Reduced Anionic Mn₁₂Molecules with Half-Integer Ground States as Single-Molecule Magnets

Abstract

The preparation, characterization, and X-ray structure are reported for the single-molecule magnet (PPh4)[Mn12O12(O2CPh)16(H2O)4]·8(CH2Cl2) (2). Complex 2 crystallizes in the triclinic space group P1̄, which at 213 K has a = 17.2329(2), b = 17.8347(2), c = 26.8052(2) Å, α = 90.515(2), β = 94.242(2), γ = 101.437(2)°, and Z = 2. The salt consists of PPh4+ cations and [Mn12O12(O2CPh)16(H2O)4]- anions. The (Mn12O12)15+ core of the anion is formed by an external ring of eight Mn atoms bridged by μ3−O2- ions to an internal tetrahedron of four Mn atoms. Because of disorder in both phenyl rings and solvate molecules, it was difficult to use bond valence sum values to determine definitively the oxidation state of each Mn atom. There is a Mn4O4 cubane unit in the internal part of the molecule and these Mn atoms are all MnIV ions. For the eight "external" Mn atoms the bond valence sum values did not define well their oxidation states. For these eight Mn atoms, it was not possible to determine whether a trapped-valence MnIIMnIII7 or an electronically delocalized description is appropriate. High-frequency EPR (HFEPR) data were collected for the previously structurally characterized MnIV4MnIII7MnII valence-trapped salt (PPh4)[Mn12O12(O2CEt)16(H2O)4] (1) at 328.2 and 437.69 GHz. In the high magnetic field the crystallites orient and the HFEPR spectra are pseudo−single-crystal like, not powder patterns. The spectral features are attributed to the fine structure expected for a S = 19/2 complex experiencing axial zero-field splitting DŜz2, where D = −0.62 cm-1. The sign of D was definitively determined by the temperature dependence of the spectrum. Complex 2 exhibits one out-of-phase ac magnetic susceptibility (χ' 'M) signal in the 3−6 K range. The temperature of the χ' 'M peak is frequency dependent, as expected for a single-molecule magnet. The rate at which the direction of magnetization reverses from "up" to "down" was evaluated from χ' 'M data collected at various frequencies (1−1512 Hz) of oscillation of the ac magnetic field. This gives magnetization relaxation rates in the 2.86−4.51 K range for complex 2 and in the 3.2−7.2 K range for complex 1. Rates were also determined in the 1.80−2.50 K range for complex 1 via magnetization decay experiments. In this latter case, the polycrystalline sample is magnetically saturated in a large dc field. After the magnetic field is rapidly decreased to zero, the decay of the magnetization to zero is monitored. The rates evaluated by both the frequency dependence of the out-of-phase ac signal and dc relaxation decay experiments for complex 1 fit on an Arrhenius plot to give an activation energy of Ueff = 57 K and a preexponential rate of 1/τ0 = 7.2 × 107 s-1. From the HFEPR data, complex 1 has a S = 19/2 ground state with D = −0.62 cm-1. This gives a potential-energy barrier of U = 79 K for the double-well potential-energy diagram. The value of Ueff is less than the barrier height U, because when individual [Mn12-] anions convert from spin "up" to spin "down", they can not only be thermally activated to go over the U = 79 K barrier, they can also quantum mechanically tunnel through the barrier between ms = −n and ms = n levels. A multiphonon Orbach process involving molecules absorbing phonon energies and being excited incrementally from one ms level to another is likely involved at these low temperatures below 10 K.