A far infrared investigation of anionic species present in solution during the crystallization of a polynuclear bromocuprate(I) cluster containing three-coordinated copper (I)

Ethanolic solutions of tetrapropylammonium bromide and copper(I) bromide, from which bis(tetrapropylammonium) hexa--bromo-tetrahedro-tetracuprate(I), N(C₃H₇)₄]₂Cu₄Br₆], a bromocuprate(I) cluster comprised of three-coordinated copper(I), crystallizes have been shown, by means of far i.r. spectroscopy, to contain the centrosymmetric, monomeric CuBr₂]^– anion as the major, and probably the sole, bromocuprate(I) species, ₃=323 cm^–1. Crystalline N(C₃H₇)₄]₂Cu₄Br₆], N(CH₃)₄]₃Cu₂Br₅] and P(CH₃)₄]₂CuBr₃], all of which contain anions in which copper(I) is three-coordinated, dissolve in nitro-methane to give solutions of the dibromocuprate(I) monomer. Variation of the coordination number of copper(I) in bromocuprates(I) crystallizing with symmetrically substituted quaternary ammonium and related unipositive cations is discussed in terms of dilution of the bromide ions by the cations, and a tentative mechanism for the formation of such compounds is proposed.Toward the end of the precipitation of N(C₃H₇)₄]₂Cu₄Br₆] from the ethanolic solution of tetrapropylammonium bromide and copper(I) bromide, a few crystals of a second bromocuprate(I) phase were obtained. This compound has been identified as tetrapropylammonium dibromocuprate(I), and its crystal structure determined from X-ray diffraction data. N(C₃H₇)₄]CuBr₂] crystallizes in space groupP2/n, witha=13.010(15),b=7.577(5),c=8.858(4) Å,=91.09(7)° andZ=2. The anion is a monomer in which Cu-Br is 2.194(3) Å and Br-Cu-Br 178.4(1)°, copper(I) being situated on a twofold axis.