Delocalization and Valence Tautomerism in Vanadium Tris(iminosemiquinone) Complexes

Abstract

To survey the noninnocence of bis(arylimino) acenaphthene (BIAN) ligands (L) in complexes with early metals,the homoleptic vanadium complex, [V(L) 3 ](1), and its monocation, [V(L) 3 ]PF 6 (2), were synthesized. These complexes were found to have av ery rich electronic behavior, whereby 1 displays strong electronic delocalization and 2 can be observed in unprecedented valence tautomeric forms.T he oxidation states of the metal and ligand components in these complexes were assigned by using spectroscopic,c rystallographic, and magnetic analyses.C omplex 1 was identified as [V IV (L red )(LC) 2 ]( L red = N,N'-bis(3,5-dimethylphenylamido)acenaphthylene;L C = N,N'-bis(3,5-dimethylphenylimino)acenaphthenesemiquinonate). Complex 2 was determined to be [V V (L red )(LC) 2 ] + at T < 150 Ka nd [V IV (LC) 3 ] + at T > 150 K. Cyclic voltammetry experiments reveal six quasi-reversible processes,t hus indicating the potential of this metal-ligand combination in catalysis or materials applications. Scheme 1. Syntheses of 1 and 2.

Ligand Field-Actuated Non-Innocence of Acetylacetonate

“…The crystallographic, magnetic, spectroscopic, and computational results all point toward the occurrence of concerted metal-ligand redox chemistry in the square-planar [Cr(acac)2] upon axial coordination of strongly donating pyridine ligands. In the ideally square-planar ligand field of 1, the singly occupied dz 2 orbital remains energetically low-lying, whereas, as previously hypothesized, 20 the pyridines of 2-4 raise the dz 2 orbital above the ligand-based LUMO, leading to a partial {Cr II -(acac)2 2-} → {Cr III -(acac)2 34,35 but the manifestation of ligand noninnocence upon alteration of the metal ligand sphere is exceedingly rare. 36 The valence tautomers 2-4 exhibit extremely strong Cr(III)-radical exchange interactions, as demonstrated by magnetization measurements and DFT calculations.…”

“…Displacement of the pyridine ligands in 2-4 demonstrates the reversibility of the {Cr II -(acac)2 2-} ⇄ {Cr III -(acac)2 •3-} valence tautomerization process. A handful of transition metal complex families demonstrate temperature-induced valence tautomerism, 34,35 but the manifestation of ligand noninnocence upon alteration of the metal ligand sphere is exceedingly rare. 36 The valence tautomers 2-4 exhibit extremely strong Cr(III)-radical exchange interactions, as demonstrated by magnetization measurements and DFT calculations.…”