Low-Dimensional Magnetism in Multivariate Copper/Zinc MOF-74 Materials Formed via Different Mechanochemical Methods

MOF-74 is an archetypal magnetic metal-organic framework (MOF) family, with metal nodes bridged by 2,5-dioxido-1,4-benzenedicarboxylic acid (H₄dobdc) and arranged into one of the simplest representations of the 1D Ising magnetic model. Recently, a novel mechano-synthetic approach opened a pathway toward a series of bimetallic multivariate (1:1) M1M2-MOF-74 materials, with the uniform distribution of metal cations in the oxometallic chains, offering a unique opportunity to investigate low-dimensional magnetism in these heterometallic MOFs. We explore here how different mechanochemical procedures affect the interaction between the metal nodes of the model system of three multivariate copper(II)/zinc(II)-MOF-74 materials, two of which were obtained through a template-controlled procedure, and the third one was obtained by recently developed mechanical MOF-alloying combined with subsequent accelerated aging. While the three Cu/Zn-MOF-74 products have almost identical powder X-ray diffraction (PXRD) diffractograms and Fourier transform infrared spectra, they differ significantly in their magnetic properties, as revealed through detailed magnetization and X-band and multifrequency high-field electron spin resonance measurements. The magnetic results of the three multivariate Cu/Zn-MOF-74s were compared to the properties of the monometallic Cu-MOF-74, which shows antiferromagnetic intrachain and weaker ferromagnetic interchain interactions. Energy-dispersive X-ray spectroscopy/scanning electron microscopy and solid-state nuclear magnetic resonance spectroscopy helped rationalize the observed differences in magnetization, and in situ synchrotron PXRD monitoring of template-controlled MOF formation revealed different reaction pathways when using the zinc or copper intermediates, involving even the fleeting occurrence of a rare MOF-74 polymorph.