Supramolecular architectures assembled by the interaction of purine nucleobases with metal-oxalato frameworks.: Non-covalent stabilization of the 7H-adenine tautomer in the solid-state

The synthesis, crystal structure and variable-temperature magnetic measurements of the compounds [Mn(mu-ox)(H2O)(7H-pur-kappa N-9)](n) (1), {[Mn(mu-ox)(H2O)(2)]center dot(7H-ade).(H2O)}(n) (2) and {[Cu(mu-ox)(H2O)-(7H-ade-kappa N-9)][Cu(mu-ox)(mu-H2O)(7H-ade-kappa N-9)]center dot similar to 10/3H(2)O}(n) (3), (where ox: oxalato dianion, pur: purine, and ade: adenine) are reported. Compounds 1 and 2 contain one-dimensional chains in which manganese(n) atoms are bridged by bis-bidentate oxalato ligands. The distorted octahedral geometry around each metal centre is completed in compound 1 by one water molecule and the imidazole N9 donor site of the purine ligand, which is a rare example of direct binding between the Mn(n) ion and the N donor site of an isolated nucleobase. Unlike 1, the adenine moiety in compound 2 is not bonded to manganese atoms and the metal coordination polyhedron is filled by two water molecules in a cis-arrangement. Its crystal building is constructed from pi-stacked layers of Watson-Crick hydrogen-bonded adenine center dot center dot center dot (H2O)(2) center dot center dot center dot adenine aggregates and zig-zag Mn(II)-oxalato chains held together by means of a strong network of hydrogen bonding interactions. The nucleobase exists in the lattice as the 7H-adenine tautomer which represents an unprecedented solid-state characterization of this minor tautomer as free molecule (without metal coordination) stabilized through non-covalent interactions. Compound 3 consists of two slightly different [Cu(ox)(H2O)(7H-ade-kappa N-9)] units in which the nucleobase coordinates through the imidazole N9 atom. The planar complex entities are parallel stacked and joined by means of long Cu-O bonds involving oxygen atoms from the oxalato and the aqua ligands, giving one-dimensional chains with a [4 + 1] square-planar pyramidal and a [4 + 2] octahedral coordination around the metal centre, respectively. Self-assembled process of compound 3 is further driven by an in-plane network of hydrogen bonding interactions to generate a porous 3D structure containing parallel channels filled by guest water molecules. Variable-temperature magnetic susceptibility measurements of all the complexes show the occurrence of antiferromagnetic interactions between the paramagnetic centres. DFT calculations have been performed to check the influence of packing in the stability of the 7H-amino tautomer of 2 and in the complex geometry of