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

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 [ZnX2(ITr)] (X = Cl (1), Br (2), I (3)), for which the low coordination mode was confirmed in both solution and the 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)), [Zn(bdt)(ITr)] (bdt = benzene-1,2-dithiolate) (8) and cationic [Zn(2-X)(ITr)]2[BPhF4]2 (X = Cl (9), Br (10), I (11)), all of which were isolated and structurally characterized. Importantly, for all complexes 4-11, the trigonal coordination environment of the ZnII 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 long-lived luminescence from triplet excited states in the solid state at room temperature according to our photophysical studies. Our quantum chemical 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.