New Polynuclear Copper-Pyrazolate Complexes: Towards the Synthesis of Photo- and Redox-Active Metal Organic Frameworks

Abstract

The main objectives of this project are the synthesis and redox- or photo-active modification and CO2 adsorption studies of metal-organic frameworks (MOFs) based on Cu3-pyrazolate secondary building units (SBUs). Trinuclear copper(II) complexes of the formula [Cu3(µ3-O)(µ-4-R-pz)3X3]z have been studied extensively due to their redox, magnetic and catalytic properties. In earlier work, we have shown that trinuclear copper(II) complexes of the formula [Cu3(µ3-O)(µ-4-R-pz)3X3]z pz = pyrazolato anion; R = H, CH(O), Cl, Br and NO2; X = Cl, NCS, CH3COO, CF3COO and pyridine – can be oxidized to the corresponding z+1, formally CuII2CuIII, species. In this project, fourteen (14) new copper-pyrazolate complexes of varying nuclearities (Cu3, Cu6, Cu7 and Cu12), terminal ligands (-NO2, py, -N3, -Cl) and bridging ligands (4-Cl-pzH and 4-Ph-pzH) have been synthesized. Efforts have been made to prepare MOFs based on the Cu3(µ3-O)-SBUs. While attempting to design the most suitable SBU for redox-active MOF construction, it was found that the one-electron oxidation of the all-CuII complex [Cu3(µ3-O)(µ-pz)3(NO2)3]2–, [8]2-, was achieved at redox potential more cathodic than any other Cu3(µ3-O)-complexes studied in our laboratory. The mixed-valent compound, [Cu3(µ3-O)(µ-pz)3(NO2)3]–, [8]–, the easiest accessible CuII2CuIII species known to date, was characterized spectroscopically. Compound [8] and analogous [11] release NO almost quantitatively upon the addition of PhSH or acetic acid. The system is catalytic in the presence of excess nitrite. Before embarking on the study of photo-active MOFs, a simpler model compound – a dimer of trimer [{Cu3(µ3-OH)(µ-4-Cl-pz)3(py)2}2(µ-abp)](ClO4)4 [21], where abp = 4,4’-azopyridine, was synthesized and its photochemistry was studied. The absorption spectra recorded before and after irradiation indicated a structural change. Two dimensional (2D) and three dimensional (3D) materials with {[Cu3(µ3-OH)(µ-4-R-pz)3]2+}n SBUs where R = Ph or Cl , which can potentially undergo cis/trans-isomerization, have been prepared during this project. A Phenyl substituent at 4-position on the pyrazole ligand leads to the formation of new class of 2D sheets. Three new 3D porous MOFs based on {[Cu3(µ3-OH)(µ-4-Cl-pz)3]2+}n SBUs have interpenetrated- lattice structures and are capable of adsorbing CO2 selectively. Compounds FIU-1 and FIU-3 also exhibit hysteretic sorption-desorption profiles indicating the flexibility of the MOFs upon adsorption. Compound FIU-1 demonstrates the usefulness of a hexanuclear CuII -pyrazolate moiety as an SBU for generating 3-fold interpenetrated 3D polymeric network. Complexes FIU-2 and FIU-3 have novel 3-fold interpenetrating 3D hexagonal framework structures. Compound FIU-2 crystallizes in the monoclinic crystal system with the P21/c space group, whereas FIU-3 crystallizes in triclinic space group P . Both structures contain Cu3-SBUs connected by the linkers through the Cu-termini