Water and a Borohydride/Hydronium Intermediate in the Borane-Catalyzed Hydrogenation of Carbonyl Compounds with H in Wet Ether: A Computational Study.

We have computationally evaluated water as an active Lewis base (LB) and introduced the borohydride/hydronium intermediate in the mechanism of B(CF)-catalyzed hydrogenation of carbonyl compounds with H in wet/moist ether. Our calculations extend the known frustrated Lewis pair mechanism of this reaction toward the inclusion of water as the active participant in all steps. Although the definition of the zero-energy point interweaves in comparison of the scenarios with and without water, we will be able to show that (i) water (hydrogen bonded to its molecular environment) can, in principle, act as a reasonably viable LB in cooperation with the borane Lewis acid such as B(CF) but relatively a strong borane-water complexation can be the hindering factor; (ii) the herein-proposed borohydride/hydronium intermediates with the hydronium cation having three OH···ether hydrogen bonds or a combination of the OH···ether/OH···ketone hydrogen bonds appear to be as valid as the previously considered borohydride/oxonium or borohydride/oxocarbenium intermediates; (iii) the proton-coupled hydride transfer from the borohydride/hydronium to a ketone (acetone) has a reasonably low barrier.

H Cleavage by Frustrated Lewis Pairs Characterized by the Energy Decomposition Analysis of Transition States: An Alternative to the Electron Transfer and Electric Field Models.

Knowing that the Papai's electron transfer (ET) and the Grimme's electric field (EF) models draw attention to somewhat different physical aspects, we are going to systematically (re)examine interactions in the transition states (TSs) of the heterolytic H-cleavage by the Frustrated Lewis Pairs (FLPs). Our main vehicle is the quantitative energy decomposition analysis (EDA), a powerful method for elucidation of interactions, plus the analysis of molecular orbitals (MOs). Herein, the Lewis acid (LA) is B(CF) and the Lewis bases (LBs) are tBuP, ( o-CHMe)P, 2,6-lutidine, 2,4,6-lutidine, MeN═C(Ph)Me imine, MeN(H)-C(H)PhMe amine, THF, 1,4-dioxane, and acetone.

Structurally Flexible Oxocarbenium/Borohydride Ion Pair: Dynamics of Hydride Transfer on the Background of Conformational Roaming.

We apply Born-Oppenheimer molecular dynamics to the practically significant [dioxane-H-acetone][(CF)B-H] and [EtO-H-OCPr][(CF)B-H] ion pair intermediates. Dynamics of hydride transfer in cation/anion ion pair takes place on the background of large-amplitude configurational changes. Geometry of oxocarbenium/borohydride ion pairs is flexible, meaning that we uncover significant actual structural disorder at a finite temperature.

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations.

We investigate the geometry of oxonium/borohydride ion pairs [ether-H-ether][LA-H] with dioxane, THF, and EtO as ethers and B(CF) as the Lewis acid (LA). The question is about possible location of the disolvated proton, [ether-H-ether], with respect to the hydride of the structurally complex [LA-H] anion. Using Born-Oppenheimer molecular dynamics and a comparison of the potential and free energies of the optimized configurations, we show that herein considered ion pairs are much more flexible geometrically than previously thought.

Theory-Based Extension of the Catalyst Scope in the Base-Catalyzed Hydrogenation of Ketones: RCOOH-Catalyzed Hydrogenation of Carbonyl Compounds with H Involving a Proton Shuttle.

As an extension of the reaction mechanism describing the base-catalyzed hydrogenation of ketones according to Berkessel et al., we use a standard methodology for transition-state (TS) calculations in order to check the possibility of heterolytic cleavage of H at the ketone's carbonyl carbon atom, yielding one-step hydrogenation path with involvement of carboxylic acid as a catalyst. As an extension of the catalyst scope in the base-catalyzed hydrogenation of ketones, our mechanism involves a molecule with a labile proton and a Lewis basic oxygen atom as a catalyst-for example, R-C(=O)OH carboxylic acids-so that the heterolytic cleavage of H could take place between the Lewis basic oxygen atom of a carboxylic acid and the electrophilic (Lewis acidic) carbonyl carbon of a ketone/aldehyde.

Testing the nature of reaction coordinate describing interaction of H with carbonyl carbon, activated by Lewis acid complexation, and the Lewis basic solvent: A Born-Oppenheimer molecular dynamics study with explicit solvent.

Using Born-Oppenheimer molecular dynamics (BOMD), we explore the nature of interactions between H and the activated carbonyl carbon, C(carbonyl), of the acetone-B(CF) adduct surrounded by an explicit solvent (1,4-dioxane). BOMD simulations at finite (non-zero) temperature with an explicit solvent produced long-lasting instances of significant vibrational perturbation of the H-H bond and H-polarization at C(carbonyl). As far as the characteristics of H are concerned, the dynamical transient state approximates the transition-state of the heterolytic H-cleavage.

Liberation of H from (o-CHMe)P-H + H-B(p-CFH) ion-pair: A transition-state in the minimum energy path versus the transient species in Born-Oppenheimer molecular dynamics.

Using Born-Oppenheimer molecular dynamics (BOMD) with density functional theory, transition-state (TS) calculations, and the quantitative energy decomposition analysis (EDA), we examined the mechanism of H-liberation from LB-H + H-LA ion-pair, 1, in which the Lewis base (LB) is (o-CHMe)P and the Lewis acid (LA) is B(p-CFH). BOMD simulations indicate that the path of H liberation from the ion-pair 1 goes via the short-lived transient species, LB⋯H⋯LA, which are structurally reminiscent of the TS-structure in the minimum-energy-path describing the reversible reaction between H and (o-CHMe)P/B(p-CFH) frustrated Lewis pair (FLP). With electronic structure calculations performed on graphics processing units, our BOMD data-set covers more than 1 ns of evolution of the ion-pair 1 at temperature T ≈ 400 K.

Computational Elucidation of a Role That Brønsted Acidification of the Lewis Acid-Bound Water Might Play in the Hydrogenation of Carbonyl Compounds with H in Lewis Basic Solvents.

Brønsted acidification of water by Lewis acid (LA) complexation is one of the fundamental principles in chemistry. Using transition-state calculations (TS), herein we investigate the role that Brønsted acidification of the LA-bound water might play in the mechanism of the hydrogenation of carbonyl compounds in Lewis basic solvents under non-anhydrous conditions. The potential energy scans and TS calculations were carried out with a series of eight borane LAs as well as the commonly known strong LA AlCl in 1,4-dioxane or THF as Lewis basic solvents.

Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton-Transfer Dynamics of the Boroalkoxide Intermediate.

By using transition-state (TS) calculations, we examined how Lewis acid (LA) complexation activates carbonyl compounds in the context of hydrogenation of carbonyl compounds by H in Lewis basic (ethereal) solvents containing borane LAs of the type (C F ) B. According to our calculations, LA complexation does not activate a ketone sufficiently enough for the direct addition of H to the O=C unsaturated bond; but, calculations indicate a possibly facile heterolytic cleavage of H at the activated and thus sufficiently Lewis acidic carbonyl carbon atom with the assistance of the Lewis basic solvent (i.e.

A Prediction of Proton-Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules.

A ketone's carbonyl carbon is electrophilic and harbors a part of the lowest unoccupied molecular orbital of the carbonyl group, resembling a Lewis acidic center; under the right circumstances it exhibits very useful chemical reactivity, although the natural electrophilicity of the ketone's carbonyl carbon is often not strong enough on its own to produce such reactivity. Quantum chemical calculations predict that a proton shared between a ketone and the Lewis basic solvent molecule (dioxane or THF) activates carbonyl carbon to the point of enabling a facile heterolytic splitting of H . Proton-catalyzed hydrogenation of a ketone in Lewis basic solvent is the result.

Ab Initio Molecular Dynamics with Explicit Solvent Reveals a Two-Step Pathway in the Frustrated Lewis Pair Reaction.

The role solvent plays in reactions involving frustrated Lewis pairs (FLPs)-for example, the stoichiometric mixture of a bulky Lewis acid and a bulky Lewis base-still remains largely unexplored at the molecular level. For a reaction of the phosphorus/boron FLP and dissolved CO2 gas, first principles (Born-Oppenheimer) molecular dynamics with explicit solvent reveals a hitherto unknown two-step reaction pathway-one that complements the concerted (one-step) mechanism known from the minimum-energy-path calculations. The rationalization of the discovered reaction pathway-that is, the stepwise formation of PC and OB bonds-is that the environment (typical organic solvents) stabilizes an intermediate which results from nucleophilic attack of the phosphorus Lewis base on CO2 .

Ab initio molecular dynamics study of hydrogen cleavage by a Lewis base [tBu3P] and a Lewis acid [B(C6F5)3] at the mesoscopic level--dynamics in the solute-solvent molecular clusters.

With the help of state-of-the-art ab initio molecular dynamics methods, we investigated the reaction pathway of the {tBu3 P + H2 + B(C6 F5 )3 } system at the mesoscopic level. It is shown that: i) the onset of H2 activation is at much larger boron⋅⋅⋅phosphorus distances than previously thought; ii) the system evolves to the product in a roaming-like fashion because of quasi-periodic nuclear motion along the asymmetric normal mode of P⋅⋅⋅HH⋅⋅⋅B fragment; iii) transient configurations of a certain type are present despite structural interference from the solvent; iv) transient-state configurations with sub-picosecond lifetime have potentially interesting infrared activity in the organic solvent (toluene) as well as in the gas phase. The presented results should be helpful for future experimental and theoretical studies of frustrated Lewis pair (FLP) activity.

How frustrated Lewis acid/base systems pass through transition-state regions: H2 cleavage by [tBu3P/B(C6F5)3].

We investigate the transition-state (TS) region of the potential energy surface (PES) of the reaction tBu3P + H2 + B(C6F5)3 → tBu3P-H(+) + (-)H-B(C6F5)3 and the dynamics of the TS passage at room temperature. Owing to the conformational inertia of the phosphane⋅⋅⋅borane pocket involving heavy tBu3P and B(C6F5)3 species and features of the PES E(P⋅⋅⋅H, B⋅⋅⋅H | B⋅⋅⋅P) as a function of P⋅⋅⋅H, B⋅⋅⋅H, and B⋅⋅⋅P distances, a typical reactive scenario for this reaction is a trajectory that is trapped in the TS region for a period of time (about 350 fs on average across all calculated trajectories) in a quasi-bound state (scattering resonance). The relationship between the timescale of the TS passage and the effective conformational inertia of the phosphane⋅⋅⋅borane pocket leads to a prediction that isotopically heavier Lewis base/Lewis acid pairs and normal counterparts could give measurably different reaction rates.

Uncovering the role of intra- and intermolecular motion in frustrated Lewis acid/base chemistry: ab initio molecular dynamics study of CO2 binding by phosphorus/boron frustrated Lewis pair [tBu3P/B(C6F5)3].

The role of the intra- and intermolecular motion, i.e., molecular vibrations and the relative motion of reactants, remains largely unexplored in the frustrated Lewis acid/base chemistry.

Ab initio dynamics trajectory study of the heterolytic cleavage of H2 by a Lewis acid [B(C6F5)3] and a Lewis base [P(tBu)3].

Activation of H2 by a "frustrated Lewis pair" (FLP) composed of B(C6F5)3 and P(tBu)3 species has been explored with high level direct ab initio molecular dynamics (AIMD) simulations at finite temperature (T = 300 K) in gas phase. The initial geometrical conditions for the AIMD trajectory calculations, i.e.

Toward controlling water oxidation catalysis: tunable activity of ruthenium complexes with axial imidazole/DMSO ligands.

Using the combinations of imidazole and dimethyl sulfoxide (DMSO) as axial ligands and 2,2'-bipyridine-6,6'-dicarboxylate (bda) as the equatorial ligand, we have synthesized six novel ruthenium complexes with noticeably different activity as water oxidation catalysts (WOCs). In four C(s) symmetric Ru(II)(κ(3)-bda)(DMSO)L(2) complexes L = imidazole (1), N-methylimidazole (2), 5-methylimidazole (3), and 5-bromo-N-methylimidazole (4). Additionally, in two C(2v) symmetric Ru(II)(κ(4)-bda)L(2) complexes L = 5-nitroimidazole (5) and 5-bromo-N-methylimidazole (6), that is, fully equivalent axial imidazoles.

A computational study of the CO dissociation in cyclopentadienyl ruthenium complexes relevant to the racemization of alcohols.

The formation of an active 16-electron ruthenium sec-alkoxide complex via loss of the CO ligand is an important step in the mechanism of the racemization of sec-alcohols by (η(5)-Ph(5)C(5))Ru(CO)(2)X ruthenium complexes with X = Cl and O(t)Bu. Here we show with accurate DFT calculations the potential energy profile of the CO dissociation pathway for a series of relevant (η(5)-Ph(5)C(5))Ru(CO)(2)X complexes, where X = Cl, O(t)Bu, H and COO(t)Bu. We have found that the CO dissociation energy increases in the following order: O(t)Bu (lowest), Cl, COO(t)Bu and H (highest).

A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II.

Across chemical disciplines, an interest in developing artificial water splitting to O(2) and H(2), driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that 'two orders of magnitude' gap with a rationally designed molecular catalyst [Ru(bda)(isoq)(2)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline).

On the possibility of catalytic reduction of carbonyl moieties with tris(pentafluorophenyl)borane and H2: a computational study.

The study thoroughly examines the Gibbs free energy surfaces of a new mechanism for reduction of ketones/aldehydes by tris(pentafluorophenyl)borane (1) and H(2). Key elements of the proposed mechanism are the proton and the hydride transfer steps similar to Stephan's catalytic reduction of imines by 1. The proton is transferred to the ketone/aldehyde in the process of H(2) cleavage by the carbonyl-borane couple and the hydride is transferred in a nucleophilic attack on the carbonyl carbon by the hydridoborate in the ionic pair, [HOCRR'](+)[HB(C(6)F(5))(3)](-).