Structural evolution and optical/magnetic modulation of Mn(II)-based MOFs driven by the configuration of imidazolylbenzene derivatives

Three Mn(II)-based MOFs, namely, [Mn 2 (mta) 2 (1,3-bmib)(H 2 O)]·xGuest (HCU51), [Mn 2 (mta) 1 . 5 (1,3,5-tib)(H 2 O)]·xGuest (HCU52), and [Mn(mta)(1,2,4,5 -tib)]·xGuest (HCU53), have been synthesized under solvothermal conditions. The synthesis employed 2-methoxyterephthalic acid (H 2 mta) as the primary linker, together with three imidazolylbenzene derivatives of distinct denticity: 1,1'-(5-methyl-1,3-phenylene)bis(2-methyl-1H-imidazole) (1,3-mpmi), 1,3,5-tri(1H- imidazole-1-yl)benzene (1,3,5-tib), and 1,2,4,5-tetra(1H-imidazole-1-yl)benzene (1,2,4,5-tib). Structural analyses reveal that the diverse geometries of ancillary ligands dictate a evolution of the metal secondary building units (SBUs): HCU51 features a dinuclear [Mn 2 (H 2 O)] 4+ core bridged by a μ 2 -water molecule, HCU52 contains a tetranuclear [Mn 4 (OH) 2 ] 6+ cluster induced by μ 3 -OH - anions, and HCU53 adopts a dinuclear [Mn 2 (CO 2 ) 2 ] 2+ unit. Concomitantly, the framework topologies transform from a 3,7-connected network (HCU51), to an interpenetrated (3,10)-connected net (HCU52), and finally to a 4,8-connected pillared-layer architecture (HCU53). Solid-state diffuse reflectance spectroscopy gives apparent optical absorption edge gaps of 3.54 and 2.92 eV for HCU51 and HCU53, respectively, indicating a clear narrowing of the optical absorption edge gaps with increasing denticity of the ancillary ligand. Variable-temperature magnetic susceptibility measurements show that HCU51 and HCU53 exhibit only weak antiferromagnetic coupling (Weiss constants θ = −7.89 and −6.63 K, respectively), whereas HCU52 displays remarkably strong antiferromagnetic interactions (θ = −80.55 K).