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)amin e) 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 = 11 1.62(2)degrees, and Z = 4. Least-squares refinement of the structure l ed to an R factor of 2.41%. To avoid steric repulsions, the chelate si x-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 s trong hydrogen-bonding interaction involving the amine hydrogen (N3) a nd 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 sol ution of nonpolar solvents. Tight ion-pairs of structure similar to th at in the solid state are formed with PF6-, BF4-, CF3SO3-, and Cl- in chloroform, as shown by the strong dependence of the chemical shifts o f the NH, H(3), and H(3') protons of the dpa ligand on the nature of t he counterion. F-19{H-1} HOESY experiments on [PtMe(dpa)(Me2SO)](+)(PF 6)(-) in CD2Cl2 confirmed that the preferential position of the counte rion is close to the NN proton. The absorption spectra are also strong ly affected by the nature of the counterion. This allowed for a stoppe d-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 am ido species [PtMe(dpa-H)(Me2SO)] can be isolated on basification. Ion- pairing and full deprotonation of the amine ligand have remarkably lit tle effect on the reactivity, as shown by the comparison of the rates of isotopic exchange of dimethyl sulfoxide of these species followed b y H-1 MMR in chloroform. The rates of substitution of dimethyl sulfoxi de from [PtMe(dpa)(Me2SO)](+) by various charged nucleophiles were mea sured in methanol, where ion-pairing effects are absent, and compared with those of the parent [PtMe(phen)(Me2SO)](+) (phen = 1,10-phenanthr oline) complex. Because of the reduced capacity of electron withdrawal from the metal of the ancillary ligand, the dpa complex is less react ive and possesses a minor nucleophilic discrimination ability compared with the phen complex.