Interaction of Gold Acetylides with Gold(I) or Silver(I) Cations

Article pubs.acs.org/Organometallics

Interaction of Gold Acetylides with Gold(I) or Silver(I) Cations Lucie Jašíková and Jana Roithová* Department of Organic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague 2, Czech Republic S Supporting Information *

ABSTRACT: Structures and properties of complexes between (trimethylphosphino)gold acetylides and another (trimethylphosphino)gold cation have been studied in the light of the possible “dual activation” reaction mode in gold catalysis. Silver salts are often used as cocatalysts in gold catalysis; therefore, also mixed complexes of acetylides with (trimethylphosphino)gold(I) and silver(I) cations have been investigated. Energy-resolved collisioninduced dissociation experiments, ion spectroscopy, and density functional theory calculations show that the gold and silver cations preferentially coordinate to the gold acetylides rather than to neutral CC triple bonds in diyne-type substrates. The effect of the binding of two metal cations to acetylides with respect to nucleophilic additions is discussed.

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INTRODUCTION Gold-catalyzed reactions have emerged as a powerful tool for the construction of a diverse array of molecular architectures.1 Most of the gold-mediated reactions involve unsaturated substrates such as enynes2 and diynes.3 Though a considerable effort toward elucidation of reaction mechanisms is devoted to the field of gold-catalyzed reactions, there are still many open questions. It is accepted that the gold cation reacts with a triple bond through π-coordination.4 Lately, discussions were opened about gold acetylides detected in the reactions of terminal alkynes and their possible role in the mechanisms in gold catalysis. It has been suggested that the reactions can be driven by a σ,π-type dual activation of terminal alkynes by two gold(I) cations.5 Toste et al. reported the first evidence of dual σ,πactivation by a gold catalyst in the cycloisomerization of 1,5allenynes.5a Meanwhile, the mode of dual activation by AuI complexes has become one of the research fields in gold catalysis. Hence, many research groups have focused on the synthesis and characterization of digold species.5b−f The Widenhoefer group reported on the formation of a dinuclear gold species from a mononuclear gold complex and a terminal alkyne.5b As the σ,π-activated alkyne complexes were easily generated without the presence of a base, it indicated that gold acetylides might have represented intermediates in the goldcatalyzed reactions.5b The Corma group independently supported this hypothesis.5c On the other hand, Simonneau et al.5d found out that the dinuclear gold(I) σ,π-acetylide complexes appear to be nonreactive in the 1,6-enyne cycloisomerization catalytic cycle. Nowadays, several reports have contributed to the questions about the synthesis, characterization, and role of geminally diaurated unsaturated hydrocarbons in catalysis.6 Weber et al.6a have recently reported on the properties of gold vinyl © 2013 American Chemical Society

intermediates and the corresponding gem-diaurated vinyl species. They found out that electron-rich aryl and vinyl ligands form less reactive digold structures, counterions with a poorer coordinating ability are better for the formation of diaurated species, and the presence of silver salts has an effect on the rate of protodemetalation of mononuclear gold species.6a Our group has used a combination of mass spectrometry, ion spectroscopy, NMR studies, and theoretical calculations for the elucidation of the structure and role of gemdiaurated intermediates in the addition of methanol to alkynes.6b Dual activation of diynes leading to gem-diaurated intermediates represents another extensively investigated topic in mechanistic studies.7 Hashmi and co-workers have reported a synthetic protocol for identifying and isolating the gemdiaurated species of diyne-type substrates.7a These species were used as precatalysts in the given reactions, and it was shown that the reaction time was then extensively reduced.7a They prepared and isolated the first example of a gem-diaurated species bearing two different gold centers.7a σ,π-Acetylide complexes derived from propyne can be also used as precatalysts; they are air-stable solids and could be applied without further activation.7e The concept of dual activation is possible to employ also in an intramolecular C(sp3)−H activation,7f which can be combined with iodination.7g Here, we present an investigation of diaurated and mixed (containing gold and silver) complexes of diynes by infrared multiphoton dissociation spectroscopy, mass spectrometry, and theoretical calculations. Received: July 9, 2013 Published: November 18, 2013 7025

dx.doi.org/10.1021/om4006774 | Organometallics 2013, 32, 7025−7033

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results at a lower level of theory, the differences were not significant (see Figure S14 in the Supporting Information). For this reason, we have optimized all structures at the mPW1PW91/cc-pVDZ:LanL2DZ level of theory. The effect of dispersion interactions was studied using the M0622 functional containing empirical D3 dispersion correction (see Tables S8 and S9 in the Supporting Information). Nevertheless, there is no systematic difference in the results and the theoretical binding energies obtained using the mPW1PW91 functional match better with experimental binding energies than the theoretical binding energies calculated by the functional M06(GD3).

EXPERIMENTAL SECTION

Gas-phase infrared (IR) spectra of mass-selected ions were recorded using a Bruker Esquire 3000 ion trap mass spectrometer coupled to a free electron laser at CLIO (Centre Laser Infrarouge Orsay, Orsay, France).8 The ions were generated by electrospray ionization (ESI) of methanolic solutions of a metal salt (Au(PMe3)Cl, AgSbF6) and 2substituted or 2,2-disubstituted derivatives of diethyl malonate. The free electron laser (FEL) was operated at 44 MeV electron energy, which provided light in the 1000−1900 cm−1 range. Spectral resolution was in the range of 15−20 cm−1 (full width at halfmaximum, fwhm) under the given experimental setup.9,10 The ions were mass-selected and stored in the ion trap. The fragmentation was induced by 5−10 laser macropulses of 8 μs admitted to the ion trap. The dependence of the fragmentation intensities on the wavelength of the IR light gives the infrared multiphoton dissociation (IRMPD) spectra. The reported IRMPD spectra are averages of two raw spectra. Each point in a raw spectrum at a given wavelength is obtained by the evaluation of four mass spectra in that each mass spectrum is an average of five measurements. The IRMPD spectra are not corrected for the power of the free-electron laser, which slightly changes depending on the wavenumber (Figure S1 in the Supporting Information). The energy-resolved CID experiments were performed with a TSQ 7000 mass spectrometer11 with a quadrupole−octopole−quadrupole configuration. The ions were generated by electrospray ionization (ESI) of methanolic solutions of 2-substituted or 2,2-disubstituted derivatives of diethyl malonate and a metal salt (Au(PMe3)Cl, AgSbF6). The first quadrupole was used to mass-select the ions of interest. The mass-selected ions were collided with xenon at different pressures to study the pressure effect on the determined binding energies. The final binding energies were determined from a linear extrapolation to 0 pressure (Figures S2−S5 and Table S1, Supporting Information). The ionic products of collision-induced dissociation (CID) were analyzed by the second quadrupole. The collision energy was varied by changing the potential offset of the octopole. The nominal zero collision energy was determined using a retarding potential analysis (Figure S6, Supporting Information). The energy resolution was 2.0 ± 0.1 eV in the laboratory frame (fwhm). In the CID experiments, gas-phase binding energies can be determined from the dependence of the relative cross sections on the collision energy.12 We have used the L-CID program from the group of Prof. Chen for fitting of our experimental data.13 The L-CID program simulates the experimental data based on electrostatic theory,14 statistical rate theory,15 and RRKM theory.16 The simulation requires input about the parent ion (kinetic-energy resolution, number of degrees of freedom, and number of free rotors, i.e., single bonds with free rotations) and information on whether the transition state for a given dissociation is loose or tight. We have used the constraint of a loose transition state for all complexes investigated in this work. The geometry optimizations and thermochemistry calculations were performed using the density functional theory method mPW1PW9117 as implemented in the Gaussian 09 package.18 As a basis set, a combination of the cc-pVDZ basis set for C, H, O, and P and the LanL2DZ basis set for Au and Ag was used (denoted as ccpVDZ:LanL2DZ in the following). All minima and transition structures were verified by analysis of their Hessian matrixes. All optimized structures and their energies can be found in the Supporting Information. The calculated infrared spectra are scaled with a scaling factor of 0.95 for all spectra.19,20 The final energies and Mulliken population analysis were determined by single-point calculations at the mPW1PW91/cc-pVTZ:(LanL2TZ for Au and Ag) level with corrections for the basis-set superposition error (BSSE).21 Mulliken charges at the carbon atoms are summed together with the adjacent hydrogen atoms (given in red italics in the figures) and over the whole PMe3 and C(COOEt)2 groups, respectively (given in blue italics in the figures). As the last step, the geometry optimizations were performed at the mPW1PW91/cc-pVTZ:(LanL2TZ for Au and Ag) level for the isomers of [(M2-H)Au2(PMe3)2]+. When we compared the optimized structures and theoretical IR spectra at this level of theory with the

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RESULTS AND DISCUSSION The aim of this paper is to investigate the formation of complexes between gold acetylides and another gold cation. With respect to the discussion of the possible effect of silver cations in gold catalysis,23 we have also included an investigation of mixed complexes of acetylides with gold and silver cations.24,25 Note, however, that the silver effect was not confirmed and it was shown, for example, for σ,π-acetylide complexes derived from propyne that no activation by silver is required.7e As mentioned in the Introduction, many gold-catalyzed reactions deal with polyunsaturated substrates. The terminal alkynes easily form gold acetylides. Coordination of a second gold cation can proceed either at a different multiple bond, which opens the door for dual activation of the substrate,5,6b or to the same terminal triple bond, which leads to the formation of geminally diaurated acetylides.6,7 We were therefore interested in the structure of the complexes formed. For comparison of the triple CC bond activation in monoaurated alkynes on one hand and diaurated acetylides on the other, the relative binding energies of gold cations to gold acetylides were determined. Energy-Resolved CID Experiments and Determination of Binding Energies. Recently, we have performed a systematic study of binding energies between different unsaturated hydrocarbons and the [Au(PMe3)]+ cation. We have shown that the binding energy between the (trimethylphosphino)gold cation and alkynes is on the order of 1.9 eV. One of the reference alkynes in our study was phenylacetylene (PhCCH). Electrospray ionization of a solution of phenylacetylene and [Au(PMe3)Cl] leads to the formation of [(PhCCH)Au(PMe3)]+ and [(PhCC)Au2(PMe3)2]+ complexes. The binding energy of [Au(PMe3)]+ in the former complex was determined as 1.87 ± 0.04 eV.26 Here, we will concentrate on the complexes of the latter type. The collision-induced dissociation of [(PhCC)Au2(PMe3)2]+ leads to the elimination of [Au(PMe3)]+ (eq 1), [Au(PMe3)2]+ [(M−H)Au 2(PMe3)2 ]+ → [(M−H)Au(PMe3)] + [Au(PMe3)]+

(1)

[(M−H)Au 2(PMe3)2 ]+ → [(M−H)Au] + [Au(PMe3)2 ]+ (2)

[(M−H)Au 2(PMe3)2 ]+ → [(M−H)Au 2(PMe3)]+ + PMe3

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(3)


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(eq 2), or PMe3 (eq 3). The simple loss of [Au(PMe3)]+ represents the dominant fragmentation channel at all energies studied (Figure 1). Fitting of the experimental data of the

important implications. First, this binding energy is about 0.5 eV higher than the binding energy of [Au(PMe3)]+ to a triple bond of nonactivated alkynes, which is a consequence of the higher electron density in the triple bond of the gold acetylide, but it can also be partially due to the aurophilic interaction between the gold cations. Second, all experimental binding energies determined in our previous study were about 0.2−0.3 eV higher than the theoretical values. Here, the experimental value is lower than the theoretical prediction. Hence, the former theoretical underestimation suggests that the DFT methods probably underestimate the binding energies between the gold cation and triple bonds, whereas the present results can indicate an overestimation of the aurophilic interaction or a wrong electron distribution in the gold acetylide determined by the DFT calculations. We note in passing that the latter effect can be smaller than that suggested by the difference between the experimental and theoretical values, because the experimental values are often underestimated due to an insufficient internal energy cooling.27 In the fitting of the experimental data, two channels have been considered. Next to the discussed elimination of [Au(PMe3)]+ also elimination of [Au(PMe3)2]+ has been fitted. If a “loose” transition state (TS) is considered for both fragmentation channels, then the experimentally derived binding energies amount to 2.37 ± 0.03 and 2.58 ± 0.05 eV, respectively. The theoretical values are 2.66 and 2.22 eV, respectively. Clearly, the latter channel is associated with a rearrangement of a phosphine group from one gold cation to the other and hence the experimental binding energy can in fact reflect the barrier height for this rearrangement, if it is higher in energy than the dissociation limit.28 We note in passing that if the transition state for this channel is set as “tight” (TS for the elimination of [Au(PMe3)]+ is kept as loose), then the experimental value drops to 2.04 ± 0.12 eV for the elimination of [Au(PMe3)2]+, while the experimental value for the

Figure 1. Energy-resolved CID (symbols) of the [(PhCC)Au2(PMe3)2]+ complex (a) and the [(M−H)Au2(PMe3)2]+ complexes, where M = M1 (b), M2 (c), M3 (d) and X = C(COOEt)2. The solid line corresponds to the L-CID fit of the relative cross sections. The experimental data for eliminations of [Au(PMe3)]+, [Au(PMe3)2]+, and PMe3 are given in green, pink, and blue, respectively.

relative fragmentation abundances on the collision energy by the L-CID program (details can be found in the Experimental Section and Table S2 in the Supporting Information) leads to a binding energy of [Au(PMe3)]+ to the gold acetylide of 2.37 ± 0.03 eV. The experimental binding energies will be compared with the results of theoretical calculations at the DFT level of theory. The most stable structures for all optimized complexes are shown in Figure 2. The theoretical value for the binding energy of [Au(PMe3)]+ in [(PhCC)Au2(PMe3) 2]+ amounts to BDEtheor = 2.66 eV (Table 1). The determined values for the binding energy of [Au(PMe3)]+ to the gold acetylide have two

Figure 2. The most stable structures located for different [(M−H)Au2(PMe3)2]+ and [(M−H)AuAg(PMe3)]+ complexes at the mPW1PW91/ccpVDZ:LanL2DZ level of theory. Selected bond lengths are given in angstroms (black numbers). The numbers in italics indicate Mulliken charges (see the Experimental Section). 7027


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Table 1. Experimental and Theoretical Binding Energies of the [(M−H)Au2(PMe3)2]+ Complexes in eV [Au(PMe3)]+ M phenylacetylene diethyl 2-propagylmalonate diethyl 2,2-dipropargylmalonate diethyl 2-butynyl-2-propargylmalonate

BDEexp 2.37 2.53 2.63 2.80

± ± ± ±

a

[Au(PMe3)2]+ BDEtheor

0.03 0.07 0.03 0.10

2.66 2.90 2.67 2.72

b

a

BDEtheorb

0.05 0.06 0.04 0.02

2.22 2.42 2.22 2.26

BDEexp 2.58 2.46 2.21 2.22

± ± ± ±

a Transition states are set as “loose” for both channels (loss of [Au(PMe3)]+ and [Au(PMe3)2]+, respectively). bGeometry optimizations and thermochemistry calculations were performed at the mPW1PW91/cc-pVDZ:LanL2DZ level of theory. The total energies were refined by singlepoint calculations, including corrections for the basis-set superposition error at the mPW1PW91/cc-pVTZ:LanL2TZ level of theory.

elimination of [Au(PMe3)]+ changes only slightly to 2.42 ± 0.06 eV (cf. Table S1 in the Supporting Information). Next, we will address the interaction of [Au(PMe3)]+ with gold acetylides of compounds based on the diethyl malonate skeleton with one or two unsaturated hydrocarbon chains attached to the carbon atom C(2) (Scheme 1). The

binding energies of [Au(PMe3)2]+ in [(Mx−H)Au2(PMe3)2]+ (x = 1−3). The reasoning of why the energy for this channel was underestimated for the complex derived from phenylacetylene can evolve from a possible change of the fragmentation mechanism: in the [(M−H)Au2(PMe3)2]+ complexes derived from the malonate skeleton, the migration of the PMe3 group can be assisted either by the carbonyl groups or by the other triple CC bond, which can significantly lower the corresponding energy barrier. Such assistance is not possible in the complex derived from phenylacetylene. Having determined the binding energies in digold complexes, also the complexes of acetylides with one silver and one (trimethylphosphino)gold cation were investigated. Under the given experimental conditions we were not able to observe any [(PhCC)AuAg(PMe3)]+ complex; hence, only [(Mx−H)AuAg(PMe3)]+, where x = 1−3, respectively, were studied. The collision-induced dissociation of [(Mx−H)AuAg(PMe3)]+ leads to the elimination of [Ag(PMe3)]+ (eq 4),

Scheme 1. Studied Compounds, Where X = C(COOEt)2

electrospray ionizations of methanolic solutions of [Au(PMe3)Cl] and diethyl 2-propargylmalonate (M1), diethyl 2,2dipropargylmalonate (M2), or diethyl 2-butynyl-2-propargylmalonate (M3), respectively, lead dominantly to the formation of the complexes [(Mx−H)Au2(PMe3)2]+ (x = 1−3). As for the system above, the collision-induced dissociation of [(Mx− H)Au2(PMe3)2]+ leads to the elimination of [Au(PMe3)]+ (eq 1), [Au(PMe3)2]+ (eq 2), or PMe3 (eq 3). Comparison of the energy-dependent CID curves (Figure 1) reveals a fundamental difference between the complexes with M1 (monoyne) on the one hand and M2 and M3 (diynes) on the other. While the fragmentation of the [(M1−H)Au2(PMe3)2]+ complex is qualitatively similar to that of [(PhCC)Au2(PMe3)2]+ (the elimination of [Au(PMe3)]+ is dominant), the abundances in fragmentation channels are drastically changed for [(M2− H)Au2(PMe3)2]+ and [(M3−H)Au2(PMe3)2]+ (elimination of [Au(PMe3)2]+ largely prevails). The binding energies of [Au(PMe3)]+ and [Au(PMe3)2]+ in [(Mx−H)Au2(PMe3)2]+ were determined again by fitting the energy-dependent CID curves with the L-CID program (Table 1). The binding energy of [Au(PMe3)]+ in [(M−H)Au2(PMe3)2]+ increases for M in the order PhCCH < M1 < M2 < M3. On the other hand, evaluation of the binding energies of [Au(PMe3)2]+ in [(M−H)Au2(PMe3)2]+ leads to results with the opposite trend; hence, the binding energies decrease for M in the order PhCCH > M1 > M2 ≈ M3. This trend is in agreement with the fact that the elimination of [Au(PMe3)2]+ prevails for the complexes of M2 and M3, whereas the elimination of [Au(PMe3)]+ prevails for the complexes of M1 and phenylacetylene. The theoretical binding energies are derived for the most stable localized structures for the digold acetylides depicted in Figure 2 (fragments can be found in the Supporting Information). In comparison to the experiment, the binding energies of [Au(PMe3)]+ are overestimated with respect to the experimental values for [(M−H)Au2(PMe3)2]+, when M is monoyne (PhCCH or M1), whereas there is a good agreement for M being diyne M2 or M3. Similarly good agreement between the experiment and theory has been found also for the

[(M−H)AuAg(PMe3)]+ → [(M−H)Au] + [Ag(PMe3)]+ (4)

[(M−H)AuAg(PMe3)]+ → [(M−H)Ag] + [Au(PMe3)]+ (5)

[(M−H)AuAg(PMe3)]+ → [(M−H)AuAg]+ + PMe3 (6)

[(M−H)AuAg(PMe3)]+ → [(M−H)Au(PMe3)] + Ag + (7)

[(M−H)AuAg(PMe3)]+ → [(M−C3H3)Au(PMe3)] + [Ag(C3H 2)]+

(8)

[Au(PMe3)]+ (eq 5), PMe3 (eq 6), Ag+ (eq 7), or [Ag(C3H2)]+ (eq 8). The losses of [Ag(PMe3)]+ and [Au(PMe3)]+ are the most abundant channels in the fragmentation patterns (Figure 3). These eliminations can be taken as an indicator of the structure of the gold−silver complexes and also whether silver or gold acetylide is favored. The CID graphs of all investigated [(Mx−H)AuAg(PMe3)]+ complexes clearly show that the threshold energy for the elimination of [Ag(PMe3)]+ is smaller than that for the loss of [Au(PMe3)]+. Further, while the elimination of [Ag(PMe3)]+ from [(Mx−H)AuAg(PMe3)]+ (x = 1, 2) is preferred at low collision energies, the loss of [Au(PMe3)]+ starts to prevail at larger collision energies. It 7028


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ions by a tunable laser. The irradiation of the mass-selected ions leads to their fragmentation, if the photons are in resonance with a vibrational transition of the ions. Hence, the dependence of the fragmentation on the wavenumbers of the photons provides spectra similar to the IR spectra. However, it has to be stressed that the spectra are obtained by the absorption of many IR photons and therefore cannot be directly compared with single-photon spectra. In particular, the intensities of the individual bands can largely vary.29 The interaction of the (trimethylphosphino)gold cation with nonalkylated diethyl malonate has been previously studied. It has been shown that the (trimethylphosphino)gold cation coordinates unsymmetrically between two carbonyl oxygen atoms of the ester functions, which leads to two strong CO stretching bands at 1630 and 1740 cm−1. The signature of the remaining part of ethyl malonate was found as a composite band at about 1320−1340 cm−1 and corresponded to C−O stretching and the CH2 bendings.20 The IRMPD spectrum of the complex [(M2−H)Au2(PMe3)2]+ (Figure 4a) has one

Figure 3. Energy-resolved CID experiments with [(Mx−H)AuAg(PMe3)]+, where x = 1 (a), 2 (b), 3 (c) and X = C(COOEt)2. The losses of [Ag(PMe3)]+, [Au(PMe3)]+, PMe3, Ag+, and [Ag(C3H2)]+ are given in red, green, blue, orange, and brown.

most probably means that while the elimination of [Ag(PMe3)]+ is thermodynamically preferred, it is disfavored kinetically. This finding is consistent with a view that the (trimethylphosphino)gold cation is σ-bound to the acetylide, whereas the silver cation is π-coordinated to the triple bonds (or carbonyl atoms). Hence, the elimination of [Ag(PMe3)]+ is associated with the migration of the PMe3 group from the gold cation to the silver cation, which is the origin of kinetic hindrance of this channel. In contrast, the elimination of [Au(PMe3)]+ corresponds to a simple σ-bond cleavage and thus prevails at large collision energies. In agreement with the above results, the theoretical binding energies for the elimination of [Ag(PMe3)]+ are lower than those for the loss of [Au(PMe3)]+ (Table 2) and also the most Table 2. Theoretical Binding Energies of the [(MH)AuAg(PMe3)]+ Complexes in eV BDEtheora M

[Ag(PMe3)]+

[Au(PMe3)]+

diethyl 2-propagylmalonate diethyl 2,2-dipropargylmalonate diethyl 2-butynyl-2-propargylmalonate

3.51 3.36 3.40

3.59 3.53 3.58

a

Geometry optimizations and thermochemistry calculations were performed at the mPW1PW91/cc-pVDZ:LanL2DZ level of theory. The total energies were refined by single-point calculations including corrections for the basis-set superposition error at the mPW1PW91/ cc-pVTZ:LanL2TZ level of theory. Figure 4. (a) IRMPD spectrum of the mass-selected [(M2− H)Au2(PMe3)2]+ complex and theoretical IR spectra of (b) 1b, (c) 1a, (d) 2a, (e) 3a, and (f) 4. The line spectra (red bars) are folded with a Gaussian function with fwhm = 16 cm−1. Selected bond lengths are given in angstroms.

stable isomers of [(M x−H)AuAg(PMe 3 )] + (x = 1−3, respectively) correspond to the gold acetylides with silver cations π-coordinated to one or both triple bonds. As in the case of the digold complex of M1, the silver cation interacts with both carbonyl groups of diethyl malonate in the [(M1− H)AuAg(PMe3)]+ complex (Figure 2). However, the most stable isomers of [(Mx−H)AuAg(PMe3)]+ (x = 2, 3) correspond to the structures in which the silver cation interacts with both triple bonds and one carbonyl group (Figure 2 and the IRMPD spectra below). Infrared Multiphoton Dissociation (IRMPD) Spectroscopy. In order to check whether the theoretically predicted structures of the investigated complexes are correct, we have measured IRMPD spectra of selected complexes. The IRMPD spectra are obtained by irradiation of trapped mass-selected

dominant band at 1205 cm−1 and two small bands at 1260 and 1750 cm−1. Hence, from the comparison, we can surely conclude that the (trimethylphosphino)gold cation is not coordinated to the carbonyl oxygen atoms. Instead, the coordination proceeds to the alkynyl chains and it is revealed by the two bands at 1205 and 1260 cm−1. Further analysis was done on the basis of theoretical calculations (see Figures S7−S9 in the Supporting Information). We had to consider that next to the expected σ,π-coordination also other options come into play for the diyne. It is possible that each of the 7029


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activation diynes may depend strongly on their overall structure. We have also studied the structures which can be obtained by cycloisomerizations of the [(M2−H)Au2(PMe3)2]+ complex. The most stable isomers (3) correspond to the structures with a five-membered ring with an adjacent allene system. However, the theoretical IR spectrum of 3a (the most stable conformer from the manifold of the isomers 3) does not agree with the experimental IRMPD spectrum. An alternative cycloisomerization product with a six-membered ring (isomer 4) lies higher in energy than the isomer 3a, and its theoretical IR spectrum does not match with the experimental peaks either. Hence, we most probably do not observe isomers formed by cyclizations and/or rearrangements. For a confirmation of our previous results, we have explored the structure of the mixed complex of diyne M2 with the (trimethylphosphino)gold and silver cations (Scheme 3) using

(trimethylphosphino)gold cations binds to a different triple bond and cycloisomerizations should be considered as well. First, we have considered the simple formation of gold acetylides and different coordinations of the other (trimethylphosphino)gold cation. All isomers were in addition studied in several different conformations (Scheme 2). The Scheme 2. Studied Compounds, Where X = C(COOEt)2

most stable isomer is formed when the other (trimethylphosphino)gold cation coordinates to the triple bond of the acetylide (1), as could have been expected on the basis of the greater binding energies of [Au(PMe3)]+ to the acetylides than to neutral alkynes (see above). In the most stable conformation 1a the gold cation interacts with one of the carbonyl oxygen atoms of diethyl malonate. The conformation where none of the oxygen atoms interact with gold, lies about 0.09 eV higher in energy (1b). In this conformation, the other triple bond is loosely coordinated to the π-coordinated gold cation. Comparison of the theoretical IR spectra of 1a and 1b with the IRMPD spectrum shows that the agreement of the spectrum of 1b with the experiment is very good, whereas the spectrum of 1a does not match the experimental peaks. The most significant feature is that we do not see a pronounced redshifted band for the coordinated carbonyl oxygen atom. Hence, these results show that the ions sampled experimentally can correspond to the simple σ,π-coordination of two [Au(PMe3)]+ cations to one of the triple bonds of the diyne in that the second triple bond is activated only by a weak coordination to one of the [Au(PMe3)]+ cations. The fact that the theoretical calculations predict the isomer with the coordinated carbonyl function as the more stable one can be ascribed to the shortcomings of the DFT methods. As we showed in our previous study,26 the DFT methods underestimate the binding energy between [Au(PMe3)]+ and CC triple bonds. This can lead to an artificially higher relative energy of 1b with respect to 1a. The isomers in which the gold cation does not coordinate to the gold acetylide but rather to the other triple bond (2) lie higher in energy than the former complexes 1. The structure 2a is the most stable localized conformer of 2 and lies 0.40 eV higher in energy than structure 1a. Both carbonyl oxygen atoms are coordinated to gold in this conformer. It could be expected that a conformer lying lower in energy could be found if the deprotonated triple bond would coordinate to the πcoordinated gold cation; however, during the optimization process of such a conformer the gold cation always migrated to the acetylide to form some of the conformers of the manifold of isomer 1. Hashmi and co-workers assumed that isomers analogous to 2 are the key intermediates in the synthesis of benzofulvene derivatives.7a Initial activation of a diyne by the σcoordination to gold increases the nucleophilicity of the βcarbon atom of the first triple bond. The second triple bond is activated by π-coordination. Theoretical calculations of Hashmi et al. indicate that their intermediate of the type “2” lies lower in energy (0.30 eV in comparison with “1”) than our 2a complex.7e This finding indicates that the probability of dual-

Scheme 3. Studied Compounds, Where X = C(COOEt)2

IRMPD spectroscopy.30 The IRMPD spectrum of the complex [(M2−H)AuAg(PMe3)]+ (Figure 5a) has two ranges of bands (1200−1400 and 1680−1780 cm−1). Comparison of the theoretical IR spectrum of the most stable conformation found for the complex [(M2−H)AuAg(PMe3)]+ (5a) with the experimental spectrum leads to a good agreement (Figure 5b).

Figure 5. (a) IRMPD spectrum of the mass-selected complex [(M2− H)AuAg(PMe3)]+ and theoretical IR spectra of (b) 5a, (c) 5b, (d) 6a, and (e) 7a. The line spectra (red bars) are folded with a Gaussian function with fwhm = 16 cm−1. Selected bond lengths are given in angstroms. 7030


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The bands in the range 1680−1780 cm−1 correspond to the stretching of CO groups. The bands in the range 1200−1400 cm−1 represent stretching of the single C−O bonds coupled with the CH2 bending vibrations. The conformer 5b, in which the silver cation interacts with both carbonyl groups, lies 0.13 eV higher in energy than the conformer 5a and the IR spectrum of 5b does not match the experimental peaks as good as that of 5a. In particular, it is clear that the coordination of the carbonyl groups in 5b is different than that found in the experiment. The isomers in which the silver cation and gold acetylides are not coordinated to the same triple bond (6) lie higher in energy than the complexes 5. The structure 6a is the most stable localized conformer of 6, and it lies 1.03 eV higher in energy than the structure 5a. The theoretical IR spectrum of 6a does not agree with the experimental IRMPD spectrum. Similarly as found for the digold species, the theoretical IR spectra of products of cycloisomerization do not match with the experimental peaks (see Figure S12 in the Supporting Information). The best possible match has been found for the bicyclic intermediate 7a (Figure 5e), which however lies 0.43 eV higher in energy than the initial complex 5a and its formation is therefore improbable. Electronic Structure. Figure 2 shows the most stable structures of the investigated complexes. Additional insight into their reactivity can be obtained from a comparison of the bond lengths and charge distributions. First, we compare [(PhCC)Au2(PMe3)2]+ with [(PhCCH)Au(PMe3)]+. The coordination of the gold cation to the triple bond of bare phenylacetylene is distorted, with a shorter bonding distance toward the terminal carbon atom. The difference between the two Au−C bonds in the [(PhCCH)Au(PMe3)]+ complex is 0.51 Å (Figure 2). The most stable structure of the [(PhCC)Au2(PMe3)2]+ complex corresponds to the geminally diaurated acetylide, in which both gold cations are symmetrically coordinated to the terminal carbon atom of the acetylide (Figure 2). In contrast, the gold cations in the optimized structures of the [(Mx− H)Au2(PMe3)2]+ complexes are not equivalent and these complexes are thus better described as dinuclear gold(I) σ,πacetylides. Different modes of coordination of the two gold cations are associated with different distributions of electron density along the activated triple bond. According to the Mulliken population analysis (values given in red in Figure 2), geminal coordination of the gold cations in the complex with phenylacetylene leads to a strong polarization of the triple bond. The terminal carbon atom bears a large negative charge, and this complex will thus probably react in analogy to a usual acetylide. Polarization of the σ,π-acetylides derived from M1− M3 is much smaller. Both carbon atoms of the triple bonds bear negative charges, and a larger negative charge is located at the terminal carbon atom. Coordination of the silver cation to the triple bonds in the mixed gold−silver complexes is much more symmetric than that found for gold in the diaurated complexes. This is also reflected in only small polarity of the triple bonds of the corresponding acetylides. The important finding is that all diaurated acetylides (except [(PhCC)Au2(PMe3)2]+) as well as the mixed acetylides are in principle deactivated for possible nucleophilic reactions and could rather act as nucleophiles, in that the changes in polarities of the triple bonds might be important in the cascade reactions, where cyclization to six- or five-membered rings occurs.

Article

CONCLUSION We have determined the binding energies of the (trimethylphosphino)gold cation and silver cation with a series of gold acetylides. The results show that the gold(I) as well as the silver(I) cations have higher affinities to the gold acetylides than to the nonactived CC triple bonds. The energy difference between the interaction of [Au(PMe3)]+ to the triple CC bond of a gold acetylide and to the triple bond of a nonactivated alkyne is about 0.5 eV. We have also explored the structures of gold(I) and silver(I) complexes with different gold acetylides. We have shown that the structure of digold species of diynes corresponds to the isomers where one of the gold cations πcoordinates to the triple bond of an acetylide formed by σcoordination with the second gold cation; the second triple bond is loosely coordinated to the π-coordinated gold cation. In analogous complexes with one gold and one silver cation, the gold cation is σ-coordinated to the acetylide and the silver cation is π-coordinated between both triple bonds. According to the Mulliken population analysis, coordination of one gold cation to a nonactived CC triple bond is more favorable for the activation of the given triple bond toward nucleophilic addition. On the other hand, both carbon atoms of the triple bond in diaurated and mixed silver−gold complexes bear negative charges and therefore the formation of complexes between the gold acetylide and gold(I) or silver(I) cations leads to a deactivation of the studied alkynes toward nucleophilic additions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, tables, and figures giving details and data about the retarding-potential analysis, the results of the L-CID fitting, IRMPD spectroscopy, theoretical results, and the complete ref 18. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail for J.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Grant Agency of the Czech Republic (207/11/0338) and the European Research Council (StG ISORI). The results from CLIO were obtained thanks to funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under the grant agreement No. 226716. The CLIO staff, particularly Philippe Maitre and Vincent Steinmetz, are gratefully acknowledged for their help and assistance.

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REFERENCES

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Interaction of Gold Acetylides with Gold(I) or Silver(I) Cations

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