Complexes of Zinc(II) with N-Imidazolyl- and N-Pyrazolylpyrimidine Donor Ligands: Synthesis, Crystal Structures, and Theoretical Study

The synthesis and crystal-structure determination of three zinc(II) complexes with 2-(1H-imidazol-1-yl)pyrimidine(imipyr) and 2-(1H-pyrazol-1-yl)pyrimidine (pyrapyr) ligands are reported. Complexes [Zn(imipyr)2Cl2] (1) and[Zn(pyrapyr)2Cl2] (2) are mononuclear systems, and [Zn2(pyrapyr)2(Cl)4] (3) is a dinuclear complex with two chlorido bridging ligands. In complex 3, the coordination numbers of the two differently coordinated Zn ions are four and six. All complexes were characterized by X-ray crystallography. A high-level theoretical study was performed to rationalize the interesting noncovalent interactions observed in the solid state. All three structures show a peculiar pi stacking, which is characterized by the absence of pi pi ring-plane overlapping. In this slipped stacking mode, the rings are antiparallely displaced, and alternating C center dot center dot center dot N interactions are established. Moreover, complex 3 forms infinite 1D columns by means of double anion-pi interactions with pyrapyr. The atoms-in-molecules (AIM) theory proposed by Bader was used to characterize the anion-pi and pi pi interactions observed in the solid state. A high-level ab initio study (RI-MP2/def2-TZVP level of theory) was performed to analyze the anionp binding affinity of the pyrapyr ligand coordinated to the transition metal atom. Finally, a CSD search demonstrates that the structure of 3 is quite unique, because dinuclear Zn complexes that contain one tetracoordinate and one hexacoordinate metal center are rare, whereas this combination is common in trinuclear, linear complexes. With bridging chlorido ligands, this simultaneous tetra- and hexacoordination is unprecedented.