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

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 2−5 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 1−4/5 shows a monotonic decrease with electron addition that is consistent with the progressive loss of Mn^III 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.