Ligand-Mediated Interconversion of Multiply-Interpenetrating Frameworks in Cu^I/Re^VII-Oxide Hybrids

Two new copper(I)-rhenate(VII) hybrid solids, Cu(bpy)ReO4 (I) and Cu(bpy)2ReO4·0.5H2O (II) (bpy = 4,4′-bipyridine), with 2-fold and 4-fold interpenetrating networks, respectively, were prepared from hydrothermal reactions, and their structures characterized by single-crystal X-ray diffraction [I, Pbca (No. 61), Z = 8, a = 10.8513(3) Å, b = 12.9419(4) Å, c = 15.6976(5) Å; II, P1̅ (No. 2), Z = 2, a = 11.8190(4) Å, b = 12.6741(4) Å, c = 13.7585(5) Å, α = 85.8653(13)o, β = 81.6197(13)o, γ = 84.0945(11)o]. The structure of I contains 63 nets of neutral CuReO4 layers that are pillared via bpy ligands on the Cu sites {CuO3N2} to yield a 2-fold interpenetrating pillared-layered network. Conversely, the structure of II consists of a 4-fold interpenetrating diamond-type network with tetrahedral {CuN4} coordination nodes that are bridged by bpy ligands, with both H2O and ReO4− within the pores. A surprising reversible structural interconversion between these two interpenetrating structures is possible via the insertion and removal of a single bpy ligand and 1/2H2O per copper atom. The structural interconversion is accompanied by a change in color from yellow to red for I and II, respectively. Measured UV−vis diffuse reflectance spectra exhibit a significant red-shift in the absorption edge of ∼0.3 eV, with the optical bandgap size decreasing from ∼2.5 eV to ∼2.2 eV for I and II, respectively. X-ray photoelectron spectra and electronic structure calculations indicate that the valence band derived from the Cu 3d and N 2p orbitals in II are pushed higher in energy compared to those in I because of the coordination of the additional bpy ligand. There is a much smaller change in the energy of the conduction band that is derived from the Re 5d orbitals. These results demonstrate that the ligand-mediated structural transformations of (d0/d10)-hybrid solids represent a new and convenient low-temperature approach to modulate their optical bandgap sizes toward the visible wavelengths for use with solar energy.