Modulation of Magnetic Properties at Room Temperature: Coordination-Induced Valence Tautomerism in a Cobalt Dioxolene Complex

• Published in: Chemistry : a European journal Vol. 20; no. 35; pp. 11149 - 11162
• Main Authors: Witt, Alexander, Heinemann, Frank W., Sproules, Stephen, Khusniyarov, Marat M.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 25.08.2014; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; Cobalt; density functional calculations; Electronic structure; Electronics; Ligands; Magnetic properties; Mathematical analysis; Mathematical models; Modulation; Temperature; valence tautomerism; density functional calculations; cobalt; magnetic properties; valence tautomerism; electronic structure
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201402129

The valence‐tautomeric six‐coordinate complex [Co(tbdiox)2(4‐papy)2] (1; tbdiox=redox‐active 3,5‐di‐tert‐butyl‐o‐dioxolene, 4‐papy=4‐phenylazopyridine) was synthesized and its electronic structure examined. Whereas 1 shows regular thermally driven valence tautomerism in the solid state, it partially dissociates in solution to form the five‐coordinate species [Co(tbdiox)2(4‐papy)] (2) and free 4‐papy. Species 1 and 2 exhibit different electronic structures—low‐spin (ls) CoIII and high‐spin (hs) CoII, respectively—in solution at room temperature and therefore different magnetic properties. Since 1 and 2 are in an equilibrium that is 4‐papy‐dependent, the magnetic moment of the solution species can be tuned by means of the ligand content. Thus, the concept of coordination‐induced valence tautomerism (CIVT) has been introduced. The electronic structures of 1 and 2 as well as their CIVT were elucidated by X‐ray crystallography, electrochemistry, titration experiments, and all variable‐temperature SQUID susceptometry, NMR, EPR, and electronic absorption spectroscopy. The experimental findings are strongly supported by broken‐symmetry DFT calculations. The magnetic exchange interactions in different types of valence‐tautomeric cobalt complexes were explored computationally. Shifting the equilibrium between a six‐coordinate and a five‐coordinate bis(o‐dioxolene) cobalt complex in solution by addition of an auxiliary ligand results in changes in the room‐temperature magnetic properties (see figure). This effect was investigated by using a series of spectroscopic methods supported by theoretical calculations.