Copper(I)−Rhenate Hybrids:  Syntheses, Structures, and Optical Properties

The new copper(I) rhenates, CuReO₄(pyz) (I) and Cu₃ReO₄(q6c)₂ (II) (pyz = pyrazine; q6c = quinoline-6-carboxylate), were synthesized by hydrothermal methods at 140−150 °C, and their structures determined via single-crystal X-ray diffraction (I, P2₁/n, No. 14, Z = 4, a = 7.972(1) Å, b = 11.928(2) Å, c = 8.430(1) Å, β = 102.161(2)°; II, P2₁, No. 4, Z = 2, a = 8.253(2) Å, b = 6.841(2) Å, c = 18.256(6) Å, β = 101.37(2)°) and characterized by thermogravimetric analyses and UV−vis diffuse reflectance. The structure of I contains ‘CuReO₄' layers that are pillared through bridging pyrazine ligands via the Cu sites, while the structure of II is polar and contains chains of ‘Cu₂ReO₄' that are condensed into layers by coordination to linear ‘Cu(q6c)₂' bridges between the chains. In contrast to air-sensitive CuReO₄, both hybrid analogues are stable in air owing to a stabilization of the Cu¹⁺ oxidation state by N-donating ligands, but decompose upon heating with the removal of the organic ligands, which for I yields crystalline CuReO₄. UV−vis diffuse reflectance measurements and electronic structure calculations on all three copper perrhenates, I, II, and CuReO₄, show that each exhibits an optical band gap of ∼2.1−2.2 eV, with conduction and valence band levels that are primarily derived from the Re d⁰ and Cu d¹⁰ orbitals, respectively, and mixed with O p-orbital contributions. In contrast to the silver rhenates, which have relatively lower energy Ag d¹⁰ orbitals, the inclusion of the organic ligands into the structures has only a very minor effect (∼0.1 eV) on the band gap size. The optical absorptions, in combination with the air-stable open-framework layered structures, illustrate that heterometallic Cu¹⁺/Re⁷⁺ oxides can be promising candidates for investigating in visible-light photocatalytic reactions.