High-symmetry coordination cages via self-assembly

We provide a summary of our results in three-dimensional, coordination-driven self-assembly based on the directional-bonding methodology, in which the stoichiometric mixing of complementary building blocks, with appropriate, predefined geometries, leads to targeted, nanoscopic cages. Using this motif, we have synthesized high-symmetry ensembles resembling the Platonic solids, such as dodecahedra, and the Archimedean solids, such as truncated tetrahedra and cuboctahedra, as well as other cages, like trigonal bipyramids, adamantanoids, and trigonal prisms. The synthesis and characterization of these compounds is discussed, as is some host-guest chemistry.

Metalla-supramolecular rectangles as electron reservoirs for multielectron reduction and oxidation

The electron-transfer capacity of molecular rectangle ions [Pt(II)(4)(PEt(3))(8)(mu-anth(2-))(2)(mu-L)(2)](4+) with anth = anthracene-1,8-diyl and L = 4,4'-bipyridine (bp) or 1,2-bis(4-pyridyl)ethene (bpe) was investigated in acetonitrile and dichloromethane using cyclic voltammetry, EPR, and UV-vis-near-IR spectroelectrochemistry. The compounds can be reversibly reduced, first in a two-electron process and then via two closely separated one-electron steps. Oxidation was also possible at rather low potentials in a reversible two-electron step, followed by an electrochemically irreversible process. The spectroscopic results indicate reduction at the neutral acceptor ligands L and oxidation at the formally dianionic anthracene "clips". In contrast, the prototypical molecular square ([Pt(triphos)(mu-bp)](4))(8+) undergoes only irreversible reduction.

Coordination-driven self-assembly: solids with bidirectional porosity

Coordination-driven self-assembly reactions have been used in the preparation of a variety of discrete supramolecular species, some of which have shown promise as synthetic receptors. Many highly ordered coordination polymers and porous networks have been prepared in a similar fashion. While a few of these solids are capable of the uptake of small organic molecules in the resultant molecular channels, the formation of truly porous structures has frequently been thwarted by lattice interpenetration. A strategy for the formation of porous solids that may circumvent this problem is based on the covalent construction of nanoscale macrocycles which, when eclipsed in the solid state, may lead to porous, tubular assemblies. We have incorporated these concepts toward the realization of a bidirectionally porous solid. The metal-directed, self-assembly of a conjugated, macrocyclic ligand provides a discrete, supramolecular entity in solution and the solid state. X-ray crystallographic analysis establishes that this assembly packs such that bidirectional channels are realized, and the incorporation of only ClCH2CH2Cl into the crystal lattice demonstrates that these channels are potentially suitable for the selective uptake of small organic guests.

Engineering the structure and magnetic properties of crystalline solids via the metal-directed self-assembly of a versatile molecular building unit

We report the supramolecular chemistry of several metal complexes of N-(4-pyridyl)benzamide (NPBA) with the general formula [Ma(NPBA)2AbSc], where M = Co2+, Ni2+, Zn2+, Mn2+, Cu2+, Ag+; A = NO3-, OAc-; S = MeOH, H2O; a = 0, 1, 2; b = 0, 1, 2, 4; and c = 0, 2. NPBA contains structural features that can engage in three modes of intermolecular interactions: (1) metal-ligand coordination, (2) hydrogen bonding, and (3) pi-pi stacking. NPBA forms one-dimensional (1-D) chains governed by hydrogen bonding, but when reacted with metal ions, it generates a wide variety of supramolecular scaffolds that control the arrangement of periodic nanostructures and form 1- (2-4), 2- (5), or 3-D (6-10) solid-state networks of hydrogen bonding and pi-pi stacking interactions in the crystal. Isostructural 7-9 exhibit a 2-D hydrogen bonding network that promotes topotaxial growth of single crystals of their isostructural family and generates crystal composites with two (11) and three (12) different components. Furthermore, 7-9 can also form crystalline solid solutions (M,M')(NPBA)2(NO3)2(MeOH)2 (M, M' = Co2+, Ni2+, or Zn2+, 13-16), where mixtures of Co2+, Ni2+, and Zn2+ share the same crystal lattice in different proportions to allow the formation of materials with modulated magnetic moments. Finally, we report the effects that multidimensional noncovalent networks exert on the magnetic moments between 2 and 300 K of 1-D (4), 2-D (5), and 3-D (7, 8, 10, and 13-16) paramagnetic networks.

Supramolecular assemblies of dimetal complexes with polydentate N-donor ligands: from a discrete pyramid to a 3D channel network

Reactions of a dinuclear metal complex in the form of dirhodium(II) tetra(trifluoroacetate), [Rh(2)(O(2)CCF(3))(4)] (1), with a number of strong N-donor ligands having functional groups rigidly oriented at different directing angles have been found to yield supramolecular architectures of differing complexity. All structures have been established by X-ray crystallography. From reaction of 1 with neutral tris(4'-pyridyl)methylsilane ligand, CH(3)Si(C(5)H(4)N)(3) (L1), a discrete pyramid-shaped hexanuclear complex [[Rh(2)(O(2)CCF(3))(4)](3)CH(3)Si(C(5)H(4)N)(3)(eta(1)-C(6)H(6))(3)].C(6)H(6) (2.C(6)H(6)) has been isolated from benzene. In 2 three molecules of 1 are strongly coordinated to one L1 ligand at only one axial position of each dirhodium unit at the Rh-N distances of 2.152(6) A. The second rhodium atom of each dimetal complex in 2 weakly coordinates a benzene molecule with an Rh-C distance of 2.69(2) A. A supramolecular complex of the composition [[Rh(2)(O(2)CCF(3))(4)](2)(C(6)H(5))(2)Si(C(5)H(4)N)(2)] (3) has been prepared by reacting the dinuclear units 1 with a potentially bidentate ligand, bis(4'-pyridyl)diphenylsilane, (C(6)H(5))(2)Si(C(5)H(4)N)(2) (L2), having two pyridyl groups rigidly oriented at 109 degrees. In 3, one L2 ligand coordinates two dirhodium molecules 1 through their axial positions with the Rh-N distance of 2.150(5) A. An interesting extended 2D layered motif is formed by additional contacts of open axial positions of dirhodium units with phenyl groups of the neighboring ligands at Rh-C distances which average to 2.88(1) A. A supramolecular compound of the composition [[Rh(2)(O(2)CCF(3))(4)](3)(HO)C(C(5)H(4)N)(3)(eta(1)-C(6)H(6))].(1)/(2)C(6)H(6) (4.(1)/(2)C(6)H(6)) has been formed when linear dirhodium units 1 were reacted with tris(4'-pyridyl)methanol (L3) having tetrahedral directing angles that average to 110 degrees. A building block in the solid structure of 4 is a hexanuclear molecule in which one L3 ligand binds three dimetal units of 1 through N atoms of pyridyl groups at the average Rh-N distance of 2.143(7) A. A unique extended pseudo-3D structure in 4 is created by additional Rh-O coordination bonds as well as by weak metal-arene interactions.

Solution and solid state studies of a triangle-square equilibrium: anion-induced selective crystallization in supramolecular self-assembly

We report that an equilibrium mixture of triangular and square supramolecular species results when the flexible, ditopic donor unit, trans-bis(4-pyridyl)ethylene, is reacted with the ditopic acceptor unit, cis-(Me(3)P)(2)Pt(OTf)(2), in a one-to-one ratio. Both products are characterized in solution, by way of multinuclear NMR, and in the solid state, via X-ray crystallography. The effects of water and anions, cobalticarborane versus triflate, on the equilibrium are probed. The selective crystallization of either of the two species can be accomplished via the appropriate choice of solvents and ratio of anions present in the system. The dominant species in solution is not always consistent with the most prevalent species in the solid state.