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

• Published in: Inorganica Chimica Acta 2000-06, Vol.304 (1), p.87-98
• Main Authors: Vamvounis, George, Caplan, Jennifer F, Cameron, T.Stanley, Robertson, Katherine N, Aquino, Manuel A.S
• Format: Article
• Language: English
• Subjects: Acetate complexes; Crystal structures; Electrochemistry; Mixed-valence complexes; Ruthenium complexes
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(00)00076-1

Diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru 2(μ-O 2CCH 3) 4L 2](PF 6), 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–L ax 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 [Ru 2(μ-O 2CCH 3) 4(H 2O) 2](PF 6)) to 61 in 1. UV–Vis measurements show a very slight increase in energy of the π(Ru–O, Ru 2)→π*(Ru 2) 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 Ru 2 4+/5+ redox couple show a decrease in the E 1/2 of 292 mV on going from complex 5 (weakest N-donor) to complex 1 (strongest N-donor). The E 1/2 range is over 400 mV when the unligated [Ru 2(μ-O 2C(CH 2) 6CH 3) 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.