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

New 1,2-azolylamidino complexes fac -[RuCl(DMSO) 3 (NHC(R)az*-κ 2 N,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 -[RuCl 2 (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 1 H NMR, 13 C 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 CO 2 media is consistent with CO 2 electrocatalytic reduction. The catalytic activity expressed as [ i cat (CO 2 )/ 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 CO 2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO 2 -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 CO 2 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.