New routes to polymetallic clusters: Fluoride-based tri-, deca-, and hexaicosametallic MnIII clusters and their magnetic properties

The syntheses, structures and magnetic properties of three new Mn III clusters, [Mn26O17(OH)8(OMe) 4F10-(bta)22(MeOH)14(H 2O)2] (1), [Mn10O6-(OH) 2(bta)8(py)8F8] (2) and [NHEt 3]2-[Mn3O(bta)6F3] (3), are reported (bta = anion of benzotriazole), thereby demonstrating the utility of MnF3 as a new synthon in Mn cluster chemistry. The "melt" reaction (100°C) between MnF3 and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O 17(OH)8(OMe)4F10-(bta) 22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1̄, and consists of a complicated array of metal tetrahedra linked by μ3-O2- ions, μ3- and μ2- OH- ions, μ2-MeO- ion and μ2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50°C) also produces complex 1. If this reaction is repeated in the presence of pyridine, the decametallic complex [Mn10O6(OH)2(bta) 8(Py)8F8] (2) is produced. Complex 2 crystallizes in the triclinic space group P1̄ and consists of a "supertetrahedral" [MnIII10] core bridged by six μ3-O2- ions, two μ3-OH- ions, four μ2-F- ions and eight μ2-bta - ions. The replacement of pyridine by triethylamine in the same reaction scheme produces the trimetallic species [NHEt3] 2[Mn3O(bta)6F3] (3). Complex 3 crystallises in the monoclinic space group P21/c and has a structure analogous to that of the basic metal carboxylates of general formula [M 3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with μ2-bta- ligands bridging each edge of the triangle and the fluoride ions acting as the terminal ligands. DC magnetic susceptibility measurements in the 300-1.8 K and 0.1-7 T ranges were investigated for all three complexes. For each, the value of χMT decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of χMT is 10 cm3 Kmol-1 and fitting of the magnetisation data gives S = 4, g = 2.0 and D = -0.90 cm-1. For complex 2, the value of χMT falls to a value of approximately 5.0 cm3 Kmol-1 at 1.8 K, which is consistent with a small spin ground state. For the triangular complex 3, the best fit to the experimental χMT versus T data was obtained for the following parameters: Ja = -5.01 cm-1, Jb = +9.16 cm-1 and g = 2.00, resulting in an S = 2 spin ground state. DFT calculations on 3, however, suggest an S = 1 or S = 0 ground state with Ja = -2.95 cm-1 and Jb = -2.12 cm-1. AC susceptibility measurements performed on 1 in the 1.8-4.00 K range show the presence of out-of-phase AC susceptibility signals, but no peaks. Low-temperature single-crystal studies performed on 1 on an array of micro-SQUIDS show the time- and temperature-dependent hysteresis loops indicative of single-molecule magnetism behaviour.