Novel iridium complexes with N-heterocyclic dicarbene ligands in light-driven water oxidation catalysis: photon management, ligand effect and catalyst evolution

Abstract

Iridium complexes [IrClCp*diNHC]PF₆, with N-heterocyclic dicarbene (diNHC) and pentamethylcyclopentadienyl (Cp*) ligands, have been investigated in light driven water oxidation catalysis within the Ru(bpy)₃²⁺/S₂O₈²⁻ cycle (bpy = 2,2′-bipyridine). In particular, the effect of different diNHC ligands was evaluated by employing the complex 1a (diNHC = 1,1′-dimethyl-3,3′-ethylenediimidazol-2,2′-diylidene) and the novel and structurally characterised 2 (diNHC = 1,1'-dimethyl-3,3′-ethylene-5,5′-dibromodiimidazol-2,2′-diylidene) and 3 (diNHC = 1,1′-dimethyl-3,3'-ethylene-dibenzimidazol-2,2′-diylidene). The presented results include: (i) a photon management analysis of the 1a/Ru(bpy)₃²⁺/S₂O₈²⁻ system, revealing two regimes of O₂ evolution rate, being dependent on the light intensity at low photon flux, where the system reaches an overall quantum yield up to 0.17 ± 0.01 (quantum efficiency 34 ± 2%), while being independent of light intensity at high photon flux thus indicating a change of limiting step; (ii) a trend of O₂ evolution activity that follows the order 1a > 2 > 3 both under low and high photon flux conditions, with the reactivity that is favoured by the electron donating nature of the diNHC ligand, quantified on the basis of the carbene carbon chemical shift; (iii) an analogous trend also in the bimolecular rate constants of electron transfer k_ET from the iridium species to photogenerated Ru(bpy)₃³⁺, with k_ET values in the range 4.2–6.1 × 10⁴ M⁻¹ s⁻¹, thus implying a significant reorganisation energy to the iridium sphere; (iv) the evolution of 1a, as the most active Ir species in the series, to mononuclear iridium species with lower molecular weight and originating from oxidative transformation of the organic ligand scaffold, as proven by converging UV-Vis, MALDI-MS and ¹H-NMR evidences. These results can be used for the further design and engineering of novel catalysts.