Lucia Carlucci, Gianfranco Ciani, Davide M. Proserpio

The development of rational synthetic routes to novel supramolecular devices, like molecular boxes, catenanes, rotaxanes, knots and helicates, as well as to new two- and three-dimensional frameworks (crystal engineering of target architectures), has recently been pursued in inorganic and coordination chemistry. These new metal containing materials are theoretically interesting for their new topologies, but also have many potential applications in electrical conductivity, magnetism, host-guest chemistry, ion-exchange and catalysis.

The basic concepts in coordination polymer chemistry use metal centres with different selected coordination geometries and suitable spacer ligands. Zeolite-like polymeric species can be obtained, that are tailored or functionalized for molecular selectivity or catalytic applications.

Our former investigations were mainly concerned with the self-assembly of Ag(I) or Cu(I) metal centres (having poorly coordinating counterions) with bidentate aromatic bases, like pyrazine, pyridazine, bipyridyl, triazine, and many others. A variety of polymeric network, exhibiting interesting topological types and intertwining phenomena, have been isolated and characterized. The most fascinating among them are 3D frames topologically related to diamond (fourfold interpenetrated), a-ThSi₂ (threefold interpenetrated), SrSi₂ (twofold interpenetrated) and a-polonium. In another strategy, we use metallic synthons to join polydentate bases, i.e. d¹⁰ metal ions or complexes with two transoid free sites, acting as spacers between organic centres. Accordingly we have studied Ag(I)- hexamethylenetetramine systems obtaining new noteworthy two- and three-dimensional polymeric architectures.

The self-assembly of Ag(I) or Cu(I) with other polydentate bases containing CN donor groups, like tetracyanoethene and tetracyanobenzene, has also produces interesting polimeric motifs.

Synthetic efforts are currently aimed towards novel ligands containing two or more 4-pyridyl groups, for a better control of the metal spacing and of the topology of the array. Metal centres as copper, cobalt, zinc, cadmium and other dications are also used. New polymeric structures with unprecedented topologies have already been characterized, showing in some cases peculiar nanoporous properties.

The a-polonium net of [Ag(pyz)₃](SbF₆)

The eightfold interpenetration in [Ag(sebaconitrile)₂](BF₄)

Many of the isolated 3D networks show large channels or cavities which contain solvent molecules. The space occupied by a single frame is usually smaller than the free volume, but interpenetration of nets drastically reduces the available space. We are trying to avoid interpenetration or to reduce its effects, to reach open networks with larger channels.

Developments in the crystal engineering of networked polymers will require further information on:

1. new topological types, to be related to the nature of the metal centres, the dimensions and conformational flexibility of the ligands, the dimensions and donor ability of the counterions, the presence of guest molecules;
2. the effect of supramolecular interactions (hydrogen bonding, p-stacking, and others);
3. the effects of interdigitation (complementary molecular shapes and steric requirements)
4. the degree of interpenetration and its control.

Schematic view of the entanglement of the ladder polymer [Ag₂(bpethy)₅](BF₄)₂

Threefold interpenetration of layers in [Cd₂(bpethy)₃(NO₃)₄]

triply interpenetration of a copper polymer with the 'dense' topology

Highlight

Some fascinating three-dimensional polymers, with remarkable topologies, have been synthesized and characterized including: i) diamondoid nets with interpenetration degrees up to 8-fold; ii) a networked ‘solid state racemate’, of 10³-a topology, hypotized many years ago by A. F. Wells, but never observed before; iii) a three-fold interpenetrated ‘dense’ net of 7⁵9 topology, theoretically suggested by O’Keeffe, but unprecedented both in inorganic chemistry and in minerals; iv) an a-polonium net comprised of uncommon octahedral silver centres; v) interesting non interpenetrated three-dimensional Ag(I)- hexamethylenetetramine arrays.

New types of supramolecular entanglements have also been envisaged including: interpenetration of motifs with different topology, polycatenated ladders, polycatenated multiple layers, polythreaded ladders with sideharms, infinite double-helices and polypseudorotaxane tubular species.

Relevant publications

L. Carlucci, G. Ciani, D.M. Proserpio, A. Sironi, Novel networks of unusually coordinated silver(I) cations: the wafer-like structure of [Ag(pyz)2][Ag2(pyz)5](PF6)3 .2G and the simple cubic frame of [Ag(pyz)3](SbF6). Angewandte Chemie - International Edition in English, 1995 34, 1895-1898.

L.Carlucci, G.Ciani, D.M.Proserpio, A. Sironi, A three-dimensional, three-connected cubic network of the SrSi2 topological type in coordination polymer chemistry: [Ag(hmt)](PF6). H2O (hmt = hexamethylenetetraamine). Journal of the American Chemical Society, 1995 117, 12861-12862.

L. Carlucci, G. Ciani, D. W. v. Gudenberg, D.M. Proserpio, Self-assembly of infinite double helical and tubular coordination polymers from Ag(CF₃SO₃) and 1,3-bis(4-pyridyl)propane. Inorganic Chemistry, 1997 36, 3812-3813.

L.Carlucci, G. Ciani, P. Macchi, D.M. Proserpio, An unprecedented triply interpenetrated chiral network of square-planar metal centres from the self-assembly of copper(II) nitrate and 1,2-bis(4-pyridyl)ethyne. Chem. Commun., 1998, 1837-1838.

L.Carlucci, G. Ciani, P. Macchi, D.M. Proserpio, S. Rizzato, Complex interwoven polymeric frames from the self-assembly of silver(I) cations and sebaconitrile. Chem. Eur. J., 1999, 5, 237-243.

L.Carlucci, G. Ciani, D.M. Proserpio, A new type of supramolecular entanglement in the silver(I) coordination polymer [Ag₂(bpethy)₅](BF₄)₂ [bpethy = 1,2-bis(4-pyridyl)ethyne]. Chem. Commun., 1999, 449-450.

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