The co-ordination chemistry of mixed pyridine–phenol ligands; spectroscopic and redox properties of mononuclear ruthenium complexes with (pyridine)6–x(phenolate)xdonor sets (x= 1 or 2)

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Abstract
Two new ruthenium complexes containing N5O and N4O2 donor sets (where N is a pyridyl donor and O a phenolate donor) were prepared, in order to study the electrochemical and spectroscopic consequences for the ruthenium centre of stepwise replacement of pyridyl ligands by phenolates. Reaction of the N,O bidentate ligand 2-(2-hydroxyphenyl)pyridine (HL2)(for which a new synthesis is presented) with [Ru(bipy)2Cl2](bipy = 2,2′-bipyridine) in refluxing ethanol followed by treatment with KPF6 gave [RuII(bipy)2L2][PF6] in near-quantitative yield. The crystal structure shows the expected pseudo-octahedral geometry, with the pyridyl ring trans to the phenolate exhibiting the shortest Ru–N distance [2.022(4)Å] due to enhanced dπ(Ru)–bipy(π*) back bonding. The cyclic voltammogram shows that the presence of the phenolate ligand stabilises the ruthenium(III) state by +0.86 V relative to [Ru(bipy)3]2+; the UV/VIS spectrum is also assigned. Reaction of RuCl3 with 2 equivalents of the N,N,O-terdentate ligand 6-(2-hydroxyphenyl)-2,2′-bipyridine (HL1) in refluxing ethylene glycol followed by treatment with KPF6 yielded [RuIIIL1 2][PF6], which also has a pseudo-octahedral structure. The cyclic voltammogram shows that the presence of a second phenolate results in stabilisation of the ruthenium(III) state by a further +0.57 V, and also brings a usually inaccessible RuIII–RuIV couple into the solvent window. Initial attempts to prepare [RuL2 3], with an N3O3 donor set, were unsuccessful.