Mn7 Species with an S = 29/2 Ground State: High-Frequency EPR Studies of a Species at the Classical/Quantum Spin Interface

A high spin (S) compound has been synthesized whose properties straddle the interface between the classical and quantum mechanical spin descriptions. The cluster [Mn7O4(pdpm)6(N3)4](ClO4)2 (Mn 7 ) has an unprecedented core structure comprising an octahedral [MnIII 6(μ4-O)(μ3-O)3(μ3-N3)4]6+ unit with one of its faces capped by a MnII ion. Magnetization and susceptibility studies indicate an S = 29/2 ground state, the maximum possible. Variable-temperature, high-frequency electron paramagnetic resonance (HF-EPR) spectra on powder and single-crystal samples of Mn 7 exhibit sharp spectral features characteristic of a quantum spin that are well resolved in a certain temperature range but which transform to a continuum of peaks characteristic of a classical spin in another; these features have been well reproduced by computer simulations. A fast Fourier transform analysis of the sharp spectral features and the low temperature EPR spectra suggests that more than one spin state are involved.