Molecular structure, acidic properties, and kinetic behavior of the cationic complex (methyl)(dimethyl sulfoxide)(bis-2-pyridylamine)platinum(II) ion

The complex [PtMe(dpa)(Me2SO)](+)(CF3SO3)(-) (dpa = bis(2-pyridyl)amine) crystallizes in the monoclinic space group P2(1)/c with a = 11.010(2) Angstrom, b = 18.366(2) Angstrom, c = 10.333(5) Angstrom, beta = 111.62(2)degrees, and Z = 4. Least-squares refinement of the structure led to an R factor of 2.41%. To avoid steric repulsions, the chelate six-membered ring assumes a boat configuration in which the two pyridyl rings are folded with a dihedral angle of 46.4(1)degrees. There is a strong hydrogen-bonding interaction involving the amine hydrogen (N3) and a triflate union oxygen O3, 2.898(5) Angstrom. The tendency by the NH group of the ligand moiety to attract anions is maintained in a solution of nonpolar solvents. Tight ion-pairs of structure similar to that in the solid state are formed with PF6-, BF4-, CF3SO3-, and Cl- in chloroform, as shown by the strong dependence of the chemical shifts of the NH, H(3), and H(3') protons of the dpa ligand on the nature of the counterion. F-19{H-1} HOESY experiments on [PtMe(dpa)(Me2SO)](+)(PF6)(-) in CD2Cl2 confirmed that the preferential position of the counterion is close to the NN proton. The absorption spectra are also strongly affected by the nature of the counterion. This allowed for a stopped-flow measure of the PF6- for Cl- exchange rate at the NH site, which is a bimolecular process with k(2) = 96.4 +/- 4 M-1 s(-1). The cation [PtMe(dpa)(Me2SO)](+) shows acidic properties in water (pK(a) = 12.1 +/- 0.2, at 25 degrees C, mu = 0.1 M, NaNO3), and the corresponding amido species [PtMe(dpa-H)(Me2SO)] can be isolated on basification. Ion-pairing and full deprotonation of the amine ligand have remarkably little effect on the reactivity, as shown by the comparison of the rates of isotopic exchange of dimethyl sulfoxide of these species followed by H-1 MMR in chloroform. The rates of substitution of dimethyl sulfoxide from [PtMe(dpa)(Me2SO)](+) by various charged nucleophiles were measured in methanol, where ion-pairing effects are absent, and compared with those of the parent [PtMe(phen)(Me2SO)](+) (phen = 1,10-phenanthroline) complex. Because of the reduced capacity of electron withdrawal from the metal of the ancillary ligand, the dpa complex is less reactive and possesses a minor nucleophilic discrimination ability compared with the phen complex.