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

Abstract

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) Co(III) and high-spin (hs) Co(II), 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.

Electric Field Control of the Valence‐Tautomeric Transformation in Cobalt Complexes

“…Moreover, for the transition from [ ls ‐Co III (3,5‐DTBsq)(3,5‐DTBcat)(phen)] to [ hs ‐Co II (3,5‐DTBsq) 2 (phen)] (3,5‐DTBsq – and 3,5‐DTBcat 2– are the semiquinonate and catecholate forms of 3,5‐di‐ tert ‐butyl‐ o ‐benzoquinone, respectively, and phen is 1,10‐phenanthroline) that occurs in solution, the enthalpy change Δ H = 2238 cm –1[16] determined from the observed magnetic susceptibility data is very close to the value Δ 4 used in further calculations. The broken‐symmetry DFT calculations performed by Witt et al gave for Δ 4 a value of 2512 cm –1 , which also slightly differs to that used in our calculations.…”

“…In the work of Azzaroli et al, a transient IR spectroscopic study supported by DFT calculations evidenced that two Co–dioxolene complexes display optically induced valence tautomerism. The experimental findings concerning the valence‐tautomeric transformation in the six‐coordinate complex trans ‐[Co(tbdiox) 2 (4‐papy) 2 ] (tbdiox = redox‐active 3,5‐di‐ tert ‐butyl‐ o ‐dioxolene, 4‐papy = 4‐phenylazopyridine) were also confirmed and supported by DFT studies . The computational design of transition‐metal complexes that have magnetic properties controlled by valence‐tautomeric mechanism has also been discussed previously …”

Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism

“…Moreover, the increased χ m T value can also be due to the chosen slow temperature sweep rates, where rearrangements of the charge distributions may occur 28 . Furthermore, we assume that ferromagnetic exchange mechanisms between the SQ π * -electrons and the residual HS-Co II ions could also play a role here 29 . For No thermal hysteresis has been observed between cooling (green) and warming (orange).…”

“…Therefore, it is of crucial importance to create a large surface to volume ratio and accordingly thin samples. Solving the Co VT molecules and drying out on a thin nonmagnetic substrate may not work as these kinds of complexes can deteriorate in solution 29,39 . Quantitative light power dependent investigations can be carried out on a pellet sample owing to its well-defined shape.…”

“…These compounds usually undergo the intramolecular ET between the coordinated metal center and the surrounding ligands, while this phenomenon is often accompanied by SCO behaviour of the metal ion, owing to the closely spaced d levels. The coordination compounds of the Co-catecholate/semiquinone families are known to display VT, and several examples have been reported, attracting considerable interest 7,10,21,22,[28][29][30][31][32] . Usually, the VT mechanisms implies the electron transfer from the ligands, leading to the interconversion between the lowspin Co III -catechole (Cat) and high-spin Co II -semiquinone (SQ) triggered by the presence of redox-active quinone ligands.…”