Binuclear Mn2+ complexes of a biphenyltetracarboxylic acid with variable N-donor ligands: syntheses, structures, and magnetic properties

Abstract

Four new manganese(II) metal–organic frameworks based on H₄bpta (H₄bpta = 3,3′,5,5′-biphenyltetracarboxylic acid) and N-donor ligands, namely [Mn₂(4,4′-bipy)(bpta)]_n (1), [Mn₂(1,4-bib)₂(bpta)]_n·2n(H₂O·DMF) (2), [Mn₂(1,3-bimb)₂(bpta)]_n·2nH₂O (3), and [Mn₂(1,4-bimb)₂(bpta)]_n·2nH₂O (4) (4,4′-bipy = 4,4′-bipyridine, 1,4-bib = 1,4-bis(imidazol-1-yl)benzene, 1,3-bimb = 1,3-bis((1H-imidazol-1-yl)methyl)benzene, and 1,4-bimb = 1,4-bis((1H-imidazol-1-yl)methyl)benzene), have been obtained under solvothermal conditions. In these complexes, the fully deprotonated bpta⁴⁻ ligands link binuclear SBUs to construct 2D layers with (4,4)-connected topologies. Complex 1 is a 3D network with 1D zigzag chains built up from 4-connected bpta⁴⁻ ligands that contain carboxylate groups bridging Mn²⁺ ions in a rather rare syn–syn geometry. Complex 2 exhibits a 3D porous network with syn–anti bridging dimeric subunits. Complex 3 is a (3,4)-connected 2D novel network with double helix chains formed by cis-1,3-bimb linkers. The structure of complex 4 is similar to that of 2, but it consists of tiny pores due to trans-1,4-bimb linkers. Magnetic studies reveal that 1 is perfectly fitted by the modified Fisher model to yield an effective intrachain antiferromagnetic coupling (J = −2.21(1) cm⁻¹). Complexes 2–4 show antiferromagnetic interactions (J = −1.57(2) cm⁻¹ for 2, and J = −0.68(2) cm⁻¹ for 3) in dimeric units of Mn₂(μ-COO)₂²⁺ related to double syn–anti and syn–syn carboxylate bridges. An important repetitive feature of all these complexes with syn–anti or syn–syn carboxylate bridges is that the antiferromagnetic coupling is very small. In addition, the gas sorption exploration of 2 exhibits relatively high sorption of CO₂ but very low sorption of N₂, making it a promising material for CO₂/N₂ separation.