2AB Reaction Research Articles

Sequential Insertion of Alkynes and CO or Isocyanides into the Pd–C Bond of Cyclopalladated Phenylacetamides. Synthesis of Eight-Membered Palladacycles, Benzo[d]azocine-2,4(1H,3H)-diones, and Highly Functionalized Acrylonitrile and Acrylamide Derivatives

The cyclopalladated complexes [Pd{κ2C,O-C6H4CH2C(O)NRR′-2}(tmeda)]TfO [R = R′ = H (1a), R = Me, R′ = H (1b), R = R′ = Me (1c)] react with alkynes XC≡CX′ in 1:3 molar ratio to give the eight-membered palladacycles [Pd{κ2C,O-C(X)═C(X′)C6H4CH2C(O)NRR′-2}(tmeda)]TfO [R = R′ = H and X = X′ = Ph (2aa), CO2Me (2ab), Et (2ac) or X = Ph, X′ = Me (2ad); R = Me, R′ = H and X = X′ = Ph (2ba), CO2Me (2bb), Et (2bc) or X = Ph, X′ = Me (2bd); R = R′ = Me and X = X′ = Ph (2ca)]. The treatment of complexes 2aa, 2ac, 2ad, 2ba, 2bc, and 2bd with CO at 50 °C affords the corresponding benzo[d]azocine-2,4(1H,3H)-diones (3), which result from the insertion of a molecule of CO into the Pd–C bond and subsequent C–N reductive coupling and formation of (tmedaH)TfO. The reaction of 2ab with CO in MeOH gives (MeO2C)2C═C(CO2Me)C6H4CH2C(O)NH2-2 (4ab). Complexes 2aa and 2ab react with 1 equiv of R″NC 1:1 to give [Pd{C(X)═C(X′)C6H4CH2C(O)NH2-2}(CNR″)(tmeda)]TfO [R″ = t-Bu, X = X′ = Ph (5a), CO2Me (5b); R = Xy, X = X′ = Ph (5a′), CO2Me (5...

Alkylthio Bridged 44 cve Triangular Platinum Clusters: Synthesis, Oxidation, Degradation, Ligand Substitution, and Quantum Chemical Calculations

Acetylplatinum(II) complexes trans-[Pt(COMe)Cl(L)2] (L = PPh3, 2a; P(4-FC6H4)3, 2b) were found to react with dialkyldisulfides R2S2 (R = Me, Et, Pr, Bu; Pr = n-propyl, Bu = n-butyl), yielding trinuclear 44 cve (cluster valence electrons) platinum clusters [(PtL)3(mu-SR)3]Cl (4). The analogous reaction of 2a-b with Ph2S2 gave SPh bridged dinuclear complexes trans-[{PtCl(L)}2(mu-SPh)2] (5), whereas the addition of Bn2S2 (Bn = benzyl) to 2a ended up in the formation of [{Pt(PPh3)}3(mu3-S)(mu-SBn)3]Cl (6). Theoretical studies based on the AIM theory revealed that type 4 complexes must be regarded as triangular platinum clusters with Pt-Pt bonds whereas complex 6 must be treated as a sulfur capped 48 ve (valence electrons) trinuclear platinum(II) complex without Pt-Pt bonding interactions. Phosphine ligands with a lower donor capability in clusters 4 proved to be subject to substitution by stronger donating monodentate phosphine ligands (L' = PMePh2, PMe2Ph, PBu3) yielding clusters [(PtL')3(mu-SR)3]Cl (9). In case of the reaction of clusters 4 and 9 with PPh2CH2PPh2 (dppm), a fragmentation reaction occurred, and the complexes [(PtL)2(mu-SMe)(mu-dppm)]Cl (12) and [Pt(mu-SMe)2(dppm)] (13) were isolated. Furthermore, oxidation reactions of cluster [{Pt(PPh3)}3(mu-SMe)3]Cl (4a) using halogens (Br2, I2) gave dimeric platinum(II) complexes cis-[{PtX(PPh3)}2(mu-SMe)2] (14, X = Br, I) whereas oxidation reactions using sulfur and selenium afforded chalcogen capped trinuclear 48 ve complexes [{Pt(PPh3)}3(mu3-E)(mu-SMe)3] (15, E = S, Se). All compounds were fully characterized by means of NMR and IR spectroscopy, microanalyses, and ESI mass spectrometry. Furthermore, X-ray diffraction analyses were performed for the triangular cluster 4a, the trinuclear complex 6, as well as for the dinuclear complexes trans-[{Pt(AsPh3)}2(mu-SPh)2] (5c), [{Pt(PPh3)}2(mu-SMe)(mu-dppm)]Cl (12a), and [{{PtBr(PPh3)}2(mu-SMe)2] (14a).

Kinetic phase transition in A2 + B2 -->2AB reaction system.

Three lattice gas models for A2 + B2 -->2AB reaction system are studied by Monte Carlo simulation in two-dimensional triangular lattice surface. When both A2 and B2 adsorb and dissociate on the catalytic surface in the random dimer-filling mechanism or in the end-on dimer-filling mechanism, there is no reactive window in the reaction system and just a discontinuous phase transition appears from a "B+vacancy" poisoned state to an "A+vacancy" poisoned state. However, a reactive window appears when one of the two dimers, A2, dissociates in the random dimer-filling mechanism but another dimer, B2, is in the end-on dimer-filling mechanism and the system exhibits a discontinuous phase transition from the active reaction state to a B+ vacancy poisoned state and a continuous phase transition to an "A+vacancy" poisoned state. Furthermore, we show that the critical behavior of the continuous phase transition with infinitely many absorbing states belongs to the robust directed percolation universality class.

Pattern formation in the $ \mathsf {A+B_{2}}$ reaction with anisotropic lateral adsorbate-adsorbate interactions

We present Monte Carlo simulations of the formation of (1×2) islands in the case of the 2 A + B 2 → 2 AB reaction occurring via the Langmuir-Hinshelwood mechanism on a square lattice under steady-state conditions. The model employed takes into account the effect of anisotropic lateral B-B interactions on the rates of B diffusion and elementary reaction events. The results obtained with qualitatively realistic ratio of the rate of elementary reaction steps indicate that the average island size depends on the details of diffusion and reaction dynamics in a similar way as in the earlier studied case of the simplest A + B reaction running via the Eley-Rideal mechanism.

Formation of Grignard Reagents from Aryl Halides: Effective Radical Probes Hint at a Nonparticipation of Dianions in the Mechanism

We have prepared highly efficient radical probes 2a-b involving the hex-5-enyl rearrangement. The reaction of 2a-b with active magnesium leads to the cyclized products 4a-b, providing a direct evidence of radical intermediates during the formation of aryl Grignard reagents. The variations of yields for cyclized products 4a-b as a function of structural modifications in 2a-b suggest that the intervention of dianions is not necessary to explain the observed results.

Monte Carlo simulations of the kinetics of catalytic reactions on nanometer-sized particles, with diffusion over facet boundaries

Using the Monte Carlo technique, we comprehensively investigate the steady-state kinetics of the 2A+B2→2AB reaction on a nanometer-sized catalyst particle. The attention is focused on the adsorbate-diffusion-mediated interplay of reaction kinetics occurring on different facets. This effect is explored for different ratios of the rate constants for adsorption, desorption, reaction, and diffusion. The results obtained demonstrate that the reaction kinetics can be uniquely different on nanometer particles compared to those on macroscopic single-crystal surfaces.

SIMULATION OF KINETIC OSCILLATIONS IN CATALYTIC REACTIONS ACCOMPANIED BY OXIDE FORMATION

One of the generic mechanisms of oscillations in heterogeneous catalytic reactions with participation of oxygen is based on slow oxidation and reduction of the metal surface. We present Monte Carlo simulations of spatiotemporal patterns corresponding to this mechanism. As an example, we analyze the 2A+B2 → 2AB reaction mimicking CO (or H 2) oxidation on Pt. Calculations taking into account the nonideality of the kinetics of oxidation and reduction of the substrate are executed with a qualitatively realistic ratio of the rates of elementary steps. The results obtained indicate that the reaction scheme under consideration possesses the ability of spatial self-organization even if cooperative effects in nucleation, growth and decay of oxide islands are neglected. In this case, however, the islands are small and rather disordered. With the cooperative effects, the size of islands becomes larger but not dramatically. A very strong tendency to form oxide islands is found to suppress oscillations.

Simulation of kinetic oscillations in catalytic reactions accompanied by adsorbate-induced surface restructuring

We present Monte Carlo simulations of phase separation on the nanometer scale during kinetic oscillations in catalytic reactions accompanied by adsorbate-induced surface restructuring. As an example, we analyze the 2A+B 2→2AB reaction mimicking CO oxidation on noble metals. The lattice–gas model employed to describe surface restructuring and the reaction steps takes into account substrate–substrate, substrate–adsorbate and adsorbate–adsorbate lateral interactions and predicts the formation of restructured islands even at low A coverages [as in the case of CO adsorption on Pt(100)].

Non-thermal surface roughening in catalytic reactions

We identify a new non-thermal, purely dynamic mechanism of surface roughening, possible in exothermic catalytic reactions when the adsorption or reaction energy is transferred to substrate atoms. This “exothermic” mechanism can induce thermodynamically allowed roughening at T> T R ( T R is the roughening temperature), when the latter is kinetically hindered to occur through thermal excitation. Competing with thermal smoothing, it can also induce roughening at T< T R. Using Monte Carlo simulations of the 2A+B 2→2AB reaction on the substrate described in the framework of the restricted-solid-on-solid model, we show that in both cases the mean-square surface-height difference increases with time as w 2∝ ln t .

Kinetics of the 2A+B 2→2AB reaction complicated by spatial constraints for adsorption, formation of islands, and steps

We present Monte Carlo simulations of the steady-state kinetics of the 2A+B 2→2AB reaction, occurring via the Langmuir–Hinshelwood (LH) mechanism on perfect and stepped surfaces. The calculations are carried out with a realistic ratio between the rates of the elementary steps (the LH step is rapid compared with A and B 2 adsorption and slow compared with A diffusion). The main novel factor introduced into the model is that B 2 adsorption is considered to occur on next-nearest-neighbour (nnn) sites (location of B particles in nearest-neighbour sites is prohibited). This constraint, mimicking O 2 adsorption on Pt(111), changes qualitatively the reaction regimes compared with those predicted by the conventional reaction scheme. In particular, the model does not predict complete poisoning of the surface by B even if the relative B pressure is high and B diffusion is negligible. On the other hand, the model easily predicts formation of B islands if the nnn B–B lateral interaction is attractive. The size of islands is found to be very sensitive with respect to the details of the LH step. Simulating the reaction on a stepped surface, we consider that the step sites are much more reactive compared with terrace sites and that the B 2 sticking coefficient for steps is much higher than for terraces. In this case, the model predicts gradients in the distribution of B particles on the surface. Such non-uniform distributions are, however, generated only in a limited range of the model-parameter values. Dramatic changes in the distribution of B particles due to formation of B islands or B diffusion towards steps are demonstrated to result in nearly negligible changes in the apparent reaction kinetics.

Surface roughening: Kinetics, adsorbate-induced effects, and manifestation in catalytic reactions

Using the Monte Carlo (MC) technique, we have analyzed the kinetics of roughening of a clean surface at T&amp;gt;TR (TR is the roughening temperature), the kinetics of relaxation from a roughened to a flat surface at T&amp;lt;TR, the effect of adsorption on surface roughening, and the influence of adsorbate-induced roughening on the kinetics of the catalytic 2A+B2→2AB reaction. (i) For a clean surface, the results were obtained by employing the standard restricted-solid-on-solid (RSOS) model with surface diffusion occurring via jumps to nearest-neighbor and next-nearest-neighbor sites. The kinetics of surface roughening at T&amp;gt;TR and of surface relaxation at T&amp;lt;TR are found to be logarithmic in time. In particular, the mean-square height difference at T&amp;gt;TR increases with time as w2∝ln t. (ii) Complementing the RSOS model by terms describing adsorption, we demonstrate that adsorbate-induced surface roughening may occur with significant lowering of TR, due, respectively, to indirect next-nearest-neighbor adsorbate–substrate repulsion, attractive nearest- neighbor adsorbate–substrate lateral interaction, and/or repulsive adsorbate-adsorbate lateral interaction. In all these cases, the ratio of TRnear adsorbate saturation and for the clean surface, respectively, is given by an analytical expression that is a simple combination of the corresponding interactions. The coverage dependence of TR in MC simulations is nonlinear and slightly different compared to the mean-field-type prediction. (iii) The effect of adsorbate-induced roughening on a catalytic reaction can be dramatic even at weak roughening, with qualitative and quantitative changes in the kinetics. For example, the absolute conversion rate at maximum rate, and the reactant mixture corresponding to this maximum can be very different on the flat and roughened surface. The behavior of the adsorbate coverages near the points corresponding to the kinetic phase transitions can change character qualitatively.

Surface restructuring, thermal desorption, kinetic bistability, and chemical waves

Adsorbate-induced surface restructuring is treated in the framework of a statistical lattice-gas model taking into account the possibility of formation of a metastable substrate structure on the clean surface and stabilization of this structure by adsorbate–substrate interaction. With these assumptions, surface restructuring is described in terms of the theory of first-order phase transitions. The proposed model is then employed to analyze (i) the influence of adsorbate-induced changes in the surface on thermal desorption spectra and (ii) the effect of surface restructuring on the propagation of chemical waves in the 2A + B2 → 2AB reaction. The interplay between reaction-diffusion kinetics and surface restructuring is shown to result in formation of chemical waves with atomistically sharp spatial features.

Oscillatory surface reaction kinetics due to coupling of bistability and diffusion limitations

Considering as an example the 2A+B2→2AB reaction occurring on the surface of a catalyst, we demonstrate that heterogeneous reactions may exhibit oscillatory behavior at isothermal conditions if the bistability of the nonoscillatory surface kinetics is combined with diffusion limitations in the gas phase. Exploring this general phenomenon for a spherical catalyst, we derive necessary conditions for its observation, estimate analytically the period of oscillations, and study numerically in detail both transient and sustained oscillatory regimes. In addition, we show that one can obtain damped oscillations in the regions where the necessary conditions for sustained oscillatory kinetics are not fulfilled (in this case the period of the oscillations monotonically increases with increasing time, and then the oscillations disappear). The requirements on experimental systems to observe this phenomenon are also discussed.

Kinetic phase transitions and bistability in a model surface reaction II. Spatially inhomogeneous theories

We introduce a general framework for describing surface reactions which can be used to understand spatially inhomogeneous states. This theory is applied to reactions with and without surface diffusion. Our approach gives rise to an infinite hierarchy of equations which in principle exactly describes the surface reaction. By truncating this hierarchy, tractable approximate theories may be developed. As an example, this approach is applied to a single model of the surface-catalyzed 2A + B 2 → 2AB reaction. We show how the metastability and nucleation phenomena which occur in the vicinity of the A-poisoning transition for the diffusionless reaction may be modeled. The effect of allowing one surface species to diffuse is also examined.

Kinetic phase transitions and bistability in a model surface reaction I. Monte Carlo simulations

We have performed Monte Carlo simulations for a model of the surface-catalyzed 2A + B 2 → 2AB reaction in which the surface reaction rate and the A surface diffusion rate are finite. The kinetic phase diagram for this model contains two poisoned phases and a reactive phase, separated by a continuous and a discontinuous kinetic phase transition. Above the discontinuous transition there is a region of near bistability in which the poisoned phase is stable and the reactive phase is strongly metastable. This regime exists in a very narrow band of impingement rates when there is no diffusion but becomes much broader when surface diffusion is fast. We present MC measurements of the range of the bistability when the diffusion and reaction rates are similar. The A diffusion rate is found to affect the bistable regime dramatically. The physical origin of this effect and its relation to experiments is analyzed. We further quantify the effects of diffusion on this reaction by measuring the surface lifetime and mobility of adsorbed A particles.

Reactant Correlation Effects Study of Hydrogen Isotope Exchange Reaction on Supported Metal Catalysts

ABSTRACTNon-classical results due to reactant correlation effects have been observed in the study of Monte Carlo simulations of the A2 + B2 → 2AB reaction. These simulation results, interpreted by the Pair Density Analysis method and the experimental kinetic studies, may provide an explanation of the early time anomalous kinetic behavior of the catalytic hydrogen isotope exchange reaction exhibited for surface-diffusion limited conditions on alumina supported platinum islands.