Simpler is better: A heterometallic (Mn-Na) metal organic framework (MOF) with a rare myc topology synthesized from bench chemicals for selective adsorption and separation of organic dyes

The present work summarizes the design of a new heterometallic (Mn-Na) metal organic framework (MOF) synthesized under solvothermal conditions. The single crystal X-ray analysis uncovers its composition as [MnNa(HCOO)₃]_n. The crystallography reveals the presence of two metal ions in the asymmetric unit i.e., Mn and Na which are connected through a bridging formate ligand. Both Mn(II) and Na(I) are coordinated by six oxygen atoms thus forming a distorted octahedral geometry around each metal ion. The HCOO⁻ ligand acts as a bridge to propagate the structure along the three axis resulting in a 3D MOF. Each oxygen of the formate is coordinated to both Mn and Na. Topological analysis shows a rare myc topology of the MOF which is very uncommon among the coordination polymers reported so far. The resulting Mn-Na shows excellent selectivity, in the aqueous phase, for adsorption and separation of cationic dye, methylene blue (MB) with an adsorption capacity to be 46.22 mg g⁻¹. Moreover, this adsorbent material showed excellent structural stability and recyclability; and the pseudo-second-order kinetics for adsorption. The plausible mechanism suggests that the HSAB concept is responsible for the adsorption process. Hard donors of dye molecules build strong interactions with hard Na⁺ ions embedded in MOF. Thus, the designed MOF is the first example in heterometallic MOF chemistry to show dye adsorption and is established as a simple, easy to prepare and low-cost polymer for environmental protection for future endeavours.