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Synthesis, Structure, Spectroscopic Properties, and Electrochemical Oxidation of Ruthenium(II) Complexes Incorporating Monocarboxylate Bipyridine Ligands

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posted on 15.10.2007, 00:00 by Nickita, Matthew J. Belousoff, Anand I. Bhatt, Alan M. Bond, Glen B. Deacon, Gilles Gasser, Leone Spiccia
[Ru(bpy)2(Mebpy-COOH)](PF6)2·3H2O (1), [Ru(phen)2(Mebpy-COOH)](ClO4)2·5H2O (2), [Ru(dppz)2(Mebpy-COOH)]Cl2·9H2O (3), and [Ru(bpy)(dppz)(Mebpy-COOH)](PF6)2·5H2O (4) (bpy = 2,2‘-bipyridine, Mebpy-COOH = 4‘-methyl-2,2‘-bipyridine-4-carboxylic acid, phen = 1,10-phenanthroline, dppz = dipyrido[3,2,-a;2‘,3-c]phenazine) have been synthesized and characterized spectroscopically and by microanalysis. The [Ru(Mebpy-COOH)(CO)2Cl2]·H2O intermediate was prepared by reaction of the monocarboxylic acid ligand, Mebpy-COOH, with [Ru(CO)2Cl2]n, and the product was then reacted with either bpy, phen, or dppz in the presence of an excess of trimethylamine-N-oxide (Me3NO), as the decarbonylation agent, to generate 1, 2, and 3, respectively. For compound 4, [Ru(bpy)(CO)Cl2]2 was reacted with Mebpy-COOH to yield [Ru(bpy)(Mebpy-COOH)(CO)Cl](PF6)·H2O as a mixture of two main geometric isomers. Chemical decarbonylation in the presence of dppz gave 4 also as a mixture of two isomers. Electrochemical and spectrophotometric studies indicated that complexes 1 and 2 were present as a mixture of protonated and deprotonated forms in acetonitrile solution because of water of solvation in the isolated solid products. The X-ray crystal structure determination on crystals of [Ru(bpy)2(MebpyCOO)][Ru(bpy)2(MebpyCOOH)]3(PF6)7, 1a, and [Ru(phen)2(MebpyCOO)](ClO4)·6H2O, 2a, obtained from solutions of 1 and 2, respectively, revealed that 1a consisted of a mixture of protonated and deprotonated forms of the complex in a 1:3 ratio and that 2a consisted of the deprotonated derivative of 2. A distorted octahedral geometry for the RuII centers was found for both complexes. Upon excitation at 450 nm, MeCN solutions of the protonated complexes 14 were found to exhibit emission bands in the 635-655 nm range, whereas the corresponding emission maxima of their deprotonated forms were observed at lower wavelengths. Protonation/deprotonation effects were also observed in the luminescence and electrochemical behavior of complexes 14. Comprehensive electrochemical studies in acetonitrile show that the ruthenium centers on 1, 2, 3, and 4 are oxidized from RuII to RuIII with reversible potentials at 917, 929, 1052, and 1005 mV vs Fc0/+ (Fc = ferrocene), respectively. Complexes 1 and 2 also exhibit an irreversible oxidation process in acetonitrile, and all compounds undergo ligand-based reduction processes.

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