Metal ion interactions with nucleobases in the tetradentate 1,4,7,10-tetraazacyclododecane (cyclen)-ligand system: Crystal structures of [Cu(cyclen)(adeninato)]·ClO4·2H2O, [{Cu(cyclen)}2(hypoxanthinato)]·(ClO4)3, [Cu(cyclen)(theophyllinato)]3·(ClO4)3·2H2O, and [Cu(cyclen)(xanthinato)]·(0.7ClO4)·(0.3ClO4)·3H2O·(0.5H2O)3

Reaction of 1,4,7,10-tetraazacyclododecane (cyclen) and Cu(ClO4)2·6H2O with nucleobases (adenine, hypoxanthine, xanthine, theophylline, cytosine, or uracil) under alkaline conditions gave four ternary cyclen–metal–nucleobase complexes, [Cu(cyclen)(adeninato)]·ClO4·2H2O (1), [{Cu(cyclen)}2(hypoxanthinato)]·(ClO4)3 (2), [Cu(cyclen)(theophyllinato)]3·(ClO4)3·2H2O (3), and [Cu(cyclen)(xanthinato)]·(0.7ClO4)·(0.3ClO4)·3H2O·(0.5H2O)3 (4), whose crystal structures were determined by X-ray diffraction. In the adenine complex 1, a cyclen-capped square–pyramidal Cu2+ ion binds to an adeninato ligand through N(9) with the formation of an intramolecular interligand hydrogen bond between the secondary amino nitrogen of cyclen and N(3) of the base. In the hypoxanthine complex 2, two cyclen-capped Cu2+ ions bind to a hypoxanthinato ligand, one through N(7) with the formation of an intramolecular N(cyclen)–H···O(6) hydrogen bond and the other through N(9) to form an intramolecular N(cyclen)–H···N(3) hydrogen bond. Similarly, in both the theophylline complex 3 and the xanthine complex 4, each cyclen-capped Cu2+ ion binds to a theophyllinato or xanthinato ligand through N(7) with the formation of an intramolecular N(cyclen)–H···O(6) hydrogen bond. However, unlike in 2, steric constraints between amino group(s) of cyclen and the methyl group at N(3) of theophylline in 3 or the proton attached to N(9) of xanthine in 4 preclude the metal bonding to N(9) in 3 or N(3) in 4. The significance of intramolecular interligand interaction as a factor that affects metal-binding site(s) on nucleobases is emphasized.