Catalytic Decarboxylative Coupling of Allyl Acetate: Role of the Metal

Article pubs.acs.org/Organometallics

Catalytic Decarboxylative Coupling of Allyl Acetate: Role of the Metal Centers in the Organometallic Cluster Cations [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+ Halah Al Sharif,†,‡,§,∥ Krista L. Vikse,†,‡ George N. Khairallah,*,†,‡ and Richard A. J. O’Hair*,†,‡,§ †

School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, Victoria 3010, Australia § ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, The University of Melbourne, Melbourne, Victoria 3010, Australia ∥ Ministry of Higher Education, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Metal-catalyzed decarboxylative coupling reactions offer new opportunities for formation of C−C bonds. Here, multistage ion trap mass spectrometry experiments together with DFT calculations are used to examine the role of the metal centers in coinage metal cluster catalyzed decarboxylative coupling of allyl acetate in the gas phase via a simple two-step catalytic cycle. In step 1, the metal acetate cluster cation [CH3CO2Cu2]+, [CH3CO2AgCu]+, or [CH3CO2Ag2]+ is subjected to collision-induced dissociation to yield the organometallic cluster cation [CH3Cu2]+, [CH3AgCu]+, or [CH3Ag2]+, respectively. Step 2 involves subjecting these organometallic cluster cations to ion−molecule reactions with allyl acetate with the aim of generating 1-butene and re-forming the metal acetate cluster cations to close the catalytic cycle. Experiment and theory reveal the role of the two metal centers in both steps of the gas-phase catalytic reaction. All three metal acetates undergo decarboxylation (step 1), although when competing reactions are taken into account, the yield of [CH3Cu2]+ is highest (83.3%). Ion−molecule reactions of the organometallic cations with allyl acetate all proceed at the collision rate; however, the types of products formed and their yields vary considerably. For example, only [CH3Cu2]+ and [CH3AgCu]+ undergo the C−C bond-coupling reaction (step 2) in yields of 52.7% and 1.2%, respectively. Overall the dicopper clusters are the superior decarboxylative coupling catalysts, since they give the highest yields of the desired products for both steps 1 and 2. These results highlight that the reactivity of organometallic coinage metal clusters can be “tuned” by varying the composition of the metal core.

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INTRODUCTION Decarboxylation reactions of metal carboxylates, RCO2M, to yield organometallics, RM,1 continue to attract considerable interest in synthetic applications2 as inexpensive and environmentally benign sources of R for incorporation into organic molecules. With appropriate metals and substrates, these reactions can be made catalytic. One of the earliest examples of such a catalytic process is the copper-catalyzed protodecarboxylation reaction (eq 1), which was first reported by Shepard et al. in 19303 and which has subsequently been used to transform aromatic carboxylic acids to aromatic compounds.4 Early mechanistic studies have identified that arylcopper species are formed as intermediates.5−7 Both Cu(I) and Cu(II) salts have been used, but Cohen suggested that Cu(I) complexes are the active catalysts and that a chelating ligand such as 1,10-phenanthroline increases the rate of decarboxylation.6a ArCO2 H → ArH + CO2

catalyzed cross-coupling reactions and C−X bond-forming reactions;2,9 and the use of mass spectrometry and DFT calculations to examine mechanistic aspects of decarboxylation in the gas phase10 and the subsequent unimolecular and bimolecular reactions of the resultant organometallics.11 Catalytic cycles can be studied in the gas phase using multistage mass spectrometry experiments in ion-trapping instruments12,13 where the catalytic intermediates or byproducts may be isolated and their unimolecular or bimolecular reactivity can be probed. In conjunction with theoretical modeling14 this approach can shed light on the mechanisms of the various elementary steps involved in catalysis.15 For example, a recent gas-phase study reported a new decarboxylative coupling reaction of allyl acetate (eq 2)16 catalyzed by the copper carboxylate anion [CH3CuO2CCH3]−.11h CH3CO2 CH 2CHCH 2 → CH3CH 2CHCH 2 + CO2

(1)

(2)

Research into coinage metal catalyzed decarboxylation reactions continues on several fronts, including: the recent use of copper nanoparticles;8 development of a range of metal© 2013 American Chemical Society

Received: July 19, 2013 Published: September 16, 2013 5416

dx.doi.org/10.1021/om400712n | Organometallics 2013, 32, 5416−5427

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procedures using the advanced scan function of the LCQ software, which allows the Q value and the activation time to be varied. The neutral substrate allyl acetate was introduced at various concentrations into the ion trap via the helium inlet line, as reported previously.15a Rates of reaction of allyl acetate with the organometallic cluster reactant cation [CH3M1M2]+ (where M1 and M2 are any combination of Cu and Ag) were measured by varying the time delay between isolation of the reactant cation and mass analysis of the resulting products (“reaction delay”, RD). The reaction of the organometallic cluster cation [CH3M1M2]+ was monitored over at least seven values of RD. The intensity of the reactant ion was calculated by integration of its ion count within the mass-selected window. Approximate pseudo-first-order rates were determined from plots of −ln(([CH3M1M2]+ intensity)/(total ion intensity)) vs RD. Rate constants were calculated by dividing the pseudo-first-order rate coefficient by the calculated concentration of allyl acetate in the ion trap. The rate constants reported are the average of five independent measurements conducted on four separate days. The branching ratios reported are the average of three independent measurements conducted on three separate days. Standard deviations for rate constants were typically around 14%, and we conservatively estimate an error of ±25%. However, relative rates are expected to be more accurate, due to the cancellation of errors. Theoretical collision rates were calculated via the average dipole orientation (ADO) theory of Su and Bowers using the COLRATE program.19 Efficiency values were calculated for each reaction by dividing the average absolute reaction rate by the theoretical ADO collision rate and multiplying by 100%. Accurate mass measurements on the precursor ions as well as isobaric ions encountered were performed on a LTQ-FT hybrid mass spectrometer (Thermo, Bremen, Germany) described previously,11h in order to unequivocally determine elemental composition of the ions. DFT Calculations. DFT calculations were carried out using Gaussian 09.20 The M06 functional was used21 in conjunction with a 6-31+G(d) basis set for C, H, and O and the Stuttgart/Dresden (SDD) effective core potential for Ag and Cu.22 Vibrational frequency calculations were carried out on each optimized structure at the same level of theory. All minima were characterized by the presence of no imaginary frequencies, while the transition states (TS) were characterized by the presence of a single imaginary frequency. Intrinsic reaction coordinate (IRC) calculations were carried out to confirm the relationship between the transition state and the reactant(s) and product(s). All quoted relative energies (E0) include zero-point vibrational energy (unscaled) calculated at the optimization level and are not corrected for basis set superposition error. Full data (Cartesian coordinates, energies, and imaginary frequencies for transition states) are given in the Supporting Information. Visualizations of the structures used in the figures were created using Gaussview.

Step 1 involved decarboxylation to give the dimethyl cuprate anion, [CH3CuCH3]−, which then underwent allylic alkylation in step 2 (Scheme 1). Scheme 1. Two-Step Catalytic Cycles for Decarboxylative Coupling of Allyl Acetate Catalyzed by the Copper Carboxylate Anion [CH3CuO2CCH3]− 11h or the Coinage Metal Cluster Cations [CH3CO2M1M2]+, Where M1 and M2 Are Any Combination of Cu and Ag (This Work)

Since related allylic alkylation reactions have been observed for the reactions of allyl iodide with the organometallic cluster cations [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [RCO2Ag2]+ (where R = CH3,11b CCR′11i) we were interested in evaluating whether these cluster cations might catalyze the decarboxylative coupling reaction of allyl acetate (Scheme 1). Thus the aims of this work were to use mass spectrometry based experiments and DFT calculations to examine (i) the decarboxylation of [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [CH3CO2Ag2]+ and (ii) the reactions of allyl acetate with the organometallic cluster cations [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+. This work provides a unique opportunity to examine the role of the metal centers in catalysis by stoichiometrically equivalent and structurally related metal complexes.

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EXPERIMENTAL SECTION

Reagents. Silver nitrate, copper(II) acetate, allyl acetate, and acetic acid were obtained from Aldrich and used without further purification. Methanol was purchased from Merck and used without further purification. The following gases were used in the mass spectrometry experiments: helium (ultra high purity, Coregas) as the bath gas and nitrogen (high purity, Coregas) as the auxiliary gas. Mass Spectrometry. Ion−molecule reactions (IMR) were performed on a modified Finnigan LCQ quadrupole ion trap mass spectrometer equipped with a Finnigan ESI (electrospray ionization) source and modified to allow the study of ion−molecule reactions as previously described.15a Gronert has shown that the temperatures of ions trapped in these quadrupole ion trap mass spectrometers without ion activation are at near-thermal conditions (i.e., room temperature).18 For silver solutions silver nitrate and acetic acid were dissolved in methanol in a molar ratio of ca. 1:3, respectively. For copper solutions copper(II) acetate and acetic acid were dissolved in methanol in a molar ratio of ca. 2:3, respectively. For mixed-metal solutions silver nitrate, copper(II) acetate, and acetic acid were dissolved in methanol in a molar ratio of ca. 1:2:3, respectively. The resulting mixtures were diluted with methanol to a maximum concentration of acetic acid of ca. 1 mM. These solutions were introduced into the electrospray source via a syringe pump at a flow rate of 5 μL min−1. Typical electrospray conditions involved needle potentials of 4.5−5 kV and a heated capillary temperature of 200 °C. Copper-containing clusters react rapidly with water and methanol. In an attempt to minimize these background reactions, the auxiliary gas was set to 30 (arbitrary units). Mass selection, collisional activation, and ion−molecule reactions were carried out using standard isolation and excitation

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RESULTS AND DISCUSSION

Since the catalytic cycle studied involves two steps (Scheme 1), the results are discussed in the following order: (i) fragmentation of [CH3CO2Cu2]+ (1), [CH3CO2AgCu]+ (2), and [CH3CO2Ag2]+ (3) with an emphasis on the productive decarboxylation channel and (ii) reaction of allyl acetate with [CH3Cu2]+ (4), [CH3AgCu]+ (5), and [CH3Ag2]+ (6). While the focus will be on the dicopper system 1, the mixed-metal and disilver systems 2 and 3, respectively, will be described as they relate to 1. 1. Fragmentation of [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [CH3CO2Ag2]+: Competition between Decarboxylation and Other Pathways. The low-energy collision-induced dissociation (CID) reactions of the metal acetate cluster cations 1−3 have been previously reported.11b Here we have reexamined them using energy-resolved CID experiments to unambiguously establish the relative energies associated with the different fragmentation pathways. In addition, potential 5417


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Scheme 2. Isomers of the Coinage Metal Acetate Cluster Cations [CH3CO2M1M2]+, Where M1 and M2 Are Any Combination of Cu and Ag

S1). Table 1 gives the relative energies of each isomer and the relative activation energies for their interconversion (TS1− TS4).

mechanisms for the decarboxylation of 1−3 in the gas phase are examined for the first time using DFT calculations. Finally, before discussing the observed fragmentation it is important to consider the structure of the reactant ions. 1.1. Isomers of Reactant Ions [CH 3 CO 2 Cu 2 ] + , [CH3CO2AgCu]+, [CH3CO2Ag2]+ and Their Interconversion. There are three possible isomers for the bimetallic ions [CH3CO2Cu2]+ and [CH3CO2Ag2]+ (Scheme 2, isomers a−c). In our previous work on the C−O bond formation reaction between [CH3CO2Ag2]+ and allyl iodide we carried out DFT calculations on the three isomers of [CH3CO2Ag2]+.23 The same three isomers are also possible for [CH3CO2Cu2]+, and they differ in the relative positions of the two metal atoms. In isomer a both metals point away from the methyl group and could be said to represent a “down−down” orientation. This is the only isomer in which calculations predict a direct metal− metal interaction (2.75 Å). Isomer b has an “up-down” configuration, while isomer c has an “up-up” configuration. Since for the mixed-metal system [CH3CO2AgCu]+ M1 is not the same as M2, an additional fourth isomer (d in Scheme 2) is possible. Figure 1 shows the calculated structures for [CH3CO2Cu2]+ as a representative example (all other structures are given in the Supporting Information, Figure

Table 1. DFT-Calculated (M06/SDD-6-31+G(d)) Relative Energies for the Isomers of [CH3CO2M1M2]+ and the Associated Transition States for Interconversion E (eV) structure

[CH3CO2Cu2]+

[CH3CO2AgCu]+

isomer a isomer b isomer c isomer d TS1 (a → b) TS2 (b → c) TS3 (a → d) TS4 (c → d)

0.17 0.00 0.27

0.25 0.00 0.32 0.045 0.35 0.36 not found 0.42

0.30 0.34

[CH3CO2Ag2]+ 0.30 0.00 0.34 not found 0.38

Isomer b is the most stable isomer for 1 (Figure 1), and this is also true for the metal acetate cluster cations 2 and 3 (Table 1). The transition states associated with interconversion between the various isomers are all relatively low in energy and involve an in-plane rocking motion of the CH3CO2M moiety. See the Supporting Information (Figure S1) for images of isomers of 2 and 3 and their related transition states. Importantly, for each of the systems 1−3 the transition states for interconversion between the various isomers are all significantly lower in energy than the relevant barrier for decarboxylation (compare calculated energies in Tables 1 and 2). As the low-energy CID process in the ion trap involves multiple collisions in the millisecond time frame, this suggests that the isomers will have the opportunity to interconvert prior to fragmentation. Therefore, the question of which of these isomers is the best precursor to decarboxylation is discussed in section 1.5. 1.2. Fragmentation Pathways of Reactant Ions [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [CH3CO2Ag2]+. Isolation within an ion trap followed by low-energy CID of the metal acetates 1−3 yielded the mass spectra shown in Figure 2. Three types of fragmentation pathways are observed in all cases and correspond to (1) formation of the organometallic cluster cation [CH3M1M2]+ via decarboxylation (eq 3), (2) cluster fragmentation to yield [CO2M1]+ or [CO2M2]+ (eqs 4a and 4b), and (3) formation of the bare metal cation M1+ or M2+ via loss of a neutral metal acetate (eqs 5a and 5b). In the case of [CH3CO2Cu2]+ (1, Figure 2a, m/z 185) the major fragmentation pathway is decarboxylation (m/z 141, eq 3). Minor peaks are observed for formation of the bare ion Cu+ (m/z 63, eq 5) and for [CO2Cu]+ (m/z 107, eq 4). For the mixed-metal cluster [CH3CO2AgCu]+ (2, Figure 2b, m/z 229), decarboxylation is less prominent. Instead, loss of a bare silver cation becomes the dominant product ion (m/z 107, eq 5a).

Figure 1. DFT-calculated (M06/SDD-6-31+G(d)) structures of the isomers of [CH3CO2Cu2]+ (1) and the relevant transition state structures for their interconversion. Ground state and transition state energies are given in eV and are relative to the lowest energy structure. The displacement vectors for the imaginary frequency of each TS are also shown. 5418


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[CH3CO2 M1M2]+ → [CH3M1M2]+ + CO2 +

(3)

→ [CO2 M1] + CH3M2

(4a)

→ [CO2 M2]+ + CH3M1

(4b)

→ M1+ + CH3CO2 M2

(5a)

+

→ M2 + CH3CO2 M1

(5b)

[CO2Cu]+ have the same nominal mass. Accurate mass experiments performed on a LTQ FT-ICR instrument confirm that the peak observed at (m/z 107 in Figure 2b) is due entirely to Ag+ (data not shown). Finally, for [CH3CO2Ag2]+ (3, Figure 2c, m/z 273) decarboxylation (m/z 229, eq 3) and loss of Ag+ (m/z 107, eq 5) appear to be competitive processes and only a small signal is observed for the formation of [CO2Ag]+ (m/z 151, eq 4). In each of these experiments, additional peaks are observed due to reaction with background gases present in the ion trap (marked with a # in Figure 2). 1.3. Branching Ratios and Relative Energetics for Fragmentation of [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [CH3CO2Ag2]+ from DFT Calculations and Energy-Resolved CID Experiments. In order to better understand the observed fragmentation pathways for [CH3CO2M1M2]+ and the associated relative energetics, we compared the experimental relative intensities of the various product ions (branching ratios, BR) with the DFT-calculated activation energies and final reaction endothermicities for each fragmentation pathway. Table 2 summarizes the key data for all of the fragmentation pathways for ions 1−3. Branching ratios are reported at the collision energy where the intensity of the precursor ion, not including solvent adducts, reaches ca. 10%. Consider the case where M1 = M2 = Cu (Table 2, column 2). Decarboxylation (eq 3, 83.3% BR) is clearly favored experimentally over generation of [CO2Cu]+ (eq 4, 0.7% BR), and this is consistent with the overall DFT-calculated endothermicities of the two pathways, which highly favor decarboxylation (0.36 eV) over generation of [CO2Cu]+ (2.04 eV). Loss of Cu+ (eq 5, M1 = M2 = Cu) is calculated to be a highly endothermic process (2.83 eV) and is therefore not expected to be observed in low-energy CID experiments. The fact that it is observed experimentally with a branching ratio of 16% can be explained by considering the collision energy at which the branching ratios were measured. Energy-resolved CID experiments (Figure 3) show the relative intensities of the product ions for fragmentation of 1 as a function of collision energy, and the vertical dotted line indicates the collision energy at which the reported branching ratios were calculated. This collision energy was chosen to minimize error in the branching ratio measurement; however, it does lead to an overemphasis in the importance of the Cu+ fragmentation pathway. From Figure 3 it is clear that generation of Cu+ does not occur to a significant extent until the normalized collision energy is higher than 27 (arbitrary units). In fact, the branching ratios at a collision energy of 20 (arbitrary units) are approximately 100% for decarboxylation (eq 3) and 0% for generation of [CO2Cu]+ (eq 4) and Cu+ (eq 5). Thus, consistent with the high endothermicity of formation for Cu+, it is not formed initially; rather, when the collision energy is increased sufficiently there is enough energy to form Cu+ from the reactant ion [CH3CO2Cu2]+. Furthermore, at higher

Figure 2. CID spectra for (a) [CH3CO2Cu2]+ (1; m/z 185), (b) [CH3CO2AgCu]+ (2; m/z 229), and (c) [CH3CO2Ag2]+ (3; m/z 273). The precursor ion is designated with an asterisk (*). The collision energy was chosen such that the intensity of the remaining precursor ion was approximately 10%. The number marks (#) represent peaks due to reactions with background solvents and gases present in the ion trap. The peaks selected for CID were the most intense in their isotopic clusters. In the case of 1 and 2 the diacetate complex [(CH3CO2)2M1M2]+ was preselected and fragmented to access the metal acetate of interest.

Interestingly, only Ag+ is lost while loss of Cu+ (m/z 63, eq 5b) is not observed. Similarly, a small peak is observed for formation of [CO2Ag]+ (m/z 151, eq 4a), but there is no signal for [CO2Cu]+ (m/z 107, eq 4b). This may be an effect of the lower ionization energy of silver (ca. 0.15 eV less than copper) and/or the generally weaker ligand binding energies of silver relative to copper.24 It is important to note that Ag+ and 5419


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Table 2. Relative Intensities of Fragmentation Products (Branching Ratios, BR) and Relevant Calculated Energies of the Associated Fragmentation Pathways for Ions 1−3, [CH3CO2M1M2]+ [CH3CO2Cu2]+ (1) fragmentation pathway

BR (%)

+

[CH3M1M2] + CO2 (eq 3) [CO2M1]+ + CH3M2 (eq 4a) [CO2M2]+ + CH3M1 (eq 4b) M1+ + CH3CO2M2c (eq 5a) M2+ + CH3CO2M1c (eq 5b)

83.3 0.7 n/a 16 n/a

a

E (eV)

[CH3CO2AgCu]+ (2) b

BR (%)

2.01 (0.36) (2.04) n/a (2.83) n/a

20.4 1.8 trace 77.8 trace

a

E (eV)

[CH3CO2Ag2]+ (3) b

1.85 (0.41) 1.85 (1.70) 1.85 (2.34) (2.69) (2.99)

BR (%)a

E (eV)b

44.5 1.8 n/a 53.9 n/a

2.06 (0.51) 2.06 (1.93) n/a (2.16) n/a

a

Branching ratios are calculated as the intensity of a fragmentation product divided by the sum of the intensities of all fragmentation products multiplied by 100%. The reported ratios are an average of three separate experiments. Errors range between ±0.04 and ±2.09%. bDFT-calculated activation energies (and reaction endothermicities in parentheses). Energies were calculated at the M06/SDD-6-31+G(d) level. cThe reaction is assumed to be barrierless.

observed to be the major product ion on fragmentation of [CH3CO2AgCu]+ and [CH3CO2Ag2]+ (Figure 2b, c). A discrepancy can be observed on comparing the results in Table 2 for fragmentation pathway 5a for [CH3CO2AgCu]+ and [CH3CO2Ag2]+. Theory predicts that we should observe the formation of more Ag+ from CID of [CH3CO2Ag2]+, but in fact we observe significantly more Ag+ from CID of [CH3CO2AgCu]+. This may be due to a failure of our chosen level of theory to accurately predict Ag−O bond strengths.25 To sum up, in terms of developing a catalyst for the decarboxylative coupling of allyl acetate (Scheme 1), the most effective system for decarboxylation (step one of the catalytic cycle) is where M1 = M2 = Cu. Both the overall yield of and selectivity for the desired organometallic cluster cation [CH3Cu2]+ is highest. 1.4. Isomers of Product Ions [CH3Cu2]+, [CH3AgCu]+, [CH3Ag2]+ and Their Interconversion. As is the case with the reactant ions, the organometallic product ions [CH3M1M2]+ could also exist in isomeric forms. There are two possibilities: cyclic and linear (Scheme 3, isomers e and f, respectively).

Figure 3. Energy-resolved MS2 CID experiments on [CH3CO2Cu2]+ (1). The relative intensities of the fragmentation products of 1 with respect to the total ion current are given as a function of collision energy. The vertical dotted line indicates the collision energy at which the reported branching ratios were calculated. Note that the ion intensities shown include the solvent adducts.

collision energies the product ions [CH3Cu2]+ and [CO2Cu]+ may have enough internal energy to undergo secondary fragmentation reactions and lose Cu+ (eqs 6 and 7). Indeed, subsequent CID of the product ions [CH3Cu2]+ and [CO2Cu]+ results in loss of Cu+ (data not shown). [CH3M1M2]+ → M1+ + CH3M2

(6)

[CO2 M1]+ → M1+ + CO2

(7)

Scheme 3. Isomers of the Coinage Metal Organometallic Cluster Cations [CH3M1M2]+, Where M1 and M2 Are Any Combination of Cu and Ag

A careful examination of entropy considerations for the three fragmentation pathways can also provide key insight into the experimentally observed formation of Cu+, and this is discussed in section 1.5. The product branching ratios and calculated fragmentation pathway energies for CID of ions 2 and 3 (Table 2, columns 3 and 4) follow trends similar to those discussed for ion 1 (see Figures S2 and S3 in the Supporting Information for the energy-resolved CID plots). There is good agreement between experiment and theory for decarboxylation and cluster fragmentation (eqs 3 and 4). Once again, as in the case with [CH3CO2Cu2]+, the amount of bare metal cation formed by CID of [CH3CO2AgCu]+ and [CH3CO2Ag2]+ is greater than what was predicted by theory. However, the energies required to lose Ag + from [CH 3 CO 2 AgCu] + (2.69 eV) and [CH3CO2Ag2]+ (2.16 eV) are less than that for losing Cu+ from [CH3CO2Cu2]+ (2.83 eV), reflecting the weaker Ag−O bond strengths.10b,23 A noteworthy effect of substituting one or both copper atoms for silver atoms in the metal clusters is that the loss of a single metal cation (eq 5) becomes increasingly competitive with decarboxylation (eq 3). Indeed, loss of Ag+ is

A previous gas-phase UV−vis spectroscopy study on [CH3Ag2]+ in an ion trap mass spectrometer has revealed that it mainly exists as the cyclic isomer (isomer e, Scheme 3), and this is consistent with DFT calculations, which reveal that the cyclic isomer is the most stable11f (Supporting Information, Figure S4). The isomeric structures for [CH3Cu2]+ (4) and the transition state for their interconversion are shown in Figure 4, while the related structures for [CH3AgCu]+ (5) and [CH3Ag2]+ (6) are shown in the Supporting Information (Figure S4). The DFT-calculated energies for all organometallic cluster cations are given in Table 3. An examination of Figure 4 and Table 3 reveals that in all cases the cyclic isomers 4e−6e are more stable than the linear isomers 4f−6f. The key feature of the transition state for isomerization involves changing the M1−C−M2 bond angle, which is associated with a significant barrier in each case: 0.87 eV for [CH3Cu2]+, 0.77 and 1.01 eV for [CH3AgCu]+, and 0.82 5420


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related transition states are also largely unchanged (Supporting Information, Figure S5). With an understanding of the structures of the reactant and product ions, in the next section we use DFT calculations to examine how these structures might be related via transition states for decarboxylation. 1.5. Insights into the Detailed Mechanisms of the Decarboxylation Reaction of [CH3CO2Cu2]+, [CH3CO2AgCu]+, and [CH3CO2Ag2]+ from DFT Calculations. Since isomers a−d can interconvert prior to decarboxylation, we were prompted to search for geometrically different decarboxylation transition states. We discovered two distinct classes of decarboxylation transition states, as follows. (1) The lowest energy transition states (TS7, TS10, TS11, and TS13 in Figure 5) in all cases correspond to a four-centered transition state which involves cleavage of the H3C−CO2 bond and formation of a H3C−metal bond while one of the oxygen atoms of the CO2 group interacts with both metal centers. These transition states give rise to the cyclic isomer of the organometallic cation [CH3M1M2]+ (isomer e). For the heterometallic cluster [CH3CO2AgCu]+, direct involvement of the copper atom in the four-centered transition is favored over involvement of silver, as TS10 is 0.41 eV lower in energy than TS11. (2) For [CH3CO2AgCu]+ and [CH3CO2Ag2]+, transition states TS12 and TS14 were found, which are reminiscent of a bicyclo[2.2.0]hexane structure. In both cases these transition states are higher in energy than the four-centered transition states. These transition states give rise to the linear isomer of the organometallic cation [CH3M1M2]+ (isomer f). A related transition state structure was not located for [CH3CO2Cu2]+. Figure 6 shows a DFT-calculated potential energy diagram for the decarboxylation of 1 together with the energies associated with the other experimentally observed fragmentation pathways (eqs 4 and 5). Similar potential energy diagrams are provided in the Supporting Information for the decarboxylation of 2 and 3 (Figure S6), and all relative activation energies and endothermicities for the three systems are summarized in Table 2. Of the reaction pathways examined, the lowest energy pathway for decarboxylation of 1 occurs from the lowest energy conformer [CH3CO2Cu2]+ (1b). The decarboxylation begins with copper-mediated H3C−CO2 bond cleavage (TS7, 1.94 eV) and H3C−Cu bond formation. The resulting intermediate (Figure 6, INT1) is interesting in a number of respects. (1)

Figure 4. DFT-calculated (M06/SDD-6-31+G(d)) isomers of the coinage metal cluster cation [CH3Cu2]+ (4) and the relevant transition state structure for their interconversion. Ground state and transition state energies are given relative to the lowest energy structure.

Table 3. DFT-Calculated (M06/SDD-6-31+G(d)) Relative Energies for the Isomers of [CH3M1M2]+ and the Associated Transition States for Interconversion E (eV) +

structure

[CH3Cu2] (4)

[CH3AgCu]+ (5)

[CH3Ag2]+ (6)

isomer e isomer f TS5 (e → f) TS6 (e → f)

0.00 0.24 0.87

0.00 0.23 0.77 1.01

0.00 0.08 0.82

eV for [CH3Ag2]+. In the case of [CH3AgCu]+ the presence of two different metals breaks the symmetry of the cyclic structure. This is most apparent in the carbon−metal bond lengths (1.95 Å for CH3−Cu and 2.35 Å for CH3−Ag). As a result, there are two potential transition states (TS5 and TS6) which lead to the linear isomer (Table 3 and Figure S4a (Supporting Information)). An interesting aspect of these decarboxylation reactions which is discussed in more detail in section 1.5 is that after C− C bond cleavage the CO2 initially remains coordinated to one of the metal centers as a neutral ligand before it is finally lost (see 4e·CO2 in Figure 6). Thus, we have also considered how the coordinating CO2 might change the relative energies of the isomer and the transition states for their interconversion. A comparison of the energetics for the bare organometallic cluster cations (Table 3) and CO2-ligated species (Table S1) reveals that coordination of CO2 does not have a dramatic effect on either the relative energies of the isomers or the associated barriers for isomerization. The geometries of the isomers and

Figure 5. DFT-calculated (M06/SDD-6-31+G(d)) geometries, imaginary frequencies, and energies of possible decarboxylation transition states relative to the associated lowest energy ground state acetate structure. 5421


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2. Ion−molecule Reactions of [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+ with Allyl Acetate: Competition between Carbon−Carbon Bond-Forming and Other Reactions. Here we describe step 2 of the catalytic cycle: reaction of allyl acetate (C3H5CO2CH3) with each of the organometallic cluster ions [CH3Cu2]+ (4), [CH3AgCu]+ (5), and [CH3Ag2]+ (6). We first discuss the experimentally determined competing reaction pathways, branching ratios, and kinetics for the ion− molecule reactions of these cluster cations with allyl acetate. We then describe the results of DFT calculations aimed at determining the mechanisms for C−C bond formation. 2.1. Ion−Molecule Reactions of [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+ with Allyl Acetate. The organometallic ions 4− 6 were allowed to undergo gas-phase ion−molecule reactions with allyl acetate, and Figure 7 shows a representative spectrum Figure 6. DFT-calculated (M06/SDD-6-31+G(d)) potential energy diagram for the fragmentation reactions of 1. Energies for decarboxylation (eq 3), formation of [CO2Cu]+ (eq 4) and formation of Cu+ (eq 5, assumed barrierless) are given relative to the calculated ground-state energy of the mass-selected ion [CH3CO2Cu2]+ (1b).

There is no direct Cu−Cu bond, reminiscent of proposed mechanisms for binuclear gold activation of alkynes.26 (2) Although the H3C−CO2 bond has been cleaved, the CO2 moiety remains bent (O−C−O bond 141°) and coordinates to methylcopper via the central carbon atom. It is only at TS9 that the linear CO2 molecule is formed. (3) It is easy to imagine facile loss of [CO2Cu]+ from INT1, and this intermediate is most likely a major precursor to [CO2Cu]+ formation via the loss of CH3Cu  a reaction calculated to have an overall endothermicity of 2.04 eV. The other potential precursor to formation of [CO2Cu]+ via CH3Cu loss is INT2. Moving forward, a two-step rearrangement via TS8 (2.01 eV) and TS9 (1.92 eV) leads to the most stable CO2-coordinated cyclic isomer of [CH3Cu2]+ (4e·CO2, −0.49 eV). Loss of CO2 gives the organometallic cluster cation [CH3Cu2]+ (4e + CO2, 0.36 eV). The DFT calculations show that decarboxylation is the lowest energy pathway and thus most likely to occur from a thermodynamic perspective, and this is consistent with experimental results. However, as mentioned in section 1.1, DFT predicts the formation of Cu+ (eq 5) to be energetically unfavorable in comparison to decarboxylation (eq 3) or formation of [CO2Cu]+ (eq 4), when in fact formation of Cu + is the secondmost prominent fragment observed experimentally. On further consideration of the DFT results, we note two possible explanations for this. (1) It is important to consider the differences in the processes which form 4e, [CO2Cu]+, and Cu+. The first two products necessitate a tight transition state (TS7), whereas loss of Cu+ could occur directly from 1b as a barrierless process. This barrierless process would be significantly favored with respect to entropy considerations, and this could explain the fact that Cu+ is observed experimentally.27 (2) Fragmentation to yield the bare Cu+ cation (2.83 eV) could presumably occur from either or both of INT1 and INT2 and, to a lesser extent, 1b and 4e·CO2. Furthermore, since each of these structures has the same mass and charge, the CID process will energize each of these structures equally for the duration of the activation time. Thus, fragmentation to yield Cu+especially from INT1 or INT2 appears feasible.

Figure 7. Ion−molecule reaction of [CH3Cu2]+ (m/z 141) with allyl acetate at a concentration of ca. 2 × 108 molecules cm−3 and reaction time of 1500 ms. Primary reaction channels are labeled with filled shapes: (▲) association of allyl acetate (m/z 241); (●) C−C bond formation to give 1-butene and [CH3CO2Cu2]+ (m/z 185); (◆) Formation of [Cu(C3H5CO2CH3)]+ (m/z 163). Secondary reactions with allyl acetate are labeled with empty shapes that correspond to the related primary product peak. Reactions with background water and methanol are labeled with the number sign (#). The mass-selected ion is marked with an asterisk (*).

for [CH3Cu2]+, m/z 141 (see Figures S7 and S8 in the Supporting Information for analogous spectra for [CH3AgCu]+ (m/z 185) and [CH3Ag2]+ (m/z 229)). At first glance, a large number of reactions seem to occur, but on closer inspection three primary reaction channels were identified corresponding to (1) association of allyl acetate (▲) [C6H11O2M1M2]+ (m/z 241, eq 8, M1 = M2 = Cu), (2) C−C [CH3M1M2]+ + C3H5CO2 CH3 → [C6H11O2 M1M2]+

(8)

→ [CH3CO2 M1M2]+ + C4 H8

(9)

→ [M1(C3H5CO2 CH3)]+ + CH3M2

(10a)

→ [M2(C3H5CO2 CH3)]+ + CH3M1

(10b)

bond formation to generate 1-butene and regenerate the active catalyst (●) [CH3CO2Cu2]+ (m/z 185, eq 9, M1 = M2 = Cu), 5422


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Table 4. Experimental Relative Product Yields and Calculated Energetics of the Ion−Molecule Reactions of [CH3M1M2]+ with Allyl Acetate [CH3Cu2]+ (4) product channel +

[C6,H11,O2,M1,M2] (eq 8) [CH3CO2M1M2]+ + C4H8 (eq 9) [M1(C3H5CO2CH3)]+ + CH3M2 (eq 10a) [M2(C3H5CO2CH3)]+ + CH3M1 (eq 10b)

yield (%) 23.4 29.3 47.3

a

E (eV)

[CH3AgCu]+ (5) b

yield (%)

−1.37 (−3.66) −1.37 (−0.83) (−0.83)

c

23.1 1.2 74.2 1.6

a

[CH3Ag2]+ (6)

E (eV)

b

−0.81 (−3.22) (−2.22) −0.98 (−0.80) (−0.58) (−0.46) c

d

yield (%)a

E (eV)b

46.9 0.00 53.1

(−1.98)d −0.10 (−0.84) (−0.32)

Relative product yields are calculated at a late stage in the reaction (2800−3000 ms reaction delay) at an allyl acetate concentration of ca. 2 × 108 molecules cm3. bDFT-calculated reaction activation energies (and exothermicities in parentheses). Energies are calculated at the M06/SDD-631+G(d) level. cEnergetic data are given for the product ion−molecule complex with 1-butene. dEnergetic data are given for the reactant ion− molecule complex with allyl acetate. a

this peak corresponds to the product association complex [CH3CO2Cu2(C4H8)]+. For the mixed-metal system M1 = Ag, M2 = Cu, the peak corresponding to [C6H11O2AgCu]+ (m/z 285) fragments primarily by allyl acetate assisted core fission to form [Cu(CH3CO2C3H5)]+ (m/z 163) and the neutral product AgCH3 (eq 12) in addition to a small channel corresponding to loss of butene and formation of [CH3CO2AgCu]+ (m/z 229, eq 11). This suggests that this peak corresponds primarily to the reactant association complex [CH3AgCu(C3H5CO2CH3)]+. When M1 = M2 = Ag, the peak corresponding to [C6H11O2Ag2]+ (m/z 329) fragments by allyl acetate assisted core fission to form AgCH3 and [Ag(CH3CO2C3H5)]+ (m/z 207, eq 12), suggesting that this peak also corresponds to the reactant association complex [CH3Ag2(C3H5CO2CH3)]+.

and (3) abstraction of a copper cation to form the copper−allyl acetate complex [Cu(C3H5CO2CH3)]+ (◆, m/z 163, eq 10, M1 = M2 = Cu). The other peaks in the spectrum can be attributed to secondary reactions with water and methanol and association of further molecules of allyl acetate. For example, the primary product [Cu(C3H5CO2CH3)]+ (m/z 163) reacts with water to give [Cu(C3H5CO2CH3)·H2O]+ (m/z 181), with methanol to give [Cu(C3H5CO2CH3)·MeOH]+ (m/z 195), and with allyl acetate to give [Cu(C3H5CO2CH3)2]+ (m/z 263). Primary and secondary reactions were assigned on the basis of the order of appearance of the ions with respect to reaction time (see Figure S9 in the Supporting Information), as well as by isolation of individual primary products to allow direct observation of the secondary reactions (data not shown). Table 4 gives the relative yields of the primary reactions by summing the relative intensities of the primary reaction product peaks with all related secondary product peaks for each of the organometallic cations 4−6. Table 4 also includes the relevant thermochemistry data (E in eV) from DFT calculations, which are described in detail in the next section. Several measurements of the rate of consumption of the organometallic cations 4−6 upon reaction with allyl acetate were undertaken (see Figure S10 in the Supporting Information) and were averaged to yield rate constants of 3.7 × 10−9, 4.2 × 10−9, and 3.3 × 10−9 cm3 molecule−1 s−1 for the reactions of [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+, respectively. These reactions are very fast, with measured rates 2−3 times higher than the theoretical ADO rate28 (see Table S2 in the Supporting Information). With regard to the catalytic cycle, arguably the most important consideration is the relative yield of the desired C−C bond coupling reaction (eq 9) with respect to the other product channels. The copper system 4 is superior in this regard, with a branching ratio of 29.3% for 1-butene formation. The mixed-metal system 5 produces a meager yield of 1.2% 1butene, while the silver system 6 produces no 1-butene at all. It is noteworthy that there is a substantial yield of the complex of formula [C6H11O2M1M2]+ (eq 8) for all systems, which raises an interesting question: are these reactant association complexes formed between the organometallic ion and allyl acetate, [CH3M1M2(C3H5CO2CH3)]+, or are they product association complexes between the metal acetate complexes and 1-butene, [CH3CO2M1M2(C4H8)]+? In order to address this question, each of the complexes [C6H11O2M1M2]+ were mass-selected and subjected to CID (see Figures S11−S13 in the Supporting Information). In the case of M1 = M2 = Cu, the peak corresponding to [C6H11O2Cu2]+ (m/z 241) fragments by loss of butene (m/z 185) (eq 11), and thus we propose that

[CH3CO2 M1M2(C4 H8)]+ → [CH3CO2 M1M2]+ + C4 H8 (11)

[CH3M1M2(C3H5CO2 CH3)]+ → [M2(C3H5CO2 CH3)]+ + CH3M1

(12)

From the results of these CID experiments, we can now assign the experimentally observed complexes [C 6 H 11 O 2 M1M2] + (eq 8) to the product complex [CH3CO2M1M2(C4H8)]+ in the case of M1 = M2 = Cu and to a reactant association complex ([CH 3 M1M2(C3H5CO2CH3)]+) for M1 = M2 = Ag and M1 =Ag, M2 = Cu. Thus, the overall product yield for the C−C bond forming reaction promoted by [CH3Cu2]+ (4) requires summing the yields for eqs 8 and 9 and corresponds to 52.7%. For the case of [CH3AgCu]+ (5) only a small increase to the yield of eq 9 is expected and, therefore, the branching ratio is slightly over 1.2%. For the case of [CH3Ag2]+ (6) the yield of 1-butene remains essentially 0%. This further confirms that [CH3Cu2]+ is superior in promoting C−C bond formation. Finally, for all systems, allyl acetate-assisted fission of the metal core (eq 10) is a major reaction pathway. For the copper system 4, core fission (47.3% yield) represents a reaction that is competitive with C−C bond formation, while for the mixedmetal (5) and silver (6) systems core fission is the dominant reaction channel upon reaction with allyl acetate. Thus, in order to design a viable bimetallic catalyst for decarboxylative coupling of allyl acetate, this reaction channel must be suppressed. 2.2. DFT Calculations on the Reactions of [CH3Cu2]+, [CH3AgCu]+, and [CH3Ag2]+ with Allyl Acetate. Here we discuss the results of DFT calculations for step 2 of the catalytic cycle. Since the experiments revealed that [CH3Cu2]+ is the superior catalyst, we focus our discussions on this system and 5423


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Figure 8. (a) DFT-calculated (M06/SDD-6-31+G(d)) potential energy diagrams for the gas-phase reaction of [CH3Cu2]+ (4e) (blue), [CH3Ag2]+ (6e) (brown), and [CH3AgCu]+ (5e) (purple for reductive elimination on Cu and green for reductive elimination on Ag) with allyl acetate. The dotted lines represent the competing product channel corresponding to eq 10: (A) formation of [Ag(C3H5CO2CH3)]+ (−0.32 eV) from 6e; (B) formation of [Ag(C3H5CO2CH3)]+ (−0.46 eV) from 5e; (C) formation of [Cu(C3H5CO2CH3)]+ (−0.58 eV) from 5e; (D) formation of [Cu(C3H5CO2CH3)]+ (−0.84 eV) from 4e. (b) DFT-calculated structures of all intermediates and transition states. The numbers in parentheses represent the energies relative to the separated reactants.

highlight important observations about the other two systems where appropriate. Figure 8 represents potential energy

diagrams for the gas-phase reactions of allyl acetate with the organometallic cluster cations 4−6.29 These potential energy 5424


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Table 5. Summary of the Experimental Observations for the C−C Cross-Coupling Reactions for Mono- and Bimetallic Systems monometallic substrate allyl acetate rel yield, %a rateb allyl iodide rel yield, %a rateb

bimetallic

[CH3AgCH3]−

[CH3CuCH3]−

N/A N/A

8111h 1111h

33300

8111g 3.211g

2411g 86311g

10011b c

[CH3Ag2]+

d

[CH3Cu2]+

[CH3AgCu]+

52.7d 37300d

1.2d 41700d

10011b c

10011b c

The relative yield of the C−C cross-coupling product in the reaction. bOverall rate constant for the decay of the reactant ion, in units of 10−13 cm3 molecules−1 s−1. cRates were comparable to those of [CH3Cu2]+ and [CH3AgCu]+ in this work.30 dThis work. a

mixed-metal system, i.e. TS20, involves both metal atoms and is only 0.30 eV lower in energy than the separated reactants. The silver system M1 = M2 = Ag also proceeds via a reductive elimination step which involves only one of the metal atoms (TS22), which is 0.10 eV lower in energy than the separated reactants. Additionally, by a comparison of these reductive elimination transition states (Figure 8b, TS16, TS19, TS20, and TS22) the following observations can be made: in the case of M1 = M2 = Cu, an early transition state (TS16, Cmethyl−Callyl bond = 2.88 Å) is observed, whereas late transition states are observed in all the other cases where the Cmethyl−Callyl bond has already formed (TS19, TS20, and TS22, C−C bonds ranging from 2.14 to 2.25 Å). The [CH3CO2M1M2·(1-butene)]+ product complexes formed from the reductive elimination transition states are similar for all three systems and are observed to be very stable, with relative energies ranging from 2.76 to 3.66 eV lower than the separated reactants. These complexes appear to be stabilized by a combination of π-bonding and agostic interactions between the metal sites and 1-butene. Loss of 1butene from this complex, which is likely favored entropically as it involves simple ligand dissociation, results in regeneration of the metal acetate [CH3CO2M1M2]+ . 3. Comparisons with Previously Reported Carbon− Carbon Cross-Coupling Reactions of Organometallic Ions with Allylic Substrates. It is interesting to compare the carbon−carbon bond forming ability of the organometallic ions [CH3M1M2]+ (M1 and M2 are any combination of Ag and Cu) and [CH3MCH3]− (M = Ag, Cu) toward allyl iodide11a,g and allyl acetate.11h Table 5 summarizes these C−C coupling reactions. Both the overall reactivity and selectivity of the metal complexes are considered. The reaction of the bimetallic systems [CH3M1M2]+ with allyl acetate, studied above, proceed via any or all of the three competing channels (i.e. C−C cross-coupling, cluster fragmentation, and association of the neutral substrate). Hence, when the bimetallic cluster was reacted with allyl acetate where one or both of the metals (M1 and M2) were silver, the C−C crosscoupling reaction channel was either very weak (1.2% yield, M1 = Cu and M2 = Ag) or did not proceed at all (M1 = Ag and M2 = Ag), whereas the other two competing channels (cluster fragmentation and association of the neutral substrate) were prominent. When both metals are copper, all three channels are accessible and competitive. On the other hand, the reactions of [CH3M1M2]+ with allyl iodide11b proceeded cleanly via a C−C bond forming reaction in all cases (eq 13) (Table 5, 100%

diagrams for C−C bond coupling are relatively similar in that they involve discrete oxidative addition and reductive elimination steps. It is worth noting that for the mixed-metal cluster [CH3AgCu]+ (5) two viable coupling reactions were found that differ in the site of attack by allyl acetate (i.e. oxidative addition at copper or at silver). Initially, allyl acetate forms a reactant association complex with the organometallic cation in which the carbonyl group coordinates to one of the metal centers and the allyl group coordinates to the other (Figure 8; 4e·(allyl acetate), 5e·(allyl acetate), 6e·(allyl acetate)). Formation of the association complex is highly exothermic (e.g. −2.65 eV in the case of 4e·(allyl acetate)) due to the low coordination number of both metal centers in the reactant cation. The oxidative addition transition states (Figure 8, TS15, TS17, TS18, and TS21) involve oxidative addition of the C−O ester bond onto one of the metal centers, while the second metal center acts as a second binding point for the acetate ligand. For instance, in the case of [CH3Cu2]+ (4e), the C−O ester bond oxidatively adds onto one of the copper centers, while the second copper center acts as a second binding point for the acetate ligand. This transition state leads to the formation of an organometallic intermediate in which both acetate and methyl groups bridge the binuclear metal core while the allyl group coordinates in a η3 fashion to only one metal center (Figure 8, INT). The reductive elimination transition state for the all-copper system TS16 (−1.49 eV) clearly involves both metal centers in the C− C bond forming process. From TS16, an intermediate is formed (1a·(1-butene), −3.66 eV), where 1-butene is now attached to a copper acetate cluster. In the final step, loss of 1butene proceeds with an overall reaction exothermicity of 0.83 eV. Also of interest in Figure 8 is the observation that the thermochemistry of the C−C bond-forming reaction is calculated to be the same as the abstraction of Cu+ by allyl acetate to form the complex [Cu(C3H5CO2CH3)]+ (eq 10, −0.83 eV) (Figure 8a, dotted line D). This is consistent with its significant experimental yield (ca. 47%). This abstraction of Cu+ by allyl acetate is also likely favored entropically, as it involves simple bond cleavages, unlike the pathway to form 1butene, which requires two tight transition states. The mechanistic similarity between the reactions of the three [CH3M1M2]+ systems ends at the intermediate complex (INT). Figure 8b shows the calculated structures for the reductive elimination transition states for all three systems (including two transition states for the mixed-metal system, TS19 and TS20). Specifically, the lowest energy transition state for the mixed-metal system (M1 = Ag, M2 = Cu) involves reductive elimination solely from the single copper atom (TS19), which is 0.98 eV lower in energy than the separated reactants. The other reductive elimination transition state in the

[CH3M1M2]+ + C3H5I → [IM1M2]+ + C4 H8

5425

(13)


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complex [CH3AgCu]+, and [CH3Ag2]+ did not react to form 1butene at all. The low yield in the mixed-metal and disilver cases appears to be due, in large part, to a competitive allyl acetate assisted cluster fragmentation reaction which generates the metal complexes [M(C3H5CO2CH3)]+ and destroys the potential active catalyst. DFT calculations provided insights into the observed reactivity trends. The formation of 1-butene reactions proceeds via oxidative addition of allyl acetate to the organometallic cation followed by reductive elimination of 1butene. A marked difference was observed in the case of the reductive elimination step of the dicopper system, which proceeds via an early transition state that involves both metal centers. Comparison with previously reported C−C crosscoupling reactions mediated by coinage metal ions highlight that, of the complexes studied, the dicopper cation is the most effective in promoting of C−C bond formation. We propose that the success of the dicopper system may be due to a number of key factors, including a higher Cu−O bond strength,24 which prevents the competitive loss of the metal cation (eq 5) in the decarboxylation step, thereby increasing the yield of [CH3Cu2]+ (eq 3), lower barriers for oxidative addition to copper, likely due to a higher electron density at copper,17 and the involvement of both copper centers in the reductive elimination step, which significantly lowers its barrier. Given the significant recent interest in bimetallic coinage metal catalysts, we are currently examining the formation and bimolecular reactivity of a range of bare and ligated cluster cations of the types [RM1M2] + and [RM1M2(L)n ] + , respectively (where R is a range of organic groups, M1 and M2 are any combination of Ag and Cu, Au and L = auxiliary ligand(s)). It will be interesting to systematically explore how auxiliary ligands further tune the reactivity of organometallic clusters toward organic substrates. In particular, it will be important to establish if a more selective bimetallic catalyst for decarboxylative coupling of allyl acetate can be designed which avoids competing reactions in both the decarboxylation and C− C bond-coupling steps.

yield) with rates comparable to those observed in the reactions with allyl acetate.30 These reactions, however, are stranded in the product [IM1M2]+ and cannot close a catalytic cycle to reform [CH3CO2M1M2]+ . The reaction of the monometallic [CH3MCH3]− (M = Cu) with allyl acetate proceeds via several potential competing channels,11h including C−C cross-coupling (eq 14). Cross[CH3MCH3]− + C3H5CO2 CH3 → [CH3CO2 CuCH3]− + C4 H8

(14)

coupling was by far the most favored channel (Table 5, 81% yield), generating the product ion [CH3CO2CuCH3]− and thus completing a catalytic cycle. However, the rate of the reaction was slow (11 × 10−13 cm3 molecules−1 s−1) with a reaction efficiency of only 0.032%. 11 h Thus, the bimetallic [CH3M1M2]+ clusters are well over 1000 times more efficient on the basis of the rate of decay of the parent ion. It is worth noting that the reaction of [CH3MCH3]− (M = Ag) with allyl acetate has not been studied. Moreover, the reactions of the monometallic [CH3MCH3]− (M = Cu, Ag) with allyl iodide proceed via several competing channels, including C−C crosscoupling (eq 15).11g In the case where M = Ag, the cross[CH3MCH3]− + C3H5I → [ICuCH3]− + C4 H8

(15)

coupling reaction was the major channel with a yield of 81%. However, kinetics show it to be slow (3.2 × 10−13 cm3 molecules−1 s−1) with an efficiency of 0.028% (relative to the collision rate). When M = Cu, cross-coupling accounted for 24% of the product yield with a rate ca. 250 times faster than for M = Ag (863 × 10−13 cm3 molecules−1 s−1) and an efficiency of 6.6% (Table 5). Hence, we can show experimentally that the nature of the leaving group (iodide versus acetate) and the metal site(s) play important roles in controlling reactivity. Additionally, Table 5 highlights that the dicopper system ([CH3Cu2]+)31 appears to have a good efficiency, good selectivity for the C−C crosscoupling reaction, and the ability to complete a catalytic cycle.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables referred to in the text. A file of all computed molecule Cartesian coordinates in a format for convenient visualization. Cartesian coordinates, energies and vibrational frequencies for reactants, intermediates, products and transition states (Figure S14−S35) and the full citation for ref 19. This material is available free of charge via the Internet at http:// pubs.acs.org.

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CONCLUSIONS As noted by Hashmi, many of the previous studies of coinage metal catalyzed reactions provide data that do not allow a direct comparison of the catalytic activity, since not only the metal but also the counterions and/or the ligands and/or the solvents are often changed in a random fashion.17 The current work highlights the value of multistage mass spectrometry experiments and DFT calculations in establishing the mechanistic details of the individual steps for decarboxylative coupling of allyl acetate catalyzed by the stochiometrically equivalent and structurally related bimetallic cations [RM1M2]+. In the first step, the organometallic cations (R = CH3, M1, M2 = Ag or Cu) were synthesized in the gas phase via decarboxylation of the metal acetate precursors [RCO2M1M2]+. In the second step these organometallic cations were allowed to react with allyl acetate with the aim of closing the catalytic cycle via reformation of the metal acetates. Dicopper complexes were found to be superior in both of these key steps. Specifically, reaction of [CH3Cu2]+ with allyl acetate produced the highest yield of C−C bond coupling. In contrast, the C−C bond formation channel is only a minor pathway for the mixed-metal

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AUTHOR INFORMATION

Corresponding Authors

*G.N.K.: e-mail, [email protected]. *R.A.J.O.: tel, +61 3 8344-2452; fax, +61 3 9347-5180; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the ARC for financial support via grant DP110103844 and DP1096134 (to R.A.J.O. and G.N.K.). We thank The University of Melbourne IT Services for computer access. H.A.S. thanks the Ministry of Higher Education of Saudi Arabia for financial support. 5426


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Catalytic Decarboxylative Coupling of Allyl Acetate: Role of the Metal

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