Magnetic Clusters

Tri- and Tetranuclear Clusters: Introduction. Single-molecule magnets (SMMs) are an interesting class of molecular clusters. These soluble, molecular, single domain super­paramagnetic clusters exhibit high spin ground states, large and negative axial magnetic anisotropy (D < 0), low-symmetry molecular shapes (e.g. disk-shaped, butterfly), and high spin reversal barriers (S²D < 50 cm^-1). The magnetic hysteresis and bistability exhibited by these clusters can theoretically be exploited for nanoscale memory applications. Unfortunately, thermally activated magnetization reversal becomes energetically favorable at extremely low temperatures (ca. 4 K or less) and increasing this “blocking temperature” is of fundamental interest and technological importance.

To significantly enhance the apparent blocking temperatures of SMM clusters, insertion of transition metal centers that possess even greater single-ion anisotropy, either via spin state (large zero-field splitting parameters, D) or orbital anisotropy (large spin-orbit coupling parameters, l) is required. Cyanometalates are excellent building blocks for constructing molecule-based clusters because cyanides generally form linear m-CN linkages between two metal centers, stabilize a variety of transition metal centers and oxidation states, and efficiently communicate spin density information. The sign and magnitude of the local exchange interactions can be controlled via substitution and predicted using simple orbital symmetry arguments. The cyanometalate ions used to construct the SMMs exhibit significant orbital anisotropy, an apparent requirement for SMM clusters, and this suggests that single-molecule magnetism is a general phenomenon.

Recent Results. Given the robust nature of most transition metal cyanide linkages and the relative ease in which they assemble into well-defined structures, we we have found that facially-capped tris- and tetra(pyrazolyl)borate tricyanide complexes are useful for constructing magnetic clusters. The low-spin Tp^R,RFe^III(CN)₃^- (R = H, Me; S = 1/2) building blocks exhibit significant orbital contribution to the magnetic moment (g = 2.9) and upon treatment with a variety of divalent trifluoromethanesulfonate salts, afford a series of isostructural clusters.

The X-ray structures of a trinuclear ("V-shaped"), rectangular, and distorted tetranuclear SMMs are illustrated in Figure 3.

Figure 3a	Figure 3b	Figure 3c

We have prepared several such clusters and are investigating the synthetic and magnetic properties of each. Magnetic data obtained for a rectangular SMM cluster is illustrated in Figure 4.

Octanuclear Clusters: Magnetic Boxes. Utilizing ligands that are less demanding sterically, we have also prepared a variety of molecular boxes. Treatment of LFe^III(CN)₃^- (L = Tp or pzTp) with divalent triflates or perchlorates affords a series of cationic box-shaped clusters of M^III₄M^II₄ stoichiometry (Figure 5).

Additional anionic, neutral, and cationic cluster derivatives can also be prepared via Scheme 3. Magnetic measurements on the sixteen structurally related analogues indicated that one is a single-molecule magnet.

Recent efforts are also investigating how molecular shape anisotropy impacts the magnetic properties exhibited by structrally distorted clusters; structrally distorted (twisted) clusters (rectangles and boxes) appear to be better SMMs than more symmetrical ones (rectangular or box-shaped). Substitition of poly(pyrazolyl)borate and -methane ligands for other polydentate ligands afford even more distorted anaolgues. Magnetic and theoretical studies underway are conducted in collaboration with Gordon T. Yee, Rodolphe Clerac, Corine Mathoniere, and Kyungwha Park.

Photomagnetic Clusters and Networks

Introduction. Several Prussian blue analogues and recently, a cluster containing cobalt and iron centers, were reported to exhibit charge-transfer-induced spin transitions (CTIST). Cyanometalates that exhibit CTIST behavior have intense metal-to-metal charge transfer absorptions in the visible region and these long-lived (below a critical temperature) electron transfer processes are either thermally (cluster) or photochemically (networks) initiated. When electron transfer between electronically coupled metal centers is induced by an external stimuli, dramatic spin state changes are often observed; these electron transfers can be reversed via irradiation or heating of the sample. Consequently, judicious choice of building blocks can in theory, tune the energy of this absorption and allow for the rational design of materials that are photoresponsive. Elaboration of these materials into electronic circuitry may ultimately afford a practical route for constructing photo­responsive molecule-based devices.

Recent Results. We have recently prepared several clusters (Figure 5) and a few networks that appear to exhibit photomagnetic behavior. These studies are conducted in collaboration with Corine Mathoniere and Rodolphe Clerac.