Olimpia Mamula

Supramolecular Coordination Compounds with Chiral Pyridine and Polypyridine Ligands Derived from Terpenes

Abstract This review summarizes the principles and the results in a special field of metallosupramolecular architectures. It deals with the stereochemical aspects of chiral assemblages that are formed using chiral derivatives of pyridine and polypyridine ligands. The modularity of the ligands that are discussed results in a variety of different structures that point in most cases to a high diastereoselectivity in the self-assembling process. Some aspects of solid state properties of such metallosupramolecular materials are discussed. Finally, the possibility of the stereoselective formation of topologically chiral organic molecules via metallosupramolecular architectures is presented.

Stereoselective Synthesis of Coordination Compounds: Self-Assembly of a Polymeric Double Helix with Controlled Chirality.

Its information content is infinitely smaller than that of DNA, but its structure (see picture) ressembles the double-stranded helix produced by nature. This self-assembled, configurationally predetermined coordination polymer is built up from enantiopure chiral bipyridine-type ligands and silver ions.

Completely Stereospecific Self-Assembly of a Circular Helicate

A nanoscale turbine wheel is an apt description of the structure of the enantiomerically pure helicate [Ag6 (L2)6 ]6+ (1), which precipitates as PF6 salt on mixing dissolved AgPF6 with the ligand L2 (a bis-bidentate ligand comprising two condensed α-pinene/bipyridine units linked by a xylylene bridge). The helicate has an outer diameter of about 3 nm and an inner diameter of 0.84 nm, and is a potential model for the study of stereospecific recognition processes.

Self-assembly of multinuclear coordination species with chiral bipyridine ligands: silver complexes of 5,6-CHIRAGEN (o,m,p-xylidene) ligands and equilibrium behaviour in solution

The complexation reactions between Ag- and a series of enantiopure ligands belonging to the CHIRAGEN (from CHIRAlity GENerator) family (L1, L2, L3, based on (-)-5,6-pinene bipyridine) have been studied in solution. It has been shown that the length of the bridge plays a fundamental role in the self-assembly processes leading to different compounds: mononuclear complexes (with L3), mixtures of polynuclear complexes (with L2) and circular helicates (with L 1). Although the absolute configuration of the chiral centres in all three ligands is the same, the metal-centred chirality of L3 (delta) is inverted with respect to that in the other two complexes with L1 and L2 (delta). The metal configuration is thus opposite in the mononuclear complex with respect to the polynuclear species. Detailed thermodynamic studies were carried out for the Ag+ and L1 ligand system by 1H and 109Ag NMR spectroscopy (as a function of concentration, temperature and pressure). At low temperature and high pressure, the [Ag6L1(6)]6+ hexanuclear circular helicate forms a tetranuclear circular helicate [Ag4L1(4)]4+: 2[Ag6L1(6)]6+ <=> 3 [Ag4L1(4)]4+. The thermodynamics parameters, obtained by temperature and pressure variation, have the following values: K298 = (8.7 +/- 0.7) x 10(-5) mol x kg(-1), deltaHo = -15.65 +/- 0.8 kJ x mol(-1), deltaSo = -130.2 +/- 3 J x mol(-1) x K(-1) and deltaVo(256 K)= -160 +/- 12 cm3 x mol(-1). The reaction volume calculated according to Connollys method indicates that the calculated structure of [Ag4L1(4)]4+ is plausible. Both the signs and large magnitudes of deltaSo and deltaVo are counterintuitive, yet can be understood by modelling methods.

Luminescence-detected phase transitions in lanthanide-containing liquid crystals

Complexes between lanthanide nitrates and a pro-mesogenic 18-membered diaza-substituted coronand are luminescent both as powders and liquid crystals (between 87 and 195 degrees C), and the phase transitions are detected by monitoring luminescence intensity and lifetime.

The bright future of stereoselective synthesis of co-ordination compounds

Transfer of chirality during the build-up of molecules has been applied innumerable times in organic chemistry since the end of the 19th century, when it was introduced in the so-called asymmetric synthesis by E. Fischer. Although analogous reactions were introduced in co-ordination chemistry in its early development, diastereoselective reactions have not been applied in a very systematic way for co-ordination species. The highly versatile co-ordination geometry of metal centres makes the synthesis of a selected stereoisomer in general a formidable task. In the present article an account on new developments in the field is given, focussing on recently synthesized molecules, where natural chiral products are used to create a large number of chiral ligands which predetermine the chirality at metal centres.

Lanthanide luminescent mesomorphic complexes with macrocycles derived from diaza-18-crown-6

Four tetracatenar (L1–L4) and one hexacatenar (L5) ligands, derived from the diaza-18-crown-6 framework, are synthesized and characterized. In these ligands, the amine functions are fitted with benzoyloxybenzyl linker groups, attached either with a carbonyl function (L1) or a methylene bridge (L2–L5) and bearing alkoxy chains, R, of various lengths: R = OCH3 for L2, OC10H21 for L3 and L5, OC12H25 for L1, and OC16H33 for L4. The non-mesomorphic ligands L1 and L3–L5 react with various lanthanide salts to give complexes forming thermotropic hexagonal columnar phases, as ascertained by thermal, optical and small-angle X-ray diffraction methods. The length of the alkoxy chains (L3 and L4) does not much influence the mesogenic behaviour, irrespective of the linker function, the number of alkoxy chains, the counterion or the lanthanide ion. The best systems proved to be the nitrato lanthanide complexes with L3, which present a Colh phase over 100 °C (up to 147 °C for La) with melting transition temperatures between 58 (La) and 86 (Tb) °C. In the case of [Eu(NO3)3L3], chosen as a representative example of all the complexes in this analysis, the inter-column separation of 29.2 A agrees well with the packing of cylindrical columns resulting from an alternated stacking of the molecules, in which the two mesogenic arms extend on the same side, i.e. stacking the molecules in a bent conformation. The liquid crystalline phases containing Eu and Tb display metal-centred emission, meaning that these complexes are interesting building blocks for the design of luminescent liquid crystalline materials.

Vollständig stereospezifische Selbstorganisation eines circularen Helicates

Wie ein Turbinenrad sieht die Struktur des enantiomerenreinen Helicats [Ag6(L2)6]6+1 aus, das als 1⋅(PF6)6-Salz beim Mischen von gelostem AgPF6 und dem Liganden L2 (zwei uber eine Xylylenbrucke verknupfte kondensierte α-Pinen/Bipyridineinheiten) entsteht. Das Helicat hat einen Ausendurchmesser von ca. 3 und einen Innendurchmesser von 0.84 nm und konnte vielleicht fur stereospezifische Erkennungsprozesse genutzt werden.

Enantiopure, Supramolecular Helices Containing Three-Dimensional Tetranuclear Lanthanide(III) Arrays: Synthesis, Structure, Properties, and Solvent-Driven Trinuclear/Tetranuclear Interconversion

The enantiomerically pure pinene-bipyridine-based receptor, (-) or (+) L(-), diastereoselectively self-assembles in dry acetonitrile in the presence of Ln(III) ions (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb) to give a C3-symmetrical, pyramidal architecture with the general formula [Ln4(L)9(mu3-OH)](ClO4)2) (abbreviated as tetra-Ln4L9). Three metal centers shape the base: an equilateral triangle surrounded by two sets of helically wrapping ligands with opposite configurations. This part of the structure is very similar to the species [Ln3(L)6(mu3-OH)(H2O)3](ClO4)2) (recently reported by us and abbreviated as tris-LnL2) formed by the ligand and the Ln(III) ions when the reactions are performed in methanol. The tetranuclear structure is completed by a capping, helical unit LnL3 whose chirality is also predetermined by the chirality of the ligand. A complete characterization of these isostructural, chiral compounds was performed in solid state (X-ray, IR) and in solution (ES-MS, NMR, CD, UV-vis and emission spectroscopies). The sign and the intensity of the CD bands in the region of the pi pi* transitions of the bipyridine (absolute Delta epsilon values at 327 nm are about 280 M(-1) x cm(-1)) are highly influenced by the helicity of the capping fragment LnL3. The photophysical properties (lifetime, quantum yield) of the visible (Eu and Tb complexes) and NIR (Nd complex) emitters indicate a good energy transfer between the ligands and the metal centers. The two related superstructures tetra-Ln4L9 and tris-LnL2 can be interconverted in acetonitrile, the switching process depending on the amount of water present in the solvent, the size of the Ln(III) ion, and the concentration. The weak chiral recognition capabilities of the self-assembly leading to the formation of tetra-Ln4L9 either by direct synthesis from a racemic mixture of the ligand and Ln(III) ions or by the conversion of a tris-Ln[(+/-)-L]2 racemate were likewise demonstrated.

A C3-symmetric chiral hexadentate podand ligand based on a tris(pyrazolyl)borate core

A hexadentate podand ligand having C3 symmetry has been prepared, consisting of a tris(pyrazolyl)borate core bearing pinene-functionalised 2-pyridyl units attached to the C3 position of each pyrazolyl ring such that each arm is a chiral bidentate chelate; complexes with Tl(I) and Tb(III) have been prepared and structurally characterised.