A new twist on an old ligand: a [Mn16] double square wheel and a [Mn10] contorted wheel

Ligand design remains key to the synthesis of coordination compounds possessing specific topologies, nuclearities and symmetries that direct targeted physical properties. N,O-chelates based on ethanolamine have been particularly prolific in constructing a variety of paramagnetic 3d transition metal complexes with fascinating magnetic properties. Here, we show that combining three ethanolamine moieties within the same organic framework in the form of the pro-ligand 1,3,5-tri(2-hydroxyethyl)-1,3,5-triazacyclohexane (LH3) leads to the formation of two highly unusual Mn wheels. Reaction of Mn(NO3)2·6H2O with LH3 in basic methanolic solutions leads to the formation of [MnIII12MnII4(μ3-O)6(μ-OH)4(μ3-OMe)2(μ-OMe)2(L)4(LH)2(H2O)10](NO3)6(OH)2 (1) and [MnIII10(μ3-O)4(μ-OH)4(μ-OMe)4(L)4(H2O)4](NO3)2 (2), the only difference in the synthesis being the ratio of metal:ligand employed. The structure of the former describes two offset [MnIII6MnII2] square wheels, linked through a common centre, and the latter a single [MnIII10] wheel twisted at its centre, such that the top half is orientated perpendicular to the bottom half. In both cases the L3−/LH2− ligands dictate the orientation of the Jahn-Teller axes of the MnIII ions which lie perpendicular to the triazacyclohexane plane. Direct current magnetic susceptibility and magnetisation data reveal the presence of competing exchange interactions in 1 and strong antiferromagnetic interactions in 2. Given the simplicity of the reactions employed and the paucity of previous work, the formation of these two compounds suggests that LH3 will prove to be a profitable ligand for the synthesis of a multitude of novel 3d transition metal complexes.
CJM and TGT thank the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant” (Project Number: 400). EKB thanks the EPSRC for financial support under grant reference numbers EP/I03255X/1 and EP/I031421/1. GL and ME thank the Ministerio de Ciencia e Innovación (RTI2018-094909-J-I00) and CSIC (PIE 201960E002).