Alkyl group dependence on structure and magnetic properties in layered cobalt coordination polymers containing substituted glutarate ligands and 4,4′-bipyridine

Five two-dimensional divalent cobalt coordination polymers containing 4,4′-bipyridine (bpy) and substituted or unsubstituted glutarate ligands have been prepared hydrothermally and structurally characterized by single-crystal X-ray diffraction. Co(mg)(bpy)]_n (1, mg=3-methylglutarate) forms a (4,4) rhomboid grid structure based on the connection of {Co₂(CO₂)₂} dimeric units. Using the more sterically encumbered ligands 3,3-dimethylglutarate (dmg) and 3-ethyl, 3-methylglutarate (emg) generated {Co(dmg)(bpy)(H₂O)]·2H₂O}_n (2) and {Co(emg)(bpy)(H₂O)]·H₂O}_n (3), respectively. These complexes manifest {Co(CO₂)}_n chains linked into 2-D by aliphatic dicarboxylate and bpy ligands. The “tied-back” substituted glutarate ligand 1,1-cyclopentanediacetate (cda) afforded Co(cda)(bpy)]_n (4), and the unsubstituted glutarate (glu) generated Co(glu)(bpy)]_n (5), both of which exhibit a topology similar to that of 1. The magnetic properties of complexes 1-4 were analyzed successfully with a recently developed phenomenological chain model accounting for both magnetic coupling (J) and zero-field splitting effects (D), even though 1 and 4 contain isolated, discrete {Co₂(CO₂)₂} dimers. The D parameter in this series varied between 21.8(8) and 48.0(9) cm⁻¹. However weak antiferromagnetic coupling was observed in 1 (J=-2.43(4) cm⁻¹) and 4 (J=−0.89(2) cm⁻¹), while weak ferromagnetic coupling appears to be operative in both 2 (J=0.324(5) cm⁻¹) and 3 (J=0.24(1) cm⁻¹).