LAUSR

Abstract Two bis-tridentate pincer complexes [Ru(H 2 bip)(Mebib)](PF 6 ) 2 ( 1 (PF 6 ) 2 ) and [Ru(H 2 bip)(H 2 bib)](PF 6 ) 2 ( 2 (PF 6 ) 2 ) were prepared and studied, where H 2 bip is 2,6-bis(benzimidazol-2-yl)pyridine, Mebib is the 2-deprotonated 1,3-bis( N -methylbenzimidazol-2-yl)benzene, and H 2 bib is 2-deprotonated 1,3-bis(benzimidazol-2-yl)benzene. These complexes were isolated as cyclometalated Ru(III) salts with a [(NCN)(NNN)] coordination due to a low Ru(III/II) redox potential. Complex 1 (PF 6 ) 2 has two N−H protons on the NNN ligand side, while complex 2 (PF 6 ) 2 has four N−H protons. Absorption spectral analysis showed that two deprotonation steps were distinguished for 1 (PF 6 ) 2 with p K a1 and p K a2 of 3.46 and 5.88, respectively. In comparison, four deprotonation steps were observed for 2 (PF 6 ) 2 with four p K a values of 3.45, 5.95, 8.95, and 10.98, respectively. When the solution was changed from acidic to basic, the Ru(III/II) redox wave of 1 (PF 6 ) 2 and 2 (PF 6 ) 2 shifted from +0.20 V vs Ag/AgCl to a less positive region by 540 and 850 mV, respectively. These studies suggest that the electronic absorption and electrochemical properties of these complexes are very sensitive to pH effect.