Bivariate metal-organic frameworks with tunable spin-crossover properties.

In this work, we have screened out pyrazine (A), aminopyrazine (B), quinoxaline (C), and 5,6,7,8-tetrahydroquinoxaline (D) among a large number of pyrazine derivatives to construct Hofmann-type MOFs Fe(L)[M(CN)4] (M = Pt, Pd) with similar 3D pillared-layer structures. X-ray single crystal diffraction reveals that the alternate linkage between M and Fe(II) ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligands A-D acted as vertical columns to connnect 2D sheets to give 3D pillared-layer structures. Subsequently, a series of bivariate MOFs were constructed by pairwise combination of the four ligands A-D , which were confirmed by 1 H NMR, PXRD, FT-IR and Raman. The results demonstrated that ligand size and crystallization velocity play a dominant role in constructing bivariate Hofmann-type MOFs. More importantly, the SCO properties of the bivariate MOFs can be finely tuned by adjusting the proportion of two pillared ligands in the 3D Hofmann-type structures. Remarkably, the spin transition temperatures, Tc ↑ and Tc ↓ of the Fe( A)x(B)1-x [Pt(CN)4] (x = 0-1) can be adjusted from 239 to 254 K and 248 K to 284 K, respectively. Meanwhile the width of hysteresis loops can be widened from 9 to 30 K. Changing Pt to Pd, the hysteresis loops of Fe(A)x(B)1-x [Pd(CN) 4] can be tuned from 9 K (T c ↑ = 215 K, Tc ↓ = 206 K) to 24 K ( Tc ↑ = 300 K, Tc ↓ = 276 K). This research provides wider implications in the development of advanced bistable materials, especially in precisely regulating SCO properties.