Protic NNN and NCN Pincer-Type Ruthenium Complexes Featuring (Trifluoromethyl)pyrazole Arms: Synthesis and Application to Catalytic Hydrogen Evolution from Formic Acid

NNN and NCN pincer-type ruthenium(II) complexes featuring two protic pyrazol-3-yl arms with a trifluoromethyl (CF3) group at the 5-position were synthesized and structurally characterized to evaluate the impact of the substitution on the properties and catalysis. The increased Bronsted acidity by the highly electron-withdrawing CF3 pendants was demonstrated by protonation-deprotonation experiments. By contrast, the IR spectra of the carbonyl derivatives as well as the cyclic voltammogram indicated that the electron density of the ruthenium atom is negligibly influenced by the CF3 group. Catalysis of these complexes in the decomposition of formic acid to dihydrogen and carbon dioxide was also examined. The NNN pincer-type complex 1a with the CF3 group exhibited a higher catalytic activity than the tBu-substituted analogue 1b. In addition, the bis(CF3-pyrazolato) ammine derivative 4 catalyzed the reaction even in the absence of base additives.