Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {Co III -cat-cat-Co III } ⇌ {Co III -cat-SQ-Co II }⇌{Co II -SQ-SQ-Co II } (cat 2− = catecholate, SQ •− = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3′,3′tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol (spiroH 4 ) or 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br 4 spiroH 4 ) with Me n tpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0−3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {Co III -cat-cat-Co III } states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {Zn II -cat-SQ-Zn II } analog have allowed elucidation of thermodynamic parameters governing the one-and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.

The ability to identify promising candidate switchable molecules computationally, prior to synthesis, represents a considerable advance in the development of switchable molecular materials. Even more useful would be the possibility of predicting the switching temperature. Cobalt-dioxolene complexes can exhibit thermally induced valence tautomeric switching between low-spin CoIII-catecholate and high-spin CoII-semiquinonate forms, where the half-temperature (T 1/2) is the temperature at which there are equal amounts of the two tautomers. We report the first simple computational strategy for accurately predicting T 1/2 values for valence tautomeric complexes. Dispersion-corrected density functional theory (DFT) methods have been applied to the [Co­(dbdiox)­(dbsq)­(N2L)] (dbdiox/dbsq•– = 3,5-di-tert-butyldioxolene/semiquinonate; N2L = diimine) family of valence tautomeric complexes, including the newly reported [Co­(dbdiox)­(dbsq)­(MeO-bpy)] (1) (MeO-bpy = 4,4′-dimethoxy-2,2′-bipyridine). The DFT strategy has been thoroughly benchmarked to experimental data, affording highly accurate spin-distributions and an excellent energy match between experimental and calculated spin-states. Detailed orbital analysis of the [Co­(dbdiox)­(dbsq)­(N2L)] complexes has revealed that the diimine ligand tunes the T 1/2 value primarily through π-acceptance. We have established an excellent correlation between experimental T 1/2(toluene) values for [Co­(dbdiox)­(dbsq)­(N2L)] complexes and the calculated lowest unoccupied molecular orbital energy of the corresponding diimine ligand. The model affords accurate T 1/2(toluene) values for [Co­(dbdiox)­(dbsq)­(N2L)] complexes, with an average error of only 3.7%. This quantitative and simple DFT strategy allows experimentalists to not only rapidly identify proposed VT complexes but also predict the transition temperature. This study lays the groundwork for future in silico screening of candidate switchable molecules prior to experimental investigation, with associated time, cost, and environmental benefits.

Solvatochromic models of tunable charge transfer bands illuminate environmental interactions that are key to potential sensing or switching applications for a family of cobalt complexes.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal− ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN 0 (L 0 ), monoanionic radical Ar-BIAN •− (L •− ), and dianionic Ar-BIAN 2− (L 2− ) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN) 3 ] n+ complexes (Ar = Ph, n = 2 (1 2+ ), 1 (1 + ), 0 (1); Ar = 3,5-CF 3 -Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (3 2+ ), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-Co II -L 0 containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-Co III -(L •− ) 3 ⇌ HS-Co II -(L •− ) 2 (L 0 ) VT interconversion. Experimentally, structural and magnetic data reveal a HS-Co II -L 0 containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-Co II -L 0 -containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-Co II -(L •− ) 2 (L 0 ) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-Co III -(L •− ) 3 ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.