Quantification of the Magnetic Anisotropy of a Single‐Molecule Magnet from the Experimental Electron Density

Abstract

Reported here is an entirely new application of experimental electron density (EED) in the study of magnetic anisotropy of single-molecule magnets (SMMs). Among those SMMs based on one single transition metal, tetrahedral Co^II-complexes are prominent, and their large zero-field splitting arises exclusively from coupling between the d <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>x</mml:mi> <mml:msup><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msup> <mml:mtext>-</mml:mtext> <mml:mi>y</mml:mi> <mml:msup><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:msub> </mml:math> and d_xy orbitals. Using very low temperature single-crystal synchrotron X-ray diffraction data, an accurate electron density (ED) was obtained for a prototypical SMM, and the experimental d-orbital populations were used to quantify the d_xy-d <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>x</mml:mi> <mml:msup><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msup> <mml:mtext>-</mml:mtext> <mml:mi>y</mml:mi> <mml:msup><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:msub> </mml:math> coupling, which simultaneously provides the composition of the ground-state Kramers doublet wave function. Based on this experimentally determined wave function, an energy barrier for magnetic relaxation in the range 193-268 cm^-1 was calculated, and is in full accordance with the previously published value of 230 cm^-1 obtained from near-infrared spectroscopy. These results provide the first clear and direct link between ED and molecular magnetic properties.