Host–Guest Systems An Unstable Ligand-Unsupported Cu Dimer Stabilized in a Supramolecular Framework**

The attractive interactions that exist between d “closedshell” elements of Group 11 have received extensive attention. 2] The term “aurophilicity” has been coined to describe the Au Au bonding interactions that are evident in many molecular and solid-state structures and have been attributed to correlation and relativistic effects. However, relativistic effects cannot be invoked for Cu···Cu interactions, which are generally referred to as “cuprophilicity”. These interactions are found in inorganic lattices, polynuclear compounds with bridging ligands or stabilized by electrostatic interactions, and polymeric systems, such as infinite ···Cu-ligand-Cu··· chains with short sideways Cu···Cu interactions. The existence of ligand-bridged Cu···Cu interactions is supported by spectral evidence and has been examined by a series of theoretical calculations, which have sometimes reached different conclusions. However, a recent comprehensive analysis has shown that only a small part of the intermolecular interaction energy is often gained in the ground state from the Cu···Cu interaction, whereas a large component results from Cu···ligand interactions, even in the case of ligand-unsupported Cu···Cu species. Only a few examples of ligand-unsupported Cu aggregates have been described, most of which have been reported quite recently. These examples include the dimer of trimers based on a substituted pyrazolate; the dimer of dimers with a substituted phenanthroline/bipyridyl ligand; the infinite arrays of [Cu(tBu)(CN)Li(OEt2)2], in which neighboring Cu dimers are connected through Li ions; the oneand three-dimensional infinite arrays of Cu ions in [Cu(NH3)2]Br and [Cu(NH3)Cl], respectively; [12] the one-dimensional chains of Cu atoms in [Cu(C6F6)(py)] (py= pyridine); [13] and the [(CuCl2)2] 2 dimer cocrystallized with the monomeric [CuCl3] ion and with amidinium cations. Except for the [(CuCl2)2] 2 dimer, all the ligand-unsupported Cu dimers (“dimer” refers to two Cu fragments with a short Cu···Cu distance) form one-dimensional infinite Cu···Cu or p–pstacked chains.