COORDINATION CHEMISTRY OF MIXED PYRIDINE-PHENOL LIGANDS - ELECTROCHEMICAL, ELECTRON-PARAMAGNETIC-RESONANCE AND STRUCTURAL STUDIES ON MONONUCLEAR RUTHENIUM(III) AND CHROMIUM(III) COMPLEXES

Syntheses of the new complexes [RuL3(1)] 1, [RuL2(1)(acac)] 2, [RuL2(1 )(bipy)][PF6] 3, [CrL3(1)] 4, [CrL2(1)(acac)] 5 and [CrL2(2)][PF(6)] 6 [HL1 = 2-(2-hydroxyphenyl)pyridine, HL2 = 6-(2-hydroxyphenyl)-2,2'-bi pyridine, Hacac = pentane-2,4-dione, bipy = 2,2'-bipyridine] have been carried out. The ruthenium(III) complexes 1-3 all show reversible +3/ +4 and +2/+3 waves in their cyclic voltammograms. Together with some p reviously reported complexes, a full set of electrochemical data is no w available for ruthenium complexes with donor sets varying from N6 to N3O3 (where N denotes a pyridyl donor and O a phenolate donor); there is a monotonic decrease of 0.75 V in the Ru(II)-Ru(III) couple per ad ditional phenolate in the co-ordination sphere. Both 1 and 2 have typi cal rhombic EPR spectra, but the spectrum of 3 is only consistent with formation of a mixture of cis and trans isomers. The chromium(III) co mplexes show similar variations in the potentials of their metal-centr ed reductions with donor set although the relationship is not linear. X-Ray analysis showed that both 4 and 6 have pseudo-octahedral crystal structures. Complex 4 crystallises in the chiral space group Fdd2 and therefore spontaneously resolves on crystallisation; 6 has a pi-stack ing interaction between aromatic residues of adjacent complex units, w ith an average separation of 3.71 angstrom between the atoms of one ar omatic ring and the mean plane of the other.