Luminescent Complexes of Iridium(III) Containing N∧C∧N-Coordinating Terdentate Ligands

Abstract

A family of bis-terdentate iridium(III) complexes is reported which contain a cyclometalated, NCN-coordinating 1,3-di(2-pyridyl)benzene derivative. This coordination mode is favored by blocking competitive cyclometalation at the C4 and C6 positions of the ligand. Thus, 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyxH) reacts with IrCl3·3H2O to generate a dichlorobridged dimer [Ir(dpyx-N,C,N)Cl(-Cl)]2, 1. This dimer is cleaved by DMSO to give [Ir(dpyx)(DMSO)Cl2], the X-ray crystal structure of which is reported here, confirming the NCN coordination mode of dpyx. The dimer 1 can also be cleaved by a variety of other ligands to generate novel classes of mononuclear complexes. These include charge-neutral bis-terdentate complexes of the form [Ir(NCN)(CNC)] and [Ir(NCN)(CNO)], by reaction of 1 with CNC-coordinating ligands (e.g., 2,6-diphenylpyridine and derivatives) and CNO-coordinating ligands (based on 6-phenylpicolinate), respectively. Treatment of 1 with terpyridines leads to dicationic complexes of the type [Ir(NCN)(NNN)]2+, while 2-phenylpyridine gives [Ir(dpyx-NCN)(ppy-C,N)Cl]. All of the charge-neutral complexes are luminescent in fluid solution at room temperature. Assignment of the emission to charge-transfer excited states with significant MLCT character is supported by DFT calculations. In the [Ir(NCN)(CNC)] class, fluorination of the CNC ligand at the phenyl 2' and 4' positions leads to a blue-shift in the emission and to an increase in the quantum yield (max = 547 nm, = 0.41 in degassed CH3CN at 295 K) compared to the nonfluorinated parent complex (max = 585 nm, = 0.21), as well as to a stabilization of the compound with respect to photodissociation through cleavage of mutually trans Ir-C bonds. [Ir(dpyx-NCN)(ppy-C,N)Cl] is an exceptionally bright emitter: = 0.76, max = 508 nm, in CH3CN at 295 K. In contrast, the [Ir(NCN)(CNO)] complexes are much less emissive, shown to be due to fast nonradiative decay of the excited state, probably involving reversible Ir-O bond cleavage. The [Ir(NCN)(NNN)]2+ complexes are very feeble emitters even at 77 K, probably due to the almost exclusively interligand charge-transfer nature of the lowest-energy excited state in these complexes.