Benzoate substituents effects on the structure of Zn(II) complexes and 1D 4,4′-bipyridine derived coordination polymers

The aim of this work is to study the influence of different substituents in two closely related ligands, 3,5-dihydroxybenzoate (3,5-(HO) ₂Bz) and 3,5-dimethoxybenzoate (3,5-(MeO) ₂Bz), in the structure of zinc complexes. The compound [Zn(3,5-(MeO) ₂Bz) ₂(CH ₃OH)] ₂ (2) has been synthesized, characterized and its crystal structure elucidated, showing a paddle wheel type dinuclear molecule, with four bridging carboxylate ligands in a syn-syn coordination mode and methanol molecules in the apical positions. This discrete structural array is different than the polymeric structure observed in {[Zn(µ-3,5-(HO) ₂Bz)(µ-OH ₂)(H ₂O) ₂]·3,5-(HO) ₂Bz·4H ₂O} _n, previously reported in our group. These two zinc complexes were used as precursor for the synthesis of coordination polymers (CPs) with the 4,4′-bipyridine (4,4′-bpy) linker. The solvent used in their synthesis influences the chemical composition of this CPs. Thus, {[Zn(3,5-(HO) ₂Bz) ₂(µ-4,4′-bpy)]·2DMSO} _n (3) was achieved in dimethylsulfoxide (DMSO) while {[Zn(3,5-(HO) ₂Bz) ₂(µ-4,4′-bpy)]·4,4′-bpy·1/2CH ₃OH} _n (4) was formed in methanol (MeOH) as solvent. The reaction of 2 with 4,4′-bpy yielded [Zn ₂(μ-3,5-(MeO) ₂Bz) ₄(µ-4,4′-bpy)] _n (5) in methanol, while after recrystallization in boiling water, complex {[Zn(3,5-(MeO) ₂Bz) ₂(µ-4,4′-bpy)]·H ₂O} _n (6) was isolated in low yield. This compound 6 can also be synthesized by a mechanochemical approach, using water in a liquid assisted grinding. The crystal structures of complexes 3, 4 and 6 were elucidated, all three showing zigzag polymeric arrays. Both polymers containing 3,5-(MeO) ₂Bz (5 and 6), showed solid state luminescence.