pH and Auxiliary Ligand Influence on the Structural Variations of 5­(2′-Carboxylphenyl) Nicotate Coordination Polymers

A ligand 5-(2′-carboxylphenyl) nicotic acid (H₂cpna) has been successfully applied to construct a series of coordination complexes {[Cd­(Hcpna)₂(H₂O)₂]·3H₂O}_n (1), [Cd­(cpna)­(H₂O)]_n (2), [M­(cpna)­(2,2′-bipy)­(H₂O)]_n (M = Cd (3), Co (4), and Mn (5)), [Co­(cpna)­(phen)­(H₂O)]_n (6), [Mn­(cpna)­(phen)­(H₂O)]_n (7), {[Nd­(Hcpna)­(cpna)­(H₂O)₂]·3H₂O}_n (8), and {[Ln­(Hcpna)­(cpna)­(phen)]·2H₂O}_n (Ln = Pr (9), Nd (10), Eu (11), and Gd (12), 2,2′-bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline) under hydrothermal conditions. By adjusting the reaction pH, H₂cpna ligand is partially deprotonated to form Hcpna^– in 1 and completely deprotonated to create cpna^2– in 2–7, and both forms are observed in 8–12. Complexes 1–5 and 8 possess two-dimensional (2D) layered structures, which are further extended into 3D metal–organic supramolecular frameworks by C–H···O hydrogen bond and/or π–π stacking interactions. Complexes 6, 7, and 9–12 exhibit one-dimensional (1D) chain structures, which further build three-dimensional (3D) supramolecular architecture via C–H···O hydrogen-bonding and/or π–π stacking interactions. The results revealed that the pH value of the reaction system and auxiliary ligand play an important role in determining the structures of the complexes. Magnetic susceptibility measurements indicate that compounds 4–10 and 12 have dominating antiferromagnetic couplings between metal centers. Furthermore, thermal stabilities for 1–12 and luminescent properties for 1–3, and 11 are also discussed in detail.