ABSTRACT
The ability of molecular switches to reversibly interconvert between different forms promises potential applications at the scale of single molecules up to bulk materials. One type of molecular switch comprises cobalt-dioxolene compounds that exhibit thermally-induced valence tautomerism (VT) interconversions between low spin Co(iii)-catecholate (LS-CoIII-cat) and high spin Co(ii)-semiquinonate (HS-CoII-sq) forms. Two families of these compounds have been investigated for decades but have generally been considered separately: neutral [Co(diox)(sq)(N2L)] and cationic [Co(diox)(N4L)]+ complexes (diox = generic dioxolene, N2L/N4L = bidentate/tetradentate N-donor ancillary ligand). Computational identification of promising new candidate compounds prior to experimental exploration is beneficial for environmental and cost considerations but requires a thorough understanding of the underlying thermochemical parameters that influence the switching. Herein, we report a robust approach for the analysis of both cobalt-dioxolene families, which involved a quantitative density functional theory-based study benchmarked with reliable quasi-experimental references. The best-performing M06L-D4/def2-TZVPP level of theory has subsequently been verified by the synthesis and experimental investigation of three new complexes, two of which exhibit thermally-induced VT, while the third remains in the LS-CoIII-cat form across all temperatures, in agreement with prediction. Valence tautomerism in solution is markedly solvent-dependent, but the origin of this has not been definitively established. We have extended our computational approach to elucidate the correlation of VT transition temperature with solvent stabilisation energy and change in dipole moment. This new understanding may inform the development of VT compounds for applications in soft materials including films, gels, and polymers.
ABSTRACT
Valence tautomerism (VT) involves the stimulated reversible intramolecular electron transfer between a redox-active metal and ligand. Dinuclear cobalt complexes bridged by bis(dioxolene) ligands can undergo thermally-induced VT with access to {CoIII-cat-cat-CoIII}, {CoIII-cat-SQ-CoII} and {CoII-SQ-SQ-CoII} states (cat2- = catecholate, SQË- = semiquinonate, CoIII refers to low spin CoIII, CoII refers to high spin CoII). The resulting potential for two-step VT interconversions offers increased functionality over mononuclear examples. In this study, the bis(dioxolene) ligand 3,3',4,4'-tetrahydroxy-5,5'-dimethoxy-benzaldazine (thMH4) was paired with Mentpa (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation at 6-position of the pyridine rings) to afford [{Co(Mentpa)}2(thM)](PF6)2 (1a, n = 0; 2a, n = 2; 3a, n = 3). Structural, magnetic susceptibility and spectroscopic data show that 1a and 3a remain in the temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} forms in the solid state, respectively. In contrast, 2a exhibits incomplete thermally-induced VT between these two tautomeric forms via the mixed {CoIII-cat-SQ-CoII} tautomer. In solution, room temperature electronic absorption spectra are consistent with the assignments from the solid-state, with VT observed only for 2a. From electrochemistry, the proximity of the two 1e--processes for the thMn- ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thMn- containing complexes. Comparison of the redox potentials of the Co and thMn- processes suggests that only 2a has these processes in sufficient proximity to afford the thermally-induced VT observed experimentally. Density functional theory calculations are consistent with the prerequisite energy ordering for a two-step transition for 2a, and temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} states for 1a and 3a, respectively. This work presents the third example, and the first formally conjugated example, of a bridging bis(dioxolene) ligand that can afford two-step VT in a Co complex, suggesting new possibilities towards applications based on multistep switching.
