A DFT study of CO2 electroreduction catalyzed by hexagonal boron-nitride nanosheets with vacancy defects.

In addition to providing a sustainable route to green alternative energy and chemical supplies from a cheap and abundant carbon source, recycling CO2 offers an excellent way to reduce net anthropogenic global CO2 emissions. This can be achieved via catalysis on 2D materials. These materials are atomically thin and have unique electrical and catalytic properties compared to bigger nanoparticles and conventional bulk catalysts, opening a new arena in catalysis. This paper examines the efficacy of hexagonal boron nitride (h-BN) lattices with vacancy defects for CO2 electroreduction (CO2RR). We conducted in-depth investigations on different CO2RR electrocatalytic reaction pathways on various h-BN vacancy sites using a computational hydrogen model (CHE). It was shown that CO binds to h-BN vacancies sufficiently to ensure additional electron transfer processes, leading to higher-order reduction products. For mono-atomic defects VN (removed nitrogen), the electrochemical path of (H+ + e-) pair transfers that would lead to the formation of methanol is most favorable with a limiting potential of 1.21 V. In contrast, the reaction pathways via VB (removed boron) imposes much higher thermodynamic barriers for the formation of all relevant species. With a divacancy VBN, the hydrogen evolution reaction (HER) would be the most probable process due to the low rate-determining barrier of 0.69 eV. On the tetravacancy defects VB3N the pathways toward the formation of both CH4 and CH3OH impose a limiting potential of 0.85 V. At the same time, the HER is suppressed by requiring much higher energy (2.15 eV). Modeling the edges of h-BN reveals that N-terminated zigzag conformation would impose the same limiting potential for the formation of methanol and methane (1.73 V), simultaneously suppressing the HER (3.47 V). At variance, the armchair conformation favors the HER, with a rate-determining barrier of 1.70 eV. Hence, according to our calculations, VB3N and VN are the most appropriate vacancy defects for catalyzing CO2 electroreduction reactions.

Improving a Methane C-H Activation Complex by Metal and Ligand Alterations from Computational Results.

We present results for a series of complexes derived from a titanium complex capable of activating C-H bonds under mild conditions (PNP)Ti═CHtBu(CH2tBu), where PNP = N[2-PiPr2-4-methylphenyl]2-. In addition to the initial activation of methane, a tautomerization reaction to a terminal methylidene is also explored due to methylidene's potential use as a synthetic starting point. Analogous complexes with other low-cost 3d transition metals were studied, such as scandium, titanium, vanadium, and chromium as both isoelectronic and isocharged complexes. Our results predict that V(IV) and V(V) complexes are promising for methane C-H bond activation. The V(V) complex has a low rate-determining barrier for methane activation, specifically 16.6 kcal/mol, which is approximately 12 kcal/mol less than that for the Ti complex, as well as having a moderate tautomerization barrier of 29.8 kcal/mol, while the V(IV) complex has a methane activation barrier of 19.0 kcal/mol and a tautomerization barrier of 31.1 kcal/mol. Scandium and chromium complexes are much poorer for C-H bond activation; scandium has very high barriers, while chromium strongly overstabilizes the alkylidene intermediate, potentially stopping the further reaction. In addition to the original PNP ligand, some of the most promising ligands from a previous work were tested, although (as shown previously) modification of the ligand does not typically have large effects on the activity of the system. Our best ligand modification improves the performance of the V(V) complex via the substitution of the nitrogen in PNP by phosphorus, which reduces the tautomerization barrier by 5 to 24.4 kcal/mol.

Electrocatalytic reduction of CO2 on size-selected nanoclusters of first-row transition metal nanoclusters: a comprehensive mechanistic investigation.

Recycling CO2 back to fuels offers an ideal solution to control anthropogenic global CO2 emissions as well as providing a sustainable green solution to alternative energy resources from a cheap and earth-abundant carbon source. Size-selected nanoclusters open a novel area in catalysis as these atomically precise nanoclusters possess unique electronic and catalytic properties different from larger nanoparticles and traditional bulk catalysts. In this work, we have investigated the ability of first-row transition metal nanoclusters (Sc-Cu) of varying sizes (3 to 10 atoms) for CO2 electroreduction (CO2RR). Employing computational hydrogen model (CHE), we have performed detailed analyzes on various CO2RR electrocatalytic reaction pathways on all nanocluster surfaces. We have identified a general trend of decreasing adsorption energies while moving across the periodic table from Sc to Cu. Moreover, we have found a general preference for CHO* mediated pathways over COH* mediated pathways for methane formation. The CHO* mediated pathways prefer the reaction route via CHO* → CH2O* → CH2OH* → CH2* → CH3* → CH4 + * on most of the nanocluster surfaces. In addition, we have established that methanol formation is greatly disfavored on all nanocluster surfaces, and the release of CO and HCOOH is greatly suppressed on all nanoclusters. We have identified several nanoclusters as potential nanocluster-based electrocatalysts for CO2RR for methane formation with relatively lower limiting potential values below 0.50 V. CO2 electroreduction versus hydrogen evolution reaction (HER) competition was also evaluated on various nanoclusters, and we identified a number of nanoclusters (Ti6, V5, V6, Mn4, Mn7, Mn10, Fe4, Fe8, Fe10, Ni4, and Cu5) that can suppress the formation of HER over CO2RR. We have also established a linear scaling relationship between the adsorption free energies of various CO2RR adsorbates to the adsorption free energies of CO2*, O*, and C* adsorbates. We have found that scaling free energy relationships that exit on heterogeneous catalysts such as the correlation between the adsorption energies of AHx with the adsorption energies of atom A (A = C, N, O, S, etc.) often breaks on nanocluster surfaces, especially for adsorbates with more than one binding motifs.

Comparing Density Functional Theory Metal-Ligand Bond Dissociation Enthalpies with Experimental Solution-Phase Enthalpies of Activation for Bond Dissociation.

The predictive ability of density functional theory is fundamental to its usefulness in chemical applications. Recent work has compared solution-phase enthalpies of activation for metal-ligand bond dissociation to enthalpies of reaction for bond dissociation, and the present work continues those comparisons for 43 density functional methods. The results for ligand dissociation enthalpies of 30 metal-ligand complexes tested in this work reveal significant inadequacies of some functionals as well as challenges from the dispersion corrections to some functionals. The analysis presented here demonstrates the excellent performance of a recent density functional, M11plus, which contains nonlocal rung-3.5 correlation. We also find a good agreement between theory and experiment for some functionals without empirical dispersion corrections such as M06, r2SCAN, M06-L, and revM11, as well as good performance for some functionals with added dispersion corrections such as ωB97X-D (which always has a correction) and BLYP, B3LYP, CAM-B3LYP, and PBE0 when the optional dispersion corrections are added.

The Chemistry of Short-Lived α-Fluorocarbocations.

The present study of the chemistry of short-lived α-fluorocarbocations reveals that even inactive methyl carbons can serve as nucleophiles, attacking a cationic center. This, in turn, facilitates the synthesis of a cyclopropane ring in certain triterpene backbones. We report the synthesis of compounds similar to 2, containing a bridgehead cyclopropane, and compounds of type 3 with an 11 membered bicyclic ring consisting of two bridgehead double bonds (anti-Bredt) within a triterpene skeleton. The synthesis involves three unconventional chemical processes: (a) a methyl group serving as a nucleophile; (b) the unexpected and unprecedented synthesis of a strained system in the absence of an external neighboring trigger; and (c) the formation of an 11-membered bicyclic diene ring within a triterpenoid skeleton. An α-fluorocarbocation mechanism is proposed and supported by density functional theory calculations.

Delayed Linker Addition (DLA) Synthesis for Hybrid SOD ZIFs with Unsubstituted Imidazolate Linkers for Propylene/Propane and n-Butane/i-Butane Separations.

We present a novel synthesis strategy termed delayed linker addition (DLA) to synthesize hybrid zeolitic-imidazolate frameworks containing unsubstituted imidazolate linkers (Im) with SOD topology (hereafter termed Im/ZIF-8). Im linker incorporation can create larger voids and apertures, which are important properties for gas storage and separation. To date, there have been only a handful of reports of Im linkers incorporated into ZIF-8 frameworks, typically requiring arduous and complicated post synthesis approaches. DLA, as reported here, is a simple one-step synthesis strategy allowing high incorporation of Im linker into the ZIF-8 framework while still retaining its SOD topology. We fabricated mixed-matrix membranes (MMMs) with 6FDA-DAM polymer and Im/ZIF-8 obtained via DLA as a filler. The Im/ZIF-8-containing MMMs showed excellent performance for both propylene/propane and n-butane/i-butane separation, displaying permeability and ideal selectivity well above the polymer upper bound. Moreover, highly detailed molecular simulations shed light to the aperture size and flexibility response of Im/ZIF-8 and its improved diffusivity as compared to ZIF-8.

Tunable model promoters in DFT simulations of catalysts.

Promoter atoms can tune a catalyst's activity and selectivity by transferring charge to and from the active site. Rational design of promoted catalysts, using density functional theory calculations, is today limited by the need to simulate many catalyst and promoter configurations. We present a simple approximation that rapidly captures some trends in promoter effects, at a cost of complexity comparable with simulating unpromoted catalysts. Negative (positive) noninteger point charges introduced into the catalyst simulate how electropositive (electronegative) promoters might affect each predicted intermediate. Calculations return Sabatier plots, relating promoters' predicted efficacy to readily measured properties such as catalyst work functions. We illustrate our approach for two reactions associated with the Fischer-Tropsch process, hydrogen-deuterium scrambling, and carbon monoxide dissociation over ruthenium. Consistent with experiment, electropositive promoters are predicted to accelerate hydrogen scrambling and unassisted CO dissociation. Simulations also provide a new prediction: electronegative promoters accelerate hydrogen-assisted CO dissociation over hydrogen-precovered surfaces by stabilizing the initial CO adsorption. © 2019 Wiley Periodicals, Inc.

Influence of chelate ring type on chelate-chelate and chelate-aryl stacking: the case of nickel bis(dithiolene).

Chelate-aryl and chelate-chelate stacking interactions of nickel bis(dithiolene) were studied at the CCSD(T)/CBS and DFT levels. The strongest chelate-aryl stacking interaction between nickel bis(dithiolene) and benzene has a CCSD(T)/CBS stacking energy of -5.60 kcal mol-1. The strongest chelate-chelate stacking interactions between two nickel bis(dithiolenes) has a CCSD(T)/CBS stacking energy of -10.34 kcal mol-1. The most stable chelate-aryl stacking has the benzene center above the nickel atom, while the most stable chelate-chelate dithiolene stacking has the chelate center above the nickel atom. Comparison of chelate-aryl stacking interactions of dithiolene and acac-type nickel chelate shows similar strength. However, chelate-chelate stacking is stronger for dithiolene nickel chelate than for acac-type nickel chelate, which has a CCSD(T)/CBS interaction energy of -9.50 kcal mol-1.

Unraveling the Role of a Flexible Tetradentate Ligand in the Aerobic Oxidative Carbon-Carbon Bond Formation with Palladium Complexes: A Computational Mechanistic Study.

Mechanistic details of the aerobic oxidative coupling of methyl groups by a novel (MeL)PdII(Me)2 complex with the tetradentate ligand, MeL = N, N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane, has been explored by density functional theory calculations. The calculated mechanism sheds light on the role of this ligand's flexibility in several stages of the reaction, especially as the oxidation state of the Pd changes. Ligand flexibility leads to diverse axial coordination modes, and it controls the availability of electrons by modulating the energies of high-lying molecular orbitals, particularly those with major d z2 character. Solvent molecules, particularly water, appear essential in the aerobic oxidation of PdII by lowering the energy of the oxygen molecule's unoccupied molecular orbital and stabilizing the PdX-O2 complex. Ligand flexibility and solvent coordination to oxygen are essential to the required spin-crossover for the transformation of high-valent PdX-O2 complexes. A methyl cation pathway has been predicted by our calculations in transmetalation between PdII and PdIV intermediates to be preferred over methyl radical or methyl anion pathways. Combining an axial and equatorial methyl group is preferred in the reductive elimination pathway where roles are played by the ligand's flexibility and the fluxionality of trimethyl groups.

Probing the Carbon-Hydrogen Activation of Alkanes Following Photolysis of Tp'Rh(CNR)(carbodiimide): A Computational and Time-Resolved Infrared Spectroscopic Study.

Carbon-hydrogen bond activation of alkanes by Tp'Rh(CNR) (Tp' = Tp = trispyrazolylborate or Tp* = tris(3,5-dimethylpyrazolyl)borate) were followed by time-resolved infrared spectroscopy (TRIR) in the υ(CNR) and υ(B-H) spectral regions on Tp*Rh(CNCH2CMe3), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh(CNMe). The major intermediate species were: κ3-η1-alkane complex (1); κ2-η2-alkane complex (2); and κ3-alkyl hydride (3). Calculations predict that the barrier between 1 and 2 arises from a triplet-singlet crossing and intermediate 2 proceeds over the rate-determining C-H activation barrier to give the final product 3. The activation lifetimes measured for the Tp*Rh(CNR) and Tp*Rh(CO) fragments with n-heptane and four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) increase with alkanes size and show a dramatic increase between C6H12 and C7H14. A similar step-like behavior was observed previously with CpRh(CO) and Cp*Rh(CO) fragments and is attributed to the wider difference in C-H bonds that appear at C7H14. However, Tp'Rh(CNR) and Tp'Rh(CO) fragments have much longer absolute lifetimes compared to those of CpRh(CO) and Cp*Rh(CO) fragments, because the reduced electron density in dechelated κ2-η2-alkane Tp' complexes stabilizes the d8 Rh(I) in a square-planar geometry and weakens the metal's ability for oxidative addition of the C-H bond. Further, the Tp'Rh(CNR) fragment has significantly slower rates of C-H activation in comparison to the Tp'Rh(CO) fragment for the larger cycloalkanes, because the steric bulk of the neopentyl isocyanide ligand hinders the rechelation in κ2-Tp'Rh(CNR)(cycloalkane) species and results in the C-H activation without the assistance of the rechelation.

What Happens Without Nickel? Cyclization Reactions of Ethylene with Ethanedithial and Related Molecules.

We present a computational study of the mechanism of the formation of 6-member heterocycles through the binding of ethylene to oxaldehyde, ethanedithial, and 2-thioxoacetaldehyde. This process is related to the olefin separation technology by metal dithiolenes and dioxolenes, being the formation of those heterocycles the main decomposition route. We also present a benchmark of 26 density functionals (spanning hybrid, double-hybrid, range-separated, semilocal, and local functionals) related to CCSD(T)/CBS reference values. Both the cyclization reaction and the isomerization of the cyclic product are included in the benchmark. The best functional among those tested for these reactions is ωB97XD, and the effect of the basis set is also investigated for it. © 2017 Wiley Periodicals, Inc.

Tailoring the gas separation efficiency of metal organic framework ZIF-8 through metal substitution: a computational study.

The influence of a zeolitic imidazolate framework (ZIF)'s metal identity on its gas separation performance is studied extensively through molecular simulations for a variety of gases. ZIF-8 is used as the original framework for alterations of different metal substitutes of the Zn2+ metal. ZIF-8 consists of cages connected by narrow apertures that exhibit flexibility through "swelling", allowing for relatively large penetrants to diffuse. Replacing the central metal atom in the basic tetrahedral unit of ZIF-8 with Cd, Co or Be results in three different structures with increasing bonding stiffness with their neighboring atoms. The metal modification approach offers a way to control the flexibility and the size of the aperture, which constitutes the main energy barrier of the penetrant's hop-like diffusion between the framework's cages. Newly developed force fields are reported and utilized here; the new frameworks are compared to the original one, in terms of the diffusivity of various gas molecules as a function of their size (from He to n-butane). The correlation of the gas diffusivity with the aperture flexibility-molecular size relation is investigated. The results reveal that the aperture flexibility-molecular size relation governs the diffusivity, which shapes a common trend along all modifications. Furthermore, a new generalized method is employed for the screening of the various modifications for specific gas separations. This method is useful to detect optimum separation performance for the various modifications: CdIF-1 (Cd) for n-butane/iso-butane mixture; ZIF-67 (Co) for propylene/n-propane and ethylene/ethane mixtures; BeIF-1 (Be) for CO2/C2H6, CO2/CH4 and CO2/N2 mixtures.

When Hartree-Fock exchange admixture lowers DFT-predicted barrier heights: Natural bond orbital analyses and implications for catalysis.

The conventional wisdom in density functional theory (DFT) is that standard approximations systematically underestimate chemical reaction barrier heights and that exact (Hartree-Fock-like, HF) exchange admixture improves this. This conventional wisdom is inconsistent with the good performance of functionals without HF exchange for many reactions on metal catalyst surfaces. We have studied several "anomalous" gas-phase reactions where this conventional wisdom is upended, and a HF exchange admixture decreases or does not affect the predicted barrier heights [Mahler et al., J. Chem. Phys. 146, 234103 (2017)]. Here we show how natural bond orbital analyses can help identify and explain some factors that produce anomalous barriers. Applications to pnictogen inversion, standard benchmark reaction barrier datasets, and a model Grubbs catalyst illustrate the utility of this approach. This approach is expected to aid DFT users in choosing appropriate functionals, and aid DFT developers in devising DFT approximations generally applicable to catalysis.

Unexpected Importance of Aromatic-Aliphatic and Aliphatic Side Chain-Backbone Interactions in the Stability of Amyloids.

The role of aromatic and nonaromatic amino acids in amyloid formation has been elucidated by calculating interaction energies between ß-sheets in amyloid model systems using density functional theory (B3LYP-D3/6-31G*). The model systems were based on experimental crystal structures of two types of amyloids: (1) with aromatic amino acids, and (2) without aromatic amino acids. Data show that these two types of amyloids have similar interaction energies, supporting experimental findings that aromatic amino acids are not essential for amyloid formation. However, different factors contribute to the stability of these two types of amyloids. In the former, the presence of aromatic amino acids significantly contributes to the strength of interactions between side chains; interactions between aromatic and aliphatic side chains are the strongest, followed by aromatic-aromatic interactions, while aliphatic-aliphatic interactions are the weakest. In the latter, that is, the amyloids without aromatic residues, stability is provided by interactions of aliphatic side chains with the backbone and, in some cases, by hydrogen bonds.

The Distinctive Electronic Structures of Rhenium Tris(thiolate) Complexes, an Unexpected Contrast to the Valence Isoelectronic Ruthenium Tris(thiolate) Complexes.

The noninnocent 2-diphenylphosphino-benzenethiolate (DPPBT) ligand containing both phosphorus and sulfur donors delocalizes the electron density in a manner reminiscent of dithiolenes. The electronic structure of the [ReL3]n (L = DPPBT, n = 0, 1+, 2+) complexes was probed with density-functional theory (DFT) and high-level ab initio methods including complete active space self-consistent field (CASSCF and CASPT2) and coupled cluster (CCSD and CCSD(T)). DFT predicts a slight preference for a closed-shell (CS) singlet ground state for the neutral [ReL3]0 and stronger preferences for low-spin ground states for the oxidized [ReL3]+ and [ReL3]2+. High-level ab initio methods confirm a CS singlet with a Re(III) (d4, S = 0) center as the ground state of [ReL3]0. Thus, this neutral Re species has considerably less thiyl radical character than the valence isoelectronic [RuL3]+, which is mainly a Ru(III) (d5, S = 1/2) anti-ferromagnetically (AF) coupled to a thiyl radical (S = 1/2). However, the oxidized derivatives [ReL3]+ and [ReL3]2+ show significant metal-stabilized thiyl radical character like [RuL3]+. Both [ReL3]+ and [ReL3]2+ have major contributions from Re(III) (d4, S = 1) centers AF coupled to thiyl and dithiyl DPPBT ligands. These findings are consistent with the experimental chemistry as [RuL3]+, [ReL3]+, and [ReL3]2+ can add ethylene to form the new C-S bonds, but [ReL3]0 cannot. The thiyl radicals on the S2 position (the S trans to a P donor) serve as the intrinsic electron acceptors in the actual ethylene addition reactions with Ru and Re tris(thiolate) complexes.

Methane Activations by Titanium Neopentylidene Complexes: Electronic Resilience and Steric Control.

The titanium neopentylidene complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2-) is capable of activating both sp2 and sp3 C-H bonds under mild conditions. In addition to methane C-H activation, competition between the initial hydrogen abstraction reaction to form the methane activation product and the tautomerization reaction of this product to form a terminal methylidene was also explored. Several modifications of the PNP and CHtBu ligands were explored to determine the effect of these changes on C-H bond activation. In general, on the one hand, the modifications involving electronic effects have small and inconsistent influence on the stability of the intermediates and products and on the reaction barriers. On the other hand, the use of bulky groups in the ligands favors the methane activation process. By replacing the iPr groups in the PNP ligand with tBu groups, both methane activation and tautomerization reactions become more energetically favorable than in the unmodified complex. On the one hand, the largest acceleration of the methane C-H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine. On the other hand, replacing the nitrogen in the PNP ligand by phosphorus results in lower barriers for the tautomerization reaction and the stabilization of the product of the tautomerization although it remains slightly less stable than product of methane C-H activation. While several ligand modifications related to the electronic effects were examined, it is interesting that most of them did not make a significant change on the barriers for either reaction, indicating a significant resilience of this titanium complex, which could be used to enhance the practical aspects of the complex without a significant loss of its activity.

Ancillary Ligand Effects upon the Photochemistry of Mn(bpy)(CO)3X Complexes (X = Br-, PhCC-).

The photochemistry of two Mn(bpy)(CO)3X complexes (X = PhCC-, Br-) has been studied in the coordinating solvents THF (terahydrofuran) and MeCN (acetonitrile) employing time-resolved infrared spectroscopy. The two complexes are found to exhibit strikingly different photoreactivities and solvent dependencies. In MeCN, photolysis of 1-(CO)(Br) [1 = Mn(bpy)(CO)2] affords the ionic complex [1-(MeCN)2]Br as a final product. In contrast, photolysis of 1-(CO)(CCPh) in MeCN results in facial to meridional isomerization of the parent complex. When THF is used as solvent, photolysis results in facial to meridional isomerization in both complexes, though the isomerization rate is larger for X = Br-. Pronounced differences are also observed in the photosubstitution chemistry of the two complexes where both the rate of MeCN exchange from 1-(MeCN)(X) by THFA (tetrahydrofurfurylamine) and the nature of the intermediates generated in the reaction are dependent upon X. DFT calculations are used to support analysis of some of the experiments.

Why are GGAs so accurate for reaction kinetics on surfaces? Systematic comparison of hybrid vs. nonhybrid DFT for representative reactions.

"Jacob's Ladder" of approximate exchange-correlation (XC) functionals in Kohn-Sham density functional theory are widely accepted to have systematic errors in reaction barriers. The first-rung local spin-density approximation (LDA) typically predicts barriers below generalized gradient approximations, which in turn predict barriers below experiment and below fourth-rung hybrid functionals incorporating a fraction of exact exchange. We show that several reactions from previous literature reports, as well as new simulations of carbon-carbon coupling in the Fischer-Tropsch process, do not follow this conventional picture. We introduce the AB9 test set of nine abnormal reaction barriers, in which density gradient corrections and exact exchange admixture tend to lower rather than to raise predicted barriers. Comparisons of normal and abnormal reactions rationalize this phenomenon in terms of how density gradient and exact-exchange corrections stabilize transition states relative to reaction intermediates. Multireference diagnostics confirm that this behavior is not merely a consequence of multireference character. Benchmarks of the AB9 set, using the best available ab initio reference values, highlight the role of symmetry breaking and show surprisingly good performance from both the LDA and "Rung 3.5" functionals. This motivates benchmarks of the AB9 set in future XC functional development.

Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex.

The mechanism and energetics of CO, 1-hexene, and 1-hexyne substitution from the complexes (SBenz)2 [Fe2 (CO)6 ] (SBenz=SCH2 Ph) (1-CO), (SBenz)2 [Fe2 (CO)5 (η(2) -1-hexene)] (1-(η(2) -1-hexene)), and (SBenz)2 [Fe2 (CO)5 (η(2) -1-hexyne)] (1-(η(2) -1-hexyne)) were studied by using time-resolved infrared spectroscopy. Exchange of both CO and 1-hexyne by P(OEt)3 and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate-determining step in both cases is assumed to be the rotation of the Fe(CO)2 L (L=CO or 1-hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1-hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (SN 1) and associative (SN 2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (µ-pdt)[Fe(CO)3 ]2 (pdt=S(CH2 )3 S) underscores the unique characteristics of a three-atom S-S linker in the active site of diiron hydrogenases.

Computational Study of the Palladium-Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination.

We describe herein computational studies on the unusual ability of Pd(PtBu3 )2 to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu3 that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR3 )Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu3 favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium-catalyzed carbonylation reactions with simple phosphine ligands, including the key rate-determining steps and palladium(0) catalyst resting state in carbonylations.