Complexes of a Noncyclic Carbazole-based N5-donor Schiff base: Structures, Redox, EPR and Poor Activity as Hydrogen Evolution Electrocatalysts

A new noncyclic pentadentate N5-donor Schiff-base ligand, HL^2Etpyr (1,1'-(3,6-di<i>tert</i>-butyl-9H-carbazole-1,8-diyl)bis(N-(2-(pyridin-2-yl)ethyl)methanimine)), prepared from 1,8-diformyl-3,6-ditertbutyl-carbazole (HU^tBu) and two equivalents of 2-(2-pyridyl)ethylamine, along with four tetrafluoroborate complexes, [M^IIL^2Etpyr](BF₄), where M = Co, Ni, Cu, and Zn, and two [Co^IIL^2EtPyr]·1/2[Co^IIX₄] complexes where X = NCS or Cl, isolated as solvates, are reported. All six complexes were structurally characterized, revealing the cations to be isostructural, with M(II) in a trigonal bipyramidal N5-donor environment. Only the Zn(II) complex is fluorescent. Cyclic voltammograms of [M^IIL^2Etpyr](BF₄) in MeCN reveal reversible redox processes at positive potentials: 0.61 (Zn), 0.62 (Cu), 0.57 (Ni), and 0.25 V (Co), and for the cobalt complex a second quasi-reversible process occurs at 0.92 V vs Fc⁺/Fc. EPR data for the first oxidation product clearly demonstrate that the Zn complex undergoes a ligand centered oxidation, and support this being the case for the Ni and Cu complexes, although this is not definitively shown. After both oxidations the EPR data shows that the Co complex is best described as a low spin Co(III)-ligand radical. In the presence of 80 mM acetic acid, controlled potential electrolysis carried out on [M^IIL^2Etpyr](BF₄) at -1.68 V in MeCN shows some electrocatalytic hydrogen evolution reaction (HER) performance in the order Ni(II) > Cu(II) > Co(II) - but the control, Ni(II) tetrafluoroborate, is more active than all three of the complexes.