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The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+). Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru(III) ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK(a) values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru(III) appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ(1)*)(2) with a low-lying, paramagnetic triplet state (dπ(1)*)(1)(dπ(2)*)(1). Systematic structural-magnetic-IR correlations are observed between ν(sym)(RuORu) and ν(asym)(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with ∠Ru-O-Ru dictated by the distortion and electron-electron repulsion.

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Ligands containing groups derived from bis(aryl)diols are widely used in asymmetric catalysis; however, few studies of the conformations of these ligands in transition-metal complexes have been reported. In this paper, the nucleophilic displacement reactions of cis-Mo(CO)4(2,2‘-C12H8O2PCl)2 (1) have been used to prepare a variety of complexes with [1,3,2]dioxaphosphepin ligands, and the conformations of these ligands have been studied by NMR spectroscopy and X-ray crystallography. The nucleophilic substitution reactions yield both the expected disubstituted complexes cis-Mo(CO)4(2,2‘-C12H8O2PXR)2 (XR = NPrn (2), OMe (4), SC6H4-4-Me (6)) and the unexpected hydrolysis products [R‘3NH][cis-Mo(CO)4(2,2‘-C12H8O2PO)(2,2‘-C12H8O2PXR)] (R‘3 = PrnH2, XR = NPrn, 3; R‘3 = Et3; XR = OMe, 5). NMR studies have demonstrated that the hydrolysis product is the major product when more than a minute amount of water is present, even in the presence of a large excess of the nucleophiles. This reaction is complete in approximately 90 min at 25 °C. A very surprising feature of this reaction is that substitution of one chloride in 1 by the RX- nucleophile greatly enhances the rate of substitution of the second chloride either by water or by another RX- nucleophile. NMR studies of the [1,3,2]dioxaphosphepin complexes in chloroform-d solution suggest that the R* and S* enantiomers of the ligands interconvert via a low-energy pathway. Crystal structures of the complexes demonstrate that both the R*S* diastereomer (1) and racemic mixtures of the R*R* and S*S* diastereomers (2−4) are observed in the solid state. These results suggest that bulkier biaryl groups are needed to prevent the racemization of the [1,3,2]dioxaphosphepin ligands in solution.