NEW LIGAND SCAFFOLDS FOR COMBINING ARENE C-H ACTIVATION AND AEROBIC OXIDATION AT PLATINUM

Abstract

Extensive research over the past half-century has proven the utility of late transition metal complexes in the activation and functionalization of alkanes and arenes. Homogeneous platinum compounds have been particularly promising as they readily form air and water stable complexes that can react with some of the strongest C-H bonds (e.g., CH3-H, Ph-H) under relatively benign conditions. Yet, the development of methods for the application of O2 or air as the terminal oxidant in the oxidative functionalization of inert C-H bonds remains an elusive but important goal. The focus of this work is to enable the direct involvement of O2 with PtII-mediated C-H activation processes through computation directed intelligent ligand design, with the end goal of selective aerobic C-H functionalization.

Prior experience with the hemi-labile tripodal ligand di-(2-pyridyl)methanesulfonate (dpms) lead us to develop a new class of sulfonated k3-CNN pincer pre-ligand, 6-phenyl-di-(2-pyridyl)methanesulfonate (ph-dpms). The ph-dpms derived PtII-aqua complex, (C6H4-dpms)PtII(H2O), was shown to be an especially active hydrogen/deuterium exchange catalyst with arene substrates. While facile, the arene C-H activation chemistry was also selective for aryl C(sp2)-H bonds over benzyl C(sp3)-H, despite severe steric protection of the former in some cases.

Ph-dpms also supports the aerobic oxidation chemistry for which the earlier generation ligand, dpms, was engineered. An anionic [PtII(Ph)]- complex derived from ph-dpms undergoes relatively fast oxidation in trifluoroethanol (TFE) solvent resulting in oxidative C-C coupling between the phenyl substituent and ligand. Changing the solvent to MeOH allows for isolation of the PtIV-Ph intermediate. Furthermore, the ability to support both C-H and O2 activation was combined in the one-pot aerobic C-H oxidation of both electron rich and electron poor arenes to give (C6H4-dpms)PtIV(Aryl)(OH) complexes from (C6H4-dpms)PtII(H2O); a feat never accomplished before by a PtII complex without the use of co-catalysts or reagents to mediate the O2 chemistry.

The limits of the new ligand scaffold were then explored through the reactivity of PtII chloro and aqua complexes derived from more rigid analogs of ph-dpms. The rigidity of the ligand was found to be intimately tied to both C-H and O2 activation chemistry as well as some detrimental bimolecular decomposition pathways.