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.