A computational study of oxidation of ruthenium porphyrins via ORuIV and ORuVIO species

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

Single-Atomic Ruthenium Catalytic Sites on Nitrogen-Doped Graphene for Oxygen Reduction Reaction in Acidic Medium

The cathodic oxygen reduction reaction (ORR) is essential in the electrochemical energy conversion of fuel cells. Here, through the NH₃ atmosphere annealing of a graphene oxide (GO) precursor containing trace amounts of Ru, we have synthesized atomically dispersed Ru on nitrogen-doped graphene that performs as an electrocatalyst for the ORR in acidic medium. The Ru/nitrogen-doped GO catalyst exhibits excellent four-electron ORR activity, offering onset and half-wave potentials of 0.89 and 0.75 V, respectively, vs a reversible hydrogen electrode (RHE) in 0.1 M HClO₄, together with better durability and tolerance toward methanol and carbon monoxide poisoning than seen in commercial Pt/C catalysts. X-ray adsorption fine structure analysis and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy are performed and indicate that the chemical structure of Ru is predominantly composed of isolated Ru atoms coordinated with nitrogen atoms on the graphene substrate. Furthermore, a density function theory study of the ORR mechanism suggests that a Ru-oxo-N₄ structure appears to be responsible for the ORR catalytic activity in the acidic medium. These findings provide a route for the design of efficient ORR single-atom catalysts.

Computationally designed tandem direct selective oxidation using molecular oxygen as oxidant without coreductant

A newly designed two-step selective oxidation process was computationally tested for propene epoxidation using molecular oxygen as oxidant without co-reductant.

Mechanistic study of methanol oxidation by RuIV–oxo complexes

The catalytic conversion of methanol to formaldehyde by three kinds of non-porphyrin Ru complexes, Ru IV O ( TPA ) (TPA = tris(2-pyridylmethyl)amine) (1a), Ru IV O (6- COO - TPA ) (6-COO-TPA = 2-(6-carboxyl-pyridyl)methyl-bis(2-pyridylmethyl)amine) (1b), and Ru IV O ( N4Py ) (N4Py = N,N-bis(2-pyridyl-methyl)-N-bis(2-pyridyl)methylamine) (1c), is discussed by using density functional theory (DFT) calculations. There are two possible reaction pathways for the oxidation of methanol to formaldehyde with respect to the first hydrogen abstraction from the methyl group (path 1) and the hydroxyl group (path 2). Path 1 and path 2 involve the hydroxymethyl radical (• CH 2 OH ) and the methoxyl radical ( CH 3 O •), respectively, as an intermediate. DFT calculations demonstrate that the two pathways are energetically comparable in the reactions by the three Ru IV –oxo complexes. The reactions with 1a and 1c are initiated by the C – H bond dissociation with activation barriers of 22.2 and 21.4 kcal/mol, respectively, while the reaction with 1b is initiated by the O – H bond dissociation with an activation barrier of 18.1 kcal/mol. However, the calculations showed that the rate-determining step is the H -atom abstraction from the CH 3 group of methanol in all the pathways. These results are in good agreement with kinetic analysis of the reactions by the Ru IV –oxo complexes, being useful for considering the mechanism of methanol oxidation.

Substitution behaviour of amine-bridged dinuclear Pt(II) complexes with bio-relevant nucleophiles

Complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(HO)2]4+ and [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(Cl)2]2+ were used to study their reactions with a series of bio-relevant nucleophiles, viz. thiourea, L-methionine and guanosine-5'-monophosphate (5'-GMP2-) as a function of nucleophile concentration and temperature. The reactions with the sulfur containing nucleophiles (thiourea and L-methionine) were followed under pseudo-first-order conditions by stopped-flow and UV-Vis spectrophotometry. The reaction with 5'-GMP2- was carried out under second order conditions and studied by NMR spectroscopy. The results indicate that the bridged dinuclear complexes remain intact after coordination of the studied nucleophiles for an extended period of time, which differs significantly from that reported for other multinuclear platinum complexes in the literature.

Influence of the bridging ligand on the substitution behaviour of dinuclear Pt(ii) complexes. An experimental and theoretical approach

A series of dinuclear Pt(II) complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(H2O)2]4+ were synthesized. Acid-base titrations, and concentration and temperature dependent stopped-flow measurements of the reaction with chloride were performed to study the thermodynamic and kinetic behaviour of the dinuclear bridged complexes. The results indicate that there is a clear interaction between the two Pt(II) centres, which becomes weaker as the aliphatic chain increases in length. From a certain chain length onwards, the Pt(II) centres become independent of each other and exhibit identical thermodynamic and kinetic properties. The experimental results are discussed in reference to structures obtained by DFT (BP86/LACVP*) calculations.