Owing to the increasing emissions of carbon dioxide (CO2), human life and the ecological environment have been affected by global warming and climate changes. To mitigate the concentration of CO2 in the atmosphere various strategies have been implemented such as separation, storage, and utilization of CO2. Although it has been explored for many years, hydrogenation reaction, an important representative among chemical conversions of CO2, offers challenging opportunities for sustainable development in energy and the environment. Indeed, the hydrogenation of CO2 not only reduces the increasing CO2 buildup but also produces fuels and chemicals. In this critical review we discuss recent developments in this area, with emphases on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field (319 references).

Recent advances in catalytic hydrogenation of carbon dioxide

Two major energy-related problems confront the world in the
next 50 years. First, increased worldwide competition for
gradually depleting fossil fuel reserves (derived from past
photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Frustrated Lewis pair chemistry: development and perspectives.

The chemistry of heavier main-group elements such as aluminium, silicon and phosphorus is very different from that of the lighter ones such as boron, carbon and nitrogen, yet discussions of this topic have been dominated by comparisons with the light elements. Philip Power's review focuses on advances in chemistry of the heavier main-group elements that reveal them as having more in common with the transition metals than the lighter members of the main groups. The concept of heavier main-group elements as 'transition metals' is supported by recent work showing that many of the new compounds react with small molecules such as H2, NH3, C2H4 and CO under mild conditions and display potential as catalysts. The last quarter of the twentieth century and the beginning decade of the twenty-first witnessed spectacular discoveries in the chemistry of the heavier main-group elements. The new compounds that were synthesized highlighted the fundamental differences between their electronic properties and those of the lighter elements to a degree that was not previously apparent. This has led to new structural and bonding insights as well as a gradually increasing realization that the chemistry of the heavier main-group elements more resembles that of transition-metal complexes than that of their lighter main-group congeners. The similarity is underlined by recent work, which has shown that many of the new compounds react with small molecules such as H2, NH3, C2H4 or CO under mild conditions and display potential for applications in catalysis.

Main-group elements as transition metals

Frustrated Lewis pairs comprising phosphine and borane react to reversibly bind and release CO2, offering a rare example of metal-free CO2 sequestration. The mechanism of formation of CO2 derivatives by almost simultaneous P-C and O-B bond formation was characterized by quantum chemical calculations.

/pdf/reversible-metal-free-carbon-dioxide-binding-by-frustrated-2ne6f1s66g.pdf

Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs

Reversible, nicht metallunterstützte Bindung von Kohlendioxid durch frustrierte Lewis‐Paare

Hydroboration of dimesitylvinylphosphine with Piers’ borane [HB(C6F5)2] gives the ethylene-bridged intramolecular frustrated P/B Lewis pair 1. It adds pyridine, tert-butyl isocyanide or pivalonitrile to the strongly electrophilic boron center to yield the respective adducts 5–7. Compound 1 undergoes a 1,1-phosphine/borane addition to the terminal nitrogen center of phenyl azide to yield the five-membered heterocycle 8, featuring a pendant –NN–Ph. This can be regarded as a boron-stabilized intermediate of a Staudinger reaction. Benzaldehyde and phenyl isocyanate undergo 1,2-P/B additions of 1 to their reactive carbonyl groups to yield the corresponding six-membered heterocycles 9 and 10, respectively. The P/B Lewis pair reacts with nitrosobenzene by 1,2-addition to the –NO unit under P–N and O–B bond formation to give the six-membered heterocycle 11. The compounds 5–11 were characterized by X-ray crystal structure analyses.

Addition reactions to the intramolecular mesityl2P–CH2–CH2–B(C6F5)2 frustrated Lewis pair

The frustrated Lewis pair induced formation of a pentafulvene [6 + 4] cycloaddition product

The frustrated Lewis pair Mes(2)PCH(2)CH(2)B(C(6)F(5))(2) reacts readily with 6-dimethylamino-6-methylfulvene at room temperature to yield the trans-1-[B(C(6)F(5))(2)]-2-[CH(2)CH(2)PMes(2)] disubstituted fulvene derivative 9 that features an internal N-B contact. Thermolysis (80 °C in toluene) results in a complete isomerization to the respective 1-[B(C(6)F(5))(2)]-3-[CH(2)CH(2)PMes(2)] isomer 10. Both compounds were characterized by using X-ray diffraction. A reaction scheme is formulated to rationalize the specific formation of these compounds, involving a retro-hydroboration/hydroboration sequence. The reaction of the 6-dimethylaminofulvene with HB(C(6)F(5))(2) yielded the corresponding parent compound 13 that was also characterized by X-ray diffraction.

Reaction of a 6-dimethylaminopentafulvene with the Mes2PCH2CH2B(C6F5)2 frustrated Lewis pair.

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