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 Fe(II)ions through cyano bridges forms the 2D extended metal cyanide sheets, and ligandsA-Dacted 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 ligandsA-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,T-c up arrow andT(c)down arrow 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 (T-c up arrow=215 K,T-c down arrow=206 K) to 24 K (T-c up arrow=300 K,T-c down arrow=276 K). This research provides wider implications in the development of advanced bistable materials, especially in precisely regulating SCO properties.