Tuning the Spin State of Cobalt in a Co–La Heterometallic Complex through Controllable Coordination Sphere of La

Abstract

Paramagnetic molecular materials with electronic structures sensitive to the environment are very attractive owing to potential applications in molecular devices and sensors. [1] Great efforts have been made in synthesizing bistable paramagnetic molecules or polymers that exist in two different spin states, which can be tuned by an external stimulus such as temperature, pressure, or light. [2][3][4] Cobalt ions can exist in multiple oxidation states, and hence are good candidates for construction of molecular materials that can switch between two different oxidation states, typically between Co III and Co II. A number of cobalt semiquinonate complexes undergo valence transition in response to external stimuli such as temperature and light (valence tautomerism). [5,6] In a few bimetallic compounds, such as Co-Fe(Os) cyanometalates, thermally induced charge transfer occurs between the two redox-active metal ions. [7] The shift of the equilibrium between Co III and Co II is accompanied by a drastic change in their magnetic and optical properties. As far as we are aware, valence changes in cobalt coordination polymers induced by a second metal coordination sphere have never been described in the literature.Here we present a novel approach to manipulate the oxidation state of cobalt by a controllable second metal coordination sphere. Polytopic ligand 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid) ( Compound 1 a is a rare example of metal phosphonates with both 3d and 4f metal ions.[8] It crystallizes in monoclinic space group P2 1 /c.[9] The octahedral Co III ion is surrounded by three phosphonate oxygen atoms (O2, O5, and O8) and three nitrogen atoms (N1, N2, and N3) from the same notp 6À ligand in a mononuclear Co (4) ). This unit behaves as a tetradentate "ligand" and links to four La III ions through phosphonate oxygen atoms O1, O3, O4, and O7 to form a two-dimensional waved layer containing six-and eight-membered rings. Each La atom is coordinated by eight oxygen atoms, four of which are phosphonate oxygen atoms from four Co(notp) 3À units, and the remainder oxygen atoms of water molecules. The interlayer space is filled by the lattice water molecules with extensive hydrogen-bonding interactions (Figure 1 a and b). Thermal analysis and XRD measurements confirm that both the coordinated and lattice water molecules can be removed on heating to 220 8C with retention of the network structure. The fully dehydrated sample H[hs-Co II La III (notp)] (1 a-220) can be redissolved in water, from which crystalline 1 a can be obtained within one day in the presence of oxygen (Supporting Information).Magnetic susceptibilities, measured in the temperature range 1.8-300 K under an external field of 2 kOe (Figure 1 c), reveal a diamagnetic nature for compound 1 a, in agreement with the presence of a low-spin Co III (m eff /Co(300 K) = 0 m B ). After heating 1 a at 120 8C for 1 h, the resulting sample 1 a-120 becomes paramagnetic. The paramagnetic behavior becomes more significant if 1 a is heated at a higher ...

Proton Transport in Metal–Organic Frameworks

“…In 231 but with different interlayer contents. Hence, in the dehydration and rehydration process, CoLa-II is irreversible, where the PXRD pattern of rehydrated CoLa-II is similar to the CoLa-I previously reported.…”

Co–Ca Phosphonate Showing Humidity-Sensitive Single Crystal to Single Crystal Structural Transformation and Tunable Proton Conduction Properties

“…The compound Co(notpH 3 )•3H 2 O was prepared according to the literatures. 70 All the other starting materials were of reagent grade quality and were obtained from commercial sources without further purification. Elemental analyses for C, H, and N were carried out on a PE 240C analyzer.…”

Tuning the magneto-structural properties of non-porous coordination polymers by HCl chemisorption

“…A different approach to tune the magnetic properties of porous solids consists of the modification of the magnetic ground state of the MOF, which can be achieved through gas sorption processes that subtly affect the geometry of either the bridge between metal centres 20 or the ligand substituents 21 . The change of the oxidation state of the metal centre 22 or the variation of the coordination number 23 on hydration/dehydration has also been used to modify the magnetic properties of porous materials. In all these cases, the presence of pores is essential to accommodate the gas molecules that induce the minor structural changes in the framework provoking major changes in the magnetic properties.…”