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

2007-02-19T00:00:00Z (GMT) by Haisheng Lin Paul A. Maggard
The new copper(I) rhenates, CuReO<sub>4</sub>(pyz) (<b>I</b>) and Cu<sub>3</sub>ReO<sub>4</sub>(q6c)<sub>2</sub> (<b>II</b>) (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 (<b>I</b>, <i>P</i>2<sub>1</sub>/<i>n</i>, No. 14, <i>Z</i> = 4, <i>a</i> = 7.972(1) Å, <i>b</i> = 11.928(2) Å, <i>c</i> = 8.430(1) Å, β = 102.161(2)°; <b>II</b>, <i>P</i>2<sub>1</sub>, No. 4, <i>Z</i> = 2, <i>a</i> = 8.253(2) Å, <i>b</i> = 6.841(2) Å, <i>c</i> = 18.256(6) Å, β = 101.37(2)°) and characterized by thermogravimetric analyses and UV−vis diffuse reflectance. The structure of <b>I</b> contains ‘CuReO<sub>4</sub>' layers that are pillared through bridging pyrazine ligands via the Cu sites, while the structure of <b>II</b> is polar and contains chains of ‘Cu<sub>2</sub>ReO<sub>4</sub>' that are condensed into layers by coordination to linear ‘Cu(q6c)<sub>2</sub>' bridges between the chains. In contrast to air-sensitive CuReO<sub>4</sub>, both hybrid analogues are stable in air owing to a stabilization of the Cu<sup>1+</sup> oxidation state by N-donating ligands, but decompose upon heating with the removal of the organic ligands, which for <b>I</b> yields crystalline CuReO<sub>4</sub>. UV−vis diffuse reflectance measurements and electronic structure calculations on all three copper perrhenates, <b>I</b>, <b>II</b>, and CuReO<sub>4</sub>, 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 d<sup>0</sup> and Cu d<sup>10</sup> orbitals, respectively, and mixed with O p-orbital contributions. In contrast to the silver rhenates, which have relatively lower energy Ag d<sup>10</sup> 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 Cu<sup>1+</sup>/Re<sup>7+</sup> oxides can be promising candidates for investigating in visible-light photocatalytic reactions.