Diruthenium Complexes [{(acac)<sub>2</sub>Ru<sup>III</sup>}<sub>2</sub>(μ-OC<sub>2</sub>H<sub>5</sub>)<sub>2</sub>], [{(acac)<sub>2</sub>Ru<sup>III</sup>}<sub>2</sub>(μ-L)](ClO<sub>4</sub>)<sub>2</sub>, and [{(bpy)<sub>2</sub>Ru<sup>II</sup>}<sub>2</sub>(μ-L)](ClO<sub>4</sub>)<sub>4</sub> [L = (NC<sub>5</sub>H<sub>4</sub>)<sub>2</sub>−N−C<sub>6</sub>H<sub>4</sub>−N−(NC<sub>5</sub>H<sub>4</sub>)<sub>2</sub>, acac = Acetylacetonate, and bpy = 2,2‘-Bipyridine]. Synthesis, Structure, Magnetic, Spectral, and Photophysical Aspects
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Paramagnetic diruthenium(III) complexes (acac)2RuIII(μ-OC2H5)2RuIII(acac)2 (6) and [(acac)2RuIII(μ-L)RuIII(acac)2](ClO4)2, [7](ClO4)2, were obtained via the reaction of binucleating bridging ligand, N,N,N‘,N‘-tetra(2-pyridyl)-1,4-phenylenediamine [(NC5H4)2−N−C6H4−N-(NC5H4)2, L] with the monomeric metal precursor unit (acac)2RuII(CH3CN)2 in ethanol under aerobic conditions. However, the reaction of L with the metal fragment RuII(bpy)2(EtOH)22+ resulted in the corresponding [(bpy)2RuII (μ-L) RuII(bpy)2](ClO4)4, [8](ClO4)4. Crystal structures of L and 6 show that, in each case, the asymmetric unit consists of two independent half-molecules. The Ru−Ru distances in the two crystallographically independent molecules (F and G) of 6 are found to be 2.6448(8) and 2.6515(8) Å, respectively. Variable-temperature magnetic studies suggest that the ruthenium(III) centers in 6 and [7](ClO4)2 are very weakly antiferromagnetically coupled, having J = −0.45 and −0.63 cm-1, respectively. The g value calculated for 6 by using the van Vleck equation turned out to be only 1.11, whereas for [7](ClO4)2, the g value is 2.4, as expected for paramagnetic Ru(III) complexes. The paramagnetic complexes 6 and [7]2+ exhibit rhombic EPR spectra at 77 K in CHCl3 (g1 = 2.420, g2 = 2.192, g3 = 1.710 for 6 and g1 = 2.385, g2 = 2.177, g3 = 1.753 for [7]2+). This indicates that 6 must have an intermolecular magnetic interaction, in fact, an antiferromagnetic interaction, along at least one of the crystal axes. This conclusion was supported by ZINDO/1-level calculations. The complexes 6, [7]2+, and [8]4+ display closely spaced Ru(III)/Ru(II) couples with 70, 110, and 80 mV separations in potentials between the successive couples, respectively, implying weak intermetallic electrochemical coupling in their mixed-valent states. The electrochemical stability of the Ru(II) state follows the order: [7]2+ < 6 < [8]4+. The bipyridine derivative [8]4+ exhibits a strong luminescence [quantum yield (φ) = 0.18] at 600 nm in EtOH/MeOH (4:1) glass (at 77 K), with an estimated excited-state lifetime of approximately 10 μs.
