Synthesis, Structural Characterization, and Phosphorescence Properties of Trigonal Zn(II) Carbene Complexes

Abstract

The sterically demanding N-heterocyclic carbene ITr (N,N′-bis(triphenylmethyl)imidazolylidene) was employed for the preparation of novel trigonal zinc(II) complexes of the type [ZnX 2 (ITr)] [X = Cl (1), Br (2), and I (3)], for which the low coordination mode was confirmed in both solution and solid state. Because of the atypical coordination geometry, the reactivity of 1− 3 was studied in detail using partial or exhaustive halide exchange and halide abstraction reactions to access [ZnLCl(ITr)] [L = carbazolate (4), 3,6-di-tert-butyl-carbazolate (5), phenoxazine (6), and phenothiazine ( 7 (9), Br (10), and I (11)], all of which were isolated and structurally characterized. Importantly, for all complexes 4−11, the trigonal coordination environment of the Zn II ion is maintained, demonstrating a highly stabilizing effect due to the steric demand of the ITr ligand, which protects the metal center from further ligand association. In addition, complexes 1−3 and 8−11 show longlived luminescence from triplet excited states in the solid state at room temperature, according to our photophysical studies. Our quantum chemical density functional theory/multireference configuration interaction (DFT/MRCI) calculations reveal that the phosphorescence of 8 originates from a locally excited triplet state on the bdt ligand. They further suggest that the phenyl substituents of ITr are photochemically not innocent but can coordinate to the electron-deficient metal center of this trigonal complex in the excited state.

Enhanced Intersystem Crossing, Yet Still Fluorescence Upon Introduction of Intermediate Charge-Transfer States in Hemicaged [Zn(bpy)3]2+

“…It is therefore not surprising that only a few phosphorescent Zn­(II) complexes have been reported in the literature. Among the older ones are Zn porphyrins , and a series of [Zn­(thiolate) 2 (diimine)] − and [Zn­(dithiolate)­(diimine)] complexes. , Very recently, a new class of phosphorescent zinc complexes entered the field, i.e., Zn II carbene complexes featuring an atypical trigonal coordination mode. , …”