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

The syntheses, structures and photophysical characterizations of new tris-chelated Zn(II) ionic complexes based on substituted bipyridine ligands (L1 = 4,40-bis(hydroxymethyl)-2,20-bipyridine; L2 = 4,40-bis(methoxy)- 2,20-bipyridine) are reported. Owing to the different hydrogen bonding ability and steric hindrance of the 4,40-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-toligand stoichiometric ratio afforded the expected octahedral complex [Zn(L2)3](PF6)2 2 as white crystalline powder. Conversely, the species {[Zn(L1)3](PF6)2}2L12H2O (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}2L12H2O (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.