A solvent-controlled switch of manganese complex assemblies with a β-diketonate-based ligand

The coordination properties of the new polynucleating ligand H3L (1,3-bis(3-oxo-3-phenylpropionyl)-2-hydroxy-5-methylbenzene) with Mn-II/III are described. Depending on the solvent used, the reaction of H(3)L1 with Mn(OAC)(2) yields either of the two new multinuclear assemblies [Mn-2(HL1)(2)(py)(4)] (1) and [Mn-3(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 Mn-III/pyridine, partial degradation of H(3)L1 occurs via oxidative cleavage, and the new complex [Mn-2(L2)(2)(py)(4)] (3) is formed. The crystal structure of this complex has shown the fully deprotonated form of the new donor H(3)L2 (3-(3-oxo-3-phenylpropionyl)-5-methylsalicylic acid). From the same reaction, the Mn-II complex 1 is also obtained. A rational synthesis of H(3)L2 is reported, and this has been used to prepare 3 in high yields, directly from its components. Variable-temperature magnetic susceptibility (chi(m)) measurements were performed on complexes 1-3 under a magnetic field of 1 kG. The data for each complex were fit to the appropriate chi(m) vs T theoretical equation, respectively. In 1, the Mn-II ions are uncoupled, with g = 2.01. The data from 2 were fit by assuming the presence of an exchange coupled Mn-II...Mn-II pair next to a magnetically isolated Mn-II center. The fit gave J = -2.75 cm(-1), g(12) = 1.97, and g(3) = 1.92, respectively. In 3, two models fit the experimental data. In the most satisfactory, the Mn-III 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 Mn-III ions.