Bis(acetylacetonato)ruthenium complexes of noninnocent 1,2-dioxolene ligands: qualitatively different bonding in relation to monoimino and diimino analogues

Abstract

Coordination compounds [Ru(acac)₂(Q)] (acac=acetylacetonate; Q=o-benzoquinone) were prepared as complexes 1 (Q=o-benzoquinone), 2 (Q=3-methoxy-o-benzoquinone), 3 (Q=4-methyl-o-benzoquinone), and 4 (Q=3,5-di-tert-butyl-o-benzoquinone). The structures of 1 and 2 were determined to reveal a Ru^III/o-benzosemiquinone formulation, supported by analysis of experimental data (spectroscopy, magnetism of 1) and by DFT calculations. The S=1 ground state calculated for 1 stands in contrast to the spin-paired analogues with arylimino-o-benzosemiquinonato and diimino-o-benzoquinone ligands. The close contacts of about 5.3 Å possible between semiquinone O atoms of different molecules in the crystal allow for intermolecular spin-spin interactions and an overall complex magnetic behavior. One quasireversible oxidation and two reversible one-electron reductions yielded the corresponding molecular ions, which were characterized by UV-visible-NIR and EPR spectroelectrochemistry in terms of [Ru^III(acac)₂(Q°)]⁺, [Ru^III(acac)₂(Q^2-)]^-, and [Ru^II(acac)₂(Q^2-)]^2- descriptions in agreement with DFT results. The use of acceptor-substituted 1,2-dioxolenes resulted in the isolation of ionic species Na[Ru(acac)₂(Q)] (Na(5); Q=4-chloro-o-benzoquinone) and Na(6) (Q=4-nitro-o-benzoquinone), which were similarly investigated as compounds 1-4. Magnetic susceptibility and EPR results confirm an S=½ ground state based on ruthenium(III). The combined studies reveal a remarkable substituent sensitivity, and in comparison to recently analyzed Ru(acac)₂ complexes with o-benzoquinone monoimine and diimine ligands, the all-O-donor-containing new systems are distinguished by a qualitatively different metal-ligand interaction based on closer intermolecular radical-radical contacts and on weaker intramolecular dπ-π interactions.