Bis(purine) complexes of trans-a(2)Pt(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(2)Pt(II), trans-[(NH3)(2)Pt(9-MeA-N7)(9-MeGH-N7)](NO3)(2) . H2O (1b) and trans-[(NH3)(2)Pt(9-MeA-N7)(2)](ClO4)(2) . H2O (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)degrees, beta = 86.62(6)degrees, gamma = 70.15(6)degrees, V = 568.3(7) Angstrom(3), Z = 1. 3c: Space group P2(1)/c, a = 8.312(2) Angstrom, b = 15.386(3) Angstrom, c = 12.365(2) Angstrom, beta = 94.83(3)degrees, 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 Il 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(2)Pt(II) entities (a = NH3 or CH3NH2). 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 NH2(6) of 9-MeA and exocyclic groups of the other nucleobases. The chemical shifts of the NH2 protons of 9-MeA in DMSO-d(6) or DMF-d(7) permit a differentiation between the various possibilities (no H bonding, single H bond, 2 Il 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 = 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(2)Pt(II) entities is briefly discussed, as are aspects of the stabilization of tripler nucleic acid structures by metal ions.