Co-ordination chemistry of mixed pyridine–phenol ligands; electrochemical, electron paramagnetic resonance and structural studies on monounclear ruthenium( III ) and chromium( III ) complexes

Syntheses of the new complexes [RuL13]1, [Rul12(acac)]2, [RuL12(bipy)][PF6]3, [CrL13]4, [CrL12(acac)]5 and [CrL22][PF6]6[HL1= 2-(2-hydroxyphenyl)pyridine, HL2= 6-(2-hydroxyphenyl)-2,2′-bipyridine, 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 previously reported complexes, a full set of electrochemical data is now 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 RuII-RuIII couple per additional phenolate in the co-ordination sphere. Both 1 and 2 have typical rhombic EPR spectra, but the spectrum of 3 is only consistent with formation of a mixture of cis and trans isomers. The chromium(III) complexes show similar variations in the potentials of their metal-centred reductions with donor set although the relationship is not linear. X-Ray analysis showed the 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 π-stacking interaction between aromatic residues of adjacent complex units, with an average separation of 3.71 Å between the atoms of one aromatic ring and the mean plane of the other.