Xiangyang Chen

A mechanistic study and computational prediction of iron, cobalt and manganese cyclopentadienone complexes for hydrogenation of carbon dioxide

A series of cobalt and manganese cyclopentadienone complexes are proposed and examined computationally as promising catalysts for hydrogenation of CO2 to formic acid with total free energies as low as 20.0 kcal mol-1 in aqueous solution. Density functional theory study of the newly designed cobalt and manganese complexes and experimentally reported iron cyclopentadienone complexes reveals a stepwise hydride transfer mechanism with a water or a methanol molecule assisted proton transfer for the cleavage of H2 as the rate-determining step.

Computational Design of Iron Diphosphine Complexes with Pendant Amines for Hydrogenation of CO2 to Methanol: A Mimic of [NiFe] Hydrogenase.

Inspired by the active-site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P(tBu) 2 N(tBu) 2 )Fe(CN)2 CO] by using an experimentally ready-made diphosphine ligand with pendant amines for the hydrogenation of CO2 to methanol. Density functional theory calculations indicate that the rate-determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol(-1) in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low-cost catalyst for the formation of methanol from CO2 and H2 under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H2 by forming a Fe-H(δ-) ⋅⋅⋅H(δ+) -N dihydrogen bond in a fashion of frustrated Lewis pairs.

Newly designed manganese and cobalt complexes with pendant amines for the hydrogenation of CO2 to methanol: a DFT study

A series of manganese and cobalt complexes with pendant amines, (PtBu2NtBu2)M(R1)(R2)(R3) (M = Mn, R1 = R2 = CO; M = Co, R1 = R2 = CN; R3 = H, CN, NO2, CH3, NH2, OH, CHO, COOH, COCH3, and COOCH3), were proposed and examined as potential catalysts for the production of methanol from CO2 and H2. Detailed mechanisms with three cascade catalytic reactions, the hydrogenation of CO2 to formic acid, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol, are predicted and analyzed through density functional theory calculations. Among all proposed complexes, (PtBu2NtBu2)Co(CN)2(COOH) (1Co–COOH) and (PtBu2NtBu2)Co(CN)2(NH2) (1Co-NH2) are the two most active with total free energy barriers of 24.9 and 25.0 kcal mol−1, respectively. (PtBu2NtBu2)Mn(CO)2(COOH) (1Mn–COOH) and (PtBu2NtBu2)Mn(CO)2(NO2) (1Mn-NO2) are the most active manganese complexes with total free energy barriers of 26.6 and 27.7 kcal mol−1, respectively. Such low barriers indicate that these newly designed cobalt and manganese catalysts are promising low-cost catalysts for the conversion of CO2 and H2 to methanol under mild conditions.

Synthesis, Reactivity, and Catalytic Transfer Hydrogenation Activity of Ruthenium Complexes Bearing NNN Tridentate Ligands: Influence of the Secondary Coordination Sphere

By the introduction of −OH group(s) into different position(s) of 6-(pyridin-2-ylmethyl)-2,2′-bipyridine, several NNN-type ligands were synthesized and then introduced to ruthenium (Ru) centers by reactions with RuCl2(PPh3)3. In the presence of PPh3 or CO, these ruthenium complexes reacted with NH4PF6 in CH2Cl2 or CH3OH to give a series of ionic products 5–9. The reaction of Ru(L2)(PPh3)Cl2 (2) with CO generated a neutral complex [Ru(L2)(CO)Cl2] (10). In the presence of CH3ONa, 10 was further converted into complex [Ru(L2)(HOCH3)(CO)Cl] (11), in which there was a methanol molecule coordinating with ruthenium, as suggested by density functional theory calculations. The catalytic transfer hydrogenation activity of all of these new bifunctional metal–ligand complexes was tested. Dichloride complex 2 exhibits best activity, whereas carbonyl complexes 10 and 11 are efficient for selectively reducing 5-hexen-2-one, suggesting different hydrogenation mechanisms. The results reveal the dramatic influence for the reactivity and catalytic activity of the secondary coordination sphere in transition metal complexes.

Mechanistic Insights and Computational Design of Transition‐Metal Catalysts for Hydrogenation and Dehydrogenation Reactions

Catalytic hydrogenation and dehydrogenation reactions are fundamentally important in chemical synthesis and industrial processes, as well as potential applications in the storage and conversion of renewable energy. Modern computational quantum chemistry has already become a powerful tool in understanding the structures and properties of compounds and elucidating mechanistic insights of chemical reactions, and therefore, holds great promise in the design of new catalysts. Herein, we review our computational studies on the catalytic hydrogenation of carbon dioxide and small organic carbonyl compounds, and on the dehydrogenation of amine-borane and alcohols with an emphasis on elucidating reaction mechanisms and predicting new catalytic reactions, and in return provide some general ideas for the design of high-efficiency, low-cost transition-metal complexes for hydrogenation and dehydrogenation reactions.