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.

Abstract

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.