High-Nuclearity Metal-Cyanide Clusters: Synthesis, Magnetic Properties, and Inclusion Behavior of Open-Cage Species Incorporating [(tach)M(CN)3] (M = Cr, Fe, Co) Complexes

The use of 1,3,5-triaminocyclohexane (tach) as a capping ligand in generating metal-cyanide cage clusters with accessible cavities is demonstrated. The precursor complexes [(tach)M(CN)3] (M = Cr, Fe, Co) are synthesized by methods similar to those employed in preparing the analogous 1,4,7-triazacyclononane (tacn) complexes. Along with [(tach)Fe(CN)3F~, the latter two species are found to adopt low-spin electron configurations. Assembly reactions between [(tach)M(CN)3] (M Fe, Co) and [120)6] (Mʹ = Ni, Co) in aqueous solution afford the clusters [(tach)4((H)20)12(Ni)4C(o)4(CN)i2r, [(tach)4((H)20)i2C(o)8(CN)i2] and [(tach)4((H)20)i2(Ni)4(Fe)4(CN)i2], each possessing a cubic arrangement of eight metal ions linked through edge-spanning cyanide bridges. This geometry is stabilized by hydrogen-bonding interactions between tach and water ligands through an intervening solvate water molecule or bromide counteranion. The magnetic behavior of the NL cluster indicates weak ferromagnetic coupling (J = 5.5 cm) between the Ni" and Fe centers, leading to an S = 6 ground state. Solutions containing [(tach)Fe(CN)3] and a large excess of [NiOe] instead yield a trigonal pyramidal [(tach)((H)20)i5(Ni)3Fe(CN)3] cluster, in which even weaker ferromagnetic coupling (J = 1.2 cm) gives rise to an S = ground state. Paralleling reactions previously performed with [(Me3tacn)Cr(CN)3], [(tach)Cr(CN)3] reacts with [NidOe] in aqueous solution to produce [(tach)8Cr8Ni6(CN)24F featuring a structure based on a cube of Cr ions with each face centered by a square planar [Ni(CN)4 unit. The metal-cyanide cage differs somewhat from that of the analogous (Me)3tacnligated cluster, however, in that it is distorted via compression along a body diagonal of the cube. Additionally, the compact tach capping ligands do not hinder access to the sizable interior cavity of the molecule, permitting hostguest chemistry. Mass spectrometry experiments indicate a 1:1 association of the intact cluster with tetrahydrofuran (THF) in aqueous solution, and a crystal structure shows the THF molecule to be suspended in the middle of the cluster cavity. Addition of THF to an aqueous solution containing [(tach)Co(CN)3] and [CuOe] templates the formation of a closely related cluster, [(tach)8((H)20)6(Cu)6C(o)8(CN)24DTHF] in which paramagnetic Cu" ions with square pyramidal coordination are situated on the face-centering sites. Reactions intended to produce the cubic [(tach)4((H)2O)12C(o)8(CN) cluster frequently led to an isomeric two-dimensional framework, [(tach)((H)20)3C(o)2(CN)3, exhibiting mer rather than fac stereochemistry at the [0020)3] subunits. Attempts to assemble larger edgebridged cubic clusters by reacting [(tach)Cr(CN)3] with [Ni(cyclam)] (cyclam = 1,4,8,11-tetraazacyclotetradecane) complexes instead generated extended one- or two-dimensional solids. The magnetic properties of one of these solids, two-dimensional [(tach)2(cyclam)3(Ni)3(Cr)2(CN)6]l2, suggest metamagnetic behavior, with ferromagnetic intralayer coupling and weak antiferromagnetic interactions between layers.