Mu Jeng Cheng

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

Oxidative Aliphatic C-H Fluorination with Fluoride Ion Catalyzed by a Manganese Porphyrin

Fluorines Smooth Introduction Carbon-fluorine bonds are emerging as increasingly versatile constituents of drugs, agrochemicals, and positron emission tomography tracers. Elemental F2 gas is in principle an efficient reagent for their preparation, but its extreme reactivity requires special handling precautions. Substantial research has therefore focused on promoting selective reactivity of more conveniently handled fluoride ion salts. Liu et al. (p. 1322) present a manganese catalyst that transfers fluoride to a range of hydrocarbons in conjunction with a hypervalent iodine-based oxidant. Mechanistic studies implicate a manganese difluoride intermediate that reacts with alkyl radicals generated by a preceding manganese oxo. A catalyst introduces fluorine in a convenient, mild fashion to a range of relatively inert hydrocarbons. Despite the growing importance of fluorinated organic compounds in drug development, there are no direct protocols for the fluorination of aliphatic C-H bonds using conveniently handled fluoride salts. We have discovered that a manganese porphyrin complex catalyzes alkyl fluorination by fluoride ion under mild conditions in conjunction with stoichiometric oxidation by iodosylbenzene. Simple alkanes, terpenoids, and even steroids were selectively fluorinated at otherwise inaccessible sites in 50 to 60% yield. Decalin was fluorinated predominantly at the C2 and C3 methylene positions. Bornyl acetate was converted to exo-5-fluoro-bornyl acetate, and 5α-androstan-17-one was fluorinated selectively in the A ring. Mechanistic analysis suggests that the regioselectivity for C-H bond cleavage is directed by an oxomanganese(V) catalytic intermediate followed by F delivery via an unusual manganese(IV) fluoride that has been isolated and structurally characterized.

A computational study on the stability of diaminocarbenes

Abstract The 1,2-H shift, 1,2-Me shift, and dimerization reactions of several diaminocarbenes have been studied using density functional theory and ab initio methods. It was observed that the activation energies ( E a ) for the dimerization of diaminocarbenes are much smaller than those of 1,2-H shift, 1,2-Me shift and insertion. Dimerization is thus the most likely course of reaction for diaminocarbenes. The activation energies for the dimerization of diaminocarbenes which exhibit 6π-electron delocalization are larger than those of the non-aromatic ones. For cyclic diaminocarbenes there is a proportional relation between E a and the singlet–triplet gap (Δ E S–T ), i.e., E a of dimerization is larger as Δ E S–T increases.

The Critical Role of Phosphate in Vanadium Phosphate Oxide for the Catalytic Activation and Functionalization of n-Butane to Maleic Anhydride

We used density functional theory to study the mechanism of n-butane oxidation to maleic anhydride on the vanadium phosphorus oxide (VPO) surface. We found that O(1)═P on the V(V)OPO4 surface is the active center for initiating the VPO chemistry through extraction of H from alkane C-H bonds. This contrasts sharply with previous suggestions that the active center is either the V-O bonds or else a chemisorbed O2 on the (V(IV)O)2P2O7 surface. The ability of O(1)═P to cleave alkane C-H bonds is due to its strong basicity coupled with large reduction potentials of nearby V(V) ions. We examined several pathways for the subsequent functionalization of n-butane to maleic anhydride and found that the overall barrier does not exceed 21.7 kcal/mol.

Computational study on the stability of imidazol-2-ylidenes and imidazolin-2-ylidenes

The dimerization reactions of 12 cyclic diaminocarbenes have been studied using density functional theory (DFT). The activation energies (Ea) and reaction energies (ΔE) for the dimerizations of imidazol-2-ylidenes are larger than those of imidazolin-2-ylidenes. It was observed that Ea of dimerization is approximately proportional to the singlet–triplet energy separation (ΔES–T), aromatic stabilization energy (ASE), and ΔE of the carbene. Excellent linear correlation is seen between Ea and ASE. Contrary to previous suggestions, we found that 4,5-dichloro substitutions decrease the stability of imidazol(in)-2-ylidenes. Steric effects on Ea occur noticeably as isopropyl (i-Pr) substitutions are introduced to the carbenes.

B2P2 rings: through-space π bond or stable diradical? A theoretical study

Theoretical study of a series of B2P2 ring molecules shows that bulky substituent groups facilitate the existence of bond-stretch isomers. With the largest substituent group, the long bond (LB) isomer is more stable and adopts a mainly through-space B-B bond. Among these LB isomers we observed extraordinarily large singlet-triplet energy separation, a small number of effectively unpaired electrons, and the convergence of spin symmetry-broken (UDFT) computations to RDFT. The T 1 diagnostic for the LB isomer of prototype compound obtained at the CCSD/6-311G** level of theory is smaller than 0.02. We thus conclude that these B2P2 ring molecules do not characterize as diradicals.

In Silico Design of Highly Selective Mo-V-Te-Nb-O Mixed Metal Oxide Catalysts for Ammoxidation and Oxidative Dehydrogenation of Propane and Ethane.

We used density functional theory quantum mechanics with periodic boundary conditions to determine the atomistic mechanism underlying catalytic activation of propane by the M1 phase of Mo-V-Nb-Te-O mixed metal oxides. We find that propane is activated by Te═O through our recently established reduction-coupled oxo activation mechanism. More importantly, we find that the C-H activation activity of Te═O is controlled by the distribution of nearby V atoms, leading to a range of activation barriers from 34 to 23 kcal/mol. On the basis of the new insight into this mechanism, we propose a synthesis strategy that we expect to form a much more selective single-phase Mo-V-Nb-Te-O catalyst.

Theoretical study of the Wanzlick equilibrium

A theoretical study of the tetraaminoethylene–diaminocarbene dissociation reaction shows that equilibrium is more readily attained for benzimidazol-2-ylidenes and doubly bridged imidazol-2-ylidenes. For imidazolin-2-ylidenes, the ‘Wanzlick equilibrium’ is less likely to occur due to the high reaction barrier. We demonstrate that the equilibrium reaction can be catalyzed by an electrophile.

Intermolecular hydrogen transfers in 2,3-dihydroimidazol-2-ylidene and 2,3-dihydrothiazol-2-ylidene: A theoretical study

Abstract The remarkable stability of N-heterocyclic carbenes has been an area of great interest in chemistry. However, prototype carbenes of these molecules, i.e. those with hydrogen atoms bearing on the nitrogens, have not been isolated as stable compounds. It is clear from our theoretical study that these prototype carbenes (2,3-dihydroimidazol-2-ylidene and 2,3-dihydrothiazol-2-ylidene) should undergo intermolecular hydrogen transfers (forming imidazole and thiazole) easily at room temperature.

Design and validation of non-metal oxo complexes for C–H activation

We use our recent discovery of the reduction-coupled oxo activation (ROA) principle to design a series of organometallic molecules that activate C-H bonds through this unique proton/electron-decoupled hydrogen abstraction mechanism, in which the main group oxo moiety binds to the proton while the electron is transferred to the transition metal. Here we illustrate this general class of catalyst clusters with several examples that are validated through quantum mechanics calculations.