Triple-stranded helical metal 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]. 

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]

The racemization of dinuclear triple-stranded helicates has been studied. These complexes adopt either AA or AA configurations arising from the tris-bidentate coordinated metal centers. A mononuclear tris-catecholate complex ML3 racemizes via a Bailar twist mechanism. Linked tris-catecholate metal centers would be expected to racemize at the same rate as that of the mononuclear complex in the absence of mechanical coupling. However, when two tris-catecholate complexes are linked in a helicate, the racemization of the M2L3 structure (from AA to AA) slows by a factor of one hundred, while racemization of the four tris-catecholate metal centers in an M4L6 tetrahedron M4L6 is not observed. Although the components of these assemblies are labile, the chiral tetrahedral structure displays structural inertness. 

Double-stranded and triple-stranded helicates as well as double helical and triple helical metal complexes, are formed by the spontaneous organisation of two or three linear polybipyridine ligands of suitable structure into a double or a triple helix by binding of specific metal ions displaying respectively tetrahedral (Cu ) and octahedral (Ni ) coordination geometry. These species are illustrated by the trinuclear double helicate 1 [27] and triple helicate 2 [28] (see also [29]). 

Triple Helicates. The steric information contained in the oligo-bipy strands based on bipy units connected in the 6,6 positions is designed to yield double helices on complexation of metal ions undergoing tetrahedral coordination. Steric effects due to the 6,6 -disubstitution hinder the binding of metal ions of octahedral coordination geometry, which would be expected to lead to triple helical complexes. 

Inorganic double or triple helices are formed by two or three ligand strands wrapped around linearly disposed metal ions [13], Among cyclic transition metal complexes, circular helicates [nJ cH [ ] "cH is a general notation characterizing circular helicates (cH) with n = number of metal ions and m = helicity (m = 2 for a double helix) have specific features and may be considered as toroidal helices [34]. There are two different kinds of circular helical systems. Some structures self-assemble from the metal ions and the ligands only in the presence of an anion, which could act as a template [34,35,64-67], whereas, in other cases, the circular helicates self-assemble from the metal ions and the ligands alone [68-70]. 

SCHEME 2.1 Self-assembly of triple stranded dinuclear helicate from three linear ligand strands and two metal ions (top). Hierarchical self-assembly of helicates by initial formation of mononuclear complexes followed by metal coordination of two units to several bridging metal ions (bottom). 

Further developments involve the investigation of the mechanism of formation of double- and triple helicates and of the effect of variations in ligand structure on their features, the determination of their physico-chemical (thermodynamic, kinetic, electrochemical, photochemical) properties, the exploration of the coordination chemistry of the ligand strands. For instance, it may be possible to obtain quadruple helical complexes with ions of high coordination number such as the lanthanides and linear ligands containing bipy or terpy units. Using cubic metal ions would also be of interest. 

The use of semirigid tridentate binding units restricts the structural issues to [] (L ) ] whereby n = 1-3 characterizes the number of strands wrapped about the central metal. The target Cs-symmetrical triple-helical complex [f (L )3] can be further produced as the major species thanks to a strict control of the conditional information expressed in the stoichiometric ratio f L = l 3 (i.e., entropic selection). 

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