Investigations of magnetic and thermal expansion properties of two cobalt(II) metal-organic frameworks constructed by mixed bipyridyl-tetracarboxylate strategy

Abstract

We report the syntheses, crystal structures, adsorption behavior, thermal expansion behavior, and magnetic properties of two cobalt(II) metal-organic frameworks (MOFs), {[Co(btca)1/2(bpe)(H2O)2]·bpe}n (1) and {Co(btca)1/2(bpe)(DMF)}n (2, H4btca = 1,2,4,5-benzenetetracarboxylic acid; bpe = 1,2-di(4-pyridyl)ethane), constructed using mixed bipyridyl and tetracarboxylate ligands. Single-crystal X-ray diffraction (SC-XRD) indicates that the change of synthetic solvents leads to a significant tuning of structural dimensionality from 2D to 3D framework. Temperature-dependent SC-XRD reveals anisotropically negative and positive thermal expansion of the compounds identified along different crystallographic axes within the frameworks over a 120–400 K temperature range. N2 adsorption measurements indicate a nonporous and ultramicroporous framework for 1 and 2, respectively. Magnetic measurements indicate an easy-plane magnetic anisotropy of the Co2+ ions in the MOFs, with experimental D values of 63.1 and 87.1 cm−1. The ac susceptibility measurements of 1 and 2 show field-induced slow magnetic relaxation below 6 K through different relaxation dynamics. This work not only provides two new cobalt(II) SIM-MOFs, but also presents an effective bipyridyl-tetracarboxylate strategy for designing and building framework materials with interesting magnetic and mechanical properties.