2' was also probed by electrochemical methods.