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

Abstract

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)]X3.4Y0.6·nH2O (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)⋯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.