trans-a₂Pt(II)-(a = NH₃, CH₃NH₂) modified purine base pairs and triples: Hydrogen bonding between self-complementary pairs and triples and heterometal (Ag⁺) coordination leading to a 1 D helix

A series of complexes containing the purine model nucleobases 9-methyladenine (9-MeA), 9-methylhypoxanthine (9-MeHxH), and 9-methylguanine (9-MeGH) have been prepared and in part (2, 3a, 4b) X-ray structurally characterized: trans-[(NH₃)₂Pt(9-MeA-N7)(9-MeHxH-N7)](ClO₄)₂ (2), trans, trans-[a₂(9-MeGH-N7)Pt(9-MeA-N7, N1)Pta₂(9-MeGH-N7)](ClO₄)₄·nH₂O (a = NH₃, n=2 (3a); a=CH₃NH₂, n =2 (3b)), trans,trans-[a₂(9-MeHxH-N7)Pt(9-MeA-N7,N1)Pta₂(9-MeHxH-N7)]X_3.4Y_0.6nH ₂O (a = NH₃, X = Y = ClO₄^-, n = 3 (4a); a = NH₃, X = ClO₄^-, Y = Cl^-, n = 2.5 (4b); a = CH₃NH₂, X = Y = ClO₄^-, n = 2 (4c)). In addition, a helical coordination polymer derived from 2 and AgNO₃, trans-[{(NH₃)₂(9-MeHxH-N7)(9-MeA-N7)}Ag(NO₃)-(H₂O)](ClO₄)(NO₃) (5), has been obtained and studied by X-ray analysis. The two bases in 2 are oriented head-head and display a donor (D), acceptor (A) sequence for H bonding of DADA, making it thus self-complementary and causing it to crystallize as a dimer, hence as a dimetalated base quartet. Both 3a and 4b associate through pairs of O(6)<dummy>N(1) intermolecular H bonds between 9-MeHxH and 9-MeGH ligands, respectively, into metalated base sextets. Ag⁺ coordination in 5 takes place in a bridging fashion through N(1) and N(3) of 9-MeA, leading to a helical [Ag-N-C-N]∞ backbone with 9-MeA acting as a tridentate ligand. (C) 2000 Elsevier Science S.A.