Bivariate Metal-Organic Frameworks with Tunable Spin-Crossover Properties.

In this work, pyrazine (A), aminopyrazine (B), quinoxaline (C), and 5,6,7,8-tetrahydroquinoxaline (D) have been screened out among a large number of pyrazine derivatives to construct Hofmann-type metal-organic frameworks (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 FeII ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligands A-D acted as vertical columns to connect the 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, FTIR, and Raman spectroscopy. The results demonstrated that ligand size and crystallization rate play a dominant role in constructing bivariate Hofmann-type MOFs. More importantly, the spin-crossover (SCO) properties of the bivariate MOFs can be finely tuned by adjusting the proportion of the two pillared ligands in the 3D Hofmann-type structures. Remarkably, the spin transition temperatures, Tc ↑ and Tc ↓ of Fe(A)x (B)1-x [Pt(CN)4 ] (x=0 to 1) can be adjusted from 239 to 254 K and from 248 to 284 K, respectively. Meanwhile, the width of the 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 (Tc ↑=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.