The Inorganic, Bioinorganic and Organometallic Chemistry Program supports the research of Andrei N. Vedernikov, Department of Chemistry, University of Maryland - College Park, investigating the selective aerobic functionalization of alkanes, olefins, and arenes using water tolerant platinum complexes. These platinum complexes will employ semi-labile, facially chelating NNO-donor ligands such as di(2-pyridyl)methanesulfonates and di(2-pyridyl)- or di(2-imidazolyl)phosphinates. Computational studies (DFT) will be correlated with the experimental observations to explore the mechanistic aspects of this complicated system. These proposed catalytic reactions, in particular, are expected to fundamentally influence the chemical and petroleum industries, enabling the production of methanol from methane, glycols from olefins and phenol derivatives from arenes, using environmentally-friendly methods. Undergraduate and graduate research students will participate in every aspect of the synthesis, characterization and reactivity studies, gaining important insights into responsible alternatives for the preparation of chemical and petrochemical feedstocks.
Atom economy and the use of ‘green’ reagents in organic oxidation, including oxidation of hydrocarbons are requirements of both current and future economies, and they remain challenges for organic synthesis. Solution to this problem would lead to a more sustainable economy including improved access to still abundant energy resources such as natural gas, about third of which is currently lost because of lack of necessary methodologies. Two readily available ‘atom-economical’ ‘green’ oxidants are dioxygen and hydrogen peroxide, but few methodologies have been effective in utilizing these oxidants in selective organic transformations. Hydrocarbon oxidation reactions rely on PdII and PtII complexes that are considered ‘practical’ for their ability to tolerate moisture and atmospheric oxygen. This ‘practicality’ creates the challenge for the development of novel palladium and platinum–catalyzed C?H oxidation reactions utilizing O2 or H2O2 that is, in significant part, the challenge of the discovery of new reactions of organopalladium(II) and organoplatinum(II) compounds with O2 and H2O2. These challenges have been met in this project: novel and highly selective M?C bond (M = PtII, PdII) functionalization and related selective hydrocarbon C?H oxidations with O2 or H2O2 were discovered. To solve efficiently the problem of direct M?C oxidation and functionalization with O2 and H2O2 the authors introduce the use of facially chelating semilabile ligands such as di(2-pyridyl)methanesulfonate and the hydrated form of di(2-pyridyl)ketone that enable selective and facile MII?C(spx) bond functionalization with O2 (M = Pt, x = 3; M = Pd, x = 3 (benzylic)) or H2O2 (M = Pd, x = 2). The reactions discovered are efficient in solvents such as water, methanol or acetic acid. With the exception of benzylic PdII complexes, organometallic substrates studied form isolable high – valent PtIV or PdIV intermediates as a result of an oxidant attack at the MII atom. The resulting high – valent MIV intermediates undergo high-yielding C?O reductive elimination. Some guidelines to the synthesis of products containing other C?X bonds (X = OAc, Cl, Br) while using O2 or H2O2 as oxidants are also discussed. While the MII?C bond functionalization reactions including high – valent intermediates are well understood, the mechanism for the aerobic functionalization of benzylic PdII complexes still needs to be explored in more detail. A ‘fine tuning’ of some systems suggested for stoichiometric MII?C bond functionalization allowed to develop catalytic reactions suitable for selective functionalization of benzylic C?H bonds with O2 as the oxidant and hydroxylation of aromatic C?H bonds with H2O2 in acetic acid solutions. Both reactions are efficient with substrates that contain a directing nitrogen atom. In addition, catalytic methods for ethylene dioxygenation with H2O2 utilizing MII complexes supported by facially chelating ligands were developed. Mechanistic studies of the reactions found constitute important ways to improve the substrate scope of the discovered reactions and to develop ‘green’ CH functionalization chemistry.
