Magnetic and structural studies on copper(II) dialkyldithiocarbamates

The magnetic susceptibilities of a series of copper(II) dialkyldithiocarbamates have been measured over the range 4-290 K, the alkyl groups being methyl, ethyl, isopropyl, and n-butyl. Contrary to a previous report of strong ferromagnetic interaction in the diethyl derivative, we find no evidence of any significant exchange interaction in this compound. The dimethyl and di-isopropyl analogues show weak antiferromagnetic interactions. Only the di-n-butyl derivative in the dialkyl series shows evidence of strong magnetic exchange interaction but of an antiferromagnetic nature; this effect is peculiar to the phase recrystallized from chloroform-light petroleum (α), the phase obtained from chloroform-ethanol (β) showing no such interaction. To seek the origin of the exchange interaction in the α derivative, its crystal structure has been determined by single-crystal X-ray diffraction methods at 295 K and refined by least squares to a residual 0.032 for 1 976 'observed' reflections. Crystals are triclinic, P1, a = 15.29(1), b = 9.963(7), c = 9.243(7) Å, α = 67.94(7), β = 82.92(7), γ = 71.55(7)°, and Z = 2. The copper environment is the usual pseudo-square-planar array of four sulphur atoms from two bidentate ligands (〈Cu-S〉, 2.31 Å), but there is a long fifth interaction [Cu ⋯ S, 2.899(4) Å] through the inversion centre leading to pseudo-dimer formation. Although similar to the diethyl analogue in this respect, differences are observed in regard to (a) the bridging geometry in the 'dimer' and (b) the proximity to the 'dimer sulphur ligands of sulphur atoms from neighbouring dimers at ca. 3.8 Å. The likely relative importance of these two features in determining the origin of the antiferromagnetic coupling is discussed. The structure of the p phase has also been determined, the final residual being 0.036 for 1 324 'observed' reflections. Crystals are monoclinic, P2 ₁ /n, a = 14.593(5), b = 7.840(2), c = 10.822(5) Å, (3 = 101.55(3)°, and Z = 2. The mdecules are located with the copper atoms on crystallographic centres of symmetry, and the only significant intermolecular interactions observed are S ⋯ H contacts. The CuS ₄ entity is planar with 〈Cu-S〉 2.30 Å.