Influence of pH on the Photochemical and Electrochemical Reduction of the Dinuclear Ruthenium Complex, [(phen)₂Ru(tatpp)Ru(phen)₂]Cl₄, in Water: Proton‐Coupled Sequential and Concerted Multi‐Electron Reduction

Abstract

The dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P; in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']-pentacene) undergoes a photodriven two-electron reduction in aqueous solution, thus storing light energy as chemical potential within its structure. The mechanism of this reduction is strongly influenced by the pH, in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled, bielectronic process. In this complex, the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH. The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments. Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data, estimates of the various reduced species' pKa values were obtained and the mechanism to form the doubly reduced, doubly protonated complex, [(phen)2Ru(H2tatpp)Ru(phen)2]4+ (H2P) at low pH (< or =7) could be shown to be a two-proton, two-electron process. Importantly, H2P is also formed in the photochemical reaction with sacrificial reducing agents, albeit at reduced yields relative to those at higher pH.

Preferential DNA Cleavage under Anaerobic Conditions by a DNA-Binding Ruthenium Dimer

“…40,42 As seen in Figure 3, P 4+ does not induce appreciable DNA cleavage under aerobic or anaerobic conditions (lanes 2 and 3). P 3+ (lanes 4 and 5) does show some enhanced nicking ability, however, H 2 P 4+ is clearly the most potent nicking agent (lanes 6 and 7).…”

“…As seen in Figure 2, the absorption spectra of P 4+ in aqueous buffer (pH 7.0) after addition of GSH is identical to that of H 2 P 4+ as prepared by stoichiometric reduction and protonation or electrochemically. 40,42 Thus, it appears that P 4+ is a prodrug, which is converted to the H 2 P 4+ by in situ reduction.…”

Polypyridyl Metal Complexes with Biological Activity

“…[40] In the presence of glutathione, 27 is reduced and cleaves DNA, while under an O 2 atmosphere the cleaving reaction is significantly less efficient, despite the fact that the derivative is still able to bind tightly to DNA (K = 1.1 10 7 m -1 in 25 mm NaCl). As confirmed by UV/Vis and electrochemistry, [41] the reduction reaction with glutathione (pH = 7.0) is a two-electron process that provides the species H 2 Ru 4+ . This species is a strong single-strand nicking agent, and its mechanism of action is likely to involve the formation of Cbased radicals.…”

Driving Multi-electron Reactions with Photons: Dinuclear Ruthenium Complexes Capable of Stepwise and Concerted Multi-electron Reduction

“…The Zn II complex is free of any MLCT type transitions and the observed LC transitions are likely only to be slightly perturbed by the Zn II coordination. The molar extinction coefficients for these two components, [Ru(phen) 3 ] 2+ and tatpp, at 450 nm in MeCN are 19,200 M )1 cm )1 (Torres et al 1999) and 17,300 M )1 cm )1 (de Tacconi et al 2005), respectively. A summation of the molar extinctions for these three components, two [Ru(phen) 3 ] 2+ and one tatpp, gives an e 445 of 55,700 M )1 cm )1 which compares reasonably well with the observed extinction coefficient for P of 65,100 M )1 cm )1 at 445 nm in MeCN (Kim et al 2002) and the sharp bands with vibronic fine structure characteristic of the free ligand are clearly visible.…”

“…We have been able to estimate the pK a 's of the following species, HP, H 2 P, HP ) at 8, 9, 10, respectively (de Tacconi et al 2005). Protonation of the parent complex P does not occur to any appreciable extent (pK a of HP + < 0) in aqueous solution.…”