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)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.

NMR spectra were recorded on a Bruker Avance III HD 400 or 500 at 293 K and processed with MestReNova software (Version 14.2.0). The chemical shifts δ are reported in parts per million (ppm). 1H NMR and 13C NMR shifts are referenced according to the applied deuterated solvent as internal standard. Constants J are presented as absolute values in Hz. For the characterization of the NMR signals the following abbreviations are used: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet and dd = doublet of doublets.
High resolution mass spectrometry.

Electrochemistry
Electrochemical data were obtained by cyclic voltammetry using a conventional single-compartment three-electrode cell arrangement in combination with a CH Instruments CHI 620E electrochemical workstation. A Pt wire was used as counter electrode, an Ag wire as quasi reference electrode and a glassy carbon electrode as the working electrode. The measurements were carried out in anhydrous and argon-saturated acetonitrile (ACN). Tetrabutylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte at ambient temperature (20 ± 5 °C). All potentials are referenced to ferrocene/ferricenium [E(Fc/Fc + ) = 0.00 V].

Absorption and Emission spectroscopy.
Absorption spectra were recorded on a JASCO Spectrometer V-670. Continuous absorption spectra were recorded on a single-channel fiber-optic spectrometer (AvaSpec-ULS2048CL-EVO or AvaSpec-ULS2048XL). For illumination, a deuterium-halogen light source was used (AvaLight DH-S-BAL, Avantes Inc., USA). Emission spectroscopic investigations were performed with a JASCO Spectrofluorometer FP-8500. All samples were measured in quartz cuvettes with a path length of 10 mm.

Resonance Raman spectroscopy.
For rR spectroscopy, a single longitudinal mode diode laser at 405 nm (TopMode-405-HP, Toptica, Germany) was used for excitation. A grating spectrometer (IsoPlane 160, Princeton Instruments, USA) with an entrance slit width of 50 µm and 160 mm focal length was used for spectral detection employing diffraction gratings with 2400, 1200 and 600 grooves/mm as indicated in the figure captions. The laser power was attenuated to approximately 5 mW to reduce photodegradation of the analyte. The Raman scattering signals were collected in transmission and filtered from laser light with dielectric long-pass filters (Semrock, USA) before being focused on the entrance slit of the spectrograph. The Raman scattered light was spectrally dispersed, and the photons were detected by a thermoelectrically cooled CCD camera with 1340 x 100 pixels (PIXIS eXcelon, Princeton Instruments, USA). The band of the solvent ACN at 1375 cm -1 was used for normalizing the Raman intensities and calibrating the wavenumber scale. Spectral post-processing includes background correction, normalization, and subtraction of the solvent spectrum.

Transient absorption (TA) spectroscopy.
The femtosecond-TA data were recorded using a specially designed optical setup. For details see. 1,2 A regenerative Ti:sapphire amplifier (Libra, Coherent, USA) at 1 kHz pulse repetition rate was used for excitation. The output of the laser is split in two parts. The first fraction is focused into a rotating CaF2 plate to generate a broadband white light supercontinuum. This broadband pulse is divided into reference and probe pulse. The other part of the laser output is used to generate the pump pulses at 400 and 470 nm of about 100 fs pulse duration. SHG of the fundamental in a nonlinear crystal is used for generation of the 400 nm pump pulse. The 470 nm pump pulse is generated in an optical parametric amplifier (Topas, Light conversion, Lithuania) by parametric conversion. A mechanical chopper is used to reduce the repetition rate of the pump pulses to 0.5 kHz and the polarization of pump and probe pulse is adjusted to the magic angle of 54.7° with respect to the white light probe beam using a Berek compensator and a polarizer. The probe pulse is focused into the 1 mm cuvette by a concave mirror of 500 mm focal length. The spectra of probe and reference pulses are recorded by a Czerny-Turner spectrograph with 150 mm focal length (SP2150, Princeton Instruments) equipped with a diode array detector (Pascher Instruments AB, Sweden). In the time range of 300 fs around time zero strong contributions of coherent artifact signals 3 are observed, which prevent the analysis of pump-probe data by multiexponential fitting algorithms with shorter time delay.
The TA data analysis first includes a spectral preprocessing step for chirp correction. Then a sum of exponential functions is fitted to the data by a least squares regression analysis using python software (python tool KiMoPack). 4 The pulse overlap range of ±150 fs is removed from the data analysis due to the coherent artifacts mentioned above. The amplitudes of the exponential fitting correspond to the decay associated spectra (DAS). For the investigation of the primary photoinduced processes, the sample was dissolved in anhydrous ACN or DCM (OD (400 nm) = 0.3 in a cell with 1 mm path length).

Nanosecond transient emission spectroscopy.
Nanosecond spectroscopy of transient emissions was used to study the lifetime of the long-lived species. The setup was used as described in the literature. 5 The pump pulses centered at 355 nm were generated by a continuum surelite Nd:YAG laser system (pulse duration 5 ns, repetition rate 10 Hz). A Continuum OPO Plus, pumped by a Continuum Surelite Nd:YAG laser, generated the pump pulses at 470 nm. The power of the pump beam was maintained at 0.2 mJ per pulse. The probe light is provided by a 75 W xenon arc lamp. Spherical concave mirrors are used to focus the probe beam into the samples and then send the beam to the monochromator (Acton, Princeton Instruments). The spectrally selected probe light is detected by a Hamamatsu R928 photomultiplier. Time-resolved emission spectra were recorded using a 475 nm long pass filter in front of the detector to eliminate pump scatter. The signal is amplified and processed by a commercially available detection system (Pascher Instruments AB). Each sample was freshly prepared, and its optical density was maintained at about 0.2 at the excitation wavelength. All measurements were performed in fluorescence cuvettes with 1 cm pathlength. The stability of the samples during all spectroscopic experiments was checked by measuring the absorption spectra before and after each spectroscopic run.
All spectroscopic measurements were performed on individual sample solutions and the same sample solutions were not measured repeatedly.

Procedure for determining the selectivity of NADH formation:
For determination of the NADH selectivity of the different catalytic processes, a calibration curve using commercially available NADH (Sigma Aldrich, 97 % purity) was recorded as follows: Samples of different NADH concentrations (0 µM, 25 µM, 50 µM, 100 µM, 150 µM, 200 µM, 250 µM and 300 µM) were prepared in either ACN:H2O = 1:9 (v:v) or ACN:H2O = 1:2 (v:v) at air and an UV/vis absorption spectrum was recorded. Afterwards from each sample an aliquot was taken out and diluted with water to a tenth of its initial concentration using deionized water. With these samples, emission spectra were recorded using λexc = 340 nm. After considering the 97 % purity of the utilized commercially available NADH, a plot of E(340 nm) vs. emission intensity(462 nm) could be plotted that gives the expected emission intensity for a certain absorbance increase at 340 nm if during the catalytic process NADH would have been generated in 100 % selectivity. To determine the true selectivity of the catalytic process, the actually recorded emission intensity was divided by the emission intensity that would have been expected for 100 % NADH-selectivity based on the recorded absorbance increase at 340 nm.