Oxidative addition of the ethane CC bond to Pd. Anab initiobenchmark and DFT validation study

Rational design of iron catalysts for C-X bond activation

We have quantum chemically studied the iron-mediated CX bond activation (X = H, Cl, CH₃ ) by d⁸ -FeL₄ complexes using relativistic density functional theory at ZORA-OPBE/TZ2P. We find that by either modulating the electronic effects of a generic iron-catalyst by a set of ligands, that is, CO, BF, PH₃ , BN(CH₃ )₂ , or by manipulating structural effects through the introduction of bidentate ligands, that is, PH₂ (CH₂ )_n PH₂ with n = 6-1, one can significantly decrease the reaction barrier for the CX bond activation. The combination of both tuning handles causes a decrease of the CH activation barrier from 10.4 to 4.6 kcal mol^-1 . Our activation strain and Kohn-Sham molecular orbital analyses reveal that the electronic tuning works via optimizing the catalyst-substrate interaction by introducing a strong second backdonation interaction (i.e., "ligand-assisted" interaction), while the mechanism for structural tuning is mainly caused by the reduction of the required activation strain because of the pre-distortion of the catalyst. In all, we present design principles for iron-based catalysts that mimic the favorable behavior of their well-known palladium analogs in the bond-activation step of cross-coupling reactions.

Palladium-catalyzed activation of HnA–AHn bonds (AHn = CH3, NH2, OH, F)

We have quantum chemically studied activation of HnA–AHn bonds (AHn = CH3, NH2, OH, F) by PdLn catalysts with Ln = no ligand, PH3, (PH3)2, using relativistic density functional theory at ZORA-BLYP/TZ2P. The activation energy associated with the oxidative addition step decreases from H3C–CH3 to H2N–NH2 to HO–OH to F–F, where the activation of the F–F bond is barrierless. Activation strain and Kohn–Sham molecular orbital analyses reveal that the enhanced reactivity along this series of substrates originates from a combination of (i) reduced activation strain due to a weaker HnA–AHn bond; (ii) decreased Pauli repulsion as a result of a difference in steric shielding of the HnA–AHn bond; and (iii) enhanced backbonding interaction between the occupied 4d atomic orbitals of the palladium catalyst and σ* acceptor orbital of the substrate.

C(spn)-X (n = 1-3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C( sp n )-X bonds (n = 1-3 and X = H, CH 3 , Cl) in archetypal model substrates H 3 C-CH 2 -X, H 2 C=CH-X and HC≡C-X by catalysts PdL n  with L n  = no ligand, Cl - , and (PH 3 ) 2 , using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C( sp 3 )-X, to C( sp 2 )-X, to C( sp )-X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from  sp 3  to  sp 2  to  sp , originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C( sp n )-X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst-substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C( sp 3 )-X to C( sp )-X.

cis-[(η5-C5H5)Fe(η1-CO)(μ-CO)]2, the poor relative between cis and trans tautomers. A theoretical study of the gas-phase Fe L3-edge and C and O K-edge XAS of trans-/cis-[(η5-C5H5)Fe(η1-CO)(μ-CO)]2

The relative stability of trans-[(η⁵-C₅H₅)Fe(η¹-CO)(μ-CO)]₂ (trans-I) and cis-I tautomers in a vacuum and in solvents with different dielectric constants (ε) has been investigated by exploiting density functional theory (DFT). Theoretical results indicate that, in agreement with experimental evidence, trans-I is more stable than cis-I in a vacuum (∼1.5 kcal mol^-1; ε = 1), while the opposite is true in media with ε > 7. Differently from solution, DFT outcomes pertaining to the vapor-phase cis-I ⇆ trans-I equilibrium at T = 368 K, the temperature at which the Fe L_2,3-edges and the C and O K-edge X-ray absorption spectroscopy (XAS) data of I have been recorded, ultimately indicate the trans-I predominance (∼93%). Compositions, oscillator strengths (f) and excitation energy (EE) values of cis-I transitions substantially mirror those of trans-I; nevertheless, the weighted cis-If(EE) distributions negligibly contribute to the diverse simulated XA spectra of I.

Understanding chemical reactivity using the activation strain model

Understanding chemical reactivity through the use of state-of-the-art computational techniques enables chemists to both predict reactivity and rationally design novel reactions. This protocol aims to provide chemists with the tools to implement a powerful and robust method for analyzing and understanding any chemical reaction using PyFrag 2019. The approach is based on the so-called activation strain model (ASM) of reactivity, which relates the relative energy of a molecular system to the sum of the energies required to distort the reactants into the geometries required to react plus the strength of their mutual interactions. Other available methods analyze only a stationary point on the potential energy surface, but our methodology analyzes the change in energy along a reaction coordinate. The use of this methodology has been proven to be critical to the understanding of reactions, spanning the realms of the inorganic and organic, as well as the supramolecular and biochemical, fields. This protocol provides step-by-step instructions-starting from the optimization of the stationary points and extending through calculation of the potential energy surface and analysis of the trend-decisive energy terms-that can serve as a guide for carrying out the analysis of any given reaction of interest within hours to days, depending on the size of the molecular system.

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN 2 Reaction of NH 2 - and CH3 Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N 2) reactions of M_n NH₂ ^(n-1) + CH₃ Cl (M⁺  = Li⁺ , Na⁺ , K⁺ , and MgCl⁺ ; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N 2-b) versus frontside attack (S_N 2-f). The results were compared to the corresponding ion-pair S_N 2 reactions involving F^- and OH^- nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N 2 reactivity along the nucleophiles F^- , OH^- , and NH 2 - , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.

Arylic C-X Bond Activation by Palladium Catalysts: Activation Strain Analyses of Reactivity Trends

We have quantum chemically explored arylic carbon-substituent bond activation via oxidative insertion of a palladium catalyst in C₆H₅X + PdL_n model systems (X = H, Cl, CH₃; L_n = no ligand, PH₃, (PH₃)₂, PH₂C₂H₄PH₂) using relativistic density functional theory at ZORA-BLYP/TZ2P. Besides exploring reactivity trends and comparing them to aliphatic C-X activation, we aim at uncovering the physical factors behind the activity and selectivity. Our results show that barriers for arylic C-X activation are lower than those for the corresponding aliphatic C-X bonds. However, trends along bonds or upon variation of ligands are similar. Thus, bond activation barriers increase along C-Cl < C-H < C-C and along Pd < Pd(PH₃) or Pd(PH₂C₂H₄PH₂) < Pd(PH₃)₂. Activation strain analyses in conjunction with quantitative molecular orbital theory trace these trends to the rigidity and bonding capability of the various C-X bonds, model catalysts, and ligands.

Ion-Pair SN 2 Reaction of OH- and CH3 Cl: Activation Strain Analyses of Counterion and Solvent Effects

We have theoretically studied the non-identity S_N 2 reactions of M_n OH^(n-1) +CH₃ Cl (M⁺ =Li⁺ , Na⁺ , K⁺ , and MgCl⁺ ; n=0, 1) in the gas phase and in THF solution at the OLYP/6-31++G(d,p) level using polarizable continuum model (PCM) implicit solvation. We want to explore and understand the effect of the metal counterion M⁺ and solvation on the reaction profile and the stereoselectivity of these processes. To this end, we have explored the potential energy surfaces of the backside (S_N 2-b) and frontside (S_N 2-f) pathways. To explain the computed trends, we have carried out analyses with an extended activation strain model (ASM) of chemical reactivity that includes the treatment of solvation effects.

Ion-Pair SN 2 Substitution: Activation Strain Analyses of Counter-Ion and Solvent Effects

The ion-pair SN 2 reactions of model systems MnF(n-1) +CH3Cl(M(+) =Li(+), Na(+), K(+), and MgCl(+); n=0, 1) have been quantum chemically explored by using DFT at the OLYP/6-31++G(d,p) level. The purpose of this study is threefold: 1) to elucidate how the counterion M(+) modifies ion-pair SN 2 reactivity relative to the parent reaction F(-) +CH3Cl; 2) to determine how this influences stereochemical competition between the backside and frontside attacks; and 3) to examine the effect of solvation on these ion-pair SN2 pathways. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. The ASM has been extended to treat reactivity in solution. These findings contribute to a more rational design of tailor-made substitution reactions.

Catalytic Carbon-Halogen Bond Activation: Trends in Reactivity, Selectivity, and Solvation

We have theoretically studied the oxidative addition of all halomethanes CH3X (with X = F, Cl, Br, I, At) to Pd and PdCl(-), using both nonrelativistic and zeroth-order-regular-approximation-relativistic density functional theory at BLYP/QZ4P. Our study covers the gas phase as well as the condensed phase (water), where solvent effects are described with the conductor-like screening model. The activation of the C*-X bond may proceed via two stereochemically different pathways:  (i) direct oxidative insertion (OxIn) which goes with retention of the configuration at C* and (ii) an alternative SN2 pathway which goes with inversion of the configuration at C*. In the gas phase, for Pd, the OxIn pathway has the lowest reaction barrier for all CH3X's. Anion assistance, that is, going from Pd to PdCl(-), changes the preference for all CH3X's from OxIn to the SN2 pathway. Gas-phase reaction barriers for both pathways to C-X activation generally decrease as X descends in group 17. Two striking solvent effects are (i) the shift in reactivity of Pd + CH3X from OxIn to SN2 in the case of the smaller halogens, F and Cl, and (ii) the shift in reactivity of PdCl(-) + CH3X in the opposite direction, that is, from SN2 to OxIn, in the case of the heavier halogens, I and At. We use the activation strain model to arrive at a qualitative understanding of how the competition between OxIn and SN2 pathways is determined by the halogen atom in the activated C-X bond, by anion assistance, and by solvation.

Reaction Coordinates and the Transition-Vector Approximation to the IRC

The appearance of a reaction profile or potential energy surface (PES) associated with the reaction path (defined as the path of steepest descent from the saddle point) depends on the choice of reaction coordinate onto which the intrinsic reaction coordinate is projected. This provides one with the freedom, but also the problem, of choosing the optimal perspective (i.e., the optimal reaction coordinate) for revealing what is essential for understanding the reaction. Here, we address this issue by analyzing a number of different reaction coordinates for the same set of model reactions, namely, prototypical oxidative addition reactions of C-X bonds to palladium. We show how different choices affect the appearance of the PES, and we discuss which qualities make a particular reaction coordinate most suitable for comparing and analyzing the reactions. Furthermore, we show how the transition vector (i.e., the normal mode associated with a negative force constant that leads from the saddle point to the steepest descent paths) can serve as a useful and computationally much more efficient approximation (designated TV-IRC) for full IRC computations, in the decisive region around the transition state.

E2 and SN2 Reactions of X(-) + CH3CH2X (X = F, Cl); an ab Initio and DFT Benchmark Study

We have computed consistent benchmark potential energy surfaces (PESs) for the anti-E2, syn-E2, and SN2 pathways of X(-) + CH3CH2X with X = F and Cl. This benchmark has been used to evaluate the performance of 31 popular density functionals, covering local-density approximation, generalized gradient approximation (GGA), meta-GGA, and hybrid density-functional theory (DFT). The ab initio benchmark has been obtained by exploring the PESs using a hierarchical series of ab initio methods [up to CCSD(T)] in combination with a hierarchical series of Gaussian-type basis sets (up to aug-cc-pVQZ). Our best CCSD(T) estimates show that the overall barriers for the various pathways increase in the order anti-E2 (X = F) < SN2 (X = F) < SN2 (X = Cl) ∼ syn-E2 (X = F) < anti-E2 (X = Cl) < syn-E2 (X = Cl). Thus, anti-E2 dominates for F(-) + CH3CH2F, and SN2 dominates for Cl(-) + CH3CH2Cl, while syn-E2 is in all cases the least favorable pathway. Best overall agreement with our ab initio benchmark is obtained by representatives from each of the three categories of functionals, GGA, meta-GGA, and hybrid DFT, with mean absolute errors in, for example, central barriers of 4.3 (OPBE), 2.2 (M06-L), and 2.0 kcal/mol (M06), respectively. Importantly, the hybrid functional BHandH and the meta-GGA M06-L yield incorrect trends and qualitative features of the PESs (in particular, an erroneous preference for SN2 over the anti-E2 in the case of F(-) + CH3CH2F) even though they are among the best functionals as measured by their small mean absolute errors of 3.3 and 2.2 kcal/mol in reaction barriers. OLYP and B3LYP have somewhat higher mean absolute errors in central barriers (5.6 and 4.8 kcal/mol, respectively), but the error distribution is somewhat more uniform, and as a consequence, the correct trends are reproduced.

Density Functional Calculations of E2 and SN2 Reactions: Effects of the Choice of Method, Algorithm, and Numerical Accuracy

Herein we provide a detailed account on how the potential energy surfaces of the E2 and SN2 reactions of X(-) + CH3CH2X (X = F, Cl) depend on various methodological and technical choices in density functional calculations. We cover a choice of density functionals (OLYP, various M06-types, and the new SSB-D), basis sets (up to quintuple- and quadruple-ζ for Gaussian- and Slater-type orbitals, respectively, plus polarization and diffuse functions), and other aspects of the computations (among others: nonrelativistic versus zeroth-order regular approximation relativistic; numerical integration accuracy; all-electron versus frozen core; self-consistent field (SCF) versus post-SCF). The program codes ADF and NWChem are used for calculations with Slater- and Gaussian-type basis sets, respectively. The fluoride systems (X = F) appear to not only depend extremely sensitively on the basis set size (especially the presence of diffuse functions) but also on other technical settings, especially in the case of hybrid meta-generalized gradient approximation functionals. This work complements a recent contribution (Y. Zhao, D. G. Truhlar, J. Chem. Theory Comput. 2010, 6, 1104) and provides recommendations for density functionals, basis sets, and technical settings.

Selective C-H and C-C Bond Activation: Electronic Regimes as a Tool for Designing d(10) MLn Catalysts

We wish to understand how a transition-metal catalyst can be rationally designed so as to selectively activate one particular bond in a substrate, herein, C-H and C-C bonds in ethane. To this end, we quantum chemically analyzed the activity and selectivity of a large series of model catalysts towards ethane and, for comparison, methane, by using the activation strain model and quantitative molecular orbital theory. The model catalysts comprise d(10) MLn complexes with coordination numbers n=0, 1, and 2; metal centers M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), and Au(+); and ligands L=NH3, PH3, and CO. Our analyses reveal that rather subtle electronic differences between bonds can be exploited to induce a lower barrier for activating one or the other, depending, among other factors, on the catalysts electronic regime (i.e., s-regime versus d-regime catalysts). Interestingly, the concepts and design principles emerging from this work can also be applied to the more challenging problem of differentiating between activation of the C-H bonds in ethane versus those in methane.

Addition of C-C and C-H bonds by pincer-iridium complexes: a combined experimental and computational study

We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C-C bond of biphenylene. The sterically crowded species ((tBu)PCP)Ir ((R)PCP = κ(3)-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)-H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C-C addition product, ((tBu)PCP)Ir(2,2'-biphenyl), is observed. The much less crowded ((iPr)PCP)Ir undergoes relatively rapid C-C addition at room temperature. The large difference in the apparent barriers to C-C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the (R)PCP ligands. Based on DFT calculations the large difference in the barriers to C-C addition can be explained in terms of a "tilted" transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°-60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the (tBu)PCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C-C bond of biphenyl by ((R)PCP)Ir.

Indenyl effect due to metal slippage? Computational exploration of rhodium-catalyzed acetylene [2+2+2] cyclotrimerization

The mechanism of CpRh (Cp=cyclopentadienyl) and IndRh (Ind=indenyl)-catalyzed acetylene [2+2+2] cyclotrimerization has been revisited aiming at finding an explanation for the better performance of the latter catalyst found experimentally. The hypothesis that an ancillary ligand of the precatalyst remains bonded to the metal center throughout the whole catalytic cycle, based on the experimental evidence that the nature of this ligand can exert some control in cocyclotrimerization of different alkynes, is considered. Strong hapticity variations occur in both the CpRh- and IndRh-catalyzed processes. As the Ind ligand undergoes a more facile slippage than Cp, the energy profile is far smoother in the IndRh-catalyzed cyclotrimerization. This difference in the energetics of the process translates into an enhanced activity of the IndRh catalyst, in nice agreement with experiment.

In silico design of heteroaromatic half-sandwich RhI catalysts for acetylene [2+2+2] cyclotrimerization: evidence of a reverse indenyl effect

A mechanistic density functional theory study of acetylene [2+2+2] cyclotrimerization to benzene catalyzed by Rh(I) half metallocenes is presented. The catalyst fragment contains a heteroaromatic ligand, that is, the 1,2-azaborolyl (Ab) or the 3a,7a-azaborindenyl (Abi) anions, which are isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, but differ from the last ones on having two adjacent carbon atoms replaced with a boron and a nitrogen atom. The better performance of either the classic hydrocarbon or the heteroaromatic catalysts is found to depend on the different mechanistic paths that can be envisioned for the process. The present analyses uncover and explain general structure-reactivity relationships that may serve as rational design principles. In particular, we provide evidence of a reverse indenyl effect.

Benchmark study of the performance of density functional theory for bond activations with (ni,pd)-based transition-metal catalysts

The performance of 23 density functionals, including one LDA, four GGAs, three meta-GGAs, three hybrid GGAs, eight hybrid meta-GGAs, and ten double-hybrid functionals, was investigated for the computation of activation energies of various covalent main-group single bonds by four catalysts: Pd, PdCl⁻, PdCl₂, and Ni (all in the singlet state). A reactant complex, the barrier, and reaction energy were considered, leading to 164 energy data points for statistical analysis. Extended Gaussian AO basis sets were used in all calculations. The best functional for the complete benchmark set relative to estimated CCSD(T)/CBS reference data is PBE0-D3, with an MAD value of 1.1 kcal mol⁻¹ followed by PW6B95-D3, the double hybrid PWPB95-D3, and B3LYP-D3 (1.9 kcal mol⁻¹ each). The other tested hybrid meta-GGAs perform less well (M06-HF: 7.0 kcal mol⁻¹; M06-2X: 6.3 kcal mol⁻¹; M06: 4.9 kcal mol⁻¹) for the investigated reactions. In the Ni case, some double hybrids show larger errors due to partial breakdown of the perturbative treatment for the correlation energy in cases with difficult electronic structures (partial multi-reference character). Only double hybrids either with very low amounts of perturbative correlation (e.g., PBE0-DH) or that use the opposite-spin correlation component only (e.g., PWPB95) seem to be more robust. We also investigated the effect of the D3 dispersion correction. While the barriers are not affected by this correction, significant and mostly positive results were observed for reaction energies. Furthermore, six very recently proposed double-hybrid functionals were analyzed regarding the influence of the amount of Fock exchange as well as the type of perturbative correlation treatment. According to these results, double hybrids with <50–60 % of exact exchange and ∼30 % perturbative correlation perform best.

Nonlinear d(10)-ML2 Transition-Metal Complexes

We have investigated the molecular geometries of a series of dicoordinated d(10)-transition-metal complexes ML2 (M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), Au(+); L=NH3, PH3, CO) using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Not all complexes have the expected linear ligand-metal-ligand (L-M-L) angle: this angle varies from 180° to 128.6° as a function of the metal as well as the ligands. Our main objective is to present a detailed explanation why ML2 complexes can become bent. To this end, we have analyzed the bonding mechanism in ML2 as a function of the L-M-L angle using quantitative Kohn-Sham molecular orbital (MO) theory in combination with an energy decomposition analysis (EDA) scheme. The origin of bent L-M-L structures is π backdonation. In situations of strong π backdonation, smaller angles increase the overlap of the ligand's acceptor orbital with a higher-energy donor orbital on the metal-ligand fragment, and therefore favor π backdonation, resulting in additional stabilization. The angle of the complexes thus depends on the balance between this additional stabilization and increased steric repulsion that occurs as the complexes are bent.

How Accurate Can a Local Coupled Cluster Approach Be in Computing the Activation Energies of Late-Transition-Metal-Catalyzed Reactions with Au, Pt, and Ir?

To improve the accuracy of local coupled cluster (LCC) methods in computing activation energies, we propose herein a new computational scheme. Its applications to various types of late-transition-metal-catalyzed reactions involving Au, Pt, and Ir indicate that the new corrective approach for LCC methods can downsize the mean unsigned deviation and maximum deviation, from the CCSD(T)/CBS reference, to about 0.3 and 0.9 kcal/mol. Using this method, we also calibrated the performance of popular density functionals, with respect to the same test set of reactions. It is concluded that the best functional is the general-purpose double hybrid functional B2GP-PLYP. Other well-performing functionals include the "kinetic" functionals M06-2X and BMK, which have a large percentage of HF exchange, and general-purpose functionals like PBE0 and wB97X. Comparatively, general-purpose functionals like PBE0 and TPSSh perform much better than the tested "kinetic" functionals for Pt-/Ir-catalyzed reactions, while the opposite is true for Au-catalyzed reactions. In contrast, wB97X performs more uniformly in these two classes of reactions. These findings hint that even within the scope of late transition metals, different types of reactions may require different types of optimal DFT methods. Empirical dispersion correction of DFT was found to have a small or no effect on the studied reactions barriers.

Steric nature of the bite angle. A closer and a broader look

The bite angle (ligand-metal-ligand angle) is known to greatly influence the activity of catalytically active transition-metal complexes towards bond activation. Here, we have computationally explored how and why the bite angle has such effects in a wide range of prototypical C-X bonds and palladium complexes, using relativistic density functional theory at ZORA-BLYP/TZ2P. Our model reactions cover the substrates H(3)C-X (with X = H, CH(3), Cl) and, among others, the model catalysts, Pd[PH(2)(CH(2))(n)PH(2)] (with n = 2-6) and Pd[PR(2)(CH(2))(n)PR(2)] (n = 2-4 and R = Me, Ph, t-Bu, Cl), Pd(PH(3))X(-) (X = Cl, Br, I), as well as palladium complexes of chelating and non-chelating N-heterocyclic carbenes. The purpose is to elaborate on an earlier finding that bite-angle effects have a predominantly (although not exclusively) steric nature: a smaller bite angle makes more room for coordinating a substrate by bending away the ligands. Indeed, the present results further consolidate this steric picture by revealing its occurrence in this broader range of model reactions and by identifying and quantifying the exact working mechanism through activation strain analyses.

Tandem mass spectrometry of silver-adducted ferrocenyl catalyst complexes

Ferrocene is a popular template in material science due to its exceptional characteristics that offer the ability to optimize the selectivity and activity of catalysts by the addition of carefully selected substituents. In combinatorial catalyst development, the high susceptibility to electrophilic substitution reactions offers the opportunity for the rapid introduction of molecular diversity. Mass spectrometry (MS)-based continuous-flow systems can be applied to rapidly evaluate catalyst performance as well as to (provisionally) identify the introduced catalyst complexes. Herein, we describe the fragmentation characteristics of the [ferrocenyl bidentate + Ag](+) catalyst complexes in dedicated (high-resolution) MS(n) experiments. The investigation of the fragmentation patterns of a selected number of catalyst classes is accompanied with a density functional theory investigation of fragmentation intermediates in order to assess the viability of a selected fragmentation mechanism.

A first principle study of the structural, vibrational and electronic properties of tetrathiafulvalene adsorbed on Ag(110) and Au(110) surfaces

We have studied the adsorption properties of a charge donor organic molecule, tetrathiafulvalene (TTF), on the (110) surfaces of silver and gold by means of the generalized gradient approach of the density functional theory using periodic slab models. This molecule is the core building block of a host of molecular materials exhibiting extremely reach phase diagrams with a variety of ground states. The interfaces formed with metallic surfaces have received only limited attention, despite of their relevance. We have determined the stable adsorption sites for two unit cells representing high and low coverage, which are determinant for the adsorption properties of TTF on the surface. The preferential chemisorption is via the direct interaction of sulfur atoms with the Ag or Au atoms on top sites. All adsorbed TTF are more stable than gas phase TTF. The simulation of the vibrational spectra has permitted us to find the fingerprints of these structures to characterize them on this surface. The donor nature of TTF induces charge transfer to the metallic surfaces.

Bond activation by group-11 transition-metal cations

We have computationally explored C–X bond activation by the group-11 transition-metal cations Cu+, Ag+, and Au+, and, for comparison, Pd, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Oxidative insertion of the second-row transition-metal species Ag+ and Pd leads, for a given bond, to the highest overall reaction barriers. On the other hand, if we compare the different bonds oxidative insertion into the C–F bond is associated with (one of the) highest overall barriers whereas insertion into the C–Cl bond leads to the lowest overall barrier for any transition metal. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity, which is an extension of the fragment approach to reaction profiles. In this model, the shape of the reaction profile ΔE(ζ) and the height of the overall reaction barrier ΔE≠ = ΔE(ζ=ζTS) are interpreted in terms of the strain energy ΔEstrain(ζ) associated with deforming the reactants along the reaction coordinate ζ plus the interaction energy ΔEint(ζ) between these deformed reactants: ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ).

Frontside versus Backside S(N)2 substitution at group 14 atoms: origin of reaction barriers and reasons for their absence

We have theoretically studied the gas-phase nucleophilic substitution at group-14 atoms (S(N)2@A) in the model reactions of Cl(-)+AH(3)Cl (A=C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. Firstly, we wish to explore and understand how the reaction coordinate zeta, and potential energy surfaces (PES) along zeta, vary as the center of nucleophilic attack changes from carbon to the heavier group-14 atoms. Secondly, a comparison between the more common backside reaction (S(N)2-b) and the frontside pathway (S(N)2-f) is performed. The S(N)2-b reaction is found to have a central barrier for A=C, but none for the other group-14 atoms, A=Si-Pb. Relativistic effects destabilize reactant complexes and transition species by up to 10 kcal mol(-1) (for S(N)2-f@Pb), but they do not change relative heights of barriers. We also address the nature of the transformation in the frontside S(N)2-f reactions in terms of turnstile rotation versus Berry-pseudorotation mechanism.

Benchmark study of DFT functionals for late-transition-metal reactions

The performance of a wide variety of DFT exchange-correlation functionals for a number of late-transition-metal reaction profiles has been considered. Benchmark ab-initio reference data for the prototype reactions Pd + H2, Pd + CH4, Pd + C2H6 (both C-C and C-H activation), and Pd + CH3Cl are presented, while ab-initio data of lesser quality were obtained for the catalytic hydrogenation of acetone and for the low-oxidation-state and high-oxidation-state mechanisms of the Heck reaction. "Kinetics" functionals such as mPW1K, PWB6K, BB1K, and BMK clearly perform more poorly for late-transition-metal reactions than for main-group reactions, as well as compared to general-purpose functionals. There is no single "best functional" for late-transition-metal reactions, but rather a cluster of several functionals (PBE0, B1B95, PW6B95, and TPSS25B95) that perform about equally well; if main-group thermochemical performance is additionally considered, then B1B95 and PW6B95 emerge as the best performers. TPSS25B95 and TPSS33B95 offer attractive performance compromises if weak interactions and main-group barrier heights, respectively, are also important. In the ab-initio calculations, basis set superposition errors (BSSE) can be greatly reduced by ensuring that the metal spd shell has sufficient radial flexibility in the high-exponent range. Optimal HF percentages in hybrid functionals depend on the class of systems considered, increasing from anions to neutrals to cations to main-group barrier heights; transition-metal barrier heights represent an intermediate situation. The use of meta-GGA correlation functionals appears to be quite beneficial.

Transition-State Energy and Position along the Reaction Coordinate in an Extended Activation Strain Model

We investigate palladium-induced activation of the C-H, C-C, C-F, and C-Cl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants' properties. To this end, we extend the activation strain model (in which the activation energy Delta E(not equal) is decomposed into the activation strain Delta E(not equal)(strain) of the reactants and the stabilizing TS interaction Delta E(not equal)(int) between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate zeta, that is, Delta E(zeta)=Delta E(strain)(zeta)+Delta E(int)(zeta). This extension enables us to understand qualitatively, trends in the position of the TS along zeta and, therefore, the values of the activation strain Delta E(not equal)(strain)=Delta E(strain)(zeta(TS)) and TS interaction Delta E(not equal)(int)=Delta E(int)(zeta(TS)) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated C-C versus C-H activation originates from steric shielding of the C-C bond in ethane by C-H bonds. Thus, before a favorable stabilizing interaction with the C-C bond can occur, the C-H bonds must be bent away, which causes the metal-substrate interaction Delta E(int)(zeta) in C-C activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the C-H bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative S(N)2 pathway, and the effect of ring strain.