We have designed and developed new, sustainable and green catalytic reactions, which do not generate waste, use sustainable starting materials such as alcohols, proceed under mild conditions, and do not harm the environment. These processes are based on novel pincer-type metal complexes that we have developed. Several of these processes generate hydrogen gas, valuable by itself, or consume it, leading to unprecedented, industrially relevant methodologies in chemical synthesis. Our research has had a strong impact on the rapid development of (a) sustainable catalysis by pincer complexes of earth-abundant metals (b) use of water as a sustainable oxidant of organic compounds, liberating H2, and other novel sustainable reactions. In sustainable energy directions, we have developed novel Liquid Organic Hydrogen Carriers (LOHC), an efficient process for methanol reforming , and a process for dehydrogenation of neat formic acid.
Specific examples, catalyzed by novel pincer complexes, include:
1. New environmentally benign, sustainable synthetic reactions catalyzed by pincer complexes of earth-abundant metals, capable of metal-ligand cooperation:
(a) Manganese is the second most abundant metal on earth’s crust. Examples of novel reactions catalyzed by Mn complexes: (a) waste-free synthesis of amides by coupling of amines with alcohols or esters, or by reaction of benzyl alcohols with ammonia, evolving hydrogen gas. Noteworthy, amide synthesis is one of the most frequently used synthetic operations, but it generatess waste. (b) Mild hydrogenation of the CO2-derived organic carbonates to methanol and alcohols, providing a green two-step process to the generation of methanol fuel from CO2, using a base-metal (rather than ruthenium) (c) C–C bond-formation via α-alkylation of ketones, amides, and esters, using primary alcohols, are synthetically important reactions.
(b) iron complexes: (a) Selective hydrogenation of nitriles to imines, avoiding over hydrogenation (b) Preparation of valuable formamides by N-formylation of amines with H2 and CO2.
(c) Cobalt complexes: Synthesis of benzimidazoles by coupling of aromatic diamines and alcohols generating H2. Benzimidazole and its derivatives are important building blocks for the pharmaceutical industry, because of their prominent biological activity such as antihistaminic, anticancer activity
2. Oxidation of organic compounds using water as oxidant evolving hydrogen gas rather than using polluting, toxic oxidation reagents or potentially unsafe oxygen pressure: (a) oxidation of alkenes by water forming ketones; while the scope is currently limited, further development is warranted; Noteworthy, such a process using oxygen pressure is practiced industrially (“Wacker Process”). (b) Ester synthesis from enol ethers and water c) Selective oxidative deamination of amines by water to form ketones or carboxylic acids. (d) oxidation of the biomass derived Furfural and 5-hydroxymethyl furfural to the corresponding carboxylic acids, with various potential applications, such as synthesis of biomass-derived polyesters.
3. Other synthetically important sustainable transformations catalyzed by ruthenium pincer complexes, liberating hydrogen gas: (a) Synthesis of thioesters by coupling of thiols and alcohols. Thioesters are valuable building blocks in synthetic processes towards heterocyclic compounds and new materials. (b) synthesis of N-heteroaromatics by coupling of ammonia with diols (c) synthesis of oxalamides by coupling of ethylene glycol and amines (d) Mild synthesis of amides under ambient conditions by coupling of alcohols and amines, enabling waste-free preparation of amide-functionalized pharmaceuticals (e) synthesis of primary amides from alcohols and ammonia, and via coupling of epoxides with amines
4. Treatment of environmental hazardous agents: (a) Abatement of the potent greenhouse gas nitrous oxide (N2O) by hydrogenation or hydrosilylation, as well as by oxidation of CO (b) Methodology for Use of formamides as surrogates for highly toxic isocyanates (c) Hydrogenative depolymerization of nylons, forming aminoalcohols, providing a method for cyclical treatment of nylon waste.
5. Novel Liquid organic hydrogen carriers (LOHCs): Employing our pincer ruthenium catalysts, we have developed several new LOHCs, including a novel, efficient and reversible liquid-to-liquid organic hydrogen carrier system based on cheap, readily available and renewable ethylene glycol, enabling efficient discharge and loading of hydrogen under relatively mild conditions using a single ruthenium pincer catalyst, with a high theoretical hydrogen storage capacity of 6.5 wt% exceeding the DOE target for light vehicles of 5.5 wt%.
6. Highly efficient, additive-free, dehydrogenation of neat formic acid, capable of generation high hydrogen pressure, catalyzed by a ruthenium pincer complex.
