Structure and optical properties of a trimetallic cobalt-molybdenum-sodium metal-organic framework

The crystallization of a trimetallic cobalt-molybdenum-sodium metal-organic framework, poly[μ-benzene-1,3,5-tricarboxylato-tetra-μ-oxido-cobaltmolybdenumtrisodium], UOW-10 or [Na₃Co(MoO₄)(BTC)]_n, is achieved by solvothermal synthesis using benzene-1,3,5-tricarboxylic acid (H₃BTC, C₉H₆O₆) as a ligand precursor, Na₂MoO₄·2H₂O and Co(NO₃)₂·2H₂O as metal sources, and N,N-dimethylformamide (DMF) as the solvent. 3D electron diffraction (3D ED) reveals that the structure crystallizes in the monoclinic space group P2₁/c, with lattice parameters of a = 9.718 (2), b = 18.250 (3), c = 6.892 (9) Å, α = γ = 90, β = 96.156 (15)°, V = 1214.7 (4) ų and Z = 4. The phase purity of the bulk sample was confirmed using synchrotron powder X-ray diffraction. The organic ligands form a 2D layer, where cobalt and molybdenum are found, with sodium cations located between the layers. There are four crystallographically distinct sodium sites: three exhibit a distorted octahedral coordination geometry, while the remaining site is seven-coordinate. The cobalt has trigonal bipyramidal coordination geometry and molybdenum exhibits a tetrahedral coordination geometry. Half the sodium cations in the structure forms 1D column-like motifs via shared oxygen edges along the crystallographic c axis, which are cross-linked in b by the cobalt and molybdenum sites via bridging O atoms, while the other half of the sodium cations form 2D ribbons in the ac plane, propagating along c, linked by sharing oxygen edges and faces. The optical properties of UOW-10 were investigated through the use of UV-Vis spectroscopy, showing a bandgap of 1.8 eV. Deconvolution of the features in the visible-light region reveals that four peaks are present, which can all be ascribed to the d-d transitions from the trigonal bipyramidal cobalt. By means of thermogravimetric analysis (TGA) and variable-temperature powder X-ray diffraction (VT-PXRD), it is demonstrated that the material has thermal stability to 410 °C, after which structure collapse occurs, leading to a mixture of Na₂MoO₄, CoO and Co₃Mo above 900 °C.