Triple-stranded helical complexes

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]. 

Zclikovich, L., Libman, J., and Shanzer, A. Molecular redox switching based on chemical triggering of iron translocation in triple-stranded helical complexes. Nature 199.5, 374, 790-792. 

Renaud F, Piguet C, Bemardinelli G, Bunzli JCG, Hopfgartner G (1997) In search fw mononuclear helical lanthanide building blocks with predetermined properties triple-stranded helical complexes with N,N,N, N -tetraethylpyridine-2, 6-dicarboxamide. Chem Eur J... 

Reaction of the bis-tridentate receptor [L14-2H]2 with R(C104)3 in water produces the highly stable homobimetallic triple-stranded helicates [R2(L14-2H)3] (R = La-Tb except Pm and R = Er-Yb-Lu, log/S[S2(Li4 2H)3] = 51(4), Elhabiri et al. (1999)). NMR spectra point to rigid D3-symmetrical complexes for which exchange between the helical enantiomers is slow on the NMR time scale (i.e., the methylene protons of the ethyl residues are systematically diastereotopic, fig. 55). 

In 1994, Hanessian and co-workers [50] reported the first examples of metal-free three-dimensional triple-stranded helicates through spontaneous self-assembly of chiral C2-symmetrical diols and chiral C2-symmetrical diamines. The initial observation resulted from the utilization of enantiopure C2-symmetrical vicinal trans-1,2-diaminocyclohexane [51,52] as ligands in the asymmetric dihydroxylations of olefins [53] and as reagents for asymmetric synthesis [54], When equimolar amounts of (5,5)-frfl x-l,2-diaminocyclohexane (28) and its (i ,i )-enantiomer (29) were individually mixed with (5,5)-frfl x-l,2-cyclohexanediol and heated in refluxing benzene, crystals of the respective supraminol complexes 28 30 and 29 30 were formed quantitatively (Scheme 12). This was the physical basis for the separation of racemic diols with tr[Pg.104]

A lower level of organization in the supramolecular structure is evident even if the layered structure of the core and the stacking of the cyclohexane residues are retained. In this context, ordered H-bonded ionic triple-stranded helicates, involving (R,R)-frdicarboxylic acid, (R,R)-44, in D2O afforded the complex (R.R)-29 (R,R)-44 as large, colourless crystals suitable for X-ray analysis (Scheme 20). The same reaction performed in the presence of the mismatched (5,5)-tra -cyclohexane-l,2-dicarboxylic acid (45) did not give any solid material. A similar result was obtained with (S)-methylsuccinic acid. 

The space-filling representation of the complex (R,R)-29 (R,R)-44 in which the triple-stranded helicate super-structure is more evident is shown in Figure 54. 

However, it is inadequate to believe that any mixture of bis-tridentate ligands with will produce triple-stranded helicates. We already mentioned in the introduction how some competing coimter-ions or solvent molecules may prevent the fixation of the third strand, thus leading to unsaturated complexes (Section 1.3, Figure 13B). A second illustrative example is shown in Figure 53, whereby the use of a 1-4-disubstituted phenyl spacer in L33 makes the output of the assembly process extremely sensitive to the stoichiometric k L33 ratio (Ronson et al., 2007). 

FIGURE 50 Formation of binuclear triple-stranded helicates with neutral bis-tridentate homotopic C2 -symmetrical ligands (A) NNN-S-NNN and (B) ONN-S-NNO. The structures of the complexes correspond to the crystal structures of [Eu2(Lll)3] and [Tb2(L13 )3] (redrawn after Martin et al., 1998 Piguet et al., 1993a,b,c. 

FIGURE 52 Formation of binuclear triple-stranded helicates with a neutral bis-triden-tate heterotopic Q-symmetrical ligand. The structure of the complex corresponds to the crystal structure of [Eu2(L30 )3] (redrawn after Andre et al., 2004). 

Both through-bond and pseudocontact contributions can be easily factorized into a series of products of two terms, each term depending either on the nucleus i (topologic and geometric location) or from the lanthanide j (electronic structure and crystal-field effects). For axial complexes, that is, possessing at least a three-fold axis as found in triple-stranded helicates, the molecular magnetic susceptibility tensor written in the principal magnetic axes system is symmetrical xx = mag-... 

FIGURE 69 Thermodynamic modeling of the formation of the eight microspecies contributing to the binuclear triple-stranded helicates [(R )x(R )2-xL3] in acetonitrile (L is a nonaxial ligand). (A) HHT HHH isomerization process (Eq. (16)) and thermodynamic cycle and (B) complexation reactions. For the sake of simplicity, the unbound R " cations are systematically considered as tricapped-trigonal prismatic nine-coordinate solvates. 

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