Two Series of Isostructural Coordination Polymers with Isomeric Benzenedicarboxylates and Different Azine Based N,N′-Donor Ligands: Syntheses, Characterization and Magnetic Properties

Seven
coordination polymers (CPs) with two types of framework structures,
namely, {[M­(4-bpdb)­(1,3-bdc)]­·(4-bpdb)0.5}n [M = Mn (1), Fe (2), and Co (3)], and {[M­(4-bpdh)­(1,4-bdc)]}n (M = Mn (4), Fe (5), Co (6), and Cd (7)] have been synthesized
through the slow diffusion technique using 1,3-bdc and 1,4-bdc ligands
with two different azine based N,N′-donor linkers [1,3-bdc
= benzene-1,3-dicarboxylate, 1,4-bdc = benzene-1,4-dicarboxylate,
4-bpdb = N,N′-bis-pyridin-4-ylmethylene-hydrazine
and 4-bpdh = N,N′-bis-(1-pyridin-4-yl-ethylidene)-hydrazine].
Their structures have been determined by single-crystal X-ray diffraction
analysis and further characterized by elemental analysis, IR spectra,
and powder X-ray diffraction (PXRD) analysis. Compounds 1–3 are isostructural and feature a two-dimensional
(2D) framework structure formed by pillaring the one-dimensional [M­(1,3-bdc)]n double chains through 4-bpdb linkers. In
all cases of compounds 1–3, free
4-bpdb linkers are present in the lattice and lattice 4-bpdb ligands
are involved in π–π and C–H···π
interactions with bridging 1,3-bdc and 4-bpdb ligands, to afford three-dimensional
(3D) supramolecular structures. Compounds 4–7 are also isostructural and here the bridging of 1,4-bdc
with the divalent metal centers forms a 2D layer of [M­(1,4-bdc)]n which is further pillared by 4-bpdh to form
a 3D pillared-layer framework. Variable temperature magnetic measurements
of two sets of isostructural complexes (1–3 and 4–7) have been carried
out. Compounds 1–6 clearly indicate
the existence of a weak antiferromagnetic interaction between the
metal ions through the bridging 1,3-bdc and 1,4-bdc ligands. The magnetic
properties depend strongly on the nature of the metal center. The
luminescence spectra of the complexes have been measured, which indicates
a ligand based emission for all seven compounds.