Determining the Impact of Ligand and Alkene Substituents on Bonding

Article pubs.acs.org/JPCA

Determining the Impact of Ligand and Alkene Substituents on Bonding in Gold(I)−Alkene Complexes Supported by N‑Heterocyclic Carbenes: A Computational Study John T. York* Department of Chemistry, Stetson University, DeLand, Florida 32723, United States S Supporting Information *

ABSTRACT: The nature of the gold(I)−alkene bond in [(NHC)Au(alkene)]+ complexes (where NHC is the Nheterocyclic carbene 1,3-bis(2,6-dimethylphenyl)imidazole-2ylidine and its derivatives) has been studied using density functional theory. By utilization of a series of electronwithdrawing and electron-donating substituents ranging from −NO2 to −NH2, an examination of substituent effects has been undertaken with 4-substituted NHC ligands, monosubstituted ethylene derivatives, and 4-substituted styrene derivatives. Natural population, natural bond orbital (NBO), molecular orbital, and bond energy decomposition analysis (EDA) methods have been used to quantify a number of important parameters, including the charge of the coordinated alkenes and the magnitude of alkene→[(NHC)Au]+ and [(NHC)Au]+→ alkene electron donation. EDA methods have also been used to quantify the strength of the [(NHC)Au]+−(alkene) bond and the impact of both ligand and alkene substitution on different components of the interaction, including polarization, orbital, electrostatic, and Pauli repulsive contributions. Finally, molecular orbital analysis has been used to understand the activation of the alkenes in terms of orbital composition and stabilization within the [(NHC)Au(alkene)]+ complexes relative to the free alkenes. These results provide important insight into the fundamental nature of gold(I)−alkene bonding and the impact of both ligand and alkene substitution on the electronic structure of these complexes.

1. INTRODUCTION Cationic gold(I) complexes have attracted significant interest due to their ability to activate C−C multiple bonds for reactivity with a variety of nucleophiles.1−6 The active catalysts generally consist of a single monodentate phosphine (PR3) or N-heterocyclic carbene (NHC) ligand bound to the gold(I) ion, with a weakly coordinating anion providing a second coordination site for binding unsaturated molecules to form a linear two-coordinate gold π complex.1−6 The enhanced electrophilic character of π-bound organic molecules in these complexes has been broadly attributed to strong Lewis acidity of the gold(I) cation.1−7 However, important questions still remain regarding the fundamental nature of activation by these catalysts and how they might be altered for a desired effect. Two variables that have significant impacts on the function of these and other similar catalysts are the electron richness of the supporting ligand and substituents on the substrate.8−14 Experiments have shown that changes to either of these can affect the stability of gold π complexes that are formed during catalysis15−17 as well as the reaction rates and product distribution observed in these processes.1−6,14,18−21 Elucidation of the fundamental impact of ligand electronics and π-substrate substituents on bonding and the electronic structure of these complexes is therefore an important goal. © 2016 American Chemical Society

To understand the fundamental manner in which gold(I) complexes activate C−C π-bonds, we have explored alkenes in this study as a representative example of such chemistry. In general, there are multiple factors that contribute to the electrophilic character of a metal-bound alkene. First, binding of an alkene to a metal can decrease unfavorable electron− electron repulsion between the alkene CC(π) orbital and a nucleophile.22 One component of this contribution is the transfer of π-electron density from the alkene to a Lewis acidic metal center. This electron donation is often described by the Dewar−Chatt−Duncanson (DCD) model of bonding, with both alkene→metal and metal→alkene donation contributing to the alkene activation.9,22−27 In the case of the gold(I) ion, previous computational studies of [Au(C2H4)]+ and related species demonstrated that DCD bonding between the metal and ethylene is dominated by ethylene→Au(I) donation (∼70−80%), with Au(I)→ethylene donation providing the minor contribution.28−32 While these findings support the role of the gold(I) ion in activating alkenes through net π-electron depletion, related analyses of [(L)Au(C2H4)]n+ complexes revealed that the supporting ligand can have a substantial Received: April 14, 2016 Revised: June 27, 2016 Published: July 25, 2016 6064

DOI: 10.1021/acs.jpca.6b03819 J. Phys. Chem. A 2016, 120, 6064−6075

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The Journal of Physical Chemistry A impact on the net charge transfer, even resulting in negatively charged bound alkenes. For instance, ethylene→[(L)Au]+ donation contributes as much as 69% for [(NHC)Au(C2H4)]+ 33 but as little as 45% for [(bipy)Au(C2H4)]+ (where NHC and bipy are an N-heterocyclic carbene and 2,2′-bipyridine).25 Because the magnitude of the charge transfer can be dramatically altered by changes in the ancillary ligand, a more systematic understanding of ligand effects on charge transfer in gold(I) complexes is needed. Although the electron distribution and charge on a metalcoordinated alkene contribute to its reactivity, computational studies have shown that alkene charge alone does not reliably predict electrophilic activation.22 In fact, some metal-coordinated ethylene moieties are predicted to be deactivated toward reactivity with a nucleophile in spite of possessing a significant positive charge.22 Ultimately, it is the frontier molecular orbitals of the metal−alkene adduct that determine the reactivity of the complex. Of particular significance to electrophilic activation are the low-lying unoccupied molecular orbitals of either alkene CC(π*) or CC(π) character, which may interact favorably with the HOMO of a nucleophile to yield reaction products.7,22,34 Changes in either ligand or alkene substituents that maximize the stabilization of these orbitals and their localization on the alkene fragment may enhance the electrophilic nature of the bound alkene.22,34 Thus, an analysis of the pertinent molecular orbitals of the gold(I)−alkene complexes is essential to fully explain their reactivity.22 Finally, because the formation of Au(I)−alkene adducts and the subsequent attack by a nucleophile can be a reversible process,18,20 the impact of ligand and alkene substituents on the strength of the gold−alkene bond is also important. Both ligand and alkene substitution have been shown to affect the binding between alkenes and gold(I) ions, potentially changing the products observed and the rates of catalysis.11,17−21,26,35−38 Noncovalent factors such as Pauli repulsion and electrostatic attraction that are not described within the DCD model also play a crucial role in bonding in these systems. Indeed, previous computational studies have shown that electrostatic attraction is the most important stabilizing component of gold(I)−alkene bonding.28,30,31 Determining the impact of ligand and alkene substitution on the strength and nature of bonding in gold(I)− alkene complexes could provide important insight into their reactivity. Herein we report the results of a DFT investigation of the bonding in two coordinate [(NHC)Au(alkene)]+ complexes (where NHC is the 1,3-bis(2,6-dimethylphenyl)imidazole-2ylidine ligand and its derivatives). Utilizing a series of electronwithdrawing and electron-donating substituents ranging from −NO2 to −NH2 (Chart 1),39 a systematic examination of substituent effects has been undertaken with 4-substituted NHC ligands (1a−g), monosubstituted ethylene derivatives (2a−g), and 4-substituted styrene derivatives (3a−g). Natural population,40 natural bond orbital (NBO),41 molecular orbital, and multiple bond energy decomposition analyses (EDAs) have been used to quantify the charge of the gold(I)-coordinated alkenes and the amount of alkene→[(NHC)Au]+ and [(NHC)Au]+→alkene donation. EDA methods have been used to quantify the strength of the [(NHC)Au]+−(alkene) bond and the impact of both ligand and alkene substitution on the various components of the interaction, including polarization, orbital interactions and charge transfer, electrostatic attraction, and Pauli repulsion. Finally, molecular orbital analysis has been used to understand the electrophilic activation

Chart 1. Gold(I)−Alkene Complexes Examined in This Study

of the alkene in terms of orbital composition and stabilization in the [(NHC)Au(alkene)]+ adducts relative to the free alkenes. These results provide important insight into the fundamental nature of gold(I)−alkene bonding and the impact of both ligand and alkene substitution on the electronic structures of these complexes.

2. COMPUTATIONAL METHODOLOGY All calculations were performed in the gas phase as spinrestricted calculations using the PBE0 functional42,43 (keyword PBE1PBE in Gaussian 09W44). This functional has been shown to be superior to many other common functionals for calculating the geometries and binding energies in gold(I) complexes of alkenes and other related molecules.45 Geometry optimizations were performed with Gaussian 09W44 (revision B.01) using the Stuttgart/Dresden (SDD) basis set and effective core potential46 for Au and the 6-31g(d,p) basis set47,48 for all other atoms (Gaussian keyword GENECP).49 All optimizations were completed using tight convergence criteria, the pruned (99 590) integration grid (Gaussian keyword int = grid = ultrafine) and with the molecular geometries constrained to a particular point group symmetry when appropriate (see Supporting Information for details on specific compounds). Frequency calculations were performed using the same computational method to verify optimized geometries as minima by the absence of imaginary frequencies. Natural population40 and natural bond orbital41 analyses were performed in Gaussian 09W using the SDD basis set and effective core potential for Au and the 6-311g(2d,p) basis set50 for all other atoms. Compositions of molecular orbitals and the contributions from fragment orbitals were calculated using the Mulliken population analysis method in AOMix51,52 (version 6.46) from single point energy calculations in Gaussian 09W using this same functional and basis set combination. Selected single point energy calculations using the CPCM solvation model53 with dichloromethane as the solvent were completed using the SDD basis set and effective core potential for Au and the 6-311g(2d,p) basis set50 for all other atoms. Solvated electronic energies were converted to Gibbs free energies using corrections obtained from the original unscaled frequency 6065


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The Journal of Physical Chemistry A calculations (298.15 K, 1 atm). Bonding interaction analysis using the EDA-NOCV method based on the extended transition state energy decomposition analysis (ETS-EDA) of Morokuma,54 Ziegler and Rauk28,55 and the natural orbitals for chemical valence (NOCV) method of Mitoraj and Ziegler56,57 was carried out using the Amsterdam Density Functional 2012 program.58−60 Scalar relativistic effects were considered using the zero-order regular approximation (ZORA).61,62 A Slater type orbital (STO) all-electron basis set of triple-ζ quality with two polarization functions was used for all atoms (ADF basis set designation ZORA/TZ2P).63 Counterpoise-corrected interfragment binding energies were calculated using the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) method of Khaliullin and co-workers64 as implemented in Q-Chem 4.065 using the SRSC basis set46 and effective core potential for Au and the 6-311g(2d,p) basis set for all other atoms. The (70 302) grid size was utilized. Geometric distortion (preparation) energies were computed from gas-phase single point calculations in Q-Chem for structures optimized in Gaussian 09W and were not corrected for zero-point vibrational energy. Atomic coordinates for the optimized geometries for all complexes are included in the Supporting Information. Counterions present in the experimental systems and their effects9,11,12 were not included in these calculations.

Figure 1. Calculated structures of complexes 1c, 2g, and 3e (only selected alkene H atoms shown for clarity).

3. RESULTS AND DISCUSSION 3.1. Calculated Structures. Selected geometric data for the calculated structures are listed in Table 1, with the representative complexes 1c, 2g, and 3e shown in Figure 1. The

average computed Au−CNHC and Au−Calkene bond lengths of 2.02 and 2.27 Å are in excellent agreement with the solid-state structures of [(NHC)Au(alkene)]+ complexes having the 1,3bis(2,6-diisopropylphenyl)imidazole-2-ylidine ligand (2.00 and 2.24 Å, respectively).15,49 The alkenes are predicted to bind in an approximately η2 geometry, with varying degrees of Au− C alkene bond asymmetry and accompanying η 2 → η 1 slippage22,37,38 observed for the substituted alkenes in 2a−g and 3a−g (Table 1). All alkenes are also predicted to bind with the CC bond oriented approximately perpendicular to the N−C−N plane of the NHC ligand, with the exact dihedral angle varying with alkene substitution (Table S1, Supporting Information). The predicted asymmetry and slippage are similar to the experimental structures of [(NHC)Au(alkene)]+ and [(PR3)Au(alkene)]+ complexes.15,16,37,38,66−68 Moreover, the X-ray crystal structures of [(NHC)Au(alkene)]+ complexes contained substituted alkenes with CC bond orientations ranging from roughly perpendicular to the N−C−N ligand plane to coplanar with the NHC ligand.15 With these structures it was suggested that steric effects may play a major role in the observed alkene orientation.15 Consistent with this hypothesis, binding of the ethylene molecule coplanar with the N−C−N ligand plane in 1d is calculated to be less than 3 kJ/mol higher in energy than the perpendicular geometry, indicating minimal electronic stabilization of a particular alkene orientation in these complexes. Substitution of the 4-position of the imidazole-2-ylidine ring of the NHC ligand with increasingly electron-donating substituents in 1a−g results in essentially no change in the Au−Cethylene and ethylene CC bond lengths (Table 1). However, substitution at the C2-carbon in the ethylene derivatives 2a−g results in varying degrees of η2 → η1 slippage, where 0% slippage indicates equal Au−Calkene bond lengths (η2 coordination), 100% indicates an Au−C1−C2 angle of 90° with

Table 1. Calculated Geometric Data for Au(I)−Alkene Complexes bond length (Å) complexa

Au−CNHC

Au−C1

Au−C2

CCalkene

% sippagec

1a 1b 1c 1db 1e 1f 1g 2a 2b 2c 2db 2e 2f 2g 3a 3b 3c 3d 3e 3f 3g 4

2.029 2.027 2.025 2.025 2.024 2.023 2.021 2.021 2.023 2.021 2.025 2.024 2.022 2.025 2.021 2.021 2.021 2.022 2.022 2.022 2.023 N/A

2.248 2.247 2.245 2.246 2.245 2.243 2.240 2.244 2.239 2.236 2.246 2.226 2.186 2.157 2.227 2.234 2.220 2.220 2.216 2.207 2.197 2.188

2.248 2.247 2.245 2.246 2.245 2.243 2.241 2.224 2.227 2.259 2.246 2.299 2.506 2.687 2.298 2.296 2.315 2.317 2.327 2.348 2.382 2.188

1.372 1.372 1.372 1.372 1.372 1.373 1.373 1.369 1.373 1.372 1.372 1.376 1.387 1.402 1.381 1.381 1.383 1.383 1.384 1.387 1.391 1.400

0 0 0 0 0 0 0 −5 −3 6 0 17 78 131 17 15 23 23 26 33 44 0

a

X = (a) NO2; (b) CF3; (c) Br; (d) H; (e) CH3; (f) OCH3; (g) NH2. Complexes 1d and 2d are identical but are listed throughout as separate complexes in the tables and figures for ease of comparison in each series. cReferences 37 and 38. b

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The Journal of Physical Chemistry A the Au ion directly above C1 (η1 coordination), and slippage greater than 100% indicates the Au ion shifted beyond the end of the alkene double bond.37,38 The bond asymmetry is particularly large with the strongly donating −OCH3 and −NH2 substituents, resulting in 78% slippage in 2f and 131% in 2g, in good agreement with the slippage observed in crystal structures of related gold(I)−enol ether (∼70−99%)37 and gold(I)−enamine (∼120−172%)38 complexes with phosphine ligands (Table 1, Figures 2 and 3). In the styrene adducts 3a−g

donation contributes roughly 64% to DCD bonding, while [(X−NHC)Au]+→ethylene(π*) back-donation provides the remainder (Table 2). While smaller, it is important to note that Table 2. Calculated Atomic Charges and Natural Bond Orbital (NBO) Occupancies for Au(I)−Alkene Complexesa complex

Au charge

C1 charge

C2 charge

alkene charge

CC(π) occupancy

CC(π*) occupancy

1a 1b 1c 1d 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 3a 3b 3c 3d 3e 3f 3g 4

0.483 0.477 0.475 0.471 0.468 0.471 0.466 0.491 0.481 0.461 0.471 0.460 0.403 0.374 0.470 0.471 0.463 0.464 0.460 0.451 0.442 0.757

−0.411 −0.412 −0.414 −0.414 −0.416 −0.417 −0.418 −0.392 −0.389 −0.471 −0.414 −0.472 −0.701 −0.748 −0.461 −0.471 −0.489 −0.489 −0.502 −0.528 −0.562 −0.385

−0.411 −0.412 −0.414 −0.414 −0.416 −0.417 −0.419 −0.164 −0.357 −0.349 −0.414 −0.179 0.278 0.174 −0.209 −0.200 −0.190 −0.188 −0.179 −0.166 −0.147 −0.385

0.123 0.119 0.113 0.112 0.107 0.105 0.100 0.060 0.085 0.112 0.112 0.128 0.203 0.254 0.110 0.116 0.128 0.131 0.139 0.154 0.174 0.243

1.756 1.757 1.758 1.759 1.760 1.759 1.759 1.753 1.760 1.782 1.759 1.754 1.754 1.738 1.747 1.749 1.746 1.746 1.744 1.742 1.738 1.599

0.129 0.132 0.137 0.136 0.139 0.142 0.148 0.182 0.172 0.226 0.136 0.164 0.280 0.358 0.207 0.209 0.225 0.219 0.229 0.250 0.277 0.147

Figure 2. Calculated Au−Calkene bond lengths (Å) for [(NHC)Au(X− CHCH2)]+ complexes 2a−g (C2 is the substituted carbon atom).

a

All occupancies in electrons.

[(X−NHC)Au]+→ethylene donation is substantial in these complexes and should be considered in explanations of their reactivity.9 Consistent with dominant ethylene(π)→[(X− NHC)Au]+ donation, the ethylene fragment is positively charged in all complexes, with the magnitude of the charge increasing linearly with the electron-withdrawing strength of the ligand substituent (Figure 4a). Given the description of the gold(I) ion primarily as a Lewis acid, this trend might be incorrectly attributed to an increase in ethylene→Au donation with more strongly electron-withdrawing ligands across the series. However, the NBO data show that ethylene(π)→[(X− NHC)Au]+ donation is relatively unaffected by ligand substitution across the series (Figure 4b). Instead, it is a larger decrease in [(X−NHC)Au]+→ethylene(π*) back-donation with electron-withdrawing substituents that yields the increasing alkene charge (Figure 4c). A similar impact on ethylene(π)→Au and Au→ethylene(π*) donation was reported previously.33 3.2.2. Impact of Alkene Substitution in 2a−g and 3a−g. The substituted alkenes in 2a−g and 3a−g are also predicted to have a positive charge (Table 2), indicating that alkene→Au charge donation dominates for all complexes in this study. In contrast to substitution on the NHC ligand, increasing the electron-withdrawing strength of alkene substituents generally decreases the alkene charge in both series (Figures S1a and S1b, Supporting Information). The increasing alkene charge with more electron-rich alkenes could potentially result from increased alkene→Au donation, decreased Au→alkene donation, or a combination of both. However, because the alkene CC(π) and CC(π*) NBOs in 2a−g and 3a−g directly

Figure 3. Au−Calkene bond lengths (Å) and calculated natural atomic charges (in parentheses) for selected [(NHC)Au(X−CHCH2)]+ complexes (L = NHC).

increasingly electron-donating para-substituents generally yield increasing slippage, with the 26% predicted for 3e comparing very well to the 28% observed in the crystal structure of the analogous phosphine complex {[P(t-Bu)2(o-biphenyl]Au(Me− C6H4−CHCH2)}+SbF6−.66 The excellent agreement between the important structural parameters in these calculated structures and related crystal structures supports the reported performance of the PBE0 functional in geometry optimizations of organometallic gold(I) complexes.45 3.2. Natural Population and Natural Bond Orbital Analysis. To gain insight into charge distribution within the Au−alkene adducts, natural population40 analysis was used to determine the atomic charges. Moreover, for complexes 1a−g, natural bond orbital (NBO)41 analysis was used to quantify the electron occupancy of the alkene CC(π) and CC(π*) NBOs, which are related to the amount of ethylene(π)→Au and Au→ethylene(π*) back-donation, respectively.25 3.2.1. Impact of NHC Ligand Substitution in 1a−g. The occupancies of the ethylene CC(π) and CC(π*) NBOs in 1a−g indicate that, on average, ethylene(π)→[(X−NHC)Au]+ 6067


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The Journal of Physical Chemistry A

Figure 4. (a) Ethylene charge, (b) ethylene π-NBO electron depletion (e−), and (c) ethylene π*-NBO electron occupancy (e−) versus Hammett parameter (σp) for [(X−NHC)Au(C2H4)]+ complexes 1a−g.

occupied fragment orbital) with the LUFO (lowest unoccupied fragment orbital) of the [(CH3 −NHC)Au]+ fragment. Consistent with a Lewis acidic nature for the gold center, the [(CH3−NHC)Au]+ LUFO is localized predominantly on the Au ion and composed of contributions from Au(6s) and Au(6p) orbitals (54% and 19%, respectively). This interaction yields a number of stabilized MOs in the adduct, including the HOMO−7 and HOMO−14. Of particular significance is the low energy of these two MOs (Figure 6) relative to the πHOFO of free ethylene; these occupied MOs constitute the majority of contributions from the ethylene π-electron density to the complex, yet are 4−5 eV lower in energy than the πorbital of free ethylene. Moreover, the alkene π-electron density in these orbitals is significantly delocalized across the entire [(CH3−NHC)Au(C2H4)]+ complex rather than localized on the alkene. This stabilization of the ethylene HOFO and delocalization of the π-electron density away from the alkene are consistent with the gold(I) ion’s role as a Lewis acid. Also important are the LUMO+7 and LUMO+11, the lowest energy unoccupied MOs having ethylene HOFO character. The relatively low energy of these orbitals increases their ability to participate in reactivity with a nucleophile.22 By use of a similar analysis, the [(CH3−NHC)Au]+→C2H4 back-bonding interaction involves two filled orbitals on the [(CH3−NHC)Au]+ fragment: the HOFO−16 (79% Au(5d)) and the HOFO−4 (6% Au(5d); 81% CNHC ligand). These two orbitals, along with the LUFO+1, interact with the ethylene π*LUFO to yield the LUMO of the complex, which would most directly interact with a nucleophile (Figures 5 and 6). Importantly, the LUMO is over 5 eV lower in energy than the π*-orbital of free ethylene, yet it still retains 50% ethylene π*-LUFO character. The ability of the gold fragment to

interact with substituent atoms even in the free alkenes, the magnitude of alkene(π)→[(NHC)Au]+ and [(NHC)Au]+→ alkene(π*) electron donation is not easily determined using the electron population of these NBOs. While the alkene CC carbons generally have a negative atomic charge, two notable exceptions are CH3O−CHCH2 and NH2−CHCH2. The substituted C2 of these alkenes is predicted to have a significant positive charge, while the unsubstituted C1 has a much larger negative charge than the other alkenes (Table 2, Figure 3). The magnitude and sign of these atomic charges are consistent with the significant asymmetry observed in the binding geometry of these two alkenes both in this computational study and in related X-ray crystal structures.37,38,68 3.3. Molecular Orbital Analysis. Although the positive alkene charge predicted by NBO calculations is consistent with their electrophilic reactivity, a more detailed understanding of alkene activation in these complexes is obtained through analysis of their molecular orbitals. The calculated composition and energy of the molecular orbitals (MOs) of the [(NHC)Au(alkene)]+ complexes are analyzed in terms of contributions from occupied and unoccupied fragment orbitals (FOs) on the [(NHC)Au]+ and alkene fragments.51,52 In this fashion, bonding can be interpreted in terms of alkene→[(NHC)Au]+ and [(NHC)Au]+→alkene donation within the DCD framework, and the impact of ligand and alkene substituents can be compared. The bonding is qualitatively similar for all complexes in this study, and [(CH3−NHC)Au(C2H4)]+ (1e) is discussed here as a representative example (Figures 5 and 6). First focusing on the main Lewis acid component of the bonding, the C2H4→[(CH3−NHC)Au]+ interaction primarily involves overlap of the filled ethylene π-HOFO (highest 6068


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Figure 5. Orbital interaction diagram for [(CH3−NHC)Au(C2H4)]+, composed of orbitals from the [(CH3−NHC)Au]+ and C2H4 fragments (only selected orbitals and interactions shown).

arises from the alkene(π*)−gold interaction and not from the alkene(π)→gold σ-donation.26 3.3.1. Impact of NHC Ligand Substitution in 1a−g. To help understand ligand effects on the reactivity of these complexes, several metrics can be used to compare the orbital interactions between ethylene and the [(X−NHC)Au]+ fragments. First, the percent contribution of the occupied ethylene FOs to unoccupied MOs of the [(X−NHC)Au(C2H4)]+ complex provides a combined measure of ethylene→[(X−NHC)Au]+ donation and polarization within the ethylene molecule.51 Analogously, the percent contribution of the unoccupied ethylene FOs to occupied MOs provides a measure of [(X−NHC)Au]+→ethylene electron donation and ethylene polarization. Additionally, the LUMO energy reflects the stabilization of the ethylene π*-LUFO by the gold(I) center, while the energies of the occupied and unoccupied MOs having substantial ethylene π-HOFO character indicate the stabilization of the ethylene π-orbital by the [(X−NHC)Au]+ fragment. Although changes in π→Au σ-donation are often implicated in ligand-induced reactivity differences for these catalysts, the NHC substituent has a minimal effect on the contribution of occupied ethylene FOs to unoccupied MOs of the complex (Table S2, Supporting Information), suggesting little change in the Lewis acidity of the Au center with ligand substitution in this series. This does not mean, however, that ligand electronics would not affect the activity of these catalysts. Electronwithdrawing ligand substituents do yield a small and consistent decrease in the contribution of the unoccupied ethylene FOs to occupied MOs of the complex, signifying decreased [(X−

Figure 6. Selected [(CH3−NHC)Au(C2H4)]+ orbitals.

stabilize the ethylene LUFO while allowing it to remain largely localized on the alkene atoms is also consistent with its effectiveness in catalyzing the electrophilic reactivity of the alkene moiety. It is notable that this component of activation 6069


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Figure 7. (a) Percent (%) ethylene unoccupied fragment orbitals (UFOs) in occupied molecular orbitals (OMOs) and (b) LUMO energy versus Hammett parameter for [(X−NHC)Au(C2H4)]+ complexes 1a−g.

Table 3. EDA-NOCV Results for Au(I)−Alkene Complexesa complex

ΔEPAULI

ΔEELEC

ΔEORBb

ΔEORB(alkene→Au)b

ΔEORB(Au→alkene)b

ΔEPREP

ΔEBOND

1a 1b 1c 1d 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 3a 3b 3c 3d 3e 3f 3g 4

441.8 444.6 450.4 449.5 452.8 455.3 459.8 433.6 462.9 440.3 449.5 448.4 421.7 430.3 427.0 432.1 428.7 428.6 428.2 428.4 425.2 595.3

−397.1 −398.4 −401.9 −400.8 −402.3 −404.5 −407.4 −321.0 −374.4 −366.1 −400.8 −411.4 −409.4 −449.6 −352.3 −368.1 −375.0 −383.6 −388.5 −396.1 −404.2 −528.7

−239.7 −239.2 −240.0 −239.1 −239.5 −240.6 −242.4 −253.7 −251.6 −247.5 −239.1 −235.4 −223.7 −230.3 −246.6 −246.4 −246.7 −244.4 −245.3 −246.2 −248.2 −368.7

−129.3 −128.0 −127.1 −126.6 −125.9 −125.8 −126.0 −96.8 −112.5 −118.8 −126.6 −127.9 −142.6 −160.8 −125.9 −125.8 −129.5 −129.8 −131.8 −135.2 −141.4 −211.0

−70.4 −71.7 −73.6 −73.4 −74.4 −75.3 −76.9 −112.6 −94.2 −79.3 −73.4 −65.4 −40.9 −32.8 −72.9 −69.7 −68.2 −66.9 −65.2 −62.3 −57.6 −86.0

15.9 15.5 18.6 17.3 17.6 16.0 16.6 21.3 22.8 20.2 17.3 19.6 26.3 40.6 18.8 17.8 19.4 19.4 19.5 21.9 27.1 25.2

−179.2 −177.6 −173.0 −173.2 −171.4 −173.7 −173.5 −119.8 −140.3 −153.1 −173.2 −178.8 −185.1 −209.1 −153.1 −164.6 −173.6 −180.1 −186.0 −192.0 −200.2 −276.8

All energies in kJ/mol. bOnly the two largest components of ΔEORB are provided here. Further EDA-NOCV data are provided in Table S4 and Figure S10, Supporting Information.

a

NHC)Au]+→ethylene back-donation and ethylene polarization (Figure 7a), consistent with the NBO results.51 Moreover, electron-withdrawing ligand substituents substantially lower the energy of [(X−NHC)Au(C2H4)]+ orbitals most connected to electrophilic reactivity. This includes increased stabilization of the LUMO by nearly 0.7 eV across the series (Figure 7b) and stabilization of the occupied and unoccupied MOs having the largest ethylene π-HOFO character by ∼0.4 eV (Table S3, Supporting Information). Electron-withdrawing ligand substituents can therefore enhance the electrophilic activation of the ethylene without having a significant effect on ethylene→[(X−NHC)Au]+ donation and polarization.20,22 3.3.2. Impact of Alkene Substitution in 2a−g and 3a−g. For both series of substituted alkenes, electron-donating alkene substituents generally yield increases in combined alkene→[(NHC)Au]+ donation and alkene polarization with simultaneous decreases in combined [(NHC)Au]+→alkene back-donation and alkene polarization that largely offset one

another. Contribution of the alkene OFOs to UMOs increases by 6% from 2a−g and by 3% from 3a−g, while contribution of the alkene UFOs to OMOs decreases by 7% from 2a−g and by 2% from 3a−g (Figure S2a and Figure S2b, Supporting Information). Moreover, the LUMO of the complex is progressively stabilized by electron-withdrawing substituents in both series (by 1.9 eV in 2a−g and by 1.0 eV 3a−g), with the energy strongly correlated with electronic nature of the substituent (Figure S3a and Figure S3b, Supporting Information). Interestingly, these two major impacts of alkene substitution would be in opposition for enhancing electrophilic reactivity, with electron-donating substituents producing a more electron-deficient alkene fragment while increasing the complex LUMO energy. 3.4. Bond Energy Decomposition Analyses. The NBO and MO analyses provide important insight into orbital interactions and charge transfer in these systems. However, because noncovalent interactions dominate the bonding in 6070


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The Journal of Physical Chemistry A these complexes, a more complete description of bonding should be considered for a better understanding of their reactivity. Toward this goal, interactions between the [(NHC)Au]+ and alkene fragments have been studied using two complementary bond energy decomposition analyses (EDA) methods. First, the EDA-NOCV method combines the Morokuma−Ziegler ETS-EDA28,54,55 with the NOCV charge decomposition analysis.56,57 Within this method, the total bonding energy between the [(NHC)Au]+ and alkene fragments is determined using eq 1: ΔE BOND = ΔE PAULI + ΔE ELEC + ΔEORB + ΔE PREP

(1)

Here, ΔEPAULI is the Pauli repulsion between the two fragments, ΔEELEC is the electrostatic interaction between the fragments, and ΔEORB is the total favorable orbital interactions, including [(NHC)Au]+→alkene and alkene→[(NHC)Au]+ contributions as well as polarization within each fragment.58 The NOCV analysis allows ΔEORB to be further quantified in terms of pairwise orbital interaction energies and for charge flow between orbitals to be visualized as the deformation density (Δρ) associated with these orbital interactions.57,69 ΔEPREP is an additional term that represents the energy required to bring the isolated fragments into their respective geometries and electronic states within the [(NHC)Au(alkene)]+ complex. The EDA-NOCV method has been used successfully to study bonding interactions in other gold(I) complexes.69−71 The second method, the ALMO-EDA,64 allows the bonding energy between the [(NHC)Au]+ and alkene fragments to be calculated using eq 2: ΔE BOND = ΔE FRZ + ΔE POL + ΔECT + ΔE PREP

Figure 8. EDA-NOCV-calculated deformation density plots (Δρ) associated with the two major pairwise orbital interactions in 1d. The flow of charge is red → blue, with approximate orbital assignments shown along with orbital interaction energies (ΔE) and eigenvalues (ν) for each.

accumulation, with charge flowing from red to blue. These two interactions represent the ethylene(π)→Au(s,p) donation and Au(d)→ethylene(π*) donation, which constitute ∼84% of total orbital stabilization effects. From 1a to 1g, ethylene(π)→[(X−NHC)Au]+ stabilization energy decreases slightly, while [(X−NHC)Au]+→ethylene(π*) stabilization increases more substantially to yield an ∼3 kJ/mol increase in ΔEORB across the series. However, while electrostatic attraction is also enhanced by 10 kJ/mol, unfavorable Pauli repulsion increases by 18 kJ/mol from from 1a to 1g (Figure S4a and Figure S4b, Supporting Information). The significantly lower Pauli repulsion with −NO2 offsets the weaker electrostatic and orbital stabilization to yield a marginally stronger bond compared to −NH2. The ALMO-EDA predicts a similar weakening of the bond with the more electron-donating −NH2 substituent due to an increase in unfavorable ΔEFRZ that outweighs the enhanced [(X−NHC)Au]+→ethylene CT stabilization. Thus, while changes in Au→ethylene or ethylene→Au donation are often implicated as primary contributors to ligand-induced differences in gold−alkene bond strengths, this is not the case here. These results might seem to contradict the experimental observation that more electron-donating ligands yield more stable Au(I)−alkene adducts.2,17,67,75 However, it is worthwhile to note that a stronger gas-phase gold(I)−alkene bond does not necessarily mean that more gold(I)−alkene adduct would be observed under experimental conditions. [(L)Au(alkene)]+ complexes are typically synthesized in situ by metathesis reactions from (L)AuCl precursors and AgSbF6 or related salts. Widenhoefer and co-workers demonstrated that these metathesis reactions can result in complicated equilibria between the [(L)Au(alkene)]+ adducts, unreacted (L)AuCl starting material, and various decomposition products, depending on reaction

(2)

In this method, ΔEFRZ represents the combined exchange− correlation and Coulomb terms.64 ΔEPOL gives the polarization energy change resulting from the intramolecular relaxation of fragment orbitals.64 ΔECT is the charge-transfer (CT) stabilization between the two fragments, which is divided into the directional [(NHC)Au]+→alkene (ΔECT(Au→alk)) and alkene→[(NHC)Au]+ (ΔECT(alk→Au)) components, as well as higher order relaxation effects (ΔECT(HO)).64 For comparison, the Au(I)−ethylene bond energy in the free [Au(C2H4)]+ cation (4) is calculated to be −276.8 kJ/mol with the EDA-NOCV (Table 3 and Table S4, Supporting Information) and −251.7 kJ/mol with the ALMO-EDA (Tables S5, Supporting Information). These values compare well with the reported lower limits of the bond dissociation energy for this complex of 247−272 kJ/mol.72−74 Consistent with previous reports,30 the EDA-NOCV predicts electrostatic attraction to dominate stabilization (59%) and for ethylene(π)→Au(s) σ-donation to provide the major contribution to ΔEORB. The ALMO-EDA similarly predicts ΔECT(eth→Au) to play the dominant role in charge transfer stabilization within the complex.30 3.4.1. Impact of NHC Ligand Substitution in 1a−g. As with the bare [Au(C2H4)]+ cation, the EDA-NOCV predicts electrostatic attraction to dominate favorable bonding contributions in the NHC-ligated complexes, providing ∼62% of the stabilization. The overall [(X−NHC)Au]+−(C2H4) bond energy is predicted to weaken by ∼6 kJ/mol as the electrondonating ability of the substituent increases from 1a to 1g. Figure 8 shows the deformation densities, Δρ, for the two most important pairwise orbital contributions to ΔEORB in 1d; red indicates charge depletion while blue indicates charge 6071


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The Journal of Physical Chemistry A stoichiometry.17 Coordination strength of the anion has also been shown to affect the formation equilibria of the [(L)Au(alkene)]+ and related adducts,9,11,12 with more strongly coordinating anions reducing the amount of [(L)Au(alkene)]+ adduct observed.17 While the more facile formation of [(L)Au(alkene)]+ adducts with electron-rich ligands could be interpreted as a sign of stronger gold−π interactions, enhanced stability of the (L)AuCl precursors or unwanted products like [(L)Au−Cl−Au(L)]+ with less electron-donating ligands is an alternative possibility. To explore this option, the Au−Cl bond strength was calculated using the ETS-EDA for the representative complexes (NO2−NHC)AuCl and (NH2−NHC)AuCl (Table S6). The Au−Cl bond in (NO2−NHC)AuCl is predicted to be 39 kJ/ mol stronger than that in (NH2−NHC)AuCl, with most of the increase resulting from enhanced electrostatic attraction between the gold(I) and chloride ions. The breaking of (L)Au−Cl bonds would therefore be less favorable for (NO2− NHC)AuCl than for (NH2−NHC)AuCl, potentially leading to reduced formation of [(L)Au(alkene)]+ adducts with electronpoor ligands. Similarly, [(L)Au−Cl−Au(L)]+ or related species having Au−Cl interactions would be stabilized by electronwithdrawing ligands, potentially contributing to the perceived greater instability of [(L)Au(alkene)]+ complex with electronpoor ligands.17 Consistent with this hypothesis, the calculated free energy change for the equilibrium exchange reaction (Scheme 1) predicts the formation of [(NH2−NHC)Au-

equilibrium binding constant for 4-substituted styrene complexes with more electron-donating substituents.15,66 Interestingly, the stronger bonds are not due to enhanced orbital stabilization as might be anticipated with more electronrich alkenes. Instead, as predicted by the MO analyses, increases in alkene(π)→Au(s,p) stabilization with more electron-donating groups are offset by simultaneous decreases in Au(d)→alkene stabilization to yield a net decrease in ΔEORB from 2a to 2g (Figure 9b and Figure 9c) and no net change in ΔEORB from 3a to 3g (Figure S5b, Supporting Information). Rather, increases in electrostatic attraction of 129 and 52 kJ/ mol primarily contribute to the stronger interaction with electron-rich alkenes in these two series (Figures 9d and S5d, Supporting Information). These factors result in a strong correlation between the bond energy and the electrostatic attraction in both series (Figure S6a and Figure S6b, Supporting Information). The ALMO-EDA predicts similar trends in ΔEBOND resulting from enhanced alkene→[(NHC)Au]+ CT stabilization, decreased [(NHC)Au]+→alkene CT stabilization, and a decrease in unfavorable ΔEFRZ with the more electron-donating substituents (Figures S7a−d and S8a− d, Supporting Information). The bond strength is also wellcorrelated with ΔEFRZ (Figure S9a and Figure S9b, Supporting Information), providing further evidence that changes in Au− alkene bond strengths in the [(NHC)Au(alkene)]+ adducts are dominated by changes in electrostatic or Pauli repulsive effects and not orbital interactions.

Scheme 1. C2H4 and Cl− Exchange Reaction with [(NO2− NHC)Au]+ and [(NH2−NHC)Au]+ Fragments

4. CONCLUSION The PBE0 functional yields geometries of [(NHC)Au(alkene)]+ adducts that are in excellent agreement with related experimental structures, including the marked degree of η2 → η1 slippage observed in adducts of substituted alkenes. Natural population analysis of these complexes shows that alkene→[(NHC)Au]+ electron donation dominates the DCD bonding for all alkenes in this study, yielding positively charged alkene moieties. Electron-withdrawing ligand substituents in [(X−NHC)Au(C2H4)]+ complexes increase the positive charge on the alkene, with NBO calculations showing that this is due to a decrease in Au→ethylene back-donation and not from enhanced ethylene→Au donation as might be assumed for a Lewis acidic metal center. In contrast, more electron-donating alkene substituents in [(NHC)Au(X−CHCH2)]+ and [(NHC)Au(X−C6H4−CHCH2)]+ adducts yield a more positively charged alkene. Molecular orbital analysis reveals that binding of alkenes to an [(NHC)Au]+ fragment has several effects that would likely enhance their electrophilicity, including stabilization and delocalization of the alkene π-orbital and lowering of the alkene π*-LUFO by ∼5 eV. Both the EDANOCV and ALMO-EDA methods predict that electronwithdrawing ligand substituents yield a slightly stronger Au(I)−alkene bond, while electron-withdrawing alkene substituents result in a significantly weaker Au(I)−alkene bond. Consistent with standard DCD rationale, alkene→[(NHC)Au]+ stabilization is enhanced by electron-rich alkenes and electron-withdrawing ligand substituents. However, these increased orbital interactions are largely negated by simultaneous decreases in [(NHC)Au]+→alkene stabilization, yielding only a minor impact of orbital effects on the total bond energy. Instead, it is decreased Pauli repulsion (with electronwithdrawing ligand substituents) or increased electrostatic attraction (for electron-donating alkene substituents) that dominates the trends in overall bond stabilization. These

(C 2 H 4 )] + and (NO 2 −NHC)AuCl to be favored over [(NO2−NHC)Au(C2H4)]+ and (NH2−NHC)AuCl in both the gas phase and in dichloromethane despite the slightly stronger Au−C2H4 bond with the −NO2 substituent (ΔGgas = −33 kJ/mol; ΔGDCM = −12 kJ/mol). The equilibrium is therefore dominated by the significant differences in strength of the Au−Cl interaction and not the relatively small difference in Au−(C2H4) bond strengths. 3.4.2. Impact of Alkene Substitution in 2a−g and 3a−g. As with ethylene, electrostatic attraction is the major contributor to bonding in 2a−g and 3a−g, accounting for 56−66% of the total stabilization. However, while ligand substitution only has a minor effect on Au−alkene bond strengths in 1a−g, the EDA-NOCV predicts ΔEBOND to become more favorable by 89 kJ/mol from 2a to 2g and by 47 kJ/mol from 3a to 3g. This increase in bond strength is strongly correlated with the σp value of the substituent (Figures 9a and S5a, Supporting Information) and is consistent with reports by Widenhoefer and co-workers of an increase in the 6072


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Figure 9. EDA-NOCV calculated values of (a) ΔEBOND, (b) ΔEORB(alk→Au), (c) ΔEORB(Au→alk), and (d) ΔEELEC versus Hammett parameter (σp) for [(NHC)Au(X−CHCH2)]+ complexes 2a−g. (4) Corma, A.; Leyva-Pérez, A.; Sabater, M. J. Gold-Catalyzed Carbon−Heteroatom Bond-Forming Reactions. Chem. Rev. 2011, 111, 1657−1712. (5) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Ligand Effects in Homogeneous Au Catalysis. Chem. Rev. 2008, 108, 3351−3378. (6) Shen, H. C. Recent Advances in Syntheses of Heterocycles and Carbocycles via Homogeneous Gold Catalysis. Part 1: Heteroatom Addition and Hydroarylation Reactions of Alkynes, Allenes, and Alkenes. Tetrahedron 2008, 64, 3885−3903. (7) Gorin, D. J.; Toste, F. D. Relativistic Effects in Homogeneous Gold Catalysis. Nature 2007, 446, 395−403. (8) Zuccaccia, D.; Belpassi, L.; Rocchigiani, L.; Tarantelli, F.; Macchioni, A. A Phosphine Gold(I) π-Alkyne Complex: Tuning the Metal−Alkyne Bond Character and Counterion Position by the Choice of the Ancillary Ligand. Inorg. Chem. 2010, 49, 3080−3082. (9) Zuccaccia, D.; Belpassi, L.; Macchioni, A.; Tarantelli, F. Ligand Effects on Bonding and Ion Pairing in Cationic Gold(I) Catalysts Bearing Unsaturated Hydrocarbons. Eur. J. Inorg. Chem. 2013, 2013, 4121−4135. (10) Ciancaleoni, G.; Biasiolo, L.; Bistoni, G.; Macchioni, A.; Tarantelli, F.; Zuccaccia, D.; Belpassi, L. NHC-Gold-Alkyne Complexes: Influence of the Carbene Backbone on the Ion Pair Structure. Organometallics 2013, 32, 4444−4447. (11) Biasiolo, L.; Del Zotto, A.; Zuccaccia, D. Toward Optimizing the Performance of Homogeneous L-Au-X Catalysts through Appropriate Matching of the Ligand (L) and Counterion (X−). Organometallics 2015, 34, 1759−1765. (12) Zuccaccia, D.; Belpassi, L.; Tarantelli, F.; Macchioni, A. Ion Pairing in Cationic Olefin−Gold(I) Complexes. J. Am. Chem. Soc. 2009, 131, 3170−3171. (13) Marchione, D.; Belpassi, L.; Bistoni, G.; Macchioni, A.; Tarantelli, F.; Zuccaccia, D. The Chemical Bond in Gold(I)

results provide new insight into the binding of alkenes to [(NHC)Au]+ fragments and the nature of the electrophilic activation of the bound alkene.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03819. Supporting figures, tables, and optimized atomic coordinates and energies for all compounds (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 386-822-8181. Notes

The author declares no competing financial interest.

■ ■

ACKNOWLEDGMENTS Acknowledgement is made to Stetson University and to the ACS-PRF (Award 48483-GB3) for financial support. REFERENCES

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Determining the Impact of Ligand and Alkene Substituents on Bonding

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