Computational Design of Frustrated Lewis Pairs as a Strategy for Catalytic Hydrogen Activation and Hydrogenation Catalyst

Catalytic hydrogenation is one of the most important reaction types commonly used in chemistry and chemical industry. Recently, there has been significant interest in developing a metal-free hydrogenation catalyst to avoid the problems caused by using heavy transition metal catalysts. On the basis of the advances of metal-free hydrogen activation with frustrated Lewis pairs (FLPs, e.g. tBu₃P/B(C₆F₅)₃) which often uses boron as a Lewis acid center, we computationally explored the prospect for phosphorus(V) and sulfur(VI) as Lewis acid centers to construct FLPs for hydrogen activation and hydrogenation. We found out that the proposed FLPs with P(V)- or S(VI)-centered Lewis acid can also activate H₂ with a mechanism similar to that used by the conventional FLPs. A heterolytic cleavage of H-H is achieved when electrons are donated simultaneously from the σ orbital of H₂ to the empty orbital of the Lewis acid center and from the lone-pair orbital of the Lewis base center to the σ* orbital of H₂. The multiple C-H···F hydrogen bonds further aid the association of the pairs for H₂ activation. Some of our designed FLPs possess kinetics and thermodynamics for developing hydrogenation catalysts. This computational exploration could inspire experimental development of a new type of FLPs with P(V) or S(VI) or a Lewis acid partner for FLPs for reversible H₂ activation.

A hydroboration route to geminal P/B frustrated Lewis pairs with a bulky secondary phosphane component and their reaction with carbon dioxide

The secondary aryl-P(H) phosphanyl substituted tert-butylacetylenes 7a,b (aryl: Mes or Mes*) undergo hydroboration with [HB(C₆F₅)₂] to give the geminal vinylidene-bridged P/B Lewis pairs 8a,b. The treatment of 8a,b with benzonitrile, N-sulfinylaniline, and phenyl isothiocyanate, respectively, gives the addition products 12a,b, 13a,b, and 14 with proton transfer from the phosphorus to the more basic nitrogen site. The reaction of the FLPs 8a,b with carbon dioxide yields a doubly boron bonded addition product. The reaction of 8b with a conjugated ynone formally proceeded by trans-1,2-hydrophosphination of the alkyne at the geminal FLP framework to give the seven-membered heterocycle 21.

A strategy for developing metal-free hydrogenation catalysts: a DFT proof-of-principle study

Using DFT computations, a metal-free strategy has been formulated to activate hydrogen reversibly and to construct hydrogenation catalysts, calling for experimental realizations.

Metal-free homolytic hydrogen activation: a quest through density functional theory computations

DFT computations reveal that heavier analogs of 1,3-butadiene could activate H₂homolyticallyvia1,4-addition.

Assessing the performance of commonly used DFT functionals in studying the chemistry of frustrated Lewis pairs

The benchmark study has assessed the performance of 18 density functional theory (DFT) functionals, including GGAs, hybrid-GGAs, meta-GGAs, and hybrid meta-GGAs, in predicting bonding strength, barrier height and structure for systems involving Lewis acids and bases. Three databases were built for the study, including 15 bonding enthalpies of dative bonds (DBH15), 10 reaction barriers (BH10) and 10 X-ray structures (XCS10). Wavefunction-based ab initio calculations were also carried out for comparisons. The benchmark data were computed at the CCSD(T)/BSI//MP2/BSI(BSI=aug-cc-pVTZ) level. The 6-311++G(d,p)(BSII) and 6-31G(d,p)(BSIII) basis sets were employed in DFT calculations. Generally, M06-2X/BSII and M05-2X/BSII outperform the other tested DFT methods. M05-2X/BSIII and M06-2X/BSIII are less accurate than M05-2X/BSII and M06-2X/BSII (or M05-2X/BSII//M05-2X/BSIII and M06-2X/BSII//M06-2X/BSIII), suggesting that large basis sets (e.g. BSII) are necessary to improve energetics. SCS-MP2 is less accurate than MP2, consistently overestimating bonding enthalpies and reaction barriers. Moreover, six DFT functionals (M05-2X, M06-2X, B3LYP, ωB97X-D, BMK and B97-D) were examined by comparing with the experimental bonding enthalpies of eighteen donor–acceptor complexes, which indicate that M05-2X and M06-2X are still better than others. Nevertheless, M05-2X and M06-2X significantly overestimate or underestimate the bonding enthalpies of F-substituted complexes, implying the necessity of improving the two functionals for describing fluorides. Using the six basis sets (BSI, cc-pVTZ, aug-cc-pVDZ, cc-pVDZ, TZVP and BSII) and DBH15 and BH10 databases, the influence of the basis sets on the performance of M06-2X functional was examined, which reveals that BSII is the most suitable basis set for the functional.

Gold catalyzed hydrogenations of small imines and nitriles: enhanced reactivity of Au surface toward H2via collaboration with a Lewis base

Au (111) surface can serve as a Lewis acid to couple with a Lewis base (e.g. imine or nitrile) to form the Au-coupled FLP (frustrated Lewis pair, left) which can cleave H₂, further achieving hydrogenation of small imines and nitriles.