Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism

Abstract

In this article, the synthesis and X-ray crystal structures of two cis/trans isomers of valence tautomeric (VT) cobalt dioxolene compounds are reported. The cis isomer (1) was isolated from the polar protic methanol solvent as a kinetic product, whereas the less polar nonprotic solvent acetone yielded the trans isomer (2). It should be noted that, although some coordination polymers involving cobalt bis(dioxolene) with the cis disposition are known for bridging ancillary ligands, such an arrangement is unprecedented for mononuclear compounds. A careful study of intermocular interactions revealed that the methanol solvent does not have much influence on the crystal growth in 1, whereas acetone forms strong halogen-bonding interactions that are crucial in the solid-state architecture of 2. This behavior can likely be used in crystal engineering to design new organic-inorganic hybrid materials. The energy difference between the two isomers was examined using DFT calculations, confirming that the trans form is in the thermodynamic state whereas the cis isomer is a kinetic product that can be converted into the trans isomer with time. Finally, both isomers exhibit solvent loss at elevated temperatures that is accompanied by a change in magnetic properties, associated with an irreversible valence tautomerism. Our results highlight the crucial role of the solvents for the isolation of cis/trans isomers in cobalt dioxolene chemistry, as well as the distinguishing effects of intermolecular forces and the solid-state packing on VT behavior.

Valence‐Tautomerism‐Driven Aromatic Nucleophilic Substitution in Cobalt‐Bound Tetrabromocatecholate Ligands – Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity

“…These studies support the fact that, at higher temperatures, the valence‐tautomeric conversion from manganese(III) tetrabromocatecholate to manganese(II) tetrabromosemiquinonate increases the electrophilic character of the aromatic carbon atom at the para position with respect to the OH group of the semiquinonate ligand and, thereby, favours the aromatic nucleophilic substitution of the Br atom by pyridine. In our recent report, we examined the valence‐tautomeric behaviour of a cobalt(III) tetrabromocatecholate system and found that trans ‐[pyH][Co(Br 4 Cat) 2 (py) 2 ] · 2H 2 O · 2C 3 H 6 O ( A ) is susceptible to a valance‐tautomeric conversion from cobalt(III) tetrabromocatecholate to cobalt(II) tetrabromosemiquinonate at elevated temperatures . Such entropy‐driven valence‐tautomeric conversion is very favourable in the gas or liquid phase.…”

“…The efficient biomimetic catalytic activity of 1 motivated us to examine the catalytic activity of trans ‐[pyH][Co(Br 4 Cat) 2 (py) 2 ] · 2H 2 O · 2C 3 H 6 O ( A ), from which 1 was synthesized, as similar studies with manganese(III) catecholate systems revealed that the positive charge at the ligand backbone significantly enhances the catalytic activity upon pyridyl substitution . However, the time‐dependent spectra for identical reaction condition showed that the biomimetic catalytic activity of A related to the function of phenoxazinone synthase is almost absent (Figure S3).…”

“…Recently, we observed the valence‐tautomeric‐induced aromatic nucleophilic substitution of a Br atom of a coordinated tetrabromocatecholate ligand by a pyridine molecule . We also observed a thermally induced valence‐tautomeric transition in a cobalt tetrabromocatecholate system in the solid state . Therefore, we anticipated that a valence‐tautomeric‐driven aromatic nucleophilic substitution similar to that in the manganese system may be possible in cobalt complexes in the solution state.…”

“…We envisioned that cobalt complexes derived from tetrabromocatecholate ligand may exhibit similar activities. Accordingly, as a part of our ongoing program on transition‐metal dioxolene chemistry and biomimetic catalysis,, , we report herein the synthesis, structure and phenoxazinone synthase like activity of the mononuclear complex [Co(Br 3 Cat‐py) 2 (py) 2 ]Cl · 8H 2 O ( 1 , py = pyridine, Br 3 pyCatH 2 = 3,5,6‐tribromo‐4‐pyridiniumcatechol). A theoretical study was also performed to support the experimental facts and especially to explore the mechanistic aspects associated with the chemical reactivity.…”

Introducing Supramolecular Interactions into Robust Bis(tetrabromocatecholate) Chelated Manganese(III) Systems and Biomimetic Catalytic Activity

“…Recently, we have explored how the crystal packing governed by different kinds of noncovalent forces affects the valence tautomeric transition in cobalt‐dioxolene systems . Further, the chemistry of manganese complexes with tetrabromocatecholate ligands leads to the formation of robust monoanionic bis(catecholate)‐chelated manganese(III) platforms as major products ,,.…”