Article pubs.acs.org/Organometallics
Interaction of the Gold(I) Cation Au(PMe3)+ with Unsaturated Hydrocarbons 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: The binding energies of unsaturated hydrocarbons (benzene, styrene, 1-pentene, 1-pentyne, 2-pentyne, phenylacetylene, cyclooctene, 1,5cyclooctadiene, 1,3-cyclooctadiene) with the (trimethylphosphino)gold cation have been investigated by means of mass spectrometry and density functional theory. It is shown that the interactions of the gold(I) cation with C−C triple bonds are stronger than those with C−C double bonds or with an aromatic ring. Further, the gold cation preferentially interacts with the isolated C−C double bonds in 1,5-cyclooctadiene and cyclooctene in comparison to the conjugated double bonds in 1,3-cyclooctadiene. In contrast to these results, the binding energies of Au(PMe3)+ to the simplest unsaturated hydrocarbons increase in the order acetylene < ethylene < butadiene. The effect of the alkyl and aryl substitution and the consequence of these results for the gold-catalyzed reactions of unsaturated hydrocarbons are discussed.
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INTRODUCTION During the past decade, gold chemistry has grown into a powerful tool for many organic transformations through a π activation of carbon−carbon multiple bonds.1 The use of gold as the catalyst enables mild, chemoselective, and efficient reactions. However, many of the suggested mechanisms of gold-catalyzed reactions are still speculative. One of the pertinent questions in mechanisms of gold-catalyzed reactions concerns the role of carbene-like and/or carbocation intermediates.2 Cycloisomerizations of unsaturated hydrocarbons have represented one of the most attractive areas in gold catalysis, because these reactions can be used for the construction of a broad variety of cyclic compounds with often remarkable selectivities.3 Biologically active compounds and other important compounds can be prepared in a sequence combining cycloisomerization and another reaction (for example, Diels−Alder reactions or 1,2-acyloxy rearrangements).4 A highly selective gold-catalyzed (4 + 2) cycloaddition between nonactivated 1,3-dienes and allenamides have been successfully developed.5 Another important area in gold catalysis is highly enantioselective transformations.6 Although gold catalysis has been intensively studied for several years, it still represents a dynamic, rapidly growing field.7 While the advantages of gold catalysis are well-known, the reaction mechanisms are still often not clear. A minireview published by Hashmi8 last year summarized the known intermediates identified by direct observation and their characterization. NMR spectroscopy, X-ray crystal structure analysis, and kinetic studies represent the most common methods for the research of reaction mechanisms. These © 2012 American Chemical Society
experimental methods are frequently complemented by theoretical studies. Reactions catalyzed by gold proceed mostly in an ionized state; therefore, they are perfectly suited for mass spectrometric studies. Nevertheless, relatively few gold-catalyzed reactions have so far been studied by means of mass spectrometry.9,10 It has been demonstrated repeatedly that mass spectrometry is a powerful tool for fishing out the ionic reaction intermediates.11 The intermediates can be identified directly from the corresponding reaction mixtures, and moreover their uni- and bimolecular reactivities can be studied in great detail. In the past decade, ion spectroscopy has also grown in importance. It combines mass spectrometry with infrared or UV spectroscopy.12 Such a combination offers yet another tool for the investigation of reaction intermediates. This work aims at the investigation of complexes of unsaturated hydrocarbons with the (trimethylphosphino)gold cation and to determine the corresponding binding energies. This information can help in the estimation of the differential reactivity of multiple bonds in gold catalysis and thereby shed light on the relevant reaction mechanisms.
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EXPERIMENTAL DETAILS
The experiments were performed with a TSQ 7000 mass spectrometer13 with a quadrupole−octopole−quadrupole configuration. The ions were generated by electrospray ionization (ESI) of methanolic solutions of the unsaturated hydrocarbon CnHm and AuCl(PMe3). The first quadrupole was used to mass-select the ions of interest [Au(PMe3)(CnHm)]+. Received: December 13, 2011 Published: February 6, 2012 1935
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The mass-selected ions were collided with xenon at a pressure of pXe ≅ 0.05 mTorr; in addition to the ion-gauge value, the pressure was finely adjusted by setting the transmission of the parent ions at ELAB = 30 eV to 90%. An evaluation of the effect of the xenon pressure on the determination of binding energies can be found in the Supporting Information. The ionic products of collision-induced dissociation (CID) were analyzed by the second quadrupole. CID of the massselected complexes [Au(PMe3)(CnHm)]+ leads to a dominant elimination of CnHm and a minor elimination of trimethylphosphine. The collision energy was varied by changing the potential offset between the first quadrupole and the octopole. The nominal zero collision energy was determined using a retarding potential analysis (see the Supporting Information). The energy resolution was 2.0 ± 0.1 eV in the laboratory frame (full width at half-maximum). The collisionenergy-dependent CIDs have been measured in at least two independent experiments done on two different days in order to eliminate possible systematic errors. Each experiment was three times repeated (i.e., six energy-dependent CIDs were measured for each complex; all results can be found in Table S2 of the Supporting Information). In a single experiment, each experimental point is obtained as an average of 25 spectra. The standard deviations in all threshold-energy measurements reported here are less than ±0.07 eV. In order to eliminate a possible effect of the kinetic energy resolution on the relative binding energies of Au(PMe3)+ to various hydrocarbons, the experimental conditions were kept as constant as possible. The same holds true for possible effects of the internal energy content of the parent ions generated in the ESI source. In CID experiments, gas-phase binding energies can be determined from the dependence of the relative cross sections on the collision energy.10,14 We have used the L-CID program from the group of Chen for fitting of our experimental data.15 The L-CID program simulates the experimental data based on the electrostatic theory,16 statistical rate theory,17 and RRKM theory.18 The simulation requires an 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. As an example, Figure 1 shows the energy-resolved CID for the loss of 1-pentyne from the complex [Au(PMe3)(1-pentyne)]+ (red
the elimination of a possible effect of pressure fluctuations as well as the uncertainty of the pressure measurement, and the rate constants can be thus directly compared. Calculations were performed using the density functional theory method mPW1PW91,19 as implemented in the Gaussian 09 package.20 As a basis set, the combination of the cc-pVTZ basis set for C, H, and P and the LanL2DZ basis set for Au was used (denoted as ccpVTZ:LanL2DZ in the following). The final binding energies are corrected for the basis-set superposition error (see Table S10 in the Supporting Information).21 All reported structures represent genuine minima on the respective potential-energy surfaces, as confirmed by analysis of the corresponding Hessian matrices. All optimized structures and their energies can be found in the Supporting Information (only the most stable structures are discussed here; less stable conformers can be found in the Supporting Information). We have also performed all calculations at the M06 level of theory22 for comparison; the results can be found in the Supporting Information. All computed binding energies are smaller than the corresponding experimental values. The experimental binding energies determined by L-CID are usually slightly underestimated in our setup, which we assign to the lack of postsource thermalization such that the ions selected from the ESI source can possess an elevated internal energy. The overestimation of the binding energies derived from experiment could be associated with an improper description of kinetic shifts23 in the threshold modeling. However, we did not observe such effects in any of our previous, similar studies, where a simple elimination of a ligand from an ionic complex was investigated.24 For comparison, we list also experimental binding energies determined by L-CID with the constraint of a tight transition state (see Table S2 in the Supporting Information). These values, however, substantially underestimate the binding energies. We therefore tend to rather ascribe the differences between the experimental and theoretical values to the uncertainties within the density functional theory method.
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RESULTS AND DISCUSSION We have first investigated complexes of the Au(PMe3)+ cation with styrene and phenylacetylene. The experimental binding energies amount to 1.85 ± 0.04 and 1.87 ± 0.04 eV, respectively. Figure 2 shows various computed structures of the corresponding complexes of styrene and phenylacetylene with the gold(I) cation. The most stable isomer of the styrene complex bears a structure in which gold interacts with the vinylic C−C bond. The theoretical binding energy amounts to 1.61 eV (Table 1), which is lower than that found experimentally. The coordination is not symmetrical; the gold cation has a shorter bond to the terminal methylene group. Isomers with the gold cation coordinated to the aromatic ring lie 0.25 and 0.26 eV higher in energy. Note that, using the mPW1PW91 functional, we were not able to localize an isomer in which the gold cation would be η6 coordinated to the aromatic ring. A similar situation has been already described for complexes of copper(I) and phenol.25 Benchmark calculations of Cu(C6H6)+ at the CCSD(T) level of theory predict the η6 coordination as the global minimum, whereas the DFT methods lead to almost isoenergetic complexes with η1, η2, and η6 coordinations, among which the η6 isomer is the highest in energy. Hence, the DFT potential energy surface is rather flat as far as the coordination of copper(I) to the aromatic ring is concerned and a free “copper ring-walk” has been predicted. A similar situation might thus be expected for the gold(I) complexes investigated here.26 The most stable isomer of the gold complex of phenylacetylene corresponds to a structure in which gold interacts with the triple bond. The theoretical binding energy amounts to 1.64 eV (Table 1). The symmetry of the coordination is even more distorted than in the styrene complex, in that the distance
Figure 1. Energy-resolved CID (symbols) of the [Au(PMe3)(1pentyne)]+ complex. The solid line corresponds to the L-CID fit of the relative cross sections. The experimental bond dissociation energy for the loss of 1-pentyne is 1.88 ± 0.07 eV. squares). The L-CID fit is represented by the solid line. The complete set of data and fitting parameters is given in the Supporting Information (S3−S10). In addition, ligand-exchange reactions were performed at nominally zero collision energy. The pressure of the hydrocarbon reactant was kept at 0.2 mTorr. The reaction rates were normalized relative to the reaction of the mass-selected [Au(PMe3)(CH3OH)]+ ion with the given hydrocarbon (kCH3OH). This ion is always present among the ions generated by ESI of the investigated methanolic solutions. The normalization to kCH3OH for each given neutral reactant should lead to 1936
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Figure 2. Optimized structures of the complexes of styrene, phenylacetylene, benzene, 1-pentene, 1-pentyne, and 2-pentyne with the (trimethylphosphino)gold cation at the mPW1PW91/cc-pVTZ:Au-LanL2DZ level of theory. The selected bond lengths are given in angstroms.
respectively. The experimental results clearly demonstrate that the gold cation coordinates most strongly to the triple bonds; the coordination to the double bonds is about 0.1 eV weaker, and the coordination to the aromatic ring is about 0.2 eV weaker. The theoretical binding energies of the complexes of the gold cation with benzene, 1-pentene, 1-pentyne, and 2pentyne are lower than those found experimentally (see Table 1). However, the relative order of the binding energies as well as the energy differences between them are analogous in experiment and theory. The coordination of the gold cation to the C−C double bond of 1-pentene and to the C−C triple bond of 1-pentyne, respectively, is again slightly distorted, with a shorter bonding distance toward the terminal carbon atom. On the other hand, the coordination of the gold cation to the internal triple bond of 2-pentyne is almost symmetrical (there is only 0.03 Å difference between the two Au−C bonds). The difference most probably stems from a small stabilization of the positive charge on the terminal carbon atom, which hence results in a tight binding between gold and the terminal carbon atom and partial positive charge localization on the neighboring carbon atom.26 Our results show that this effect is much stronger for terminal alkynes, with either alkyl or aryl substituents, than for terminal alkenes (the calculated Mulliken charges at individual atoms can be found in the Supporting Information; see also the discussion below). Next, we address the interactions of Au(PMe3)+ with two double bonds. As a model, we have chosen eight-membered cyclic hydrocarbons, because these allow us to compare the coordination of the gold cation with the single double bond in cyclooctene, the conjugated system of 1,3-cyclooctadiene, and the two isolated double bonds in 1,5-cyclooctadiene. The experimental binding energy of the gold cation with cyclooctene is 1.89 ± 0.02 eV. This value is larger than that found for 1-pentene. This is probably due to the strain in the eightmembered ring, which makes the C−C double bond very reactive27 and therefore also the interaction energies with reactant partners are larger. The experimental interaction energy with 1,5-cyclooctadiene is determined as 1.93 ± 0.03 eV and thus is almost identical with the interaction energy found for the single double bond in cyclooctene. This finding suggests
Table 1. Binding Energies of Unsaturated Hydrocarbons with the (Trimethylphosphino)gold Cation CnHm styrene phenylacetylene benzene 1-pentene 1-pentyne 2-pentyne cyclooctene 1,5-cyclooctadiene 1,3-cyclooctadiene
BDEexptl (eV)
BDEtheora (eV)
krelb
± ± ± ± ± ± ± ± ±
1.61 1.64 1.31 1.57 1.60 1.69 1.72 1.83 1.63
0.435 ± 0.025
1.85 1.87 1.68 1.82 1.88 1.91 1.89 1.93 1.83
0.04 0.04 0.03 0.05 0.07 0.04 0.02 0.03 0.03
0.461 0.315 0.129 0.015 0.018 0.006 0.072
± ± ± ± ± ± ±
0.017 0.029 0.011 0.002 0.002 0.002 0.007
a
Calculations were performed using density functional theory (mPW1PW91/cc-pVTZ:LanL2DZ) and include corrections for the basis-set superposition error. bReactions between [(CnHm)Au(PMe3)]+ and phenylacetylene (pressure: 0.2 mtorr). For the determination of the relative reaction rate constants see Experimental Details.
between gold and the terminal carbon atom is 0.40 Å shorter than that to the inner carbon atom of the triple bond. The isomers with the gold cation coordinated to the aromatic ring lie 0.38 eV higher in energy. If we take the ring-coordinated isomers as a relative anchor, the difference between the binding of the gold cation to the double bond and that to the triple bond is about 0.13 eV, which is much more than that found as a difference between the binding energies in [Au(PMe3)(styrene)]+ and [Au(PMe3)(phenylacetylene)]+. The difference can stem from the effect of conjugation. Accordingly, below we investigate the interactions of Au(PMe3)+ with aliphatic systems and benzene independently. To this end, we have generated complexes of the gold cation with benzene, 1-pentene, 1-pentyne, and 2-pentyne. In agreement with the above results, the smallest binding energy is found for the complex between the gold cation and benzene. The experimental binding energy amounts to 1.68 ± 0.03 eV. The experimental binding energy between 1-pentene and the gold cation is determined as 1.82 ± 0.05 eV, whereas the binding energies between the gold cation and 1- and 2pentyne amount to 1.88 ± 0.07 and 1.91 ± 0.04 eV, 1937
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relative rates of the reactions were normalized to the reaction rate of the exchange reaction between [Au(PMe3)(CH3OH)]+ and phenylacetylene (reaction 2). This normalization should
that gold might interact with only one double bond of the diene, which would be also expected on the basis of the preference of gold(I) for a linear coordination of only two ligands (here the phosphine and one of the double bonds).28 Surprisingly, the smallest binding energy is found for the gold complex of 1,3-cyclooctadiene. The experimental binding energy amounts to 1.83 ± 0.03 eV. According to the calculations, the gold cation coordinates only to one of the double bonds of 1,3-cyclooctadiene (Figure 3). This result is
[Au(PMe3)(CH3OH)]+ + Ph−CCH k CH3OH
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Au(PMe3)(Ph−CCH)]+ + CH3OH
(2)
account for a possible variation of the pressure in the collision cell (see Experimental Details). The relative rate constants (krel = ki/kCH3OH) are given in Table 1. Figure 4 shows the dependence of ln(krel) on the determined binding energies. As expected, the rate constants decrease with
Figure 3. Optimized structures of the gold complexes with cyclooctene, 1,3-cyclooctadiene, and 1,5-cyclooctadiene at the mPW1PW91/cc-pVTZ:LanL2DZ level of theory. Selected bond lengths are given in angstroms.
consistent with the findings of Sanguramath et al.,29 who reported an X-ray structure of 2,5-dimethylhexa-2,4-diene with a gold cation and showed that the metal is η2 coordinated to only one of the C−C bonds. The smaller binding energy can be again explained by the conjugation, which decreases the binding energy similarly as found for phenylacetylene. Again, the computed binding energies of the complexes of Au(PMe3)+ with cyclooctene, 1,5-cyclooctadiene, and 1,3cyclooctadiene, respectively, are lower than the experimental values (see Table 1). However, the theoretical binding energies of the eight-membered cyclic hydrocarbons in the gold complexes decrease in the order 1,5-cyclooctadiene, cyclooctene, and 1,3-cyclooctadiene, which is in full agreement with the experimental findings. Notwithstanding these trends, all of the determined binding energies are very close to each other. In view of experimental and theoretical errors, some of the binding energies might thus be misordered. We have therefore decided to perform an independent cross-check of the relative binding energies by investigating ligand-exchange reactions in the gas phase.30 The reactions are performed at nominally zero collision energy and hence under thermal energies. Under these conditions, Bouchoux and co-workers have demonstrated that it is possible to relate the rate constants of the ligand-exchange reactions with the associated ΔG values.31 In the present study, the absolute values of the binding energies derived from the CID experiments can be thus compared with a sensitive determination of the relative binding energies of the various ligands in the ligand-exchange reactions. We have selected phenylacetylene as a reactant for the ligand-exchange reactions, because its binding energy to Au(PMe3)+ is in the middle of the determined binding energies. The mass-selected complexes [Au(PMe3)(CnHm)]+ were reacted with neutral phenylacetylene (reaction 1) and the
Figure 4. Dependence of ln(krel) on the binding energies of the investigated unsaturated hydrocarbons to the gold cation.
increasing binding energy of the hydrocarbon to the gold cation. In agreement with the previous results, it can be clearly seen that the binding energies decrease in the order 2-pentyne >1-pentyne >1-pentene (blue symbols). The binding energies of the eight-membered cyclic hydrocarbons are again clearly in the order 1,5-cyclooctadiene > cyclooctene > 1,3-cyclooctadiene. The weakest binding is again found for benzene. The only disagreement is found for styrene, which is predicted to be bound more weakly from the ligand-exchange reactions than from the CID experiments. The fact that 1,3-cyclooctadiene is more weakly bound to the gold cation in comparison to plain cyclooctene is remarkable. The effect was studied also in the ligand-exchange reactions, in which the cyclooctadienes and cyclooctene were exchanged among themselves (reaction 3). The reaction rate was again [Au(PMe3)(C8H12)]+ + C8H14 ki
→ [Au(PMe3)(C8H14)]+ + C8H12
normalized to the reaction rate of the ligand-exchange reaction of the methanol complex (reaction 4). The reaction between [Au(PMe3)(CH3OH)]+ + C8H14 k CH3OH
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Au(PMe3)(C8H14)]+ + CH3OH
→ [Au(PMe3)(Ph−CCH)]+ + CnHm
(4)
1,5-cyclooctadiene and cyclooctene is much slower (krel = 0.070 ± 0.012) than that of 1,3-cyclooctadiene and cyclooctene (krel =
[Au(PMe3)(Cn Hm)]+ + Ph−CCH ki
(3)
0.186 ± 0.020), which again suggests a larger binding energy for 1,5-cyclooctadiene to the gold cation than for the
(1)
conjugated 1,3-diene. 1938
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As a further complement, we computationally and also experimentally studied the interactions of Au(PMe3)+ with the smallest unsaturated hydrocarbons, namely acetylene, propyne,26c,d ethylene, and butadiene. The binding energies increase in the order acetylene < ethylene < butadiene < propyne (Table 2). The finding that acetylene is even less tightly bound than
Additional insight can be gained from the charge distributions in the complexes. According to Mulliken population analysis (see numbers in italics in Figure 5), a substantial part of the positive charge is delocalized to the methyl substituents of the multiple bonds (about +0.2e per methyl substituent). Further, there is a fundamental difference between terminal alkenes and alkynes. Coordination of the gold cation to alkenes with either terminal or internal double C−C bonds leads to a more or less equal charge distribution in the double bond. However, the coordination of Au(PMe3)+ to terminal alkynes leads to a strong polarization of the C−C triple bond and buildup of a large positive charge at the C(2) carbon (see the charge distribution in the complex of propyne). This explains the large differences between the Au−C bonds in the complexes of terminal alkynes and also the large substituent effects on the binding energies in the gold−alkyne complexes. Analogous charge distributions are obtained for all other complexes studied here (see the Supporting Information). As far as conjugated dienes are concerned, the introduction of one methyl group increases the binding energy of the olefin to the gold cation from 1.50 eV determined for butadiene to 1.65 eV determined for 1,3-pentadiene. Interestingly, the Au(PMe3)+ cation preferentially coordinates to the terminal methylene group. The coordination to the internal double bond leads to a drop of the binding energy by 0.13 eV (see Figure 5). In agreement, the binding energy determined for the complex of Au(PMe3)+ with 2,4-hexadiene, i.e. a conjugated diene bearing methyl groups on both ends of the conjugated system, amounts to 1.62 eV (note that this value is close to the binding energy determined for the gold complex of 1,3-cyclooctadiene). These results clearly show that the gold cation preferentially coordinates to the terminal methylene group of conjugated dienes. Our findings hence provide experimental and theoretical evidence for the generally accepted view that gold(I) catalysts preferentially bind to carbon−carbon triple bonds in comparison to carbon−carbon double bonds. Nevertheless, this is true only for alkyl- or aryl-substituted multiple bonds. If we compare only acetylene and ethylene, then the binding to ethylene is substantially preferred. Similarly, competitive binding of gold to a terminal triple C−C bond and an internal double bond can lead to the complexation to the double bond (cf. the binding energies in the complexes of 1-propyne and 2butene). This finding could shed light on some gold(I)catalyzed reactions of enynes, in which substrates with terminal and internal triple bonds yield different products or the reactions proceed via presumably different mechanisms.33 Further, this effect is also important with respect to model studies, which are often performed with smaller homologues of the reactants and could lead to misleading interpretations if substitution effects are not taken into account.
Table 2. Binding Energies of Unsaturated Hydrocarbons with the (Trimethylphosphino)gold Ion CnHm
BDEtheora (eV)
krelb
acetylene ethylene 1-propyne 1-propene 2-butyne 2-butene 1,3-butadiene 1,3-pentadiene 2,4-hexadiene
1.27 1.41 1.51 1.52 1.64 1.56 1.50 1.65 1.62
0.52 ± 0.05 0.41 ± 0.05
0.11 ± 0.01
a
Calculations were performed using density functional theory (mPW1PW91/cc-pVTZ:LanL2DZ) and include corrections for the basis-set superposition error. bRelative reaction rate constants for the ligand-exchange reaction with propyne in the collision cell (pressure 0.2 mTorr), the rate constants are normalized to the rate constant of the reference reaction between [Au(PMe3)(MeOH)]+ and propyne.
ethylene, whereas propyne is the most strongly bound ligand in the series, demonstrates the tremendous effect of the substitution of the triple bond. Butadiene is more strongly bound than ethylene. Unlike the case of the eight-memberedring systems, butadiene turns out to be more strongly bound to Au(PMe3)+ than ethylene, which is attributed to the greater polarizability of the larger diene in comparison to ethylene. The relative binding energies of the small hydrocarbons were also investigated by means of ligand-exchange reactions (Table 2). Propyne was selected as the neutral reactant, and the relative rate constants were determined from reactions 5 and 6. [Au(PMe3)(CnHm)]+ + C3H 4 ki
→ [Au(PMe3)(C3H 4)]+ + CnHm
(5)
[Au(PMe3)(CH3OH)]+ + C3H 4 k CH3OH
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Au(PMe3)(C3H 4)]+ + CH3OH
(6)
Clearly, the experimental results fully support the theoretical binding energies and show that the binding energies for the simplest unsaturated hydrocarbons increase in the order acetylene < ethylene < butadiene. We have studied a series of other exchange reactions, which show the same effect; the results can be found in the Supporting Information. We have further explored computationally the effect of the alkyl substituents on the binding energies (Table 2). The binding energies between Au(PMe3)+ and propene and propyne, respectively, are very similar (1.52 and 1.51 eV, respectively). If we formally add methyl groups to the terminal carbon atoms of both multiple bonds, the binding energy of 2butyne to Au(PMe3)+ is already considerably larger than for 2butene (1.64 vs 1.56 eV, respectively). Hence, the alkyl substituents appear to be crucial for the large affinity of triple C−C bonds toward cationic gold complexes.32
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CONCLUSIONS We have determined the binding energies of the (trimethylphosphino)gold cation with a series of unsaturated C5−C8 hydrocarbons. The results show that the gold cation preferentially coordinates to C−C triple bonds, whereas the weakest binding energies are found for the coordination to aromatic rings. The interaction energies of Au(PMe3)+ to C−C triple bonds are roughly 0.1 eV larger than those to C−C double bonds and about 0.2 eV larger than those to aromatic rings. We have also explored the interaction energies between Au(PMe3)+ and small unsaturated hydrocarbons, such as 1939
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Figure 5. Optimized structures of complexes of acetylene, ethylene, 1-propene, 1-propyne, 2-butene, 2-butyne, 1,3-butadiene, 1,3-pentadiene, and 2,4-hexadiene with (trimethylphosphino)gold at the mPW1PW91/cc-pVTZ:LanL2DZ level of theory. The selected bond lengths are given in angstroms. Numbers in red italics give Mulliken charges. The charges at the carbon atoms are summed together with the adjacent hydrogen atoms. Numbers in blue italics give Mulliken charges summed for the whole PMe3 groups.
Notes
acetylene, ethylene, propyne, and propene. It is shown that alkyl or aryl substitution of the C−C triple bond is crucial for the generally accepted view of the strong interaction between gold and C−C triple bonds. In fact, acetylene itself has a substantially smaller binding energy to Au(PMe3)+ than does ethylene. These findings are of importance for the conceptual explanation of various enyne reactions catalyzed by gold(I) compounds in the condensed phase. While we can usually expect preferential coordination of gold to internal C−C triple bonds, reactions of systems containing terminal C−C triple bonds and internal C−C double bonds may thus be initiated by an attack of the gold cation at the C−C double bond, rather than the alkyne.34 A similarly important role of alkylation is found in the interactions of Au(PMe3)+ with conjugated double bonds. For terminal dienes, however, coordination to the terminal methylene group appears to be generally preferred. While the present results from gas-phase experiments certainly cannot be transferred to the condensed phase in a strict 1:1 fashion, the described intrinsic trends can form a starting point in future discussions of reaction mechanisms in homogeneous gold catalysis.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grant No. 207/11/0338 from the Grant Agency of the Czech Republic and the Ministry of Education of the Czech Republic (No. MSM0021620857).
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ASSOCIATED CONTENT
S Supporting Information *
Figures, tables, and text giving details and data about the retarding-potential analysis, the effect of the collision-gas pressure in the determination of the binding energies, relative cross-section measurements, the results of the L-CID fitting, relative reaction rate constants for the ligand-exchange reactions, theoretical results (binding energies, geometries, and energetics obtained by the alternative M06 functional and geometries, energetics and Mulliken charges for all calculated structures at the mPW1PW91 level of theory) and the complete ref 20. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 1940
dx.doi.org/10.1021/om2012387 | Organometallics 2012, 31, 1935−1942
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(34) Note that reactions are influenced not only by thermochemical preferences but also by kinetic factors, which we did not study here.
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