Interionic solution structure of [PtMe(eta(2)-olefin)(N,N-diimine)]BF4 complexes by F-19{H-1}-HOESY NMR spectroscopy: Effect of the substituents on the accessibility of the counterion to the metal

The relative cation-anion position in [Pt(Me)(eta(2)-olefin)(N,N)]BF4 compl exes (where N,N = 2,6-(R')(2)C6H3N=C(R ")C(R ")=N-2,6-(R')(2)C6H3, R' = H, Me, Et, and i-Pr, R " = H, Me; olefin = CH2=CHR, R = H, Me, COOMe) in methy lene chloride has been investigated by detecting specific interionic dipola r interactions in the F-19{H-1}-HOESY NMR spectra. The counterion shows str ong interionic contacts with R " and R' protons and weak contacts with R an d olefinic protons only when R' < Et and R " = Me. For R' greater than or e qual to Et the accessibilty to the metal center is completely inhibited and the counterion is located above or below the backside of the diimine ligan d. The same position is also observed when R " = H and R' = R = Me despite the limited steric hindrance of the substituents due to specific interactio ns between H " and the fluorine of BF4-. In the other cases (R' < Et), the counterion also interacts with Me, R, and olefinic protons, indicating that the accessibility to the metal center is not forbidden.