A major, pH-induced stereochemical switch of pairs of trans-oriented ligands in complexes of trans-a(2)Pt(II) (a = NH3, CH3NH2)

trans-[Pt(CH3NH2)(2)(1-MeC-N-4)(2)]X-2 (3a, X = NO3-; 3b, X = ClO4-), conta ining the model nucleobase 1-methylcytosine (1-MeC) platinated at N4 and pr otonated at N3, hence in its rare tautomeric form, has been prepared from t he Pt-IV precursor trans,trans,trans-[Pt(CH3NH2)(2)(1-MeC-N-4)(2)(OH)(2)](N O3)(2). 2H(2)O (2) upon reduction with H-2. Crystallization of 3a from 1 M NaOH afforded trans-[Pt(CH3NH3)(2)(1-MeC--N-4)(2)]. 4H(2)O (4a) or, followi ng lyophilization and deprotonation in CH3OH by means of (CH3)(3)CONa, gave trans-[Pt(CH3NH2)(2)(1-MeC--N-4)(2)]. 2CH(3)OH (4b). While dihedral angles between the coplanar bases and the PtN4 planes are large in the case of 2 (84.8(1)degrees) and 3b (73.9(1)degrees), they become markedly smaller in 4 a (55.5(2)degrees) and 4b (26.6(2)degrees) as a consequence of pairwise int ramolecular H bonding between the NH protons of the CH3NH2 groups and the N 3 positions of the cytosine nucleobases. DFT calculations for the correspon ding NH3 complex gave a dihedral angle of 22.3 degrees. The switch of the m utually trans-oriented ligand pairs from approximately perpendicular to rou ghly coplanar appears to take place during the crystallization process, pro bably because of competition between intramolecular H bonding and intermole cular H bonding with the solvent. 2: triclinic, P (1) over bar, a = 5.937(1 ) Angstrom, b = 8.228(2) Angstrom, c = 12.470(2) Angstrom, alpha = 80.36(3) degrees, beta = 80.80(3)degrees, gamma = 80.54(3)degrees, Z = 2. 3b, tricli nic, P (1) over bar, a = 7.392(1) Angstrom, b = 9.072(2) Angstrom, c = 10.0 47(2) Angstrom, alpha = 112.40(3)degrees, beta = 106.07(3)degrees, gamma = 94.66(3)degrees, Z = 2. 4a, triclinic, P (1) over bar, a = 7.104(1) Angstro m, b = 7.549(2) Angstrom, c = 9.209(2) Angstrom, alpha = 87.74(3)degrees, b eta = 88.04(3)degrees, gamma = 85.92(3)degrees, Z = 2. 4b, triclinic. P (1) over bar, a = 7.045(1) Angstrom, b = 7.421(1) Angstrom, c = 9.966(2) Angst rom, alpha = 109.25(3)degrees, beta = 99.22(3)degrees, gamma = 95.02(3)degr ees, Z = 2.