Isomer-Directed Structural Diversity and Its Effect on the Nanosheet Exfoliation and Magnetic Properties of 2,3-Dimethylsuccinate Hybrid Frameworks

The structures of seven new transition metal frameworks
featuring
Mn, Co, or Zn and either the meso or chiral d and l isomers of the 2,3-dimethylsuccinate ligand are reported.
Frameworks that exhibit two-dimensional covalently bonded layers with
weak interlayer interactions can be made with all three cations by
incorporation of the chiral isomers of the 2,3-dimethylsuccinate ligand.
The formation of such structures, suitable for the creation of nanosheets via exfoliation, is, however, not as ubiquitous as is the
case with the 2,2-dimethylsuccinate frameworks since frameworks that
incorporate the meso-2,3-dimethylsuccinate ligand
form three-dimensional structures. This clear distinction between
the formation of structures with covalent connectivity in two and
three dimensions, depending on the choice of 2,3-dimethylsuccinate
isomer, is due to the different conformations adopted by the backbone
of the ligand. The chiral isomer prefers to adopt an arrangement with
its methyl and carboxylate groups gauche to the neighboring
functional groups of the same type, while the meso-ligand prefers to adopt trans geometry. A gauche-arrangement of the methyl groups places them on the
same side of the ligand, making this geometry ideal for the formation
of layered structures; a trans-relationship leads
to the methyl groups being further apart, reducing their steric hindrance
and making it easier to accommodate them within a three-dimensional
structure. The ease of exfoliation of the layered frameworks is examined
and compared to those of known transition metal 2,2-dimethylsuccinate
frameworks by means of UV–vis spectroscopy. It is suggested
that layered frameworks with more corrugated surfaces exfoliate more
rapidly. The size, structure, and morphology of the exfoliated nanosheets
are also characterized. The magnetic properties of the paramagnetic
frameworks reveal that only the three dimensionally covalently bonded
phases containing meso-2,3-DMS in trans-arrangements order magnetically. These frameworks are antiferromagnets
at low temperatures, although the Co compound undergoes an unusual
antiferromagnetic to ferromagnetic transition with increasing applied
magnetic field.