Ruthenium(II) Complexes Incorporating 2-(2′-Pyridyl)pyrimidine-4-carboxylic Acid

A new bidentate ligand bearing a single carboxylate functionality, 2-(2′-pyridyl)pyrimidine-4-carboxylic acid (cppH), has been prepared and applied in the synthesis of a series of ruthenium(II) complexes. Reaction of this new ligand with Ru^II(bpy)₂Cl₂ led to the unexpected oxidation of the starting material to give [Ru^III(bpy)₂Cl₂]Cl·H₂O and a low yield of [Ru^II(bpy)₂(cppH)](PF₆)₂·H₂O (1) on addition of an aqueous KPF₆ solution (bpy = 2,2′-bipyridine and cpp = 4-carboxylate-2′-pyridyl-2-pyrimidine). An X-ray crystal structure determination on crystals of 1a, [Ru^II(bpy)₂(cpp)](PF₆), obtained from slow evaporation of an aqueous solution of 1 revealed that the nitrogen para to the carboxylate group in the cpp⁻ ligand coordinates to the ruthenium(II) center rather than that ortho to this group. The same complex was prepared via decarbonylation of [Ru^II(cppH)(CO)₂Cl₂]·H₂O in the presence of bpy and an excess of trimethylamine-N-oxide (Me₃NO), as the decarbonylation agent. The coordination of cppH in the precursor is the same as in the final product. The related complex [Ru^II(phen)₂(cppH)](PF₆)₂·2H₂O (2) (phen = 1,10-phenanthroline) was similarly synthesized. [Ru^II(bpy)(dppz)(cppH)](PF₆)₂·CH₃CN (3) (dppz = dipyrido[3,2,-a;2′,3-c]phenazine) was also prepared by photochemical decarbonylation of [Ru^II(bpy)(CO)₂Cl₂] giving [Ru^II(bpy)(CO)Cl₂]₂ followed by bridge splitting with dppz to generate [Ru^II(bpy)(dppz)(CO)Cl](PF₆)·H₂O. This intermediate was then reacted with cppH to produce 3, as a mixture of geometric isomers. In contrast to 1, X-ray crystallography on the major product isolated from this mixture, [Ru^II(bpy)(dppz)(cpp)](NO₃)·10H₂O, 3^N3 indicated that the nitrogen adjacent to the carboxylate was coordinated to ruthenium(II). Full characterization of these complexes has been undertaken including the measurement of UV−visible and emission spectra. Electrochemical and spectroelectrochemical studies in acetonitrile show that these complexes undergo reversible oxidation from Ru^II to Ru^III at potentials of 983 ± 3 mV, 1004 ± 5 mV, and 1023 ± 3 mV versus Fc^0/+ (Fc = Ferrocene) for 1, 2, and 3^N3, respectively.