Correspondence of Ru ^IIIRu ^II and Ru ^IVRu ^III mixed valent states in a small dinuclear complex

The diruthenium(III) compound [(μ-oxa){Ru(acac) ₂} ₂] [1, oxa ^2-=oxamidato(2-), acac ^-=2,4- pentanedionato] exhibits an S=1 ground state with antiferromagnetic spin-spin coupling (J=-40 cm ^-1). The molecular structure in the crystal of 1â2C ₇H ₈ revealed an intramolecular metal-metal distance of 5.433 Å and a notable asymmetry within the bridging ligand. Cyclic voltammetry and spectroelectrochemistry (EPR, UV/Vis/NIR) of the two-step reduction and of the two-step oxidation (irreversible second step) produced monocation and monoanion intermediates (K _c=10 ^5.9) with broad NIR absorption bands (Îμ ca. 2000M ^-1cm ^-1) and maxima at 1800 (1 ^-) and 1500 nm (1 ⁺). TD-DFT calculations support a Ru ^IIIRu ^II formulation for 1 ^- with a doublet ground state. The 1 ⁺ ion (Ru ^IVRu ^III) was calculated with an S=3/2 ground state and the doublet state higher in energy (ÎE=694.6 cm ^-1). The Mulliken spin density calculations showed little participation of the ligand bridge in the spin accommodation for all paramagnetic species [(μ-oxa){Ru(acac) ₂} ₂] ⁿ, n=+1, 0, -1, and, accordingly, the NIR absorptions were identified as metal-to-metal (intervalence) charge transfers. Whereas only one such NIR band was observed for the Ru ^IIIRu ^II (4d ⁵/4d ⁶) system 1 ^-, the Ru ^IVRu ^III (4d ⁴/4d ⁵) form 1 ⁺ exhibited extended absorbance over the UV/Vis/NIR range.