Helicate complexes

Figure Bl.17.10. Principles of 3D reconstruction methods, (a) Principle of single axis tomography a particle is projected from different angles to record correspondmg images (left panel) this is most easily realized in the case of a helical complex (right panel), (b) Principle of data processing and data merging to obtain a complete 3D structure from a set of projections.

Spontaneous self assembly of a dinuclear triple helical complex is observed with linked bis-[4,5]-pineno-2,2 -bipyridines. Studies by electrospray mass spectrometry, CD and NMR determined that the major species in solution was a complex of Zn L = 2 3 stoichiometry with a triple helical structure and an enantiomerically pure homochiral configuration at the metal centers. The preference for the formation of one of the possible stereoisomers over the other is of interest.265 Another binuclear triple helical complex is formed from zinc addition to bis[5-(l-methyl-2-(6-methyl-2 -pyridyl)benzimidazolyl)]methane. Spectrophotometric titrations with a zinc solution... 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

Helical complexes, chirality in, 26 803-804 Helical polypeptide, 24 58 Helical ribbon impeller, 16 690, 691 Helicobacter pylori, 15 303 antibiotic resistant, 3 36 Helio-photocatalysis, 19 78, 95 Heliotridine, 2 80... 

The first example of a helical complex with pre-determined chirality was the dinuclear complex [Fe2(rdt)3], where rdtFl2 is the fungal iron chelator rhodotorulic acid, (15), a dihydroxamate siderophore. Several more helical and chiral Fe " " and Fe complexes are documented in the diimine and in the hydroxamate and catechol sections. A doubly looped ( bow tie ) complex has been constructed with the aid of a tris-terimine ligand (Section 5.4.3.5.7). 

Macrocyclic ligands of biological importance as thiophenolate-containing Schiff-base macrocycles and their amine analogs (see review [138]) and new helical complexes with bis(bidentate) SchifF-base ligand [139] were also described. 

Piguet, C. Biinzli, J.-C. G. Bernardinelli, G. Hopfgartner, G. Williams, A. F. Self-assembly and photophysical properties of lanthanide dinuclear triple-helical complexes. J. Am. Chem. Soc. 1993,115, 8197-8206. 

ASSEMBLING/DISASSEMBLING OF HELICATE COMPLEXES DRIVEN BY THE Cu /Cu" COUPLE... 

Figure 2.14 The molecular structure of the [Cu2I(16)2]2 + douhle-strand helicate complex cation. Cu1 metal centers are represented as spheres. The hydrogen atoms of the two strands have been omitted for clarity. Structure redrawn from data deposited at the Cambridge...

The occurrence of the redox-driven reversible assembling-disassembling process involving copper complexes of 16 has been verified through cyclic voltammetry experiments at a platinum electrode in a MeCN solution. Figure 2.17 shows the CV profile obtained with a solution of the double-strand helicate complex [ Cu 21 (16)212 +. 

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

Figure 2.18 A square scheme illustrating the disassembling of the [Cu2(16)2]2 + double helicate complex, following Cu -to-Cu" oxidation, and the consequent assembling of two [Cun(16)]2+ mononuclear complexes, following the Cu"-to-Cu reduction. The process ultimately derives from the geometrical coordinative preferences of the two oxidation states Cu1 prefers a tetrahedral coordination, which can be achieved with the double helicate arrangement Cu11 prefers a square coordination geometry, which is fulfilled by the coordination of a single molecule of 16.

The bidentate ligand 17 forms bis with Cu1 a double-strand helicate complex, whose structure was elucidated through X-ray diffraction studies and is shown in Fig. 2.19. 

Figure 2.20 Cyclic voltammogram of a MeCN solution of [Cu2 (17)2 2 1 double-strand helicate complex. Supporting electrolyte [Bu4N]C104 scan rate 0.2 V/s internal reference electrode Fc+/Fc. Diagram adapted from Ref. 21.

Double-strand dicopper helicate complexes are interesting systems in that they may show hysteresis (as observed with ligand 16), thus giving rise to a rare example... 

During the 1980s several laboratories prepared and investigated double-stranded helical complexes, systems containing either pyirolic ligands [75, 76] and derivatives [77-79] (with Zn2+, Ag+, Cu+) or oligomers of 2,2 -bipyridine [80, 81]. Helicates [80-84] may consist of up to five copper centers and these systems are reminiscent of the DNA double helix. 

More recently, oligobidentate ligands were also used for the synthesis of various triple-stranded helical complexes around metals with a preference for octahedral coordination, e.g. Co(II) [85] or Ni(II) [86], or around Ag(I) [87] and Ga(III) ions [88]. 

---