Gas-Phase Unimolecular Reactions of Pallada- and Nickelalactone

Article pubs.acs.org/Organometallics

Gas-Phase Unimolecular Reactions of Pallada- and Nickelalactone Anions Krista L. Vikse,†,‡ George N. Khairallah,†,‡ and Richard A. J. O’Hair*,†,‡ †

School of Chemistry and ‡Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, Victoria 3010, Australia S Supporting Information *

ABSTRACT: Electrospray ionization in combination with multistage mass spectrometry experiments in a linear ion trap mass spectrometer was used to generate and study the gasphase ion chemistry of the metallalactones [(CH3CO2)Ni(CH2CO2)]− (m/z 175, 5a) and [(CH3CO2)Pd(CH2CO2)]− (m/z 223, 5b). Low-energy collision-induced dissociation (CID) resulted in decarboxylation to produce novel organometallic ions at m/z 131 (Ni) and m/z 179 (Pd). Isotope labeling experiments, bimolecular gas-phase reactions with allyl iodide, and DFT calculations reveal that decarboxylation primarily occurs from the acetato ligand to yield ions of the form [(CH3)M(CH2CO2)]− (M = Pd, Ni). Further CID experiments on [(CH3)M(CH2CO2)]− together with DFT calculations highlight the following. (1) Both palladium and nickel can facilitate C−C bond formation, with elimination of ethylene being observed. (2) The mechanism for formation of ethylene from [(CH3)M(CH2CO2)]− is inherently different for M = Pd versus M = Ni. Elimination of ethylene is competitive with further decarboxylation in the case of nickel, and nickel remains in the 2+ oxidation state throughout. In contrast, the palladium complex is reduced to palladium(0) upon C−C bond formation and undergoes a second decarboxylation before ethylene is released. Finally, the products of the ion−molecule reactions of [(CH3)Pd(CH2CO2)]− (7b) with allyl iodide provide evidence for the formation of the Pd(IV) intermediate [(CH3)(I)(CH2CHCH2)Pd(CH2CO2)]−, which decomposes via a range of processes, including losses of iodide, propionate, allyl, and methyl radicals and reductive elimination of butane and methyl iodide.

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INTRODUCTION Palladacycles are a well-known class of compounds, and they have been used extensively as catalyst precursors, due to their easy synthesis and handling (Scheme 1). They most commonly

nickelalactones, which have been well studied for over 20 years with regard to their potential role in the formation of acrylic acid from ethylene and CO2.4 The combination of electrospray ionization and multistage mass spectrometry experiments has proved to be a potent way of forming and studying the unimolecular and bimolecular chemistry of organometallic ions.5 There are two simple ways to synthesize these species by mass spectrometry for subsequent investigation. First, CID of metal carboxylates to induce decarboxylation has been used as a straightforward method to create a range of elusive organometallic ions in the gas phase, including complexes of copper,6 silver,7 gold,8 nickel, palladium, platinum,9 magnesium,10 calcium, strontium, and barium.11 For example, the coinage metal dimethylmetallates have been “synthesized” via double decarboxylation (eqs 1 and 2a) and

Scheme 1. Examples of Palladacycles Used as Catalyst Precursors

[CH3CO2 MO2 CCH3]− → [CH3MO2 CCH3]− + CO2

consist of a five-membered ring containing palladium bound on one side to carbon and on the other to a two-electron donor such as nitrogen, oxygen, sulfur, or phosphorus. While they are most commonly thought of as a controlled source of Pd(0) for catalytic C−C and C−X bond coupling reactions,1 there are also numerous cases in which palladacycles have been reported to be directly involved in catalysis.2 One subclass of palladacycles that has been largely overlooked thus far is the palladalactones. To our knowledge there are only two reports in the literature in which a palladalactone was characterized.3 This is in contrast with the analogous © 2012 American Chemical Society

(1)

[CH3MO2 CCH3]− → [CH3MCH3]− + CO2

→ CH3CO2− + CH3M

(2a) (2b)

their unimolecular and bimolecular chemistry has been compared. While all metals underwent bond homolysis (eq 3a), Received: August 5, 2012 Published: October 15, 2012 7467

dx.doi.org/10.1021/om300741n | Organometallics 2012, 31, 7467−7475

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[CH3MCH3]− → [CH3M]·− + CH3·

(3a)

[CH3MCH3]− → [CH3CH 2MH]−

(3b)

[CH3CH 2MH]− → [HMH]− + CH 2CH 2

(3c)

Two methods were employed to determine the site of decarboxylation from the ion [(CH3CO2)Pd(CH2CO2)]−. Experimental details for the methods are given here, and a full discussion is provided in section 2a of the Results and Discussion. (a). Isotope Labeling. A solution containing palladium acetate (∼1 mM), d4-acetic acid (∼170 mM), and 1-13C-acetic acid (∼170 mM) in methanol was electrosprayed in the negative ion mode. The peak at m/z 226, which primarily represents the two isobaric ions [(CD3CO2)105Pd(CH213CO2)]− and [(CH313CO2)106Pd(CD2CO2)]−, was isolated14 and subjected to CID. The resulting product ions were detected by FT-ICR in order to distinguish between isobaric products. (b). Ion−Molecule Reactions with Allyl Iodide. Ion−molecule reactions of mass-selected ions were examined in an LTQ mass spectrometer. Palladium(II) acetate was dissolved in methanol/water (3/1) containing 0.5% acetic acid to give a 1.0 mM solution. Allyl iodide was introduced into the ion trap via the ion−molecule line at a flow rate of 10 μL h−1 (concentration in the trap of ca. 6.6 × 109 molecules cm−3). The desired ion was mass-selected and allowed to interact with allyl iodide in the ion trap for times ranging from 30 to 500 ms. 3. DFT Calculations. DFT calculations were performed using Gaussian 0915 and the M06 functional.16 The M06 functional was selected because it has been shown to provide more accurate predictions than B3LYP for organometallic systems.17 The Stuttgart− Dresden (SDD) basis set with effective core potential was used for palladium and nickel.18 The AUG-cc-pVDZ basis set was used for carbon, hydrogen, and oxygen.19 Vibrational frequencies were calculated for all optimized structures to check that they corresponded to true ground-state or transition-state structures (having zero imaginary frequencies or one imaginary frequency, respectively). IRC calculations were performed to ensure the correct transition states were reported. All energies were corrected with zero-point vibrational energies (Ereported = Eelectronic + Ezpve).

copper6e and to a lesser extent gold,8 also fragmented via a novel 1,2-dyotropic rearrangement reaction (eqs 3b and 3c), in which both methyl groups underwent dehydro C−C bond coupling. Dimethylcuprate has been shown to be more reactive in C−C bond coupling reactions with methyl iodide6a and allyl iodide.6f A second, less implemented method for producing interesting gas-phase organometallic ions is deprotonation of metal acetates during the ESI process or by CID. Butschke and Schwarz recently reported an example of ESI deprotonation of platinum acetate. They generated organometallic platinum dimers of the form [Pt2(μ-OOCCH3)3(μ-CH2COO)·2CH3OH]+ where deprotonation of one of the acetato ligands resulted in an unsymmetric dianionic C,O-bridging ligand.12 It is this type of process that we have used to gain access to mononuclear metallalactones in the gas phase. Here we report the formation of novel pallada- and nickelalactones and compare their subsequent unimolecular reactivities, with special attention given to C−C bond coupling reactions.

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EXPERIMENTAL SECTION

1. Reagents. Reagents were obtained from the following sources: palladium(II) acetate from Precious Metals Online, nickel(II) acetate tetrahydrate, d4-acetic acid, and 1-13C-acetic acid from Aldrich, glacial acetic acid from BDH, and methanol from Merck. All were used without further purification. 2. Mass Spectrometry. All mass spectrometry experiments were carried out using a Finnigan LTQ FT hybrid linear ion trap (Finnigan, Bremen, Germany) fitted with the standard factory electrospray ionization source that was previously modified to allow the introduction of neutral reagents into the ion trap.13 Palladium(II) acetate or nickel(II) acetate tetrahydrate was dissolved in methanol/water (3/1) containing 0.5% acetic acid to give a 0.5−1.0 mM solution. The solution was transferred via syringe pump to the electrospray source at 5−10 μL min−1. The isotope patterns of palladium (102Pd, 1.02%; 104Pd, 11.14%; 105 Pd, 22.33%; 106Pd, 27.33%; 108Pd, 26.46%; 110Pd, 11.72%) and nickel (58Ni, 68.08%; 60Ni, 26.22%; 61Ni, 1.14%; 62Ni, 3.63%; 64Ni, 0.93%) were used to identify the metal-containing species. In addition, accurate mass measurements and high-resolution mass spectrometry data were obtained by transferring ions of interest from the linear ion trap into the FT-ICR cell. In order to obtain accurate mass measurements, calibration was performed via the in-built automatic calibration function using the recommended LTQ-FT calibration solution, consisting of caffeine, the short peptide MRFA, and Ultramark 1621. CID experiments were performed in the linear ion trap by massselecting the desired precursor anion with a window of m/z 1.1 and subjecting it to CID using standard isolation and excitation procedures (Q = 0.25−0.35, activation time 30 ms).

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RESULTS AND DISCUSSION 1. Electrospray of Nickel and Palladium Acetates. The negative ion electrospray ionization mass spectrum of a solution of nickel(II) acetate is dominated by a signal corresponding to the expected triacetato anion [Ni(O2CCH3)3]− (Scheme 2 and Figure S1 (Supporting Information), m/z 235, 4a). Monoanionic nickel dimers, trimers, tetramers, and pentamers bearing an increasing number of acetato or methoxy ligands account for the other major peaks in the spectrum, with one significant exception: an ion corresponding to [NiC4H5O4]− is observed at m/z 175. On the other hand, when a solution of palladium(II) acetate was subjected to negative ion mode ESI-MS, the analogous palladium-containing ion [PdC4H5O4]− appears at m/z 223 as the dominant palladium-containing anion (Figure 1). In fact, the expected triacetato anion [Pd(O2CCH3)3]− (m/z 283, 4b, Scheme 2) is not observed, even under extremely mild ESI conditions and after tuning the instrument specifically for such an ion. The elemental composition [PdC4H5O4]−, verified by accurate mass and good isotope pattern match,20 suggests a palladium acetate complex that has undergone deprotonation.

Scheme 2. Some Possible Structures for Nickel- and Palladium-Containing Anions Formed by Negative Ion Mode ESI of Nickel(II) Acetate or Palladium(II) Acetate Solutions

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decarboxylation from the acetato ligand to yield [(CH3)Pd(CH2CO2)]− (7b) or (b) decarboxylation from the lactone moiety to yield [(CH3CO2)Pd(CH2)]− (7b′). This scenario is reminiscent of the competitive decarboxylation reactions of mixed carboxylates of copper and silver. In previous work, the site of decarboxylation in mixed carboxylates of copper (eqs 4a and 4b) [CH3CO2 CuO2 CR]− → [CH3CuO2 CR]− + CO2

(4a)

→ [CH3CO2 CuR]− + CO2

(4b)

was probed by allowing the resultant organocuprates to undergo C−C bond coupling reactions with allyl iodide (eqs 5 and 6).6b [CH3CuO2 CR]− + C3H5I → [ICuO2 CR]− + CH3C3H5 (5)

[CH3CO2 CuR]− + C3H5I → [CH3CO2 CuI]− + RC3H5

Figure 1. ESI(−) mass spectrum of a palladium acetate solution in 3/1 methanol/water with 0.5% v/v acetic acid. The ion at m/z 223 corresponds to [Pd,C4,H5,O4]− (confirmed using a high-resolution accurate mass measurement (HRMS): experimental m/z 222.9229, calculated m/z 222.9228, error 0.4 ppm). The expanded region from m/z 218 to 230 shows the full experimental (black line) and calculated (gray bars) isotope pattern for [Pd,C4,H5,O4]−.20

(6)

In followup work, a more direct way of determining the site of decarboxylation in mixed carboxylates of silver was introduced by 13C labeling of the acetate, which readily allows 13CO2 loss (eq 7a) versus 12CO2 loss (eq 7b) to be differentiated.7b

This ion is likely formed by loss of acetic acid from 4b in the ion source (compare with the unimolecular reactivity of 4a in section 4). DFT calculations indicate that the lowest energy structure for this anion in the gas phase is the palladalactone 5b (Scheme 2; see Figure S2 (Supporting Information) for GaussView structures), in which one of the acetato ligands has been deprotonated at carbon and binds through one carbon and one oxygen atom. This structure is 1.43 eV lower in energy than the other possible O, O-bound isomer 6b. DFT calculations predict that in the gas phase the triacetatopalladium anion 4b should be slightly more stable than 5b (plus one molecule of acetic acid) by 0.32 eV; however, only 5b is observed experimentally. Since studies on the structure of palladium(II) acetate in various solvents and at different concentrations and temperatures do not suggest 5b as a typical solution-phase structure,21 the electrospray process appears to favor the irreversible formation of the palladalactone 5b. For the analogous nickel system the calculated energy difference more strongly favors [Ni(O2CCH3)3]− (4a) over the nickelalactone (5a) plus acetic acid by 1.03 eV (Figure S2 in Supporting Information). This is consistent with experimental results, where the triacetato anion 4a dominates in the spectrum of nickel(II) acetate and the nickelalactone 5a has a relative abundance of only 13% after optimization of the signal. Isolation of 5a and 5b in the gas phase provides a unique opportunity to probe the inherent reactivity of organometallic nickel and palladium systems containing a metallalactone moiety, and the following sections further discuss our work in this area. 2. Unimolecular Fragmentation Reactions of [(CH3CO2)Pd(CH2CO2)]−. The dominant peak from the isotope pattern corresponding to the palladalactone 5b (m/z 223) was isolated and subjected to CID. Under these conditions it undergoes decarboxylation cleanly to afford an ion at m/z 179, [Pd,C3,H5,O2]−, as essentially the sole product ion (Figure S3 in the Supporting Information).22 On examination of the structure of 5b it is clear that decarboxylation could be occurring from two different sites: (a)

[CH313CO2 AgO2 CR]− → [CH3AgO2 CR]− + 13CO2

(7a)

→[CH313CO2 AgR]− + CO2

(7b)

In this work, both of these approaches were used to determine the site of decarboxylation of [(CH3CO2)Pd(CH2CO2)]−, as described in the next sections. In addition to these experiments, DFT calculations were carried out to examine the energetics associated with decarboxylation from the acetate and the lactone ligands. (a). Site of Decarboxylation of [(CH3CO2)Pd(CH2CO2)]− from Labeling Studies. Isotope labeling studies were performed to determine the site of decarboxylation. Unfortunately, due to the greater number of Pd isotopes, this proved to be a more challenging experiment than in the case of the mixed silver carboxylate.7b A consideration of the Pd isotopes suggested a double-labeling experiment using deuterated acetate, CD3CO2−, in conjunction with the 13C-labeled acetate, CH313CO2−. Thus, mass selection of the isobaric ions [(CD 3 CO 2 ) 105 Pd(CH 2 13 CO 2 )] − and [(CH 3 13 CO 2 ) 106 Pd(CD2CO2)]− at m/z 22614 followed by low-energy CID gave rise to fragment ions at m/z 182 and 181 from losses of CO2 and 13CO2, respectively (Figure S4 in the Supporting Information). These were confirmed via high-resolution accurate mass measurements. In the cases of [(CD3CO2)105Pd(CH213CO2)]− (where the lactone was 13C labeled) unlabeled CO2 was preferentially lost (85% CO2 loss and 15% 13CO2 loss), indicating that decarboxylation from the acetato ligand is favored (eq 8a). In the case of [(CH313CO2)106Pd(CD2CO2)]−, [(CD3CO2 )105Pd(CH 213CO2 )]−

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→ [(CD3)105Pd(CH 213CO2 )]− + CO2

(8a)

→[(CD3CO2 )105Pd(CH 2)]− + 13CO2

(8b)


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where the acetato ligand was 13C labeled, the result was reversed (eq 9a). This again supports the conclusion that [(CH313CO2 )106 Pd(CD2 CO2 )]− → [(CH3)106 Pd(CD2 CO2 )]− + 13CO2

(9a)

→[(CH313CO2 )105Pd(CD2 )]− + CO2

(9b)

decarboxylation from the acetato ligand is favored and suggests that the ion at m/z 179 corresponds primarily to [(CH3)Pd(CH2CO2)]− (7b) with a minor amount of [(CH3CO2)Pd(CH2)]− (7b′) likely present. (b). Site of Decarboxylation of [(CH3CO2)Pd(CH2CO2)]− from Ion−Molecule Reactions with Allyl Iodide. Ion− molecule reactions were also used to differentiate between the two possible isomers [(CH3)Pd(CH2CO2)]− (7b) and [(CH3CO2)Pd(CH2)]− (7b′). Hence, the ions formed at m/z 179 (Figure S3 in the Supporting Information) were isolated in the ion trap and allowed to react with the neutral reagent, allyl iodide. From previous work we know that if an M−CH3 moiety exists in the ion (as in 7b), we are likely to see coupling of the methyl group with the allyl group of allyl iodide.6 This coupling would be observed as a loss of butene (56 Da) from the ion− molecule complex [Pd,C3,H5,O2]− (7b or 7b′) + allyl iodide (Scheme 3, eq 12). Thus, allyl iodide vapor was entrained into the helium bath gas flow as described previously in order to provide a constant concentration of the neutral reagent within the ion trap for reaction with [Pd,C3,H5,O2]−.13 The addition product of allyl iodide to [Pd,C3,H5,O2]− (m/z 179) was observed at m/z 347 (Figure 2 and Scheme 3, eq 10), and indeed subsequent loss of butene (m/z 291, eq 12) was seen as a major fragment. In addition, losses corresponding to iodide (m/z 127, eq 11), a methyl radical (m/z 332, eq 13), an allyl radical (m/z 306,

Figure 2. ESI(−) mass spectrum of an ion−molecule reaction of [Pd,C3,H5,O2]− (m/z 179) with allyl iodide in the ion trap for 500 ms. The mass-selected ion is marked with an asterisk.

eq 14), methyl iodide (m/z 205, eq 15), and propionate (m/z 73, eq 16) were also detected. All of this is consistent with a precursor ion of the form [(CH3)Pd(CH2O2)]− (7b). Once again, this suggests decarboxylation occurs predominantly from the acetato ligand and not the lactone. The ion observed at m/z 347 in Figure 2 is particularly interesting in its own right, since it corresponds to the organometallic palladium(IV) species [(CH3)(I)(CH2CHCH2)Pd(CH2CO2)]− (8b, Scheme 3). To further substantiate this assignment, the full isotope pattern centered at m/z 179 was mass-selected and allowed to react with allyl iodide. The peak observed at m/z 347 exhibits an excellent isotope pattern match with the calculated isotope pattern for 8b. In addition, accurate

Scheme 3. Product Ions Observed for the Reaction of 7b with Allyl Iodide

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mass measurement further supports our assignment (calculated 346.8769, experimental 346.8771, error ca. 0.75 ppm). (c). DFT Calculations on the Energetics for Decarboxylation of [(CH3CO2)Pd(CH2CO2)]− from the Acetato and Lactone Sites. DFT calculations support our experimental observations that decarboxylation of the acetato ligand is favored over decarboxylation from the palladalactone. The energy required for decarboxylation from the palladalactone is approximately 0.62 eV (or ca. 60 kJ mol−1) higher than decarboxylation from the acetato ligand (Figure 3a).

Figure 4. ESI(−) mass spectrum of the collisionally activated ion [(CH3)Pd(CH2CO2)]− (m/z 179) (7b). The mass-selected ion is marked with an asterisk.

correspond to loss of methane (m/z 163, eq 18), loss of ethylene (m/z 151, eq 19), and loss of C3H4 (m/z 139, eq 20). [(CH3)Pd(CH 2CO2 )]− → [Pd,C2 ,H5]− → [PdC2HO2 ]− + CH4

(17) (18)

→ [(H)Pd(CO2 )]− + C2H4 (19)

→ [PdHO2 ]− + C3H4

Figure 3. (a) DFT (M06/SDD/AUG-cc-pVDZ)-calculated potential energy diagram showing the lowest energy pathways for decarboxylation from the lactone (left) or the acetato (right) ligands in the complex [(CH3CO2)Pd(CH2CO2)]−. (b) Reaction scheme illustrating both possible decarboxylation pathways.

(20)

[Pd,C2,H5]− (m/z 135) appears to fragment further via loss of ethylene (m/z 107), and this was confirmed by performing CID on the isolated ion at m/z 135 (Figure S5 in the Supporting Information). The presence of palladium in the ions m/z 179, 135, and 107 was confirmed by repeating the experiment shown above in Figure 5 while selecting the entire isotope pattern of the peak at m/z 179 (Figure S6 in the Supporting Information). DFT calculations were carried out to gain insights into the structure of [Pd,C2,H5]− and to determine the most likely mechanism for generation of the ions. Somewhat surprisingly, the lowest energy pathway for decarboxylation from [(CH3)Pd(CH2CO2)]− (7b) first requires a C−C bond-forming step (Figure 5a) which is 1.6 eV higher in energy than the entrance channel. The three-centered C−C bond-forming transition state (TS(7b-9)) is a reductive elimination type process in which palladium is reduced from Pd(II) to Pd(0) and in which a new propionate ligand, CH3CH2CO2−, is created (Figure 5, 9). The Pd(0) center can then undergo facile decarboxylation via a second transition state (TS(9-10)), followed by an ion− molecule complex intermediate (10, 0.35 eV lower in energy than 7b) where the Pd center is bound to CO2 on one side and CH3CH2 on the other. Formation of the product, [Pd(CH2CH3)]−, is endothermic by 0.82 eV. To further understand the dissociation of [Pd(CH2CH3)]− into [PdH]− + CH2CH2, DFT was also employed. Thus, from [Pd(CH2CH3)]− (11), formed in the step before (Figure 5), a further input of energy (1.7 eV) is required for β-hydride elimination to occur (Figure 6a, TS(11-12b)). The initially formed

The two transition states TS(5b-7b′) (1.86 eV) and TS(5b-7b) (1.24 eV) in Figure 3 are similar in nature; both involve direct decarboxylation via a three-centered transition state. Consequently, the calculated preference for decarboxylation from the acetato ligand likely arises from the difference in stability of the product ions. Specifically, decarboxylation from the acetato ligand yields a methyl-containing palladium complex with a relative energy of 0.96 eV with respect to 5b, whereas decarboxylation from the palladalactone moiety leads to a comparatively high energy carbene complex (1.86 eV). It is worth noting that there are two other transition states between 5b and TS(5b-7b), and these are represented in Figure 3a by a dotted line. They are both related to rotation of the acetato ligand and have energies of 0.45 and 0.81 eV relative to 5b (see Figure S14 in the Supporting Information). As they are both lower in energy than TS(5b-7b), they are excluded from Figure 3a for clarity. 3. Unimolecular Fragmentation Reactions of [(CH3)Pd(CH2CO2)]− (7b). The organopalladium species [(CH3)Pd(CH2CO2)]− (m/z 179) can be isolated in the absence of a neutral reagent and subjected to a further stage of CID (Figure 4). In this case, the dominant product ion corresponds to a second decarboxylation reaction to yield an ion of stoichiometry [Pd,C2,H5]− (m/z 135, eq 17). Minor ions are observed, and on the basis of accurate mass measurements they 7471


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The nickelalactone [(CH3CO2)Ni(CH2CO2)]− (5a) may be obtained directly from ESI of a solution of nickel acetate or via CID of [Ni(O2CCH3)3]− (4a, eq 21) (in the latter case 5a is not the major product ion (Figure S7 in the Supporting Information)). The branching ratios of the product ions observed upon CID of 4a are 70% for decarboxylation (eq 22), 19% for loss of acetic acid to form the nickelalactone 5a (eq 21), 8% for loss of the acetic acid radical (eq 23), and 3% for the combined loss of both CO2 and acetic acid (eq 24). [Ni(O2 CCH3)3 ]− → [(CH3CO2 )Ni(CH 2CO2 )]− + CH3CO2 H

(21)

→ [(CH3)Ni(O2 CCH3)2 ]− + CO2 (22)

→ [Ni(O2 CCH3)2 ]·− + CH3CO·2

(23)

→[(CH3)Ni(CH 2CO2 )]− + CH3CO2 H + CO2

Figure 5. DFT (M06/SDD/AUG-cc-pVDZ)-calculated potential energy diagram (a) and reaction scheme (b), showing the lowest energy pathway for decarboxylation from [(CH3)Pd(CH2CO2)]− and the proposed intermediate structures.

(24)

These clearly highlight the preference toward decarboxylation and interestingly show a significant loss of the neutral acetic acid radical. It is worth noting that the CID spectra for 5a were identical whether they were obtained by isolating the nickelalactone from the full ESI spectrum (Figure S8a in the Supporting Information) or after generating it from fragmentation of [Ni(O2CCH3)3]− (Figure S8b in the Supporting Information). In addition, these spectra were analogous to the CID spectrum of the palladalactone 5b, with decarboxylation being the only observed process, generating [(CH3)Ni(CH2CO2)]− (m/z 131). DFT calculations for the nickel system indicate that (a) the lactone structure (5a) is the lowest energy structure for [NiO4C4H5]− by 0.76 eV (Figure S2 in the Supporting Information), (b) similar to the palladium system, decarboxylation is favored from the acetato ligand over the lactone by 0.91 eV (Figure S9 in the Supporting Information), and (c) removal of CO2 from [(CH3CO2)Ni(CH2CO2)]− (5a) is energetically equivalent to removal of CO2 from [(CH3CO2)Pd(CH2CO2)]− (5b). The decarboxylated nickelalactone [(CH3)Ni(CH2CO2)]− (7a) was isolated and subjected to CID (Figure 7). In contrast to the palladium analogue, [(CH3)Ni(CH2CO2)]− (7a) does not undergo decarboxylation as the dominant fragmentation pathway. Instead, loss of ethylene to form [(H)Ni(CO2)]− (m/z 103, eq 25) is competitive with decarboxylation ([(H)Ni(C2H4)]−, m/z 87, eq 26).

Figure 6. DFT (M06/SDD/AUG-cc-pVDZ)-calculated potential energy diagram (a) and reaction scheme (b) showing the lowest energy pathway for loss of ethylene from [(CH3)Pd(CH2)]− to generate [PdH]−.

product has a side-bound ethylene ligand, similar to that observed in the β-hydride elimination reactions of ethylcuprates,6b,e which can then dissociate from palladium. This energetically demanding process TS(11-12b) is consistent with the low-intensity signal for loss of ethylene observed after decarboxylation from [(CH3)Pd(CH2CO2)]−. The dissociation of ethylene leads to the palladium(0) hydride [PdH]− (13b). 4. Comparison with the Nickel Congener. The propensity of the palladalactone [(CH3)Pd(CH2CO2)]− (7b) to undergo reductive elimination and form a new carbon−carbon bond is noteworthy, and in order to more fully understand the fundamental reactivity of this system, its reactivity was compared to that of its nickel congener.

[(CH3CO2 )Ni(CH 2CO2 )]− → [(H)Ni(CO2 )]− + C2H4 (25)

→[(H)Ni(C2H4)]− + CO2 (26)

Further fragmentation of the ion at m/z 103 (Figure S10 in the Supporting Information) gives rise to peaks corresponding to [NiH]− (13a, m/z 59, eq 27), [NiO]− (m/z 74, eq 28), [NiOH]− (m/z 75, eq 29), [Ni,C,H,O]− (m/z 87, eq 30), [NiO2]− (m/z 90, eq 31), and [Ni,H,O2]− (m/z 91, eq 32). All of these channels are consistent with the assignment of m/z 103 to [(H)Ni(CO2)]−. 7472


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Figure 7. ESI(−) mass spectrum of the collisionally activated ion [(CH3)Ni(CH2CO2)]− (7a). The mass-selected ion is marked with an asterisk.

[(H)Ni(CO2 )]− → [NiH]− + CO2

Figure 8. DFT (M06/SDD/AUG-cc-pVDZ)-calculated potential energy diagram (a) and reaction scheme (b) showing the lowest energy pathways for decarboxylation or loss of ethylene from [(CH3)Ni(CH2CO2)]−.

(27)

→ [NiO]− + HCO

(28)

→ [NiOH]− + CO

(29)

→ [NiCHO]− + O

(30)

→ [NiO2 ]− + CH

(31)

→ [NiHO2 ]− + C

(32)

transition state (TS(7a-14a)). Additionally, while a pathway like the one shown above for nickel was successfully calculated for the palladium analogue, the transition state was 0.47 eV higher in energy than the three-centered TS(7b-9) shown in Figure 5. From the four-centered nickel-containing transition state (TS(7a-14a)), a nickelalactone possessing a five-membered ring is formed (14a) reminiscent of those proposed in the catalytic conversion of ethylene and CO2 to acrylic acid. In fact, the second half of the reaction pathway shown in Figure 8 is consistent with the reverse of the proposed solution-phase reaction to form acrylic acid.23 The five-membered nickelalactone (14a) rearranges via a three-centered transition state (TS(14a-15a), 0.90 eV) to a nickel species which contains both a ligated ethylene and CO2 group (15a). From this ion− molecule complex, two thermodynamically competitive processes can occur: loss of CO2 to yield 16a (+1.18 eV) or loss of ethylene to yield 12a (+1.15 eV). These calculations are consistent with our experimental observation of competitive loss of ethylene and CO2 upon CID of [(CH3)Ni(CH2CO2)]− (7a). This is yet another example of the value of DFT in interpreting gas-phase events in mass spectrometry, providing the necessary added level of mechanistic detail and energetics to explain the different unimolecular reactivity of the metallalactone complexes 7a and 7b.

Further fragmentation of m/z 87 (Figure S11 in the Supporting Information) gives rise to peaks corresponding to [NiH]− (m/z 59, eq 33) and [NiC2H]− (m/z 83, eq 34), which are both consistent with the assignment [(H)Ni(C2H4)]− (eq 26). [(H)Ni(C2H4)]− → [NiH]− + C2H4 → [NiC2H]− + 2H 2

(33) (34)

Thus, while [(CH3)Pd(CH2CO2)]− (7b) undergoes a second decarboxylation before release of ethylene, the analogous nickel complex [(CH3)Ni(CH2CO2)]− (7a) fragments to yield both CO2 and ethylene competitively, which poses the question: Why the dif ferent reactivity for nickel? DFT calculations provide a compelling answer. Hence, analyzing the potential energy diagram in Figure 8a, we can conclude that, in the case of nickel, C−C bond formation is once again involved in the first step of the lowest energy pathway for decarboxylation and ethylene loss. However, this step does not proceed via reductive elimination. Alternatively, a four-centered transition state 1.63 eV higher in energy than 7a is formed, in which one C−H bond is sacrificed to form the new C−C bond (Figure 8a, TS(7a-14a)). Nickel, then, remains in a 2+ oxidation state, whereas palladium was reduced to Pd(0). It is worth noting that a careful search was conducted for a three-centered transition state for the nickel system which was analogous to that calculated for palladium, but all attempts to find such a transition state converged again to the four-centered

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CONCLUSIONS Electrospray ionization coupled to an ion trap mass spectrometer modified to allow for ion-molecule reactions continues to prove its worth as a tool for the study of highly reactive organometallic complexes. In this case, members of a virtually unstudied class of compounds (the palladalactones) were formed via ESI, and subsequent investigation by CID experiments indicated that this class of compounds is capable of facilitating carbon−carbon bond-forming reactions in the gas phase. Ion-molecule reactions allowed identification of the site of decarboxylation from [(CH3CO2)Pd(CH2CO2)]− (5b) and also facilitated the observation of a reactive palladium(IV) intermediate during reaction of the palladalactone 7b with allyl iodide. 7473


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(4) Bruckmeier, C.; Lehenmeier, M. W.; Reichardt, R.; Vagin, S.; Rieger, B. Organometallics 2010, 29, 2199−2202. (5) O’Hair, R. A. J. Chem. Commun. 2006, 1469−1481. (6) (a) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761−2764. (b) Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069−1079. (c) Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. 2009, 2832−2836. (d) Rijs, N. J.; Yates, B. F.; O’Hair, R. A. J. Chem. Eur. J. 2010, 16, 2674−2678. (e) Rijs, N. J.; O’Hair, R. A. J. Organometallics 2010, 29, 2282−2291. (f) Rijs, N. J.; Yoshikai, N.; Nakamura, E.; O’Hair, R. A. J. J. Am. Chem. Soc. 2012, 134, 2569−2580. (g) Rijs, N. J.; O’Hair, R. A. J. Dalton Trans. 2012, 41, 3395−3406. (h) Sraj, L. O.; Khairallah, G. N.; da Silva, G.; O’Hair, R. A. J. Organometallics 2012, 31, 1801−1807. (7) (a) O’Hair, R. A. J. Chem. Commun. 2002, 20−21. (b) Rijs, N. J.; O’Hair, R. A. J. Organometallics 2009, 28, 2684−2692. (c) Brunet, C.; Antoine, R.; Broyer, M.; Dugourd, P.; Kulesza, A.; Petersen, J.; Röhr, M. I. S.; Mitrić, R.; Bonačić-Koutecký, V.; O’Hair, R. A. J. J. Phys. Chem. A 2011, 115, 9120−9127. (d) Röhr, M. I. S.; Petersen, J.; Brunet, C.; Antoine, R.; Broyer, M.; Dugourd, P.; Bonačić-Koutecký, V.; O’Hair, R. A. J.; Mitrić, R. J. Phys. Chem. Lett. 2012, 3, 1197−1201. (8) Rijs, N. J.; Sanvido, G. B.; Khairallah, G. N.; O’Hair, R. A. J. Dalton Trans. 2010, 39, 8655−8662. (9) Woolley, M. J.; Khairallah, G. N.; Donnelly, P. S.; O’Hair, R. A. J. Rapid Commun. Mass Spectrom. 2011, 25, 2083−2088. (10) (a) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173−12183. (b) Thum, C. C. L.; Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 48, 9118−9121. (c) Khairallah, G. N.; Thum, C.; O’Hair, R. A. J. Organometallics 2009, 28, 5002−5011. (d) Khairallah, G. N.; Yoo, E. J. H.; O’Hair, R. A. J. Organometallics 2010, 29, 1238−1245. (e) Leeming, M. G.; Khairallah, G. N.; da Silva, G.; O’Hair, R. A. J. Organometallics 2011, 30, 4297− 4307. (11) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255−256, 45−52. (12) Butschke, B.; Schwarz, H. Chem. Eur. J. 2011, 17, 11761−11772. (13) (a) Donald, W. A.; McKenzie, C. J.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2011, 50, 8379−8383. (b) Lam, A. K. Y.; Li, C.; Khairallah, G.; Kirk, B. B.; Blanksby, S. J.; Trevitt, A. J.; Wille, U.; O’Hair, R. A. J.; da Silva, G. Phys. Chem. Chem. Phys. 2012, 14, 2417− 2426. (14) Two other lower abundant isotopologues also have a nominal m / z value o f 226: [(CD 3 CO 2 ) 1 0 4 P d ( CD 2 CO 2 )] − a n d [(CH313CO2)108Pd(CH2CO2)]−. All ions with a nominal m/z of 226 were selected as one and subjected to CID in the LTQ. Analysis of the resultant product ions by FT-ICR allowed us to uniquely identify each product. (15) Frisch, M. J., et al. Gaussian 09, Revision B.01; Gaussian, Inc., Wallingford, CT, 2010. See the Supporting Information for complete citation. (16) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (17) (a) Maestri, G.; Motti, E.; Della Ca, N.; Malacria, M.; Derat, E.; Catellani, M. J. Am. Chem. Soc. 2011, 133, 8574−8585. (b) Benitez, D.; Tkatchouk, E.; Goddard, W. A. Organometallics 2009, 28, 2643−2645. (18) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1−28. (b) Fuentealba, P.; Preuss, H.; Stoll, H.; Szentpaly, L. v. Chem. Phys. Lett. 1982, 89, 418−22. (19) (a) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796−6806. (b) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358−1371. (20) The measured and calculated isotope patterns differ at m/z 224, 226, and 228, indicating that there exists another overlapping ion with similar m/z values. Isolation and CID experiments on these signals (data not shown) indicate that the signals are likely due to the Pd(I) diacetate complex [Pd(O2CCH3)2]−. (21) (a) Stoyanov, E. S. J. Struct. Chem. 2000, 41, 440−445. (b) Nosova, V. M.; Ustynyuk, Y. A.; Bruk, L. G.; Temkin, O. N.; Kisin, A. V.; Storozhenko, P. A. Inorg. Chem. 2011, 50, 9300−9310.

Palladium(IV) complexes have recently been implicated as key intermediates in many palladium-catalyzed reactions, but experimental evidence to support these claims is rare and often inconclusive.24 This is primarily due to the fact that isolating and directly observing a small amount of a highly reactive species is impractical by most experimental techniques. Where other techniques often fail, ion trap mass spectrometry provides the ability to generate, isolate, and characterize these highly reactive intermediates. We have further evidence to suggest that this type of palladium(IV) intermediate is also observed in the catalytic conversion of allyl acetate to butane in the gas phase, which is the subject of a forthcoming paper. Finally, experimental and theoretical results demonstrate that both nickela- and palladalactones are capable of facilitating the formation of new carbon−carbon bonds, although the two metals do so in dramatically different ways. While the nickelalactone 7a favors the formation of a new C−C bond via a fourcentered transition state in which one C−H bond is broken and one C−C bond is formed, the palladalactone 7b undergoes reductive elimination in a process that mimics the formation of a catalytically active Pd(0) species from a Pd(II) precatalyst. In light of these different modes of reactivity, it seems desirable to examine condensed-phase reactions of palladalactonecontaining complexes, as they cannot be expected to behave in the same manner as their nickelalactone analogues and may have untapped potential as C−C bond-forming catalysts.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Text giving the complete ref 15, and Figures S1 − S20, which include mass spectra and the Cartesian coordinates of all DFT optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel: +61 3 8344-2452. Fax: +61 3 9347-5180. E-mail: rohair@ unimelb.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the ARC for financial support via grants DP110103844 (to R.A.J.O. and G.N.K.) and DP1096134 (G.N.K.). We thank the Victorian Partnership for Advanced Computing and the University of Melbourne for generous allocation of computer time.

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REFERENCES

(1) Dupont, J.; Flores, F. R. Palladacycles in catalysis. In Green Catalysis, Vol. 1: Homogeneous Catalysis; Crabtree, R. H., Ed.; WileyVCH: Weinheim, Germany, 2009; Vol. 1, pp 319−342. (2) (a) Iglesias, Á .; Á lvarez, R.; de Lera, Á . R.; Muñiz, K. Angew. Chem., Int. Ed. 2012, 51, 2225−2228. (b) Rauf, W.; Thompson, A. L.; Brown, J. M. Dalton Trans. 2010, 39, 10414−10421. (c) Dupont, J., Pfeffer, M.; Eds. Palladacycles: Synthesis, Characterization and Applications; Wiley-VCH: Weinheim, Germany, 2008; p 417. For a recent review highlighting gas-phase studies on a special case of cyclometalation, i.e. “rollover” cyclometalation, see: (d) Butschke, B; Schwarz., H. Chem. Sci. 2012, 3, 308−326. (3) (a) Kakino, R.; Nagayama, K.; Kayaki, Y.; Shimizu, I.; Yamamoto, A. Chem. Lett. 1999, 7, 685−686. (b) Nagayama, K.; Kawataka, F.; Sakamoto, M.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1999, 72, 573−580. 7474


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(22) On isolating 5b in the ion trap for 30, 300, and 3000 ms, no appreciable signals (i.e. >0.1% intensity relative to the selected ion) can be observed for reaction with residual ESI solvents in the ion trap (water and methanol). The same can be said for 7b when isolated for 30 ms; however, small signals (

Gas-Phase Unimolecular Reactions of Pallada- and Nickelalactone

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