Octahedral Co(III) salen complexes: the role of peripheral ligand electronics on axial ligand release upon reduction

Abstract

A series of octahedral CoIII salen complexes (where salen represents a N2O2 bis-Schiff-base bis-phenolate framework) were prepared with axial imidazole ligating groups. When using 1-methylimidazole (1-MeIm) axial ligands, the CoIII/CoII reduction potential could be altered by 220 mV via variation of the electron-donating ability of the para-ring substituents (R = H (1), OMe (2), tBu (3), Br (4), NO2 (5), and CF3 (6)). In addition, the irreversibility of the reduction process suggested substantial geometrical changes and axial ligand exchange upon reduction to the more labile CoII oxidation state. Installing an imidazole-coumarin conjugate as the axial ligands resulted in fluorescence quenching when bound to the CoIII centre (R = H (7), OMe (8), and CF3 (9)). The redox properties and fluorescence increase upon ligand release for 7–9 were studied under reducing conditions and in the presence of excess competing ligand (1-MeIm). It was determined that the Lewis acidity of the CoIII centre was the dominant factor in controlling axial ligand exchange for this series of complexes.

Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes

“…This electrochemical process has been observed for related Co(III) complexes. 16,23,95 Consistent with the d z2 LUMO, the potential of the Co(III)/Co(II) couple is most strongly dependent on the nature of the axial ligand, a phenomenon that has been documented for similar complexes. 98,99 As expected, the more electron-donating NH 3 ligands give rise to more negative reduction potentials compared to the analogous 3F-BnNH 2 complexes.…”

“…These complexes have also been studied as reductionactivated prodrugs. 81,95 Despite these previous investigations, few studies have systematically evaluated the general activity of the [Co(Schiff base)(L) 2 ] + scaffold. In this study, a small set of [Co(Schiff base)(L) 2 ] + complexes was prepared and thoroughly investigated.. Two families of Co(III) Schiff base complexes, bearing either ammonia (NH 3 ) or 3-fluorobenzylamine (3F-BnNH 2 ) axial ligands, were studied.…”

A kinetic study and mechanisms of reduction of N, N′-phenylenebis(salicyalideneiminato)cobalt(III) by L-ascorbic acid in DMSO-water medium

“…The inability of Co II and other pro compounds to bio-reductive tune reduction potential beyond lower end of range (-420 to -150mV) to pave way for the selective activation in solid-tumor hypoxic environment ( Denny, 2010 ; Failes et al., 2007 ; Kizaka-Kondoh et al., 2003 ; Renfrew et al., 2013 ; Ware et al., 1997 ). Some researchers previously proposed and investigated the probability of tuning reduction potential of metal complexes beyond large lower range (-420 to -150mV) through structural alteration at ring para-position ( Chiang et al., 2014 , 2012 ; Clarke and Storr, 2016 ; Zhang et al., 2017 ). Their research lead to the metal complexes of octahedral Co III salen series.…”