The influence of trinuclear complexes on light-induced hydrogen production

Abstract

Inspired by nature, artificial molecular systems are designed to produce hydrogen for chemical storage or as a primary source of energy. A class of new trinuclear complex based on a PNP-ligand 3 was synthesised, characterised and investigated for light-induced hydrogen production. To demonstrate the influence of multiple metal centres on the chromophoric and catalytic behaviour, the ability of the systems to produce hydrogen was investigated under various conditions. We found that multiple metal centres hinder each other in the case of chromophores and support each other in the case of WRCs (water reduction catalysts), which was apparent in the form of the received TON (turnover number). UV/Vis spectroscopy and photophysical measurements were conducted to gain insights into the light-induced transitions of the excited chromophoric units. A full photophysical characterisation of the trinuclear chromophore [(Cu(phenanthrolinederivate))₃(1,3,5-tris(PNP-Me)-benzene)](PF₆)₃ (4d) is presented and the hydrogen evolution ability was tested when combined with the literature-known and benchmarked catalysts 1 [Ni(py-S)₃](NEt₄) and 2 [Fe₃(CO)₁₂]. Trimetallic WRCs based on Pd, Pt, Co, Ni, and Fe were developed and the ability of the non-noble and noble metal-based systems to produce hydrogen was studied. Different sunlight imitating light sources were used to optimise the final TON and turnover frequency (TOF). Additionally, the redox states of the most promising WRCs were analysed by cyclic voltammetry (CV) to gather information about their water reduction ability. A major preparative effort has been undertaken in order to obtain at least some excellent chromophores and/or WRCs. For the trinuclear chromophore 4d, only a modest TON of 48 could be achieved. However, the trinuclear WRC [(Pd(ACN)₂)₃(tris(PNP-Me)benzene)](PF₆)₆ (8b) showed excellent TONs up to 8899 and a turnover frequency (TOF) of 2737 h⁻¹ with a correlating incident photon conversion efficiency (IPCE) of 2.1%. These values are among the best regarding molecular WRCs. The structurally similar non-noble metal-based WRCs with iron and nickel showed TONs of 290 and 460, respectively.