A robust microporous 3D cobalt(ii) coordination polymer with new magnetically frustrated 2D lattices: single-crystal transformation and guest modulation of cooperative magnetic properties

Abstract

A microporous 3D cobalt(II) coordination polymer featuring pillared layers [Co₂(ma)(ina)]_n·2nH₂O (1·2H₂O) (ma = malate, ina = isonicotinate) was generated by hydrothermal treatment with a void volume of 25.8%, in which the in-situ generated ma ligands connect the Co^II ions into a 2D lattice with mixed and multiple exchange-bridges, affording a new geometrical topology different from the Kagomé lattice and leading to spin frustration. The rigid ina-pillared metallic-layered structure could retain 3D structural ordering upon guest removal and exchange. By soaking guest-free host (1) in MeOH and methanamide (HCONH₂) solutions, single crystals of dehydrated 1 were transformed into single crystals of 1·MeOH and 1·HCONH₂, respectively, without apparent host-structural changes. 1 can also be rehydrated into 1·2H₂O′. The guest-inclusion crystals have been characterized by X-ray single-crystal diffraction at 293 K and 93 K, confirming the single-crystal-to-single-crystal transformations and providing detailed information of the guest molecules confined in the subnanospace and host–guest and/or guest–guest hydrogen-bonding interactions. The magnetic behaviours of this family of porous magnetic materials are complex due to the influences of multiple metal sites, intra- and inter-layer exchanges, spin–orbit coupling, as well as geometrical frustration, which show magnetic ordering at <2 K, 3.5 K, 3.5 K, and 8 K for 1, 1·MeOH, 1·HCONH₂, and 1·2H₂O, respectively, due to the different size of guest molecules along with the different host–guest interaction, which may slightly modify the path of magnetic exchange, decrease the intensity of the spin-frustration in the 2D lattice, and cooperatively enhance the magnetic ordering temperature.