The synthesis of a metalled double-helix containing exclusively silver-mediated C*-C* base pairs is reported herein (C*=N hexylcytosine). Remarkably, it is the first crystal structure containing infinite and consecutive C*-Ag -C* base pairs that form a double helix. The Ag ion occupies the center between two C* residues with N(3)-Ag bond lengths of 2.1 Å and short Ag -Ag distances (3.1 Å) suggesting an interesting argentophilic attraction as a stabilization source of the helical disposition. The solid-state structure is further stabilized by metal-mediated base-pairs, hydrogen bonding and π-stacking interactions. Moreover, the angle N(3)-Ag-N(3) is almost linear in the [Ag(N hexylcytosine) ] motif and the bases are not coplanar, thus generating a double-strand helical aggregate in the solid state. The noncovalent and argentophilic interactions have been rationalized based on DFT calculations.

We report the first gold(iii) complex with a cytosine derivative since 35 years. In the crystal structure, the complex stacks by reciprocal regium-bonding interactions. These interactions appear to be common in the Cambridge structure database.

We have recently communicated the important role of lone pair-p, p-p and hydrophobic interactions in the solid architecture of 5-fluoro-1-hexyluracil and 1-hexyluracil (CrystEngComm, 2010, 12, 362-365). As a matter of fact, the simple substitution of a hydrogen atom by a fluorine atom has an enormous consequence in the solid state structure. It has been demonstrated that this is due to an increase in the p-acidity of the ring. In this article we extend the study to other uracil derivatives, where we have changed the hydrophobicity of the hexyl chain by introducing hydrophilic groups in the substituent, such as hydroxyl or carboxylic groups. The latter compounds, i.e. (N 1 -(3-hydroxypropyl)-5-fluorouracil and N 1 -(4-hydroxycarbonylbutyl)-5-fluorouracil monohydrate present interesting fluorine-fluorine interactions that are very important in determining the crystal packing.

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The monomeric [M(ACV) 2 (H 2 O) 4 ]Cl 2 ؒ2ACV (M = Ni II 1 or Co II 2), [Zn(ACV)Cl 2 (H 2 O)] 3 and the polymeric [Cd(ACV)Cl 2 ]ؒH 2 O 4 [ACV = acyclovir = 9-(2-hydroxymethoxymethyl)guanine] complex have been prepared and characterised by X-ray diffraction and IR data; 1 H and 13 C NMR have been used to interpret the structural characteristics of the complexes in solution. Compounds 1 and 2 exist as octahedral complexes with four H 2 O ligands in the basal plane [Ni-OW 2.053(2) and 2.057( 7) Å] and two axial ACV molecules linked to Ni through N( 7) [Ni-N(7) 2.160(2) Å]. Two additional ACV molecules are included in the outer sphere of the complex, interacting by means of hydrogen bonds with the co-ordinated ACVs. This reveals the unprecedented recognition of free acyclovir molecules by Ni(or Co)-ACV (1 and 2). The monomeric zinc() complex 3 exhibits a distorted-tetrahedral geometry, involving two chlorides, the N(7) of the ACV ligand [Zn-N(7) 2.009(2) Å] and a water molecule. The hydrogen bonding of two guanine bases via NH 2 and N(3) (unit 1) with N(3) and NH 2 (unit 2) represents a novel type of interaction between nucleobases. In the case of the cadmium() complex 4 the structure is built by polymeric (CdCl 2 ) n chains which are held together by ACV ligands. The cadmium cation is octahedrically coordinated by four chlorides, the N(7) from an ACV molecule and the hydroxylic oxygen from another ACV molecule, the latter two atoms being placed in cis disposition. On the other hand, the complex [{Hg(ACV)Cl 2 } x ] 5 can be tentatively assigned as a polymer by comparison with analogous guanosine systems and spectroscopic and conductometric data.

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