Outpacing conventional nicotinamide hydrogenation catalysis by a strongly communicating heterodinuclear photocatalyst

Abstract

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)2Ru-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 RhIII center but rather the rate at which a two-fold reduced RhI 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 β-hydride elimination step. Moreover, for this photocatalyst loss of the alkynyl functionality under photocatalytic conditions is identified as an important deactivation pathway.

Influence of the Linker Chemistry on the Photoinduced Charge‐Transfer Dynamics of Hetero‐dinuclear Photocatalysts

“…Upon excitation of the Rh I ‐MLCT at 600 nm the choice of the bridging ligand, that is, −≡− versus −trz−, significantly impacts the photoinduced dynamics in the Ru‐BL−Rh complexes. Considering Ru II −trz−Rh I , the TA signatures upon 600 nm excitation differ only slightly in the structure of the ESA above 670 nm in comparison to the TA spectra recorded upon 403 nm excitation (Figure 2A) [11] . We rationalize this result considering that the excited‐state properties are determined by the reduced phen moiety, which acts as charge accepting moiety in both the Ru II ‐MLCT and Rh I ‐MLCT transitions.…”

“…Excitation of Ru II −trz−Rh I yields the spectral signatures depicted in Figure 2: a ground‐state bleach (GSB) following the steady‐state absorption is accompanied by a rather broad, unstructured excited state absorption (ESA) in the red part of the visible spectrum and a comparably sharp ESA band at 355 nm. While the spectral features of Ru II −trz−Rh III do not differ significantly from the data recorded for [Ru(bpy) 2 (phen)] 2+ and [Ru(phen) 3 ] 2+ chromophores (Figure S4; bpy=2,2′‐bipyridine), electrochemical reduction of the Rh center (Rh III /Rh I ) in Ru II −trz−Rh I affects the charge transfer dynamics compared to [Ru(bpy) x (phen) y ] 2+ ‐chromophores [11,15,20] . This is evident when looking at the kinetics of the ESA above 500 nm.…”

“…The triazole linker, on the other hand, slows down the charge transfer from the photocenter to the catalytic center, but stabilizes the charge‐separated state and blocks the charge transfer back to the Ru center upon excitation at 600 nm. This results in a long‐lived, charge‐separated state, and explains the similar catalytic activity of Ru II −≡−Rh III and Ru II −trz−Rh III [11] . Finally, the sequence of electrochemical two‐electron reduction followed by Ru‐MLCT excitation triggering hole transfer from the Ru to the Rh center, paves the way towards understanding the electron‐transfer kinetics of a twofold‐reduced Ru−BL−Rh complex.…”

“…In contrast, excitation of the Ru II center in the reduced complex Ru II −≡−Rh I (Figure 3A) leads to TA spectra significantly different from the non‐reduced complex Ru II −≡−Rh III [11] . Following excitation of the Ru II ‐MLCT, a strong excited state absorption feature above 620 nm and negative differential absorption features at 460 and 565 nm appear immediately (Figure 3A and C).…”

“…The first wave is assigned to a two‐electron reduction of the Rh III /Rh I redox couple accompanied by a loss of a chloride ligand [12] . The second reduction wave causes reduction of the bridging ligand, that is, of the 1,10‐phenathroline (phen) fragment in the ruthenium sphere [3,11,13] . At even more negative potential the peripheral tbbpy ligands become reduced [13a,14] …”