Oriented External Electric-Field Effects on the Activation of Aryl CO Bond in Anisole using Rh(PEP) (E=Al, B, Ga) Catalysts.

The present work reports a DFT-based mechanistic investigation of aryl C-O bond activation in anisole catalysed by a Rh-Al pincer-type complex at room temperature. The study is extended to analogues Rh-E complexes based on Group 13 elements (E=B/Ga). Our results show a preference for the heterolytic cleavage pathway over oxidative addition in the C-O bond activation. The calculated barriers range from 16 to 36 kcal/mol, following the order: E=Al

CO2 capture, activation and dissociation on the Ti2C surface and Ti2C MXene: the role of surface structure.

Atmospheric CO2 is one of the main components of the greenhouse effect. To overcome this problem there are ongoing efforts to convert CO2 to some other useful and harmless products. The capture, activation and dissociation of CO2 are the preliminary steps in this process. In an effort to understand the role of surface composition and structure in CO2 adsorption and dissociation, in this work, with the help of first principles density functional theory based calculations, we have studied the same on the {100} surface of cubic Ti2C and MXene (also the {0001} surface of trigonal Ti2C). Our results show that CO2 undergoes barrierless chemisorption on both of these surfaces with a preference towards {100} cubic Ti2C. We attribute the reason for this to a lower value of the work function of the {100} surface. Furthermore, on MXene, the barrier for CO2 dissociation is lower compared to that on the {100} surface. Coverage dependent CO2 chemisorption studies on these two surfaces show that on the Ti2C surface the CO2 molecules form clusters around the C-vacancies while on MXene they are uniformly spread on the surface.

Characterization of Rh-Al Bond in Rh(PAlP) (PAlP = Pincer-type Diphosphino-Aluminyl Ligand) in Comparison with Rh(L)(PMe3)2 (L = AlMe2, Al(NMe2)2, BR2, SiR3, CH3, Cl, or OCH3): Theoretical Insight.

The unique Rh-Al bond in recently synthesized Rh(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand Al[NCH2(P iPr2)]2(C6H4)2NMe} was investigated using the DFT method. Complex 1 has four doubly occupied nonbonding d orbitals on the Rh atom and one Rh d orbital largely participating in the Rh-Al bond which exhibits considerably large bonding overlap between Rh and Al atoms like in a covalent bond. Interestingly, Rhδ--Alδ+ polarization is observed in the bonding MO of 1, which is reverse to Rhδ+-Eδ- (E = coordinating atom) polarization found in a usual coordinate bond. This unusual polarization arises from the presence of the Al valence orbital at significantly higher energy than the Rh valence orbital energy. Characteristic features of 1 are further unveiled by comparing 1 with similar Rh complexes RhL(PMe3)2 (2 for L = AlMe2, 3 for L = Al(NMe2)2, 4 for L = BMe2, 5 for L = SiMe3, 6 for L = SiH3, 7 for L = CH3, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rhδ+-Eδ- polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rhδ--Eδ+ polarization is observed in the Rh-E bonding MOs of 2-5 like in 1, while the Rh-Si bond is polarized little in 6. These results are clearly understood in terms of the valence orbital energy of the ligand. Because the LUMO of 1 mainly consists of the Rh 4dσ, 5s, and 5p orbitals and the Al 3s and 3p orbitals, both Rh and Al atoms play the role of coordinating site for a substrate bearing a lone pair orbital. For instance, NH3 and pyridine coordinate to both Al and Rh atoms with considerably large binding energy. PAlP exhibits significantly strong trans influence, which is as strong as that of SiMe3 but moderately weaker than that of BMe2. The trans influence of these ligands is mainly determined by the valence orbital energy of the ligand and the covalent bond radius of the coordinating E atom.

Rhodium Complexes Bearing PAlP Pincer Ligands.

We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and single-crystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable σ-donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP-Rh complexes catalyze C2-selective monoalkylation of pyridine with alkenes.

Can substituted allenes be highly efficient leaving groups in catalytic processes? A computational investigation.

There is considerable interest presently in the chemistry of allenes. The current computational investigation looks into the possibility of using allenes and their derivatives as leaving groups. As it is well known, leaving groups are significant in catalytic processes for generating the active site. A full quantum mechanical study using density functional theory shows that allenes and their derivatives can function as excellent leaving groups. Indeed, the calculations show that they can be several orders of magnitude more effective than existing ligands for this purpose. The modification of second generation Grubbs' catalysts with these ligands suggests that the allene ligand cases that would be most effective are those having electron withdrawing groups, especially those that have the potential for supramolecular interactions between the substituent groups in the free state.

Can molecular cages be effective at small molecule activation? A computational investigation.

Several approaches have been investigated in recent years in the area of small molecule activation. The current work introduces a new concept in this field: that of molecular cages, containing only main group elements, serving to activate small molecules. Full quantum mechanical calculations employing DFT/SCS-MP2 methods indicate that recently synthesized Ga-N cage compounds would be excellent candidates for mediating in important reactions such as the catalysis of ammonia borane dehydrogenation. The current work therefore opens up exciting new possibilities in small molecule activation research.

A theoretical study of metal-metal cooperativity in the homogeneous water gas shift reaction.

The possibility of metal-metal cooperativity in improving the yield of the homogeneous water gas shift reaction (WGSR) has been investigated through full quantum mechanical density functional theory calculations. The calculations indicate that bimetallic catalysts would be likely to be more highly active than mononuclear metal-based catalysts for the WGSR. The results have implications for the design of improved WGSR catalysts in the future.

Can main group systems act as superior catalysts for dihydrogen generation reactions? A computational investigation.

The protolytic cleavage of the O-H bond in water and alcohols is a very important reaction, and an important method for producing dihydrogen. Full quantum chemical studies with density functional theory (DFT) reveal that germanium based complexes, such as HC{CMeArB}2GeH (Ar = 2,6-(i)Pr2C6H3), with the assistance of silicon based compounds such as SiF3H, can perform significantly better than the existing state-of-the-art post-transition metal based systems for catalyzing dihydrogen generation from water and alcohols through the protolysis reaction.

New insights into small molecule activation by acyclic silylenes: a computational investigation.

Recently synthesized acyclic silylenes have the potential to rival transition metal complexes in performing single site small molecule activation, which is significant because of the need to find cheap and green alternatives to transition metal complexes for this important class of reactions. However, the current computational study, a full quantum chemical investigation with density functional theory, demonstrates that undesired side reactions would be competitive in these systems during small molecule activation. The current investigation, in addition to shedding light on this problem, also provides solutions on how the undesired side reactions during small molecule activation can be avoided.