Metal-Modified Base Pairs Involving Different Donor Sites of Purine Nucleobases:  <i>trans</i>-[a<sub>2</sub>Pt(7,9-DimeG-<i>N1</i>)(9-EtGH-<i>N7</i>)]<sup>2+</sup> and <i>trans</i>-[a<sub>2</sub>Pt(7,9-DimeG-<i>N1</i>)(9-EtG-<i>N7</i>)]<sup>+</sup> (a = NH<sub>3</sub> or CH<sub>3</sub>NH<sub>2</sub>; 9-EtGH = 9-Ethylguanine; 7,9-DimeG = 7,9-Dimethylguanine). Possible Relevance to Metalated DNA Triplex Structures

The X-ray crystal structures of four mixed-nucleobase complexes <b>1a</b>, <b>1b</b>, <b>2</b>, and <b>4</b> of <i>trans</i>-a<sub>2</sub>Pt<sup>II</sup> (a = NH<sub>3</sub> or CH<sub>3</sub>NH<sub>2</sub>) with N1-bonded 7,9-dimethylguanine (7,9-DimeG) and N7-bonded 9-ethylguanine (9-EtGH) are reported. The compounds are discussed in terms of their possible model character for platinated nucleobase triplets of the pu × pu·pym (pu = purine, pym = pyrimidine) type and are compared with known guanine·guanine pairs. Structural differences resulting from a “metal modification” have been studied, as have been stacking and H bonding motifs. <b>1a</b> and <b>1b</b> are two modifications of the same compound, <i>trans</i>-[(NH<sub>3</sub>)<sub>2</sub>Pt(7,9-DimeG)(9-EtGH)](ClO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O. X-ray data for <b>1a</b>:  orthorhombic, <i>Pca</i>2<sub>1</sub>, <i>a</i> = 13.833(3) Å, <i>b</i> = 14.357(3) Å, <i>c</i> = 13.293(3) Å, <i>Z</i> = 4, <i>R</i> = 0.027. <b>1b</b>:  monoclinic, <i>P</i>2<sub>1</sub>/<i>n</i>, <i>a</i> = 13.987(3) Å, <i>b</i> = 12.582(3) Å, <i>c</i> = 15.613(3) Å, <i>β</i> = 104.61(3)°, <i>Z</i> = 4, <i>R</i> = 0.034. <i>trans</i>-[(CH<sub>3</sub>NH<sub>2</sub>)<sub>2</sub>Pt(7,9-DimeG)(9-EtGH)](ClO<sub>4</sub>)<sub>2</sub> (<b>2</b>):  orthorhombic, <i>Pca</i>2<sub>1</sub>, <i>a</i> = 13.886(1) Å, <i>b</i> = 14.698(1) Å, <i>c</i> = 13.008(3) Å, <i>Z</i> = 4, <i>R</i> = 0.034. <i>trans</i>-[a<sub>2</sub>Pt(7,9-DimeG)(9-EtG)](ClO<sub>4</sub>)·<i>n</i>H<sub>2</sub>O (<b>3</b>, a = NH<sub>3</sub>, <i>n</i> = 2; <b>4</b>, a = CH<sub>3</sub>NH<sub>2</sub>, <i>n</i> = 3). X-ray data for <b>4</b>:  monoclinic, <i>C</i>2/<i>c</i>, <i>a</i> = 23.296(5) Å, <i>b</i> = 10.566(2) Å, <i>c</i> = 22.327(4) Å, <i>β</i> = 93.73(3)°, <i>Z</i> = 8, <i>R</i> = 0.047. The metalated base triplet <b>2</b>·(1-MeC) (1-MeC = 1-methylcytosine) has been detected in DMSO-<i>d</i><sub>6</sub> solution through <sup>1</sup>H NMR spectroscopy, and an ab initio optimized structure of an analogous platinated base triplet has been obtained.