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.

(C5Me5)2Yb x OEt2 reacts with terpyridine and tetrapyridinylpyrazine to afford new mixed-valent systems.

The high-yield preparation, by double oxidative addition, of nine novel platinum and palladium bis(trans-M(PR3)2X)aryl (M = Pt or Pd; R = PPh3 or PEt3; X = Br or I; aryl = 1,4-benzene, 4,4‘-biphenyl, 4,4‘‘-ter-p-phenyl, 4,4‘-tolane, or 4,4‘-benzophenone) complexes from the reaction of Pt(PPh3)4, Pt(PEt3)4, or Pd(PPh3)4 with the respective dihalo aromatic in toluene is described. These complexes were fully characterized by elemental analysis, mass spectrometry, and NMR (1H, 13C{1H}, and 31P{1H}) and vibrational (IR or Raman) spectroscopies. The single-crystal molecular structure of 4,4‘-bis(trans-Pt(PEt3)2I)biphenyl (2a) was determined by X-ray crystallography. The key structural feature of this complex is the dihedral angle of 18.9° between the two planes defined by the phenyl groups of the biphenyl linkage. The nature of the palladium−carbon bond is investigated by 13C{1H} NMR spectroscopy; Taft's σR parameter is found to correlate in a linear fashion with [δ(C ipso ) − δ(C o )] for these palladium complexes. These data indicate the 13C chemical shift of C ipso is linearly related to the amount of π-electron density of the carbon bound to the palladium center. The potential utility of these bimetallic platinum and palladium complexes as subunits in the generation of organometallic macrocycles is described.

A systematic study of the novel charge-transfer [(f)14-(pi)0-(f)14 --> (f)13-(pi)2-(f)13] electronic state found in 2:1 metal-to-ligand adducts of the type [(Cp)2Yb](BL)[Yb(Cp)2] [BL = tetra(2-pyridyl)pyrazine (tppz) (1), 6',6' '-bis(2-pyridyl)-2,2':4',4'':2'',2'''-quaterpyridine (qtp) (2), 1,4-di(terpyridyl)-benzene (dtb) (3), Cp = (C5Me5)] has been conducted with the aim of determining the effects of increased Yb-Yb separation on the magnetic and electronic properties of these materials. The neutral [(f)13-(pi)2-(f)13], cationic [(f)13-(pi)1-(f)13] and dicationic [(f)13-(pi)0-(f)13] states of these complexes were studied by cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, NMR, X-ray crystallography, and magnetic susceptibility measurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistent with an [(f)13(pi)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donates one electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. The voltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1-3, as manifested in the separation between successive metal reduction waves, is large compared to analogous transition metal systems. Electronic spectra for the neutral and monocationic bimetallic species are dominated by pi-pi and pi-pi transitions, masking the f-f bands that are expected to best reflect the electronic metal-metal interactions. However, these metal-localized transitions are observed when the electrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, and they suggest very little electronic interaction between metal centers in the absence of pi electrons on the bridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagnetic coupling of the type Yb(alpha)(alphabeta)Yb(beta) at approximately 13 K.

A new N-heterocyclic complex of ytterbocene (Cp(2)Yb(II), Cp = C(5)Me(5)) has been prepared by the addition of 4'-cyano-2,2':6',2' '-terpyridine (tpyCN) to Cp(2)Yb(II)(OEt(2)) in toluene to give a dark blue species designated as Cp(2)Yb(tpyCN). The effect of the electron-withdrawing group (-CN) on the redox potentials of the charge-transfer form of this complex [in which an electron is transferred from the f(14) metal center to the lowest unoccupied (pi) molecular orbital of the tpyCN ligand to give a 4f(13)-pi(1) electronic configuration] has been quantified by cyclic voltammetry. The tpyCN ligand stabilizes this configuration by 60 mV more than that in the unsubstituted tpy ligand complex and by 110 mV more than that in the unsubstituted bpy ligand complex. Magnetic susceptibility measurements corroborate the enhanced stabilization of the 4f(13)-pi(1) configuration by the substituted terpyridyl ligand complex. Furthermore, the temperature dependence of the magnetic data is most consistent with a thermally induced valence tautomeric equilibrium between this paramagnetic 4f(13)-pi(1) form that dominates near room temperature and the diamagnetic 4f(14)-pi(0) form that dominates at low temperature. Differing coordination modes for the tpyCN ligand to the ytterbocene center have also been confirmed by isolation and X-ray crystallographic characterization of complexes binding through either the cyano nitrogen of tpyCN or the three terpyridyl nitrogen atoms of tpyCN.