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...

trans-1,4-Cyclohexanedicarboxylic acid (t-1,4-chdcH) or the commercially available mixture of the cis and trans isomers (c,t-1,4-chdcH) has been used in the synthesis of a series of 14 uranyl ion complexes, all obtained under solvohydrothermal conditions, some in the presence of additional metal cations and/or 2,2'-bipyridine (bipy). With its two isomeric forms having very different shapes and its great sensitivity to the experimental conditions, 1,4-chdc appears to be suitable for the synthesis of uranyl ion complexes displaying a wide range of architectures. Under the conditions used, the pure trans isomer gives only the complexes [UO(t-1,4-chdc)(HO)] (1) and [UO(t-1,4-chdc)] (2), which crystallize as one- and two-dimensional (1D and 2D) species, respectively. Complexes containing either the cis isomer alone or mixtures of the two isomers in varying proportion were obtained from the isomer mixture. The neutral complexes [UO(c-1,4-chdc)(DMF)] (3) and [UO(c-1,4-chdc)(bipy)] (4) are 2D and 1D assemblies, respectively, while all the other complexes are anionic and include various counterions. [C(NH)][HNMe][(UO)(c-1,4-chdc)]·HO (5) crystallizes as a three-dimensional (3D) framework with {10} topology. While [HNMe][(UO)(c-1,4-chdc)(t-1,4-chdc)]·DMF·2HO (6) is a 1D ladderlike polymer, [HNMe][(UO)(c-1,4-chdc)(t-1,4-chdc)]·2HO (7), which differs in the cis/trans ratio, is a 3-fold 2D interpenetrated network with {6} honeycomb topology. The related [HNMe][(UO)(c,t-1,4-chdc)]·2.5HO (8), with one disordered ligand of uncertain geometry, is a 3-fold 3D interpenetrated system. The two isomorphous complexes [Co(bipy)][(UO)(c-1,4-chdc)]·1.5HO (9) and [Cd(bipy)][(UO)(c-1,4-chdc)]·1.5HO (10) form 3D frameworks with the {10} srs topological type. In contrast, [Ni(bipy)][(UO)(c-1,4-chdc)(t-1,4-chdc)(NO)]·2HO (11) is a molecular, tetranuclear complex due to the presence of terminal nitrate ligands. A 2-fold 3D interpenetration of frameworks with {10} ths topology is observed in [Cu(bipy)][(UO)(c-1,4-chdc)(t-1,4-chdc)]·2HO (12), while [Zn(bipy)][(UO)(c-1,4-chdc)]·4HO (13) crystallizes as a 2D net with the common {4.8} fes topological type. The additional Pb cation is an essential part of the 3D framework formed in [UOPb(c-1,4-chdc)(t-1,4-chdc)(bipy)] (14), in which uranyl and its ligands alone form 1D subunits. Together with previous results, the solid-state uranyl emission properties of seven of the present complexes evidence a general trend, with the maxima for the complexes with O equatorial environments being blue-shifted with respect to those for complexes with O environments.

An overview is given of recent structural studies on heterometallic uranyl-containing species, with particular emphasis on those formed with carboxylate ligands, showing the effect of the additional metal cations and ancillary ligands on the dimensionality, geometry and topology of the assemblies generated. These examples illustrate the level of elaboration which is now possible in the design and synthesis of both uranyl clusters and coordination polymers, properties and potential applications of which are also briefly discussed.

International audienceUranyl nitrate was reacted with several polycarboxylic acids under solvo-/hydrothermal conditions using N-methyl-2-pyrrolidone (NMP) as the organic component to give six novel complexes, which were crystallographically characterized. NMP is coordinated to the uranyl ion in all cases but one. The complex with terephthalic acid (H2tph), [UO2(tph)(NMP)] (1), crystallizes as a three-dimensional (3D) framework, an unprecedented feature in uranyl complexes with this ligand. The two complexes obtained with 2,5-thiophenedicarboxylic acid (H2thd), [UO2(thd)(NMP)] (2 and 3), crystallize as 3D frameworks having the same formula and topology but different packings. Two complexes were also obtained with 1,3,5-benzenetriacetic acid (H3bta), [(UO2)3(bta)2(NMP)3]·0.5H2O (4) and [Hbipy][UO2(bta)]·H2O (5). Complex 4, with NMP coordinated, is a 2D assembly, with a sheet thickness of ?12 Å arising from the superposition of three sublayers. Complex 5, obtained in the presence of 2,2?-bipyridine (bipy), is a 1D polymer with a nanotubular shape subtended by π-stacking interactions. In the additional presence of nickel(II) nitrate, nitrilotriacetic acid (H3nta) gives the complex [(UO2)2Ni(nta)2(NMP)2]·NMP (6), which crystallizes as a 3D framework that, like complexes 1 and 3, displays channels containing coordinated or free NMP molecules. The emission spectra under excitation at 350 nm were measured for all of the complexes. The usual vibronic fine structure in the ?460?600 nm range is apparent for 1?5, while 6 shows only a weak and featureless band indicative of quenching of the uranyl luminescence by Ni(II). The properties of these complexes illustrate the potential of solvo-/hydrothermal methods, particularly those involving coordinating organic solvents, for the synthesis of new uranyl?organic species