Redox behavior of a binuclear ruthenium complex having a di-μ-nitrosyl ligand, [Ru(acac)22(μ-NO)2] (acac = acetylacetonato)

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

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.

Synthesis and X-ray structural characterization of the ruthenium β-diketonato complexes: Ru(hfac)3, cis-Ru(hfac)2(MeCN)2, and cis-Ru(hfac)(acac)(MeCN)2

Ru(hfac)3 (2) was synthesized via peroxide oxidation of the Ru(II) species [Na][Ru(hfac)3] (1) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate). Treatment of either 1 or 2 with CF3SO3H in MeCN generated cis-Ru(hfac)2(MeCN)2 (3). Ru(acac)2(hfac) (4) (acac = acetylacetonate) was generated by addition of hfac to an EtOH solution of cis-[Ru(acac)2(MeCN)2][CF3SO3]. Subsequent treatment of 4 with CF3SO3H in MeCN yielded cis-Ru(acac)(hfac)(MeCN)2 (7). Syntheses of [Na][Ru(hfac)2(acac)] (5) and Ru(hfac)2(acac) (6) are also reported. The complexes were characterized generally by elemental analysis, cyclic voltammetry, UV-vis, NMR, and IR spectroscopies. The structures of 2, 3, and 7 were established by X-ray crystallographic analyses. Crystals of 2 are monoclinic with a = 8.7781(4), b = 13.0760(11), c = 19.1857(5) Å, β = 92.2275(5)°, Z = 4, and space group P21/n; those of 3 are monoclinic with a = 25.731(4), b = 8.8332(13), c = 18.1955(4) Å, β = 93.3395(6)°, Z = 8, and space group C2/c; and those of 7 are triclinic with a = 7.6812(9), b = 10.680(2), c = 12.578(2) Å, α = 88.062(6)°, β = 83.874(3)°, γ = 69.5898(15)°, Z = 2, and space group Pbar over 1. The structures were solved by Patterson methods and refined by full-matrix least-squares procedures to R(F) = 0.036, 0.035, and 0.068 (Rw(F2) = 0.061,Rw(F) = 0.052, and Rw(F) = 0.089), respectively.Key words: ruthenium, β-diketonato complexes, acetylacetonate, 1,1,1,5,5,5-hexafluoroacetylacetonate, acetonitrile complexes.