Systematic Modulation of a Bichromic Cyclometalated Ruthenium(II) Scaffold Bearing a Redox-Active Triphenylamine Constituent

The syntheses and physicochemical properties of nine bis-tridentate ruthenium(II) complexes containing one cyclometalating ligand furnished with terminal triphenylamine (TPA) substituents are reported. The structure of each complex conforms to a molecular scaffold formulated as {[}Ru-II(TPA-2,5-thiophene-pbpy)(Me(3)tctpy)] (pbpy = 6-phenyl-2,2'-bipyridine; Me(3)tctpy = trimethyl-4,4',4 `'-tricarboxylate-2,2':6',2 `'-terpyridine), where various electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are installed about the TPA unit and the anionic ring of the pbpy ligand. It is found that the redox chemistry of the Ru center and the TPA unit can be independently modulated by (i) placing EWGs (e.g., -CF3) or EDGs (e.g., -OMe) on the anionic ring of the pbpy ligand (substituted sites denoted as R-2 or R-3) and/or (ii) installing electron-donating substituents (e.g., -H, -Me, -OMe) para to the amine of the TPA group (i.e., R-1). The first oxidation potential is localized to the TPA unit when, for example, EDGs are placed at R-1 with EWGs at R-2 (e.g., the TPA(center dot+)/TPA(0) and Ru-III/Ru-II redox couples appear at +0.98 and +1.27 V vs NHE, respectively, when R-1 = -OMe and R-2 = -CF3). This situation is reversed when R-3 = EDG and R-1 = -H: TPA-based and metal-centered oxidation waves occur at +1.20 and +1.11 V vs NHE, respectively. The UV-vis spectrum for each complex is broad (e.g., absorption bands are extended from the UV region to beyond 800 nm in all cases) and intense (e.g., epsilon similar to 10(4) M-1 . cm(-1)) because of the overlapping intraligand charge-transfer and metal-to-ligand charge-transfer transitions. The information derived from this study offers guiding principles for modulating the physicochemical properties of bichromic cyclometalated ruthenium(II) complexes.