Outpacing conventional nicotinamide hydrogenation catalysis by a strongly communicating heterodinuclear photocatalyst

Unequivocal assignment of rate-limiting steps in supramolecular photocatalysts is of utmost importance to rationally optimize photocatalytic activity. By spectroscopic and catalytic analysis of a series of three structurally similar [(tbbpy)(2)Ru-BL-Rh(Cp*)Cl](3+) photocatalysts just differing in the central part (alkynyl, triazole or phenazine) of the bridging ligand (BL) we are able to derive design strategies for improved photocatalytic activity of this class of compounds (tbbpy = 4,4 '-tert-butyl-2,2 '-bipyridine, Cp* = pentamethylcyclopentadienyl). Most importantly, not the rate of the transfer of the first electron towards the Rh-III center but rather the rate at which a two-fold reduced Rh-I species is generated can directly be correlated with the observed photocatalytic formation of NADH from NAD(+). Interestingly, the complex which exhibits the fastest intramolecular electron transfer kinetics for the first electron is not the one that allows the fastest photocatalysis. With the photocatalytically most efficient alkynyl linked system, it is even possible to overcome the rate of thermal NADH formation by avoiding the rate-determining beta-hydride elimination step. Moreover, for this photocatalyst loss of the alkynyl functionality under photocatalytic conditions is identified as an important deactivation pathway. Although multinuclear, supramolecular photocatalysts show promise in their ability to separate the processes required for lightdriven energy production into light absorption, charge separation and fuel production by individual parts of the molecule, mechanistic understanding of the performance limiting processes are lacking. Here the authors synthesize two new dinuclear catalysts and compare them to a benchmark through detailed spectroscopic studies, obtaining significant chemical insight.

Automated summary

Unequivocal assignment of rate-limiting steps in supramolecular photocatalysts is crucial for optimizing photocatalytic activity. By studying a series of structurally similar photocatalysts, we discovered that the rate at which a two-fold reduced Rh-I species is generated is directly correlated with the observed photocatalytic formation of NADH from NAD(+). Interestingly, the fastest intramolecular electron transfer kinetics for the first electron does not necessarily lead to the fastest photocatalysis.