Gas-Phase Synthesis of Organoargenate Anions ... - ACS Publications

2684

Organometallics 2009, 28, 2684–2692

Gas-Phase Synthesis of Organoargenate Anions and Comparisons with Their Organocuprate Analogues Nicole J. Rijs†,‡,§ and Richard A. J. O’Hair*,†,‡,§ School of Chemistry, The UniVersity of Melbourne, Victoria 3010, Australia, Bio21 Institute of Molecular Science and Biotechnology, The UniVersity of Melbourne, Victoria 3010, Australia, and ARC Centre of Excellence in Free Radical Chemistry and Biotechnology ReceiVed January 23, 2009

The gas-phase double decarboxylation of the silver carboxylate centers [RCO2AgO2CR]- and [MeCO2AgO2CR]- was investigated as a means of synthesizing homo- [RAgR]- (R ) Me and Et) and heteroargenates [MeAgR]- (R ) Et, Pr, iPr, tBu, Allyl, PhCH2, Ph). The formation of these organoargenates was examined by multistage mass spectrometry experiments employing collision-induced dissociation (CID) and by density functional theory. A key side reaction in competition with the second stage of decarboxylation involves the loss of the anionic carboxylate ligand. Interpretation of the decarboxylation pathway of the heterocarboxylates [MeCO2AgO2CR]- was more complex due to the possibility of decarboxylation occurring at either of the two different carboxylate ligands, giving rise to the possible isomers [MeAgO2CR]- and [MeCO2AgR]-. This difficulty was overcome through the use of 13C labeling experiments in which [Me13CO2AgO2CR]- was subjected to CID, thereby providing direct evidence for the relative population of the isomers through the losses of 13CO2 and CO2. For example, [Me13CO2AgO2CtBu]- underwent exclusive loss of 13CO2, indicating decarboxylation from the MeCO2ligand, while [Me13CO2AgO2CAllyl]- underwent exclusive loss of CO2, indicating decarboxylation from the AllylCO2- ligand. MeCO2- was preferentially decarboxylated when R ) Et, Pr, iPr, and tBu, while RCO2- was preferred for R ) Allyl, PhCH2, and Ph. Subsequent fragmentation was in agreement with the assigned structures, including in cases where sufficient yields of both the isomeric products [Me13CO2AgR]- and [MeAgO2CR]- were formed (R ) PhCH2 and Ph), each being independently isolated and subjected to CID. Detailed DFT calculations were carried out on the potential energy surfaces for the first and second decarboxylation reactions of all homo- and heteroargenates, as well as possible competing reactions. These reveal that in all cases the first decarboxylation reaction is favored over loss of the carboxylate ligand. In contrast, carboxylate ligand loss can become favored over the second decarboxylation reaction. Organoargenate species formed included [MeAgMe]- and [MeAgR]- (where R ) Et, Pr, Allyl, PhCH2, and Ph). The organoargenate [MeAgR]- is the principle product for R ) Allyl, PhCH2, and Ph but is only a minor product when R ) Et and Pr. Finally, comparisons are made with previous results on the gas-phase formation of organocuprates via double decarboxylation of the related copper carboxylate centers [RCO2CuO2CR]- and [MeCO2CuO2CR]-. Introduction While organocuprates, formulated as “R2CuLi” and known as Gilman reagents,1 are widely used in a range of C-C bond coupling reactions,2 the same cannot be said for their silver counterparts.3,4 In order to understand the differences in reactivity of organocuprates and organoagenates, we have turned * Corresponding author. Fax: +613 9347 5180; Tel: +61 3 8344 2452; E-mail: [email protected]. † School of Chemistry, The University of Melbourne. ‡ Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne. § ARC Centre of Excellence in Free Radical Chemistry and Biotechnology. (1) For a historical account of Gilman’s work, see: (a) Eisch, J. J. Organometallics 2002, 21, 5439. (b) Corey, E. J.; Posner, G. H. J. Am. Chem. Soc. 1967, 89, 3911. (c) Corey, E. J.; Posner, G. H. J. Am. Chem. Soc. 1968, 90, 5615. (2) For reviews and monographs on organocopper species see: (a) Modern Organocopper Chemistry; Krause, N., Ed., Wiley-VCH: Weinheim, 2002. (b) Lipshutz, B. H. Organometallics in Synthesis; Schlosser, M., Ed.; Wiley: Chichester, U.K., 1994; pp 283-382. (c) Organocopper Reagents: A Practical Approach; Taylor, R. J. K., Ed.; Oxford University Press: Oxford, U.K., 1994. (d) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3751.

to fundamental gas-phase studies in which mass-selected, mononuclear organometallates are examined using mass spectrometry-based approaches.5 Thus the complicating effects of solvent and counterions are absent, and the possibility for dimerization and clustering is avoided. Since simple dialkylmetallate anions, [RMR]- (where M ) Cu or Ag), are challenging to generate via direct electrospray ionization of

(3) The use of coinage metals in organic synthesis is blossoming, with an entire recent issue of Chemical ReViews devoted to this theme: Lipshutz, B. H.; Yamamoto, Y. Chem. ReV. 2008, 108. (4) Organoargenates are involved in catalytic C-X bond coupling reactions. For a recent example where X ) Si, see: Murakami, K.; Hirano, K.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2008, 47, 5833. (5) (a) O’Hair, R. A. J. Chem. Commun. 2002, 20. (b) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (c) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761. (d) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255-256, 45. (e) O’Hair, R. A. J.; Waters, T.; Cao, B. Angew. Chem., Int. Ed. 2007, 46, 7048. (f) Rijs, N.; Waters, T.; Khairallah, G. N.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069. (g) Thum, C. C. L.; Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118. (h) Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. 2009, DOI: 10.1039/b822371h.

10.1021/om900053c CCC: $40.75  2009 American Chemical Society Publication on Web 04/02/2009

Gas-Phase Synthesis of Organoargenate Anions

Organometallics, Vol. 28, No. 9, 2009 2685

Scheme 1. Attempted Syntheses of Organometallates (M ) Cu5f and Ag)

A and B. Since these isomers have the same m/z value, we used C-C bond coupling reactions with allyliodide to establish which isomer(s) was formed. Here we (i) examine whether the related organoargenates 1-9 (Scheme 1 where M ) Ag) can be formed; (ii) introduce a new and simple 13C labeling strategy to establish which isomer is formed in the first decarboxylation step in the synthesis of heteroargenates; (iii) compare the gas-phase synthesis of the organoargenates to their copper counterparts.

Experimental Section Scheme

2. Potential Fragmentation Pathways [MeCO2MO2CEt]- (M ) Cu and Ag)

of

organometallic solutions,6 we have adopted an alternative approach whereby metal carboxylate anions are subjected to collision-induced dissociation (CID) to “synthesize” the organometallates via a decarboxylation reaction.5 These decarboxylation reactions are carried out inside a quadrupole ion trap mass spectrometer, and thus the multistage mass spectrometry capabilities can be exploited to further study the reactivity of the organometallate ion via ion-molecule reactions or via additional stages of CID.7 Using this approach we have compared the reactivity of the dimethylcuprate, [MeCuMe]-, and dimethylargenate, [MeAgMe]-, anions in the Corey-Posner reaction. We found that while [MeCuMe]- underwent C-C bond coupling with methyl iodide, [MeAgMe]- did not.5c DFT calculations indicated that the energy of the transition state associated with the Corey-Posner reaction was below that of the separated reactants in the case of [MeCuMe]-, but not for [MeAgMe]-. Subsequently Nakanishi et al. provided additional insights into the differences in reactivity of [MeCuMe]- and [MeAgMe]-.8 We have been interested in extending the decarboxylation strategy to “synthesize” a wide range of organometallate anions, including both homo- and hetero-organometallates. In a recent study we used detailed experiments and DFT calculations to investigate the “synthesis” of the organocuprates 1-9, Scheme 1 (where M ) Cu).5f All organocuprates were successfully formed, except for 6, with the steric bulk of the tBu group disfavoring decarboxylation. These “syntheses” require two stages of decarboxylation, and for the heterocuprates, competition between various reaction channels occurs, as illustrated for the case of [MeCO2MO2CEt](Scheme 2). Perhaps the most interesting feature is that the first stage of decarboxylation can occur from either the MeCO2 or EtCO2 ligands, giving rise to the isomeric organometallate ions (6) Some success has been had in the direct use of ESI/MS on solutions of organometalates: (a) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999, 18, 1571. (b) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 100. (c) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 771. (7) For reviews see: (a) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (b) O’Hair, R. A. J. Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species. In MS InVestigations of ReactiVe Intermediates in Solution; Santos, L. S., Ed.; Wiley-VCH: New York,in press. (8) Nakanishi, W.; Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 1446.

Reagents. Silver(I) acetate, vinylacetic acid, and trimethylacetic acid were obtained from Aldrich. Propionic acid, phenylacetic acid, and n-butyric acid were obtained from BDH Laboratory Supplies. Isobutyric acid and methanol were obtained from Ajax. Benzoic acid was obtained from May Baker. Acetic acid-1-13C was obtained from Isotech. All reagents were used without further purification. Mass Spectrometry. Mass spectra were recorded using either (i) a Finnigan LCQ quadrupole ion trap mass spectrometer (Finnigan MAT, San Jose, CA) or (ii) a Finnigan LTQ FT hybrid linear ion trap (Finnigan, Bremen, Germany). Both instruments were fitted with the standard factory Finnigan electrospray ionization sources. Silver(I) acetate and the various carboxylic acids were dissolved in methanol in a 1:2 molar ratio, with typical concentrations of 0.5-1 mM. Where acetic acid-1-13C was added to these solutions, it was in a 1:2:4 ratio. These solutions were transferred to the electrospray source via a syringe pump, at a rate of 5 µL/min. Typical electrospray source conditions: (i) LCQ, needle potentials of 4.0-5.0 kV and a heated capillary temperature of 180 °C; (ii) LTQ, needle potentials of 4.0-5.0 kV and a heated capillary temperature of 180-200 °C. The desired precursor ion was mass selected and subjected to CID using standard isolation and excitation procedures. The silver isotope pattern (107Ag, 51.8%, 109Ag, 48.2%) was used to identify silver-containing species. For 13C labeling experiments performed on the LTQ, the [Me13CO2109AgO2CR]- ions were mass selected with a window of 1 Da. DFT Calculations. Theoretical calculations were carried out to provide insights into the fragmentation mechanisms of the silver carboxylates. Calculations were performed with the Gaussian 03 package9 utilizing DFT at the B3LYP level.10 The Stuttgart effective core potential and basis set (SDD)11 was used for Ag and the 6-31+G* basis set for all remaining atoms (C, H, and O).12 Vibrational frequencies were calculated for all systems and had either no imaginary frequencies (for all minima) or one imaginary frequency (for transition states). Reaction endothermicities are corrected for zero-point energies (unscaled), but are not corrected for basis set superposition errors. Our main reasons for the choice of this method and basis set are as follows: they allow for a direct comparison with the previously reported fragmentation endothermicities of the related copper carboxylates; the related 6-31G* has been used to compare the chemistry of organocuprates, organoargentates, and organoaurates.8 Indeed this combination of method and basis set has been shown to be effective in calculating bond lengths and geometries and relative endothermicities in related (9) Frisch M. J.; et al. Gaussian-03; Gaussian, Inc.: Pittsburgh, PA, 2003. (10) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (11) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866. (12) For transition metal complexes it has been suggested to use an ECP for metals and an all-electron basis set for ligands: (a) Cundari, T. R.; Leza, H. A. R.; Grimes, T.; Steyl, G.; Waters, A.; Wilson, A. K. Chem. Phys. Lett. 2005, 401, 58. For discussions on the addition of diffuse functions for negatively charged systems and the importance of d-polarization functions for ligated bonds see: (b) ComprehensiVe Organometallic Chemistry III, Volume 1; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: New York, 2007; pp 639-669.

2686 Organometallics, Vol. 28, No. 9, 2009

Rijs and O’Hair

Figure 1. LCQ mass spectra showing collision-induced dissociation (CID) of the homocarboxylates [MeCO2AgO2CMe]- and [EtCO2AgO2CEt]-: (a) MS2 CID spectrum of [MeCO2AgO2CMe]-, m/z 225/227; (b) MS3 CID spectrum of [MeCO2AgMe]-, m/z 181/183; (c) MS2 CID spectrum of [EtCO2AgO2CEt]-, m/z 253/255; (d) MS3 CID spectrum of [EtCO2AgEt]-, m/z 209/211. The mass-selected precursor ion is marked with an * in each case. copper systems,5f and Nakamura, who has examined a range of different basis sets and theoretical methods for oragnocuprates, has noted that “we conclude that B3LYP method using the relativistic ECP basis set for Cu and the 6-31G* basis set for C, H, O...is currently the most appropriate...”.13 We note that we have added diffuse functionals as these benefit calculation of anionic systems.12b One of the challenges in testing the performance of this combination of method and basis set for silver systems is that there is a lack of experimental data. Thus we have been unable to compare the calculated DFT geometries directly with those determined by X-ray crystallography (although the general features of the calculated argenates (Scheme 1) are consistent with X-ray crystal structures of several related argenates14) or to benchmark the predicted thermochemistry.

Results and Discussion Electrospray of a methanolic solution silver(I) acetate resulted in a complex negative ion mode mass spectrum. Silvercontaining anions were identified by their characteristic isotope pattern (107Ag, 51.8%, 109Ag, 48.2%). An ion assigned as [MeCO2AgO2CMe]- containing the expected dual silver isotope pattern was observed (m/z 225/227). Multinuclear silver carboxylate cluster anions of the type [(MeCO2Ag)n+MeCO2]were also observed at higher m/z values, but their gas-phase chemistry was not examined. Addition of other carboxylic acids to the silver acetate electrospray solution resulted in a range of silver carboxylate anions. For example, addition of propionic acid yielded the species [MeCO2AgO2CEt]- and [EtCO2(13) Yamanaka, M.; Inagaki, A.; Nakamura, E. J. Comput. Chem. 2003, 24, 1401. (14) (a) Hwang, C.-S.; Power, P. P. J. Organomet. Chem. 1999, 589, 234. (b) Aboulkacem, S.; Tyrra, W.; Pantenburg, I. J. Chem. Crystallogr. 2006, 36, 141.

AgO2CEt]-, together with multinuclear silver carboxylate cluster anions (see Supporting Information, Figure S1). Our current method for synthesizing gas-phase organoargenates requires a double decarboxylation of silver carboxylate centers under low-energy CID conditions (e.g., Scheme 2). This method was explored for the generation of homoargenates from [MeCO2AgO2CMe]- and [EtCO2AgO2CEt]- and a series of heteroargenates from [MeCO2AgO2CR]- (R ) Et, Pr, iPr, tBu, allyl, PhCH2, Ph; Scheme 1). First we detail the experimental and theoretical results from the attempted syntheses of the homoargenates [MeAgMe]- and [EtAgEt]- by discussing decarboxylation within the context of other competing reaction pathways. Then the role of the RCO2- group on the formation of heteroargenates, [MeAgR]-, is described. Finally comparisons with the related gas-phase “syntheses” of homo- and heterocuprates are made.5f (a) Formation of the Homoargenates [MeAgMe]- and [EtAgEt]-. The dimethylargenate anion [MeAgMe]- was successfully formed via double decarboxylation of [MeCO2AgO2CMe]-, as previously described.5a,c During the first stage of CID, the sole fragmentation pathway observed corresponded to a single decarboxylation of an acetate ligand (eq 1, Figure 1a). This is in agreement with the energy differences calculated, with loss of the acetate anion (eq 2) predicted to be significantly higher in energy (0.69 eV) than the transition state associated with decarboxylation (Figure 2a). The second stage of CID (Figure 1b) reveals decarboxylation of the remaining acetate ligand as the preferred pathway (eq 3), with loss of the acetate ligand as a minor competing pathway (eq 4). The DFT calculations, which are summarized in Figure 2b, support the experiments since acetate anion loss is predicted to be only negligibly higher in energy (0.02 eV) than the energy barrier associated with decarboxylation. Although a third



Figure 3. DFT-calculated energies for minima and transition states relevant to fragmentation of (a) [EtCO2AgO2CEt]- via decarboxylation (right-hand side) and carboxylate loss (left-hand side); (b) [EtCO2AgEt]- via decarboxylation (right-hand side) and carboxylate loss or β-hydride transfer (left-hand side); and (c) structures of minima and transition states relevant to the β-hydride transfer pathway in (b).

Figure 2. DFT-calculated energies for minima and transition states relevant to fragmentation of (a) [MeCO2AgO2CMe]- and (b) [MeCO2AgMe]- via decarboxylation (right-hand side) and acetate loss (left-hand side); (c) structures of minima and transition states relevant to the first (ci) and second (cii) decarboxylation pathway.

possible fragmentation pathway resulting in the formation of the methyl anion, Me-, via the loss of silver acetate, [MeCO2Ag] (eq 5), cannot be excluded due to the low mass cutoff of the quadrupole ion trap, the DFT calculations predict that this pathway is much higher in energy (4.05 eV), and thus it seems unlikely to occur under low-energy CID conditions.

[MeCO2AgO2CMe]- f [MeCO2AgMe]- + CO2 (1) fMeCO2-+ [MeCO2Ag]

(2)

[MeCO2AgMe]- f [MeAgMe]-+ CO2

(3)

fMeCO2 + [MeAg]

(4)

fMe- + [MeCO2Ag]

(5)

As noted above, addition of propionic acid results in the formation of the silver proprionate anion, [EtCO2AgO2CEt]-

(m/z 253/255). This anion was mass selected and subjected to low-energy CID to give the spectrum shown in Figure 1c. Decarboxylation is the major fragmentation pathway (eq 6), with loss of propionate (eq 7) being a very minor channel. These experimental results are consistent with the DFT calculations shown in Figure 3a, which reveal that the energy associated with the formation of the propionate anion (eq 7) is higher (0.57 eV) than that associated with the decarboxylation transition state barrier. The second stage of CID on [EtCO2AgEt]- (m/z 209/ 211) results in the loss of the propionate ligand (eq 9) with essentially no decarboxylation occurring to form the desired organoargenate [EtAgEt]- (eq 8) (see the MS3 spectrum shown in Figure 1d). There is no evidence for the occurrence of a β-hydride transfer reaction (eq 10), which contrasts with the cuprate analogue, where [EtCO2CuH]- was a significant product.5f Loss of Et- (eq 11), which is predicted to be the most endothermic pathway at +4.08 eV, cannot be observed due to the low mass cutoff of the quadrupole ion trap. The DFT calculations (Figure 3b) support the experimental observations, predicting that the preferred pathway is loss of the propionate ligand (eq 9). This pathway is predicted to be 0.18 eV lower in energy than the barrier to decarboxylation (eq 8). Finally both β-hydride transfer (eq 10) and loss of Et- (eq 11) are predicted to be higher in energy than decarboxylation and loss of the propionate ligand.


Rijs and O’Hair

[EtCO2AgO2CEt]- f [EtCO2AgEt]-+CO2

(6)

fEtCO2-+[EtCO2Ag]

(7)

[EtCO2AgEt]- f [EtAgEt]- + CO2

(8)

fEtCO2- + [EtAg]

(9)

(eqs 14 and 15). All seven heterocarboxylates (3-9 of Scheme 1) fragment via decarboxylation as the dominant pathway (Supporting Information, Figure S2). Indeed, only the aliphatic carboxylates where R ) Pr, iPr, and tBu (4-6 of Scheme 1) gave trace amounts of loss of the carboxylate ligand (eq 15, Supporting Information, Figure S2a-c), while the low mass cutoff prevented detection of loss of the acetate ligand (eq 14) for most systems.

f[EtCO2AgH]-+ CH2dCH2

(10)

[MeCO2AgO2CR]- f [MeAgO2CR]-+ CO2

(12)

fEt-+ [EtCO2Ag]

(11)

f[MeCO2AgR]-+ CO2

(13)

fMeCO2- + [RCO2Ag]

(14)

fRCO2- + [MeCO2Ag]

(15)

Both the experiments and DFT calculations show that β-hydride transfer does not occur for [EtCO2AgEt]- compared with [EtCO2CuEt]-. Since this trend is opposite that calculated for Ni versus Pd complexes,15 we were interested in comparing known differences in the properties of copper and silver compounds. Wang and Andrews have calculated essentially identical hydride anion affinities for CuH and AgH,16 and thus the hydride binding energy is unlikely to play a role in destabilizing the TS for the argenate. In contrast, it is known that silver forms weak and labile alkene adducts17 and that the gas-phase binding energies of ethene to Ag+ are about 0.4 eV less than for Cu+.18 Thus the weaker π-interaction seems the more likely explanation for the higher energy of the transition state associated with β-hydride transfer for [EtCO2AgEt]compared to that of [EtCO2CuEt]-. Indeed, the product complexes arising from the β-hydride transition states associated with [EtCO2CuEt]- and [EtCO2AgEt]- are quite different. The former is a π-complex5f that must surmount another, low-lying transition state for CH2dCH2 loss. In contrast, the latter (15 of Figure 3b and c) is a weakly bound ion-molecule complex that can directly dissociate via CH2dCH2 loss. (b) Formation of Heteroargenates [MeAgR]-. Double decarboxylation of the heterocarboxylates [MeCO2AgO2CR]was also examined as a way of generating the heteroargenates, [MeAgR]-. We have adopted our previous approach5f of keeping one ligand as constant (acetate), while varying the other (Scheme 1, R ) Et, Pr, iPr, tBu, Allyl, PhCH2, and Ph). This allowed the role of the R groups to be probed while reducing the number of carboxylates to be examined. We first describe experimental and DFT calculations aimed at establishing the site(s) of decarboxylation and other competing reaction pathways associated with the first stage of CID. Then experiments and theory are described for the “synthesis” of the heteroargenate, [MeAgR]-. The First Stage of CID: Establishing the Site of Decarboxylation and Understanding Other Competing Pathways. On the basis of our previous studies on the formation of heterocuprates,5f we anticipated that the first stage of CID on the mixed carboxylates [MeCO2AgO2CR]- might produce four possible competing fragmentation pathways (e.g., Scheme 2 for the case of R ) Et). These included decarboxylation at either of the two nonequivalent carboxylate groups to yield the isomeric species [MeAgO2CR]- and [MeCO2AgR]- (eqs 12 and 13) or the loss of either of the carboxylate ligands (MeCO2- or RCO2-) to yield neutrals RCO2Ag or MeCO2Ag, respectively (15) Niu, S.; Hall, M. B. Chem. ReV. 2000, 100, 353. (16) Wang, X.; Andrews, L. Angew. Chem., Int. Ed. 2003, 42, 5201. (17) Dias, H. V. R.; Wu, J. Eur. J. Inorg. Chem. 2008, 509. (18) (a) Sievers, M. R.; Jarvis, L. M.; Armentrout, P. B. J. Am. Chem. Soc. 1998, 120, 1647. (b) Manard, M. J.; Kemper, P. R.; Bowers, M. T. Int. J. Mass Spectrom. 2005, 241, 109.

Isotopic Labeling for Isomer Distinction. The isomers [MeAgO2CR]- (isomer A) and [MeCO2AgR]- (isomer B) have identical molecular formulas and thus cannot be distinguished by their m/z values. Since the main fragmentation pathway for all of the mixed carboxylates [MeCO2AgO2CR]- involved decarboxylation, establishing the site(s) of decarboxylation was paramount. In our previous study we indirectly probed the structures of the related organocuprate isomers [MeCuO2CR]and [MeCO2CuR]- via the use of ion-molecule reactions with allyl iodide and CID. Here we introduce a new and simple strategy to directly establish which isomer is formed in the first decarboxylation step in the synthesis of heteroargenates. It involves 13C labeling, in which the precursor ion, [Me13CO2AgO2CR]-, is formed via ESI on a mixture of silver acetate, acetic acid-1-13C, Me13CO2H, and the carboxylic acid, RCO2H. CID on the mass-selected precursor ion, [Me13CO2AgO2CR]-, gives information on isomer formation since the site(s) of decarboxylation are directly determined via the loss of 13CO2 (eq 16) and/or 12CO2 (eq 17).

[Me13CO2AgO2CR]- f [MeAgO2CR]-+ 13CO2 (16) f[Me13CO2AgR]-+ CO2

(17)

Figure 4 illustrates this approach for four different cases, while the remaining data are given in the Supporting Information (Figure S3). In addition, the predicted activation energies for the two competing decarboxylation reactions (eqs 12 and 13) and the reaction endothermicities for loss of the carboxylate ligands (eqs 14 and 15) are listed in Table 1. Figure 4a shows that CID of the monoisotopic ion [Me13CO2109AgO2CtBu]- (m/z 270) yields [Me109AgO2CtBu]- (m/z 225) via loss of 13CO2 (45 Da), giving clear evidence for liberation of carbon dioxide from the Me13CO2- ligand (eq 16). A very minor loss of the trimethyl acetate ligand (m/z 101) is also observed. These experimental results are supported by the DFT calculations, which show that decarboxylation from the acetate ligand has a lower activation energy (eq 12, 1.62 eV) than decarboxylation from the trimethyl acetate ligand (eq 13, 2.03 eV). Loss of the carboxylate ligands (eqs 14 and 15) is predicted to be higher in energy (2.34 and 2.23 eV, respectively), consistent with these products not being significant in the CID spectrum. The rest of the anionic series [Me13CO2109AgO2CR]- (R ) Pr, iPr, tBu, Allyl, PhCH2, and Ph) were also mass selected and subjected to CID (Figure S2 and Figure 4b,c). The remaining alkyl carboxylates almost exclusively lost 13CO2 (Figure S2), thereby establishing that decarboxylation is kineti-



Figure 4. LTQ mass spectra showing the results of 13C labeling experiments to establish the site of CO2 loss in the first stage of CID of the heterocarboxylates [Me13CO2AgO2CR]-: (a) R ) tBu, m/z 270; (b) R ) Allyl, m/z 254; (c) R ) PhCH2, m/z 304; (d) R ) Ph, m/z 290. The mass-selected precursor ion is marked with an * in each case. A single peak was mass selected in each case and contained the 109Ag isotope. Table 1. DFT-Predicted Energies for Competing Decarboxylation and Carboxylate Anion Loss Reactions for [MeCO2AgO2CR][MeAgO2CR]- + CO2a

[MeCO2AgR]- + CO2a

MeCO2- + RCO2Agb

RCO2- + MeCO2Agb

R)

(eq12)

(eq13)

(eq14)

(eq15)

Et Pr iPr tBu Allyl PhCH2 Ph

1.62 (0.97) 1.62 (0.96) 1.62 (0.96) 1.62 (0.96) 1.62 (0.94) 1.63 (0.94) 1.62 (0.94)

1.72 (1.15) 1.71 (1.10) 1.85 (1.20) 2.03 (1.17) 1.57 (0.78) 1.52 (0.70) 1.45 (0.99)

2.31 2.31 2.31 2.34 2.38 2.39 2.40

2.29 2.28 2.26 2.23 2.18 2.14 2.14

a Activation energy (eV) for decarboxylation reactions. The values in parentheses refer to the overall endothermicity of the reaction. endothermicity (eV) for carboxylate anion loss (assumed as barrierless).

cally favored from the acetate ligand (eq 16). These results are consistent with the DFT calculations (Table 1), which show that the transition state energy associated with loss of CO2 from the acetate ligand remains unchanged (1.62 eV) across the series of heterocarboxylates. In contrast, the transition state energy associated with loss of CO2 from the RCO2 ligand of [MeCO2AgO2CR]- is always higher in energy and increases as the steric bulk of the alkyl group increases. Thus in all of the alkyl cases isomer A is formed. The CID spectra of the remaining heterocarboxylates, [Me13CO2109AgO2CR]- (R ) Allyl, PhCH2, Ph), are shown in Figures 4b, c, and d. In all cases, loss of the unlabeled CO2 dominates, suggesting that decarboxylation is favored from the RCO2 ligand. Once again, these results are consistent with the DFT calculations (Table 1), which show that the transition state energy associated with loss of CO2 from the RCO2 ligand of [MeCO2AgO2CR]- is lower that that from the acetate in the cases of R ) Allyl, PhCH2, and Ph. Thus in all of these cases the main isomer formed is B.

b

Reaction

The Second Stage of CID: Which Heteroargenate Can Be “Synthesized” via Double Decarboxylation? Although the 13 C labeling experiments and DFT calculations show the preferential formation of isomer A, [MeAgO2CR]-, or B, [MeCO2AgR]-, in the first stage of CID for specific heterocarboxylates, we have carried out DFT calculations on the structures and energies of species associated with various potential fragmentation reactions of both isomers. The reactions we have considered for isomer A are decarboxylation (eq 18) and loss of the carboxylate ligand (eq 19), and the energetics associated with these reactions are summarized in Table 2. The reactions we have considered for isomer B are decarboxylation (eq 20), loss of the carboxylate ligand (eq 21), and β-hydride transfer in the case of the alkyl ligands R ) Et, Pr, iPr, and tBu (eq 22). The energetics associated with each of these reactions are summarized in Table 3. Before describing the experimental results, we briefly highlight key aspects of the DFT data for each of the isomers.

[MeAgO2CR]- f [MeAgR]-+ CO2

(18)


Rijs and O’Hair

fRCO2- + [MeAg]

(19)

[MeCO2AgR]- f [MeAgR]-+CO2

(20)

fMeCO2- + [RAg]

(21)

f[MeCO2AgH]- + [R - H]

(22)

DFT Calculations on Potential Fragmentation Pathways for Isomer A, [MeAgO2CR]-. The DFT-calculated data for the energetics associated with the decarboxylation (eq 18) and carboxylate loss (eq 19) reactions for isomer A, [MeAgO2CR]-, are given in Table 2. In all cases, the energetics for loss of the carboxylate anion is significantly lower (by around 0.6 eV) for [MeAgO2CR]- (eq 19) than for the [MeCO2AgO2CR]- precursor (eq 15). In contrast, the barrier for decarboxylation for [MeAgO2CR]- (eq 18) is the same or larger than for the [MeCO2AgO2CR]- precursor (eq 13). As a consequence, decarboxylation is only expected to be potentially competitive with carboxylate loss for the following isomers A, [MeAgO2CR]-, R ) Allyl, PhCH2, and Ph. These results are consistent with the experimental and theoretical data presented for the homoargenates. DFT Calculations on Potential Fragmentation Pathways for Isomer B, [MeCO2AgR]-. The DFT-calculated data for the energetics associated with the decarboxylation (eq 20), acetate loss (eq 21), and β-hydride transfer (eq 22) reactions for isomer B, [MeCO2AgR]-, are given in Table 3. A number of interesting trends emerge: (i) the alkyl ligand has essentially no influence on the barrier for decarboxylation of [MeCO2AgR](eq 20), which is nearly identical for all R (range is 1.66-1.69 eV), a phenomenon previously noted for the related cuprates;5f (ii) β-hydride transfer (eq 22) is much higher in energy and is not expected to be experimentally observed under conditions of low-energy CID; (iii) loss of the acetate anion (eq 21) is only lower in energy than the decarboxylation reaction (eq 20) for the aliphatic R groups (R ) Et, Pr, iPr, and tBu). As a consequence, decarboxylation is only expected to be potentially preferred over carboxylate loss for the following isomers B, [MeCO2AgR]-, R ) allyl, PhCH2, and Ph. Can [MeAgR]- Be Formed? Experiments on the CID of the Unlabeled [MeCO2AgO2CR - CO2]- Ions. Table 4 summarizes the CID spectra of [MeCO2AgO2CR - CO2]- ions (where R ) Et, Pr, iPr, tBu, Allyl, PhCH2, and Ph). [MeAgR]can only be formed for the following: R ) Et, Pr, allyl, PhCH2, and Ph. The CID spectra of all of the aliphatic precursors [MeCO2AgO2CR - CO2]- (R ) Et, Pr, iPr, and tBu) are dominated by the loss of the carboxylate ligand, RCO2-, with decarboxylation to form the desired argenate [MeAgR]- being a very minor channel for R ) Et and Pr and virtually nonexistent for all the remaining species. The dominant loss of the carboxylate ligand (RCO2-) suggests the loss of the first CO2 occurs from the acetate ligand of [MeCO2AgO2CR]-, consistent with both the 13C labeling studies and the DFT calculations, which predict that the [MeAgO2CR]- isomer should be formed preferentially. It is also noteworthy that it becomes increasingly difficult to form the argenate, [MeAgR]-, as the size of the alkyl group increases. The DFT calculations described above (Tables 2 and 3) point to two factors operating against the formation of [MeAgR]-: (i) as the steric bulk of the RCO2- ligand increases, decarboxylation (eq 18) at this site becomes more difficult, thereby leading to an increase in the energy of the transition state, resulting in poor yields of the desired argenate, [MeAgR]-;

Table 2. DFT-Predicted Energies for Competing Decarboxylation and Carboxylate Loss for Isomers A, [MeAgO2CR]products from isomer A, [MeAgO2CR]R)

[MeAgR]- + CO2a

RCO2- + MeAgb


1.79 1.77 1.94 2.10 1.57 1.52 1.55

1.67 1.66 1.64 1.62 1.58 1.54 1.55

a Activation energy (eV) for decarboxylation and β-hydride transfer reactions. b Reaction endothermicity (eV) for carboxylate anion loss (assumed as barrierless).

Table 3. DFT-Predicted Energies for Competing Decarboxylation, Carboxylate Loss, and β-Hydride Transfer Reactions for Isomers B, [MeCO2AgR]products from B, [MeCO2AgR]R)

[MeAgR]- + CO2a MeCO2- + RAgb [MeCO2AgH]- + R-Ha


1.67 1.67 1.67 1.67 1.66 1.66 1.69

1.61 1.66 1.61 1.65 1.81 1.88 2.03

2.30 2.24 2.34 2.40 c c c

a Activation energy (eV) for decarboxylation and β-hydride transfer reactions. b Reaction endothermicity (eV) for carboxylate anion loss (assumed as barrierless). c Hydride formation pathway not calculated.

Table 4. Products from the Second Stage of CID on the Heterocarboxylates Ions [MeCO2AgO2CR - CO2]- (formed via eqs 12 and/or 13)a R)

CO2 loss (eqs 18 or 20)

RCO2- loss (eq 19)


1 2 0 0 100 100 100

100 100 100 100 0 0 25

a Relative abundances were calculated by integrating the area under each peak (LTQ).

(ii) as the size of the RCO2- ligand increases, the stability of the ligand increases (due to the enhanced acidity of the conjugate acid, RCO2H), making loss of RCO2- (eq 19) even more favorable. Isotopic Labeling Allows Differences in Reactivity of Isomers to Be Probed. The gas-phase fragmentation reactions of the isomers [MeAgO2CR]- (isomer A) and [MeCO2AgR](isomer B) can be distinguished from each other by using the 13 C labeling strategy outlined above. This allows a direct comparison of the experimental data with the DFT calculations shown in Tables 2 and 3. Figure 5 shows the CID spectra of isomers A, [MeAgO2CR]-, and B, [Me13CO2AgR]-, derived from the first stage of decarboxylation of the heterocarboxylates shown in Figure 4. Low-energy CID on mass-selected “pure” A or B isomers resulted in product ions that are consistent with those expected to arise from the assigned structure. For example, CID on [Me109AgO2CtBu]- (m/z 225), formed in Figure 4a, resulted in loss of tBuCO2- (Figure 5a, cf. eq 19), as would be expected for isomer A, based on the related unlabeled spectrum, as discussed above (Table 4). This is also consistent with DFT predictions that the barrier to decarboxylation is much higher in energy (0.68 eV) than ligand loss (Table 2). The low-energy



Figure 5. LTQ mass spectra showing collision-induced dissociation (CID) of isomers A, [MeAgO2CR]-, and B, [MeCO2AgR]-, derived from the first stage of decarboxylation in the spectra shown in Figure 4: (a) MS3 CID spectrum of isomer A where R ) tBu, m/z 225; (b) MS3 CID spectrum of isomer B where R ) Allyl, m/z 210; (c) MS3 CID spectrum of isomer A where R ) PhCH2, m/z 259; (d) MS3 CID spectrum of isomer B where R ) PhCH2, m/z 260; (e) MS3 CID spectrum of isomer A where R ) Ph, m/z 245; (f) MS3 CID spectrum of isomer B where R ) Ph, m/z 246. The mass -selected precursor ion is marked with an * in each case. A single peak was mass selected in each case and contained the 109Ag isotope.

CID spectrum of isomer B [Me13CO2AgAllyl]-, formed in Figure 4b, reveals decarboxylation as the sole pathway (Figure 5b, cf. eq 20) as also predicted by the DFT calculations, being 0.15 eV lower in energy than ligand loss. In a number of cases where both A and B isomers were formed in sufficient abundance from the same precursor, it is possible to directly compare the isomer specific fragmentation chemistry. For example, CID on [Me13CO2AgO2CCH2Ph]- produced both isomers A and B (Figure 4c). While [MeAgO2CCH2Ph](isomer A, Figure 5c) and [Me13CO2AgCH2Ph]- (isomer B, Figure 5d) both fragment via decarboxylation (cf. eqs 18 and 20), the former also fragments via loss of PhCH2CO2- (cf eq 19). In a similar fashion, [MeAgO2CPh]- (isomer A, Figure 5e) fragments via both decarboxylation (cf. eq 18) and loss of PhCO2- (cf. eq 19), while [Me13CO2AgPh]- (isomer B, Figure 5f) fragments only via decarboxylation (cf. eq 20). Organoargenate and Organocuprate Formation via the Double Decarboxylation Strategy: What Are the Trends and Can We Understand the Differences? In our previous study we successfully formed 10 of the 11 cuprates shown in Scheme 1, with only the heterocuprate [MeCutBu]-, 6, eluding

synthesis via the double decarboxylation strategy.5f In contrast, we have been able to form only the following six of the 11 argenates shown in Scheme 1: [MeAgMe]-, 1; [MeAgEt]-, 3; [MeAgPr]-, 4; [MeAgAllyl]-, 7; [MeAgCH2Ph]-, 8; and [MeAgPh]-, 9. Of these, 3 and 4 are formed only as very minor products. Why is it more challenging to form the argenates than the cuprates? A comparison of the DFT calculations for the formation of cuprates versus argenates reveals that the key lies in the energetics of the various competing pathways in the second stage of decarboxylation of [MeMO2CR]- (Figure 6).19 Thus while the barriers for decarboxylation of [MeMO2CR]and [MeCO2MR]- are remarkably similar for M ) Cu (a black line) and Ag (b blue line), the energetics associated with loss of the carboxylate, RCO2-, from [MeMO2CR]-, decrease (by around 0.1 eV) on moving from Cu (c red line) to Ag (d green (19) A slightly more stable TS was found for decarboxylation of [MeCuO2CPr]-, and this is given in the Supporting Information, Figure S17. This is due to a different conformation of the propyl group and results in a slightly lower decarboxylation energy (1.76 eV) than that reported in ref 5f.


Rijs and O’Hair

Figure 6. Comparison of the DFT-calculated decarboxylation activation energy versus the energetics of carboxylate loss from isomers A, [MeMO2CR]-, where M ) Ag (this study) and Cu (from ref 5f) and R ) Me, Et, Pr, iPr, tBu, Allyl, PhCH2, and Ph.

line).18,20 As a consequence, the balance is tipped in favor of carboxylate loss for [MeAgO2CR]- in the cases of R ) Et, Pr, iPr, and tBu.

Conclusions The combined use of experiments and theory has been used to unravel the competing pathways in the first and second decarboxylation steps required for the attempted formation of the homo- and heteroargenates shown in Scheme 1. For the heterocarboxylates [MeCO2AgO2CR]- decarboxylation can occur at either of the two different carboxylate ligands, giving rise to the possible isomers [MeAgO2CR]- or [MeCO2AgR]-. The relative populations of these isomers were directly established via the novel use of 13C labeling experiments in which [Me13CO2AgO2CR]- was subjected to CID and the losses of 13 CO2 and CO2 were directly monitored. 13CO2 loss is preferred when R ) Et, Pr, iPr, and tBu, consistent with DFT calculations that suggest steric effects operate. In contrast, CO2 loss is preferred for R ) Allyl, PhCH2, and Ph. Furthermore, these 13 C labeling experiments allow the isolation of isomerically “pure” ion populations for further reactivity studies. When compared to previously published work on the synthesis of organocuprates,5f two key differences emerge: (i) Fewer organoargenates can be formed via double decarboxylation of silver carboxylate anions. Thus while 10 of the 11 organocuprates shown in Scheme 1 can be synthesized, only six of the 11 organoargenates can be formed ([MeAgMe]-, [MeAgEt]-, [MeAgPr]-, [MeAgAllyl]-, [MeAgCH2Ph]-, and [MeAgPh]-). A simple bond energy explanation can be used to explain this difference. Thus the silver-carboxylate ligand bond energy is lower than that for copper, with the consequence (20) Weaker gas-phase ligand bond energies are also observed on moving from [Cu(L)2]+ to [Ag(L)2]+ for a wide range ligands, L: (a) Deng, H.; Kebarle, P. J. Am. Chem. Soc. 1998, 120, 2925. (b) Deng, H.; Kebarle, P. J. Phys. Chem. A 1998, 102, 571.

that loss of RCO2- can become favored in the second stage of decarboxylation of [MeAgO2CR]-. (ii) Fewer fragmentation reactions compete with the second decarboxylation reaction in the case of the argenates. For example, [EtCO2CuEt]- underwent loss of C2H4 by β-hydride transfer to give [EtCO2CuH]-,5f but this reaction is completely absent for [EtCO2AgEt]-. A possible explanation is the weaker π interaction in the case of silver. The successful synthesis of a series of organocuprates and organoargenates sets the stage for an examination of their gasphase bimolecular and unimolecular reactivity. Thus previous studies on the C-C bond coupling reactions with methyl iodide5c can now be extended to a range of organometallates and organic substrates. Since C-C bond coupling is often hampered by decomposition reactions of organosilver and copper reagents, it will also be interesting to examine the gasphase unimolecular fragmentation reactions of mass-selected organometallates. Preliminary studies suggest that [MeMR](where M ) Cu and Ag) undergo a range of reactions under CID conditions, including loss of R-, β-hydride reactions, and bond homolysis to give radical anions.

Acknowledgment. We thank the ARC for financial support via grant DP0558430 (to R.A.J.O.). N.R. thanks the Faculty of Science for a Science Faculty Scholarship. VICS is acknowledged for the Chemical Sciences High Performance Computing Facility. We thank Dr. Koszinowski for providing a preprint of ref 6c. Supporting Information Available: Complete citation for ref 9. Mass spectra for silver acetate and propionic acid mentioned in the text. Mass spectra for fragmentation of [Me13CO2AgO2CR]-. Cartesian coordinates and energies (au) for species relevant to each of the fragmentation pathways described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. OM900053C

Gas-Phase Synthesis of Organoargenate Anions ... - ACS Publications

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