2,2′-Bipyridine Zn(ii) complexes: effect of the 4,4′ substituents on the crystalline solid state properties

Abstract

The syntheses, structures and photophysical characterizations of new tris-chelated Zn(II) ionic complexes based on substituted bipyridine ligands (L₁ = 4,4′-bis(hydroxymethyl)-2,2′-bipyridine; L₂ = 4,4′-bis(methoxy)-2,2′-bipyridine) are reported. Owing to the different hydrogen bonding ability and steric hindrance of the 4,4′-bipyridine substituent groups, the L₁ and L₂ ligands cause a substantial difference in terms of stoichiometry, supramolecular organizations and solid state photophysical properties of the final products. The reaction performed in solution between the Zn(CH₃COO)₂ salt and the ligand L₂ in a 1 : 3.5 metal-to-ligand stoichiometric ratio afforded the expected octahedral complex [Zn(L₂)₃](PF₆)₂2 as white crystalline powder. Conversely, the species {[Zn(L₁)₃](PF₆)₂}₂·L₁·2H₂O (1a) in the form of pink single crystals is the exclusive product of the reaction performed under the same conditions as those for complex 2 by using L₁, the compound including un-coordinated ligands into the crystalline framework. The resulting molecular structure of the new product 1a consists of a sort of supramolecular tetra-cationic adduct, the intercalation of the un-coordinated L₁ ligand between two crystallographically equivalent [Zn(L₁)₃]²⁺ cations being mainly supported by stacking interactions and cooperative hydrogen bonds. Additionally, compound {[Zn(L₂)₃](PF₆)₂}₂·L₁·2H₂O (2a) has also been prepared through targeted synthetic approaches by combining complex 2 with the free L₁ ligand. In particular, the liquid-assisted grinding (LAG) method has been successfully applied by mixing complex 2 with a twofold molar amount of L₁.