Guest modulation of spin-crossover transition temperature in a porous Iron(II) metal-organic framework: Experimental and periodic DFT studies

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)₄]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29?K wide and average critical temperatures T_c=201?K (1fur), 167?K (1pyr), and 114.6?K (1thio) well below that of the parent compound 1 (T_c=295?K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1fur, while 1pyr and 1thio show 50?% spin transition. For 1fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1pyr and 1thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1fur, 1pyr, 1thio, and previously reported derivatives 1CS₂, 1I, 1bz(benzene), and 1pz(pyrazine) have been carried out to investigate the electronic structure and nature of the host-guest interactions as well as their relationship with the changes in the LS-HS transition temperatures of 1Guest. Geometry-optimized lattice parameters and bond distances in the empty host 1 and 1Guest clathrates are in general agreement with the X-ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T_c and calculated LS-HS electronic energy gap was observed. Finally, specific host-guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.