Ruthenium(ll) complexes incorporating 2-(2′-Pyridyl)pyrimidine-4- carboxylic acid

Abstract

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(ll) complexes. Reaction of this new ligand with RuII(bpy)2CI2 led to the unexpected oxidation of the starting material to give [RuIII(bpy) 2CI2]CI · H2O and a low yield of [RuII(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(ll) center rather than that ortho to this group. The same complex was prepared via decarbonylation of [RuII(cppH)(CO)2CI2] · 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 [RuII(phen)2(cppH)](PF6) 2 · 2H2O (2) (phen = 1,10-phenanthroline) was similarly synthesized. [RuII(bpy)(dppz)(cppH)](PF6) 2 · CH3CN (3) (dppz = dipyrido[3,2,- a;2′,3-c]phenazine) was also prepared by photochemical decarbonylation of [RuII(bpy)(CO)2CI2] giving [Ru II(bpy)- (CO)CI2]2 followed by bridge splitting with dppz to generate [RuII(bpy)(dppz)(CO)CI](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, [RuII(bpy)(dppz) (cpp)](NO3) · 10H2O, 3N3 indicated that the nitrogen adjacent to the carboxylate was coordinated to ruthenium(ll). Full characterization of these complexes has been undertaken including the measurement of UV-visible and emission spectra. Electrochemical and spectroelectro-chemical studies in acetonitrile show that these complexes undergo reversible oxidation from RuII to RuIII at potentials of 983 ± 3 mV, 1004 ± 5 mV, and 1023 ± 3 mV versus Fc0/+ (Fc = Ferrocene) for 1, 2, and 3N3, respectively. © 2009 American Chemical Society.