Cobalt(II), Copper(II), and Zinc(II) Framework Systems Derived from Ditopic Pyridyl-Acetylacetone and Pyridyl-Pyrazole Ligands

Abstract

The synthesis of two new bifuctional ligand sytems incorporating two different “classical” coordination units connected by a bent (but flexible) thioether linkage is reported. The new systems are the β-diketone-pyridyl derivative, 3-(pyridin-4-ylthio)pentane-2,4-dione (L1), and its hydrazine condensation product, the pyridyl-pyrazole species 4-(3,5-dimethyl-1H-pyrazol-4-ylthio)pyridine (L2). Both systems were designed for use in metal−organic framework studies. With L1, four metal-containing products were crystallized, [Cu(L1−H)2]n (1), {[Cu(L1−H)2]·0.5H2O·CHCl3}n (2), {[Zn(L1−H)2]·2H2O}n (3), and [Zn(L1−H)2]·2MeOH}n (4); these display three different array types. Compound 1 incorporates 5-coordinate Cu(II) ions with square pyramidal stereochemistry. The β-diketonato groups from two ligands coordinate in the basal plane with the apical position occupied by a pyridyl nitrogen from a third ligand that bridges to an adjacent copper center. Propagation of this coordination motif produces a 1D chain. In 2, which is a pseudo-polymorphic phase of 1, each Cu(II) ion is 6-coordinate with each metal center being bound by two bidentate β-diketonato groups and two pyridyl donors from four different ligands in a cis configuration. The ligand arrangement bridges each metal to four others producing an infinite 2D rhombic grid [(4,4) net]. The grids stack to form channels containing disordered water and chloroform molecules; the solvent-accessible space is 33% of the crystal volume. Crystals of 3 are isostructural with 2, except that they incorporate water as the only solvate in the channels. The solvent-accessible voids constitute 22% of the crystal volume, reflecting closer packing of adjacent grids. A TGA study showed that the water molecules are removed by 375 K and that the resulting desolvated complex is stable up to 573 K. Compound 4 is a pseudo-polymorph of 3 in which the 6-coordinate Zn(II) coordination is similar to that in 3 but with polymeric connectivity producing a chiral [8,4] type 3D network (space group P41212). Helical channels corresponding to 25% of the crystal volume run parallel to the 4-fold axes and are occupied by methanol guests. With 4-(3,5-dimethyl-1H-pyrazol-4-ylthio)pyridine (L2), an extended array consisting of zwitterionic [Co(L2+H)Cl3]n (5) units linked by hydrogen bonds and the metal-linked molecular framework species{[Zn(L2)Cl2]·0.5H2O}n (6) were obtained. The cobalt center in 5 is tetrahedrally coordinated by three chloride ions and a pyrazole nitrogen from (L2+H); the protonation is present at the pyridyl nitrogen atom. This protonated pyridyl nitrogen forms a charge-assisted hydrogen bond of type N−H+···Cl with a coordinated chloride from an adjacent complex to yield a hydrogen-bonded chainlike array. Intermolecular N−H···Cl hydrogen bonds also exist between the non-deprotonated pyrazole nitrogen and adjacent chloro ligands. Each tetrahedral Zn(II) center in 6 is coordinated to two chloride ions, a pyridyl nitrogen from one L2 ligand and a pyrazole nitrogen from another. Each L2 thus bridges adjacent Zn(II) centers, forming 1D polymeric chains that aggregate in the crystal to produce narrow channels occupied by water molecules.