Hydroxo-Bridged Dimers of Oxo-Centered Ruthenium(III) Triangle: Synthesis and Spectroscopic and Theoretical Investigations

The homometallic hexameric ruthenium cluster of the formula [Ru-6(III)((3)-O)(2)(-OH)(2)((CH3)(3)CCO2)(12)(py)(2)] (1) (py=pyridine) is solved by single-crystal X-ray diffraction. Magnetic susceptibility measurements performed on 1 suggest that the antiferromagnetic interaction between the Ru-III centers is dominant, and this is supported by theoretical studies. Theoretical calculations based on density functional methods yield eight different exchange interaction values for 1: J(1)=-737.6, J(2)=+63.4, J(3)=-187.6, J(4)=+124.4, J(5)=-376.4, J(6)=-601.2, J(7)=-657.0, and J(8)=-800.6cm(-1). Among all the computed J values, six are found to be antiferromagnetic. Four exchange values (J(1), J(6), J(7) and J(8)) are computed to be extremely strong, with J(8), mediated through one -hydroxo and a carboxylate bridge, being by far the largest exchange obtained for any transition-metal cluster. The origin of these strong interactions is the orientation of the magnetic orbitals in the Ru-III centers, and the computed J values are rationalized by using molecular orbital and natural bond order analysis. Detailed NMR studies (H-1, C-13, HSQC, NOESY, and TOCSY) of 1 (in CDCl3) confirm the existence of the solid-state structure in solution. The observation of sharp NMR peaks and spin-lattice time relaxation (T1 relaxation) experiments support the existence of strong intramolecular antiferromagnetic exchange interactions between the metal centers. A broad absorption peak around 600-1000nm in the visible to near-IR region is a characteristic signature of an intracluster charge-transfer transition. Cyclic voltammetry experiments show that there are three reversible one-electron redox couples at -0.865, +0.186, and +1.159V with respect to the Ag/AgCl reference electrode, which corresponds to two metal-based one-electron oxidations and one reduction process.