A new bis(thioether)-dipyrrin N2S2 ligand and its coordination behaviors to nickel, copper and zinc

Abstract

Tetradentate N₂S₂ coordination platforms are widespread in biological systems and have endowed metalloenzymes and metalloproteins with abundant reactivities and functions. However, there are only three types of N₂S₂ scaffolds respectively based on the bipyridine, aryl and alkyl amine derivatives, which are significantly underdeveloped for coordination chemistry. With the objective of developing a new N₂S₂ coordination platform to assemble a series of first-row transition metal complexes, we have designed a novel tetradentate N₂S₂ ligand containing a central dipyrrin donor functionalized with two thioether-substituted aryl units. Interestingly, complexation of the N₂S₂ ligand with the chloride salts of Ni(II), Cu(II) and Zn(II) yields various geometries with various coordination numbers. The reaction between the ligand and NiCl₂ readily forms two chloride-bridged centrosymmetric dinickel complexes in which the nickel centers are hexacoordinated by an N₂S₂Cl₂ coordination environment in distorted octahedron geometry. In contrast, metalation of the ligand with CuCl₂ gives a mononuclear copper complex consisting of a pentacoordinated copper center in a trigonal bipyramidal geometry with an N₂S₂Cl coordination sphere. Unexpectedly, the complexation of the ligand with ZnCl₂ forms a homoleptic zinc complex in which the zinc center is surrounded by an N₄ coordination sphere from two dipyrrin units in a non-planar pseudo-tetrahedral geometry despite the steric hindrance of two bulk thioether-substituted aryl units. These various geometries illustrate the potential structural flexibility of this new ligand. In addition, the optical properties of these compounds were also examined. This work thus provides a new N₂S₂ coordination platform with geometric flexibility.