Paul N. W. Baxter

THE DESIGN AND GENERATION OF INORGANIC CYLINDRICAL CAGE ARCHITECTURES BY METAL-ION-DIRECTED MULTICOMPONENT SELF-ASSEMBLY

The multicomponent self-assembly of ditopic linear ligands, circular ligands and copper(I) or silver(I) ions leads to the exclusive formation of a series of cylindrical inorganic architectures containing an internal cavity whose height may be modulated by the nature of the ligands used (see picture: 1: 20 A; 2: 33 A). The same reactions with tritopic and tetratopic linear ligands and circular tritopic ligands generates tri- (3) and tetracompartmental (4) cylindrical architectures of nanometric size in one stroke by a process involving the spontaneous positioning of up to 19 particles and the formation of up to 48 coordination bonds.

Self-Assembly and Structure of Interconverting Multinuclear Inorganic Arrays: A [4×5]-AgI20 Grid and an AgI10 Quadruple Helicate

Coordination of the pentatopic ligand 3 with AgI leads to the simultaneous self-assembly of two polynuclear architectures: a [4×5] grid-type species 10 and a quadruple-helicate 11, which contain twenty and ten silver ions, respectively. Their structures have been established by X-ray diffraction analysis of the crystals obtained as a mixture on crystallisation. Complex 10 contains two [2×5]-AgI10 rectangular subgrids located on opposite sides of an array of parallel ligands of 3 that are twisted into a transoid N=CC=N arrangement around the central CC bond; it may thus be formulated as a grid of grids: [2×(2×5)]. Complex 11 is an inorganic quadruple helicate that consists of two sets of two parallel ligands of 3 connected by an array of ten silver ions. Both compounds 10 and 11 are novel types of polynuclear complexes that are composed of two subunits. Their formation points to the possibility of generating specific arrays of metal ions by self-assembly, involving, in particular, a combination of subunits within the overall entity. They represent organised patterns of ion dots of special significance in view of their formal relationship to quantum dots.

Adaptive Self-Assembly: Environment-Induced Formation and Reversible Switching of Polynuclear Metallocyclophanes

Ligand 3 has been shown to self-assemble under coordination of copper(II) cations in a 1:1 ratio in acetonitrile to give equilibrating mixtures of a [2 x 2] grid-type tetranuclear structure 1 and a hexanuclear achitecture of hexagonal shape 2. The latter was confirmed by determination of the crystal structure which further indicated that 2 contained acetonitrile molecules and hydroxo groups bound to the copper(II) centers, which are therefore five-coordinate. The structures assigned to 1 and 2 were further supported by the spectral (mass, UV/Vis) data. The self-assembly process is strongly dependent on the conditions of the medium. An increase in concentration in acetronitrile increases the relative amount of hexamer 2, which appears to be the favored entity at the highest concentrations that can be reached before precipitation occurs. On the other hand, in nitromethane only the tetranuclear complex 1 was detected by mass spectrometry. Replacement of nitromethane by acetonitrile and vice versa indicated the reversible switching between a solution containing either 1 alone or an equilibrium mixture of 1 and 2, respectively. In conclusion, the system described presents several remarkable features: 1) self-assembly with substrate binding, 2) dynamic combinatorial structure generation, and 3) environment-induced structural switching amounting in effect to a process of adaptive self-assembly.

The Designed Self‐Assembly of Multicomponent and Multicompartmental Cylindrical Nanoarchitectures

The multicomponent self-assembly of ditopic linear ligands, circular ligands and copper(I) or silver(I) ions leads to the exclusive formation of a series of cylindrical inorganic architectures containing an internal cavity whose height may be modulated by the nature of the ligands used (see picture: 1: 20 A; 2: 33 A). The same reactions with tritopic and tetratopic linear ligands and circular tritopic ligands generates tri- (3) and tetracompartmental (4) cylindrical architectures of nanometric size in one stroke by a process involving the spontaneous positioning of up to 19 particles and the formation of up to 48 coordination bonds.

Self-assembly in self-organized inorganic systems: a view of programmed metallosupramolecular architectures

This review summarizes the recent advances that have been performed on self-assembled inorganic systems, especially on helical systems, metallo-exoreceptors and on architectures formed from multiple components. In addition, applications as well as the perspectives for these designed systems are also discussed.

Self-Assembly, Characterisation, and Crystal Structure of Multinuclear Metal Complexes of the [2×3] and [3×3] Grid-Type

The self-assembly of new multimetallic complexes of grid-type architecture is described. The binding of a set of tris-terdentate ligands, 1 a-1 d, based on terpyridine-like subunits, with different octahedrally coordinated metal ions leads to the formation of species whose structure depends strongly on the ligand, the metal ion, the counterion, the solvent, and the reaction conditions. Under suitable conditions, the [3 x 3] grid was obtained from the reaction of ligand 1 a with zinc tetrafluoroborate and from ligand 1 b with mercury triflate. The other ligands led to the formation of mainly one compound of composition [M(6)L(5)](12+), which has the structure of an incomplete [2 x 3] grid. The crystal structure of such a [2 x 3] grid, [Co(6)(1 d)(5)](12+), has been determined. In this complex, the three central pyrimidine-pyridine-pyrimidine non-coordinating sites adopt transoid NCbond;CN conformations. The much less stable cisoid conformations, the pinching of the coordination sites in the complex, the weaker donor strength of the central binding site, and the steric demand of the substituents are all factors contributing to the reluctance to produce the [3 x 3] structure. A subtle interplay between the nature of the metal, the steric demand of the ligand, the reaction conditions, and the type of counterion determine the product of self-assembly. The results obtained show that by tuning the parameters, complexes containing six or nine octahedrally coordinated metal ions in a well-defined grid-type arrangement are accessible. Both types of arrays, [2 x 3] and [3 x 3 ], are of interest as self-assembled inorganic architectures of well-defined structure and nuclearity that may be suitable prototypes for selective information storage media.

Oriented Crystalline Monolayers and Bilayers of 2×2 Silver(I) Grid Architectures at the Air-Solution Interface: Their Assembly and Crystal Structure Elucidation

Oriented crystalline monolayers, approximately 14 A thick, of a 2 x 2 Ag+ grid complex, self-assembled at the air-solution interface starting from an water-insoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counter-ions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial self-assembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.

Twistophane Macrocycles with Integrated 6,6′-Connected-2,2′-Bipyridine Units: A New Lead Class of Fluorescence Sensors for Metal Ions

The new twistophane macrocycles 2 and 3 have been synthesised; these compounds are composed of a cyclically conjugated dehydrobenzoannulene framework that incorporates 6,6-connected-2,2-bipyridine moieties for the purpose of coordinating metal ions. The cyclophanes were characterised by spectroscopic techniques, and shown by molecular mechanics calculations to be helically twisted and chiral molecules that may exist in several possible ground state conformations. UV/vis spectroscopic studies revealed that 2, 3 and precursor 9 bind with different selectivities to particular members of the following small group of metal analytes: CuII, AgI, HgII, Tl1 and PdII. Significantly, 2, 3 and 9 signal the presence of CuII ions through fluorescence emission quenching output responses. Furthermore, cyclophane 3 exhibited a particularly sensitive protontriggered chromogenic fluorescence response. With respect to their unique structural features, high analyte selectivity coupled with their enhanced and characteristic fluorescence emission responses, these molecules are among the first examples representing a new lead class of chemosensory materials. Compounds 2, 3 and 9 and derivatives thereof may, therefore, be expected to find many future applications in the detection of metal-based environmental pollutants, biologically important trace elements and monitoring proton fluxes.

Generation of an equilibrating collection of circular inorganiccopper(i) architectures and solid-state stabilisation of the dicopperhelicate component

Cu n I n ions and ligand L self-assemble to generate in solution na mixture of the double helical 1, triangular 2 and square grid 3 ncomplexes, from which 1 has been crystallized and its crystal structure ndetermined.

Novel tetra- and hexa-dentate ligands from 6,6′-dicyano-2,2′-bipyridine

A simple, direct route from 2,2′-bipyridine to 6,6′-dicyano-2,2′-bipyridine and its conversion to the 6,6′-dithiocarboxamide and various novel tetra- and hexa-dentate ligands such as the 6,6′-bis(4-methylthiazol-2-yl) and 6,6′-bis[4-(2-pyridyl)thiazol-2-yl] derivatives have been established. Diaqua[6,6′-bis(4-methylthiazol-2-yl)-2,2′-bipyridine]zinc(II) bis(trifluoromethanesulfonate) has been prepared and its crystal structure determined. The tetra-heterocyclic ligand is planar, binding through two bipyridine nitrogen atoms and, more weakly, through two thiazolyl nitrogen atoms to zinc(II) which is in a distorted octahedral environment.