Impact of positional isomers on the selective isolation of cis-trans isomers in cobalt-dioxolene chemistry and solvation effects on the valence tautomerism in the solid state

Abstract

Three new mononuclear cobalt compounds─trans-[Co(3,5-dbcat)(3,5-dbsq)(4-Etpy)2]·CH3CN (1), cis-[Co(3,5-dbcat)(3,5-dbsq)(3-NH2py)2]·DMF (2), and trans-[Co(3,5-dbcat)(3,5-dbsq)(4-NH2py)2]·2DMF (3) (where 3,5-dbcat2– and 3,5-dbsq• – stand for 3,5-di-tert-butyl-catecholate and 3,5-di-tert-butyl-semiquinonate, respectively), derived from a redox-active o-dioxolene ligand in the presence of 4-ethylpyridine (4-Etpy), 3-aminopyridine (3-NH2py), and 4-aminopyridine (4-NH2py), respectively─have been synthesized and investigated with a view to examine possible influence of pyridine derivatives and solvation on the valence tautomeric (VT) process. Single-crystal X-ray diffraction data for all compounds at room temperature suggest Co(III)(3,5-dbcat)(3,5-dbsq) charge distribution in these complexes. Further insight into the crystal structures discloses the diverse non-covalent interactions offered by the isomers of aminopyridine in 2 and 3, leading to the first example of selective isolation of cis and trans isomers in cobalt-dioxolene chemistry induced by the positional isomers of the ancillary ligands. Variable temperature magnetic susceptibility data for all compounds between 2 K and 300 K are consistent with the structural studies. At elevated temperatures, complex 1 exhibits a VT interconversion from low spin Co(III)-(3,5-dbcat)(3,5-dbsq) to high spin Co(II)-(3,5-dbsq)(3,5-dbsq), triggered by the loss of lattice solvent molecules, while a partial interconversion is observed for complex 3, even heating up to 430 K. The present report overall highlights the impact of positional isomers on the selective isolation of cis-trans isomers in cobalt-dioxolene chemistry and solvation effects on the valence tautomerism in the solid state.