STRUCTURE AND BONDING IN BIS(QUINONE) COMPLEXES OF RUTHENIUM - SYNTHESIS AND CHARACTERIZATION OF THE RU(PPH3)2(SQ)2 (SQ = 3,5-DI-TERT-BUTYLSEMIQUINONE, TETRACHLORO-1,2-SEMIQUINONE) SERIES

Questions concerning charge distribution in the neutral bis(quinone) complexes of ruthenium have prompted investigation of members of the Ru(PPh3)2(SQ)2 (SQ = 3,5-di-tert-butylsemiquinone (DBSQ), tetrachloro-1,2-semiquinone (Cl4SQ)) series. Ru(PPh3)2(DBSQ)Cl2 has been synthesized and characterized as a precursor. Crystals of Ru(PPh3)2(DBSQ)Cl2 form in the monoclinic space group P2(1)/c with Z = 4 in a unit cell of dimensions a = 16.544 (3) angstrom, b = 12.217 (2) angstrom, c = 22.535 (5) angstrom, and beta = 100.65 (3)-degrees. Structural features of the quinone ligand indicate that it is a semiquinone and that the complex contains Ru(III). The diamagnetism of the complex results from strong antiferromagnetic coupling between the paramagnetic metal and the coordinated radical. Treatment of the complex with a second catecholate ligand leads to displacement of the chloro ligands with formation of the bis(quinone) product. Complexes containing combinations of 3,5-di-tert-butylcatechol and tetrachlorocatechol have been studied, and Ru(PPh3)2(Cl4SQ)2 has been characterized crystallographically as a dichloromethane solvate. Crystals form in the triclinic space group P1BAR with Z = 2 in a unit cell of dimensions a = 12.586 (2) angstrom, b = 12.893 (2) angstrom, c = 17.351 (2) angstrom, alpha = 87.156 (13)-degrees, beta = 82.806 (11)-degrees, and gamma = 66.285 (12)-degrees. Structural features of the quinone ligands agree with values expected for semiquinones, pointing to the Ru(PPh3)2(Cl4SQ)2 charge distribution in the solid state. Similarities in the infrared spectra of Ru(PPh3)2(DBSQ)Cl2 and Ru(PPh3)2(DBSQ)2 indicate a similar charge distribution for the (DBSQ)2 complex. Electrochemical characterization has shown that each of the bis(quinone) complexes undergoes two oxidations and two reductions with potentials for members of each four-membered series shifted progressively in a negative direction for Ru(PPh3)2(Cl4SQ)2, Ru(PPh3)2(DBSQ)(Cl4SQ), and Ru(PPh3)2(DBSQ)2, respectively. Values of these potentials are quite similar to corresponding couples of the related bipyridine complexes, Ru(bpy)(SQ)2. This result, with similarities in the electronic spectra for the two classes of complexes, has been interpreted to indicate that the bipyridine and triphenylphosphine complexes have similar charge distributions in solution and that the redox-active electronic levels are ligand-delocalized molecular orbitals.