Coordination chemistry helical structures

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. 

This concept may also be extended to polynuclear helicates [38]. When 2-amino-quinoline and 4-chloroaniline were mixed with the phenanthroline dialdehyde shown in Scheme 1.10, a dynamic library of potential ligands was observed to form. The addition of copper(I) causes this library to collapse, generating only dicopper and tricopper helicates. As in the mononuclear case of Scheme 1.9, the driving force behind this selectivity appeared to be the formation of structures in which all ligand and metal valences are satisfied. The use of supramolecular (coordination) chemistry to drive the covalent reconfiguration of intraligand bonds thus... 

A more recent example of dynamic coordination chemistry controlled by anion recognition was demonstrated with the series of ligands 2, which formed linear binuclear triple helicates with Co2+ anions (Fig. 4) [16,17]. The asymmetric ligand 2a, functionalized at one end with an amide hydrogen-bond donor group, may form two types of helicate structures head-to-head-to-head (HHH) and head-to-head-to-tail (HHT) isomers. With the weak hydrogen-bond acceptor C104 as counteranion, a 3 1 mixture of HHH HHT isomers was observed. However, with the... 

Discrete infinite double helices are quite rare in inorganic and coordination chemistry [Fig. 2(e)]. Ciani has reported that reaction of L21 with Ag salts in the ratio 2 1 gave completely different and noteworthy products, one of which is the infinite double-helical coordination polymer [Ag(L2i)]CF3S03 (50). The structure shows the balanced packing of left-handed and right-handed double helices of cationic —Ag—L—Ag—L— chains (Fig. 11). The period of the helices is... 

The spherical Cu(I) cation (d ° electronic configuration) is thus ideally suited for the formation of double-stranded helicates with LI and L4-L6, because of its lack of directional coordination bonds and its limited electrostatic factor z /R = 1.30 eu A (z is the charge of fhe cation and R is its ionic radius Shannon, 1976), which is compatible with CN = i (Figure 5). The demonstration of this novel concept in coordination chemistry, combined with the undeniable aesthetic appeal of helical strucfures, were at the origin of a considerable enthusiasm leading to the isolation and structural characterization of a plethora of binuclear double-, triple-, and quadruple-stranded helicates during the past two decades (for comprehensive reviews, see Albrecht, 2001 Constable, 1992, 1994, 1996 ... 

Among the most eye-catching architectures resulting from the self-assembly process in supramolecular coordination chemistry are helicates. The term helicate was introduced by Lehn and coworkers,who made a great contribution to this area (see below). It is believed that the double helical structure of DNA represents a continuous inspiration in this area and chemists are tempted to produce such appealing motifs, namely by the use... 

In the last decades, the design and fabrication of helical architectures that mimic some of these bio-related helices have become a very big challenge for supramolecu-lar chemists. Among the different approaches that one can use, coordination chemistry has certainly become one of the most valuable tools to obtain helical coordination structures. The first efforts were dedicated to the design of discrete coordination complexes with helicate architectures. " However, more recently, the number of reported helical CPs has increased exponentially, becoming one of the most promising ways to construct these fascinating systems. 

The stabilization of well-defined peptide microstructures is an important challenge in bioorganic chemistry. As the presented results show, metal coordination can be a simple but very effective tool for reaching this goal and fixing three dimensional molecular structures. The conformational (and stereochemical) information which is embedded in configurationally stable metal complex units can be transferred to amino acid residues or even to relatively large peptides and can induce helical-, sheet- or as discussed turn-type structures. 

The structural chemistry and solution behavior of lithium diisopropylamide (LiN(CHMe2)2, EDA) typifies the complexities of these systems. Although the base-free compound crystallizes as infinite helical chains with two-coordinate lithium and four-coordinate nitrogen, it is isolated from A,A,A, A -tetramethylethylenediamine (TMEDA)/hexane mixtures as an infinite array of... 

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