Reinforcement learning based hybrid bond-order coarse-grained interatomic potentials for exploring mesoscale aggregation in liquid-liquid mixtures.

Exploring mesoscopic physical phenomena has always been a challenge for brute-force all-atom molecular dynamics simulations. Although recent advances in computing hardware have improved the accessible length scales, reaching mesoscopic timescales is still a significant bottleneck. Coarse-graining of all-atom models allows robust investigation of mesoscale physics with a reduced spatial and temporal resolution but preserves desired structural features of molecules, unlike continuum-based methods. Here, we present a hybrid bond-order coarse-grained forcefield (HyCG) for modeling mesoscale aggregation phenomena in liquid-liquid mixtures. The intuitive hybrid functional form of the potential offers interpretability to our model, unlike many machine learning based interatomic potentials. We parameterize the potential with the continuous action Monte Carlo Tree Search (cMCTS) algorithm, a reinforcement learning (RL) based global optimizing scheme, using training data from all-atom simulations. The resulting RL-HyCG correctly describes mesoscale critical fluctuations in binary liquid-liquid extraction systems. cMCTS, the RL algorithm, accurately captures the mean behavior of various geometrical properties of the molecule of interest, which were excluded from the training set. The developed potential model along with the RL-based training workflow could be applied to explore a variety of other mesoscale physical phenomena that are typically inaccessible to all-atom molecular dynamics simulations.

Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center.

A mononuclear nonheme cobalt(ii) complex, [(TMG3tren)CoII(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMG3tren)CoIV(O)]2+-(Sc(OTf)3)n (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)3. Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(ii) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.

Ligand-Constraint-Induced Peroxide Activation for Electrophilic Reactivity.

µ-1,2-peroxo-bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron-rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of µ-1,2-peroxo-bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end-on µ-1,2-peroxodicobalt(III) complex 1 involving a non-heme ligand system, L1, supported on a Sn6 O6 stannoxane core, we now show that a peroxo-bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn6 O6 core, represents a new domain for metal-peroxide reactivity.

Transition metal-mediated O-O bond formation and activation in chemistry and biology.

Oxygen evolution and reduction reactions are fundamental processes in biological energy conversion schemes, which represent an attractive method for artificial energy conversion for a world still largely depending on fossil fuels. A range of metalloenzymes achieve these challenging tasks in biology by activating water and dioxygen using cheap and abundant transition metals, such as iron, copper, and manganese. High-valent metal-oxo/oxyl, metal-superoxo, and/or metal-(hydro)peroxo species are common reactive intermediates that are found in the O-O bond formation and activation reactions. The transient nature of the metal-oxygen intermediates has, however, prevented their isolation and characterization in most cases. As a consequence, unambiguous mechanistic assignments in the O-O bond formation and cleavage processes in biological and chemical entries remain elusive, especially for the intermediates and mechanisms involved in the O-O bond formation reactions. This viewpoint article aims at summarizing the information obtained to date in enzymatic and biomimetic systems that fuels the debate regarding the nature of the active oxidants and the mechanistic uncertainties associated with the transition metal-mediated O-O bond formation and cleavage reactions.

Investigating the validity of Schrage relationships for water using molecular dynamics simulations.

Recently, molecular dynamics (MD) simulations were utilized to show that Schrage theory predicts evaporation/condensation mass fluxes with good accuracy in the case of monoatomic and non-polar molecular fluids. Here, we examine if they are equally accurate for molecular polar fluids, such as water. In particular, using molecular dynamics (MD) simulations, we study the steady state evaporation/condensation processes of water in a one-dimensional heat-pipe geometry to ascertain the validity of Schrage relationships. Non-equilibrium mass flow is driven by controlling the temperatures of the source/sink. Equilibrium simulations are utilized to evaluate the saturation properties and the mass accommodation coefficients as a function of temperature. Our results indicate that Schrage equations predict the evaporation/condensation rates of water with good accuracy. Moreover, we show that molecular velocity distributions in the vapor phase are indeed Maxwellian distributions shifted by the velocity of the macroscopic vapor flow, as assumed in Schrage's theoretical analysis.

Catalytic dioxygen reduction mediated by a tetranuclear cobalt complex supported on a stannoxane core.

The synthesis, spectroscopic characterization (infrared, electron paramagnetic resonance and X-ray absorption spectroscopies) and density functional theoretical calculations of a tetranuclear cobalt complex Co4L1 involving a nonheme ligand system, L1, supported on a stannoxane core are reported. Co4L1, similar to the previously reported hexanuclear cobalt complex Co6L2, shows a unique ability to catalyze dioxygen (O2) reduction, where product selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 30 °C. Detailed mechanistic insights were obtained on the basis of kinetic studies on the overall catalytic reaction as well as by low-temperature spectroscopic (UV-Vis, resonance Raman and X-ray absorption spectroscopies) trapping of the end-on µ-1,2-peroxodicobalt(iii) intermediate 1. The Co4L1- and Co6L2-mediated O2-reduction reactions exhibit different reaction kinetics, and yield different ratios of the 2e-/2H+ and 4e-/4H+ products at -50 °C, which can be attributed to the different stabilities of the µ-1,2-peroxodicobalt(iii) intermediates formed upon dioxygen activation in the two cases. The deep mechanistic insights into the transition-metal mediated dioxygen reduction process that are obtained from the present study should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.

Nucleophilic versus Electrophilic Reactivity of Bioinspired Superoxido Nickel(II) Complexes.

The formation and detailed spectroscopic characterization of the first biuret-containing monoanionic superoxido-NiII intermediate [LNiO2 ]- as the Li salt [2; L=MeN[C(=O)NAr)2 ; Ar=2,6-iPr2 C6 H3 )] is reported. It results from oxidation of the corresponding [Li(thf)3 ]2 [LNiII Br2 ] complex M with excess H2 O2 in the presence of Et3 N. The [LNiO2 ]- core of 2 shows an unprecedented nucleophilic reactivity in the oxidative deformylation of aldehydes, in stark contrast to the electrophilic character of the previously reported neutral Nacnac-containing superoxido-NiII complex 1, [L'NiO2 ] (L'=CH(CMeNAr)2 ). According to density-functional theory (DFT) calculations, the remarkably different behaviour of 1 versus 2 can be attributed to their different charges and a two-state reactivity, in which a doublet ground state and a nearby spin-polarized doublet excited-state both contribute in 1 but not in 2. The unexpected nucleophilicity of the superoxido-NiII core of 2 suggests that such a reactivity may also play a role in catalytic cycles of Ni-containing oxygenases and oxidases.

Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex.

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 25 °C. A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to 1 an entropically unfavorable process. This makes the end-on µ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H2O2. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O2-to-2H2O catalytic conversion in an overall 4e-/4H+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.

A luminescent europium(III)-platinum(II) heterometallic complex as a theranostic agent: a proof-of-concept study.

A luminescent heterometallic multifunctional theranostic Eu-Pt2 complex [{cis-PtCl2(DMSO)}2Eu(L)(H2O)] has been synthesized, possessing two therapeutic Pt-centers as covalent DNA binders and one emissive Eu(3+)-center which is sensitized by platinum-based metal-to-ligand charge-transfer excited states.