Supramolecular architectures of metal-oxalato coordination polymers bearing N-tethered adenine nucleobases

Abstract Reactions of N-methylated adenine (9-methyladenine and 3-methyladenine) with water-soluble oxalato complexes in the presence of potassium oxalate yields a series of oxalato-bridged complexes with the nucleobases as pendant ligands whose crystal structures and magnetic properties are reported herein. The complexes [M 2 (µ–ox) 2 (9Meade) 3 ] n [M(II) = Zn ( 1 ), Cu ( 2 ); where ox = oxalato and 9Meade = 9-methyladenine] are composed of infinite corrugated one-dimensional chains in which two types of metal(II) centres are sequentially bridged by asymmetric bis-bidentate oxalato ligands and their coordination polyhedra are filled by N7 -coordinated 9-methyladenine molecules giving rise to two different chromophores: an octahedral N 2 O 4 donor set with the nucleobase moieties in trans arrangement, and square based pyramidal NO 4 coordination polyhedra. The cohesiveness of their supramolecular 3D architectures is achieved by means of an extensive network of non-covalent interactions including a short contact between the minor grove N3 site of the model 9-methyladenine and the oxalato bridging ligand from a neighbouring chain. This unconventional interaction between the lone pair of the pyrimidinic nitrogen atom of the nucleobase and the π-system of the oxalate anion has been analysed and discussed in the light of available structural data for analogous systems. The compounds {[M(µ –ox)(H 2 O)(3Meade)]·H 2 O} n [M(II) = Zn ( 3 ), Co ( 4 ) and Mn ( 5 ); where 3Meade = 3-methyladenine], and {[Cu(µ –ox)(3Meade) 2 ]·4H 2 O} n ( 6 ) are comprised of zig-zag metal-oxalato chains, with the oxalato adopting a symmetric bis-bidentate coordination mode. The octahedral environment of the metals is completed in a cis-arrangement by one water molecule and the imidazole N7 donor site of a 3-methyladenine ligand for 3 - 5 , and by two N7-coordinated nucleobases for 6 . The crystal packing of the chains leads to the presence of channels with dimensions of 5 x 10 A for 6, and 4 x 6 A for 3 - 5 which are occupied by crystallization water molecules. The non-covalent interactions of the nucleobases among them and with the metal-oxalato framework have been studied in detail as well as the magnetic interactions mediated by the bridging oxalato ligand.