Preparation and characterization of new Mn 6 and Mn 8 clusters obtained from the in situ formation of an unprecedented octadentate ligand

Abstract

The use of 1,3,5-trihydroxybenzene (thbH 3) in manganese carboxylate chemistry has been investigated. The reactions of thbH 3 with 4 and 6 equivalents of Mn(O 2CEt) 2 in MeOH afford the complexes [Mn 6(O 2CEt) 8(L)(MeOH) 4(H 2O) 2] ( 1) and [Mn 8O 2(O 2CEt) 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 thbH 3 group to a new octadentate ligand L 4−; the latter has never been previously reported. Both complexes possess rare topologies, with 1 containing 6Mn II ions, whereas 2 is mixed-valent 6Mn II, 2Mn III. The core of 1 consists of two [Mn 3(μ 3-OR)] 5+ triangles linked by the bulky octadentate ligand L. The [Mn 8(μ 4-O) 2(μ-OR) 8] 6+ core of 2 can be considered an extension of the common [Mn 6(μ 4-O) 2] 10+ (4Mn II, 2Mn III) 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 EtCO 2 - groups, respectively. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1 and 2 in the 1.8–300 K range. The magnetic susceptibility data for 1 were fit to the theoretical χ M vs T expression, derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, for two essentially non-interacting triangular [Mn 3(μ 3-OR)(μ-OR) 2] 3+ units. The fitting procedure revealed the two pairwise exchange parameters to be weakly ferromagnetic ( J basal = J′ = + 0.79(3) cm −1) and antiferromagnetic ( J side = 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 Mn 2 III O 2 unit.