Interaction of a predesigned molecular "clip" (4) with rigid dipyridyl bridging ligands, in acetone/water mixtures, leads to the formation of molecular rectangles (5-8) in 92-97% isolated yields via spontaneous self-assembly. Characterization was accomplished with multinuclear NMR and UV-vis spectroscopy, FAB mass spectrometry, and X-ray crystallography. The length of these metallamacrocycles ranges from 2 to 3 nm. Postmodification via non-nucleophilic counterion exchange results in enhanced structural integrity for the assemblies.

The novel charge-transfer ground state found in alpha,alpha'-diimine adducts of ytterbocene (C(5)Me(5))(2)Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f(14) metal center into the lowest unoccupied (pi*) molecular orbital (LUMO) of the diimine ligand to give an f(13)-L(*)(-) ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by approximately 1.0 V each upon complexation for both bpy and phen adducts. The separation between the ligand-based oxidation wave (L(0/-)) and the metal-based reduction wave (Yb(3+/2+)) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorption spectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with those reported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand (pi*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converse of the ground-state charge-transfer process (e.g., f(13)-L(*-) <--> f(14)-L(0)). These new bands occur at approximately 5000 cm(-1) in both adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronic bands in these transitions are consistent with the removal of an electron from the ligand pi* orbital. The unusually large intensity observed in the f --> f intraconfiguration transitions for the neutral phenanthroline adduct is discussed in terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearly reveal resonance enhancement for excitation into the low-lying pi* --> pi* ligand-localized excited states, and comparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms that the ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for the modes in the bipyridine adduct with excitation into different pi* --> pi* levels illustrate the different nodal structures that exist in the various low-lying pi* orbitals.

A family of nanoscale-sized supramolecular cage compounds with a trigonal prismatic framework was prepared by means of spontaneous self-assembly from the combination of a predesigned molecular ''clip'' with tritopic pyridyl subunits. As confirmed by x-ray crystallography, the smallest structure of the reported series is Ϸ1 ؋ 2 nm and possesses a nitrate anion incarcerated inside its molecular cavity. The largest structure has dimensions of Ϸ 1 ؋ 4 nm. T he formation of discrete supramolecular entities driven and held together by metal coordination is an intense new area of investigation at the forefront of supramolecular chemistry (1-10). Because self-assembly is guided by the chemical information encoded into the molecular subunits, diverse structures with predetermined shape, size, and functionality can readily be designed. Indeed, a wide variety of aesthetic structures have been realized, such as molecular grids, helicates, rings, catenanes, tetrahedra, cubes, cuboctahedra, etc. Once assembled, many of the hollow structures have been shown to be capable of encapsulating molecules through electrostatic and͞or dispersion forces. Often times, ions will template the formation of an assembly (11-21). When considering that metal-containing assemblies often possess magnetic, photophysical, and͞or redox properties not accessible from purely organic systems, studies in basic host-guest chemistry hold new promise for technologies in molecular sensing (22-28), separations, and catalysis (29,30).Because lower-symmetry hosts can ultimately be expected to show enhanced guest selectivity, especially toward planar aromatic guests, prismatic cages represent a natural progression in the development of this area. Although M 3 L 2 -type cages are relatively simple three-dimensional constructs, they remain uncommon. Of those that have been reported (31-40), most usually either require the use of templates to assemble in solution, or assemble only in the solid state. Part of the reason for this limitation is possibly the fact that, in most cases, flexible ligands were used. By contrast, structures derived from rigid tritopic linkers with cis-metal ions are either: (i) tetrahedral M 6 L 4 cages (41) where L is a planar ligand, or (ii) double-square M 6 L 4 cages (42) where L is a 109°linker ligand. Construction of the M 3 L 2 , D 3h species is complicated by the fact that rigid tritopic linkers with ideal mutual angles of 60°are not easily accessible. A noteworthy trigonal bipyramidal structure (35), made from Pd(II) ions and a calix[3]arene subunit, was shown to be able to reversibly bind a molecule of C 60 .By exploiting incommensurate symmetry requirements for differing metallic subunits, an alternative approach to structures of this general topology was recently reported. Raymond and Wong (43-45) successfully prepared a series of M 2 MЈ 3 L 6 supramolecular clusters where a multifunctionalized ligand (L) was cleverly designed to selectively interact with two types of metal ions (one hard and one soft).Double oxidative addition ...

Reaction of 2 or 3 equiv of potassium 1,3-bis(trimethylsilyl) with the triflate salts of Ce, Nd, Eu, Tb, and Yb gives the corresponding neutral bis-(Yb, Eu) and tris-(Ce, Nd, Tb) allyl lanthanide complexes in yields ranging from 40 to 80%. These complexes, which have been crystallographically characterized, initiate the polymerization of methyl methacrylate (MMA), but with poor turnover frequencies when compared with the corresponding salt complexes of the type K[LnA′ 3 ]. K[A′] itself initiates MMA polymerization, however, and its presence as an ion-pair in the salt complexes may contribute to the activity of heterometallic lanthanide catalysts.