High-Nuclearity Metal-Cyanide Clusters:  Assembly of a Cr₈Ni₆(CN)₂₄ Cage with a Face-Centered Cubic Geometry

The synthesis of high-nuclearity metal-cyanide clusters presents a possible means of controlling magnetic properties in the design of new single-molecule magnets. Previous work employed tridentate blocking ligands in directing the assembly of a cubic [(tacn) 8 Co 8 (CN) 12 ] 12+ (tacn = 1,4,7-triazacyclononane) cluster; an improved crystal structure now confirms the lack of a guest water molecule inside the cluster cage. The ability to generate larger clusters by using a blocking ligand on only one of the mononuclear reaction components is demonstrated with the synthesis of a fourteen-metal [(Me 3 tacn) 8 Cr 8 Ni 6 (CN) 24 ] 12+ cluster. The geometry of this cluster consists of a cube of eight Me 3 tacn-ligated Cr III ions connected via bridging cyanide ligands to six square-planar Ni II ions situated just above the center of each cube face. Surprisingly, no guest species are evident within the 284 Å 3 cavity defined by the rigid metal-cyanide cage. Assembly of the cluster in boiling aqueous solution involves a linkage isomerization wherein the carbon end of each cyanide ligand reorients from binding a Cr III center in the reactant to binding the softer Ni II center in the product. Consequently, the Ni II ions become diamagnetic, resulting in magnetic behavior at high temperatures that is consistent with eight isolated Cr III ( S = 3 / 2) ions per cluster. However, below 30 K, a drop in the χ M T is attributed to weak antiferromagnetic coupling between Cr III ions through the LUMO orbitals of the [Ni(CN) 4 ] 2 - -like units centering each cluster face. Carrying out the assembly reaction in methanol at −40 °C forestalls the linkage isomerization, yielding a high-spin green form of the cluster. Reaction of [(Me 3 tacn) 8 Cr 8 Ni 6 (CN) 24 ] 12+ with [Ni(CN) 4 ] 2 - affords an aggregate species with a tetracyanonickelate ion capping each face of the cluster through a mean Ni···Ni contact of 3.00(1) Å, an interaction that destroys the long-range antiferromagnetic coupling between Cr III ions. Efforts to construct a larger cluster with an edge-bridged cubic geometry produced a linear [(Me 3 tacn) 2 (cyclam)NiCr 2 (CN) 6 ] 2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) fragment exhibiting an S = 4 ground state. The weak ferromagnetic coupling ( J = 10.9 cm - 1 ) within this cluster leads to a more rapid decrease in the magnetization with increasing temperature at higher magnetic fields as a result of the Zeeman splitting and population of low-lying excited states.