Synthesis, structure and magnetism of two hexameric copper(II) complexes: Diaquabis[μ-(4-bromobenzoato- O,O′)]-tetrakis(4-bromobenzoato)hexakis[μ-(2-dimethylaminoethanolato)]hexacopper(II) and Bis[μ 3-(2-dibutylaminoethanolato)-μ-(2-dibutylaminoethanolato)] tetrakis-(μ-dichloroacetato- O,O′)-bis(dichloroacetato)-di-μ 3-hydroxo-hexacopper(II)

Abstract

The title compounds, 1 and 2, have been crystallized and examined by spectroscopy, magnetism and X-ray diffraction techniques. The 4-bromobenzoate compound crystallizes in the triclinic space group P 1 , with a=10.897(4), b=12.496(6), c=16.260(11) Å, α=69.33(4), β=95.59(4), γ=102.00(4)°, Z=1, M r=2146.2, D x=1.76 g cm −3, V=2025(2) Å 3 and R=0.047 for 4015 observed reflections. The centrosymmetric overall structure is composed of three dimers, which are bridged into a hexametallic unit by carboxylato oxygen atoms with apical CuO distances of 2.575(7) and 2.651(5) Å. Of the three ethanolato-bridged dimers, one is centrosymmetric and the other two are non-centrosymmetric. Within the Cu 2O 2 dimeric units the CuOCu angles are 96.7(2) and 97.5(3) for the non-symmetric dimer and 102.1(3)° for the symmetric dimer and the CuCu distances are 2.896(2) and 2.988(2) Å, respectively. The dichloroacetate compound 2 crystallizes in the triclinic space group P 1 , with a=10.414(7), b=13.884(9), c=15.809(9) Å, α=109.96(6), β=106.76(6), γ=78.21(6)°, Z=1, M r=1872.2, D x=1.52 g cm −3, V=2042(2) Å 3 and R=0.052 for 3257 observed reflections. The crystal structure consists of discrete hexameric complex molecules, considered as tetrahedral Cu 4 units sharing a common edge, arranged in a distorted octahedral geometry. The copper atoms in the centrosymmetric molecule are linked together by carboxylate groups, ethanolato oxygen atoms and triply bridging hydroxo oxygen atoms. The distances between the hydroxo O and Cu atoms are 1.972(5)–2.003(5) Å and the CuOCu angles are 99.8(3)–130.4(3)°. The six CuCu distances in the distorted tetrahedral units vary between 3.037(1) and 3.608(1) Å. The spectroscopic and magnetic properties of both compounds agree with the observed X-ray structures. For 1 the coordinated water molecules can be distinguished in the IR spectra, as can the OH groups in compound 2. The ligand-field spectra show broad bands, in agreement with different coordination geometries for the several copper compounds. Magnetic exchange in both compounds is quite strong, and only in the 80–300 K temperature range is the Curie-Weiss law observed. In 1 and 2 the values of θ are −72 and −153 K, respectively. The magnetic moment of 1 varies from 1.30 to 1.53 BM between 80 and 300 K, whereas in 2 the magnetic moment varies almost linearly from 1.0 BM at 4 K, to 1.4 BM at 300 K, indicating a complicated exchange mechanism in the hexanuclear unit. The high-temperature magnetic moments obtained by extrapolation for 1 and 2 are 1.70 and 1.75 BM, respectively, in agreement with S= 1 2 systems. EPR powder spectra show weak signals, and only at 77 K can some hyperfine splitting (probably due to minute amounts of paramagnetic species) be seen on top of the broad lines originating from the hexanuclear clusters.