Three-coordinate [(CuX3)-X-II](-) (X = Cl, Br), trapped in a molecular crystal

摘要

Mixtures of [Ph3PNPPh3]Cl-+(-) with CuBr2 (or CuBr2 + CuCl2) in ethanol/dichloromethane yield crystals containing three-coordinate copper(II) with mixed chloride and bromide ligands, namely [Ph3PNPPh3](+)[CuCl0.9Br2.1](-) (1) and [Ph3PNPPh3](+)[CuCl2.4Br0.6](-) (2). The trigonal-planar coordination of copper(H) is angularly distorted but unambiguous, as there is no other halide ligand within 6.7 Angstrom of the copper atom. Density functional theory (DFT) calculations on planar [CuClBr2](-) show that the energy surface for angle bending is very soft. Crystallisation in the presence of CH3CN yields [Ph3PNPPh3](+)[CuCl0.7Br2.3(NCCH3)](-) (3), in which there is additional secondary coordination by NCCH3 (Cu-N 2.44 Angstrom). DFT calculations of the potential energy surface for this secondary coordination show that it is remarkably flat (< 3 kcal mol(-1) for a variation of Cu-N by 0.8 &ANGS;). The crystal packing in 1. 2 and 3. which involves multiple phenyl embraces between [Ph3PNPPh3](+) ions and numerous C-H &BULL;&BULL;&BULL; Cl and C-H &BULL;&BULL;&BULL; Br motifs, is associated with intermolecular energies that are larger than the variations in intramolecular energies. For reference, the crystal structures of [Ph3PNPPh3+](2)-[Cu2Cl6](2-) (4) and [Ph3PNPPh3+](2)-[Cu2Br6](2) 12 (5) are described. We conclude 1) that three-coordinate copper(II) with monatomic halide ligands, although uncommon, can be regarded as normal, 2) that steric control by ligands is not necessary to enforce three -coordination, 3) that a hydrophobic aryl environment stabilises [Cu(Cl/Br)(3)](-), and 4) that the energy change in the transition from three- to four-coordinate copper(II) is very small (ca 5 kcal mol(-1)). [References: 19]