Syntheses, Structures, and Fluxionality of Blue Luminescent Zinc(II) Complexes: Zn(2,2‘,2‘ ‘-tpa)Cl2, Zn(2,2‘,2‘ ‘-tpa)2(O2CCF3)2, and Zn(2,2‘,3‘ ‘-tpa)4(O2CCF3)2 (tpa = Tripyridylamine)

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Abstract
Three novel Zn(II) complexes containing either 2,2‘,2‘ ‘-tripyridylamine (2,2‘,2‘ ‘-tpa) or 2,2‘,3‘ ‘-tripyridylamine (2,2‘,3‘ ‘-tpa) have been synthesized and structurally characterized. Compound 1, Zn(2,2‘,2‘ ‘-tpa)Cl2, has a tetrahedral geometry while compounds 2, Zn(2,2‘,2‘ ‘-tpa)2(O2CCF3)2, and 3, Zn(2,2‘,3‘ ‘-tpa)4(O2CCF3)2, have an octahedral geometry. The 2,2‘,2‘ ‘-tpa ligand in 1 and 2 functions as a bidentate ligand, chelating to the zinc center, while the 2,2‘ ‘,3‘ ‘-tpa ligand in 3 functions as a terminal ligand, binding to the zinc center through the 3-pyridyl nitrogen atom. All three compounds emit a blue color in solution and in the solid state. The emission maxima for the three compounds in solution are at λ = 422, 426, and 432 nm, respectively. The blue luminescence of the complexes is due to a π* → π transition of the tpa ligand as established by an ab initio calculation on the free ligand 2,2‘,2‘ ‘-tpa and complex 1. Compounds 1 and 2 are fluxional in solution owing to an exchange process between the coordinate and noncoordinate 2-pyridyl rings of the 2,2‘,2‘ ‘-tpa ligand. Compound 2 is also fluxional owing to a cis−trans isomerization process, as determined by variable-temperature 1H NMR spectroscopic analysis.