Formation of Methylmagnesium or Coordinated Ylide? Competition

Feb 1, 2010 - George N. Khairallah†‡, Ellie Jung Hwa Yoo†‡ and Richard A. J. O'Hair*†‡. † School of Chemistry, The University of Melbour...
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Organometallics 2010, 29, 1238–1245 DOI: 10.1021/om901050s

Formation of Methylmagnesium or Coordinated Ylide? Competition between Decarboxylation of Acetate and Betaine Ligands in [CH3CO2MgO2CCH2X(CH3)2]þ (where X = NCH3 and S) George N. Khairallah,†,‡ Ellie Jung Hwa Yoo,†,‡ and Richard A. J. O’Hair*,†,‡ †

School of Chemistry, The University of Melbourne, Victoria 3010, Australia , and ‡Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Victoria 3010, Australia Received December 8, 2009

Decarboxylation of the ligands in the magnesium cations [CH3CO2MgO2CCH2X(CH3)2]þ (where X = NCH3, 1a; and S, 1b) can give rise to either an organometallic cation [CH3MgO2CCH2X(CH3)2]þ, 2, or a coordinated ylide [CH3CO2MgCH2X(CH3)2]þ, 3. Collision-induced dissociation of a precursor complex in which the acetate ligand was 13C labeled allowed the direct determination of the relative ratio of formation of 2 and 3 via a comparison of the losses of 13CO2 and CO2. For the nitrogen betaine, 1a, both 2a and 3a are formed, in a ratio of ca 1:3, respectively. In contrast, for the sulfur betaine, 1b, the coordinated ylide, 3b, is almost exclusively formed. These striking differences are confirmed by DFT calculations at the B3LYP/6-31þG(d) level of theory, which show that the activation energies associated with the formation of 2a and 3a are close (2.33 eV versus 2.28 eV), but that formation of 3b (Ea =1.97 eV) is favored over formation of 2b (Ea =2.33 eV). Use of the 13Clabeled acetate ligand also allowed the bimolecular hydrolysis reactions of isomerically pure populations of mass-selected 2 and 3 to be examined. Ion-molecule reactions with water proceeded via addition/elimination reactions, but the nature of the magnesium hydroxide is different for each isomer. Thus 2a reacted to yield the charged magnesium hydroxide [HOMgO2CCH2N(CH3)3]þ, plus methane, while 3a and 3b reacted to yield the neutral magnesium hydroxide [CH3CO2MgOH] plus the charged onium ions (CH3)3Xþ. Kinetic measurements reveal a reactivity order of 2a > 3a ≈ 3b, consistent with DFT calculations on the potential energy surfaces associated with these hydrolysis reactions.

Introduction Decarboxylation reactions provide a handy way of unmasking reactive intermediates, including organometallics. Such reactions have been utilized in both the condensed1 and gas phases,2 and there has been considerable recent interest in generating catalytic decarboxylation reactions in C-C bond coupling applications such as the Heck reaction.3 We have been exploiting multistage mass spectrometry techniques and molecular modeling methods to examine the decarboxylation reactions of a wide range of metal carboxylate *Corresponding author. Fax: þ61 3 9347-5180. Tel: þ61 3 8344-2452. E-mail: [email protected]. (1) For an older review on the use of decarboxylation reactions to produce organometallics in the condensed phase see: Deacon, G. B.; Faulks, S. J.; Pain, G. N. Adv. Organomet. Chem. 1986, 25, 237. (2) For a review on the use of decarboxylation and other types of reactions to produce organometallic ions in the gas phase see: 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: Weinheim, Chapter 6, 2010; pp 199-227, ISBN: 978-3-527-32351-7. (3) For a review see: (a) Goossen, L. J.; Rodriguez, N.; Goossen, K. Angew. Chem., Int. Ed. 2008, 47, 3100. For key papers see: (b) Tanaka, D.; Romeril, S. P.; Myers, A. G. J. Am. Chem. Soc. 2005, 127, 10323. (c) Tanaka, D.; Myers, A. G. Org. Lett. 2004, 6, 433. (d) Myers, A. G.; Tanaka, D.; Mannion, M. R. J. Am. Chem. Soc. 2002, 124, 11250. pubs.acs.org/Organometallics

Published on Web 02/01/2010

ions and the subsequent reactivity of the corresponding organometallic ions. Our motivation for these studies is that they provide access to ions related to textbook organometallics in the pristine gas phase environment, where the inherent reactivity of these organometallic ions can be examined unencumbered by cluster effects, counterions, and solvent molecules. For example, we have examined the formation of mononuclear4,5 and binuclear organomagnesates6 via collision-induced dissociation (CID) of the appropriate magnesium carboxylates (eqs 1 and 2). Each of these anions acts as a base when allowed to react with water (eq 3), consistent with the known condensed phase reactivity of Grignard reagents.7 Indeed intrinsic gas phase basicities can be determined, whereby the role of R and the auxiliary ligand, L, can be investigated.

½RCO2 MgL2  - f ½RMgL2  - þ CO2

ð1Þ

(4) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (5) Thum, C. C. L.; Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118. (6) Khairallah, G. N.; Thum, C. C. L.; O’Hair, R. A. J. Organometallics 2009, 28, 5002. (7) Kosar, W. In Handbook of Grignard Reagents; Silverman, G. S., Rakita, P. E., Eds.; Dekker: New York, 1996; Chapter 23. r 2010 American Chemical Society

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½RCO2 Mg2 Cl4  - f ½RMg2 Cl4  - þ CO2

ð2Þ

each isomer to be “purified” via mass selection for subsequent reactivity studies.

½RMgL2  - þ H2 O f ½HOMgL2  - þ RH

ð3Þ

½CH3 CO2 Ag2 þ f CH3 Ag2 þ þ CO2

Other reactions can compete with decarboxylation. For example, the CID mass spectrum of the lithium diacetate anion is dominated by acetate loss (eq 4).8 One way of overcoming this problem is to redesign the precursor complex. Thus by converting the carboxylate ligand from the acetate anion to the neutral zwitterion betaine, decarboxylation became the dominant channel (eq 5). Although the resultant product is formally a coordinated ylide, it also acts as a base in reactions with water (eq 6).9 Since coordinated ylides can be the precursors of metal carbenes,10,11 methods for their synthesis are of considerable interest.

½ðCH3 CO2 Þ2 Li - f CH3 CO2 - þ CH3 CO2 Li þ

½CH3 CuO2 CR - þ C3 H5 I f ½ICuO2 CR - þ CH3 C3 H5 ð8Þ ½CH3 CO2 CuR - þ C3 H5 I f ½CH3 CO2 CuI - þ RC3 H5 ð9Þ ½CH3 13 CO2 AgO2 CR - f ½CH3 AgO2 CR - þ 13 CO2 ð10Þ

ð4Þ

þ

½ðCH3 Þ3 NCH2 CO2 Li f ½ðCH3 Þ3 NCH2 Li þ CO2 ð5Þ ½ðCH3 Þ3 NCH2 Liþ þ H2 O f ðCH3 Þ4 Nþ þ LiOH ð6Þ Apart from main group organometallic ions, we have also exploited decarboxylation reactions as a means of generating coinage metal organometallic cations (eq 7),12 organocuprates,13 and organoargentates.13a,14 The latter organometallates require two stepwise decarboxylation reactions from each of the coordinated carboxylate ligands. In the case of a heterodicarboxylate precursor, [CH3CO2MetalO2CR]-, the first decarboxylation reaction can occur at either of the carboxylate ligands, giving rise to the isomeric organometallates [CH3MetalO2CR]- and [CH3CO2MetalR]-. We have developed two approaches to establish which of these isomers are formed. The first involved ion-molecule reactions between the organocuprate(s) and allyliodide, giving rise to different C-C bond coupling products (eqs 8 and 9).13b The second, more direct approach involved a 13C labeling experiment, in which the acetate ligand is labeled.14b This method allows the direct determination of the relative ratio of isomer formation simply via a comparison of the losses of 13CO2 (eq 10) and CO2 (eq 11). Moreover, in cases where both isomers are formed, their mass difference of 1 Da allows (8) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255-256, 45. (9) O’Hair, R. A. J.; Waters, T.; Cao, B. Angew. Chem., Int. Ed. 2007, 46, 7048. (10) For key solution phase examples see: (a) Johnson, L. K.; Frey, M.; Ulibarri, T. A.; Virgil, S. C.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 8167. (b) Gandelman, M.; Naing, K. M.; Rybtchinski, B.; Poverenov, E.; Ben-David, Y.; Ashkenazi, N.; Gauvin, R. M.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 15265. (11) For recent gas phase examples see: (a) Fedorov, A; Marc-Etienne Moret, M-E; Chen, P. J. Am. Chem. Soc. 2008, 130, 8880. (b) Fedorov, A; Chen, P Organometallics 2009, 28, 1278. (12) Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. 2009, 2832. (13) (a) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761. (b) Rijs, N.; Waters, T.; Khairallah, G. N.; O'Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069. (14) (a) O’Hair, R. A. J. Chem. Commun. 2002, 20. (b) Rijs, N.; O'Hair, R. A. J. Organometallics 2009, 28, 2684.

ð7Þ

f ½CH3 13 CO2 AgR - þ CO2

ð11Þ

To date, we have been able to compare the competitive decarboxylation reactions of only two different anionic carboxylates. It occurred to us that by moving to a divalent metal ion with experiments performed in the positive ion mode, it should be possible to compare the competition between decarboxylation of an anionic versus a zwitterion ligand for the first time. Here we use a combination of CID and ion-molecule reactions in ion trap mass spectrometer and DFT calculations at the B3LYP/6-31þG(d) level of theory to examine decarboxylation of the cations [CH3CO2MgO2CCH2X(CH3)2]þ (where X = NCH3, 1a; and S, 1b). We show that (i) both the isomeric organomagnesium cation [CH3MgO2CCH2X(CH3)2]þ, 2, and the coordinated ylide [CH3CO2MgCH2X(CH3)2]þ, 3, are formed when X = NCH3, but that the coordinated ylide is the dominant product when X = S; and (ii) 2 and 3 react with water to give structurally diagnostic products (cf. eqs 3 and 6).

Experimental Section Reagents. Magnesium acetate, magnesium bromide, and betaine were obtained from Aldrich. Acetic acid-1-13C was obtained from Isotech (99% 13C isotopic purity). All chemicals were used without further purification. The betaine (CH3)2SþCH2CO2- was synthesized according to a literature procedure 15, isolated as a hydrochloride salt, and recrystallized. Mass Spectrometry. Electrospray ionization (ESI) tandem mass spectrometry experiments (MSn) were carried out to generate and study the chemistry of the complex [CH3CO2MgO2CCH2X(CH3)2]þ, X=S, NCH3, which were readily identified by the distinctive magnesium (24Mg, 78.99%; 25Mg, 10.00%; 26Mg, 11.01%) isotope patterns as well as high-resolution mass spectrometry experiments. Mass spectra were generated by mass selecting either a single isotope or an isotope cluster. In this paper, only single isotope mass spectra are presented in which the most intense peak in the cluster was selected. ESI solutions were prepared by dissolving magnesium acetate with betaine in methanol in a 2:1 molar ratio, with typical concentrations of ca. 1 mM. The 13C-labeled complex [CH313CO2MgO2CCH2X(CH3)2]þ was formed via ESI on a solution of magnesium bromide, acetic acid-1-13C, and betaine in methanol in a 2:2:1 (15) Vasudevamurthy, M.; Weatherley, L.; Lever, M. Biocatal. Biotransform. 2005, 23, 285.

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molar ratio, with typical concentrations of ca. 1 mM. These solutions were directly infused into the ESI sources of two different mass spectrometers: (1) Finnigan LCQ quadrupole ion trap mass spectrometer (Finnigan MAT, San Jose, CA) with a Finnigan electrospray ionization source: This instrument has been modified to allow for ion-molecule reactions (IMR) as described previously,16 and its use in metal-mediated studies has been recently reviewed.17 The ESI solutions were introduced into the electrospray source via a syringe pump operating at a rate of 5 μL/min. Typical electrospray source conditions involved needle potentials of 4.0-5.0 kV. The heated capillary temperature was set at ca. 160 °C. Tuning of electrospray conditions for signal optimization was often required due to low abundance of some species. Mass selection and collision-induced dissociation (CID) were carried out using standard isolation and excitation procedures using the “advanced scan” function of the LCQ software. The mass selection window width was 1.0-1.5 m/z for single isotopes and 5-9 m/z for multi-isotope selection. A normalized collision energy range of 25-40%, an activation Q value between 0.25 and 0.4, and an activation time of 10-30 ms were used for CID and 0.03-300 ms for IMR. (2) Finnigan LTQ FT hybrid linear ion trap (Finnigan, Bremen, Germany) with a Finnigan electrospray ionization source: The ESI solutions were introduced into the electrospray source via a syringe pump operating at a rate of 5 μL/min. Typical electrospray source conditions involved needle potentials of 3.5-4.5 kV. The heated capillary temperature was set at 250 °C. Tuning of electrospray conditions for signal optimization was often required due to low abundance of some species. Mass selection and collision-induced dissociation were carried out using standard isolation and excitation procedures of the LTQ software. The mass selection window width was 1.0-1.5 m/z for single isotopes and 5-9 m/z for multi-isotope selection. A normalized collision energy range of 25-40%, an activation Q value between 0.25 and 0.35, and an activation time of 10-30 ms were used for CID and 0.03-300 ms for IMR. The reaction kinetics for the hydrolysis of the isomers 2 and 3 were examined using both mass spectrometers. Ion-molecule reaction rate measurements were conducted by isolating the reactant ions, [CH3MgO2CCH2N(CH3)3]þ, 2a, [CH313CO2MgCH2N(CH3)3]þ, 3a, and [CH313CO2MgCH2S(CH3)2]þ, 3b, and allowing them to react with background water for different reaction times prior to mass analysis.6 Pseudo-first-order rates were estimated by extrapolation of plots of ln(relative reactant ion intensity) versus time delay. Several independent measurements were taken on different days. The mass selection windows and scan mass range were kept constant. Absolute rates were determined by injecting a known amount of water into the LCQ quadrupole ion trap, as previously described.6 DFT Calculations. In order to gain insights into the mechanisms, structures, and reactions of the organomagnesates, theoretical calculations were carried out using Gaussian 03.18 The density functional theory (DFT) method was used with the B3LYP functional.19 The 6-31þG(d) basis set was used for all atoms.20 Vibrational frequencies were calculated for all optimized structures and either had no imaginary frequencies (for all minima) or one imaginary frequency (for transition states). Full (16) (a) Reid, G. E.; O’Hair, R. A. J.; Styles, M. L.; McFadyen, W. D.; Simpson, R. J. Rapid Commun. Mass Spectrom. 1998, 12, 1701. (b) Waters, T.; O'Hair, R. A. J.; Wedd, A. G. J. Am. Chem. Soc. 2003, 125, 3384. (17) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (18) Frisch, M. J.; et al. et al. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (20) (a) Patterson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (b) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294.

Khairallah et al.

Figure 1. LTQ-FT mass spectra associated with the decarboxylation of the 13C-acetate-labeled magnesium complexes, [CH313CO2MgO2CCH2X(CH3)2]þ: (a) CID of [CH313CO2MgO2CCH2N(CH3)3]þ (m/z 201); (b) CID of [CH313CO2MgO2CCH2S(CH3)2]þ (m/z 204). The mass-selected precursor ion is marked with an * in each case, and peaks due to reaction with background water are marked with a # and are further described in the text. Peaks due to the addition of water are marked with a •; the X (m/z 172) corresponds to an unknown peak inherently due to a small isobaric ion present at the parent mass (m/z 204). The insets at the top left corners in a and b are expansions showing the relative intensities of the key decarboxylation products. data (Cartesian coordinates, energies, and imaginary frequencies for transition states) are given in the Supporting Information.

Results and Discussion 1. Decarboxylation of [CH313CO2MgO2CCH2X(CH3)2]þ (where X = NCH3, 1a; and S, 1b). Figure 1 shows the CID spectra of the 13C-labeled complexes [CH313CO2MgO2CCH2N(CH3)3]þ (Figure 1a) and [CH313CO2MgO2CCH2S(CH3)2]þ (Figure 1b). The peaks at higher m/z (219 in Figure 1a and 222 in Figure 1b) are due to the formation of adducts arising from reactions of the precursor complexes with adventitious background water (eq 12). The dominant fragment ions in both spectra are due to decarboxylation reactions. For [CH313CO2MgO2CCH2N(CH3)3]þ decarboxylation occurs at both the carboxylate ligand (eq 13 where X = NCH3) as well as the betaine ligand (eq 14 where X=NCH3). All other ions in Figure 1a arise from reactions of the precursor and fragments with background water, as described in further detail in section 3 below. While it is hard to determine the exact branching ratio for decarboxylation at both ligands, we have made an estimate by integrating all of

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the peaks (at m/z 156, 158, and 176) associated with decarboxylation at the acetate ligand and integrating all of the peaks (at m/z 74, 157, and 175) associated with decarboxylation at the betaine ligand (Figure S1, Supporting Information). This leads to a branching ratio of ca. 1:3, in favor of decarboxylation at the betaine ligand. The preference for decarboxylation at the betaine ligand is even more dramatic for the sulfur case, with the coordinated ylide (eq 14 where X=S) being the dominant product (at m/z 160 in Figure 1b). Decarboxylation at the acetate ligand is almost completely absent. It is worth noting that decarboxylation at the betaine ligands has allowed the first gas phase “synthesis” of the magnesium ylides [CH313CO2MgCH2N(CH3)3]þ and [CH313CO2MgCH2S(CH3)2]þ. These appear to be rare examples of magnesium-coordinated ylides.21,22 In order to help understand this preference for decarboxylation at the betaine ligand, in the next section we use DFT calculations to examine the competition between decarboxylation at both ligands in [CH3CO2MgO2CCH2N(CH3)3]þ and [CH3CO2MgO2CCH2S(CH3)2]þ.

½CH3 13 CO2 MgO2 CCH2 XðCH3 Þ2 þ þ H2 O f ½CH3 13 CO2 MgO2 CCH2 XðCH3 Þ2 þH2 Oþ ð12Þ ½CH3 13 CO2 MgO2 CCH2 XðCH3 Þ2 þ f ½CH3 MgO2 CCH2 XðCH3 Þ2 þ þ 13 CO2 f ½CH3 13 CO2 MgCH2 XðCH3 Þ2 þ þ CO2

ð13Þ ð14Þ

2. DFT-Calculated Potential Energy Surfaces Associated with the Formation of [CH3MgO2CCH2X(CH3)2]þ, 2, and [CH3CO2MgCH2X(CH3)2]þ, 3. The decarboxylation reactions of [CH3CO2MgO2CCH2X(CH3)2]þ follow similar reaction pathways to those previously studied for a wide range of mononuclear metal carboxylates.4-6,9,13b,14b Figure 2a shows the calculated potential energy surfaces (PES) for the CID reaction of the mononuclear magnesium cations [CH3CO2MgO2CCH2N(CH3)3]þ and [CH3CO2MgO2CCH2S(CH3)2]þ at both the acetate and the betaine sites, while Figure 2b shows key structures associated with these decarboxylation reactions. An examination of Figure 2a provides an immediate explanation for the experimentally observed difference in the decarboxylation reactions of the two complexes. Thus while decarboxylation at the acetate ligand of both complexes has almost identical activation energies, decarboxylation of the betaine is dependent on the betaine structure. For [CH3CO2MgO2CCH2N(CH3)3]þ, 1a, the activation energy for decarboxylation at the betaine is almost the same as decarboxylation of the acetate, which is why under low-energy CID conditions both channels operate (Figure 1a). In contrast, for [CH3CO2MgO2CCH2S(CH3)2]þ, 1b, the activation energy for decarboxylation at the betaine is less than that for decarboxylation of the acetate, which is why the coordinated ylide is formed in preference (Figure 1b). (21) For reviews on the coordination chemistry of ylides see: (a) Weber, L. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S. Eds.; Wiley: Chichester, 1982; Chapter 3, p 91. (b) Kaska, W. C. Coord. Chem. Rev. 1983, 48, 1. (c) Weber, L. Angew. Chem., Int. Ed. Engl. 1983, 22, 516. (22) Previous examples of magnesium-coordinated ligands appear to be limited to phosphine ylides (see refs 21a,21b above). The study of the coordination of (CH3)3NCH2 to Lewis acids appears to have been confined to boron and aluminium compounds. See: Musker, W. K.; Stevens, R. R. Inorg. Chem. 1969, 8, 255.

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The structural features of these decarboxylation reactions are worthy of comment. The ground-state structures for both [CH3CO2MgO2CCH2N(CH3)3]þ, 1a, and [CH3CO2MgO2CCH2S(CH3)2]þ, 1b, are ones in which the magnesium center is tetrahedral due to each of the carboxylate ligands binding in a bidentate fashion. This is consistent with the propensity for Mg to maximize its coordination sphere23 as well as our previous DFT studies on related magnesium systems.4-6 The transition state for decarboxylation of 1a at the acetate ligand (TS1a-4a) is four-centered and lies 2.33 eV higher in energy than 1a and yields a complex between [CH3MgO2CCH2N(CH3)3]þ and CO2, 4a, that requires little energy to dissociate to give the organomagnesium cation [CH3MgO2CCH2N(CH3)3]þ, 2a. The transition state for decarboxylation of 1a at the betaine ligand (TS1a-5a) is also four-centered, lies 2.28 eV higher in energy than 1a, and yields a complex between [CH3CO2MgCH2N(CH3)3]þ and CO2, 5a, that dissociates to give the coordinated ylide [CH3CO2MgCH2N(CH3)3]þ, 3a. Thus the relative energies of TS1a-4a and TS1a-5a are close, consistent with the observation that both decaboxylation reactions occur experimentally. The general features for decarboxylation of 1b are very similar to those calculated for 1a. A key difference is that the transition state for decarboxylation of the sulfur betaine, TS1b-5b, is lower in energy (1.97 eV) than that for decarboxylation of the acetate ligand, TS1b-4b (2.33 eV), consistent with the experimental observed preference for formation of the coordinated sulfur ylide, 3b, under conditions of low-energy CID. Overall it is interesting to note that the DFT calculations predict that decarboxylation at the sulfur betaine ligand is both kinetically and thermodynamically favored. This is consistent with the fact that sulfur ylides are generally more stable than nitrogen ylides due to the sulfur onium group being better able to stabilize the adjacent negative charge.24 3. Hydrolysis Reactions of [CH3MgO2CCH2X(CH3)2]þ, 2, and [CH3CO2MgCH2X(CH3)2]þ, 3, with Water. Experiment and DFT Calculations. Use of the 13C-labeled acetate ligand allows the bimolecular hydrolysis reactions of isomerically pure populations of mass-selected 2 and 3 to be examined. The results of these experiments are shown in Figure 4. The ion-molecule reactions with water are indeed hydrolyses reactions for both types of isomers and appear to proceed via addition/elimination reactions to give the magnesium hydroxides. A key difference in the mass spectra (23) For reviews on the structures of organomagnesium and inorganic magnesium structures determined via X-ray crystallography see: (a) Bickelhaupt, F. In Grignard Reagents: New Developments; Richey, H. G., Jr., Ed.; Wiley: Chichester, 2000; pp 299-328; (b) Uhm, H. L. In Handbook of Grignard Reagents; Silverman, G. S., Rakita, P. E., Eds.; Dekker: New York, 1996; pp 117-144. (c) Markies, P. R.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; Spek, A. L. Adv. Organomet. Chem. 1991, 32, 147. (d) Holloway, C. E.; Melnik, M. Coord. Chem. Rev. 1994, 135/136, 287. (e) Holloway, C. E.; Melnik, M. J. Organomet. Chem. 1994, 465, 1. (24) For a discussion on bonding in ylides see: (a) Dobado, J. A.; Martnez-Garca, H.; Molina, J. M.; Sundberg, M. R. J. Am. Chem. Soc. 2000, 122, 1144. The greater stability of sulfur ylides is suggested by the following: (b) Experiments that show a greater exothermicity for deprotonation of PhCH2S(CH3)2Br than PhCH2N(CH3)3Br; see: Arnett, E. M.; Wernett, P. C. J. Org. Chem. 1993, 58, 301. (c) A range of pKa measurements on benzyl onium salts; see: Cheng, J.-P.; Liu, B.; Zhao, Y.; Sun, Y.; Zhang, X.-M.; Lu, Y. J. Org. Chem. 1999, 64, 604. . Theoretical calculations that show the following: (d) A lower proton affinity of CH2SH2 relative to CH2NH3; see: Koppel, I. A.; Schwesinger, R.; Breuer, T.; Burk, P.; Herodes, K.; Koppel, I.; Leito, I.; Mishima, M. J. Phys. Chem. A 2001, 105, 9575. (e) A lower exothermicity for addition of CH2S(CH3)2 to benzaldehyde compared to that of CH2N(CH3)3; see: Aggarwal, V. K.; Harvey, J. N.; Robiette, R. Angew. Chem., Int. Ed. 2005, 44, 5468.

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Figure 2. DFT (B3LYP/6-31þG(d))-calculated potential energy surfaces associated with the decarboxylation of [CH3CO2MgO2CCH2X(CH3)2]þ (where X = NCH3 and S): (a) energies for decarboxylation of the acetate (left) ligand to yield isomer 2, [CH3MgO2CCH2X(CH3)2]þ, versus decarboxylation of the betaine ligand (right) to yield isomer 3, [CH3CO2MgCH2X(CH3)2]þ; (b) structures of key reactants, intermediates, and products. Electronic energies and Cartesian coordinates of all structures are available in the Supporting Information.

is that each class of isomer gives a different type of ionic product that is detected by the mass spectrometer. Thus the organometallic ion [CH3MgO2CCH2N(CH3)3]þ, 2a, reacted to give the charged metal hydroxide [HOMgO2CCH2N(CH3)3]þ plus methane (eq 15, Figure 4a). Note that this reaction is directly related to the hydrolysis of the previously studied organomagnesates (eq 34-6). The ion at m/z 176 is a secondary reaction between the hydroxide and water (eq 16). In contrast, the reaction of water with the coordinated ylides [CH3CO2MgCH2X(CH3)2]þ (isomer 3) gives a different set of ionic products. Both ylides give rise to water adducts (eq 17: m/z 175 for 3a in Figure 4b; m/z 178 for 3b in Figure 4c) as well as the onium ions (CH3)4Nþ (m/z 74 in Figure 4b) and (CH3)3Sþ (m/z 77 in Figure 4c) via a hydrolysis reaction (eq 18). The latter reaction is directly related to the hydrolysis of

the lithium ylide (eq 6,9).

½CH3 MgO2 CCH2 NðCH3 Þ3 þ þ H2 O f ½HOMgO2 CCH2 NðCH3 Þ3 þ þ CH4

ð15Þ

½HOMgO2 CCH2 NðCH3 Þ3 þ þ H2 O f ½HOMgO2 CCH2 NðCH3 Þ3 þH2 Oþ

ð16Þ

½CH3 13 CO2 MgCH2 XðCH3 Þ2 þ þ H2 O f ½CH3 13 CO2 MgCH2 XðCH3 Þ2 þH2 Oþ

ð17Þ

f ðCH3 Þ3 Xþ þ ½CH3 13 CO2 MgOH

ð18Þ

The absolute rate constants and the reaction efficiencies (j) for these hydrolysis reactions were calculated from

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Figure 3. LTQ-FT MS experiments showing the ion-molecule reactions of water with the mass-selected organomagnesium cations: (a) [CH3MgO2CCH2N(CH3)3]þ (m/z 156), 2a, to yield [HOMgO2CCH2N(CH3)3]þ (m/z 158) and its adduct with water (m/z 176); (b) [CH313CO2MgCH2N(CH3)3]þ (m/z 157), 3a, to yield (CH3)4Nþ (m/z 74) and [CH313CO2MgCH2N(CH3)3 þ H2O]þ (m/z 175); (c) [CH313CO2MgCH2S(CH3)2]þ (m/z 160), 3b, to yield (CH3)3Sþ (m/z 77) and [CH313CO2MgCH2S(CH3)2 þ H2O]þ (m/z 178); (d) relative reaction rates for all three ions ([CH3MgO2CCH2N(CH3)3]þ, [CH313CO2MgCH2N(CH3)3]þ, and [CH313CO2MgCH2S(CH3)2]þ) reacting with water, where the x-axis represents the reaction delay and the y-axis is the ln of the relative ion count. The mass-selected precursor ion is marked with an * in each case.

several independent measurements of the rate of hydrolysis of the relevant ions measured in the LCQ mass spectrometer via the direct infusion of known amounts of water. The amounts of adventitious background water present in the trap were measured on the day of the experiment and taken into account.25 The results of these studies are given in Table 1. Thus, the measured reaction efficiencies for the hydrolysis reaction studied were [CH3MgO2CCH2N(CH3)3]þ (j = 0.63); [CH313CO2MgCH2N(CH3)3]þ (j=0.37); and [CH313CO2MgCH2S(CH3)2]þ (j=0.38). These results clearly indicate that the [CH3MgO2CCH2N(CH3)3]þ reaction efficiency is almost twice those of the ylides. In order to help understand the reactivity order for the hydrolysis reactions, we turned to DFT calculations to examine the hydrolysis of [CH3MgO2CCH2N(CH3)3]þ, 2a, [CH3CO2MgCH2N(CH3)3]þ, 3a, and [CH3CO2MgCH2S(CH3)2]þ, 3b, and, for the sake of comparison, [CH3MgO2CCH2S(CH3)2]þ, 2b. The results of these studies are shown in Figure 4. The PES for each of these hydrolysis reactions procceds via a typical double well28 and closely resemble the PES for the hydrolysis of [RMgL2]- (where R = CH3 and (25) The concentration of the adventitious water (x molecules 3 cm-3) present in the trap at the time of an experiment is measured by conducting experiments where different known amounts of water were introduced into the trap and their corresponding reaction rates were measured. Corresponding equations, with one unknown (x), can then be derived and thus, the value of x can be calculated. (26) Chesnavich, W. J.; Su, T.; Bowers, M. T. J. Chem. Phys. 1980, 72, 2641. (27) Lim, K. F. Quantum Chem. Program Exch. 1994, 14, 3. (28) Brauman, J. I. J. Mass Spectrom. 1995, 30, 1649.

L = Cl or CH3CO2;3 R = HCC and L = Cl5). Each of the reactions is exothermic overall, with the energies of all calculated species on the surface (intermediates and transition states) falling below the energies of the separated reactants (water and either of 2a, 2b, 3a, or 3b). All reactions proceed via a four-centered transition state involving coordination of water at the Mg. The key structures associated with the hydrolysis reaction are shown in Figure 4c. The reactions proceed via the formation of the complexes 6a, 6b, 9a, and 9b, in which the oxygen of the water coordinates to the Mg of 2a, 2b, 3a, and 3b. The four-centered transition states for hydrolysis (TS6a-7a, TS6b-7b, TS9a-10a, and TS9b-10b) involve the breaking of the Mg-C and O-H bonds and the formation of Mg-O and C-H bonds, to yield a complex. The nature of the complex and the final products are dictated by the nature of the ligand, R, bound to the Mg center. In the case of R=CH3 (2a and 2b), the complexes that are formed are between [HOMgO2CCH2X(CH3)2]þ and methane (7a and 7b), which require only a modest amount of energy (0.12 and 0.14 eV, respectively) to separate into the hydrolysis products [HOMgO2CCH2X(CH3)2]þ, 8a and 8b, and CH4. In contrast, when R = CH2Xþ(CH3)2 (3a and 3b), the complexes that are formed are between the onium ions (CH3)3Xþ and [CH3CO2MgOH] (10a and 10b), which requires more energy (0.55 and 0.61 eV, respectively) to separate into the hydrolysis products (CH3)3Xþ (11a and 11b) and [CH3CO2MgOH]. An examination of the structures of these complexes suggests that the reason more energy is required to separate them is due to the formation

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Figure 4. DFT (B3LYP/6-31þG(d))-calculated potential energy surfaces associated with the hydrolysis of (a) [CH3MgO2CCH2N(CH3)3]þ, 2a, and [CH3CO2MgCH2N(CH3)3]þ, 3a, and (b) [CH3MgO2CCH2S(CH3)2]þ, 2b, and [CH3CO2MgCH2S(CH3)2]þ, 3b; (c) structures of key reactants, transition states, intermediates, and products of these hydrolysis reactions. Table 1. Kinetics Associated with the Ion-Molecule Reactions of Water with the Organomagnesium Cations 2a, 3a, and 3b measured abs rate kexperimentala

ion þ

[CH3MgO2CCH2N(CH3)3] , 2a [CH313CO2MgCH2N(CH3)3]þ, 3a [CH313CO2MgCH2S(CH3)2]þ, 3b

-9

1.11  10 0.65  10-9 0.66  10-9

ADO theory rate ktheoreticala,b -9

1.75  10 1.75  10-9 1.75  10-9

efficiency (j) c 0.63 0.37 0.38

Units of cm3 molecule-1 s-1. As in our previous study,4 errors are conservatively estimated as (25%. b Calculated using the theory of Chesnavich et al.26 The calculation was done using the program COLRATE.27 c Reaction efficiency (j) = kexperimental/ktheoretical  100. a

of hydrogen bonds between the coordinated hydroxide and the C-H bonds of the onium ions. By examining the PES further, it is worth noting that the transition state for the reaction of 2a with water (TS6a-7a) is lower in energy (-0.40 eV) than that of the reaction of 3a with water (TS9a-10a) (-0.3 eV); this, associated with a

smaller energy cost from 6a to TS6a-7a than that from 9a to TS9a-10a (0.5 eV versus 0.83 eV), indicates that the reaction of 2a with water is more favorable, consistent with the experimentally determined kinetics. Since the organometallic ion 2b was not observed experimentally, a similar comparison of 2b and 3b is not possible. However, the energetics

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associated with the transition state of the reaction of water with 3b can be compared to those of water with both 2a and 3a. Thus, although the energy of the transition state for the ylide 3b (TS9b-10b, -0.41 eV) is closer to the organometallic ion 2a (TS6a-7a, -0.4 eV) than to the ylide 3a (TS9a-10a, -0.3 eV), the reactant ion-molecule complex 9b lies in the deepest well (-1.28 eV). Thus the energy cost on proceeding from the reactant ion-molecule complex 9b to TS9b-10b (0.87 eV) is similar to that of 9a to TS 9a-10a (0.83 eV), but larger than that of the organometallic ion 6a to TS6a-7a (0.5 eV). Thus the overall DFT data suggest that 3a and 3b should exhibit similar reactivity with water, but react more slowly than 2a, which is consistent with the experimental rate measurements.

Conclusions This study represents the first example of the competition between decarboxylation of an anionic carboxylate ligand (acetate) and a zwitterion carboxylate ligand (the betaines (CH3)2XCH2CO2) coordinated to the same metal center in the magnesium complexes [CH3CO2MgO2CCH2X(CH3)2]þ (where X = NCH3, 1a; and S, 1b). Just as we have found for the related competing decarboxylation reactions of [CH3CO2MetalO2CR]- (where metal = Cu10b or Ag11b), the activation energy for decarboxylation of the acetate ligand is uninfluenced by a change in the nature of the other carboxylate ligand. Thus any changes to the site of decarboxylation in [CH3CO2MgO2CCH2X(CH3)2]þ and [CH3CO2MetalO2CR]- is influenced by the change in activation energy for decarboxylation at (CH3)2XCH2CO2 and RCO2,

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respectively. In the case of the sulfur betaine (X = S), the activation energy for decarboxylation at the betaine is substantially reduced, making the formation of the coordinated ylide the favored pathway. Not only are the coordinated ylides both the kinetic and thermodynamic products of decarboxylation, but they are also less reactive toward water that the organometallic cation [CH3MgO2CCH2N(CH3)3]þ. Decarboxylation continues to provide access to a rich array of organometallic cations for fundamental reactivity studies. Aims of our future work include among others extending the current and previous work to develop an intrinsic basicity scale for the reactions of organomagnesium ions with water and utilizing the coordinated betaines as precursors for the formation of a wide range of coordinated ylides, including those involving the coinage metals. Indeed preliminary experiments suggest that the coordinated sulfur betaine is an excellent precursor to the coordinated sulfur ylide, which in turn can be used to “synthesize” novel silver and gold carbenes via the loss of dimethylsulfide.

Acknowledgment. We thank the ARC for financial support and VICS for the Chemical Sciences High Performance Computing Facility and the Victorian Partnership for Advanced Computing (VPAC). Supporting Information Available: Complete citation for ref 18. Cartesian coordinates and energies (au) for species relevant to each of the reaction pathways described in the text (Figures 2 and 4). Spectrum and ion counts used to calculate the decarboxylation branching ratios. This material is available free of charge via the Internet at http://pubs.acs.org.