The mechanistic study of C-H and H-H bond activation and C-C coupling reactions at the interface of homogeneous catalysis and heterogeneous catalysis

Commodity chemicals are produced annually on a large-scale via various catalyst-based technologies. Most of the industrially relevant catalytic processes operate based on molecular transition metal complexes, solid acid catalysts, or supported transition metal solid catalysts. The shale gas revolution has provided an abundance of methane and ethane, and has shifted the chemical industry's focus from the long-standing crude oil-based raw materials supply to a cost-effective natural gas components-based source. As a result, new catalytic processes to increase the efficiency of using natural gas-derived chemicals have become a high priority.
In the last few decades, extensive research has been focused on the catalytic oxidative coupling of methane (OCM) to generate C2 products (i.e., ethylene and ethane). Efforts have focused on mixed oxide catalysts such as Li/MgO and Mn/Na2WO4/SiO2 to optimize the yield of C2 products. One of the biggest challenges for OCM development is the radical-based C–H activation and methyl coupling, which hinders the selectivity and yield of the high-value C2 products as often the products are more reactive than methane. Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C–C bond formation. In Chapter 2, we investigate C–C coupling to form ethane from various phosphine ligated gem-digold(I) methyl complexes including [Au2(µ-CH3)(PMe2Ar’)2][NTf2] and [Au2(µ-CH3)(XPhos)2][NTf2] {Ar’ = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H3-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H3-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonyl)imide)}. The gem-digold methyl complexes are synthesized through reactions between Au(CH3)L and Au(L)(NTf2) (L = phosphines listed above). For [Au2(µ-CH3)(XPhos)2][NTf2] and [Au2(µ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated by single crystal X-ray diffraction. The rate of ethane formation from [Au2(µ-CH3)(PMe2Ar’)2][NTf2] increases as the steric bulk of the phosphine substituent Ar’ decreases. Monitoring the rate of ethane elimination reactions by 1H NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational studies, it is proposed that the mechanism of C–C coupling likely involves: 1) cleavage of [Au2(µ-CH3)(PMe2Ar’)2][NTf2] to form Au(PR2Ar’)(NTf2) and Au(CH3)(PMe2Ar’), 2) phosphine exchange from a second equivalent of [Au2(µ-CH3)(PMe2Ar’)2][NTf2] aided by [Au2(PMe2Ar’)][NTf2], formed in step 1, to produce [Au2(µ-CH3)(PMe2Ar’)][NTf2], and 3) recombination of [Au2(µ-CH3)(PMe2Ar’)][NTf2] and Au(CH3)(PMe2Ar’) to eliminate ethane.
Alkyl and alkenyl arenes such as ethylbenzene and styrene were produced on a scale of ~40 million tons and ~38 million tons annually in 2018. The synthesis of ethylbenzene and styrene relies on acid-based Friedel-Crafts alkylation or zeolite-based catalysis, which are multi-step and energy-intensive processes. Transition metal based arene alkylation or alkenylation could provide an alternative strategy towards high-value chemicals synthesis. Recently, molecular Rh(I) catalysts have been reported for the synthesis of alkenyl arenes from benzene and olefins using Cu(II) salts as the in situ oxidant (e.g., Acc. Chem. Res. 2020, 53, 920-936). In Chapter 3, we focus on the synthesis of supported Rh materials and the study of their efficacy as pre-catalysts for the oxidative alkenylation of arenes. Rhodium nanoparticles supported on silica (Rh/SiO2; ~3.6 wt% Rh) and nitrogen-doped carbon (Rh/NC; ~1 wt% Rh) are synthesized via liquid ion exchange and high-temperature pyrolysis, respectively. Heating mixtures of Rh/SiO2 or Rh/NC with benzene and ethylene or α-olefins and CuX2 {X = OPiv (trimethylacetate) or OHex (2-ethylhexanoate)} to 150 °C results in the production of alkenyl arenes. When using Rh/SiO2 or Rh/NC as catalyst precursor, the conversion of benzene and propylene or toluene and 1-pentene yields a ratio of anti-Markovnikov to Markovnikov products that is nearly identical to the same ratios using the molecular precursor [Rh(μ-OAc)(η2-C2H4)2]2 as catalyst. These results and other observations are consistent with the formation of active catalysts by leaching of soluble Rh from the supported Rh materials.
In Chapter 4, we disclose efforts toward the development of nanoparticle-mediated tandem catalysis, in which a proof-of-concept study of nanoparticles catalyzed hydrogenolysis of (tbpy)Pt(OPh)Cl complex is conducted. Three silica-supported nanoparticles (5.0 wt% Pd/SiO2, 1.0 wt% Pt/SiO2, and 3.6 wt% Rh/SiO2) are synthesized and the study of their efficacy for hydrogenolysis of (tbpy)Pt(OPh)Cl complex was performed. Using (tbpy)Pt(OPh)Cl complex as a probe molecule, nanocatalysts effectively promote hydrogenolysis of Pt–OPh bonds to release HOPh at 50 °C. Monitoring the hydrogenolysis kinetics by 1H NMR spectroscopy, a first-order dependence on Pd/SiO2, and (tbpy)Pt(OPh)Cl complex was observed. The rate for the noble metals mediated hydrogenolysis of (tbpy)Pt(OPh)Cl complex and C=C bonds hydrogenation followed the same reactivity trend: Pd > Rh > Pt.