(NBu4)Ni(mnt)2: A novel bistable high temperature spin-Peierls-like system

The title compound, tetrabutylammonium bis(1,2-dimercaptoethene-1,2-dicarbonirtilato-S,S′)-nickel (III) [henceforth (NBu4)Ni(mnt)2] undergoes a hysteretic phase transition that occurs at 172 K upon cooling. Crystallographic studies show that the phase transition is an order–disorder type involving one arm of the NBu4+ cation. Both phases are monoclinic, space group C2/c with a = 30.860(3), b = 13.804(2), c = 15.678(2) Å and β = 115.642(3)° with V = 6020.8(11) Å3 at room temperature and with a = 31.073(3), b = 13.928(1), c = 14.414(1) Å and β = 115.073(2)° and V = 5652.1(9) Å3 at 84 K. In both phases, the anions form non-uniform stacks parallel to the crystallographic c direction. In the high temperature phase, the Ni–S distances between anions are accidentally equal, and the stacks behave as nearly uniform antiferromagnetic chains with nearly equal exchange constants. At the phase transition, a non-uniform compression of the stacks occurs and the magnetic moment drops rapidly, indicating a magnetic dimerization of the stacks. Susceptibility and Cp measurements show an approximate 7 °C thermal hysteresis.

The title compound contains novel stacks of antiferromagnetically coupled [Ni(mnt)2]− anions. A structural phase transition takes place at 170 K that leads to an increase in the antiferromagnetic interactions, leading to rapid drop in the magnetic susceptibility in the low temperature phase. The phase transition is driven by thermally induced disorder in one arm of the tetrabutylammonium cations. A 7° hysteresis in the phase transition is observed.Figure optionsDownload as PowerPoint slide