High Proton Mobility, Solvent Induced Single Crystal to Single Crystal Structural Transformation, and Related Studies on a Family of Compounds Formed from Mn3 Oxo-Clusters

The reaction between 4,4′-sulfonyldibenzoic acid (H2SDBA) and manganese under mild conditions resulted in the isolation of two new three-dimensional compounds, [Mn4(C14H8O6S)4­(DMA)2]­·3DMA, I, and [Mn3(C14H8O6S)3­(DMA)2­(MeOH)]­·DMA, IIa. Both structures have Mn3 trimer oxo cluster units. While the Mn3 oxoclusters are connected through octahedral manganese forming one-dimensional Mn–O–Mn chains in I, the Mn3 units are isolated in IIa. The SDBA units connect the Mn–O–Mn chains and the Mn3 clusters giving rise to the three-dimensional structure. Both compounds have coordinated and free solvent molecules. In IIa, two different solvent molecules are coordinated, of which one solvent can be reversibly exchanged by a variety of other similar solvents via a solvent-mediated single crystal to single crystal (SCSC) transformation. The free lattice DMA solvent molecules in I can be exchanged by water molecules resulting in hydrophilic channels. Proton conductivity studies on I reveals a high proton mobility with conductivity values of ∼0.87 × 10–3 Ω–1 cm–1 at 34 °C and 98% RH, which is comparable to some of the good proton conductivity values observed in inorganic coordination polymers. We have also shown structural transformation of I to IIa through a possible dissolution and recrystallization pathway. In addition, both I and IIa appear to transform to two other manganese compounds [H3O]­[Mn3(μ3-OH)­(C14H8O6S)3­(H2O)]­(DMF)5 and [H3O]2­[Mn7(μ3-OH)4­(C14H8O6S)6­(H2O)4]­(H2O)2­(DMF)8 under suitable reaction conditions. We have partially substituted Co in place of Mn in the Mn3 trimer clusters forming [CoMn2­(C14H8O6S)3­(DMA)2­(EtOH)]­·DMA, III, a structure that is closely related to IIa. All the compounds reveal antiferromagnetic behavior. On heating, the cobalt substituted phase (compound III) forms a CoMn2O4 spinel phase with particle sizes in the nanometer range.