Alex von Zelewsky

Predetermined Chirality at Metal Centers

Asymmetric synthesis in coordination chemistry was described very clearly by Smirnoff in 1920, but, contrary to the development in organic chemistry, it was almost completely neglected for several decades. The interest in chirality in coordination chemistry (see the stereoview of [Ru(bpy)3 ]2+ ) has increased rapidly in recent times as a consequence of developments in several fields where chirality is important (polynuclear systems, supramolecular structures, and enantioselective catalysis). Here we show many examples of how, through the choice of ligand, the configuration of a metal center or the chirality of a helicate can be predetermined.

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.

Charge-transfer excited state properties of chiral transition metal coordination compounds studied by chiroptical spectroscopy

Abstract Chiroptical properties of chiral coordination compounds can be used to assign absolute configuration of ground and excited states. We discuss examples of transition metal complexes where MLCT, LMCT, MMCT and exciton LCT transitions have been observed. Such models are dimolybdenum complexes, cyclometallated Pt and Pd complexes, helicates, and complexes with Schiff-bases, conjugated diimines, catecholates and hydroxamates as ligands. The theoretical methods to assign configurations, i.e. comparison methods, exciton theory or quantum mechanical calculations, are reviewed and shown for selected examples.

Luminescence of ortho-metallated platinum(II) complexes

Abstract The absorption spectra, emission spectra, and emission lifetimes of Pt(Phpy)2, Pt(Thpy)2, and Pt(Bhq)2 complexes (Phpy−, Thpy−, and Bhq− are the ortho C-deprotonated forms of 2-phenylpyridine, 2-(2-thienyl)-pyridine, and benzo(h)quinoline) have been studied and compared with those of the C-protonated neutral ligands. For all complexes examined the low-energy absorption bands in the near UV and visible region are assigned to metal-to-ligand charge-transfer transitions. The strong and structured luminescence emissions observed in the 500–600 nm region (lifetime in the microsecond range at 77 K) are assigned to metal-to-ligand charge-transfer excited states.

New versatile optically active bipyridines as building blocks for helicating and caging ligands

Abstract Short and efficient syntheses of two new optically active bipyridines have been accomplished using α-pinene as source of chirality. The new bipyridines allow easy access to chiral helicating and caging ligands of sterically well defined shapes and properties.

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.

Stereoselective synthesis of coordination compounds

Abstract After an introduction, where the general aspects of stereoselective synthesis of coordination species are shortly reviewed, an account of the use of pyridine and bipyridine ligands that are made chiral through incorporation of structural elements taken from terpenes, which occur naturally as enantiopure or at least non-racemic compounds, is given. It is shown that this ligand family, which presently comprises about 90 different characterized species, can be used for many different purposes: (i) tetradentate ligands of the so called CHIRAGEN type create a chirality element of well defined configuration in an octahedral complex, where two coordination sites are left for further binding to other didentate ligands. In this way the synthesis of polynuclear coordination species of predetermined stereochemistry becomes feasible. (ii) Didentate chiral thienyl–pyridine ligands yield dinuclear complexes with octahedral coordination sites, where one homochiral configuration is strongly preferred. Through cleavage of the bridge in these dinuclear species, mononuclear complexes with predetermined configuration are accessible. (iii) Tripodal ligands containing chiral structural elements from terpene molecules yield octahedral complexes with predetermined configuration. (iv) Terpyridine-type ligands can be synthesized that form either chiral C 2 -symmetric complexes if the two ligands are identical, or achiral S 4 -symmetric complexes, when two ligands have opposite chirality. (v) The configuration of chiral ‘square–planar’ complexes can be predetermined, using terpene-chiralized pyridine ligands in cyclometallated species. (vi) Terpene–bipyridine ligands can render a large number of d 8 metal complexes chiral. (vii) The use of certain CHIRAGEN-type ligands results in complexes with tetrahedral coordination sites, where the configuration is predetermined. (viii) It is shown that a number of terpene–pyridines and –bipyridines yield a non-racemic alcohol, when used as catalysts in ethylation reactions of a prochiral aldehyde. Enantiomeric excess values of up to 86% are reached. (ix) CHIRAGEN-type ligands can yield with labile coordination centers, either linear or circular helicates with predetermined configuration. In such self assembly reactions, complete chiral recognition was observed.

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.

Enantioselective catalytic cyclopropanation of styrenes by copper complexes with chiral pinene-[5,6]-bipyridine ligands

Abstract Substituted mono- and bis-pinene-[5,6]-bipyridines are useful ligands in asymmetric copper-catalyzed cyclopropanation by styrenes. Copper complexes were prepared either in situ or prior to the reaction. The catalytic reaction of styrene with ethyl diazoacetate and Cu – 11b yields ethyl trans -phenylcyclopropane carboxylate in >99% yield and 87% e.e. at 0°C. The corresponding cis -configured cyclopropane was produced with an e.e. of 90%. The cis / trans ratio is 22:78. Other ligands of this series are less effective. Various olefins were tested as substrates but exo -methylene olefins show the best results.