Isomerism in bis(diethylenetriamine)nickel(II) thiocyanate: synthesis, solid-state interconversion and X-ray crystallographic study of sym-fac and mer isomers

Abstract

Bis(diethylenetriamine)nickel(II) thiocyanate has been found to undergo a phase transition (162–180 °C, ΔH= 29.4 kJ mol^–1) involving a low-temperature ordered system (sym-fac isomer 1) and a high-temperature disordered system (mer isomer 2). Both isomers have been synthesised and characterized by X-ray crystallography. Isomer 2 is metastable and reverts to 1 in the presence of a humid atmosphere (relative humidity ca. 60% or more) as heterogeneous catalyst. Isomer 1 crystallizes in the triclinic system, space group P, with a= 7.240(2), b= 8.225(3), c= 8.766(3)Å, α= 63.27(2), β= 74.07(4), γ= 65.56(2)°, Z= 1, R= 0.024. In the cationic unit, the ligand geometry around nickel(II) is nearly octahedral. Two chelating tridentate diethylenetriamine (dien) molecules with terminal amino groups in cis and secondary amino groups in trans position define the sym-fac geometry. Isomer 2 crystallizes in the cubic system, space group F3m, with a=b=c= 12.373(3)Å, Z= 4, R= 0.112. The cation [Ni(dien)₂]²⁺ is three-fold disordered about the [111] direction and the two tridentate dien molecules are co-ordinated meridionally.