Structural Variation from 1D to 3D: Effects of Temperature and pH Value on the Construction of Co(II)-H₂tbip/bpp Mixed Ligands System

To determine the influence of reaction conditions on the formation of metal−organic frameworks, a series of Co(II)/H 2 tbip/bpp complexes, {[Co 2 (tbip) 2 (H 2 O) 6 ]·8H 2 O} n ( 1 ), {[Co(tbip)(bpp)(H 2 O)] 2 ·4H 2 O} n ( 2 ), {[Co(tbip)(bpp)]·H 2 O} n ( 3 ), {[Co 2 (H 2 O)(tbip)(Htbip) 2 (bpp) 2 ]·0.5(H 2 tbip)·0.5H 2 O} n ( 4 ), {[Co 3 (OH)(tbip) 2 (Htbip)(bpp)]} n ( 5 ) (H 2 tbip = 5- tert -butyl isophthalic acid, bpp = 1,3-bi(4-pyridyl)propane), have been synthesized through the hydrothermal reaction of cobalt(II) acetate, H 2 tbip, and bpp under different temperatures and pH conditions. The diverse product structures show the marked sensitivity of the structural chemistry of the aromatic carboxylate ligand to synthesis conditions. Polymer 1, which features a three-dimensional (3D) framework with CdSO 4 topology, has carboxylate bridging polymeric chains spanning two different directions and bridging water molecules between these chains, and is formed at pH = 4 at 120 °C. At pH = 6, a one-dimensional tube-like structure of 2 is formed. Further increase of the pH value to 7.5 results in a two-dimensional corrugated network structure of 3 . Raising the temperature to 160 °C results in the formation of binuclear [Co 2 (H 2 O)] 4+ and trinuclear cluster node [Co 3 (μ 3 -OH)] 5+ secondary building units in 4 and 5 . 4 has a 3D coordination network having alternate opposite chiral layers constructed from {[Co 2 (H 2 O)] 4+ -bpp -tbip-} n single-helical chains, whereas compound 5 exhibits a 3D network based on the [Co 3 (μ 3 -OH)] 5+ node. Both compounds have α-Po topology. Interestingly, the trinuclear cluster node in 5 has a unique combination between pentacoordinated, tetrahedrally coordinated, and octahedrally coordinated Co(II) sites, which has never been reported in other μ 3 -hydroxyl-bridged trinuclear or tetranuclear cobalt clusters. The thermal stabilities of 1, 2, and 4 have been studied by means of thermogravimetric analysis. Preliminary X-ray powder diffraction studies of 4 indicate that the framework is stable even to the loss of both guest H 2 tbip and coordinated water molecules. The magnetic properties of 1, 4, and 5 have been investigated and discussed in detail.