Tuneable reversible redox of cobalt(iii) carbazole complexes

Four tridentate carbazole-based ligands, HL {3,6-di(tert-butyl)-1,8-bis[5-(3-benzyl-1,2,3-triazole)]-9H-carbazole}, HL {3,6-di(tert-butyl)-1,8-bis[5-(3-(4-tert-butyl)benzyl-1,2,3-triazole)]-9H-carbazole}, HL^H/H {1,8-bis[5-(3-benzyl-1,2,3-triazole)]-9H-carbazole} and HL^H/ {1,8-bis[5-(3-(4-tert-butyl)benzyl-1,2,3-triazole)]-9H-carbazole}, were prepared and complexed with cobalt(ii) tetrafluoroborate. In situ air oxidation resulted in cobalt(iii) complexes 1-4 with the general formula [Co^III(L)₂]BF₄·xH₂O (1: L = L, x = 2; 2: L = L, x = 1; 3: L = L^H/H, x = 0.5; 4: L = L^H/, x = 2). X-ray structural characterisation confirmed that the four complexes are isostructural, with two orthogonally coordinated deprotonated tridentate ligands providing an octahedral N₆-donor set to the cobalt(iii) ion. ¹H NMR studies show that this structure is maintained in CDCl₃ and DMSO-d₆ solution. Cyclic voltammetry on 1-4 in MeCN showed that all of the complexes exhibit two reversible, one-electron oxidation processes (probably due to ligand oxidations), and an irreversible or quasi-reversible reduction process (probably due to reduction of Co(iii) to Co(ii)). As expected, the oxidations move 120-140 mV to lower potentials on adding tert-butyl substituents to the 3 and 6 positions of the carbazole rings, and unsurprisingly the potentials are far less sensitive to the nature of the benzyl ring substituents.