Partial in Situ Reduction of Copper(II) Resulting in One-Pot Formation of 2D Neutral and 3D Cationic Copper(I) Iodide–Pyrazine Coordination Polymers: Structure and Emissive Properties

On the way to copper­(I) iodide coordination polymers with specific luminescent properties, the in situ reduction of Cu­(II) in the presence of KI and bidentate N-heteroatomic ligand, either pyrazine (pyz) or 4,4′-bipyridine (bpy), resulted in one two-dimensional and two three-dimensional new coordination networks. Starting from Cu­(NO3)2·3H2O in the presence of pyz, successive precipitation of known yellow [(CuII)2(pyz)]n, new orange [CuII­(pyz)]n, and new dark blue {[CuI(pyz)2]·I5}n polymeric solids was observed. Starting from the same salt in the presence of bpy resulted in the successive precipitation of known yellow [(CuII)2(bpy)]n and new brown {[CuII(NO3)­(bpy)2]·I3·(dmf·H2O)}n coordination polymers. By using either Cu­(CH3COO)2·H2O or Cu­(BF4)2 as starting materials, both known forms, yellow [(CuII)2(bpy)]n and orange [CuII­(bpy)]n, precipitated successively. The new solids were characterized by IR spectroscopy and X-ray analysis. [CuII­(pyz)]n represents the missing member in the row of two-dimensional coordination networks with general formula [CuIX­(pyz)]n (X = Cl, Br, I). Its steady state and time-resolved characterization together with DFT and TDDFT calculations revealed that the emission at room temperature is mainly delayed fluorescence originating from mixed singlet metal-to-ligand charge transfer and halide-to-ligand charge transfer states, while that at 77 K is phosphorescence, associated with the small singlet–triplet energy differences (ΔE = 70 meV).