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Variable-temperature structural studies on valence tautomerism in cobalt bis(dioxolene) molecular complexes.
Venkatesha R Hathwar1, Marian Stingaciu1, Bo Richter1
1Center for Materials Crystallography, Department of Chemistry and iNano, Aarhus University, Langelandsgade 140, Aarhus C 8000, Denmark.
Summary
This study investigates cobalt molecular complexes, revealing how lattice solvents and halogen substitutions influence valence tautomerism (VT). The non-solvated complex exhibits a sharper VT transition compared to its solvated counterpart.
Area of Science:
- Inorganic Chemistry
- Materials Science
- Solid-State Chemistry
Background:
- Valence tautomerism (VT) in molecular complexes involves a reversible change in electron distribution and spin state.
- Cobalt complexes with semi-quinonate and pyridine ligands are known to exhibit interesting electronic and magnetic properties.
- Understanding factors influencing VT transitions is crucial for designing switchable molecular materials.
Purpose of the Study:
- To investigate the variable-temperature single-crystal structures of five cobalt molecular complexes.
- To study the phenomenon of thermally induced valence tautomerism (VT) in these complexes.
- To elucidate the effect of lattice solvent and halogen substitution on VT behavior.
Main Methods:
- Variable-temperature single-crystal X-ray diffraction was employed to study the structural changes.
- Five cobalt molecular complexes with varying pyridine ligands were synthesized and characterized.
- Differential scanning calorimetry or similar thermal analysis techniques were implicitly used to observe transition temperatures.
Main Results:
- A solvent-free polymorph of CoII(3,5-DBSQ)2(DBPy)2 was obtained.
- Thermally induced valence tautomerism (hs-CoII ↔ ls-CoIII) was observed in complexes (1S), (1), and (2S), but not in (3) and (4).
- The non-solvated complex (1) showed a sharp VT transition at ~245 K, while the solvated polymorph (1S) exhibited a broader transition centered at ~150 K, demonstrating the solvent's influence.
Conclusions:
- Lattice solvent significantly impacts the valence tautomerism transition, leading to sharper transitions in solvent-free systems.
- Halogen substitution on the pyridine ring influences the occurrence and characteristics of VT.
- The coordination environment of cobalt changes during the VT conversion, as evidenced by structural studies.