Broad-range spin-crossover modulation in guest-responsive 2D Hofmann-type coordination polymers

Abstract

Rationalizing the role of chemical functionalisation in the synergy between spin crossover (SCO) and guest adsorption properties in porous Fe^II coordination polymers is a central topic in the switchable materials field. However, obtaining meaningful magneto-structural information requires the comparison between structural platforms whose topology remains unaltered upon chemical function exchange. Here, based on the previously reported SCO two-dimensional Hofmann-type compounds {Fe(NH₂Pym)₂[M(CN)₄]}·xguest (1(NH₂)^M·xguest; 5-NH₂Pym = 5-aminopyrimidine; M = Pd^II, Pt^II; guest = H₂O, MeOH, EtOH; x = 0–1), we present the isostructural networks 1(OH)^M·xguest resulting from the replacement of the 5-NH₂Pym axial ligand with 5-OHPym (5-hydroxipryrimidine). The evaluation of their temperature-dependent magnetic and calorimetric data reveals that whereas the unsolvated counterparts (1(NH₂)^M and 1(OH)^M) undergo complete one step spin transitions with small variations in their SCO temperatures (T_c), the solvated species markedly differ not only in T_c but also in the shape of the spin transition curve. A relevant example is observed for 1(NH₂)^M·0.5MeOH and 1(OH)^M·0.5MeOH, which present incomplete one-step and complete three-step spin transitions, respectively. These changes are also complementary to the higher adsorption capacities of the hydroxy-functionalised compounds according to the guest uptake isotherms, powder X-ray diffraction and thermogravimetric data. The single crystal-to-single crystal transformations occurring upon the sorption–desorption of guest molecules have permitted us to determine the structural reasons explaining the variation of the SCO and adsorption properties with chemical functionalisation.