Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

Abstract

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.

Electronic Lability of Quinonoid‐Bridged Dinuclear 3 d‐Metal Complexes with Tetradentate N‐Donor Bases

“…The direct dependence of the character (one‐ or two‐step) and the degree of completion of VT transition on the redox‐potential of the ligand was recently shown on the example of a series of complexes with brominated spiro‐conjugated bis(dioxolene) ( Br 4 spiroH 4 ) (Scheme 16). [61] Bromine derivative [Co 2 ( Me 3 TPA ) 2 ( Br 4 spiro )] 2+ is capable to undergo a partial VT interconversion { HS Co II −(SQ−Cat)− LS Co III }→{ HS Co II −(SQ−SQ)− HS Co II }, while its unsubstituted analog [Co 2 ( Me 3 TPA ) 2 ( spiro )] 2+[60] is in the high‐spin state over a wide temperature range. Experimental and theoretical research has shown the possibility to control VT behaviour of dinuclear cobalt complexes by means of modification of the redox‐active and ancillary ligands.…”

Metal‐to‐Ligand Charge Transfer Induced Valence Tautomeric Forms of Non‐Innocent 2,2′‐Azobis(benzothiazole) in Ruthenium Frameworks

“…Such thermal stability of valence tautomers indeed facilitated their structural/spectroscopic authentication even at ambient conditions. Though interconversion was not observed up to a temperature of 373 K, heating of 2 a in 1‐butanol at ≈393 K for 12 h yielded minor quantity of 2 b , suggested the two energy minima were separated by a very substantial energy barrier [4d] …”

Large‐Amplitude Thermal Vibration‐Coupled Valence Tautomeric Transition Observed in a Conductive One‐Dimensional Rhodium–Dioxolene Complex

“…Valence tautomeric (VT) complexes derived from redox‐active ligands and metal centers, represented by metal‐dioxolene complexes, can take two valence isomers or redox isomers with different localized electronic structures exhibiting different physical properties [8] . VT transitions between these isomers accompanying intramolecular electron transfer can not only cause changes in the valence and spin states and radical formation and disappearance but also lead to interesting mixed‐valence states [9] and spin transitions, [2a–c, 10] and these features can be tuned by controlling the VT transitions by external stimuli, such as heat, [10a, f, g, 11] light, [1g, 10e, f, 12] X‐ray, [13] pressure, [3g] and electric [14] and magnetic fields [15] . Therefore, VT transitions are widely utilized as stimuli‐responsive, controllable functions on dynamic molecular crystals.…”