Carbazole complexes as catalysts for hydrogen evolution and CO2 reduction

Abstract

In chapter 1, after an introduction to Schiff bases, a literature survey of synthetic routes to the key diformyl-carbazole head units, required for all ligands made in this thesis, is discussed. An overview of the previously reported Schiff base ligands and complexes derived from 1,8-diformyl-3,6-ditertbutylcarbazole HUtBu follows. Then an overview of homogeneous molecular catalysts for hydrogen evolution (HER) and CO2 reduction (CO2RR) is provided. Finally, the systems targeted in this thesis are presented. In chapter 2 HUtBu and a back-to-back ethylene-linked diformyl-carbazole (HU2Et) are prepared via the established routes and are characterised. The low solubility of HU2Et would make further work with it challenging, but the focus in this thesis is entirely on systems derived from HUtBu. In chapter 3 a new non-cyclic pentadentate N5-donor Schiff-base ligand, HL2Etpyr, prepared from HUtBu and two equivalents of 2-(2-pyridyl)ethylamine is presented. Four tetrafluoroborate monometallic complexes, [MIIL2Etpyr](BF4), where M = Co, Ni, Cu, and Zn, along with two non-tetrafluoroborate cobalt(II) complexes, [CoIIL2EtPyr]2[CoII(NCS)4], and [CoIIL2Etpyr]2[CoIICl4], were isolated as solvates and structurally characterised. The cations in all six complexes are isostructural and feature the metal(II) centre in a trigonal bipyramidal N5-donor environment. Only the Zn(II) complex is fluorescent. Cyclic voltammograms of all four of the tetrafluoroborate complexes in MeCN show one reversible redox process at positive potentials and the Co(II) complex displays a second reversible process. Finally, the [MIIL2Etpyr](BF4), where M = Co, Ni and Cu, complexes are tested as HER electrocatalysts. Controlled potential electrolysis carried out at -1.6 V in MeCN in the presence of 80 mM acetic acid shows electrocatalytic HER performance in the order Ni(II) > Cu(II) > Co(II), but disappointingly, the control Ni(II) tetrafluoroborate salt is more active than all three catalysts. In chapter 4 the macrocyclic ligand, HLPr, prepared from HUtBu and one equivalent of bis(3-aminopropyl)amine, previously reported by Dr Malthus in his thesis, is remade due to our interest in the 3d tetrafluoroborate complexes. Reversible redox processes are seen for [MIILpr](BF4), where M = Cu, Ni and Zn. Furthermore, the M = Cu and Ni complexes are investigated as electrocatalysts for HER, and the results indicate that NiLPr performed better than CuLPr but the control, Ni(BF4)2, performed far better than both catalysts. In chapter 5 the two new N5-donor macrocycles, HLEt-Pyr and HLPrPyr, were prepared from HLEt and HLPr by alkylation with one equivalent of 2-(bromomethyl)pyridine. Five monometallic [MLR](BF4) complexes, where M = Co, Ni, and Cu; (R = Et-Pyr or Pr-Pyr) and one dinuclear [Ni2IIL2(Et-Pyr)(CH3OH)](BF4)2 complexes are prepared and characterised. The CVs of the [NiIILEt-Pyr](BF4), [CuIILEt-Pyr](BF4), and [CuIILPr Pyr](BF4)2 complexes revealed reversible redox processes at positive and negative potentials. Selected complexes are tested as HER electrocatalysts. In chapter 6 the open-armed complexes which are reported in chapter 3 are tested under photocatalytic conditions as catalysts for HER and CO2RR. The performance under HER conditions is not encouraging as the controls show the simple salts are more active than the complexes in generating hydrogen. The results indicate that CO2 is reduced to CO and CH4, although the CH4 obtained is sub-stoichiometric. HER, a competitor during CO2RR, overshadowed CO and CH4 formation. Possible formation of other products such as formate still needs to be checked by our collaborator in Montreal. Finally, in chapter 7 a summary of the key findings is provided.