New Routes to Polymetallic Clusters: Fluoride‐Based Tri‐, Deca‐, and Hexaicosametallic Mn^III Clusters and their Magnetic Properties

Abstract

The syntheses, structures and magnetic properties of three new MnIII clusters, [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1), [Mn(0O6(OH)2(bta)8(py)8F8] (2) and [NHEt3]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 degrees C) between MnF(3) and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1, and consists of a complicated array of metal tetrahedra linked by mu3-O2- ions, mu3- and mu2-OH- ions, mu2-MeO- ions and mu2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50 degrees 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 P1 and consists of a "supertetrahedral" [Mn(III)10] core bridged by six mu3-O2- ions, two mu3-OH- ions, four mu2-F- ions and eight mu2-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 P2(1)/c and has a structure analogous to that of the basic metal carboxylates of general formula [M3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with mu2-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 chi(M)T decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of chi(M)T is 10 cm(3) K mol(-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 chi(M)T falls to a value of approximately 5.0 cm(3) K mol(-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 chi(M)T 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 J(a)=-2.95 cm(-1) and J(b)=-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.

Tetrahedra, Super‐Tetrahedra, Bipyramids, Boxes and More: Polymetallic Clusters of Benzotriazole

“…The loops do not show the step-like features associated with quantum tunneling of magnetization (QTM), and appear typical for a cluster with a distribution of energy barriers. The steps may be present but broadened out, due to the inherent disorder associated with the crystal of 7, [21] sulting in a distribution of Mn 3+ environments. These observations have been reported for other large SMMs.…”

“…Repeating the reaction that produces complex 7 but in the presence of pyridine affords the decametallic complex [Mn 10 O 6 (OH) 2 (Bta) 8 (py) 8 F 8 ] (8), while the same reaction in the presence of triethylamine produces the trimetallic species [NHEt 3 ] 2 [Mn 3 O(Bta) 6 F 3 ] (9). [21,25] Complex 8 (Figure 10) consists of a "supertetrahedral" [Mn and J b = -2.12 cm -1 with two S = 1 states at 1.46 cm -1 and 5.9 cm -1 , and an S = 2 state at 6.1 cm -1 above the ground state. [21] The presence of low-lying excited states leads to obvious problems when trying to satisfactorily fit the experimental data.…”

“…[21,25] Complex 8 (Figure 10) consists of a "supertetrahedral" [Mn and J b = -2.12 cm -1 with two S = 1 states at 1.46 cm -1 and 5.9 cm -1 , and an S = 2 state at 6.1 cm -1 above the ground state. [21] The presence of low-lying excited states leads to obvious problems when trying to satisfactorily fit the experimental data. The ground state of this molecule can be changed from S = 0 to S = 2 by small changes in the exchange interaction J b .…”

“…[20] For MnF 3 this insolubility can be easily overcome by using either a "melt" reaction whereby the BtaH acts as the solvent (MPt = 100°C) in which the MnF 3 dissolves as the reaction proceeds, [21] or by the simple reflux of the reaction mixture in a particular solvent. 32+ core of 7 can be described as a central distorted tetra-face-capped octahedron either side of which are attached vertex-sharing tetrahedra.…”

Relieving frustration: The case of antiferromagnetic Mn3molecular triangles

“…The triangular [Mn 3 -(μ 3 -oxo)] motif seen at the core of the molecule in Fig. 1 has been extensively studied in this regard [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. For a long time, it was believed that this topology would result exclusively in antiferromagnetic (AF) coupling and, hence, to a low-spin ground state.…”