Structural and spectroscopic studies of halogenocuprate(I) complexes

Abstract

Reaction of tetraalkylammonium halides, PPh₃ and copper(I) halides in a 1 : 1 : 1 mole ratio in dichloromethane yields complexes [NR₄][Cu(PPh₃)X₂](R = Et, X = Cl or Br; R = Pr, X = Br, I; R = Bu, X = Br) and [NEt₄]₂[Cu₂(PPh₃)₂I₄]. These have been characterized by infrared and cross polarization magic angle spinning (CP MAS)³¹P NMR spectroscopy, and the structures of most have been determined by single-crystal X-ray diffraction. The compounds [NR₄][Cu(PPh₃)X₂] in the first group above contain mononuclear anions [Cu(PPh₃)X₂]^– with essentially trigonal planar PCuX₂ co-ordination. The P–Cu–X and X–Cu–X angles are all within 5 ° of the ideal trigonal angle of 120 °. The compound [NEt₄]₂[Cu₂(PPh₃)₂I₄] contains the centrosymmetric binuclear anion [Cu₂(PPh₃)₂I₄]^2–{a dimer of [Cu(PPh₃)I₂]^–} in which the two copper atoms are bridged by two iodine atoms, and each copper atom is also bound to a terminal iodine atom and PPh₃ molecule. The FIR spectra of the mononuclear complexes [Cu(PPh₃)X₂]^– show strong bands at 264, 195 and 175 cm^–1 for X = Cl, Br and I respectively, which are assigned to the asymmetric v(CuX) stretching mode of the bent X–Cu–X units, and weak bands at 194, 150 and 128 cm^–1 due to the symmetric v(CuX) mode. In addition, the X = Cl complex shows a strong band at 126 cm^–1 which is assigned as the δ(CuCl₂) bending mode. The dimer [Cu₂(PPh₃)₂I₄]^2– shows a remarkably simple spectrum in the v(CuI) region: a partially resolved doublet at 113 and 126 cm^–1. The above vibrational frequencies are compared with those of the species [CuX₂]^– and [Cu₂I₄]^2–, from which the present species can be considered to be derived by addition of one PPh₃ molecule per copper atom. The solid state CP MAS ³¹P NMR spectra generally consist of asymmetric quartets due to coupling of the ³¹P nucleus to the quadrupolar copper nucleus, and the splitting patterns are interpreted in terms of the copper co-ordination environment.