4′-Functionalized 2,2′:6′,2″-terpyridines as the NˆN domain in [Ir(CˆN)2(NˆN)][PF6] complexes

Abstract

The cyclometallated iridium(III) complexes [Ir(ppy)2(NˆN)][PF6] (Hppy = 2-phenylpyridine) with NˆN = 4′-chloro-2,2′:6′,2″-terpyridine (1), 4′-methoxy-2,2′:6′,2″-terpyridine (2), 4′-ethoxy-2,2′:6′,2″-terpyridine (3), 4′-methylthio-2,2′:6′,2″-terpyridine (4), 4′-phenylthio-2,2′:6′,2″-terpyridine (5) and 4′-dimethylamino-2,2′:6′,2″-terpyridine (6) are reported including the single crystal structures of 2{[Ir(ppy)2(1)][PF6]}·0.6Et2O·CH2Cl2, [Ir(ppy)2(5)][PF6]·0.5CH2Cl2 and [Ir(ppy)2(6)][PF6]. The single crystal structure of [Ir(ppy)2(3)]Cl·2H2O·MeCN is also reported. In each complex, the 2,2′:6′,2″-terpyridine (tpy) ligand binds to the metal centre in a bidentate fashion with the non-coordinated pyridine ring folded into the coordination sphere and engaging in a pyridine–phenyl π-stacking interaction. Solution NMR spectra are consistent with hindered rotation of the non-coordinated pyridine ring at 298 K, with intra-cation CH…N(pyridine) hydrogen bond formation between adjacent [ppy]– and tpy ligands. Trends in the electrochemical HOMO–LUMO gaps and emission maxima of the complexes (in CH2Cl2 solution) are consistent with the electron-withdrawing or releasing properties of the 4′-tpy substituent; in degassed solution, [Ir(ppy)2(6)][PF6] has a quantum yield of 24.8% and emission lifetime of 441 ns, while the other complexes exhibit significantly lower quantum yields and shorter lifetimes.