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. 2023 Jul 3;62(26):10031-10038.
doi: 10.1021/acs.inorgchem.3c01241. Epub 2023 Jun 16.

Independent Tuning of the p Ka or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes

Benjamin D Groff  1 , Mauricio Cattaneo  1 , Scott C Coste  1 , Chloe A Pressley  1 , Brandon Q Mercado  1 , James M Mayer  1
Affiliations

Independent Tuning of the p Ka or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes

Benjamin D Groff et al. Inorg Chem. .

Abstract

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

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Figures

Figure 1.
Figure 1.
Drawings of RuII(acac)2(py-imH) complexes (top row) and molecular structures of their oxidized, deprotonated forms RuIII(acac)2(py-im) from single-crystal X-ray diffraction (bottom row). These show the substitution patterns in acac-substituted (1-4) and py-imH-substituted (4-7) series. Complex 3 (not shown) is the hexafluoro-acac derivative Ru(hfac)2(py-imH) (the same as 4 but with CF3 instead of CH3 groups).
Figure 2.
Figure 2.
Visible region of the optical spectra of the RuIIimH forms of 1-7, showing the characteristic MLCT bands.
Figure 3.
Figure 3.
Computed Molecular orbital diagrams for 1-7. Percental contributions are shown by color: black (Ru), blue (py-imH), and red (acac).
Figure 4.
Figure 4.
Overlay of dilution-corrected UV-vis spectra from the titration of 1-RuIIIimH+ with 0.05 (blue) to 100 (pink) equivalents of 2,4,6-collidine (pKa = 15.0030), and after the addition of excess strong base (dashed black line). Inset shows the linear plot of [RuIIIim][collidine–H+]/RuIIIimH+] vs. [collidine], with slope = Keq.
Figure 5.
Figure 5.
(A) CVs of 6-RuIIIimH+ before (blue) and after addition of excess triethylamine (pKa = 18.83; red) in MeCN solution containing ferrocene and [n-Bu4N][PF6]. (B) CVs of 6-RuIIIim in buffered 2,4,6-collidine/2,4,6-collidine–H+ (pKa = 15.00) in varying ratios. (C) Plot of the E1/2 vs Fc+/0 of the 1e−/1H+ proton-coupled RuIII/II redox couple vs logarithm of the buffer ratio, with slope of 59.0 ± 0.1 mV and y-intercept (dashed line) of −701 ± 5 mV. The colors of the points correspond to those of the corresponding CVs in part (B).
Figure 6.
Figure 6.
Bar chart of the E1/2 and pKa changes from the parent complex 4 induced by substitutions at the acac (1-4) and py-imH (4-7) ligands, converted to free energies. Darker bars are experimental values and lighter bars are calculated.
Figure 7.
Figure 7.
The experimental and calculated λmax of the two visible MLCT bands (left axis) and the E1/2 (right axis) for each of RuIIimH complexes 1-7. The plot highlights the correlation of the E1/2 with the energy of the lower-energy MLCT (purple) in complexes 1-4.
Scheme 1.
Scheme 1.
Square scheme for an XH/X PCET reagent and a drawing of the class of ruthenium complexes examined here.
See this image and copyright information in PMC

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