Origin of the Unusual Ground-State Spin S = 9 in a Cr ₁₀ Single-Molecule Magnet

Abstract

The molecular wheel [Cr ₁₀(OMe) ₂₀(O ₂CCMe ₃) ₁₀], abbreviated {Cr ₁₀}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/ k _B = −0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state ( S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr ₁₀}. For powder samples, the temperature dependence of the susceptibility plotted as χ T( T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M( H) did not saturate at the expected 30 μ _B/fu for 10 FM coupled 3/2 spin Cr ³⁺ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 μ _B/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr ₁₀} wheel is formed by two semiwheels, each with four Cr ³⁺ spins FM coupled ( J _FM/ k _B = 2.0 K), separated by two Cr ³⁺ ions AF coupled asymmetrically ( J ₂₃/ k _B = J ₇₈/ k _B = −2.0 K; J ₃₄/ k _B = J ₈₉/ k _B = −0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/ k _B = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr ₁₀} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr–O–Cr alkoxy bridges coupling the Cr ³⁺ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr ³⁺ ion in the wheel.