Circular Helicate Research Articles

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 Connolly's 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.

Characterization of supramolecular complexes by high resolution electrospray ionization mass spectrometry

High resolution mass spectrometry on time-of-flight (TOF) and ion trap (IT) mass spectrometers is used for the qualitative speciation of a heterodimetallic triple-stranded helicate [EuFe(L1)3](CIO4)5 and a toroidal complex [Ni3(L2)3](CIO4)6 (= circular single-stranded helicate) dissolved in acetonitrile. The resolution of about 5000 at peak width at half height using ion trap mass spectrometry (ITMS) and time-of-flight mass spectrometry (TOFMS) combined with electrospray ionization (ESI) is sufficient to resolve sextuply charged ions leading to a straightforward characterization of the complexes in solution. The ESI-MS response is influenced by the interface design, particularly for the heated capillary interface which requires optimized conditions for the study of non-covalent complexes.

Kinetic and Thermodynamic Control in Self-Assembly: Sequential Formation of Linear and Circular Helicates.

NMR and MS analysis as a function of time has shown that the self-assembly of a linear ligand with Fe2+ or Ni2+ , metal ions of octahedral coordination geometry, generates first a triple helicate and thereafter the circular helicate 1 as kinetic and thermodynamic products, respectively. The results provide insight into features of the energy hypersurface on which this self-assembly operates and point to the general role of kinetic and thermodynamic control in such processes.

Kinetic and Thermodynamic Control in Self-Assembly: Sequential Formation of Linear and Circular Helicates

Kinetic and Thermodynamic Control in Self-Assembly: Sequential Formation of Linear and Circular Helicates

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 Tetra- and Hexanuclear Circular Helicates

The self-assembly of the tris-bipyridine ligands BI and BII with iron(II) salts yields polynuclear complexes displaying structures of cyclic double-helix type, termed circular helicates [n]cH (of order n). With BI in which the bipyridine units in the ligand are connected by ethylene bridges, penta- or hexanuclear architectures [5]cH and [6]cH are obtained, depending on the anion present during the self-assembly process. The elongated tris-bipyridine ligand BII with oxypropylene bridges forms a tetranuclear circular helicate [4]cH, whose structure has been confirmed by crystal structure determination. The possible oligomeric combinations of tris-bipy ligands and iron(II) ions may be considered to constitute the potential members of a virtual combinatorial library, generated via dynamic combinatorial chemistry, from which a specific real constituent of the virtual set of circular helicates is expressed in given conditions.

Self‐Assembly of a Circular Double Helicate

Self‐Assembly of a Circular Double Helicate