Supramolecular copper hydroxide tennis balls:: Self-assembly, structures, and magnetic properties of octanuclear [Cu8L8(OH)4]4+ clusters (HL = N-(2-pyridylmethyl)acetamide)

The self-assembly of supramolecular copper "tennis balls" that possess unusual magnetic properties using a small pyridyl amide ligand is described. Copper(II) complexes of N-(2-pyridylmethyl)acetamide (HL) were synthesized in methanol. In the absence of base, the mononuclear complex [Cu(HL)(2)](ClO4)(2) (1) was prepared. The structure of 1, determined by X-ray crystallography, contains a copper(II) ion surrounded by bidentate HL ligands coordinated via the pyridyl N atom and the carbonyl O atom in a trans, square planar arrangement. Reactions carried out in the presence of triethylamine resulted in cluster complexes [Cu8L8(OH)(4)](ClO4)(4) and [Cu8L8(OH)(4)](CF3SO3)(4) [2(ClO4)(4) and 2(OTf)(4), respectively]. The cationic portions of 2(ClO4)(4) and 2(OTf)(4) are isostructural, containing eight copper(II) ions, eight deprotonated ligands (L-), and four mu(3)-hydroxide ligands. The top and bottom halves of the cluster are related by a pseudo-S-4 symmetry operation and are held together by bridging L- ligands. Solutions of 2(ClO4)(4) and 2(OTf)(4), which were shown to contain the full [Cu8L8(OH)(4)](4+) fragment by electrospray mass spectrometry and conductance experiments, are EPR silent. Magnetic susceptibility measurements for 2(ClO4)(4) as a function of temperature and magnetic field showed the Cu ions all to exhibit magnetic moments in the range expected for the d(9) configuration. At low temperatures, the magnetization was reduced due to predominantly antiferromagnetic interactions between ions. Analysis showed that partially frustrated interactions among the four Cu ions making up each half of the cluster gave good agreement with the data once a large molecular anisotropy was taken into account, with J(c) = 106 cm(-1,) D = 27 cm(-1,) and g = 2.17.