Single Site Isomeric Ru WOCs with an Electron-Withdrawing Group: Synthesis, Electrochemical Characterization, and Reactivity

The synthetic intermediate cis­(out),cis-[Ru­(Cl)2(HL)­(DMSO)2], 1 (DMSO = dimethyl sulfoxide), and four new mononuclear ruthenium
complexes with general formula out/in-[Ru­(HL)­(trpy)­(X)]m+ (trpy = 4-tert-butylpyridine;
X = Cl–, m = 1, 2a+ and 2b+; X = H2O, m = 2, 3a2+ and 3b2+) based on the ligand 1H-pyrazole-3-carboxylic
acid, 5-(2-pyridinil)-, ethyl ester (HL), are synthesized and characterized
by analytical, spectroscopic, and electrochemical methods. A linkage
isomerism is observed for a DMSO moiety of 1, and relevant
thermodynamics and kinetics values are obtained through electrochemical
experiments and compared to literature. Different synthetic routes
are developed to obtain isomeric 2a+ and 2b+, with different relative yields. Water oxidation
activity of 3a2+ and 3b2+ is analyzed by means of electrochemical methods, through foot of
the wave analysis, yielding kobs values
of 1.00 and 2.23 s–1, respectively. Chemically driven
water oxidation activity is tested using [(NH4)2Ce­(NO3)6] as sacrificial electron acceptor,
and turnover number (TON) and turnover frequency (TOF) values of TON
= 10.8 and TOFi = 58.2 × 10–3 s–1 for 3a2+ and TON = 4.2 and
TOFi = 15.4 × 10–3 s–1 for 3b2+ are obtained.