Compounds [Co(L-N₄R₂)(dbdiox)](BPh₄) (L-N₄R₂ = N,N′-di-alkyl-2,11-diaza[3.3]-(2,6)pyridinophane, R = iPr (1a), Et (2a); dbdiox = 3,5-di-tert-butyldioxolene) and [M(L-N₄iPr₂)(dbdiox)](BPh₄) (M = Mn (3a), Fe (4a)) have been synthesized and investigated with a view to possible valence tautomeric (VT) or spin crossover (SCO) interconversions. Single crystal X-ray diffraction data for all compounds at 100 or 130 K indicate trivalent metal cations and di-tert-butylcatecholate (dbcat²⁻) dioxolene ligands. Variable temperature magnetic susceptibility data for all species between 2 and 340 K are consistent with these redox states, with low spin configurations for the cobalt(III) ions and high spin for the manganese(III) and iron(III) ions. Above 340 K, compound 1a exhibits an increase in magnetic susceptibility, suggesting the onset of a VT interconversion from low spin Co(III)-dbcat to high spin Co(II)-dbsq (dbsq⁻ = di-tert-butylsemiquinonate) that is incomplete up to 400 K. In solution, variable temperature electronic absorption spectra and Evans NMR method magnetic susceptibility data indicate reversible VT interconversions for 1a in several solvents, with the transition temperature varying with solvent. Variable temperature electronic absorption spectra are temperature-invariant for 3a and 4a, while spectra for 2a in 1,2-dichloroethane suggest the onset of a VT transition at the highest temperatures measured. Density functional theory calculations on all four compounds and literature analogues provide key insights into the relative energies of the different electromeric forms and the possibilities for VT versus SCO interconversions in this family of compounds.

The concurrent effects of single-ion anisotropy and exchange interactions on the electronic structure and magnetization dynamics have been analyzed for a cobalt(II)-semiquinonate complex. Analogs containing diamagnetic catecholate and tropolonate ligands were employed for comparison of the magnetic behavior and zinc congeners assisted with the spectroscopic characterization and assessment of intermolecular interactions in the cobalt(II) compounds. Low temperature X-band (ν ≈ 9.4 GHz) and W-Band (ν ≈ 94 GHz) electron paramagnetic resonance spectroscopy and static and dynamic magnetic measurements have been used to elucidate the electronic structure of the high spin cobalt(II) ion in [Co(Me₃tpa)(Br₄cat)] (1; Me₃tpa = tris[(6-methyl-2-pyridyl)methyl]amine, Br₄cat²⁻ = tetrabromocatecholate) and [Co(Me₃tpa)(trop)](PF₆) (2(PF₆); trop⁻ = tropolonate), which show slow relaxation of the magnetization in applied field. The cobalt(II)-semiquinonate exchange interaction in [Co(Me₃tpa)(dbsq)](PF₆)·tol (3(PF₆)·tol; dbsq⁻ = 3,5-di-tert-butylsemiquinonate, tol = toluene) has been determined using an anisotropic exchange Hamiltonian in conjunction with multistate restricted active space self-consistent field ab initio modeling and wavefunction analysis, with comparison to magnetic and inelastic neutron scattering data. Our results demonstrate dominant ferromagnetic exchange for 3⁺ that is of similar magnitude to the anisotropy parameters of the cobalt(II) ion and contains a significant contribution from spin–orbit coupling. The nature of the exchange coupling between octahedral high spin cobalt(II) and semiquinonate ligands is a longstanding question; answering this question for the specific case of 3⁺ has confirmed the considerable sensitivity of the exchange to the molecular structure. The methodology employed will be generally applicable for elucidating exchange coupling between orbitally-degenerate metal ions and radical ligands and relevant to the development of bistable molecules and their integration into devices.