STRUCTURAL VARIANTS OF TETRANUCLEAR L₄CU₄X₄ - INFLUENCE OF L ON THE COORDINATION MODE OF COPPER(I)

International audienceThree pi complexes between nonconjugated dienes and copper(I) chloride and/or bromide have been prepared and characterized by means of crystal structure determination. The complex between 1,4-pentadiene and copper(I) chloride (1) was prepared in ethyl vinyl ketone (1-penten-3-one), utilizing the complex between copper(I) chloride and the solvent as an in situ precursor. [Cu2Cl2(C5H8)] (1) crystallizes in the orthorhombic space group Pccn with a = 7.292 (4) angstrom, b = 14.146 (3) angstrom, c = 15.727 (4) angstrom, and Z = 8, and he structure was refined to a final R = 0.065 (R(w) = 0.080) for 82 parameters and 890 observed reflections. The compound contains a distorted cubane Cu4Cl4 core, bridged by pi-coordinated 1,4-pentadiene ligands, to form an infinite ribbon. The copper(I) centers exhibit distorted tetrahedral (trigonal pyramidal) coordination geometry. Compounds 2 and 3, [Cu4Br2Cl2(C7H8)4] and [Cu4Br4(C7H8)4], respectively, prepared by direct reaction between copper(I) halide and norbornadiene, are isostructural, crystallizing in the tetragonal space group P42(1)cBAR, with Z = 2. Cell constants are a = 12.187 (3) and c = 9.668 (2) angstrom (2) and a = 12.245 (2) and c = 9.759 (2) angstrom (3). The structures were refined to final R = 0.038 (R(w) = 0.043) (2) and R = 0.029 (R(w) = 0.033) (3). Both 2 and 3 are discrete tetramers with distorted Cu4X4 cores, each norbornadiene ligand being pi-coordinated to copper(I) through one olefinic linkage. The factors which favor a regular or a distorted cubic cage in tetranuclear L4Cu4X4 are discussed by means of extended Huckel calculations (EHT). It is shown that a regular cubane-type structure is preferred when L is a pure sigma-donor ligand. In contrast, when L is a pi-acceptor ligard, two types of distortion in which four Df the cube edges are elongated are possible. These two structures, which differ in the geometrical arrangement of the four long bonds, are shown to be the only possible isomers. It is demonstrated that these distortions are due to the presence of a moderately active d block and low-lying s and p blocks on the Cu(I) center. This specific orbital pattern makes the coordination of copper(I) highly variable