Coupling Molecular Photocatalysis to Enzymatic Conversion

Abstract

A hetero‐binuclear dyad that contains a ruthenium polypyridyl moiety bound through an aromatic bridging ligand to an organometallic catalytic center has been used for the light‐driven reduction of the N‐benzyl‐3‐carbamoylpyridinium cation, NAD+, and NADP+ to yield the two‐electron‐reduced analog. Direct coupling with enzymatic conversion was proved by using UV/Vis spectroscopy and liquid chromatography, which showed cofactor‐recycling and enzymatic conversion with a turnover number of 350 per photocatalyst. First insights into the complex behavior of the catalytic system under irradiation point towards multiple prerequisites on the molecular as well as on the macroscopic level to generate highly efficient semiartificial photo‐biocatalytic systems for future energy‐storage applications.

Unraveling the Light‐Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

“…Here we exploit electrochemical reduction and chemical reduction with CoCp 2 to form this species.Itshould be noted that finding as uitable reductant to access the catalytically competent intermediate presents adelicate task, because the respective redox potentials have to be balanced to selectively reduce the Rh ion only,atask, however, easily accomplished by electrochemical reduction. [6] To the best of our knowledge,w ep resent, for the first time,t he results of early-time photodynamics of electrochemically generated intermediates of af ully competent photocatalyst, that is,t he doubly reduced Ru(tpphz)RhCp*,u nder non-catalytic conditions to gain important mechanistic insights into electron-transfer cascades occurring during the catalytic cycle. [6] To the best of our knowledge,w ep resent, for the first time,t he results of early-time photodynamics of electrochemically generated intermediates of af ully competent photocatalyst, that is,t he doubly reduced Ru(tpphz)RhCp*,u nder non-catalytic conditions to gain important mechanistic insights into electron-transfer cascades occurring during the catalytic cycle.…”

Product Selectivity in Homogeneous Artificial Photosynthesis Using [(bpy)Rh(Cp*)X]n+-Based Catalysts

“…The following discussion intends to create a coherent picture of the intricate mechanisms of hydrogen and NADH production from the available literature. Special attention will be given to slightly basic conditions, since the TONs for NAD + reduction have been observed to be higher by a factor of 100 in contrast to the TONs for hydrogen production using a heterobimetallic photocatalyst [99]. Based on other literature reports, this finding could be anticipated, since for the photocatalytic formation of hydrogen, an optimal pH value of 3.6 has been found [37].…”

“…A broad variety of photosensitizers have been applied in these systems, ranging from small organic dyes [85,[89][90][91] to functionalized graphenes [92][93][94][95], photoredoxactive polymers [96,97], red light absorbing tin porphyrines from Knör and coworkers [98], ruthenium polypyridine complexes [32,97,99], and different solid photoactive materials [100][101][102][103][104][105][106][107][108][109][110].…”

“…These could exhibit, due to their molecular preorganisation of the chromophore and catalyst, much faster collision independent electron transfers onto the latter, which would consequently decrease the amount of bioinactive NAD(P) dimers, even at low catalyst concentrations. An example of these catalysts is depicted in Figure 13, where a photoredox active ruthenium polypyridine chromophore is linked to a [(NN)Rh(Cp*)Cl] catalyst via a tetrapyridophenazine bridging ligand [99]. The molecular catalyst exhibits suitable redox potentials for intramolecular electron transfers [113].…”

“…On the contrary, the interaction of a proton with the d z 2 HOMO of S5 would lead to the formation of S9, where a formally electron poor Rh III center is bound to the π-accepting Cp*H ligand. Due to the possibly much less effective interplay of mutual stabilizing effects in S9 compared to S12, the energy barrier for the formation of the latter would be lower, which could explain the different observed reaction rates for hydrogen production and NAD(P)H formation at neutral pH values [99]. Finally it should be noted that the hypothesized reasons for the observed chemoselectivity of [(NN)Rh(Cp*)X] n+ catalysts are based on literature reports and would need further theoretical calculations as support.…”