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

• Published in: Inorganic chemistry Vol. 42; no. 5; pp. 1403 - 1419
• Main Authors: Yang, Jenny Y, Shores, Matthew P, Sokol, Jennifer J, Long, Jeffrey R
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10.03.2003
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic026065f

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)3]1-, the latter two species are found to adopt low-spin electron configurations. Assembly reactions between [(tach)M(CN)3] (M = Fe, Co) and [M‘(H2O)6]2+ (M‘ = Ni, Co) in aqueous solution afford the clusters [(tach)4(H2O)12Ni4Co4(CN)12]8+, [(tach)4(H2O)12Co8(CN)12]8+, and [(tach)4(H2O)12Ni4Fe4(CN)12]8+, 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 Ni4Fe4 cluster indicates weak ferromagnetic coupling (J = 5.5 cm-1) between the NiII and FeIII centers, leading to an S = 6 ground state. Solutions containing [(tach)Fe(CN)3] and a large excess of [Ni(H2O)6]2+ instead yield a trigonal pyramidal [(tach)(H2O)15Ni3Fe(CN)3]6+ cluster, in which even weaker ferromagnetic coupling (J = 1.2 cm-1) gives rise to an S = 7/2 ground state. Paralleling reactions previously performed with [(Me3tacn)Cr(CN)3], [(tach)Cr(CN)3] reacts with [Ni(H2O)6]2+ in aqueous solution to produce [(tach)8Cr8Ni6(CN)24]12+, featuring a structure based on a cube of CrIII ions with each face centered by a square planar [Ni(CN)4]2- unit. The metal−cyanide cage differs somewhat from that of the analogous Me3tacn-ligated 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 host−guest 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 [Cu(H2O)6]2+ templates the formation of a closely related cluster, [(tach)8(H2O)6Cu6Co8(CN)24⊃THF]12+, in which paramagnetic CuII ions with square pyramidal coordination are situated on the face-centering sites. Reactions intended to produce the cubic [(tach)4(H2O)12Co8(CN)12]8+ cluster frequently led to an isomeric two-dimensional framework, [(tach)(H2O)3Co2(CN)3]2+, exhibiting mer rather than fac stereochemistry at the [Co(H2O)3]2+ subunits. Attempts to assemble larger edge-bridged cubic clusters by reacting [(tach)Cr(CN)3] with [Ni(cyclam)]2+ (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)3Ni3Cr2(CN)6]I2, suggest metamagnetic behavior, with ferromagnetic intralayer coupling and weak antiferromagnetic interactions between layers.