1,2,4,5-benzenetetracarboxylate- and 2,2′-bipyrimidine-containing cobalt(II) coordination polymers:: Preparation, crystal structure, and magnetic properties

Three new mixed-ligand cobalt(II) complexes of formula [Co-2(H2O)(6)(bta)(bpym)](n)center dot 4nH(2)O (1), [Co-2(H2O)(2)(bta)(bpym)](n) (2), and [Co-2(H2O)(4)(bta)(bpym)](n)center dot 2nH(2)O (3) (bpym = 2,2'-bipyrimidine and H(4)bta = 1,2,4,5-benzenetretracaboxylic acid) have been synthesized and characterized by single crystal X-ray diffraction. 1 is a chain compound of mertriaquacobalt(II) units which are linked through regular alternating bis-bidentate bpym and bis-monodentate bta groups. 2 and 3 are three-dimensional compounds where aquacobalt(II) (2) and cis-diaquacobalt(II) (3) entities are linked by bis-bidentate bpym (2 and 3) and tetrakis- (2 and 3) and octakis-monodentate (2) bta ligands. The cobalt atoms in 1-3 exhibit somewhat distorted octahedral surroundings. Two bpym-nitrogen atoms (1-3) and either two bta-oxygens (2) or one bta-oxygen and a water molecule (1 and 3) build the equatorial plane, whereas the axial positions are filled either by two water molecules (1) or by a bta-oxygen atom and a water molecule (2 and 3). The values of the cobalt-cobalt separation across the bridging bpym vary in the range 5.684(2)-5.7752(7) angstrom, whereas those through the bta bridge cover the ranges 5.288(2)-5.7503(5) angstrom (across the anti-syn carboxylate) and 7.715(3)-11.387(1) angstrom (across the phenyl ring). The magnetic properties of 1-3 have been investigated in the, temperature range 1.9-290 K They are all typical of an overall antiferromagnetic coupling with the maxima of the magnetic susceptibility at 14.5 (1) and 11.5 K (2 and 3). Although exchange pathways through bis-bidentate bpym and carboxylate-bta in different coordination modes are involved in 1-3, their magnetic behavior is practically governed by that across the bpym bridge, the magnitude of the exchange coupling being J = -5.59(2) (1), -4.41(2) (2), and -4.49(2) (3) with the Hamiltonian (H) over cap = -J (S) over cap (1)(S) over cap (2).