A Solvent-Controlled Switch of Manganese Complex Assemblies with a β-Diketonate-Based Ligand

The coordination properties of the new polynucleating ligand H3L1 (1,3-bis(3-oxo-3-phenylpropionyl)-2-hydroxy-5-methylbenzene) with MnII/III are described. Depending on the solvent used, the reaction of H3L1 with Mn(OAc)2
yields either of the two new multinuclear assemblies [Mn2(HL1)2(py)4] (1) and [Mn3(HL1)3] (2), as revealed by X-ray
crystallography. The structure of 2 is remarkable in that it shows a unique asymmetric triple-stranded helicate.
Complexes 1 and 2 can be interconverted by controlling the solvent of the reaction system, and therefore, this
ensemble constitutes an interesting externally addressable switch. In the presence of MnIII/pyridine, partial degradation
of H3L1 occurs via oxidative cleavage, and the new complex [Mn2(L2)2(py)4] (3) is formed. The crystal structure of
this complex has shown the fully deprotonated form of the new donor H3L2 (3-(3-oxo-3-phenylpropionyl)-5-methylsalicylic acid). From the same reaction, the MnII complex 1 is also obtained. A rational synthesis of H3L2 is
reported, and this has been used to prepare 3 in high yields, directly from its components. Variable-temperature
magnetic susceptibility (χm) measurements were performed on complexes 1−3 under a magnetic field of 1 kG. The
data for each complex were fit to the appropriate χm vs T theoretical equation, respectively. In 1, the MnII ions are
uncoupled, with g = 2.01. The data from 2 were fit by assuming the presence of an exchange coupled MnII···MnII
pair next to a magnetically isolated MnII center. The fit gave J = −2.75 cm-1, g12 = 1.97, and g3 = 1.92, respectively.
In 3, two models fit the experimental data. In the most satisfactory, the MnIII ions are coupled antiferromagnetically
with J = −1.48 cm-1 and g = 1.98 and a term for weak ferromagnetic intermolecular exchange is included with
zJ‘ = 0.39 cm-1. The other model contemplates the presence of two uncloupled zero field split MnIII ions.