A Second Metal Center Enhances the Reactivity of an

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Organometallics 2009, 28, 5002–5011 DOI: 10.1021/om900426h

A Second Metal Center Enhances the Reactivity of an Organomagnesate: Comparison of the Gas-Phase Reactions of Water with [RCCMgCl2]and [RCCMg2Cl4]- (R = H, Ph) George N. Khairallah,†,‡,§ Charlene Thum,†,‡ and Richard A. J. O’Hair*,†,‡,§ †

School of Chemistry and ‡Bio21 Institute of Molecular Science and Biotechnology and §ARC Center of Excellence in Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia Received May 22, 2009

The gas-phase formation and hydrolysis reactions of the mononuclear and binuclear organomagnesate ions [RCCMgnCl2n]- (where R = H, Ph and n = 1, 2) were studied using a combination of ion trap mass spectrometry based experiments and DFT calculations. The organomagnesates were formed via decarboxylation of the carboxylates [RCCCO2MgnCl2n]-. The binuclear organomagnesates [RCCMg2Cl4]- were found to be at least 5 times more reactive than the mononuclear organomagnesates [RCCMgCl2]- (where R = H, Ph). The DFT calculations highlight the role that a second magnesium center can have on both the formation and reaction of an organomagnesate. Thus, DFT calculations reveal that while decarboxylation of the mononuclear carboxylate [HCCCO2MgCl2]- proceeds via only one pathway involving a four-centered transition state, there are two different decarboxylation pathways possible for the isomeric binuclear carboxylates [HCCCO2Mg2Cl4]-: (i) a related four-centered transition state involving a single Mg, resulting in the organomagnesate isomer with a terminal acetylide [HCCMg(μ-Cl3)MgCl]- and (ii) a six-centered transition state involving both Mg centers, which gives rise to the organomagnesate isomer with the bridging acetylide [ClMg(μ-CCH)(μ-Cl2)MgCl]-. Decarboxylation via the six-centered transition state is slightly preferred on both thermodynamic and kinetic grounds. DFT calculations suggest that the source of the difference in the hydrolysis is the direct involvement of the second Mg center. Thus, while hydrolysis of the mononuclear organomagnesate can only proceed via a four-centered transition state, hydrolysis of the binuclear organomagnesate can proceed via a six-centered transition state in which the water binds to one Mg, while the departing acetylide coordinates to the other Mg. Introduction Organometallics such as organolithiums1 and Grignard reagents2 continue to be widely used in organic synthesis. Their roles include that of nucleophiles in C-C bond coupling reactions and of bases in deprotonation reactions. One of the challenges in fully understanding the mechanisms of these reactions is that organometallic reagents can exhibit solvent-dependent aggregation, and thus establishing the nature of the reactive species can be a challenge. Further complications include the fact that solvents are not just reaction media but can take on the role of ligands3 and the *To whom correspondence should be addressed. Fax: +613 93475180. Tel: +61 3 8344-2452. E-mail: [email protected]. (1) (a) Wu, G.; Huang, M. Chem. Rev. 2006, 106, 2596. (b) The Chemistry of Organolithium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2004. (2) (a) Grignard Reagents: New Developments; Richey, H. G., Jr., Ed.; Wiley: Chichester, U.K., 2000. (b) The Chemistry of Organomagnesium Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2008. For a recent essay on Grignard reagents see: (c) Seyferth, D. Organometallics 2009, 28, 1598. (3) Collum, D. V.; McNeil, A. J.; Ramirez, A. Angew. Chem., Int. Ed. 2007, 46, 3002. (4) (a) Gossage, R. A.; Jastrzebski, J. T. B. H.; van Koten, G. Angew. Chem., Int. Ed. 2005, 44, 1448. (b) Pratt, L. M. Mini Rev. Org. Chem. 2004, 1, 209. pubs.acs.org/Organometallics

Published on Web 08/05/2009

fact that heteroaggregates can be formed and these can exhibit different reactivity.4 Where multiple aggregates can coexist, very few studies have identified the relative reactivity order of these aggregates. A recent exception involves the reactions of the monomeric (1), dimeric (2), and tetrameric (3) organolithiums with acetylides, where relative reactivity orders were established via the rapid injection NMR technique (Scheme 1).5 Dramatic differences in reactivity were observed. For example, the dimer 2b was 3.2 108 times more reactive than the tetramer 3b toward PhSO2CCH. In order to understand organometallic reactivity trends, we have turned to fundamental gas-phase studies in which massselected organometalates are examined using mass spectrometry based approaches.6 Thus, the complicating effects of (5) Jones, A. C.; Sanders, A. W.; Bevan, M. J.; Reich, H. J. J. Am. Chem. Soc. 2007, 129, 3492. (6) (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, 2832. (i) Rijs, N.; O'Hair, R. A. J. Organometallics 2009, 28, 2684. r 2009 American Chemical Society

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Scheme 1

solvent and counterions are absent, and the role of clustering on reactivity can be readily examined by directly comparing the reactivity of mass-selected clusters.7 Since simple organometalate anions are challenging to generate via direct electrospray ionization on organometallic solutions,8 we have adopted an alternative approach whereby metal carboxylate anions are subjected to collision-induced dissociation (CID) to “synthesize” the organometalates via a decarboxylation reaction (eq 1).6 These decarboxylation reactions are carried out inside an ion trap mass spectrometer, and thus its inherent multistage mass spectrometry capabilities can be exploited to further study the reactivity of the organometalate ion via ionmolecule reactions or via additional stages of CID.9 Using this approach we have demonstrated that ion-molecule reactions of organoalkaline-earth metalates with water (eq 2) are a useful probe of intrinsic reactivity, since this reaction is sensitive to both the auxiliary ligand, L,6b as well as the metal.6d

½RCO2 MðLÞ2 - f ½RMðLÞ2 - þ CO2 ½CH3 MðLÞ2 - þ H2 O f ½HOMðLÞ2 - þ CH4

ð1Þ ð2Þ

The combination of kinetic measurements and DFT calculations provided clear evidence for the influence of the auxiliary ligand on reactivity of the organomagnesates with water (M = Mg), with the organomagnesate [CH3MgCl2]-, which contains chloride ligands, being more reactive than that with acetate ligands, [CH3Mg(O2CCH3)2]-. The DFT calculations suggested that this arises from the bidentate binding mode of acetate, which induces overcrowding of the Mg coordination sphere.6b In a followup study we demonstrated a relative reactivity order of [CH3Ba(O2CCH3)2]- ≈ [CH3Sr(O2CCH3)2]- > [CH3Ca(O2CCH3)2]- > [CH3Mg(O2CCH3)2]- toward water (eq 2).6d Here we present one of the first gas-phase studies which directly compares the reactivity of monomeric and dimeric organometallic ions. Specifically, the formation and subsequent ion-molecule reactions of the organomagnesate anions [RCCMgCl2]- and [RCCMg2Cl4]- (where R = H, Ph) with water are examined.

Experimental Section Reagents. Magnesium chloride, propiolic acid and phenyl propiolic acid were obtained from Aldrich and acetic acid was (7) Wang, F. Q.; Khairallah, G. N.; Koutsantonis, G. A.; Williams, C. M.; Callahan, D. L.; O’Hair, R. A. J. Phys. Chem. Chem. Phys. 2009, 11, 4132. (8) Some success has been had in the direct use of ESI/MS on solutions of organometalates: (a) Lipshutz, B. H.; Stevens, K. L.; James, B.; Pavlovich, J. G.; Snyder, J. P. J. Am. Chem. Soc. 1996, 118, 6796. (b) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999, 18, 1571. (c) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 100. (d) Koszinowski, K.; Boehrer, P. Organometallics 2009, 28, 771. (9) 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: Weinheim, Germany, in press.

obtained from BDH. All chemicals were used without further purification. Mass Spectrometry. Electrospray ionization (ESI) tandem mass spectrometry experiments (MSn) were carried out to generate and study the chemistry of suitable magnesium chloride carboxylate anions. The magnesium chloride containing anions were readily identified in the mass spectra via their distinctive magnesium (24Mg, 78.99%; 25Mg, 10.00%; 26Mg, 11.01%) and chlorine (35Cl, 75.77%, 37Cl, 24.23%) isotope patterns. Mass spectra were generated by either mass selecting 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 chloride and the appropriate propiolic acid in methanol in a 1:2 molar ratio, with typical concentrations of 0.2-1.0 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 as described previously,10 and its use in metal-mediated studies has been recently reviewed.9 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 were carried out by standard isolation and excitation procedures using the “advanced scan” function of the LCQ software. The mass selection window width was m/z 1.5-2 for single isotopes and m/z 5-9 for multi-isotope selection. A normalized collision energy range of 25-35%, an activation Q value between 0.25 and 0.4, and an activation time of 10-30 ms were used. 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 m/z 1.5-2 for single isotopes and m/z 5-7 for multiple isotope selection. A normalized collision energy range of 25-35%, an activation Q value between 0.25 and 0.35, and an activation time of 10-30 ms were used. The reaction kinetics for the hydrolysis of the organomagensates [RMgnCl2n]- (n = 1, 2) were examined using both mass spectrometers. Ion-molecule reaction rate measurements were conducted by isolating the reactant ion, [RMgnCl2n]-, and allowing it to react with water ([H2O] = ca. 4.5 109 molecules cm-3) for different reaction times prior to mass analysis.10b (10) (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.

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Figure 1. Collisional activation of organomagnesates using the LTQ-FT mass spectrometer: (a) [HCCCO2MgCl2]- (m/z 163) to yield [HCCMgCl2]- (m/z 119); (b) [PhCCCO2MgCl2]- (m/z 239) to yield [PhCCMgCl2]- (m/z 195); (c) [HCCCO2Mg2Cl4]- (m/z 259) to yield [HCCMg2Cl4]- (m/z 215); (d) [PhCCCO2Mg2Cl4]-(m/z 335) to yield [PhCCMg2Cl4]- (m/z 291). The mass-selected precursor ion is marked with * in each case, and peaks due to reaction with background water are marked with #. Pseudo-first-order rates were estimated by extrapolation of plots of ln(relative reactant ion intensity) vs time delay. Single and multiple isotope peaks were used in several independent measurements taken over several days. The mass selection windows and scan mass range were kept constant. DFT Calculations. In order to gain insights into the mechanisms, structures, and reactions of the organomagnesates, theoretical calculations were carried out using Gaussian 03.11 The density functional theory (DFT) method was used with the B3LYP functional.12 The 6-31+G* basis set was used for all atoms.13 Vibrational frequencies were calculated for all optimized structures and either had no imaginary frequencies (for all minima) or one imaginary frequency (for transition states). Reaction endothermicities are corrected for zero -point energies scaled by 0.9806.14 Full data (Cartesian coordinates, energies, and imaginary frequencies for transition states) are given in the Supporting Information.

Results and Discussion 1. Competition between Decarboxylation and Other Reactions in the CID Tandem Mass Spectra of [RCCCO2MgCl2]and [RCCCO2Mg2Cl4]- (where R = H, Ph). A survey of different monomagnesium carboxylates, [RCO2MgCl2]-, and dimagnesium carboxylates, [RCO2Mg2Cl4]-, revealed that acetylides are the only suitable precursors to yield both (11) Frisch, M. J.; et al. Gaussian_03; Gaussian, Inc., Pittsburgh, PA, 2003. (12) (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. (13) (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. (14) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.

the mononuclear and binuclear organomagnesates via decarboxylation reactions.15 Figure 1 shows the CID mass spectra of [RCCCO2MgCl2]- and [RCCCO2Mg2Cl4]- (where R = H, Ph). The monomers [HCCCO2MgCl2]- (m/z 163) and [PhCCCO2MgCl2]- (m/z 239) readily fragment via CO2 loss (eq 3a) to form the acetylides [HCCMgCl2]- (Figure 1a, m/z 119) and [PhCCMgCl2]- (Figure 1b, m/z 195). Virtually none of the carboxylate anion, RCCCO2 (m/z 69 for R = H and m/z 145 for R = Ph), is observed as a CID product (eq 3b), while the low mass cutoff limit of the quadrupole ion trap prevented detection of the chloride loss channel (eq 3c). Finally, in both cases, the magnesium hydroxide anion [HOMgCl2]- is observed at m/z 111, and this arises from an ion-molecule reaction of [HCCMgCl2]- and [PhCCMgCl2]with H2O, as discussed in further detail below (section 3).

½RCCCO2 MgCl2 - f ½RCCMgCl2 - þ CO2

ð3aÞ

f RCCCO2 - þ MgCl2

ð3bÞ

f Cl - þ RCCCO2 MgCl

ð3cÞ

The dimers [HCCCO2Mg2Cl4]- (m/z 259) and [PhCCCO2Mg2Cl4]- (m/z 335) also fragment via CO2 loss (eq 4a) to form the acetylides [HCCMg2Cl4]- (Figure 1c, m/z 215) and [PhCCMg2Cl4]- (Figure 1d, m/z 291). Although there are potentially a range of other competing fragmentation (15) The formation and reactivity of other mononuclear organomagnesates, [RMgCl2]-, will be the subject of a subsequent paper.

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Figure 2. Collisional activation of the organomagnesates using the LTQ-FT mass spectrometer: (a) [PhCCMgCl2]- (m/z 195) to yield [PhCC]- (m/z 101); (b) [PhCCMg2Cl4]-(m/z 291) to yield [PhCCMgCl2]- (m/z 195) and [MgCl3]- (m/z 131). The mass-selected precursor ion is marked with * in each case, and peaks due to reaction with background water are marked with #.

reactions for these dimeric clusters (e.g., eqs 4b-4e), most of these are not observed experimentally. An exception is cluster fragmentation to form MgCl3 at m/z 131 (eq 4d), which is observed as a minor fragment ion in both the CID spectra of [HCCCO2Mg2Cl4]- and [PhCCCO2Mg2Cl4](Figure 1c,d).16 Once again, the low mass cutoff limit prevented detection of the chloride loss channel (eq 4c). The hydroxide cluster [HOMg2Cl4]- (m/z 207) is observed in both mass spectra (Figure 1c,d) and arises from ion-molecule reactions of [HCCMg2Cl4]- and [PhCCMg2Cl4]- with H2O, as discussed in further detail below (section 3).

½RCCCO2 Mg2 Cl4 - f ½RCCMg2 Cl4 - þ CO2

ð4aÞ

f RCCCO2 - þ Mg2 Cl4

ð4bÞ

f Cl - þ RCCCO2 Mg2 Cl3

ð4cÞ

f MgCl3 - þ RCCCO2 MgCl

ð4dÞ

f RCCCO2 MgCl2 - þ MgCl2

ð4eÞ

Since both monomeric and dimeric organomagnesates could be formed, we were interested in investigating their structures by experiment and via the use of DFT calculations. The latter results are discussed in detail for the formation of [HCCMgCl2]- and [HCCMg2Cl4]- (see section 2). We have used both CID and ion-molecule reactions as experimental probes of structure. The ion-molecule studies are discussed in detail below (section 3), while the CID spectra of [PhCCMgCl2]- and [PhCCMg2Cl4]- are shown in Figure 2. [PhCCMgCl2]- fragments solely via loss of MgCl2 to form the acetylide anion, PhCC- (Figure 2a, m/z 101, eq 5), while CID of [PhCCMg2Cl4]- (Figure 2b) involves fragmentation into the monomers MgCl3 (m/z 131, eq 6a) and PhCCMgCl2 (m/z 195, eq 6b). These fragmentation reactions are entirely consistent with the formation of (16) Cluster fragmentation is a common reaction for inorganic clusters. For a recent example see: Ma, M. T.; Waters, T.; Beyer, K.; Palamarczuk, R.; Richardt, P. J. S.; O’Hair, R. A. J.; Wedd, A. G. Inorg. Chem. 2009, 48, 598.

the organomagnesates [PhCCMgnCl2n]- (n = 1, 2).

½PhCCMgCl2 - f PhCC - þ MgCl2 ½PhCCMg2 Cl4 - f MgCl3 - þ PhCCMgCl f PhCCMgCl2 - þ MgCl2

ð5Þ ð6aÞ ð6bÞ

-

CID on the monomer [HCCMgCl2] did not produce any detectable fragment ions, most likely due to the fact that both potential ligand loss channels (HCC - and Cl-) are below the low-mass cutoff of the ion trap mass spectrometer. However, [HCCMg2Cl4]- generated MgCl3 , similar to the [PhCCMg2Cl4]-case (cf. eq 6a and the Supporting Information, Figure S1). 2. DFT Calculated Structures of [HCCMgCl2]- and [HCCMg2Cl4]- and Potential Energy Surfaces Associated with Their Formation. In order to gain further insights into the structures of the mononuclear and binuclear organomagnesates RCCMgCl2 and RCCMg2Cl4 , we have carried out DFT calculations on the simplest system (R = H). The following aspects were considered: (i) potential structures for the organomagnasates, (ii) minimum energy structures for the reactants and products, (iii) products formation via the decarboxylation reactions (eqs 3a and 4a), and (iv) other competing fragmentation pathways (eqs 3b, 3c and 4b-4e). Mononuclear [HCCMgCl2]- (4) exhibits a trigonal-planar structure (Figure 3a), similar to that reported for [CH3MgCl2]-.6b Before describing our approach to calculating potential isomers of the hitherto unknown binuclear organomagnesates of formula [RMg2Cl4]-, it is worth noting that there are related neutral binuclear magnesium halides whose structures have been determined from experiment or theory (5-9 in Scheme 2). Thus, gas-phase electron diffraction data in combination with ab initio calculations have identified the magnesium chloride dimer as having the D2h structure “Mg2(μ-Cl2)” (5), in which there are two bridging chlorides.17 Calculations have revealed that while the C3v structure “Mg2(μ-Cl3)” (6) is a minimum, it is not the ground state.17 Nonetheless, there are examples of the “Mg2(μ-Cl3)” core, with 7 being a directly relevant organomagnesium (17) Molnar, J.; Marsden, C. J.; Hargittai, M. J. Phys. Chem. 1995, 99, 9062.

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Figure 3. DFT (B3LYP/6-31+G*) optimized structures of organomagnesates: (a) [HCCMgCl2]-; [HCCMg2Cl4]- isomers (b) 13, (c) 14, (d) 15, (e) 16a, (f) 16b, and (g) 17. Isomer 14 is the global minimum, and all relative energies of other isomers are given in eV.

determined via X-ray crystallography.18,19 Also of importance for our DFT calculations is whether acetylides adopt terminal or bridging positions in dimers. An examination of the literature has revealed several different X-ray crystal structures, which clearly show that acetylides can adopt terminal (e.g., 8 in Scheme 2,20) or bridging (e.g., 9 in Scheme 2,21) positions in dimers. Finally, different isomeric binuclear magnesium fluoride anions of formula [Mg2,F5](18) Al-Juaid, S. S.; Eaborn, C.; Hitchcock, P. B.; Jaggar, A. J.; Smith, J. D. J. Organomet. Chem. 1994, 469, 129. (19) On the basis of coldspray mass spectrometry experiments, structures related to 6 have been invoked to explain the solution-phase reactivity of organomagnesium compounds: Sakamoto, S.; Imamoto, T.; Yamaguchi, K. Org. Lett. 2001, 3, 1793. (20) Yang, K.-C.; Chang, C.-C.; Huang, J.-Y.; Lin, C.-C.; Lee, G.-H.; Wang, Y.; Chiang, M. Y. J. Organomet. Chem. 2002, 648, 176. (21) Xia, A.; Heeg, M. J.; Winter, C. H. Organometallics 2003, 22, 1793. (22) (a) Anusiewicz, I.; Skurski, P. Chem. Phys. Lett. 2007, 440, 41. (b) Anusiewicz, I. Aust. J. Chem. 2008, 61, 712. (23) For reviews on the structures of organomagenisum 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, U.K., 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.

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have been recently explored using theoretical methods, and the three stable structures 10-12 (where X = F) were found, with the following stability order: 10 (ground state) > 11 > 12,22a consistent with the preference of magnesium to maximize its coordination.23 On the basis of these structures, we initially optimized three different isomers of formula [Mg2,Cl5]-: the “Mg2(μCl3)” structure 10 (X = Cl), the “Mg2(μ-Cl2)” structure 11 (X = Cl), and the “Mg2(μ-Cl)” structure 12 (X = Cl) (optimized structures are presented in the Supporting Information).22 At the B3LYP/6-31+G* level of theory, 12 converged to 11, suggesting that 12 is unstable with respect to rearrangement. The relative stability order 10 (0.00 eV) > 11 (0.36 eV) is consistent with that of [Mg2,F5]-. During the course of our work, isomers of formula [Mg2,Cl5]- were examined at a higher level of theory (MP2/6-311+G*).22b Only 10 and 11 were found to be stable, with the former being more stable by 0.46 eV. In order to calculate the structure of the organomagnasates, we replaced one of the terminal or bridging chlorides of 10-12 with an acetylide ligand, X (X = CCH), giving isomers 13-19 in Scheme 3. Each of these geometry “guesses” were calculated at the B3LYP/6-31+G* level of theory, and fully optimized, stable structures are shown in Figure 3. The DFT calculations show that all structures contain a [MgCl3] unit, suggesting that the CID experiments (e.g. Figure 2b) cannot distinguish which of these isomers is observed experimentally. Structures with three bridging ligands (13 and 14) are more stable than structures with two bridging ligands (15-17). Starting geometry guesses with only one bridging ligand, 18 and 19 (Scheme 3), converged into 17 and 14, respectively. The four optimized structures 15, 16a,b, and 17, based on structure 11, possess a Mg atom which is four-coordinate, with the other being three-coordinate. Two structures were accessible from structure 16, due to the acetylide group adopting a slanted position (cf. 14): in 16a the acetylide forms a σ bond to the three-coordinate Mg, while in 16b it forms the σ bond to the four-coordinate Mg. Structures 1517 have the relative stability order 16a > 17 > 15 > 16b. Isomers 13 and 14 are the most stable and hence most likely candidates for the experimentally observed species. Both structures contain two four-coordinate (tetrahedral) Mg centers but differ in the position of the acetylide ligand (13, terminal; 14, bridging). The bridging acetylide in 14 adopts a slanted position to allow an interaction with one Mg via a σ bond and with the second Mg via a π interaction. Since 13 is only 0.003 eV higher in energy than 14, it is important to consider the kinetics associated with their formation via two different decarboxylation reactions. The results of these DFT studies are shown in Figure 4 and are also compared with the decarboxylation energetics associated with the monomer [HCCCO2MgCl]-. Figure 4a shows the calculated potential energy surface (PES) for the CID reaction of mononuclear magnesium carboxylate, [HCCCO2MgCl2]-, while Figure 4c shows key structures associated with the decarboxylation reaction. The ground-state structure for [HCCCO2MgCl2]- is 20, in which the magnesium center is tetrahedral due to the carboxylate binding in a bidentate fashion. In contrast, the reactive geometry for decarboxylation is 21, whereby the magnesium is trigonal planar since the carboxylate binds in a monodentate fashion. The transition state for decarboxylation

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Scheme 2

Scheme 3

(TS21-22) is four-centered and lies 1.33 eV higher in energy than the entrance channel corresponding to the separated reagents (Figure 4c and tables in the Supporting Information) and yields a complex between [HCCMgCl2]and CO2, 22, that requires little energy to dissociate to give the organomagnesate 4. The general features for decarboxylation of [HCCCO2MgCl2]- are very similar to those calculated for [MeCO2MgL2]-.6b Finally, the other two potential dissociation pathways are shown as dashed lines in Figure 4a and involve: formation of Cl- (+2.57 eV) and formation of HCCCO2 (+3.03 eV) (see tables in the

Supporting Information). Since they are much higher in energy, they are not expected to occur under conditions of low-energy CID. Figure 4b shows the calculated potential energy surface (PES) for the CID reaction of binuclear magnesium carboxylate anions, [HCCCO2Mg2Cl4]-, while Figure 4c shows key structures associated with these reactions. Two possible decarboxylation reactions were considered, giving rise to the two different [HCCMg2Cl4]- isomers 13 and 14. A total of four isomers of [HCCCO2Mg2Cl4]- were found, based on the structural motifs of 13 and 14. The ground state appears

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Figure 4. DFT calculated potential energy surfaces associated with the decarboxylation of [HCCCO2MgCl2]- and the decarboxylation of [HCCCO2Mg2Cl4]- isomers to yield [HCCMg2Cl4]- isomers 13 and 14: (a) [HCCCO2MgCl2]-; (b) isomers 23 and 24 to yield 13 and 14; (c) key reactants and intermediate structures.

to be 23, in which the carboxylate bridges between both magnesium centers-thus, this isomer is related to 14. The next most stable isomer is 24, in which the carboxylate bridges in a bidentate fashion to one of the magnesium centers-thus, this isomer is related to 13. Similar to the decarboxylation of the mononuclear system 20, the two different decarboxylation channels, to yield 13 and 14, proceed via the reactive complexes 25 and 26. Indeed, the reactive complex 25 is similar to the reactive complex 21, since both have the carboxylate binding in a monodentate fashion. The transition state for decarboxylation of 25 (TS25-27) also bears some resemblance to TS21-22, since it only involves one magnesium center, is four-centered, and results in a product complex, 27, between the organomagnesate and CO2. Loss of CO2 from 27 results in the formation of 13. The PES associated with decarboxylation to yield 14 exhibits some interesting differences: (i) an examination of the reactive geometry for decarboxylation, 26 (Figure 4c), reveals that the acetylide moiety binds to one magnesium center via one O atom and to the second Mg via a π interaction; (ii) decarboxylation involves a six-centered transition state, TS26-28, in which the CO2 binds to one Mg and the acetylide is transferred to the other Mg to form a product

complex, 28, which readily dissociates via loss of CO2 to form 14. Finally, in Figure 4b we have also considered the energetics of fragmentation reactions of [HCCCO2Mg2Cl4]- that might compete with decarboxylation (eq 4a)these are shown as dashed lines. The ligand loss reactions are the most energetically demanding and are predicted to be endothermic by +3.93 eV for carboxylate loss (eq 4b) and +3.42 eV for Cl- loss (eq 4c). In contrast, the cluster fragmentation reactions are less endothermic: formation of MgCl3 requires +2.21 eV, while formation of RCCCO2MgCl2 requires +2.44 eV. Of all these competing reactions, the energetics associated with formation of MgCl3 is closest to that of the transition state energy for decarboxylation, consistent with MgCl3 being a minor product in the experiments (Figure 1c,d). 3. Experimental Reactivity Orders of the Reactions of Water with [RCCMgCl2]- and [RCCMg2Cl4]- (R = H, Ph). The mononuclear organomagnesate [HCCMgCl2](m/z 121), formed by decarboxylation of [HCCCO2MgCl2]-, was mass-selected in a MS3 experiment and allowed to undergo an ion-molecule hydrolysis reaction with background water to form [HOMgCl2]- (m/z 113, eq 7, Figure 5a). The mass-selected binuclear organomagnesate

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Table 1. Kinetics Associated with the Ion-Molecule Reactions of Water with a Range of Organomagnesates ion [PhCCMg2Cl4][PhCCMgCl2][HCCMg2Cl4][HCCMgCl2][CH3MgCl2]-

measd abs rate kexptla 1.62 0.27 0.97 0.22 0.87

ADO theory rate ktheora,b 1.71 1.74 1.73 1.78 1.80

efficiency (j)c 0.95 0.15 0.56 0.12 0.48

a Units of 10-9 cm3 molecule-1 s-1. As in our previous study.6b errors are conservatively estimated as (25%. b Calculated using the theory of Chesnavich et al.25 The calculation was done using the program COLRATE.26 c Reaction efficiency (j) = kexptl/ktheor 100.

[HCCMg2Cl4]- (m/z 215) reacts with water in an analogous manner to form the binuclear magnesium hydroxide [HOMg2Cl4]- (m/z 207, eq 8, Figure 5b). In order to compare the relative reactivity orders of both organomagnesates, the hydrolysis reactions were conducted under identical conditions as described previously.6d Thus, in order to ensure that the reaction times were identical for both [HCCMgCl2]- and [HCCMg2Cl4]- (including the scan out time to eject ions from the trap and into the detector), the same reaction mass range window was used. The relative abundances of [HOMgCl2]- and [HOMg2Cl4]- from hydrolysis of [HCCMgCl2]- and [HCCMg2Cl4]-, respectively, provide clear evidence that the binuclear organomagnesate reacts more rapidly with water than the mononuclear organomagnesate. Indeed, comparison of the rates of hydrolysis under pseudo-first-order kinetics demonstrates that the binuclear organomagnesate is approximately 5 times more reactive than the mononuclear organomagnesate (Figure 5c). Similar reactivity studies have been conducted with the other organomagnesates [PhCCMgCl2]- and [PhCCMg2Cl4]-, and related reactivity trends are observed. Thus, the binuclear organomagnesate [PhCCMg2Cl4]- is ca. 6 times more reactive than its mononuclear counterpart [PhCCMgCl2](Supporting Information, Figures S2-S4). It is worth mentioning that plots of the total ion current versus reaction time produced linear plots for the hydrolysis of all organomagnesates, suggesting the presence of only one isomer (data not shown). In addition, the reactivity trends were identical for two different mass spectrometers (quadrupole ion trap and linear ion trap). Several independent measurements were done on separate days, including single and multiple isotope selection, all providing similar results.

½RCCMgCl2 - þ H2 O f ½HOMgCl2 - þ RCCH ð7Þ ½RCCMg2 Cl4 - þ H2 O f ½HOMg2 Cl4 - þ RCCH ð8Þ In order to establish the absolute rate constant and the reaction efficiencies (j) for these hydrolysis reactions, they were directly compared with the related hydrolysis of [CH3MgCl2]- (eq 2, where M = Mg and L = Cl) which was previously measured.6b The results of these studies are given in Table 1. The absolute rate and reaction efficiency of [CH3MgCl2]- were independently measured in the LCQ mass spectrometer via the direct infusion of known amounts of water. The efficiency was found to be j = 0.48, which is slightly different from that measured previously (j = 0.54) but is within experimental error. Thus, the measured reaction efficiencies for the hydrolysis reaction studied were as follows: [PhCCMgCl2]-, j = 0.15; [PhCCMg2Cl4]-, j = 0.95; [HCCMgCl2]-, j = 0.12; [HCCMg2Cl4]-, j = 0.56. These results indicate that the monomer reaction efficiencies are well below that of [CH3MgCl2]-, whereas the dimer reaction efficiencies are on par. This reactivity is similar to

Figure 5. Ion-molecule reactions of mass-selected organomagnesates with water, as studied on the LTQ-FT mass spectrometer: (a) [HCCMgCl2]- (m/z 121) to yield [HOMgCl2]- (m/z 113); (b) [HCCMg2Cl4]-(m/z 215) to yield [HOMg2Cl4]- (m/z 207); (c) temporal decline of [HCCMgCl2]- and [HCCMg2Cl4]-, where the x axis represents the reaction delay and the y axis is the natural log of the relative ion count. The mass-selected precursor ion is marked with * in each case.

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Figure 6. DFT (B3LYP/6-31+G*) calculated potential energy surfaces associated with the hydrolysis of (a) [HCCMgCl2]-, (b) [HCCMg2Cl4]-isomer 13, and (c) [HCCMg2Cl4]-isomer 14 and (d) key reactant and intermediate structures.

that expected for the solution-phase reactions of the corresponding Grignard reagents.24 4. DFT Calculations To Help Rationalize the Reactivity of Water with HCCMgCl2 and HCCMg2Cl4 . In order to help understand the experimental reactivity orders, we have carried out DFT calculations of the potential energy surfaces associated with the hydrolysis reactions of the organomagnesates [HCCMgCl2]- and [HCCMg2Cl4]-. The PES for the reaction of the mononuclear organomagnesates [HCCMgCl2]- with water is shown in Figure 6a and is a typical double well.27 The reaction is exothermic overall, with the energies of all calculated species on the surface (intermediates and transition state) falling below the energy of the separated reactants ([HCCMgCl2]- and water). Furthermore, the PES shown in Figure 6a closely resembles the PES for the hydrolysis of [CH3MgL2]-,6c with both reactions proceeding 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 6d. The reaction proceeds via the formation of complex 29, in which the oxygen of the water coordinates to the Mg of 4. The four-centered transition state for hydrolysis (TS29-30) is 0.51 eV lower in energy than the entrance (24) While the relative reactivities of MeMgX and HCCMgX toward water do not appear to have been measured in the condensed phase, MeMgX is expected to be a stronger base than HCCMgX, on the basis of the following. (a) Alkynylmagnesiums are prepared via the reaction of alkylmagnesiums and alkynes; see: Yanagisawa, A. Sci. Synth. 2004, 7, 523. (b) DFT calculations predict that the reactions of MeMgCl and MeMgBr with propyne are exothermic: Tuulmets, A.; Tammiku-Taul, J.; Burk, P. THEOCHEM 2004, 674, 233. (25) Chesnavich, W. J.; Su, T.; Bowers, M. T. J. Chem. Phys. 1980, 72, 2641. (26) Lim, K. F. Quantum Chemistry Program Exchange 1994, 14, 3. (27) Brauman, J. I. J. Mass Spectrom. 1995, 30, 1649.

channel corresponding to the separated reactants (Figure 6d and tables in the Supporting Information). It involves the breaking of the Mg-C and O-H bonds and the formation of Mg-O and C-H bonds. This yields complex 30 between [HOMgCl2]- and HCCH, which requires 0.37 eV to separate into the hydrolysis products [HOMgCl2]-, 31, and HCCH. For the hydrolysis of the binuclear organomagnesate [HCCMg2Cl4]-, potential energy surfaces were calculated for both isomers 13 and 14. In the case of isomer 13, the reaction profile is a typical double-well potential energy surface (PES), forming both the precomplex 32 and the postcomplex 33. In this case, however, the transition state (TS32-33) is calculated to be 0.11 eV higher in energy than the entrance channel and thus is unlikely to occur under the near thermal energies of the ion trap. In contrast, isomer 14 displays a PES profile where the transition state (TS35-36) is very close in energy to both the reactant, 35, and product complexes, 36. Indeed, when zero-point energies are taken into account, the transition-state energy is less than the energy of the initially formed complex. Since the energies of all calculated species on the surface (intermediates and transition state) fall below the energy of the separated reactants (14 and water), this reaction should readily proceed under the near-thermal conditions of the ion trap. An examination of the key structures associated with the hydrolysis reactions of 13 and 14 (Figure 6d) reveals some interesting differences. In the case of 13 the reactant complex 32 formed involves coordination of the water to the Mg containing the terminal acetylide. This complex then reacts via a four-centered transition state (TS32-33), similar to the mononuclear case 4, where only one Mg is involved. This transition state leads to the product complex 33 of [HOMg2Cl4]- and HCCH, which can then lose acetylene to form the terminal binuclear hydroxide 34 (directly related

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to 13, where “HCC” is replaced by “OH”). In the hydrolysis of isomer 14, the reactant complex formed, 35, leads to a sixcentered transition state (TS35-36) which is 0.58 eV lower in energy than the entrance channel. This involves the formation of Mg-O and C-H bonds. The product complex 36 of [HOMg2Cl4]- and HCCH can readily dissociate to form the bridging binuclear hydroxide 37 (directly related to 14, where “HCC” is replaced by “OH”). Thus, the DFT calculations predict that the hydrolysis of 14 should be much faster than that of 13, which provides additional, indirect evidence that the most likely isomer formed in our experiments is 14 and not 13. Finally, it is important to note that the DFT calculations are entirely consistent with the relative reactivity order for hydrolysis of the mono- and binuclear organomagnesates: hydrolysis of 14 is predicted to be faster than that of 4, since its transition state energy is lower and its overall reaction exothermicity is greater (compare parts a and c of Figure 6).

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binuclear organomagnesate [HCCMg2Cl4]- can proceed via a six-centered transition state in which the water binds to one Mg, while the departing acetylide coordinates to the other Mg (see TS35-36 of Figure 6). Our results are consistent with previous theoretical studies on the mechanism of the Grignard reaction between the dimer (MeMgX)2 and formaldehyde.28 Thus, Yamazaki and Yamabe found that the aldehyde coordinates to one magnesium, while the Me group is transferred from the other Mg, as shown in transition state 38.28,29 Their transition state bears similarities to those found in the current work for decarboxylation (TS26-28 of Figure 4) and hydrolysis (TS35-36 of Figure 6).

Conclusions This study represents the first example of the gas-phase formation and reactivity of both mononuclear and binuclear organomagnesate ions, [RCCMgnCl2n]- (where R = H, Ph; n = 1, 2). The work highlights the role that a second magnesium center can have on both the formation and reaction of an organomagnesate. Thus, DFT calculations reveal that while decarboxylation of the mononuclear carboxylate, [HCCCO2MgCl2]-, proceeds via only one pathway involving a four-centered transition state (see TS21-22 of Figure 4), there are two different decarboxylation pathways possible for isomeric binuclear carboxylates, [HCCCO2Mg2Cl4]-: (i) a related four-centered transition state involving a single Mg center (see TS25-27 of Figure 4), resulting in an organomagnesate isomer with a terminal acetylide (see 13 of Figure 4) and (ii) a six-centered transition state involving both Mg centers (see TS26-28 of Figure 4), which gives rise to an organomagnesate isomer with a bridging acetylide (see 14 of Figure 4). Decarboxylation via the six-centered transition state is slightly preferred on both thermodynamic and kinetic grounds. Ion-molecule reactions of the organomagnesate ions [RCCMgnCl2n]- highlight that the binuclear organomagnesates are at least 5 times more reactive than the mononuclear organomagnesates. Once again, DFT calculations suggest that the source of the difference in reactivity is the direct involvement of the second Mg center. Thus, while hydrolysis of the mononuclear organomagnesate [HCCMgCl2]- can only proceed via a four-centered transition state (see TS29-30 of Figure 6), hydrolysis of the

Finally, recent studies have shown that cluster size can have a profound effect on the type of fragmentation reactions observed in silver acetylide cluster cations, [(RCCAg)nAg]+.7 Future work will examine the role that cluster size has on the bimolecular C-C bond coupling reactions of organometallic clusters.

Acknowledgment. We thank the ARC for financial support via Grant No. DP0558430 (to R.A.J.O.) and the VICS for the Chemical Sciences High Performance Computing Facility. We thank Dr. Tom Waters for useful discussions and Ann Jacob for carrying out preliminary calculations on the [HCCMgCl2]-system. Supporting Information Available: Text, tables, and figures giving the complete citation for ref 9, mass spectra for fragmentation of [HCCMg2Cl4]- and reaction of [PhCCMgnCl2n]- with water, and Cartesian coordinates and energies (au) for species relevant to each of the fragmentation pathways described in the text (Figures 4 and 6). This material is available free of charge via the Internet at http://pubs.acs.org. (28) Yamazaki, S.; Yamabe, S. J. Org. Chem. 2002, 67, 9346. (29) A more recent theoretical study of the Grignard reaction of acetone with solvated (MeMgX)2 has suggested the involvement of dimeric transition-state structures with only one bridging halide ligand. This study also predicts that the dimer is more reactive than the monomer: Mori, T.; Kato, S. J. Phys. Chem. A 2009, 113, 6158.

A Second Metal Center Enhances the Reactivity of an

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