Synthesis, Structure, and Electronic Properties of a Dimer of Ru(bpy)₂ Doubly Bridged by Methoxide and Pyrazolate

The heterobridged dinuclear complex cis,cis-[(bpy)₂Ru(μ-OCH₃)(μ-pyz)Ru(bpy)₂]²⁺ (1; bpy = 2,2′-bipyridine; pyz = pyrazolate) was synthesized and isolated as a hexafluorophosphate salt. Its molecular structure was fully characterized by X-ray crystallography, ¹H NMR spectroscopy, and ESI mass spectrometry. The compound 1·(PF₆)₂ (C₄₄H₃₈F₁₂N₁₀OP₂Ru₂) crystallizes in the monoclinic space group P2₁/c with a = 13.3312(4) Å, b = 22.5379(6) Å, c = 17.2818(4) Å, β = 99.497(2)°, V = 5121.3(2) ų, and Z = 4. The meso diastereoisomeric form was exclusively found in the crystal structure, although the NMR spectra clearly demonstrated the presence of two stereoisomers in solution (rac and meso forms at approximately 1:1 ratio). The electronic properties of the complex in acetonitrile were investigated by cyclic voltammetry and UV−vis and NIR−IR spectroelectrochemistries. The stepwise oxidation of the Ru^II−Ru^II complex into the mixed-valent Ru^II−Ru^III and fully oxidized Ru^III−Ru^III states is fully reversible on the time scale of the in situ (spectro)electrochemical measurements. The mixed-valent species displays strong electronic coupling, as evidenced by the large splitting between the redox potentials for the Ru(III)/Ru(II) couples (ΔE_1/2 = 0.62 V; K_c = 3 × 10¹⁰) and the appearance of an intervalence transfer (IT) band at 1490 nm that is intense, narrow, and independent of solvent. Whereas this salient band in the NIR region originates primarily from highest-energy of the three IT transitions predicted for Ru(II)−Ru(III) systems, a weaker absorption band corresponding to the lowest-energy IT transition was clearly evidenced in the IR region (≈3200 cm⁻¹). The observation of totally coalesced vibrational peaks in the 1400−1650 cm⁻¹ range for a set of five bpy spectator vibrations in Ru^II−Ru^III relative to Ru^II−Ru^II and Ru^III−Ru^III provided evidence for rapid electron transfer and valence averaging on the picosecond time scale. Other than a relatively short Ru···Ru distance (3.72 Å for the crystalline Ru^II−Ru^II complex), the extensive communication between metal centers is attributed mostly to the π-donor ability of the bridging ligands (pyz, OMe) combined with the π-acceptor ability of the peripheral (bpy) ligands.