Introduction
Today, almost all of the world’s carbon-based materials and energy are generated from the oxidative conversion of petroleum or natural gas, which comprises mainly alkanes. As a result of the high strength of the alkane C-H bond, current processes for converting these raw materials employ high temperatures (>500 °C) that lead to excessive emissions, lower efficiencies and high capital costs. The development of lower temperature (<250 °C), selective, catalytic alkane C-H bond conversion chemistry could lead to a new paradigm in energy and materials technology in the 21st century that is environmentally and economically superior and allow the vast reserves of natural gas to be employed directly as alternative feed stocks for fuels and chemicals. Our group works to develop methods for the efficient conversion of these raw materials via the C-H activation reaction, mechanistic studies, and C-O and C-C functionalization chemistry to develop new routes to these useful products. Below we highlight these main focuses of research and our current progress on them.
C-H Activation
Catalysts based on the C-H activation reaction show potential for such efficient hydrocarbon conversion technology. We define CH activation chemistry as a reaction between CH bonds and a species “MX” that leads to the formation of a M-C bond. The key to this remarkable reaction is that the cleavage of the CH bond proceeds in a concerted manner within the coordination sphere of “MX”. The reaction proceeds under mild conditions and with extraordinary selectivity because high-energy intermediates such as free radicals, carbocations or carbanions are not involved. There are many systems known for the CH activation reaction but only a few can be incorporated into useful catalytic cycles, that generate products. During my research, the focus has been the design and study of new homogeneous catalysts that are based on the CH activation reaction and functionalization. We have develop systems capable of functionalizating both alkyl or aryl C-H bonds. We develop methods for the coupling of the M-R intermediates either to C-O or C-C bond formations.
Hydroxylation
The hydroxylation of hydrocarbons has been a primary goal in catalytic chemistry. One method for hydroxylating hydrocarbons is via C-H bond activation using homogenous transition metal complexes followed by oxy-functionalization. Typically, the focus of C-H activation has been on the use of soft, electrophilic, late transition metals such as Pt(II), Hg(II), Pd(II), and Au(III). These metals have shown promise in C-H activation reactions followed by oxy-functionalization, most notably via reductive functionalization. However, these electrophilic systems suffer from water and methanol inhibition that renders them unviable for commercialization. To alleviate these problems, we have moved to less electrophilic metals such as Ir, Rh, Os, Ru, and Re. However, the oxidative addition/reductive funtionalization (OA/RF) pathways typical to the more electrophilic systems (Pt, Hg, Au, and Pd) with a positively polarized metal alkyl (M-Rδ+) are often not applicable to negatively polarized metal-alkyl (M-Rδ-) complexes generated from low oxidation state group VII and VIII metals (where M = transition metal and R = any alkane or arene). The negatively polarized metal-alkyl complexes generated from early transition metals are not favorable to react via reductive functionalization pathways due to their reduced oxidation potential and electrophilicity.
The shift from electrophilic transition metal C-H activation catalysts towards more electropositive metals that do not operate via OA/RF pathways will require the development of new functionalization strategies. Recently, we showed the viability of a Baeyer-Villager (BV) O-atom insertion and a [2+3] type mechanism for functionalizing M-Rδ- metal-alkyl complexes (above Figure). Furthermore, another possible strategy for generating alcohols is by alkyl transfer to an electrophilic co-catalyst Z. The figure below depicts the proposed O-atom insertion or transalkylation reductive functionalization (TRF) cycle where 1) M activates R-H to form an M-Rδ- intermediate, 2) R undergoes transalkylation to Z and 3) oxidation by an air renewable O-atom donor (YO) occurs, followed by RF to yield product.
Hydroarylation
Homogenous iridium complexes containing oxygen donor ligands {R-Ir(acac)2-L and R-Ir(trop)2-L} were found to carry out the selective functionalization of arene C-H bonds. These complexes were found to catalyze the anti-Markovnikov, hydro-arylation of unactivated olefins by unactivated arenes to produce saturated alkyl arenes in a single step as opposed to the conventional methods involving Friedel Craft’s alkylation or acylation followed by reduction (See Figure).
All the proposed intermediates were synthesized and the mechanism was elucidated in great detail. Mechanistic details involving the hydroarylation, shown in figure, have been discussed and in depth studies reveal that: 1) all the dinuclear and mononuclear complexes follow the same mechanistic steps and share catalytic steps, 2) Initiation step involve either the cleavage of the dinuclear or the loss of the ligand L from the mononuclear species in a dissociative process to generate the 5-coordinate site, 3) These complexes undergo trans-cis isomerization of the acac groups to generate the active cis species, 4) C-H activation and trans-cis isomerization goes through a common intermediate, 5) C-H activation is not the rate determining step, and 6) bis bidentate ligands can be employed to access the benefits of a tetradentate ligand.