Fourth isolated oxidation level of the [Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets

The Mn-12 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 NPr4n+ counterions. Thus, complex 1 was treated with 1, 2, and 3 equiv of NPr4n+, and this led to the successful isolation of (NPr4n)[Mn12O12(O2CCHCl2)(16)(H2O)(4)] (2), (NPr4n)(2)[Mn12O12(O2CCHCl2)(16)(H2O)(4)] (3), and (NPr4n)(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 [Mn-12](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 [Mn-12](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 [Mn-12](3)-complexes represent a fourth isolated oxidation level of the Mn-12 family of SMMs, by far the largest range of oxidation levels yet encountered within single-molecule magnetism.