ONE-ELECTRON REDUCTION OF RUTHENIUM(II)-DIIMINE COMPLEXES - CHARACTERIZATION OF REDUCED SPECIES CONTAINING 2,2'-BIPYRIDINE, 2,2'-BIPYRIMIDINE, AND 2,2'-BIPYRAZINE IN AQUEOUS-SOLUTION

The one-electron reductions of ten Ru(II)-diimine complexes of the general formula Ru(bpy)3-m-z(bpm)m(bpz)z2+ (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0,1,2,3 and m + z less-than-or-equal-to 3), generically indicated as RuL2+, have been investigated in aqueous solution by the use of radiation chemical and electrochemical techniques, leading to the characterization of the one-electron reduced products (RuL.+) and their conjugate acids (RuLH.2+). Because the ease of reduction of the ligands follows the trend bpz > bpm > bpy, the electron added to RuL2+ is localized on the ligand that is most easily reduced. The E-degrees values for the RuL2+/RuL.+ couples range from -0.50 V to -1.22 V for Ru(bpz)3(2+) and Ru(bpy)3(2+), respectively. All the RuL.+ absorption spectra show intense bands (epsilon-max approximately 10(4) M-1 cm-1) in the 340- and 480-nm regions that arise from ligand-localized transitions within the uniquely reduced ligand, with the visible band also overlapped by a MLCT band; the absorption bands of RuLH.2+ are shifted somewhat to the blue. From the variation of the absorbance as a function of pH, the pK(a) values of RuLH.2+ are obtained; they range from 6.3 to 9.2 for Ru(bpm)2(bpmH.-)2+ to Ru(bpy)2(bpzH.-)2+, respectively, and correlate linearly with E-degrees (RuL2+/RuL.+), with separate lines for bpz and bpm acceptor ligands. From the values of pK(a) and E-degrees (RuL2+/RuL.+), E-degrees (RuL2+,H+/RuLH.2+) are obtained. The rate constants for the following electron-transfer reactions have been measured or calculated and have been correlated with the values of DELTA-G-degrees and other kinetic parameters by use of the Marcus theory: RuL2+ +.CO2- --> RuL.+ + CO2, RuL.+ + MV2+ --> RuL2+ + MV.+ and its reverse reaction, and RuLH.2+ + MV2+ --> RuL2+ + MV.+ + H+ and its reverse reaction. The relevance of the results to the use of the complexes as electron-transfer photosensitizers is discussed.