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Controlling the Oxidative Addition of Aryl Halides to Au(I) Israel Fernandez,*[a] Lando P. Wolters[b] and F. Matthias Bickelhaupt*[b,c] By means of density functional theory calculations, we computationally analyze the physical factors governing the oxidative addition of aryl halides to gold(I) complexes. Using the activation strain model of chemical reactivity, it is found that the strain energy associated with the bending of the gold(I) complex plays a key role in controlling the activation barrier of the process. A systematic study on how the reaction barrier
depends on the nature of the aryl halide, ligand, and counteranion allows us to identify the best combination of gold(I) complex and aryl halide to achieve a feasible (i.e., low barrier) oxidative addition to gold(I), a process considered as kinetiC 2014 Wiley Periodicals, Inc. cally sluggish so far. V
Introduction
interaction DEint(f) between these increasingly deformed reactants:
Homogeneous gold catalysis has emerged as a powerful synthetic tool during the last decade.[1,2] As a result, the scope of gold-catalyzed processes has been spectacularly expanded, which, in addition to the great number of studies focused on the involved reaction mechanisms,[3] has significantly improved our understanding of gold reactivity. Although gold is able to undergo transmetallation,[4] insertion,[5] and reductive elimination reactions,[6] the Au(I)!Au(III) oxidative addition remains comparatively highly elusive.[7] In this regard, no direct evidence for oxidative addition of Csp2AX bonds at a single gold center has been obtained so far, (For a computational study of Csp22F bond activation at gold, see Ref. [8]) with the notable and elegant exception of the intramolecular reaction of aryl halides bearing phosphine side arms toward gold recently described by Amgoune, Bourissou and coworkers.[9] The reluctance of gold(I) to activate Csp2ABr or Csp2AI bonds has been very recently highlighted by Echavarren and coworkers who demonstrated that this elementary transformation, although thermodynamically feasible, is kinetically very sluggish.[10] Despite that, the physical factors controlling this fundamental process, which are crucial toward the design of new catalytic systems, are not completely understood so far. For this reason, herein we aim at a deeper understanding of those factors controlling the Au(I)!Au(III) oxidative addition of aryl halides toward the ulterior design of feasible (i.e., kinetically more favorable) transformations. To this end, we have applied the so-called activation strain model (ASM) of reactivity.[11–13] This method, also known as distortion/interaction model,[12] has allowed us to gain more insight into the physical factors that control how the activation barriers arise in different fundamental processes such as SN2 and E2 reactions,[14] pericyclic reactions,[15] and bond activation reactions promoted by transition metals.[16] Within this model, the potential energy surface DE(f) can be decomposed into two contributions along the reaction coordinate f: the strain energy DEstrain(f), which is associated with the structural deformation that the reactants undergo, plus the 2140
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DEðfÞ 5 DEstrain ðfÞ 1 DEint ðfÞ It is the interplay between DEstrain(f) and DEint(f) that determines if and at which point along f a barrier arises. According to this model, the activation energy of a reaction DE‡ 5 DE(f TS) consists of the activation strain DE‡strain 5 DEstrain(f TS), which is defined as the energy needed to deform the reactants from their equilibrium geometries to the geometries they adopt in the corresponding transition structures (TS), plus the TS interaction DE‡int 5 DEint(f TS) (see Fig. 1). Therefore: DE ‡ 5DE ‡ strain 1DE ‡ int Further details of the method can be found in the literature.[13]
Theoretical Methods Computational details Geometry optimizations were carried out with the GAUSSIAN 09 suite of programs[17] using the B3LYP[18] functional in [a] I. Fern andez Departamento de Quımica Org anica I, Facultad de Ciencias Quımicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040, Madrid, Spain E-mail: [email protected] [b] L. P. Wolters F. M. Bickelhaupt Department of Theoretical Chemistry, Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, De Boelelaan 1083, 1081, HV Amsterdam, The Netherlands E-mail: [email protected] [c] F. M. Bickelhaupt Institute for Molecules and Materials (IMM), Radboud University Nijmegen, Heyendaalseweg 135, 6525, AJ Nijmegen, The Netherlands. Contract grant sponsors: Netherlands Organization for Scientific Research (NWO/CW), National Research School Combination—Catalysis (NRSC-C); Contract grant sponsor: Spanish MINECO; Contract grant number: CTQ2013-44303-P, Consolider-Ingenio2010, CSD2007-00006, and S2009/PPQ-1634 C 2014 Wiley Periodicals, Inc. V
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application, see Ref. [22]. To this end, it is further decomposed into four physically meaningful terms: DEint ðfÞ 5 DVelstat 1 DEPauli 1 DEoi 1 DEdisp
Figure 1. Illustration of the activation-strain model for the oxidative addition of a phenyl halide (PhX) to a generic gold(I) complex (LAAuAX).
combination with the double-f quality def2-SVP[19] basis sets for all atoms. Reactants and products were characterized by frequency calculations, and have positive definite Hessian matrices. TSs show only one negative eigenvalue in their diagonalized force constant matrices, and their associated eigenvectors were confirmed to correspond to the motion along the reaction coordinate under consideration using the intrinsic reaction coordinate method.[20] Single-point energy refinements were carried out using the Truhlar’s meta-hybrid exchange-correlation functional M06[21] with the triple-f quality def2-TZVPP basis sets[19] on the geometries obtained at the B3LYP/def2SVP level. This level is denoted M06/def2-TZVPP//B3LYP/def2SVP.
Energy decomposition analysis The interaction DEint(f) between the strained reactants can be further analyzed in the conceptual framework provided by the Kohn–Sham molecular orbital (MO) model. For reviews about the Energy Decomposition Analysis (EDA) method and its
Figure 2. Activation strain diagram of the oxidative addition reaction of PhCl (blue curves) and PhI (black curves) to (Me3P)AuCl projected onto the forming AuAC distance. The position of the corresponding TS’s is indicated by a dot. All data have been computed at the M06/def2-TZVPP//B3LYP/ def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
The term DVelstat corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the deformed reactants and is usually attractive. The Pauli repulsion DEPauli arises from antisymmetrization of the fragment wavefunctions. It comprises the destabilizing interactions between occupied orbitals and is responsible for any steric repulsion. The orbital interaction DEoi accounts for charge transfer (interaction between occupied orbitals on one moiety with unoccupied orbitals on the other, including HOMO–LUMO interactions) and polarization (empty-occupied orbital mixing on one fragment due to the presence of another fragment). Finally, DEdisp takes into account interaction due to dispersion forces. The EDA calculations were performed with the Amsterdam density functional program[23] using the BP86 functional[18,24] in combination with the dispersion-corrected D3 method developed by Grimme et al.[25] using the geometries computed at the B3LYP/def2-SVP level. The MOs were expanded in a large uncontracted set of Slater-type orbitals (STOs) containing diffuse functions, TZ2P. This basis is of triple-f quality and has been augmented by two sets of polarization functions, that is, p and d functions for the hydrogen atom and d and f functions for the other atoms. An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each SCF cycle. Relativistic effects were accounted for using the zeroth-order regular approximation (ZORA).[26] This level is denoted ZORA-BP86-D3/TZ2P//B3LYP/ def2-SVP.
Results and Discussion We first considered the reaction between phenyl chloride and phenyl iodide with (Me3P)AuCl. As reported,[9,10] the process occurs via a concerted transition state associated with the simultaneous formation of the AuAC and AuAX bonds with concomitant rupture of the CAX bond (see Figs. 1 and 2) instead of through the alternative backside SN2-like mechanism. The computed activation barrier for the corresponding oxidative addition reaction is relatively high (DE‡ 5 40.6 and 27.7 kcal/mol, for PhCl and PhI, respectively) which is in line with previous calculations.[10] Detailed quantitative insight into the factors controlling the process is given by the ASM. Figure 2 shows the full activation-strain diagram, that is, the reaction profile DE(f) together with its decomposition into the strain energy DEstrain(f) and the instantaneous interaction energy DEint(f) between the deformed reactants projected onto the forming AuAC bond, for the processes involving PhCl (blue curves) and PhI (black curves) with (Me3P)AuCl. As clearly shown in Figure 2, the interaction between the deformed reactants is clearly stabilizing from the very Journal of Computational Chemistry 2014, 35, 2140–2145
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Figure 3. Contributions to the total strain energy along the reaction coordinate up to the TS for the reaction between PhI and (Me3P)AuCl. All data have been computed at the M06/def2-TZVPP//B3LYP/def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
beginning of the process. This is mainly due to a significant donor–acceptor interaction between the LP of the halide atom of PhX and the low-lying 6s atomic orbital of gold. Only in the transition state region, the interaction of gold with the r*(CAX) MO of PhX becomes significant, thus leading to an additional stabilization. However, the stabilizing effect of the interaction term cannot compensate the strong destabilizing effect of the deformation energy, DEstrain (DEint‡ 5 217.2 kcal/ mol vs. DEstrain‡ 5 44.9 kcal/mol for the reaction involving PhI). Therefore, the dominant factor controlling the barrier height of the oxidative addition of aryl halides to (Me3P)AuCl is the energy needed to deform the reactants from their initial equilibrium geometries to the geometries they adopt in the corresponding transition state. The partitioning of the strain energy into contributions stemming from each reactant (Fig. 3) indicates that the major contribution to the total strain energy is the deformation associated with the gold(I) complex (DEstrain‡[Au] 5 34.2 kcal/mol vs. DEstrain‡(PhI) 5 10.7 kcal/mol), that is, with the angle change required to achieve the maximum orbital overlap with the
Figure 4. EDA of the interaction energy for the oxidative addition of PhI to (Me3P)AuCl projected onto the forming AuAC bond. All data have been computed at the ZORA-BP86-D3/TZ2P//B3LYP/def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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Figure 5. Evolution of the reactants’ frontier MOs (and corresponding computed overlap, S), involved in the oxidative addition of PhI to (Me3P)AuCl along the reaction coordinate. All data have been computed at the ZORABP86-D3/TZ2P//B3LYP/def2-SVP level.
r*(CAI) orbital of phenyl iodide. Indeed, the total strain energy DEstrain(f) curve matches the DEstrain(f)([Au]) curve along the entire reaction coordinate and only in the proximities of the ˚ ) the deformatransition state (at a CAAu distance of ca. 2.8 A tion of PhI becomes significant. The interaction energy between the deformed reactants can be further analyzed with the help of the energy decomposition analysis (EDA) method. As graphically shown in Figure 4, the main contribution to the total interaction between PhI and (Me3P)AuCl comes from the electrostatic attraction (measured by the DVelst term). Indeed, this term contributes ca. 55% to the total attraction in the transition state. Despite that, the orbital interaction term DEoi, resulting mainly from the 5d(Au)r*(CAI) interaction is also quite significant (ca. 40% to the total attraction in the TS). As expected, the corresponding orbital overlap (S) steadily increases from the beginning of the process (S 5 0.018) to a maximum value of S 5 0.075 in the WWW.CHEMISTRYVIEWS.COM
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Table 1. Activation energy values (in kcal/mol)[a] of the considered oxidative addition reactions of aryl halides to gold(I) complexes Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Aryl halide
[Au(I)]
DE‡
DEstrain‡ (aryl)
DEstrain‡([Au])
DEstrain‡(total)
DEint‡
PhAOTf PhACl PhABr PhAI pNH2AC6H4AI pNO2AC6H4AI PhAI PhAI PhAI PhAI PhAI PhAI pNO2AC6H4AI PhAI pNO2AC6H4AI
Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl (MeO)3PAAuACl Ph3PAAuACl NHCAAuACl[b] Me3PAAuABr Me3PAAuAI Me3PAAuAAt Me3PAAuAAt (CH2CHO)Me2PAAuAI (CH2CHO)Me2PAAuAAt
57.4 40.6 34.9 27.7 28.4 25.8 29.1 28.1 28.8 26.6 25.9 25.0 23.1 22.7 21.3
38.1 24.3 16.7 10.7 13.5 8.6 13.9 21.6 13.5 12.5 15.2 15.7 14.7 15.0 14.6
34.9 35.8 35.2 34.2 35.7 33.1 34.8 31.4 35.7 31.4 28.3 26.8 25.9 27.3 25.9
73.0 60.1 51.9 44.9 49.2 41.7 48.7 53.0 49.1 43.9 43.7 42.5 40.6 42.3 40.5
215.6 219.5 217.1 217.2 220.1 215.8 219.5 224.8 220.8 217.3 217.8 217.5 217.5 219.6 219.2
[a] All data have been computed at the M06/def2-TZVPP//B3LYP/def2-SVP level. [b] NHC 5 1,3-bis(phenyl)21,3-dihydro-2H-imidazol-2-ylidene.
i. Nature of the aryl halide: Not surprisingly, the activation barrier of the process decreases when using aryl iodides instead of bromides, chlorides or triflates. This can be
explained by the lower CAI bond strength compared to CACl, CABr, or CAOTf which manifests itself in a less destabilizing DE‡strain(aryl) (ranging from 38.1 to 10.7 kcal/mol, entries 1–4). As can be seen in Figure 2, breaking the stronger CAX bond causes not only more strain at a given point along the reaction coordinate but also pulls the TS more toward the product side, that is, to a later stage in the activation process. This shift goes with an additional increase of the activation strain. Interestingly, the presence of electron withdrawing groups such as NO2 group (entry 6) leads to a substantial reduction of the barrier (ca. 2 kcal/mol) compared to PhI (entry 4), whereas p-donor groups (like NH2, entry 5) exhibit the contrary effect. This is again related to the relative C2X bond strength, which is translated into a lower activation strain energy (8.6 vs. 13.5 kcal/mol). ii. Effect of the ligand: The replacement of the PMe3 by PPh3 or NHC ligand has little effect on the activation barrier (entries 4 vs. 8 and 9). Similarly, phosphite ligand P(OMe)3 has no beneficial effect either, as it leads to a slightly higher activation barrier as compared to PMe3 (entry 7). iii. Effect of the counteranion: A remarkable decrease on the oxidative addition activation barrier is found when varying the counteranion from X 5 Cl to At (entry 4 vs. entries 10–12). As the corresponding DE‡int remains practically constant, this effect can be ascribed to the lower and lower activation strain energy associated with the bending of the PAAuAX angle which steadily decreases when going down in the halogen group (from 34.2 to only 26.8 kcal/mol).
We have also calculated the oxidative addition reaction of PhI to the parent AuCl complex. Not surprisingly, this process starts with the highly exothermic coordination of PhI to AuCl through the iodide lone-pair (DE 5 33.1 kcal/mol). From this intermediate, the oxidative addition reaction occurs with an activation barrier of only 16.4 kcal/mol, as a consequence of the practically negligible activation strain associated with the AuCl fragment (DEstrain‡ 5 0.3 kcal/mol).
The above results clearly confirm that the reaction barrier associated with oxidative addition of aryl halide to gold(I) stems from activation strain but can be also modulated through the transition-state interaction. Feasible (i.e., low barrier) processes can therefore be achieved, for example, when using deactivated aryl iodides in combination with Me3PAAuAAt, that is, the systems leading to lower DE‡strain
transition state geometry (see Fig. 5). Finally, the contribution of dispersion forces, although highly important at the initial stages of the process, remains constant along the reaction coordinate, thus indicating that in the transition state region its contribution is almost negligible (only 5% of the total attractions). The data above suggest that one could design low barrier oxidative additions to Au(I) by attenuating the significant strain energy DEstrain‡ and/or by strengthening the weak interaction energy DEint‡ between the deformed reactants. The obvious solution is of course to make this process intramolecular, therefore, approaching both reactants, as reported by Bourissou and coworkers.[9] Thus, using an 8-iodo naphtyl phosphine-Au(I) complex leads to a quite favorable oxidative addition which occurs in 1 h at room temperature (computed activation barrier of only 22 kcal/mol).[9] Alternatively, a cationic complex of the type LAu(I) (for instance, R3PAu1) leads to low computed barriers as well,[10] which is mainly due to the drastic reduction of the associated strain energy of the [Au] reactant. However, these monocoordinated species are unknown.† Once the main factors controlling the barrier of the process are understood, we systematically explored the effect of the nature of the aryl halide, ligand, and counteranion on the activation barrier aiming at predicting a feasible oxidative addition to gold(I). Table 1 gathers the systems considered together with their corresponding activation energy terms.
†
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indicate that a transformation considered as kinetically sluggish can be effectively transformed into a feasible reaction with the proper selection of the catalyst and aryl halide. We predict that deactivated aryl iodides and Me3PAAuAAt or similar gold(I) catalysts having an additional weakly coordinating group constitute the best combination to achieve oxidative addition reactions. Keywords: gold oxidative addition reactivity DFT calculations activation strain model
How to cite this article: Fernandez, I., Wolters, L. P., Bickelhaupt, F. M. J. Comput. Chem. 2014, 35, 2140–2145. DOI: 10.1002/jcc.23734 Figure 6. Linear relationship between the computed activation barrier (DE‡) and the total strain energy (DEstrain‡). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
values. As a further confirmation of this finding, we computed the process involving pNO2AC6H4AI and Me3PAAuAAt which renders an activation barrier of only 23.1 kcal/mol as a result of the lowest computed total strain energy (40.6 kcal/mol, entry 13). The strain control of the process is further confirmed by the very good linear relationship found when plotting the computed activation barriers versus the total strain energy (DE‡ vs. DEstrain‡, correlation coefficient of 0.97 and standard deviation of 2.47, see Fig. 6). In a final attempt to further decrease the activation barrier, we combine the above insights with an interesting remote electronic effect that we found. The presence of a carbonyl group attached directly to the phosphine (CH2CHO)Me2PAAuAI) enhances the interaction (DEint‡) between the reactants in the corresponding transition state (compare entry 11 and 14, Table 1).[27]§ As a result, a lower barrier of 22.7 kcal/mol (compared to Me3PAAuAI, DE‡ 5 25.9 kcal/mol) has been computed for this species. Not surprisingly, even a lower barrier can be found in the reaction between (CH2CHO)Me2PAAuAAt and pNO2AC6H4AI (DE‡ 5 21.3 kcal/mol, entry 15), which sharply contrasts the “sluggish” value of >40 kcal/mol computed for the reaction between PhCl and (Me3P)AuCl.
Conclusion From the above computational study, we can conclude that the oxidative addition of aryl halides to gold(I) complexes is controlled by the strain energy associated with the deformation of the reactants from their equilibrium geometries to the geometries they adopt in the corresponding concerted transition state. This is mainly ascribed to the angle change or bending of the initially linear LAAu(I)AX complex. The strain associated with the bond-breaking in the aryl substrate is comparatively less significant, except in those cases involving strong C(aryl)2X bonds, that is, CACl, CAOTf. Our calculations § A related carbonyl enhanced interaction has been recently described by Houk and coworkers in Ni-catalyzed CAO activation reactions. See Ref. [27]
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Additional Supporting Information may be found in the online version of this article.
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Received: 25 August 2014 Accepted: 27 August 2014 Published online on 25 September 2014
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Controlling the Oxidative Addition of Aryl Halides to Au(I) Israel Fernandez,*[a] Lando P. Wolters[b] and F. Matthias Bickelhaupt*[b,c] By means of density functional theory calculations, we computationally analyze the physical factors governing the oxidative addition of aryl halides to gold(I) complexes. Using the activation strain model of chemical reactivity, it is found that the strain energy associated with the bending of the gold(I) complex plays a key role in controlling the activation barrier of the process. A systematic study on how the reaction barrier
depends on the nature of the aryl halide, ligand, and counteranion allows us to identify the best combination of gold(I) complex and aryl halide to achieve a feasible (i.e., low barrier) oxidative addition to gold(I), a process considered as kinetiC 2014 Wiley Periodicals, Inc. cally sluggish so far. V
Introduction
interaction DEint(f) between these increasingly deformed reactants:
Homogeneous gold catalysis has emerged as a powerful synthetic tool during the last decade.[1,2] As a result, the scope of gold-catalyzed processes has been spectacularly expanded, which, in addition to the great number of studies focused on the involved reaction mechanisms,[3] has significantly improved our understanding of gold reactivity. Although gold is able to undergo transmetallation,[4] insertion,[5] and reductive elimination reactions,[6] the Au(I)!Au(III) oxidative addition remains comparatively highly elusive.[7] In this regard, no direct evidence for oxidative addition of Csp2AX bonds at a single gold center has been obtained so far, (For a computational study of Csp22F bond activation at gold, see Ref. [8]) with the notable and elegant exception of the intramolecular reaction of aryl halides bearing phosphine side arms toward gold recently described by Amgoune, Bourissou and coworkers.[9] The reluctance of gold(I) to activate Csp2ABr or Csp2AI bonds has been very recently highlighted by Echavarren and coworkers who demonstrated that this elementary transformation, although thermodynamically feasible, is kinetically very sluggish.[10] Despite that, the physical factors controlling this fundamental process, which are crucial toward the design of new catalytic systems, are not completely understood so far. For this reason, herein we aim at a deeper understanding of those factors controlling the Au(I)!Au(III) oxidative addition of aryl halides toward the ulterior design of feasible (i.e., kinetically more favorable) transformations. To this end, we have applied the so-called activation strain model (ASM) of reactivity.[11–13] This method, also known as distortion/interaction model,[12] has allowed us to gain more insight into the physical factors that control how the activation barriers arise in different fundamental processes such as SN2 and E2 reactions,[14] pericyclic reactions,[15] and bond activation reactions promoted by transition metals.[16] Within this model, the potential energy surface DE(f) can be decomposed into two contributions along the reaction coordinate f: the strain energy DEstrain(f), which is associated with the structural deformation that the reactants undergo, plus the 2140
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DEðfÞ 5 DEstrain ðfÞ 1 DEint ðfÞ It is the interplay between DEstrain(f) and DEint(f) that determines if and at which point along f a barrier arises. According to this model, the activation energy of a reaction DE‡ 5 DE(f TS) consists of the activation strain DE‡strain 5 DEstrain(f TS), which is defined as the energy needed to deform the reactants from their equilibrium geometries to the geometries they adopt in the corresponding transition structures (TS), plus the TS interaction DE‡int 5 DEint(f TS) (see Fig. 1). Therefore: DE ‡ 5DE ‡ strain 1DE ‡ int Further details of the method can be found in the literature.[13]
Theoretical Methods Computational details Geometry optimizations were carried out with the GAUSSIAN 09 suite of programs[17] using the B3LYP[18] functional in [a] I. Fern andez Departamento de Quımica Org anica I, Facultad de Ciencias Quımicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040, Madrid, Spain E-mail: [email protected] [b] L. P. Wolters F. M. Bickelhaupt Department of Theoretical Chemistry, Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, De Boelelaan 1083, 1081, HV Amsterdam, The Netherlands E-mail: [email protected] [c] F. M. Bickelhaupt Institute for Molecules and Materials (IMM), Radboud University Nijmegen, Heyendaalseweg 135, 6525, AJ Nijmegen, The Netherlands. Contract grant sponsors: Netherlands Organization for Scientific Research (NWO/CW), National Research School Combination—Catalysis (NRSC-C); Contract grant sponsor: Spanish MINECO; Contract grant number: CTQ2013-44303-P, Consolider-Ingenio2010, CSD2007-00006, and S2009/PPQ-1634 C 2014 Wiley Periodicals, Inc. V
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application, see Ref. [22]. To this end, it is further decomposed into four physically meaningful terms: DEint ðfÞ 5 DVelstat 1 DEPauli 1 DEoi 1 DEdisp
Figure 1. Illustration of the activation-strain model for the oxidative addition of a phenyl halide (PhX) to a generic gold(I) complex (LAAuAX).
combination with the double-f quality def2-SVP[19] basis sets for all atoms. Reactants and products were characterized by frequency calculations, and have positive definite Hessian matrices. TSs show only one negative eigenvalue in their diagonalized force constant matrices, and their associated eigenvectors were confirmed to correspond to the motion along the reaction coordinate under consideration using the intrinsic reaction coordinate method.[20] Single-point energy refinements were carried out using the Truhlar’s meta-hybrid exchange-correlation functional M06[21] with the triple-f quality def2-TZVPP basis sets[19] on the geometries obtained at the B3LYP/def2SVP level. This level is denoted M06/def2-TZVPP//B3LYP/def2SVP.
Energy decomposition analysis The interaction DEint(f) between the strained reactants can be further analyzed in the conceptual framework provided by the Kohn–Sham molecular orbital (MO) model. For reviews about the Energy Decomposition Analysis (EDA) method and its
Figure 2. Activation strain diagram of the oxidative addition reaction of PhCl (blue curves) and PhI (black curves) to (Me3P)AuCl projected onto the forming AuAC distance. The position of the corresponding TS’s is indicated by a dot. All data have been computed at the M06/def2-TZVPP//B3LYP/ def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
The term DVelstat corresponds to the classical electrostatic interaction between the unperturbed charge distributions of the deformed reactants and is usually attractive. The Pauli repulsion DEPauli arises from antisymmetrization of the fragment wavefunctions. It comprises the destabilizing interactions between occupied orbitals and is responsible for any steric repulsion. The orbital interaction DEoi accounts for charge transfer (interaction between occupied orbitals on one moiety with unoccupied orbitals on the other, including HOMO–LUMO interactions) and polarization (empty-occupied orbital mixing on one fragment due to the presence of another fragment). Finally, DEdisp takes into account interaction due to dispersion forces. The EDA calculations were performed with the Amsterdam density functional program[23] using the BP86 functional[18,24] in combination with the dispersion-corrected D3 method developed by Grimme et al.[25] using the geometries computed at the B3LYP/def2-SVP level. The MOs were expanded in a large uncontracted set of Slater-type orbitals (STOs) containing diffuse functions, TZ2P. This basis is of triple-f quality and has been augmented by two sets of polarization functions, that is, p and d functions for the hydrogen atom and d and f functions for the other atoms. An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each SCF cycle. Relativistic effects were accounted for using the zeroth-order regular approximation (ZORA).[26] This level is denoted ZORA-BP86-D3/TZ2P//B3LYP/ def2-SVP.
Results and Discussion We first considered the reaction between phenyl chloride and phenyl iodide with (Me3P)AuCl. As reported,[9,10] the process occurs via a concerted transition state associated with the simultaneous formation of the AuAC and AuAX bonds with concomitant rupture of the CAX bond (see Figs. 1 and 2) instead of through the alternative backside SN2-like mechanism. The computed activation barrier for the corresponding oxidative addition reaction is relatively high (DE‡ 5 40.6 and 27.7 kcal/mol, for PhCl and PhI, respectively) which is in line with previous calculations.[10] Detailed quantitative insight into the factors controlling the process is given by the ASM. Figure 2 shows the full activation-strain diagram, that is, the reaction profile DE(f) together with its decomposition into the strain energy DEstrain(f) and the instantaneous interaction energy DEint(f) between the deformed reactants projected onto the forming AuAC bond, for the processes involving PhCl (blue curves) and PhI (black curves) with (Me3P)AuCl. As clearly shown in Figure 2, the interaction between the deformed reactants is clearly stabilizing from the very Journal of Computational Chemistry 2014, 35, 2140–2145
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Figure 3. Contributions to the total strain energy along the reaction coordinate up to the TS for the reaction between PhI and (Me3P)AuCl. All data have been computed at the M06/def2-TZVPP//B3LYP/def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
beginning of the process. This is mainly due to a significant donor–acceptor interaction between the LP of the halide atom of PhX and the low-lying 6s atomic orbital of gold. Only in the transition state region, the interaction of gold with the r*(CAX) MO of PhX becomes significant, thus leading to an additional stabilization. However, the stabilizing effect of the interaction term cannot compensate the strong destabilizing effect of the deformation energy, DEstrain (DEint‡ 5 217.2 kcal/ mol vs. DEstrain‡ 5 44.9 kcal/mol for the reaction involving PhI). Therefore, the dominant factor controlling the barrier height of the oxidative addition of aryl halides to (Me3P)AuCl is the energy needed to deform the reactants from their initial equilibrium geometries to the geometries they adopt in the corresponding transition state. The partitioning of the strain energy into contributions stemming from each reactant (Fig. 3) indicates that the major contribution to the total strain energy is the deformation associated with the gold(I) complex (DEstrain‡[Au] 5 34.2 kcal/mol vs. DEstrain‡(PhI) 5 10.7 kcal/mol), that is, with the angle change required to achieve the maximum orbital overlap with the
Figure 4. EDA of the interaction energy for the oxidative addition of PhI to (Me3P)AuCl projected onto the forming AuAC bond. All data have been computed at the ZORA-BP86-D3/TZ2P//B3LYP/def2-SVP level. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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Figure 5. Evolution of the reactants’ frontier MOs (and corresponding computed overlap, S), involved in the oxidative addition of PhI to (Me3P)AuCl along the reaction coordinate. All data have been computed at the ZORABP86-D3/TZ2P//B3LYP/def2-SVP level.
r*(CAI) orbital of phenyl iodide. Indeed, the total strain energy DEstrain(f) curve matches the DEstrain(f)([Au]) curve along the entire reaction coordinate and only in the proximities of the ˚ ) the deformatransition state (at a CAAu distance of ca. 2.8 A tion of PhI becomes significant. The interaction energy between the deformed reactants can be further analyzed with the help of the energy decomposition analysis (EDA) method. As graphically shown in Figure 4, the main contribution to the total interaction between PhI and (Me3P)AuCl comes from the electrostatic attraction (measured by the DVelst term). Indeed, this term contributes ca. 55% to the total attraction in the transition state. Despite that, the orbital interaction term DEoi, resulting mainly from the 5d(Au)r*(CAI) interaction is also quite significant (ca. 40% to the total attraction in the TS). As expected, the corresponding orbital overlap (S) steadily increases from the beginning of the process (S 5 0.018) to a maximum value of S 5 0.075 in the WWW.CHEMISTRYVIEWS.COM
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Table 1. Activation energy values (in kcal/mol)[a] of the considered oxidative addition reactions of aryl halides to gold(I) complexes Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Aryl halide
[Au(I)]
DE‡
DEstrain‡ (aryl)
DEstrain‡([Au])
DEstrain‡(total)
DEint‡
PhAOTf PhACl PhABr PhAI pNH2AC6H4AI pNO2AC6H4AI PhAI PhAI PhAI PhAI PhAI PhAI pNO2AC6H4AI PhAI pNO2AC6H4AI
Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl Me3PAAuACl (MeO)3PAAuACl Ph3PAAuACl NHCAAuACl[b] Me3PAAuABr Me3PAAuAI Me3PAAuAAt Me3PAAuAAt (CH2CHO)Me2PAAuAI (CH2CHO)Me2PAAuAAt
57.4 40.6 34.9 27.7 28.4 25.8 29.1 28.1 28.8 26.6 25.9 25.0 23.1 22.7 21.3
38.1 24.3 16.7 10.7 13.5 8.6 13.9 21.6 13.5 12.5 15.2 15.7 14.7 15.0 14.6
34.9 35.8 35.2 34.2 35.7 33.1 34.8 31.4 35.7 31.4 28.3 26.8 25.9 27.3 25.9
73.0 60.1 51.9 44.9 49.2 41.7 48.7 53.0 49.1 43.9 43.7 42.5 40.6 42.3 40.5
215.6 219.5 217.1 217.2 220.1 215.8 219.5 224.8 220.8 217.3 217.8 217.5 217.5 219.6 219.2
[a] All data have been computed at the M06/def2-TZVPP//B3LYP/def2-SVP level. [b] NHC 5 1,3-bis(phenyl)21,3-dihydro-2H-imidazol-2-ylidene.
i. Nature of the aryl halide: Not surprisingly, the activation barrier of the process decreases when using aryl iodides instead of bromides, chlorides or triflates. This can be
explained by the lower CAI bond strength compared to CACl, CABr, or CAOTf which manifests itself in a less destabilizing DE‡strain(aryl) (ranging from 38.1 to 10.7 kcal/mol, entries 1–4). As can be seen in Figure 2, breaking the stronger CAX bond causes not only more strain at a given point along the reaction coordinate but also pulls the TS more toward the product side, that is, to a later stage in the activation process. This shift goes with an additional increase of the activation strain. Interestingly, the presence of electron withdrawing groups such as NO2 group (entry 6) leads to a substantial reduction of the barrier (ca. 2 kcal/mol) compared to PhI (entry 4), whereas p-donor groups (like NH2, entry 5) exhibit the contrary effect. This is again related to the relative C2X bond strength, which is translated into a lower activation strain energy (8.6 vs. 13.5 kcal/mol). ii. Effect of the ligand: The replacement of the PMe3 by PPh3 or NHC ligand has little effect on the activation barrier (entries 4 vs. 8 and 9). Similarly, phosphite ligand P(OMe)3 has no beneficial effect either, as it leads to a slightly higher activation barrier as compared to PMe3 (entry 7). iii. Effect of the counteranion: A remarkable decrease on the oxidative addition activation barrier is found when varying the counteranion from X 5 Cl to At (entry 4 vs. entries 10–12). As the corresponding DE‡int remains practically constant, this effect can be ascribed to the lower and lower activation strain energy associated with the bending of the PAAuAX angle which steadily decreases when going down in the halogen group (from 34.2 to only 26.8 kcal/mol).
We have also calculated the oxidative addition reaction of PhI to the parent AuCl complex. Not surprisingly, this process starts with the highly exothermic coordination of PhI to AuCl through the iodide lone-pair (DE 5 33.1 kcal/mol). From this intermediate, the oxidative addition reaction occurs with an activation barrier of only 16.4 kcal/mol, as a consequence of the practically negligible activation strain associated with the AuCl fragment (DEstrain‡ 5 0.3 kcal/mol).
The above results clearly confirm that the reaction barrier associated with oxidative addition of aryl halide to gold(I) stems from activation strain but can be also modulated through the transition-state interaction. Feasible (i.e., low barrier) processes can therefore be achieved, for example, when using deactivated aryl iodides in combination with Me3PAAuAAt, that is, the systems leading to lower DE‡strain
transition state geometry (see Fig. 5). Finally, the contribution of dispersion forces, although highly important at the initial stages of the process, remains constant along the reaction coordinate, thus indicating that in the transition state region its contribution is almost negligible (only 5% of the total attractions). The data above suggest that one could design low barrier oxidative additions to Au(I) by attenuating the significant strain energy DEstrain‡ and/or by strengthening the weak interaction energy DEint‡ between the deformed reactants. The obvious solution is of course to make this process intramolecular, therefore, approaching both reactants, as reported by Bourissou and coworkers.[9] Thus, using an 8-iodo naphtyl phosphine-Au(I) complex leads to a quite favorable oxidative addition which occurs in 1 h at room temperature (computed activation barrier of only 22 kcal/mol).[9] Alternatively, a cationic complex of the type LAu(I) (for instance, R3PAu1) leads to low computed barriers as well,[10] which is mainly due to the drastic reduction of the associated strain energy of the [Au] reactant. However, these monocoordinated species are unknown.† Once the main factors controlling the barrier of the process are understood, we systematically explored the effect of the nature of the aryl halide, ligand, and counteranion on the activation barrier aiming at predicting a feasible oxidative addition to gold(I). Table 1 gathers the systems considered together with their corresponding activation energy terms.
†
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indicate that a transformation considered as kinetically sluggish can be effectively transformed into a feasible reaction with the proper selection of the catalyst and aryl halide. We predict that deactivated aryl iodides and Me3PAAuAAt or similar gold(I) catalysts having an additional weakly coordinating group constitute the best combination to achieve oxidative addition reactions. Keywords: gold oxidative addition reactivity DFT calculations activation strain model
How to cite this article: Fernandez, I., Wolters, L. P., Bickelhaupt, F. M. J. Comput. Chem. 2014, 35, 2140–2145. DOI: 10.1002/jcc.23734 Figure 6. Linear relationship between the computed activation barrier (DE‡) and the total strain energy (DEstrain‡). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
values. As a further confirmation of this finding, we computed the process involving pNO2AC6H4AI and Me3PAAuAAt which renders an activation barrier of only 23.1 kcal/mol as a result of the lowest computed total strain energy (40.6 kcal/mol, entry 13). The strain control of the process is further confirmed by the very good linear relationship found when plotting the computed activation barriers versus the total strain energy (DE‡ vs. DEstrain‡, correlation coefficient of 0.97 and standard deviation of 2.47, see Fig. 6). In a final attempt to further decrease the activation barrier, we combine the above insights with an interesting remote electronic effect that we found. The presence of a carbonyl group attached directly to the phosphine (CH2CHO)Me2PAAuAI) enhances the interaction (DEint‡) between the reactants in the corresponding transition state (compare entry 11 and 14, Table 1).[27]§ As a result, a lower barrier of 22.7 kcal/mol (compared to Me3PAAuAI, DE‡ 5 25.9 kcal/mol) has been computed for this species. Not surprisingly, even a lower barrier can be found in the reaction between (CH2CHO)Me2PAAuAAt and pNO2AC6H4AI (DE‡ 5 21.3 kcal/mol, entry 15), which sharply contrasts the “sluggish” value of >40 kcal/mol computed for the reaction between PhCl and (Me3P)AuCl.
Conclusion From the above computational study, we can conclude that the oxidative addition of aryl halides to gold(I) complexes is controlled by the strain energy associated with the deformation of the reactants from their equilibrium geometries to the geometries they adopt in the corresponding concerted transition state. This is mainly ascribed to the angle change or bending of the initially linear LAAu(I)AX complex. The strain associated with the bond-breaking in the aryl substrate is comparatively less significant, except in those cases involving strong C(aryl)2X bonds, that is, CACl, CAOTf. Our calculations § A related carbonyl enhanced interaction has been recently described by Houk and coworkers in Ni-catalyzed CAO activation reactions. See Ref. [27]
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Additional Supporting Information may be found in the online version of this article.
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Received: 25 August 2014 Accepted: 27 August 2014 Published online on 25 September 2014
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