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

A family of bis-terdentate iridium(III) complexes is reported which contain a cyclometalated, N∧C∧N-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(μ-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 N∧C∧N 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(N∧C∧N)(C∧N∧C)] and [Ir(N∧C∧N)(C∧N∧O)], by reaction of 1 with C∧N∧C-coordinating ligands (e.g., 2,6-diphenylpyridine and derivatives)
and C∧N∧O-coordinating ligands (based on 6-phenylpicolinate), respectively. Treatment of 1 with terpyridines
leads to dicationic complexes of the type [Ir(N∧C∧N)(N∧N∧N)]2+, while 2-phenylpyridine gives [Ir(dpyx-N∧C∧N)(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(N∧C∧N)(C∧N∧C)] class, fluorination of the C∧N∧C ligand at the phenyl 2‘ and 4‘ positions leads to
a blue-shift in the emission and to an increase in the quantum yield (λmax = 547 nm, φ = 0.41 in degassed
CH3CN at 295 K) compared to the nonfluorinated parent complex (λmax = 585 nm, φ = 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-N∧C∧N)(ppy-C,N)Cl] is an exceptionally bright emitter:  φ = 0.76, λmax = 508 nm, in CH3CN at 295 K.
In contrast, the [Ir(N∧C∧N)(C∧N∧O)] 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(N∧C∧N)(N∧N∧N)]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.