Investigation of Geminally Diaurated Arene Complexes in the Gas Phase

Aug 4, 2015 - The stability of gem-diaurated arene complexes [Au2(L)2(μ-aryl)]+ has been investigated by collision-induced dissociation (CID) experim...
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Investigation of Geminally Diaurated Arene Complexes in the Gas Phase Jiří Schulz,* Elena Shcherbachenko, and Jana Roithová* Department of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague 2, Czech Republic

Organometallics 2015.34:3979-3987. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/04/18. For personal use only.

S Supporting Information *

ABSTRACT: The stability of gem-diaurated arene complexes [Au2(L)2(μ-aryl)]+ has been investigated by collision-induced dissociation (CID) experiments and density functional theory (DFT) calculations. Performed mass spectrometric experiments revealed the influence of arene-ring substituents and a gold supporting ligand L on the stability of the corresponding diaurated complexes. From the determined appearance energies it emerged that the electron-donating substituents (p-MeO, p-Me, m-MeO, m-Me) strengthen, while the electron-withdrawing ones (p-Cl, p-CN, p-NO2, m-Cl, m-CN, m-NO2) weaken the three-center two-electron bond. More stable gem-diaurated complexes were found with electron-poor supporting ligands. It was found, however, that the electronic influences can be surpassed by the steric factors. Experimental results agree well with the performed DFT calculations at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory.

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INTRODUCTION Dinuclear gold complexes have been recently recognized as possible intermediates involved in gold-promoted catalytic transformations of alkynes. Generally, two classes of diaurated complexes are frequently mentioned in the context of gold catalysis, namely, σ,π-dicoordinated acetylides1 and gemdiaurated vinyl and aryl complexes.2 Collected experimental evidence convincingly showed that two gold centers can efficiently cooperate in the catalysis of chemical reactions via σ,π-activation of polyunsaturated substrates, typically diynes3 or allenynes.4 While the concept of dual σ,π-activation of a single substrate is being widely accepted,5,6 the role of gem-diaurated complexes in gold catalysis remains still a subject of debate. Currently, the prevailing understanding is that gem-diaurated complexes are formed during the catalytic cycle rather as catalyst resting states than genuine reactive intermediates.7 Still, the current upswing of gold catalysis brought about renewed interest in investigation of these species with particular focus on their preparation and structural characterization.8,9 The groups of Gray9a and Nolan9b have developed convenient and general synthetic procedures affording gem-diaurated complexes smoothly and in high yields starting from arylboronic acids and suitable gold precursors. Our group reported a mechanistic study concerning the role of gem-diaurated complexes in the addition of methanol to alkynes catalyzed by Au(PMe3)+.10 Based on electrospray ionization mass spectrometric (ESI-MS) experiments, NMR kinetics, infrared multiphoton dissociation spectroscopy (IRMPD), and DFT calculations, it was suggested that the gem-diaurated species are formed as reaction intermediates and © 2015 American Chemical Society

that their formation is connected with a dual activation. As a part of our ongoing research efforts aimed at the study of reaction intermediates involved in gold promoted transformations,10,11 we have decided to investigate a group of gem-diaurated complexes derived from selected arylboronic acids by means of energy-dependent collision-induced dissociation (CID) experiments. The obtained experimental results are supported by quantum chemical calculations.

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EXPERIMENTAL DETAILS

Mass spectrometry measurements were performed with a Finnigan LCQ Deca ion trap mass spectrometer.12 The mass spectrometer is equipped with a conventional ESI source consisting of a spray unit that is followed by a heated capillary and two transfer octopoles. Generated ions are stored within a Paul ion trap, which also allows for a further manipulation including various MSn experiments. For detection, the ions are ejected from the ion trap to an electron multiplier. The collisional activation in CID experiments is achieved by collisions with helium buffer gas (pressure of He within the ion trap is ∼10−3 mbar) induced by applying of an excitation ac voltage to the end-caps of the ion trap. Generally, the mass spectra were collected using soft ionization conditions (low potentials on the first set of lenses) with the heated capillary kept at 200 °C, the spraying voltage set to 6 kV, and a flow rate of 5.0 μL min−1. All spectra were recorded in the positive-ion mode. For the CID experiments an excitation period of 30 ms and a trapping parameter qz = 0.25 was used. As the excitation of massselected ions for CID experiments may be influenced also by the chosen isolation width, the isolation width of 2 amu was used uniformly for all MSn experiments (being generally sufficient for Received: April 24, 2015 Published: August 4, 2015 3979

DOI: 10.1021/acs.organomet.5b00343 Organometallics 2015, 34, 3979−3987

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Scheme 1. Generation of gem-Diaurated Complexes 2−9

separation of single isotopes). The experimental appearance energies (AEs) of the observed fragmentation channels were extracted from the energy-resolved CID experiments applying a calibration of the collision-energy scale introduced by Schröder.13,14 This calibration scheme is based on the correlation of experimental AEs with theoretical bond-dissociation energies (BDEs) for a set of reference ions. The breakdown curves were modeled by sigmoid functions using a least-squares method.15 The AEs were then obtained by a linear extrapolation of the rise of the sigmoid functions at E1/2 to the baseline. The intensities due to the ions formed by consecutive dissociations were added to the intensity of the primary fragment. The geometry optimizations and thermochemistry calculations were performed using the density functional theory method mPW1PW9116 as implemented in the Gaussian 09 package.17 As a basis set, the combination of the cc-pVDZ basis set for C, H, O, N, F, P, and Cl and the LanL2DZ basis set for Au was used (denoted as ccpVDZ:LanL2DZ(Au) in the following). All minima and transition structures were verified by the analysis of their Hessian matrixes. The geometry-optimized structures and their energies can be found in the Supporting Information. The performance of a series of DFT functionals with and without inclusion of Grimme’s D3 dispersion correction18 was tested on a selected model system (complex 2a). The geometry optimizations were performed using the functional of choice with the cc-pVDZ:LanL2DZ(Au) basis set. The used functionals encompass M06,19 B97D3 (reparametrized version of Becke’s B97 functional with built-in D3bj dispersion correction),20 BP86,21 and PBE0.22 A summary of the obtained results is given in Table S11 and Figure S41 in the Supporting Information.

yielding two fragmentation channels, A and B (Scheme 2). As a representative example, the CID spectrum of complex 2a

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RESULTS Previously, the groups of Gagné and Maier investigated the stability of gem-diaurated complexes in the solution by NMR spectroscopy. Gagné et al. studied directly the equilibrium between monoaurated and diaurated aryl complexes derived from Au(PPh3)+,7b while Maier and Zhdanko determined relative stabilities in the set of enol ether-derived gem-diaurated complexes.23 Maier and Zhdanko used equilibrium constants (Keq) of the reaction between diaurated species and suitable nucleophiles to probe various structural effects influencing the stabilities of diaurated complexes. Both research groups concluded that electron-rich ligands (either aryl or vinyl) promote the formation of gem-diaurated species, unless outweighed by the steric influences. In addition, Maier and Zhdanko focused also on the influence of the supporting ligand L on gold with the same conclusion. According to their data, electron-rich supporting ligands stabilize gem-diaurated complexes, unless they are too bulky. We have performed a gasphase study in order to determine energy demands associated with the fragmentation of gem-diaurated complexes. Our aim was to assess how the arene moiety and the ancillary ligand L influence the stability of corresponding diaurated species in the gas phase. Collision-Induced Dissociation Experiments. Geminally diaurated arene complexes 2−9 were prepared in situ by transmetalation from arylboronic acids 1a−k to the respective gold complexes using a previously described synthetic approach (Scheme 1). 9b Readily formed diaurated complexes [Au2(L)2(μ-aryl)]+ were transferred to the gas phase by electrospray ionization, isolated by the ion trap, and studied by collision-induced dissociation experiments. First of all, we focused on diaurated complexes derived from Au(PMe3)+ and selected arylboronic acids bearing various electron-donating (p-OMe, p-Me, m-OMe, m-Me) and electron-withdrawing substituents (p-Cl, p-CN, p-NO2, m-Cl, m-CN, m-NO2). Complexes 2a−k dissociate upon CID,

Scheme 2. Collision-Induced Dissociation (CID) Reactions Observed for Investigated gem-Diaurated Arene Complexes

is shown in Figure 1. Surprisingly, the preferred dissociation pathway of complexes 2a−k corresponds to the formation of Au(PMe3)2+ (dissociation channel A).

Figure 1. CID spectrum of gem-diaurated complex 2a. The inset shows the energy dependence of parent ion fragmentation and the linear extrapolation of the sigmoidal fit to determine the appearance energies (AEs) of the dissociation channels A (blue) and B (red): The dots represent the experimental data points; the lines show the sigmoid fits used for analysis. 3980

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Organometallics This reaction is a stepwise process involving transmetalation of a trimethylphosphine ligand between two gold centers and subsequent expulsion of neutral arylgold(I) (vide inf ra). The second observed dissociation channel corresponds to the simple rupture of a three-center two-electron bond yielding Au(PMe3)+ (dissociation channel B). Alternatively, gemdiaurated species can dissociate by the loss of one of the supporting ligands (dissociation channel C, vide inf ra). Dissociation channel A is common for all of the investigated complexes and represents therefore a good measure of their stability. We used energy-dependent CID experiments to determine energy demands for the dissociation of complexes 2a−k.13,14 The appearance energies obtained for the individual fragmentation channels can be related to the corresponding bond-dissociation energies predicted by quantum chemical calculations. The experimental AEs determined for the dissociation channel A for complexes 2a−k are summarized in Table 1 along with the corresponding theoretical BDE values. The obtained AEs range from 29.4 ± 0.1 to 35.0 ± 0.6 kcal mol−1 and correspond very well with the computed BDEs. Table 1. Appearance Energies (AEs) and Corresponding Bond-Dissociation Energies (BDEs) for Fragmentation Channel A (kcal mol−1) of gem-Diaurated Complexes 2a−k

Figure 2. Plots of experimental appearance energies AEs (a) and bond-dissociation energies BDEs (b) (red dots) for the fragmentation channel A of gem-diaurated complexes 2a−k vs the corresponding Hammett σ constants. The solid lines represent the linear fits of the data points; note that the data points of p-CN, m-CN, and p-NO2 substituents (blue squares) were omitted from the correlation (see the text).

dissociation channel A substituent R

Hammett σa

H (2a) p-MeO (2b) p-Me (2c) p-Cl (2d) p-CN (2e) p-NO2 (2f) m-MeO (2g) m-Me (2h) m-Cl (2i) m-CN (2j) m-NO2 (2k)

0 −0.27 −0.17 0.23 0.66 0.78 0.12 −0.07 0.37 0.56 0.71

BDEb

AE 32.5 35.0 34.4 31.8 33.8 29.4 34.0 33.9 31.4 32.9 29.4

± ± ± ± ± ± ± ± ± ± ±

0.3 0.6 0.1 0.3 0.7 0.3 0.1 0.1 0.1 0.1 0.1

33.9 38.0 35.5 32.7 28.7 28.1 34.6 34.6 32.1 29.6 29.6

excluded from the linear correlation also complex 2f for the same reason (vide inf ra). Next, we have determined the AEs for the dissociation channel B, i.e., the liberation of Au(PMe3)+, for complexes 2a− k (see Table S1 in the Supporting Information). The experimental AEs determined for this dissociation channel fall into a relatively narrow range from 33.1 ± 0.1 to 37.8 ± 0.2 kcal mol−1. The corresponding computed BDEs do not agree at all with the experimental values, being in range from 49.2 to 62.7 kcal mol−1. This finding might be attributed to a rather low abundance of the Au(PMe3)+ elimination channel (less than 7%) in the 2a−k fragmentation. More probably, however, a subsequent fragmentation of daughter ions Au(PMe3)2+ at elevated collision energies contributes to the determined Au(PMe3)+ abundances, which hampers a precise determination of the respective AEs.26,27 Our speculation follows from the shape of the energy-dependent CID curves (see inset in Figure 1 and Figures S19−29 in the Supporting Information). It can be seen that at the elevated collision energies the Au(PMe3)+ abundance grows at least partially at the expense of the Au(PMe3)2+ abundance. The Hammett plot for the dissociation channel B constructed from the theoretical BDEs can be found in the Supporting Information (Figure S37). We have added also a plot for the AEs for completeness. We continued our study by the evaluation of electronic and steric effects of the gold supporting ligands L on the stability of the corresponding diaurated species. Hence, we moved our attention to a series of gem-diaurated complexes derived from phenylboronic acid (1a) and gold cations bearing different phosphine, phosphite, or N-heterocyclic carbene ligands (see Scheme 1). The species of interest were obtained in a similar fashion to those supported by trimethylphosphine and

a Hammett para (σp) and meta (σm) constants. bmPW1PW91/ccpVDZ:LanL2DZ(Au) energies including zero-point energy corrections.

The electronic effects can be analyzed by a correlation between AEs, BDEs, and the corresponding Hammett σ constants.24 Hammett constants provide a measure of substituent resonance and inductive electron-donating and electron-withdrawing abilities. Figure 2 shows the plots of obtained AEs (a) and BDEs (b) as a function of the corresponding Hammett σ constants of the arene ring substituents. Interestingly, we found out that the correlation between the AEs and the Hammett σ constants can be significantly improved (from r = −0.76 to r = −0.95) by omitting the data points for the cyano substituents from the linear regression.25 On the contrary, the computed BDEs correlate very well (r = − 0.98) with the respective σ constants even when all of the substituents are included in the fitting procedure. The observed deviation of the experimental AEs of the cyano-substituted diaurated complexes from the computationally predicted trend suggests that they likely do not correspond to the expected gem-diaurated structures 2e and 2j. Presumably, these complexes adopt structures lacking aurophilic interaction in the gas phase, in which the cyanophenyl ring behaves as a C,N-bridging ligand. Note that we have 3981

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energy dependence of the fragmentation of complex 6 (Figure S33). Similarly to the energy-dependent CID spectra of complexes 2a−k, the abundance of the Au(JohnPhos) + daughter ion grows at the expense of the ion 12JohnPhos. The energy-dependent CID curves of 6 can be therefore evaluated by treating Au(JohnPhos)+ either as a product of the secondary fragmentation (and add together with the intensity of the primary fragment) or as a primary dissociation product of reaction B (and determine the corresponding AE). Importantly, we found out that the choice of the fitting approach does not affect significantly the obtained AEs of the remaining dissociation channels (see Table S1 in the Supporting Information). We note in passing that the liberation of Au(JohnPhos)+ is in part accompanied by the dissociation of one or even more tert-butyl groups from the phosphine ligand, releasing isobutene or tert-butyl radical (Figure S15 in the Supporting Information). The intensities of these secondary fragments were added to the intensity of the Au(JohnPhos)+ daughter ion. The dissociation behavior of complex [Au2(dppp)(Ph)]+ (9) is largely influenced by the chelating nature of the dppp bidentate ligand. The loss of phenylgold from 9 yields highly strained bent dicoordinated gold fragment Au(dppp)+, which is prone to eliminate another phenylgold molecule and yield phosphine-stabilized phosphenium cation [Ph2P(CH2)3PPh]+.28 This fragmentation was again for the fitting added to the primary fragmentation channel. Note that although we have identified the fragment ions [Au2(tBuOP)(Ph)]+ (12tBuOP) due to the loss of a phosphite ligand (dissociation channel C) in the CID spectra of complex [Au2(tBuOP)2(Ph)]+ (8), the very low intensity of this dissociation channel did not allow us to determine the AE. Density Functional Theory Calculations. With the aim to locate possible intermediates and transition states along the observed dissociation pathways of the investigated gemdiaurated complexes we have performed density functional theory calculations. For the sake of completeness we included all of the primary dissociation channels (A, B, and C) in our calculations regardless of being observed for the specific ion or

characterized by a measurement of energy-dependent CID curves. Fragmentation channel A, i.e., the loss of phenylgold, is the only observed dissociation pathway for most of the investigated complexes. The exceptions are the species derived from bulky ligands JohnPhos (6), tBuOP (8), and chelating ligand dppp (9). The appearance energies for the dissociation channel A obtained from the threshold measurements are presented in Table 2 together with BDEs predicted by the DFT calculations. Table 2. Appearance Energies (AEs) and Corresponding Bond-Dissociation Energies (BDEs) for the Fragmentation Channel A (kcal mol−1) of gem-Diaurated Complexes 3−9 dissociation channel A ligand L PPh3 (3) P(p-Tol)3 (4) P(TTPP)3 (5) JohnPhos (6) IPr (7) tBuOP (8) dppp (9)

BDEa

AE 34.1 35.5 36.5 36.0 38.3 40.5 44.0

± ± ± ± ± ± ±

0.2 0.1 0.7 0.5 1.0 0.5 0.3

32.1 30.6 36.8 37.1 36.1b 36.6 46.0

a

mPW1PW91/cc-pVDZ:LanL2DZ(Au) energies including zero-point energy corrections. bEnergy of the transition state TS7−7Int preceding the final dissociation (see the section on DFT calculations).

All of the primary dissociation channels A, B, and C can be observed in the CID spectra of complex [Au2(JohnPhos)2(Ph)]+ (6). The most abundant dissociation channel corresponds to the loss of one of the phosphine ligands (reaction C in Scheme 2) yielding digold fragment [Au2(JohnPhos)(Ph)]+ (12JohnPhos). This daughter ion can be viewed as a molecule of phenylgold tagged with the gold cation Au(JohnPhos)+. We assume that the subsequent loss of neutral phenylgold from 12JohnPhos can liberate the Au(JohnPhos)+ cation and contribute to the abundance of the primary daughter ions from reaction B. This assumption can be inferred from the

Figure 3. Schematic representation of the potential energy surface for the observed dissociation reactions of gem-diaurated complex 2a. Geometry optimizations and thermochemistry calculations were performed at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections). 3982

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Organometallics Table 3. Relative Energies for the Observed Dissociation Pathways of gem-Diaurated Complexes 3−9a dissociation channel A

B

ligand

TSInt

Int

10+AuL2+

PPh3 (3) P(p-Tol)3 (4) P(TTPP)3 (5) JohnPhos (6) IPr (7) tBuOP (8) dppp (9)

28.3 26.4 31.8 29.7 36.1 36.2

22.0 20.2 25.8 24.2 25.4 20.4

32.1 30.6 36.8 37.1 30.4 36.6 46.0

AE 34.1 35.5 36.5 36.0 38.3 40.3 44.0

± ± ± ± ± ± ±

C +

11+AuL 0.2 0.1 0.7 0.5 1.0 0.3 0.3

55.4 53.5 56.9 44.5 54.5 55.5

AE

37.7 ± 0.4

AE

12+L 56.4 57.3 51.5 35.9 70.0 31.4

36.0 ± 0.2 −b

a

mPW1PW91/cc-pVDZ:LanL2DZ(Au) energies including zero-point energy corrections. bThe AE was not determined due to the very low abundance of the dissociation channel C.

Alternative dissociation pathways proceed in a single step and correspond either to the simple rupture of the three-center twoelectron bond (pathway B) or to the loss of one of the supporting ligands L on gold (pathway C). Interestingly, the loss of PMe3 (pathway C) from 2a is favored by 1.0 kcal mol−1 over the liberation of Au(PMe3)+ (channel B). The dissociation of the ancillary ligand is even more favored in the case of diaurated complexes bridged by electron-rich aryl ligands, i.e., those bearing electron-donating substituents MeO (2b, 2g) and Me (2c, 2h) (see Table S10 in the Supporting Information). Hence, if Au(PMe3)+ would be a product of the primary dissociation of the parent diaurated complexes, we should also observe elimination of PMe3 (pathway C). The fact that the loss of PMe3 is not observed upon CID of 2a−c and 2g,h supports our assumption that Au(PMe3)+ is formed by the secondary fragmentation of primary fragment Au(PMe3)2+. From the performed DFT calculations emerges that the endothermicity of the dissociation channel A is given for most of the investigated complexes by the threshold for ultimate dissociation yielding arylgold 10 and Au(L)2+. The transmetalation step brings about an additional barrier in an excess of endothermicity only in the case of the most bulky and highly electron-donating ligand IPr. Predicted BDEs for the dissociation channel A range from 30.6 kcal mol−1 for the most loosely bound complex (4) to 46.0 kcal mol−1 for the most tightly bound complex (9). Importantly, the activation barriers for dissociation channel A lie for most of the investigated complexes about 20 kcal mol−1 below the dissociation thresholds of channels B and C. These computational results correspond well with the fact that the loss of arylgold is either the dominant or the only observed dissociation channel in CID experiments. As mentioned previously, the exceptions are digold complexes 6, 8, and 9. Dissociation behavior of these ions stems either from the ability of the supporting ligands JohnPhos and tBuOP to stabilize fragment ions by cation−π interactions or from the chelating nature of dppp (vide inf ra). The calculated activation barriers for the dissociation channels A, B, and C of complex 6 bearing the JohnPhos ligand amount to 37.1, 44.5, and 35.9 kcal mol−1, respectively. The energies found for the dissociation pathways B and C considerably drop in comparison with the other complexes due to the additional stabilization of the fragments by the cation−π interaction (see Figure 5). We have estimated the additional stabilization from different conformations of the fragment ion 12JohnPhos as 23.9 kcal mol−1 (Figure 5). Gold cation Au(JohnPhos)+ is stabilized in a similar way (stabilization

not. Figure 3 shows the schematic potential energy surface (PES) for the fragmentation of trimethylphosphine-supported diaurated complex 2a. Table 3 summarizes the relative energies calculated for the remaining diaurated ions (3−9) derived from phenylboronic acid (1a). Calculated relative energies for species 2b−k can be found in the Supporting Information (Table S10). Calculated structures of important intermediates, transition states, and fragments located along the dissociation pathways of complex 2a are shown in Figure 4.

Figure 4. Calculated intermediates and transition states along the dissociation pathways of gem-diaurated complex 2a. Relative energies are calculated at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections). Selected bond lengths are given in Å.

As illustrated by the PES depicted in Figure 3 for the dissociation of diaurated complex 2a, dissociation channel A is a stepwise, continuously endothermic process that is triggered by the transmetalation of the trimethylphospine ligand from one gold atom to another via transition state TS2a−2aInt. The transmetalation reaction yields T-shaped intermediate 2aInt, which can be viewed as a molecule of phenylgold capped through aurophilic interaction with bis(trimethylphosphino) gold cation Au(PMe3)2+. Intermediate 2aInt dissociates without any additional barrier by breakage of the gold−gold interaction, affording phenylgold 10a and Au(PMe3)2+. 3983

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Figure 5. Possible geometry-optimized structures of fragment ion 12JohnPhos in the gas phase. Relative energies are calculated at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections). Selected bond lengths are given in Å. Figure 6. Potential-energy surface for the expulsion of phenylgold from gem-diaurated complex 9. Relative energies were calculated at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections).

−1

amounts to 10.9 kcal mol ; see Figure S42 in the Supporting Information). As a result, neither the loss of the supporting ligand (channel C) nor the dissociation of Au2C bond (channel B) can be considered as a simple bond-breaking reaction, but rather as a stepwise dissociation−association process.29,30 We note that the secondary interactions contribute also to the stability of the parent ion 6 (stabilization amounts to 7.7 kcal mol−1; see Figure S43 in the Supporting Information). Stabilization due to the cation−π interactions renders the loss of the supporting ligand tBuOP from complex 8 the lowest-energy dissociation pathway (see Figure S44 in the Supporting Information). It seems, however, that this dissociation channel is kinetically hindered due to the structural rearrangement that is required along the reaction coordinate to form the fragment ion 12tBuOPA stabilized by the cation−π interaction. The loss of the supporting ligand is therefore observed only as a minor dissociation channel with very low abundance in the CID spectra of 8. In comparison with the diaurated complexes derived from the monodentate ancillary ligands, only the dissociation channel A can lead to a dissociation of complex 9 (i.e., change of m/z) due to the chelating nature of dppp. Thus, the only experimentally observable primary dissociation channel of 9 is the expulsion of phenylgold (BDE amounts to 46.0 kcal mol−1). Nevertheless, the formation of species corresponding formally to the dissociation channels B and C is less energy demanding (see Figure S45 in the Supporting Information). Our attempts to localize transition state TS9−9Int and intermediate 9Int have failed, presumably due to the highly strained disposition of these species. We therefore sought possible alternative mechanisms for the expulsion of phenylgold from 9. Alternatively, the expulsion of phenylgold from 9 is preceded by the rupture of a three-center two-electron bond that is accompanied by concomitant migration of the departing gold atom to the phenyl group of the adjacent diphenylphosphinogold unit through transition state TS9−11Int (24.1 kcal mol−1; Figure 6). This process leads to gold complex 11Int stabilized by the aurophilic interaction (Figure 7). Direct dissociation of phenylgold from intermediate 11Int yields the Au(dppp)+ cation. Finally, we have focused on the elucidation of the gas-phase structures of putative gem-diaurated complexes 2e and 2j derived from the cyanophenylboronic acids. Obvious alternative structures correspond to the C,N-dicoordinated

Figure 7. Calculated structures of transition state TS9−11Int and intermediate 11Int. Relative energies were calculated at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections). Selected bond lengths are given in Å.

complexes 2e′ and 2j′ lying about 5 kcal mol−1 lower in energy than the isomeric gem-diaurated species (see Figure 8). We were able to locate transition states TS2e′−2e (26.8 kcal mol−1) and TS2j′−2j (26.5 kcal mol−1) for the transfer of Au(PMe3)+ from the cyano group to the arene ring. On the contrary, our attempts to find any local minima or transition states for a “ring walk” of the migrating gold unit around the arene ring failed.11a We note in passing that although the rearrangement to form gem-diaurated complexes from 2e′ and 2j′ is highly energy-demanding, the activation barriers lie below the threshold energies for all of the observed dissociation reactions. The fact that the calculated BDEs for the expulsion of phenylgold from C,N-dicoordinated complexes 2e′ (34.7 kcal mol−1) and 2j′ (34.3 kcal mol−1) match well the AEs determined for this dissociation channel (33.4 and 32.9 kcal mol−1, respectively) supports our structural assignment. These computational results underline the importance to consider all of the possible linkage isomers in the case of the diaurated complexes derived from potentially ambidentate aryl ligands, i.e., those bearing methoxy, chloro, and nitro substituents. Our calculations confirm that the gem-diaurated isomers represent the preferred structure for a majority of investigated complexes 3984

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S39 in the Supporting Information). Both parent ions as well as the aryl-containing fragment are positively charged in this channel. Both structures will be therefore stabilized by the electron-donating substituents. The positive ρ value means that the fragment ion is more electronically influenced by the substituent than the parent complex. The effect of the ligand L on the stability of the diaurated complexes is less straightforward than in the case of the aryl ligands. The reason is that not only the electronic properties but also the steric bulk plays a major role. The performed DFT calculations predict that the stability of diaurated complexes derived from triarylphosphine ligands PPh3 (3), P(p-Tol)3 (4), and TTPP (5), with rather similar steric properties but substantially different electron-donating abilities, decreases with the increasing donating ability of the ligand. Hence, the least stable is complex 4, bearing the electron-rich ligand P(pTol)3, while the most stable one is complex 5, supported by the electron-poor ligand tris(p-trifluoromethylphenyl)phosphine (TTPP). The increasing BDE of [L2Au]+ toward an aryl gold moiety with decreasing electron-donating ability of L is expected based on the electronic considerations. This finding is, however, in direct contradiction with the conclusions made by Maier and Zhdanko in their recent study.23 They have compared stabilities of gem-diaurated enol ethers (E) in an equilibrium reaction with picoline:

Figure 8. Potential-energy surface for the rearrangement of C,Ncoordinated complexes 2e′ and 2j′ (energies in parentheses) to the corresponding gem-diaurated species 2e and 2j. Relative energies calculated at the mPW1PW91/cc-pVDZ:LanL2DZ(Au) level of theory (kcal mol−1, including zero-point energy corrections).

(see Figure S46 in the Supporting Information). However, there are exceptions of the cyano-substituted species and paranitro-substituted complex 2f. For the nitro-substituted complex, the alternative structure with the trimethylphosphinogold unit coordinated to the nitro group is favored by 0.5 kcal mol−1. Accordingly, the BDE for the expulsion of arylgold 10f from 2f′ (28.6 kcal mol−1) is closer to the corresponding experimental AE (29.4 kcal mol−1) than that of 2f (28.1 kcal mol−1). The increased stability of 2f′ with respect to 2f is given by the electron-withdrawing inductive effect that simultaneously destabilizes the three-center two-electron bond in 2f and increases the electron-donating ability of the nitro group in 2f′.

[(E)Au 2(L)2 ]+ + picoline → [(E)Au(L)] + [(picoline)Au(L)]+

The establishment of an equilibrium in this reaction can be interpreted as a competition between picoline and monoaurated enol ether [(E)Au(L)] for the binding with the [Au(L)]+ cation. The more electron deficient L is, the more the equilibrium is shifted toward the formation of [(picoline)Au(L)]+. This is in perfect agreement with their observations. The translation of these results for the stabilities of the diaurated complexes is however indirect and resulted in their contradicting conclusion. Rather similar BDEs were obtained for the complexes bearing bulky ligands JohnPhos (6), IPr (7), and tBuOP (8). This finding is noteworthy when we consider entirely different electronic properties of these ligands, i.e., strong σ-donating character of JohnPhos and IPr vs poor σdonating and moderate π-accepting ability of tBuOP. The most tightly bound complex from the investigated species is [Au2(dppp)(Ph)]+ (9) derived from the chelating ligand dppp. This complex owes its unexpected stability mainly to the steric constraints induced by the supporting ligand. Note that the BDEs are influenced by two factors: (i) the stability of the diaurated complexes and (ii) the stability of the fragments. Hence, the large BDE found for the complex [Au2(dppp)(Ph)]+ (9) is mainly due to the large strain in the fragment ion [Au(dppp)]+. The experimental AEs for the dissociation channel A follow the trend outlined by theoretical calculations with one exception. The AE determined for complex 4 is considerably higher than the computed BDEs. This discrepancy is in line with the fact that BDEs seem to be systematically underestimated in comparison with the experimental values.

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DISCUSSION From the performed energy-resolved CID experiments it emerges that the three-center two-electron bond Au2C is stabilized by electron-donating substituents of the arene ligand while weakened by electron-withdrawing ones. The influence of the arene-ring substituents is well illustrated by the obtained Hammett slopes (ρ) of the BDEs. Channels A and B lead to neutral arylgold fragments; the effect of the substitution is therefore more pronounced in the stabilization of the positive charge of the parent ion. The more electron-donating the substituent, the more stabilized the diaurated complex is and the more energy required for its fragmentation. Accordingly, the ρ values are negative. These findings are in line with the previous solution-phase studies by the groups of Gagné and Maier aimed at similar complexes.7b,23 The comparison of the absolute magnitudes of ρ indicates that the dissociation channel B (ρ = −12.8) is more susceptible to electronic perturbations at the arene ring than the dissociation channel A (ρ = −8.3). Note also the fairly significant difference between the Hammett slopes associated with the dissociation channel A. The ρ values of −5.4 (AEs) and −8.3 (BDEs) were obtained. Interestingly, the slope obtained by plotting the calculated activation barriers for the transmetalation step (ρ = −4.7; see Figure S38 in the Supporting Information) is closer to the value obtained from the plot of the experimental AEs. The ligand elimination channel C is associated with a positive ρ value (ρ = 4.2, Figure

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CONCLUSION In summary, we have investigated the fragmentation behavior of gem-diaurated arene complexes [Au2(L)2(μ-aryl)]+. We focused particularly on the evaluation of their stabilities with respect to their electronic structure and steric properties. Three 3985

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dissociation channels have been observed in the CID experiments. The expulsion of arylgold(I) (dissociation channel A) is common for all of the investigated species and dominant for most of them. Energy-resolved CID experiments revealed that the stability of gem-diaurated arenes is directly influenced by the electronic perturbation of the arene-ring moiety and indirectly by the electron-donating ability as well as by the steric bulk of the gold supporting ligand L. From our experimental and theoretical results emerges that the electron-rich aryl ligands form more strongly bound gemdiaurated complexes than the electron-poor ones. Further, the stability of the gaseous gem-diaurated complexes increases with the decreasing electron-donating ability of the ligand L. Precise evaluation of the electronic effects is, however, impeded by the contribution of steric influences. The steric disposition of the ligand L can either increase the energy barrier for the dissociation reaction (as was shown for the dissociation channel A of complex 7) or contribute significantly to the stability of fragment ions, as observed for the dissociation of complexes 6, 8 (stabilizing effect due to the cation−π interactions), and 9 (destabilizing effect due to the induced steric strain).

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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.organomet.5b00343. Details on the experimental procedures and theoretical calculations, potential energy surfaces, optimized geometries, and complete ref 17 (PDF) Geometry-optimized structures (MOL)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the European Research Council (StG ISORI, No. 258299). REFERENCES

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