Increasing the spin ground state and anisotropy in single-molecule magnet species

We are interested in developing metal-cyanide clusters in hopes of producing single-molecule magnets (SMMs). In their ground state, SMMs can exist with a "spin up" or a "spin down" orientation. Consequently, such systems may potentially be used in data storage, where the spin orientation of a molecule represents a single bit. The spin-reversal barrier, U, depends on the square of the total spin, S², and the axial zero-field splitting, D. In order to increase this barrier, we are making new compounds to address these two factors–molecules with a large spin ground state and both building blocks and clusters with greater axial anisotropy.

We have synthesized a jungle gym-like cyano-bridged high-spin cluster, [(cyclen)₁₂Ni₁₃Fe₆(CN)₃₆]⁸⁺, with an overall S = 16 ground state (see figure below, left) via a three-component self-assembly reaction to give a SMM species. Replacement of each metal component with other transition metal ions could represent a strategy to build up even higher spin cyano-bridged clusters. We are working on the synthesis and investigation of magnetic behavior of such clusters with diverse components.

In addition to spin, the overall symmetry of the molecule is a concern because frequently high-spin clusters also have high symmetry. In these cases, symmetry-breaking substitutions can serve to alter the second factor in determining the spin-reversal barrier, the anisotropy. Clusters of the form (MeOH)₂₄M₉M^'₆(CN)₄₈ served as an ideal test case because they have high ground state spins, but their overall symmetry hinders their ability to become single-molecule magnets. We used diamagnetic [ReCN₈]^3– to break the symmetry of these clusters (see figure, above right). This effect was demonstrated using Co₉W₆ clusters which are single molecule magnets due to the high anisotropy of Co^II. This substitution resulted in a change in the barrier to spin inversion, demonstrating that reducing magnetic symmetry affects magnetic anisotropy. 

In addition to breaking the symmetry within a given cluster, we can compare the magnetic properties of clusters in which the same metals comprise the cluster, but different molecular shape results from the blocking ligands about the metals. An example of such a study is the magnetism of low-spin Fe³⁺ and Cu²⁺ bridged by cyanide. In the Fe₈Cu₆ cubic cluster, the measured zero-field splitting, D, is –0.16 cm^–1, whereas in the analogous Fe₂Cu₃ trigonal bipyramid, D is significantly larger, –5.7 cm^–1.

Finally, we are exploring the effects of ligands on the anisotropy of the metal center. We have synthesized the complex, [Cr(dmpe)₂(CN)_X]⁺ (dmpe = bis-dimethylphosphino ethane, X = Cl, Br, I). The low temperature magnetization data were fit with ANISOFIT 2.0 to give the D values shown below. The splitting in the iso-field lines as the halide is changed from Cl to Br to I shows the increase in axial anisotropy generated entirely by the halide (due to an increase in spin-orbit coupling), with the D for the iodide complex being the largest reported for a Cr^III metal center.

Structures and reduced magnetization data for [Cr(dmpe)₂(CN)X]⁺ (X = Cl, Br, I). Solid lines represent fits to the data.