Xinzheng Yang

Monomeric and Oligomeric Amine-Borane σ-Complexes of Rhodium. Intermediates in the Catalytic Dehydrogenation of Amine-Boranes

A combined experimental/quantum chemical investigation of the transition metal-mediated dehydrocoupling reaction of H(3)B.NMe(2)H to ultimately give the cyclic dimer [H(2)BNMe(2)](2) is reported. Intermediates and model complexes have been isolated, including examples of amine-borane sigma-complexes of Rh(I) and Rh(III). These come from addition of a suitable amine-borane to the crystallographically characterized precursor [Rh(eta(6)-1,2-F(2)C(6)H(4))(P(i)Bu(3))(2)][BAr(F)(4)] [Ar(F) = 3,5-(CF(3))(2)C(6)H(3)]. The complexes [Rh(eta(2)-H(3)B.NMe(3))(P(i)Bu(3))(2)][BAr(F)(4)] and [Rh(H)(2)(eta(2)-H(3)B.NHMe(2))(P(i)Bu(3))(2)][BAr(F)(4)] have also been crystallographically characterized. Other intermediates that stem from either H(2) loss or gain have been characterized in solution by NMR spectroscopy and ESI-MS. These complexes are competent in the catalytic dehydrocoupling (5 mol %) of H(3)B.NMe(2)H. During catalysis the linear dimer amine-borane H(3)B.NMe(2)BH(2).NHMe(2) is observed which follows a characteristic intermediate time/concentration profile. The corresponding amine-borane sigma-complex, [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)], has been isolated and crystallographically characterized. A Rh(I) complex of the final product, [Rh(P(i)Bu(3))(2){eta(2)-(H(2)BNMe(2))(2)}][BAr(F)(4)], is also reported, although this complex lies outside the proposed catalytic cycle. DFT calculations show that the first proposed dehydrogenation step, to give H(2)B horizontal lineNMe(2), proceeds via two possible routes of essentially the same energy barrier: BH or NH activation followed by NH or BH activation, respectively. Subsequent to this, two possible low energy routes that invoke either H(2)/H(2)B horizontal lineNMe(2) loss or H(2)B horizontal lineNMe(2)/H(2) loss are suggested. For the second dehydrogenation step, which ultimately affords [H(2)BNMe(2)](2), a number of experimental observations suggest that a simple intramolecular route is not operating: (i) the isolated complex [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)] is stable in the absence of amine-boranes; (ii) addition of H(3)B.NMe(2)BH(2).NHMe(2) to [Rh(P(i)Bu(3))(2)(eta(2)-H(3)B.NMe(2)BH(2).NHMe(2))][BAr(F)(4)] initiates dehydrocoupling; and (iii) H(2)B horizontal lineNMe(2) is also observed during this process.

Monoiron Hydrogenase Catalysis: Hydrogen Activation with the Formation of a Dihydrogen, Fe-Hδ-···Hδ+-O, Bond and Methenyl-H4MPT+ Triggered Hydride Transfer

A fully optimized resting state model with a strong Fe-H(delta-)...H(delta+)-O dihydrogen bond for the active site of the third type of hydrogenase, [Fe]-hydrogenase, is proposed from density functional theory (DFT) calculations on the reformulated active site from the recent X-ray crystal structure study of C176A (Cys176 was mutated to an alanine) mutated [Fe]-hydrogenase in the presence of dithiothreitol. The computed vibrational frequencies for this new active site model possess an average error of only +/-4.5 cm(-1) with respect to the wild-type [Fe]-hydrogenase. Based on this resting state model, a new mechanism with the following unusual aspects for hydrogen activation catalyzed by [Fe]-hydrogenase is also proposed from DFT calculations. (1) Unexpected dual pathways for H(2) cleavage with proton transfer to Cys176-sulfur or 2-pyridinols oxygen for the formation and regeneration of the resting state with an Fe-H(delta-)...H(delta+)-O dihydrogen bond before the appearance of methenyl-H(4)MPT(+) (MPT(+)). (2) The strong dihydrogen bond in this resting state structure prevents D(2)/H(2)O exchange. (3) Only upon the arrival of MPT(+) with its strong hydride affinity can D(2)/H(2)O exchange take place as the arrival of MPT(+) triggers the breaking of the strong Fe-H(delta-)...H(delta+)-O dihydrogen bond by taking a hydride from the iron center and initiating the next H(2) (D(2)) cleavage. This new mechanism is completely different than that previously proposed (J. Am. Chem. Soc. 2008, 130, 14036) which was based on an active site model related to an earlier crystal structure. Here, Fes role is H(2) capture and hydride formation without MPT(+) while the pyridones special role involves the protection of the hydride by the dihydrogen bond.

Mechanism of Water Splitting and Oxygen−Oxygen Bond Formation by a Mononuclear Ruthenium Complex

Density functional theory (DFT) predicts a detailed mechanism for the reported potential photocatalytic system for solar hydrogen production from water, (P-da-PNN)RuH(CO) (1, P-da = dearomatized at the phosphorus side arm, PNN = (2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine) (Science 2009, 324, 74). In the initial thermal reaction, the coordination of a water molecule is followed by cleavage of an O-H bond and aromatization of the PNN ligand to form (PNN)RuH(CO)(OH) (3), the most stable complex in the reaction. This low-barrier step is followed by the rate-determining dearomatization and formation of H(2). Next, a second water molecule is activated, resulting in the formation of the cis-dihydroxo complex (PNN)Ru(CO)(OH)(2) (7), which photolytically eliminates H(2)O(2). Time-dependent DFT calculations predict that the breaking of the two strong Ru-O bonds and the formation of the O-O bond in this photolytic reaction involve low-energy triplet states and singlet-triplet crossings. Rather than regeneration of initial complex 1 after the light-induced H(2)O(2) evolution in the catalytic cycle, the DFT calculations predict a new route with a lower energy barrier via the regeneration of 1, an isomer of 1 with the unsaturated carbon at the nitrogen side arm of the PNN ligand. This new route involves hydride transfer from the methylene group at the nitrogen side, rather than the previously proposed regeneration of 1 through hydride transfer from the phosphorus side arm of the PNN ligand.

The catalytic dehydrogenation of ammonia-borane involving an unexpected hydrogen transfer to ligated carbene and subsequent carbon-hydrogen activation.

Density functional Tao−Perdew−Staroverov−Scuseria calculations with all-electron correlation-consistent polarized valence double-ζ basis set demonstrate that N-heterocyclic carbene (NHC) nickel complexes catalyze the dehydrogenation of ammonia-borane, a candidate for chemical hydrogen storage, through proton transfer from nitrogen to the metal-bound carbene carbon, instead of the B−H or N−H bond activation. This new C−H bond is then activated by the metal, transferring the H to the metal, then forming the H2 by transferring a H from B to the metal, instead the β-H transfer. This reaction pathway explains the importance of the NHC ligands in the dehydrogenation and points the way to finding new catalyst with higher efficiency, as partial unsaturation of the M-L bond may be essential for rapid H transfers.

A structurally rigid bis(amido) ligand framework in low-coordinate Ni(I), Ni(II), and Ni(III) analogues provides access to a Ni(III) methyl complex via oxidative addition.

A structurally persistent bis-amido ligand framework capable of supporting nickel compounds in three different oxidation states has been identified. A highly unusual, isolable Ni(III) alkyl species has been prepared and characterized via a rare example of a two-electron oxidative addition of MeI to Ni(I).

Unexpected direct reduction mechanism for hydrogenation of ketones catalyzed by iron PNP pincer complexes.

The hydrogenation of ketones catalyzed by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated iron pincer complexes was studied using the range-separated and dispersion-corrected ωB97X-D functional in conjunction with the all-electron 6-31++G(d,p) basis set. A validated structural model in which the experimental isopropyl groups were replaced with methyl groups was employed for the computational study. Using this simplified model, the calculated total free energy barrier of a previously postulated mechanism with the insertion of ketone into the Fe-H bond is far too high to account for the observed catalytic reaction. Calculation results reveal that the solvent alcohol is not only a stabilizer of the dearomatized intermediate but also more importantly an assistant catalyst for the formation of trans-(PNP)Fe(H)(2)(CO), the actual catalyst for hydrogenation of ketones. A direct reduction mechanism, which features the solvent-assisted formation of a trans dihydride complex trans-(PNP)Fe(H)(2)(CO), direct transfer of hydride to acetophenone from trans-(PNP)Fe(H)(2)(CO) for the formation of a hydrido alkoxo complex, and direct H(2) cleavage by hydrido alkoxo without the participation of the pincer ligand for the regeneration of trans-(PNP)Fe(H)(2)(CO), was predicted.

Computational and experimental study of the mechanism of hydrogen generation from water by a molecular molybdenum-oxo electrocatalyst.

We investigate the mechanism for the electrocatalytic generation of hydrogen from water by the molecular molybdenum-oxo complex, [(PY5Me(2))MoO](2+) (PY5Me(2) = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine). Computational and experimental evidence suggests that the electrocatalysis consists of three distinct electrochemical reductions, which precede the onset of catalysis. Cyclic voltammetry studies indicate that the first two reductions are accompanied by protonations to afford the Mo-aqua complex, [(PY5Me(2))Mo(OH(2))](+). Calculations support hydrogen evolution from this complex upon the third reduction, via the oxidative addition of a proton from the bound water to the metal center and finally an α-H abstraction to release hydrogen. Calculations further suggest that introducing electron-withdrawing substituents such as fluorides in the para positions of the pyridine rings can reduce the potential associated with the reductive steps, without substantially affecting the kinetics. After the third reduction, there are kinetic bottlenecks to the formation of the Mo-hydride and subsequent hydrogen release. Computational evidence also suggests an alternative to direct α-H abstraction as a mechanism for H(2) release which exhibits a lower barrier. The new mechanism is one in which a water acts as an intramolecular proton relay between the protons of the hydroxide and the hydride ligands. The calculated kinetics are in reasonable agreement with experimental measurements. Additionally, we propose a mechanism for the stoichiometric reaction of [(PY5Me(2))Mo(CF(3)SO(3))](+) with water to yield hydrogen and [(PY(5)Me(2))MoO](2+) along with the implications for the viability of an alternate catalytic cycle involving just two reductions to generate the active catalyst.

Trigger mechanism for the catalytic hydrogen activation by monoiron (iron-sulfur cluster-free) hydrogenase.

A fully optimized model for the resting state of the active site of the third type of phylogenetically unrelated (monoiron) hydrogenase, iron-sulfur cluster-free hydrogenase (Hmd), was constructed based on density functional calculations. This resting state structure shows good agreement with the experimental IR spectra. The calculations predict that the barrier for H2 cleavage in the presence of MPT+ is 18 kcal/mol lower than that in the absence of MPT+, a result that explains why the isotopic H2/D2O exchange catalyzed by Hmd is strictly dependent on the presence of MPT+. This difference is a result of the MPT+ triggering the pyridone to provide electron density to allow the Fe to take a proton while transferring a hydride to the MPT+. These active site models and catalytic mechanism are useful in understanding this hydrogen activation for the design of novel hydrogenation catalysts and for low cost, high efficiency hydrogen generation.

Mechanistic insights into iron catalyzed dehydrogenation of formic acid: β-hydride elimination vs. direct hydride transfer

Density functional theory calculations reveal a complete reaction mechanism with detailed energy profiles and transition state structures for the dehydrogenation of formic acid catalyzed by an iron complex, [P(CH2CH2PPh2)3FeH](+). In the cationic reaction pathway, a β-hydride elimination process is confirmed to be the rate-determining step in this catalytic reaction. A potential reaction pathway starting with a direct hydride transfer from HCOO(-) to Fe is found to be possible, but slightly less favorable than the catalytic cycle with a β-hydride elimination step.

The mechanism of alkene addition to a nickel bis(dithiolene) complex: the role of the reduced metal complex.

The binding of an alkene by Ni(tfd)(2) [tfd = S(2)C(2)(CF(3))(2)] is one of the most intriguing ligand-based reactions. In the presence of the anionic, reduced metal complex, the primary product is an interligand adduct, while in the absence of the anion, dihydrodithiins and metal complex decomposition products are preferred. New kinetic (global analysis) and computational (DFT) data explain the crucial role of the anion in suppressing decomposition and catalyzing the formation of the interligand product through a dimetallic complex that appears to catalyze alkene addition across the Ni-S bond, leading to a lower barrier for the interligand adduct.