Cobalt Complexes with Aminophenolate Ligands: Spin Crossover and Ligand Versatility

A search for switchable molecules has afforded a family
of cobalt
complexes featuring derivatives of 2-aminophenol: 4,6-di-tert-butyl aminophenol (H2LH) and 2-anilino-4,6-di-tert-butyl aminophenol (H2LPh). The
heteroleptic cobalt complexes incorporate a Mentpa ligand (tpa = tris(2-pyridylmethyl)amine; n = 0–3), which involves the methylation of the 6-position
of the pyridine ring). Eight members of this family have been synthesized
and characterized: [CoII(HLPhAP)−(tpa)](BPh4) (1), [CoII(HLPhAP)−(Me2tpa)](BPh4) (2a), [CoII(LPhISQ)•−(Me2tpa)](BPh4) (2b), [CoII(HLPhAP)−(Me3tpa)](BPh4)
(3a), [CoII(LPhISQ)•−(Me3tpa)](BPh4) (3b), [CoIII(HLHAP)−(tpa)] (BPh4)(ClO4) (4a), [CoIII(LHISQ)•−(tpa)](BPh4)(ClO4) (4b) and [CoII(HLHAP)−(Me2tpa)](BPh4) (5), where the aminophenol-derived ligands are monoanionic
in either the open shell radical iminosemiquinonate (LISQ)•− or the closed shell protonated
aminophenolate (HLAP)−. We isolate high-spin
Co(II) and low-spin Co(III) compounds with no thermal interconversions
evident in the solid state. In contrast, variable temperature electronic
spectra for solutions of 3b suggest Co(II) spin crossover.
Density functional theory calculations employing the M06L-D4/def2-TZVPP
and TD-CAM-B3LYP/def2-TZVPP levels of theory support the experimental
findings. This work presents the first example of a cobalt–iminosemiquinonate
complex to exhibit spin crossover. The electronic structure versatility
of complexes that integrate redox-active metal ions with both low-
and high-spin states, alongside ligands that can exist in various
redox and protonation states, holds significant promise for applications
in molecular materials and chemical reactivity.