Solvent-mediated secondary building units (SBUs) diversification in a series of Mn^II-based metal-organic frameworks (MOFs)

The role of auxiliary solvents in the formation of MOFs has been investigated for a series of Mn^II-based framework systems. Reactions of 4,4′,4″-nitrilotribenzoic acid (H₃L) with Mn^II through varying auxiliary solvents of the medium resulted in the formation of diversified multinuclear Mn^II subunits in four new coordination polymers: [Mn₃(L)(HCOO)₃(DEF)₃] (1), [Mn₃(L)₂(EtOH)₂]·DMF (2), [Mn₅(L)₄(H₂O)₂]·2(H₂NMe₂)⁺·4DMF·2H₂O (3), and [Mn₃(L)₂(py)₄(H₂O)]·H₂O (4) (H₃L=4,4′,4′-nitrilotribenzoic acid, DMF=dimethylformamide, DEF=N,N-diethylformamide, py=pyridine). These four compounds were fully characterized by single-crystal X-ray diffraction, showing interesting SBUs variations. For compound 1, it displays a (3,6)-connected kgd net with wheel [Mn₆] cluster serving as SBU, whereas in 2, the sequence of Mn₃(COO)₉(EtOH)₂ is repeated by inversion centers located between Mn1 and Mn3 to form an infinite Mn-carboxylate chain, which are further interlinked by L^3- ligands to form a 3D architecture. In 3, the pentanuclear Mn₅(CO₂)₁₂ clusters are interlinked to form a layer, which are further pillared by L^3- to generate a 3D network. Compound 4 has a (3,6)-connected network in which the SBU is a linear trimeric Mn₃(COO)₆(py)₄(H₂O) cluster. In addition, the thermal stabilities, X-ray powder diffraction of all the compounds were studied, photoluminescence behaviors of compounds 1, 3 and 4 are discussed.