Bis(acetylacetonato)ruthenium Complexes of Noninnocent 1,2-Dioxolene Ligands: Qualitatively Different Bonding in Relation to Monoimino and Diimino Analogues

• Published in: Chemistry : a European journal 2011-09, Vol.17 (39), p.11030-11040
• Main Authors: Das, Dipanwita, Sarkar, Biprajit, Kumbhakar, Doyel, Mondal, Tapan Kumar, Mobin, Shaikh M., Fiedler, Jan, Urbanos, Francisco A., Jiménez-Aparicio, Reyes, Kaim, Wolfgang, Lahiri, Goutam Kumar
• Format: Article
• Language: English
• Subjects: density functional calculations; Ligands; magnetic properties; ruthenium; spectroelectrochemistry; structure elucidation
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201101009

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 RuIII/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 [RuIII(acac)2(Q0)]+, [RuIII(acac)2(Q2−)]−, and [RuII(acac)2(Q2−)]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=1/2 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. Noninnocence and experience: Weak intramolecular metal–ligand interaction and close intermolecular semiquinone–semiquinone contacts result in a complex paramagnetism of [RuIII(acetylacetonate)2(Q.−)] (see figure), which can further undergo reduction and oxidation. This behavior contrasts with the diamagnetism established for monoimino and diimino analogues.