1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO₂ reduction

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)₃(NHC(R)az*-κ²N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl₂(DMSO)₃(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by ¹H NMR, ¹³C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO₂ media is consistent with CO₂ electrocatalytic reduction. The catalytic activity expressed as [i_cat(CO₂)/i_p(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO₂ reduction experiments using compounds 1b, 2b and 3b carried out in a CO₂-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)₃]²⁺ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm^-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO₂ reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.