A New Class of Iridium Complexes Suitable for Stepwise Incorporation into Linear Assemblies: Synthesis, Electrochemistry, and Luminescence

Abstract

A new family of cationic iridium(III) complexes is reported that contain two cyclometalating terdentate ligands. The complex [Ir(N--C--N-dpyx)(N--N--C-phbpy)]+ (1) contains one N--C--N-coordinating ligand, cyclometalating through the central phenyl ring, and one N--N--C-coordinated ligand, cyclometalated at the peripheral phenyl ring [dpyxH = 1,3-di(2-pyridyl)-4,6-dimethylbenzene; phbpyH = 6-phenyl-2,2'-bipyridine]. This binding mode dictates a mutually cis arrangement of the cyclometalated carbon atoms: the complexes are thus bis-terdentate analogues of the well-known [Ir(N--C-ppy)2(N--N-bpy)]+ family of complexes, which similarly contain a cis-C2N4 coordination environment. The dpyx ligand can be brominated regioselectively at the carbon atom para to the metal under mild conditions. Starting from a modified complex, [Ir(N--C--N-dpyx)(N--N--C-mtbpy-phi-Br)]+ (2), which incorporates a pendent bromophenyl group, a sequential cross-coupling-bromination-cross-coupling strategy can be applied for the stepwise introduction of aryl groups into the ligands, using in situ palladium-catalyzed Suzuki reactions with arylboronic acids [mtbpyH-phi-Br = 4-(p-bromophenyl)-6-(m-tolyl)bipyridine]. Dimetallic complexes 6 and 7 have similarly been prepared by a palladium-catalyzed reaction of complex 2 with 1,4-benzenediboronic acid and 4,4'-biphenyldiboronic acid, respectively. All five monometallic complexes and both dimetallic systems are luminescent in solution, emitting around 630 nm in MeCN at 298 K, with quantum yields in the range of 0.02-0.06, superior to [Ir(ppy)2(bpy)]+. The luminescence, electrochemistry, and singlet-oxygen-sensitizing abilities of the new family of complexes are discussed in the context of the tris-bidentate analogues and related bis-terdentate compounds that contain a trans arrangement of cyclometalated carbon atoms.