trans-a(2)Pt(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-methyladeni ne (9-MeA), 9-methylhypoxanthine (9-MeHxH), and 9-methylguanine (9-MeGH) ha ve been prepared and in part (2, 3a, 4b) X-ray structurally characterized: trans-[(NH3)(2)Pt(9-MeA-N7)(9-MeHxH-N7)](ClO4)(2) (2), trans,trans-[a(2)(9- MeGH-N7)Pt(9-MeA-N7,N1)Pta(2)(9-MeGH-N7](ClO4)(4). nH(2)O (a = NH3, n = 2 ( 3a); a = CH3NH2, n = 2 (3b)), trans, trans-[a(2)(9-MeHxH-N7)Pt(9-MeA-N7,N1) Pta(2)(9-MeHxH-N7)]X3.4Y0.6. nH(2)O (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(4 c)). 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 oriente d head-head and display a donor (D), acceptor (A) sequence for II bonding o f 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 thro ugh pairs of O(6)...N(1) intermolecular H bonds between 9-MeHxH and 9-MeGH ligands, respectively, into metalated base sextets. Ag+ coordination in 5 t akes place in a bridging fashion through N(1) and N(3) of 9-MeA, leading to a helical [Ag-N-C-N](infinity) backbone with 9-MeA acting as a tridentate ligand. (C) 2000 Elsevier Science S.A. All rights reserved.