Kinetico-Mechanistic Studies on Intramolecular C–X Bond Activation (X = Br, Cl) of Amino-Imino Ligands on Pt(II) Compounds. Prevalence of a Concerted Mechanism in Nonpolar, Polar, and Ionic Liquid Media

The C–Br and C–Cl oxidative addition reactions
of
molecules containing a set of {N-amino,N-imino} chelating donor groups (2-X,6-YC6H4CHNCH2CH2NMe2, X = Br, Cl; Y = Cl,
H) attached to a {PtII(Ar)2} (Ar = Ph, 4-MeC6H4) have been studied. The Pt­(IV) complexes formed,
[PtAr2X­{2-YCC5H3CHNCH2CH2NMe2}], containing a metalated tridentate [C,N,N′]
ligand have been fully characterized by the usual techniques, and
the X-ray crystal structure of the complex with Ar = 4-MeC6H4 and X =Y = Cl has been determined. Monitoring of the
reactions at varying temperatures and pressures and in different solvents
agrees with a mechanism that involves the preliminary decoordination
of the N-amino donor from the ligand
to produce a three-coordinated intermediate. This evolves, via a concerted
C–X bond activation, to form a second pentacoordinated intermediate
species that, on coordination of the N-amino donor,
produces the final complex. The kinetico-mechanistic parameters measured
indicate that the concerted character of the process is maintained
from nonpolar (xylene and toluene) to polar (acetone) and ionic liquid
((Bmin­(NTf2)) media. Furthermore, the ΔV⧧ values measured indicate that, for the (2,6-Cl)­C6H3CHNCH2CH2NMe2 ligand, the existence of hydrogen bonding within the metalating
molecule is a determinant for the acceleration observed.