Preparation and characterization of new Mn6 and Mn8 clusters obtained from the in situ formation of an unprecedented octadentate ligand

The use of 1,3,5-trihydroxybenzene (thbH3) in manganese carboxylate chemistry has been investigated. The reactions of thbH3 with 4 and 6 equivalents of Mn(O2CEt)2 in MeOH afford the complexes [Mn6(O2CEt)8(L)(MeOH)4(H2O)2] (1) and [Mn8O2(O2CEt)14(MeOH)4] (2), respectively. In the case of complex 1, the product of the in situ organic ligand transformation has been observed, namely the conversion of the tridentate thbH3 group to a new octadentate ligand L4−; the latter has never been previously reported. Both complexes possess rare topologies, with 1 containing 6MnII ions, whereas 2 is mixed-valent 6MnII, 2MnIII. The core of 1 consists of two [Mn3(μ3-OR)]5+ triangles linked by the bulky octadentate ligand L. The [Mn8(μ4-O)2(μ-OR)8]6+ core of 2 can be considered an extension of the common [Mn6(μ4-O)2]10+ (4MnII, 2MnIII) core, comprising two edge-sharing tetrahedra, with two additional Mn atoms at one end. Peripheral ligation in both 1 and 2 is provided by eight and fourteen bridging EtCO2- groups, respectively. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1 and 2 in the 1.8–300K range. The magnetic susceptibility data for 1 were fit to the theoretical χM vs T expression, derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, for two essentially non-interacting triangular [Mn3(μ3-OR)(μ-OR)2]3+ units. The fitting procedure revealed the two pairwise exchange parameters to be weakly ferromagnetic (Jbasal=J′=+0.79(3)cm−1) and antiferromagnetic (Jside=J=−2.04(3)cm−1), respectively, resulting in an S=5/2 spin ground state. In contrast, the data for 2 revealed dominant antiferromagnetic interactions and a resulting S=0 ground state; the latter value is rationalized in terms of the strong antiferromagnetic coupling within the central Mn2IIIO2 unit.