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Molecular Materials - Cyanide

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A major interest in physics, chemistry & materials science is the interface between macroscopic and microscopic realms. One traditional method of attaining magnetic materials at the nanoscale level is by the mechanical preparation of very small magnetic particles, the “Top-Down” approach.  Unfortunately, this approach suffers from the distribution of the size of nanoparticles, as well as their non-uniform dispersion on a surface. To further explore nanoscale magnetism, there is growing emphasis towards building these materials via a “Bottom-Up” synthetic approach, which takes advantage of the intrinsic physical properties of a molecule, not the bulk material.

Our approach towards obtaining these molecules is to rationally design magnetic architectures by bridging transition metal ions with cyanide, an excellent ligand for magnetic exchange.  Inspired by the family of Prussian Blue analogues, which exhibit interesting magnetic properties, we are able to isolate metal cyanide clusters of desired geometries that give rise to a variety of magnetic phenomena, e.g.  spin-crossover, photomagnetism, and single molecule magnetism. Our current goals are to use the building block approach (aka molecular lego®) to determine which metal combinations result in clusters with these properties, to study the effect of ion anisotropy on the magnetic properties of these clusters, and to  use the clusters as building blocks for the synthesis of multi-functional networks (1D, 2D, and 3D).

The Building Block Approach

In order to obtain discrete polynuclear cyanide complexes, one must use an appropriate capping ligand to block coordination sites on the metal ion, the result of which is a mononuclear convergent building block. This convergent precursor limits the growth of extended structures and, when combined with divergent building blocks (e.g., homoleptic cyanometalates or fully solvated metal ions), leads to molecular cyanide complexes that, in most cases, can be isolated as single crystals.

The Dunbar group has capitalized on such a building-block or "modular" approach. This method has an excellent track record for leading to families of related clusters whose analyses allow for precise correlation of structure with magnetic properties.

Our principal objectives at the present time are:

• Investigate the role of metal ions with strong, first order spin-orbit coupling in determining the magnetic properties of discrete as well as extended phase cyanide compounds.

• Test theoretical models that have been reported on new derivatives of our cyanide-based trigonal bipyramidal clusters as well as smaller clusters of the linear trinuclear and pentanuclear type, and to develop, with our collaborators in Moldova and Israel, new models to explain and predict the SMM, spin-crossover and charge-transfer induced spin transition behavior of the new cyanide clusters.

Figure 1. The Building Block Approach

The shape adopted by the cyanide-bridged core in these clusters is dictated by the topology of the available coordination sites (those occupied by labile ligands or cyanide groups) of the convergent building block.

The finite structure of the discrete clusters and their well-defined metal coordination environments allow for the establishment of magnetostructural correlations and the development of theoretical models that can be used as a starting point for the analysis of extended magnetic structures.

Figure 2. Discrete Clusters or Extended Networks Obtained Using the Building Block Approach

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Department of Chemistry | Texas A&M University | State of Texas