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

The molecular wheel [Cr 10 (OMe) 20 (O 2 CCMe 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.