Pyridine-2,4-dicarboxylate: a versatile building block for the preparation of functional coordination polymers

The hydrothermal reaction of the pyridine-2,4-dicarboxylate (2,4-pdc) dianion with various 3d metal(II) cations resulted in the formation of a series of one-, two- and three-dimensional coordination polymers with varied network topologies. 2,4-pdc reacted directly with MnCI2 or FeCl2 in water at 200 degrees C under mild basic conditions to give a dense three-dimensional polymer, [M(2,4-pdc)] (M = Mn, 1a; M = Fe, 1b), which supports a highly-connected three-dimensional magnetic exchange lattice. Both 1a and 1b undergo antiferromagnetic ordering and the latter Fe(II)-containing material exhibited spin-canting behavior below 6.5 K. Reaction of 2,4-pdc with CoCl2 under identical conditions gave an unusual zigzag chain polymer, [Co4(2,4-pdc)4(OH2)10] (2), that has large amounts of coordinated H2O. The magnetism of 2 based on isolated Co(II) dimers was modeled using a modified van Vleck approach. Adjustment of the reaction pH resulted in the formation of additional Co(II)-containing materials with strongly contrasting structures: at lower pH in the presence of oxalic acid, the same reaction components gave a protonated molecular species, [Co(2,4-pdcH)2(OH2)2]. 2H2O (3); at higher pH in the presence of excess hydroxide, a highly porous, three-dimensional material was obtained, [Co3(micro3-OH)2(2,4-pdc)2] x 9H2O (4a). 4a contains cobalt hydroxide chains, whose magnetic behavior has been studied in detail by neutron diffraction. A Ni(II)-containing analogue was also prepared using NiCI2 (4b), which shows weak antiferromagnetic coupling. Attempts to obtain a Zn(II)-based analogue of the porous material 4 gave instead a unique coordination material, [Zn(micro2-OH)2(2,4-pdc)] (5), which contains uncommon zinc hydroxide bridging modes.