Valence tautomeric transition of bis(o-dioxolene) cobalt complex in solid state and solution.

Valence tautomer transition occurs mainly in 3d metalorganic complexes with redox-active ligands and makes them potential candidates for single-molecular switches. The transition occurs under temperature, pressure, or light-induced stimuli and is strongly affected by the intermolecular interactions. However single-crystal x-ray diffraction is not always applicable to such systems when crystal structure is destroyed upon transition or system is studied in the solution. Such an example is bis(o-semiquinonato) cobalt complex with TEMPO-functionalized iminopyridine ancillary ligand. In this work we apply two complementary techniques-ligand-sensitive Fourier transform infrared spectroscopy (FTIR) and metal sensitive Co K-edge x-ray absorption spectroscopy (XAS). In a solid state, a temperature hysteresis of magnetization larger than 40 K was observed upon cyclic cooling-heating. So, the temperature of phase transition upon cooling is about 40 K lower than that upon heating. In solution, the x-ray absorption spectra for high-temperature and low-temperature states were similar to that in the solid form, but the hysteresis was absent. Two methods are can probe valence tautomer transition, but XAS has an advantage for the liquid phase analysis and FTIR has larger sensitivity to the ligand related interactions in solid.

Quantum chemical modeling of magnetically bistable metal coordination compounds. Synchronization of spin crossover, valence tautomerism and charge transfer induced spin transition mechanisms.

It has been shown that the computationally designed bimetallic complexes formed as the adducts of Co(II) diketonates and salicylaldiminates with Fe(II) chelates of 1,10-phenanthroline-5,6-dione are susceptible to the synchronized thermally induced intramolecular rearrangements between their electromeric forms LSCo(III)-SQ-LSFe(II), LSCo(III)-SQ-HSFe(II), HSCo(II)-BQ-LSFe(II), HSCo(II)-BQ-HSFe(II) and also HSCo(II)-SQ-LSFe(III), which are governed by the spin-crossover (SCO), valence tautomerism (VT) and charge-transfer-induced spin transition (CTIST) mechanisms of spin-state switching. Stability of the adducts with respect to dissociation into components, relative energies and magnetic properties of the electromers and energy barriers against VT and unprecedented one-step (SCO + VT) rearrangements (estimated as minimum energy crossing points on the seams of the intersection of the corresponding potential energy surfaces) were calculated using the DFT (B3LYP*/6-311++G(d,p)) method. The calculations showed that all these characteristics of the system as well as the energy preferred spin-state switchable mechanisms are very sensitive to the structure of the cobalt diketonate (salicylaldiminate) fragment and can be varied and interchanged by the introduction of electron withdrawing substituents into the ligands.

Theoretical modeling of valence tautomeric dinuclear cobalt complexes. Adducts of Co(II) diketonates with cyclic redox-active tetraone ligands.

The approach to the employment of the mechanism of valence tautomerism (VT) for the design of molecular 2-qubit quantum gates has been extended to adducts 5-8 of Co(II) bis-(malonates), bis-(acetylacetonates), bis-(hexafluoroacetylacetonates) and bis-(trifluoroacetylacetonates) with tetradentate tetraone (cyclic di-o-quinones) redox-active piperazine-2,3,5,6-tetraone L5 (X = NH), 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetraone L5 (X = C(CH3)2), cyclopenta[fg]acenaphthylene-1,2,5,6-tetraone L6, cyclopenta[fg]acenaphthylene-3,4,7,8-tetraone L7 and pyrene-4,5,9,10-tetraone L8 and computationally studied using the B3LYP*/6-311++G(d,p) method. The calculations reveal ferromagnetic ordering of unpaired electrons in the low-spin electromeric forms of complexes 8 (R1, R2 = H, CH3, CF3) on the basis of L8, which provides for the paramagnetic character of all three interconverting electromers of 8 and makes it possible to realize the two-step mechanism of thermally driven migration of paramagnetic centers between the pyrene-tetraone fragment and metal ions. Through the structural variation of the ancillary diketonate ligands the energy gaps between the electromeric forms of adducts 8 and energy barriers for their interconversion were adjusted to the range of values typical of thermal VT rearrangements. The best energy parameters for the occurrence of thermal two-step VT rearrangements involving all three paramagnetic electromeric forms can be achieved for complex 8 (R1 = CH3, R2 = CF3), which was shown to meet the principal conditions for compounds with a potential role of spin qubit carriers: thermal stability with respect to dissociation into the components, thermally accessible energy barriers for the interconversion of the electromers and weak coupling between their paramagnetic centers.