A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework

The synthesis of a new RuII-based water oxidation catalyst is presented, in which a nitrophenyl group is introduced into the backbone of dpp via a pH-sensitive imidazole bridge (dpp = 2,9-di-(2′-pyridyl)-1,10-phenanthroline). This modification had a pronounced effect on the photophysical properties and led to the appearance of a significant absorption band around 441 nm in the UV–vis spectrum upon formation of the monoprotonated species under neutral conditions. Theoretical investigations could show that the main contributions to this band arise from transitions involving the imidazole and nitrophenyl motif, allowing us to determine the pKa value (6.8 ± 0.1) of the corresponding, twofold protonated conjugated acid. In contrast, the influence of the nitrophenyl group on the electrochemical properties of the catalytic center was negligible. Likewise, the catalytic performance of Ru(dppip-NO2) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO2 = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline). This allowed the original catalytic properties to be retained while additionally featuring a functionalized ligand scaffold, which provides further modification opportunities as well as the ability to report the pH of the catalytic solution via UV–vis spectroscopy.


General Information
Oxygen detection: O2 concentrations were measured using a FireStingO2 optical oxygen meter (Pyroscience, Germany) using oxygen sensitive optical sensor spots (OXSP5, with optical isolation).
The spot was glued (transparent silicone glue, SPGLUE). to the inner glass wall of a screw-capped vial.
The O2 concentration was measured in μmol/L (solution) and mbar (gas-phase). Two-point calibration of the liquid phase was performed using a de-oxygenated aqueous solution (aqueous sodium dithionite solution) and air equilibrated deionized water. Two-calibration of the gas-phase was performed against Ar-atmosphere and ambient air. Solution turn over numbers (TONs) were calculated based on the detected concentration, gas-phase TONs were calculated utilizing the ideal gas equation. This method of oxygen detection has been reported previously. 2 Irradiation setup: Irradiation of the samples was performed via one LED-stick (λmax = 470 nm, 45-50 mW cm -2 ). During irradiation reaction vessels were tempered by a custom air cooling setup (25 °C). To ensure reproducible irradiation conditions, the reaction vial and LED-stick were fixated with a 3D-printed holder (also see Figure S1). calculations were performed with the ORCA 4.2 7 program package using B3LYP 8-11 -D3BJ 12,13 , the C-PCM 14 continuum solvation model (acetonitrile) and using the RIJCOSX 15-17 approximation for computational efficiency 18 together with the SARC/J 19,20 auxiliary basis set. B3LYP was chosen since it is reported to yield good results for computing absorption spectra of Ru-complexes. [21][22][23][24][25][26] The geometries were optimized with DFT using the def2-TZVP 27 basis set with the corresponding effective core potential (def2-ECP) 28 on Ru, and for C-PCM the Gaussian charge scheme 29 was used as implemented in ORCA 4.2. Frequency calculations showed that the optimized structures correspond to minima (no imaginary frequency). TDDFT in the Tamm-Dancoff approximation (TDA) 30 was used to compute the spectra with the scalar relativistic ZORA 31 Hamiltonian, and the relativistically recontracted versions of the basis sets, 32 ZORA-def2-TZVP and ZORA-TZVP on Ru. In the case of Ru(dpp), 150 singlet excited states were calculated, and 200 states for Ru(dppip-NO2).
The stick spectra of the equilibrium geometries were convoluted with Gaussian functions employing a full width at half maximum (fwhm) of 0.35 eV to obtain their absorption spectra. Furthermore, 50 geometries for each Ru-complex were sampled 33 from a temperature-dependent Wigner distribution 34,35 at 300 K excluding low-frequency vibrational modes below 40 cm -1 (50 cm -1 for 0H-Ru(dppip-NO2)). For the corresponding spectra, Gaussian functions with a fwhm of 0.30 eV were used.
An automatized charge transfer analysis of the transition density matrix was performed with the TheoDORE 36-38 package. For this, the Ru-complexes were divided into chromophoric fragments as depicted in Figure S2 based on a hierarchical clustering ansatz 39   The pH-dependent spectra of the Ru-complex were calculated using the B3LYP spectra from the 300 K Wigner ensembles of 1H-Ru(dppip-NO2) and 2H-Ru(dppip-NO2). The intensities Int Ru1H,calc. and Int Ru2H,calc. were scaled based on the relative fractional concentrations Ru1H and Ru2H of the two species at different pH values using the experimental pKa,1 of 6.8. The (relative) total intensity as a function of the pH is given by the sum of Int Ru1H (pH) and Int Ru2H (pH).

NMR Spectra
S7 Figure      Figure S13 compares the computed equilibrium spectra of Ru(dpp) and 1H-Ru(dppip-NO2). The contribution of a specific fragment to the total spectrum was analyzed with TheoDORE. States with a significant contribution of the nitrophenyl group in 1H-Ru(dppip-NO2), as specified by a sum of the charge transfer (CT) numbers on the nitrophenyl fragment equal to or greater than 0.33, were convoluted with Gaussians to the red sub-spectrum in Figure S13. The residual states, which only have a minor or no contribution from the nitrophenyl group, were combined to the "rest" sub-spectrum shown in blue.
Interestingly, this residual spectrum of 1H-Ru(dppip-NO2) in blue between ca. 275 and 700 nm closely resembles the spectrum of Ru(dpp) showing an intense band at 298 nm with a shoulder at ca. 330 nm (compared to 306 and 334 nm in Ru(dpp), cf. Table S3), followed by two smaller bands in the visible region at 398 nm and 513 nm (412 and 517 nm in Ru(dpp)). The excitation energies, oscillator strengths (fosc) and main excited-state characters of intense transitions computed for the equilibrium structures of the Ru-catalysts are summarized in Table S2 and   Table S3 below and compared to experimental values. Table S2: Selected intense states of 1H-Ru(dppip-NO2) and its protonated ("2H") and deprotonated forms ("0H"): vertical excitation energies E, wavelengths λ, oscillator strengths fosc, and main state characters; wavelengths λmax of peak maxima or shoulders "sh"

Catalysis
Turnover number: