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)Cr8Ni6(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 283 Angstrom (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 chi T-M 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 degreesC forestalls the Linkage isomerization, yielding a high-spin green form of the cluster. Reaction of [(Me(3)tacn)(8)Cr8Ni6(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) Angstrom, 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)NiCr2(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.