The DFT study on Rh–C bond dissociation enthalpies of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η⁵-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H₂ generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H₂ generation. Conversely, N, a more electron-withdrawing center, favors H₂ generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

Oilfield-produced water treatment using conventional and membrane-based technologies for beneficial reuse: A critical review

Oilfield produced water (OPW) is one of the most important by-products, resulting from oil and gas exploration. The water contains a complex mixture of organic and inorganic compounds such as grease, dissolved salt, heavy metals as well as dissolved and dispersed oils, which can be toxic to the environment and public health. This article critically reviews the complex properties of OPW and various technologies for its treatment. They include the physico-chemical treatment process, biological treatment process, and physical treatment process. Their technological strengths and bottlenecks as well as strategies to mitigate their bottlenecks are elaborated. A particular focus is placed on membrane technologies. Finally, further research direction, challenges, and perspectives of treatment technologies for OPW are discussed. It is conclusively evident from 262 published studies (1965-2021) that no single treatment method is highly effective for OPW treatment as a stand-alone process however, conventional membrane-based technologies are frequently used for the treatment of OPW with the ultrafiltration (UF) process being the most used for oil rejection form OPW and oily waste water. After membrane treatment, treated effluents of the OPW could be reused for irrigation, habitant and wildlife watering, microalgae production, and livestock watering. Overall, this implies that target pollutants in the OPW samples could be removed efficiently for subsequent use, despite its complex properties. In general, it is however important to note that feed quality, desired quality of effluent, cost-effectiveness, simplicity of process are key determinants in choosing the most suitable treatment process for OPW treatment.

Nature and Strength of M-H···S and M-H···Se (M = Mn, Fe, & Co) Hydrogen Bond

The significance of dispersion contribution in the formation of strong hydrogen bonds (H-bonds) can no more be ignored. It was illustrated that less electronegative and electropositive H-bond acceptors such as S, Se, and Te are also capable of forming strong N-H···Y H-bonds, mostly due to the high polarizabilities of H-bond acceptor atoms. Herein, for the first time, we report the evidence of formation of nonconventional M-H···Y H-bonds between metal hydrides (M-H, M = Mn, Fe, Co) and chalcogen H-bond acceptors (Y = O, S, or Se). The nature and the strength of unusual M-H···Y H-bonds were revealed by several quantum chemical calculations and H-bond descriptors. The structural parameters, electron density topology, donor-acceptor natural bond orbital (NBO) interaction energies, and spectroscopic observables such as M-H stretching frequencies and ¹H chemical shifts are well-correlated to manifest the existence and strength of M-H···Y H-bonding. The M-H···Y H-bonds are dispersive in nature, and the computed H-bond energies are found to be in the range from ∼5 to 30 kJ/mol, which can be compared to those of the conventional H-bonds such as O-H···O, N-H···O, and N-H···O═C H-bonds, etc.