Exploring Platinum(II)/Platinum(IV) Conversions Using Pyrazolyl- and Triazolyl-based Scorpionate Ligands

Our research program has focused on reactivity centering around Pt(II)/Pt(IV) conversions utilizing a pyrazolyl-based Tp'Pt system. Reactions have included C-H bond formation, alkene insertion, dehydration of alkanes, oxidative addition of alkynes, and Water Gas Shift chemistry. In this work, the mechanism for decarboxylation and protonation for the stepwise WGS reaction stemming from the Tp'Pt(Me)(CO) reagent is studied and modeled. Nucleophilic attack on the platinum-bound CO ligand by amide nucleophiles (NHR-) followed by methylation using methyl iodide results in clean conversion to carboxamido complexes of the type Tp'PtMe2(C(O)NHR). Deprotonation of the carboxamido ligand triggers isocyanate elimination and the platinum product can be methylated or protonated to form Tp'PtMe3 or Tp'PtMe2H, respectively. Furthermore, methylation of the carboxamido compounds results in rare Pt(IV) cationic carbenes. This work suggests that a Pt(IV) species is a requirement enroute to elimination in the WGSR and that the overall reaction proceeds via a step-wise mechanism including protonation, deprotonation, and decarboxylation.;To facilitate kappa3/kappa2 interconversions and promote reductive elimination from Pt(IV), the triazolyl-based scorpionate ligands TtR were synthesized using the alkyne-azide "Click" reaction. Metallation of such ligands results in Pt(II) square-planar dimethyl or diphenyl complexes that display a kappa2 coordination mode for the scorpionate ligand. Addition of electrophilic alkyl or allyl reagents results in isolable cationic Pt(IV) complexes. Protonation of the kappa 2 Pt(II) reagents generates [TtRPtR'2H] + (R' = Me, Ph), which eliminates methane or benzene under mild conditions. Trapping the resulting fragment with carbon monoxide or an olefin produces [TtRPt(R')(L)]+. When R' = Ph, phenyl migration and ortho C-H activation is observed under gentle heating to generate metallacyclic hydride complexes. Deuterium labeling studies and NOESY NMR spectra reveal the dynamic nature of the metallacycles, which undergo both aryl and alkyl C-H elimination and re-activation. Heating the mixture of isobutylene insertion isomers reveals conversion from the kinetic insertion product with substituents on the alpha-carbon to the thermodynamic insertion product with substituents on the beta-carbon.