Guanidinate Yttrium Complexes Containing Bipyridyl and Bis(benzimidazolyl) Radicals

Abstract

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 spinbased 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 radicalcontaining rare-earth guanidinate complexes by combining our recently discovered yttrium tetraphenylborate complex [{(Me 3 Si) 2 NC(N i Pr) 2 } 2 Y][(μ-η 6 -Ph)(BPh 3 )] with the redoxactive 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, {(Me 3 Si) 2 NC(N i Pr) 2 } 2 Y(bpy • ) (1), and a dinuclear bis(benzimidazolyl. The latter was achieved by an in situ reduction of [{(Me 3 Si) 2 NC(N i Pr) 2 } 2 Y] 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.

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…Multinuclear lanthanide complexes with radial-anion bridges have historically been record-setting SMMs, , and so a better understanding of the ligand (both bridging and ancillary at the metal) features that maximize desirable magnetic properties is patently important. In their contribution, Demir’s group used variable-temperature electron paramagnetic resonance spectroscopy combined with density functional theory (DFT) calculations to experimentally and computationally determine the ligand and metal spin density in mononuclear [Y­{C­(N i Pr) 2 (N­[SiMe 3 ] 2 )} 2 (2,2′-bipy • )] (2,2′-bipy • = 2,2′-bipyridine radical anion) and dinuclear [Y­{C­(N i Pr) 2 (N­[SiMe 3 ] 2 )} 2 ] 2 (Bbim • )] (Bbim • = bis-benzimidizoyl radical anion) complexes . The guanidinate ligand systems used in this study are highly customizable and represent tremendous scope for tuning the properties of future radical-bridged complexes using ligand design.…”