Synthesis, structure and electrochemistry of nitrogen base adducts of tetraacetatodiruthenium(II,III): dependence of redox potential and Ru–Ru bond length on axial ligand donor strength

Diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru2(μ-O2CCH3)4L2](PF6), with L=N-heterocyclic axial ligands quinuclidine (quin) (1), 4-methylpyridine (4-Mepy) (2), pyridine (py) (3), 4-cyanopyridine (4-CNpy) (4), 3-cyanopyridine (3-CNpy) (5) and 4-phenylpyridine (4-Phpy) (6) have been synthesized and all but 5 were characterized by X-ray crystallography to study the effect of the variation of the donor number (DN) of L on the Ru–Ru and Ru–Lax bond lengths, the magnetic moment, the electronic spectral properties and the redox potential. When data from previous studies on O-donor adducts was also included a DN range of 18–61 could be established. Over this range the Ru–Ru bond length increases slightly from 2.265(1) to 2.2917(6) Å as the donor number is increased from 18 (in [Ru2(μ-O2CCH3)4(H2O)2](PF6)) to 61 in 1. UV–Vis measurements show a very slight increase in energy of the π(Ru–O, Ru2)→π*(Ru2) transition, however, room temperature magnetic susceptibilty measurements show no change in the magnetic moment over the same range of donor numbers. Electrochemical measurements in 1,2-dichloroethane of the Ru2 4+/5+ redox couple show a decrease in the E1/2 of 292 mV on going from complex 5 (weakest N-donor) to complex 1 (strongest N-donor). The E1/2 range is over 400 mV when the unligated [Ru2(μ-O2C(CH2)6CH3)4] complex is included (DN=1 for dichloromethane). The variation of axial ligand base strength does not effect the near-degeneracy of the (π*δ*)3 HOMO or the π→π* energy gap, however, the actual (π*δ*)3 HOMO energy varies significantly and increases as the basicity of the axial ligand increases allowing selective tuning of the redox potential.