Dimeric quinoline adduct of copper(II) trifluoroacetate: Preparation, structure, and magnetism

The dimeric quinoline adduct of copper(II) trifluoroacetate has been shown to have a long Cu–Cu distance of 2·886 A and magnetic and magnetic properties similar to those of copper(II) acetate monohydrate.


Suwmary
The dimeric quinoline adduct of copper(I1) trifluoroacetate has been shown to have a long Cu-Cu distance of 2-886 and magnetic properties similar to those of copper(1r) acetate monohydrate.
CONSIDERABLE difference exists between the magnetic properties of anhydrous copper(I1) acetate and those of the corresponding anhydrous trifluoroacetate. In contrast to the depressed effective magnetic moment a t 295 K of 1-4 B.M. per Cu2+ ion and the antiferromagnetic xM us. T behaviour for copper (11) acetate,l the trifluoroacetate displays a normal room-temperature moment of 1-81 B.M. and a magnetic susceptibility which conforms to the Curie-Weiss law between 94 and 297 K., The variation in magnetic behaviour has been attributed to differences in basicity between the trifluoroacetate group and the acetate ion.3 This explanation, however, becomes untenable if the anhydrous carboxylates are not in fact isostructural. No definitive evidence of the dimeric structure exists for copper-(11) trifluoroacetate; it has been suggested on the basis of magnetic susceptibility and i.r. spectral data that anhydrous copper(I1) trifluoroacetate does not adopt the copper(r1) acetate structure.2 This situation is further complicated by the general disinclination of copper (11) trifluoroacetate to form mono-adducts of the Cu (O,CR),.L stoicheiometry typically found for the acetate and many other carboxyl-ates4 We now report the preparation and characterization by crystallographic, magnetic susceptibility, and e.s.r. methods of the first definitive example of a copper(r1) trifluoroacetate adduct of the Cu(O,CR),.L stoicheiometry and the copper(I1) acetate structure.
A light green solid material was obtained by heating the violet bisquinoline adduct Cu(O,CCF,),(quinoline),. 4 b 9 C 9 6 A crystalline sample was obtained by Soxhlet extraction and recrystallization from hot benzene. The crystals are highly hygroscopic, forming a blue product in air. and observed densities are 1-76 and 1-74(2) g ~m -~, respectively. The structure analysis was based on 1555 independent non-zero reflections collected by the 8-28 scan technique with Mo-K, radiation on a Picker diffractometer. The structure was solved in space group C2/c by Patterson and Fourier methods. Full-matrix anisotropic leastsquares refinement for all non-hydrogen atoms, with hydrogen atoms included as fixed contributions, converged to a conventional R factor of 0.062. The molecular structure (Figure) is of the familiar dimeric copper acetate hvdrate tvDe. The Cu-Cu distance is 2.886(2) A, a full 0.272 A longer than the corresponding distance in copper(I1) acetate monohydrate.6 Other structure parameters are unremarkable; the thermal parameters of the CF, groups display considerable anisotropy.
Magnetic susceptibility measurements were made from 80-300 K by the Faraday method on an Alpha/Ainsworth susceptibility system. Corrections for diamagnetic and temperature-independent paramagnetic contributions were applied. The susceptibility data were satisfactorily fitted to the usual expression for an isolated pair of interacting ions of spin l/2.' The best fit was obtained for g = 2.27 and 2 J = -310 cm-l. The effective magnetic moment a t 300 K is 1.42B.M. per Cu2+ ion.
This susceptibility behaviour is similar to that of copper (11)  View Article Online / Journal Homepage / Table of Contents for this issue magnitude of the Cu-Cu interaction, the pKa of the parent carboxylic acid cannot be a major factor in determining the strength of the interaction. (ii) The substantial difference between the magnetic properties of anhydrous copper-(11) trifluoroacetate and those of the dimeric quinoline adduct supports the view2 that the anhydrous salt does not have the bridged dimeric structure. (iii) The large difference in Cu-Cu separation between the magnetically similar acetate and trifluoroacetate adducts demonstrates that the metal-metal distance in these dimers is not an important factor in determining the strength of the Cu-Cu interaction. This point has previously been made in other contexts.1° Support of this work by the National Science Foundation is acknowledged. We thank Dr. David Jeter for experimental assistance and helpful discussions.