American Chemical Society
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A Fourth Isolated Oxidation Level of the [Mn12O12(O2CR)16(H2O)4] Family of Single-Molecule Magnets

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journal contribution
posted on 2007-12-10, 00:00 authored by Rashmi Bagai, George Christou
The Mn12 family of single-molecule magnets (SMMs) has been extended to a fourth isolated member. [Mn12O12(O2CR)16(H2O)4] (1) exhibits three quasi-reversible one-electron-reduction processes at significantly higher potentials than [Mn12O12(O2CMe)16(H2O)4]. This has allowed the previous generation and isolation of the one- and two-electron-reduced versions of 1 to now be extended to the three-electron-reduced complex. For cation consistency and better comparisons, the complete series of complexes has been prepared with NPrn4+ counterions. Thus, complex 1 was treated with 1, 2, and 3 equiv of NPrn4I, and this led to the successful isolation of (NPrn4)[Mn12O12(O2CCHCl2)16(H2O)4] (2), (NPrn4)2[Mn12O12(O2CCHCl2)16(H2O)4] (3), and (NPrn4)3[Mn12O12(O2CCHCl2)16(H2O)4] (4), respectively. Another three-electron-reduced analogue (NMe4)3[Mn12O12(O2CCHCl2)16(H2O)4] (5) was prepared by the addition of 3 equiv of NMe4I to 1. Direct current magnetization data were collected on dried microcrystalline samples of 25 and were fit by matrix diagonalization methods to give S = 19/2, D = −0.35 cm-1, and g = 1.95 for 2; S = 10, D = −0.28 cm-1, and g = 1.98 for 3; S = 17/2, D = −0.25 cm-1, and g = 1.91 for 4; and S = 17/2, D = −0.23 cm-1, and g = 1.90 for 5, where D is the axial zero-field splitting parameter. Thus, the [Mn12]3- complexes 4 and 5 possess significantly decreased absolute magnitudes of both S and D as a result of the three-electron addition to 1, which has S = 10 and D = −0.45 cm-1. The D value of the series 14/5 shows a monotonic decrease with electron addition that is consistent with the progressive loss of MnIII ions, which are the primary source of the molecular anisotropy. Nevertheless, when studied by ac susceptibility techniques, the [Mn12]3- complexes still exhibit frequency-dependent out-of-phase susceptibility signals at ≤2.5 K, indicating them to be single-molecule magnets (SMMs), albeit at lower temperatures compared with 1 (6−8 K range), 2 (4−6 K range), and 3 (2−4 K range); the shifts to lower temperatures reflect the decreasing S and D values upon successive reduction and hence the decreasing energy barrier to magnetization relaxation. Thus, the [Mn12]3- complexes represent a fourth isolated oxidation level of the Mn12 family of SMMs, by far the largest range of oxidation levels yet encountered within single-molecule magnetism.