Usual monodentate binding mode of 2,2'-dipyridylamine (L) in isomeric trans-(acac)₂Ru^II(L)₂, trans-[(acac)₂Ru^III(L)₂]ClO₄, and cis-(acac)_2Ru^II(L)₂ (acac = acetylacetonate). synthesis, structures, and spectroscopic, electrochemical, and magnetic aspects

Abstract

The reaction of cis-Ru(acac)₂(CH₃CN)₂ (acac = acetylacetonate) with 2,2'-dipyridylamine (L) in ethanolic medium resulted in facile one-pot synthesis of stable [(acac)₂Ru^III(L)]ClO₄ ([1]ClO₄), trans-[(acac)₂Ru^II(L)₂] (2), trans-[(acac)₂Ru^III(L)₂]ClO₄ ([2]ClO₄), and cis-[(acac)₂Ru^II(L)₂] (3). The bivalent congener 1 was generated via electrochemical reduction of [1]ClO₄. Although in [1]⁺ the dipyridylamine ligand (L) is bonded to the metal ion in usual bidentate fashion, in 2/[2]⁺ and 3, the unusual monodentate binding mode of L has been preferentially stabilized. Moreover, in 2/[2]⁺ and 3, two such monodentate L's have been oriented in the trans- and cis-configurations, respectively. The binding mode of L and the isomeric geometries of the complexes were established by their single-crystal X-ray structures. The redox stability of the Ru(II) state follows the order 1 < 2 «3. In contrast to the magnetic moment obtained for [1]ClO₄, μ = 1.84 μ_B at 298K , typical for low-spin Ru(III) species, the compound [2]ClO₄ exhibited an anomalous magnetic moment of 2.71 μ_B at 300 K in the solid state. The variable-temperature magnetic measurements showed a pronounced decrease of the magnetic moment with the temperature, and that dropped to 1.59 μ_B at 3K . The experimental data can be fitted satisfactorily using eq 2 that considered nonquenched spin-orbit coupling and Weiss constant in addition to the temperature-independent paramagnetism. [1]ClO₄ and [2]ClO₄ displayed rhombic and axial EPR spectra, respectively, in both the solid and the solution states at 77K.