Structural studies into the spin crossover behaviour of Fe(abpt)2(NCS)2 polymorphs B and D

Abstract

The spin-crossover behaviour of [Fe(abpt)₂(NCS)₂] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) polymorphs B and D has been studied using single crystal X-ray diffraction to monitor changes in structural features. High pressure single crystal measurements on polymorph B showed that it underwent a monoclinic P2₁/n (Z′ = 0.5) to triclinic P-1 (Z′ = 2 × 0.5) phase transition between 11.5 and 13.5 kbar, at which point it also starts to undergo a thermally inaccessible spin crossover. In polymorph D which also crystallises in the mononclinic space group P2₁/n (Z′ = 2 × 0.5) one of the unique Fe centres undergoes a thermal spin transition. It also displays light-induced excited spin-state trapping (LIESST), and a structure has been obtained at 30 K through continuous irradiation with a 670 nm 5 mW CW laser. In addition high pressure single crystal measurements on polymorph D showed a stepped pressure induced spin transition. At ∼9.6 kbar one of the unique Fe centres had undergone a spin transition and by ∼15 kbar both of the unique Fe centres are shown to be essentially low spin, a situation that is thermally inaccessible. Crystallographic data were collected for both polymorphs using variable temperature or high pressure single crystal X-ray diffraction to allow changes in cell parameters, bond lengths and distortion parameters to be monitored for the spin crossover or phase transition.