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

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 [Ru^II(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 d_π(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 [Ru^IIIL¹₂][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 Ru^III–Ru^IV couple into the solvent window. Initial attempts to prepare [RuL²₃], with an N₃O₃ donor set, were unsuccessful.