Spontaneous assembly of double-stranded helicates from oligobipyridine ligands and copper(I) cations: structure of an inorganic double helix.

Abstract

Two oligobipyridine ligands containing two and three 2,2'-bipyridine subunits separated by 2-oxapropylene bridges have been synthesized and some of their complexation properties with metal ions have been investigated. In particular, with copper(I) they form, respectively, a dinuclear and a trinuclear complex containing two ligand molecules and two or three Cu(I) ions. In view of the pseudotetrahedral coordination geometry of Cu(I)-bis(bipyridine) sites and of NMR data indicating that the present complexes are chiral, one may assign to these dinuclear and trinuclear species a double-helical structure in which two molecular strands are wrapped around two or three Cu(I) ions, which hold them together. These complexes may thus be termed "double-stranded helicates." Determination of the crystal structure of the trinuclear species has confirmed that it is indeed an inorganic double helix, possessing characteristic features (helical parameters, stacking of bipyridine bases) reminiscent of the DNA double helix. This spontaneous formation of an organized structure by oligobipyridine ligands and suitable metal ions opens ways to the design and study of self-assembling systems presenting cooperativity and regulation features. Various further developments may be envisaged along organic, inorganic, and biochemical lines.Molecular helicity is a fascinating property displayed by chemical and biological macromolecular structures such as the a-helix of polypeptides and the helical conformation of polymers (1,2). Particularly well known is the double helix present in nucleic acids (3), whose structure, formation, and dissociation have been the subject of very extensive studies. Helicity has been analyzed for twisted chains of atoms (4) and its basic geometrical features are found in several types of small molecules (5, 6).We report here results on a class of organic ligands of poly(2,2'-bipyridine)t nature, which, by binding metal ions of specific coordination geometry, undergo spontaneous organization into a helical double-stranded, polymetallic complex, in effect an inorganic double helix, reminiscent of the double-helical structure of nucleic acids (3, 7). Design Principle. Previous work on the dinuclear Cu(I) complex [Cu2(pQP)2](CI04)2 of a special quaterpyridine ligand, pQP, has shown that in this dimeric species, two pQP molecules bind two Cu(I) ions in a distorted tetrahedral coordination geometry, using a bipyridine subunit from each quaterpyridine chain (8); the two pQP molecules possess a twisted, chiral conformation and are wrapped around the two Cu(I) ions (Fig. 1). Related structural features may be found in some other dinuclear metal complexes (9, 10). Suitable modification of the pQP ligand and extension of its basic features might lead to a general class of ligands capable of forming double-helical complexes. Rather than simply using polypyridine chains, it appeared desirable to preserve the basic bipyridine units in the ligand structure and to link several such groups by a bridge that would isolate the co...

Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-β-diketonato ligands

“…2 In now classical studies, when tetrahedral copper(I) was reacted with a linked poly(bipyridyl) ligand containing flexible spacers between the individual bipyridyl groups then a double helix was generated. 3 In contrast, the use of an octahedral metal (such as high-spin nickel(II)) in an analogous study employing a closely related ligand system yielded a triple helix. 4 Besides helices, there have now been many reports 5 of metallosupramolecular assemblies exhibiting a wide range of molecular architectures whose shape is dictated by the metal directing properties of a given metal ion coupled with the steric requirements of an appropriate ligand component or components.…”

Synthetic selectivity through avoidance of valence frustration

“…A diimine formed by A may readily occupy three coplanar meridional coordination sites in an octahedral complex (36), but its geometry is ill-suited to tridentate chelation within the pseudotetrahedral coordination environment favored by copper(I). Multinuclear helical complexes, in contrast, are well adapted to the preferred geometry of this metal ion (37).…”

Dynamic Helicity Control of Oligo(salamo)-Based Metal Helicates

“…On the other hand, more stable helical structures can be obtained by metal coordination, which are useful for obtaining well-defined structures. The chemistry of helical structures based on metal coordination has been initiated from the self-assembled double helicates that are obtained by complexation of oligobipyridine ligands with metal ions [12]. To date, various kinds of helical structures including single-, double-, and triple-helicates have been synthesized by metal complexation from the appropriate organic structures containing multiple coordination sites and linkers [13,14].…”