Microporosity, Optical Bandgap Sizes, and Photocatalytic Activity of M(I)-Nb(V) (M = Cu, Ag) Oxyfluoride Hybrids

Using hydrothermal methods, a new series of five M(I)−Nb(V) oxyfluoride hybrid solids was prepared having the compositions [Cu(pyz)]2NbOF5 (1, pyz = pyrazine), Ag(pyz)NbOF4 (2), M(bpy)NbOF4·2H2O (3, M = Cu; 4, M = Ag; bpy = 4,4′-bipyridine), and [Cu(dpe)]2NbOF5·8H2O (5, dpe = 1,2-bis(4-pyridyl)-ethene). Each of the hybrid solids consists of layered structures with bridging ligands that coordinate via the Ag+ or Cu+ (i.e., M+) sites in each layer. The structure of 1 is comprised of [Cu2(pyz)22+]n double chains that are cross-linked into double layers by coordination to partially O/F-disordered [NbOF5]2− octahedra. Hybrids 2−4 contain pyz- and bpy-pillared [MNbOF4] layers, with the longer bpy ligand affording two interlayer water molecules per M+ site. These [MNbOF4] layers consist of ordered polar [NbOF4]− chains that are bridged via the M+ sites and that orient in alternating directions. The longest ligand in 5, dpe, results in a 3-fold interpenetrating pillared network consisting of [Cu(dpe)+]n chains bridged by O/F-disordered [NbOF5]2− octahedra and accommodating four interlayer water molecules per Cu+ site. The thermally-driven removal of water from 5 results in a loss of crystallinity, but which can be rapidly reversed by reabsorption of water. By contrast, the structures of 3 and 4 are retained upon dehydration, and both reversibly absorb either two waters or one methanol molecule per M+ site. Measured optical bandgap sizes for the Ag-containing hybrids occur at ∼3.13 eV and ∼3.54 eV for 2 and 4, and for the Cu-containing hybrids at ∼2.15 eV, ∼2.71 eV, and ∼2.30 eV for 1, 3, and 5, respectively. Dehydration of 3 and 4 results in a ∼0.3 eV red-shift of their bandgap sizes. Hybrids 2 and 4 are also found to be the first metal-oxyfluorides active for the photocatalytic decomposition of methylene blue with rates of 1.83 and 1.37 mg L−1 h−1, respectively.