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...

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Inorganic double helices having 2 and 2.5 turns are formed by the tetra‐ and penta(bipyridine) ligands 1 and 2, respectively, upon addition of Cu1 salts. These polynuclear complexes, in which the copper ions are aligned like a string of pearls, are estimated to be about 22 and 27 nm long, respectively.

Dedicated to Edgar Heilbronner on the occasion of his 80th birthdayWe report in this paper the coordination and kinetic properties of two oligobipyridine strands, which contain three 2,2'-bipyridine subunits separated by oxydimethylene bridges, the 4,4'-bis(CONET 2 )-substituted L and the 4,4'-bis(CO 2 Et)-substituted L'. Spectrophotometric measurements allowed the characterization of thermodynamic complexes and kinetic intermediates* which are involved in the self-assembly process of L 2 Cu 3 and L ' 2 Cu 3 helicates. The reaction presents positive cooperativity for the binding of two 2,2'-bipyridine strands to the cuprous cations. While reactive kinetic intermediates* present distorted coordination geometries around Cu I , the final rearrangement of the tricuprous bistranded helicates allows more closely tetrahedral coordination of each cation and reduces the interactions. Differences in the bulkiness and electronic properties of the L and L' substituents do not affect significantly the stability of the corresponding helicates, but greatly influence binding rates in the self-assembly process.Introduction. ± In recent years, creating molecules that are programmed by virtue of their structure and binding sites to spontaneously organize themselves into supramolecular architectures held together by metal coordination has aroused the interest of chemists [1 ± 5] (for recent reviews on metal-ion-mediated self-assembly, see, e.g., [2] ], were generated. To design such species presenting specific structural and functional features, it is of great importance to establish the rules by which control the assembly process can be achieved through chemical programming by means of suitable components and assembling algorithms [1] [5]. In the self-assembly of helicates, three main structural features determine the nature and shape of the helical species formed. The binding sites impose the number of strands able to coordinate metal ions with a given geometry. Bidentate or tridendate subunits combined, respectively, with metal ions of tetrahedral [6] [7] and of octahedral [6] [24] coordination geometry produce double helical polynuclear species, while triple helical complexes are formed with bidentate ligands and octahedrally coordinated metal ions [25]. Taking into account these properties, double-helical mixed-valence [14] heteronuclear [26] or heterostranded [27] complexes have been synthesized. However, not every combination of oligomultidentate ligands with metal ions will give helical structures, and the spacers between the metal binding sites play an important role. They should be flexible enough to allow the ligand to bind

Professor Edgar Lederer zum 80. Geburtstag gewidmetDer Entwurf molekularer Spezies, die unter Selbstassoziation wohlgeordnete Uberstrukturen aufbauen, offnet den Weg zur spontanen, zielgerichteten Bildung organisierter iibermolekularer Einheiten; dabei wird ein ,, Aufbauplan" verfolgt, der In Fortfiihrung unserer Arbeiten iiber Quaterpyridine pQPI2] zeigten wir kiirzli~h [~], daB die acyclischen Liganden BPZ 2 und BP, 3, die zwei bzw. drei bpy-Einheiten enthalten und aus 6,6'-Dimethyl-2,2'-bipyridin 1 synthetisiert wurden, Cu'-lonen in einer Weise binden, daB zwei-bzw. dreikernige doppelstrangige ,,Helicate" (Helix-Metallkomplexe) gebildet werden; dabei ,,wickeln" sich jeweils zwei Ligandmolekiile um zwei bzw. drei Cu'-Ionen (ein Ion pro bpy-Einheit jedes Liganden), die die Strange zusammenhalten. Die Strukturen des zweikernigen Komplexeswurden durch Rontgenbeugungsanalyse bestimmtlsl. Urn die Mdglichkeiten, mit Polybipyridin-Liganden Doppelhelix-Komplexe durch Spontanaufbau zu erhalten, weiter zu studieren, synthetisierten wir die nachsten beiden Glieder dieser Ligandklasse, BP,, 4 und BP5 5 und untersuchten ihre Cu'-Bindungseigenschaften.Sukzessive Umsetzung des Hydroxymethylbipyridins 613] mit NaH (THF, Raumtemp., 30 min) und einem Aquivalent der Bis(brommethy1)-Verbindung 716] (3 h RiickfluO) ergab ein Produktgemisch, aus dem durch Chromatographie an Si02 (1% MeOH in CH2CI,) die Brommethyl-Verbindung 8 (Fp= 152-I55OC) in 45% Ausbeute isoliert werden konnte; daneben wurden 15% des Trimers 3IJ1 erhalten und 30% 7 zuriickgewonnen. Reaktion von 8 mit NaOAc (Dimethylformamid, 140"C, 12 h) und Hydrolyse des Produkts mit NaOH (MeOH/H20 (9/1), 0.5 h RiickfluS) fiihrten zur Hydroxymethyl-Verbindung 9 (Fp= 178"C, 80% Ausb.). Sukzessive Umsetzung von 9 mit je einem Aquivalent KOtBu (THF, Raumtemp., 30 min) und 8 (12 h RuckfluI3) ergab als Niederschlag das Tetramer 4 (Fp>26O0C) in 65% Ausbeute. Analog kann aus 9 und 0.