A Major, pH-Induced Stereochemical Switch of Pairs of <i>trans</i>-Oriented Ligands in Complexes of <i>trans</i>-a₂Pt^II (a = NH₃, CH₃NH₂)

trans-[Pt(CH3NH2)2(1-MeC-N4)2]X2 (3a, X = NO3-; 3b, X = ClO4-), containing the model nucleobase 1-methylcytosine (1-MeC) platinated at N4 and protonated at N3, hence in its rare tautomeric form, has been prepared from the PtIV precursor trans,trans,trans-[Pt(CH3NH2)2(1-MeC-N4)2(OH)2](NO3)2·2H2O (2) upon reduction with H2. Crystallization of 3a from 1 M NaOH afforded trans-[Pt(CH3NH2)2(1-MeC--N4)2]·4H2O (4a) or, following lyophilization and deprotonation in CH3OH by means of (CH3)3CONa, gave trans-[Pt(CH3NH2)2(1-MeC--N4)2]·2CH3OH (4b). While dihedral angles between the coplanar bases and the PtN4 planes are large in the case of 2 (84.8(1)°) and 3b (73.9(1)°), they become markedly smaller in 4a (55.5(2)°) and 4b (26.6(2)°) as a consequence of pairwise intramolecular H bonding between the NH protons of the CH3NH2 groups and the N3 positions of the cytosine nucleobases. DFT calculations for the corresponding NH3 complex gave a dihedral angle of 22.3°. The switch of the mutually trans-oriented ligand pairs from approximately perpendicular to roughly coplanar appears to take place during the crystallization process, probably because of competition between intramolecular H bonding and intermolecular H bonding with the solvent. 2: triclinic, P1̄, a = 5.937(1) Å, b = 8.228(2) Å, c = 12.470(2) Å, α = 80.36(3)°, β = 80.80(3)°, γ = 80.54(3)°, Z = 2. 3b, triclinic, P1̄, a = 7.392(1) Å, b = 9.072(2) Å, c = 10.047(2) Å, α = 112.40(3)°, β = 106.07(3)°, γ = 94.66(3)°, Z = 2. 4a, triclinic, P1̄, a = 7.104(1) Å, b = 7.549(2) Å, c = 9.209(2) Å, α = 87.74(3)°, β = 88.04(3)°, γ = 85.92(3)°, Z = 2. 4b, triclinic, P1̄, a = 7.045(1) Å, b = 7.421(1) Å, c = 9.966(2) Å, α = 109.25(3)°, β = 99.22(3)°, γ = 95.02(3)°, Z = 2.