Bis(purine) complexes of trans-a sub(2)Pt super(II): Preparation and X-ray structures of bis(9-methyladenine) and mixed 9-methyladenine, 9-methylguanine complexes and chemistry relevant to metal-modified nucleobase triples and quartets

Mononuclear bis(purine) complexes of trans-a sub(2)Pt super(II), trans-[(NH sub(3)) sub(2)Pt(9-MeA-N7) (9-MeGH-N7)](NO sub(3)) sub(2)-H sub(2)O (1b) and trans-[(NH sub(3)) sub(2)Pt(9-MeA-N7) sub(2)](ClO sub(4)) sub(2)-H sub(2)O (3c), have been prepared and their structures determined by X-ray crystallography. 1b: Space group P1, a = 7.245 (5) angstrom, b = 7.715(6) angstrom, c = 10.907(8) angstrom, alpha = 82.36(6) degree , beta = 86.62(6) degree , gamma = 70.15(6) degree , V = 568.3(7) angstrom super(3), Z = 1. 3c: Space group P2 sub(1) /c, a = 8.312(2) angstrom, b = 15.386(3) angstrom, c = 12.365(2) angstrom, beta = 94.83(3) degree , V = 1575.72(55) angstrom, Z = 2. The cation of 3c is centrosymmetric. In the cation of 1b, the two purines adopt a head-head orientation with an intramolecular H bond of 2.94(3) angstrom between the exocyclic amino group of 9-MeA and the exocyclic carbonyl group of 9-MeGH. Di- and trinuclear derivatives of 1b and 3c have been synthesized and/or studied in solution. They include compounds of types Cl-N1-A-N7-GH (2a), T-N1-A-N7-GH (2b), GH-N1-A-N7-T (2c), and GH-N7-A-N1-N1-A-N7-GH (2d) as well as X-N1-A-N7-N7-A-N1-X (X = Cl (4), GH (5a), T (5b)) with - representing trans-a sub(2)Pt super(II) entities (a = NH sub(3) or CH sub(3)NH sub(2)). The fact that Pt-(A-N1) and Pt-(A-N7) vectors are at right angles and the nucleobases essentially coplanar in many cases leads to intramolecular H bonding involving NH sub(2)(6) of 9-MeA and exocyclic groups of the other nucleobases. The chemical shifts of the NH sub(2) protons of 9-MeA in DMSO-d sub(6) or DMF-d sub(7) permit a differentiation between the various possibilities (no H bonding, single H bond, 2 H bonds, 1 Pt or 2 Pt coordinated to 9-MeA). As far as intermolecular H bonding is concerned, the neutral 9-MeGH ligand in 1b forms a Watson-Crick pair with 1-MeC but a 9-MeGH identical with 9-MeG pair at pH 8. The potential usefulness of the complexes prepared with regard to the formation of two-dimensional sheet structures and molecular squares built up of purine nucleobase and trans-a sub(2)Pt super(II) entities is briefly discussed, as are aspects of the stabilization of triplex nucleic acid structures by metal ions.