Synthesis and crystal structure of cadmium(II) coordination polymers with 3- and 4-pyridylacrylic acids

Four new cadmium(II) coordination polymers, [Cd₃(4-Hpya)₄(SCN)₆(H₂O)₂] (1), [Cd(4-pya)(SCN)(H₂O)]·H₂O (2), [Cd(3-Hpya)₂(SCN)₂] (3) and [Cd(3-pya)₂] (4), have been synthesized from appropriate Cd(II) salts and 4-pyridylacrylic acid (4-Hpya) or 3-pyridylacrylic acid (3-Hpya) in the presence of thiocyanate under similar hydrothermal conditions. The use of cadmium perchlorate as metal source leads to complexes (1 and 3) with 4-Hpya and 3-Hpya, while the use of cadmium acetate leads to complexes (2 and 4) with 4-pya^- and 3-pya^-. In 1 and 3, the protonated ligands act as monodentate terminal ligands and lead to different 1D coordination chains with alternating end-to-end and end-on-N thiocyanate bridges (1) or with uniform end-to-end thiocyanate bridges (3). While 3-Hpya is in the normal carboxylic acid form in 3, it is unusual and unprecedented that 4-Hpya in 1 takes the zwitterionic form with the proton attached to the pyridyl nitrogen. The proton in either compound plays an important role in assembling the chains into higher-dimensional networks via hydrogen bonds. On the other hand, the deprotonated ligands in 2 and 4 assume μ₂ and μ₃ multidentate bridging modes, respectively, and allow the formation of 2D layers with coexistent NCS^- and 4-pya^- bridges (2) or with only 3-pya^- bridges (4).