Modeling Solvation of Magnesium Centers by Ether Ligands: Gas

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Modeling Solvation of Magnesium Centers by Ether Ligands: Gas-Phase Synthesis and Hydrolysis of the Organomagnesium Cations [CH3Mg(3X-crown-X)]+ (X = 4
6) Michael G. Leeming,†,‡ George N. Khairallah,†,‡ Gabriel da Silva,§ 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 § Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia ‡

bS Supporting Information ABSTRACT: Gas-phase decarboxylation of the acetate ligand in the magnesium acetate crown complex cations [CH3CO2Mg(3X-crown-X)]+ (where X = 4
6) has been explored as a means of synthesizing the corresponding organomagnesium cations as models for the organomagnesium core CH3Mg+, which is solvated by ether ligands. Low-energy collisioninduced dissociation (CID) of these complexes in ion trap mass spectrometers gives rise to a range of product ions. Highresolution mass measurements reveal the formation of isobaric product ions arising from decarboxylation, [CH3Mg(3X-crown-X)]+, and from loss of C2H4O from the crown ether ligand. The use of low-energy CID of the isotopically labeled complexes [CH313CO2Mg(3X-crown-X)]+ allowed for (i) energy-resolved CID studies, which demonstrated that [CH313CO2Mg(12-crown-4)]+ is more easily decarboxylated than [CH313CO2Mg(15-crown-5)]+ and [CH313CO2Mg(18-crown-6)]+ and (ii) the separation and isolation of the methyl magnesium crown ether cations [CH3Mg(3X-crown-X)]+ for subsequent reactivity studies with water. Rate constants for the hydrolysis of [CH3Mg(3Xcrown-X)]+ were experimentally determined to follow the reactivity order [CH3Mg(12-crown-4)]+ > [CH3Mg(18-crown-6)]+ > [CH3Mg(15-crown-5)]+. DFT calculations at the B3LYP/6-31+G(d) level of theory in conjunction with RRKM modeling are consistent with the experimentally determined reactivity orders for decarboxylation of [CH3CO2Mg(3X-crown-X)]+ and hydrolysis of [CH3Mg(3X-crown-X)]+ and highlight that reactivity generally decreases with increasing solvation of the organomagnesium core CH3Mg+.

’ INTRODUCTION Over 100 years ago Pesci discovered that mercury salts promote decarboxylation of phthalic acid to form an organomercury species.1 Since then, the decarboxylation of metal carboxylates has been widely studied.2,3 Initial efforts were focused on the isolation and characterization of the resultant organometallic product.2 Subsequent pioneering work by Nilsson in the 1960s and 1970s highlighted the potential of catalytic C
C bond formation in organic synthesis.4 In the past decade there has been an explosion of interest in further developing metal-catalyzed decarboxylation reactions.3 Protodecarboylation (eq 1) is one class of reaction that has been studied in both the condensed phase5 and the gas phase.6 Many of the gas-phase studies have involved multistage mass spectrometry experiments carried out on ion trap mass spectrometers equipped with electrospray ionization (ESI).7,8 Thus, low-energy collision-induced dissociation (CID) of metal carboxylate ions formed via ESI has been used to “synthesize” a wide range of organometallic anions6,9 and cations10 via decarboxylation (eq 2). This gas-phase approach has provided opportunities to examine fundamental reactivity of mass-selected organometallic ions with a range r 2011 American Chemical Society

of substrates. RCO2 H f RH þ CO2

ð1Þ

½RCO2 MLn 
=þ f ½RMLn 
=þ þ CO2

ð2Þ

Two key “textbook” organometallic reagents that we have tried to develop gas-phase models for are Gilman reagents, R2CuLi,11 and Grignard reagents, RMgL (where L is typically a halide such as chloride).12 Since mass spectrometers can only isolate and detect ions, these gas-phase studies require the design of organometallic complexes that possess an overall charge. In the case of Gilman reagents, the “bare” cuprates [CH3CuCH3]
and [CH3CuR]
have proven to be useful gas-phase models.9b,d,h
j With regard to the various organomagnesium complexes that can coexist in the solution-phase equilibria of Grignard reagents, we have focused on the ionic species 1 and 2, which are involved in Received: April 28, 2011 Published: July 28, 2011 4297

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Scheme 1. Organoalkaline Earth Complex Ions

the various ionization equilibria shown in eqs 3
5.13 þ

RMgL h RMg þ L




ð3Þ

1

2RMgL h RMgþ þ RMgL2
1

ð4Þ

2

been synthesized in the condensed phase (8, 9b, 9c and 11 of Scheme 1).16 Here we describe mass spectrometry based experiments and DFT calculations aimed at examining the formation and hydrolysis reactions of organomagnesium complexes utilizing crown ether ligands, [CH3Mg(3X-crown-X)]+ (Scheme 1: 8, where X = 4; 9a, where X = 5; 10, where X = 6).17

’ EXPERIMENTAL SECTION

The anionic mononuclear organoalkaline earth complexes 3 and 4 (Scheme 1), which are directly related to 2, have all been successfully “synthesized” in the gas phase.6a,9c,g The reactivity of the simplest model system, CH3MgCl2
, 3a, toward a wide range of substrates has been studied.6a This organomagnesate undergoes acid
base reactions with substrates containing an acidic proton (eq 6), including aldehydes and ketones. Since related reactions are well documented for Grignard reagents in solution,14 this has provided the impetus for a range of studies aimed at examining how hydrolysis reactions (eq 6, where A = HO) are influenced by the role of the auxiliary ligand, L, in CH3Mg(L)2
, 3a and 4a;6a alkaline earth metal center in the complexes CH3M(O2CCH3)2
, 4a
4d;9c and cluster size (e.g., the monomers 3c and 3d versus dimers 5a and 5b).9g ½RMLn 
=þ þ HA f ½AMLn 
=þ þ RH

ð6Þ

In solution, the “RM+” organometallic core of 1 will be ligated, most likely by several ether solvent molecules, to form complexes of the type RMg(OR0 2)n+ (where R0 2O represents either tetrahydrofuran or diethyl ether).15 In our first foray into generating a solvated “RM+” organometallic core, we attempted to use decarboxylation reactions to produce the cationic mononuclear complexes 7 in which neutral, zwitterionic betaine auxiliary ligands were used as the “solvent”.10c The betaine auxiliary ligand also underwent competitive decarboxylation to produce the coordinated ylide complexes 7. Indeed for the sulfur betaine, the barrier for decarboxylation to produce 7b was substantially lower,and none of the organomagnesium cation 6b was formed under the low-energy CID conditions used. In order to provide better model systems for a “RM+” organometallic core ligated by ether solvent molecules, we now turn our attention to crown ether ligands, where the number of ether oxygens can readily be changed by simple changing the size of the crown (8
10 in Scheme 1). Indeed we note that organomagnesium complexes that utilize macrocylic ligands containing either O or N heteroatoms have

Reagents. Magnesium bromide, 18-crown-6, 15-crown-5, and 12crown-4 were obtained from Aldrich; acetic acid was obtained from BDH; methanol was from Merck. 1-13C-Acetic acid was obtained from Isotech (99% 13C isotope purity). All chemicals were used without further purification. Mass Spectrometry. Electrospray ionization tandem mass spectrometry experiments (MSn) were carried out to generate and study the chemistry of suitable magnesium crown ether acetate cations. The magnesium-containing cations were readily identified in the mass spectra via their distinctive magnesium (24Mg, 78.99%; 25Mg, 10.00%; 26 Mg, 11.01%) isotope patterns. 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 bromide and the appropriate crown ether in methanol in a 1:1 molar ratio, with typical concentrations of 0.1
0.5 mM. Methanol was used as the solvent in all experiments since it is ESI “friendly” and facilitates the complexation of metal cations to crown ethers.18 Acetic acid was added in approximately 2-fold molar excess. The ESI solutions were directly infused into the electrospray source of the mass spectrometer via a syringe pump operating at a rate of 5 μL/min. 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 using the “advanced scan” function of the MS software. Two different mass spectrometers were used under the following operating conditions: (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,19 and its use in metal-mediated studies has been reviewed.7 Typical electrospray source conditions involved needle potentials of 4.0
5.0 kV. The heated capillary temperature was set at ca. 180 °C. The mass selection window width was 1.2
1.5 m/z for single isotopes and 5
9 m/z for multi-isotope selection. A normalized collision energy range of 30
45%, 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. Typical 4298

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Organometallics electrospray source conditions involved needle potentials of 3.5
4.5 kV. The heated capillary temperature was set at 250 °C. The mass selection window width was 1.1
1.5 m/z for single isotopes and 5
7 m/z for multi-isotope selection. A normalized collision energy range of 30
45%, an activation Q value between 0.25 and 0.35, and an activation time of 10
30 ms were used. In order to differentiate possible isobaric ions arising from losses of C2H4O and CO2, high-resolution mass spectra were obtained by transferring the CID product ions from the linear ion trap into the FT-ICR cell. Mass calibration was performed in the positive ion mode via the automatic calibration function using the recommended LTQ-FT calibration solution, consisting of caffeine, the tetrapeptide MRFA, and Ultramark 1621. Measurement of Hydrolysis Kinetics. The reaction kinetics for the hydrolysis of the organomagensates [CH3Mg(3X-crown-X)]+ (X = 4
6) were examined using both mass spectrometers. Ion
molecule reaction rate measurements were conducted by isolating the reactant ion, [CH3Mg(3X-crown-X)]+, and allowing it to react with water (>109 molecules 3 cm
3) for different reaction times prior to mass analysis.19b Pseudo-firstorder rates were estimated by least-squares regression of plots of ln(relative reactant ion intensity) vs time delay. Single isotope peaks were used in several independent measurements taken over several days. The mass selection windows and scan mass range were kept constant throughout. Threshold CID Measurements. In order to gain insights into the relative ease of decarboxylation of [CH3CO2Mg(3X-crown-X)]+ for X = 4
6, threshold energy-resolved CID studies were undertaken on the LCQ spectrometer as described previously by Colorado and Brodbelt.20 In brief, these experiments measure the appearance of decarboxylation products [CH3Mg(3X-crown-X)]+ by varying the CID energy imparted on the mass-selected precursors [CH313CO2Mg(3X-crown-X)]+ for X = 4
6 and observing the changes in product ion intensity as a function of collision energy. Reaction delays and scan window ranges were held constant throughout each experiment. These experiments are complicated by the formation of the magnesium hydroxides, [HOMg(3X-crown-X)]+, within the time frame of the CID experiment. These hydroxides are assumed to be secondary hydrolysis products formed first by decarboxylation of [CH313CO2Mg(3X-crown-X)]+ (eq 2) and subsequent reaction of the organomagnesium [CH3Mg(3Xcrown-X)]+ product ions with background water (eq 6, where HA = H2O). On the basis of this assumption, the areas of the hydrolysis and decarboxylation peaks were summed, normalized as a percentage of total ion abundance, and plotted as a function of CID energy. DFT Calculations. Theoretical calculations were carried out using Gaussian 0321 to examine the mechanisms associated with the formation and hydrolysis reactions of the organomagnesium cations [CH3Mg(3Xcrown-X)]+. Structures of reactants, transition states, and products were calculated using density functional theory (DFT) with the B3LYP functional.22 The 6-31+G* basis set was used for all atoms.23 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 0.9806.24 This level of theory was chosen (i) to allow direct comparison with our previous organomagnesium work6a,9c,e,g and10c (ii) as it is less computationally demanding than the higher levels of ab initio theory. Full data (Cartesian coordinates, energies, and imaginary frequencies for transition states) are given in the Supporting Information. Unfortunately the computational chemistry literature on Mg2+ crown ether complexes with coordinated anions is sparse, and thus there is little to guide us on the issue of preferred conformations. Previous calculations of the bare crown ether ligands have identified a range of different conformations that fall within a small band of energies. For example, conformational studies at the HF/STO-3G level of theory on the larger crown ether, 18-crown-6, revealed 29 different conformations within 0.09 eV of the ground state.25a By comparison, only three conformations

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of the smaller 12-crown-4 ligand were found within 0.09 eV of the ground state.25b While it is clear that crown ether ligands of Mg2+-crown ether complexes may also adopt a range of conformations that increase with increasing crown ether size and that the conformation of the crown ether can potentially change as the complexes undergo reactions in the gas phase (particularly those that result in a change the coordination number), a full conformational search is beyond the scope of this work. Instead we have conducted a brief search guided largely by data from X-ray crystal structures.12,26,27 “De novo” structures of [(CH3CO2)Mg(3X-crown-X)]+ for X = 4
6 were generated as follows: (1) the Mg2+ cation was placed at the center of the relevant crown ether with a high degree of symmetry and the subsequent structure optimized. (2) The acetate ligand was then added as a coordinating ligand and the structure reoptimized. (3) The geometry of the crown ether obtained at this point was used in subsequent calculations of the potential energy diagrams associated with the decarboxylation (eq 2) and the hydrolysis reactions (eq 3). Given this approach, it is possible that these “de novo” structures are not the ground-state conformations. The most significant changes in the reaction energy of a complex undergoing either decarboxylation or hydrolysis are, however, most likely influenced by the changes in the identity of the ionic groups directly coordinated to the Mg2+ center rather than due to differences in the conformation of the crown ether ligand. To provide a check, X-ray crystal structures of Mg2+ cations coordinated to crown ethers were obtained from the literature26 and modified to the complex of interest with the preservation of the geometry throughout the central Mg2+-crown ether portion of the complex. This process involved replacing axially coordinating ligands with the CH3 ligand, thereby generating the [CH3Mg(3X-crown-X)]+ complex of interest. These structures were then fully optimized. In the cases where these optimized structures were lower in energy than the de novo structures, the relative energies of [CH3CO2Mg(3X-crown-X)]+ and [HOMg(3X-crown-X)]+ along with relevant transition states were calculated using this new crown ether geometry and compared to those for the same structures generated as described earlier. In each case however, the optimized structures guided by the X-ray crystal structures were either higher or negligibly lower in energy than the de novo structures. This confirms that the overall energetics are unlikely to be dramatically influenced by changes in the crown ether conformation. Reaction Rate Theory. Decarboxylation rate constants for [CH3CO2Mg(3X-crown-X)]+ were calculated according to canonical transition-state theory, on the basis of the B3LYP/6-31+G* structures, vibrational frequencies, and energies. Calculations were performed between 300 and 2000 K, with the resultant rate constants expressed in the conventional Arrhenius form. For the bimolecular [CH3Mg(3X-crownX)]+ + H2O reactions, rate constants were determined from stochastic master equation simulations with Rice
Ramsperger
Kassel
Marcus (RRKM) theory for microcanical k(E). Densities and sums of states, as well as moments of inertia, are determined using the DFT structures and vibrations. Barrierless association of the cations with water is assumed to proceed at the collision rate and is treated using the hindered Gorin transition-state model.28 The hybrid master equation formulation is solved for the temparature and pressure conditions of the ion trap (298 K and 1.75 mTorr He) using ΔEdown = 200 cm
1, with an energy grain of 10 cm
1 up to 30 000 cm
1 (with the continuum component then solved up to 200 000 cm
1 across a further 6000 gains). Reported results represent one hundred million trials of 100 collisions. All rate constant calculations were performed using MultiWell2011.1.29

’ RESULTS AND DISCUSSION Electrospray ionization of solutions containing MgBr2, acetic acid, and a crown ether produced a range of cationic species in the positive ion mass spectrum, including the desired 4299

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Figure 1. LTQ mass spectra associated with the decarboxylation of the magnesium complexes [CH3CO2Mg(3X-crown-X)]+: (a) CID where X = 4 (m/z 259); (b) CID where X = 5 (m/z 303); (c) CID where X = 6 (m/z 347). The mass-selected precursor ion is marked with an * in each case, and peaks corresponding to the desired decarboxylation product are marked with a #. Color codes refer to proposed structures shown in Scheme 2, which are further described in the text. Panel (d) exemplifies the competition between CO2 loss and C2H4O loss in the case of [CH3CO2Mg(12-crown-4)]+ revealed by high-resolution MS experiments using the LTQ-FT MS.

[CH3CO2Mg(3X-crown-X)]+ complex “target” as well as other crown ether ions, including the protonated crown ether, [3Xcrown-X+H]+ (data not shown). [BrMg(3X-crown-X)]+ was also observed, and its abundance varied with the amount of acetic acid used. The gas-phase synthesis and hydrolysis of the desired organomagnesium cation require a sequence of two reactions: (1) mass selection followed by decarboxylation of the acetate ligand in the [CH3CO2Mg(3X-crown-X)]+ complex under lowenergy CID conditions; (2) mass selection of the organomagnesium [CH3Mg(3X-crown-X)]+ complex followed by ion
molecule reactions with water. In the next sections each of these reactions is discussed in detail. (1) Gas-Phase Synthesis of [CH3Mg(3X-crown-X)]+. LowEnergy CID Experiments. The low-energy CID mass spectra of each of the ions [CH3CO2Mg(3X-crown-X)]+ are shown in Figure 1 (a, where X = 4; b, where X = 5; c, where X = 6). CID of [CH3CO2Mg(12-crown-4)]+ (m/z 259, Figure 1a) produced a peak at m/z 215 assigned to the loss of neutral CO2 and formation of the corresponding organomagnesium species, [CH3Mg(12-crown-4)]+. Similarly, both [CH3CO2Mg(15-crown-5)]+ (m/z 303, Figure 1b) and [CH3CO2Mg(16-crown-6)]+ (m/z 347, Figure 1c) fragment via decarboxylation to produce the desired organometallic ions [CH3Mg(15-crown-5)]+ (m/z 259, Figure 1b) and [CH3Mg(18-crown-6)]+ (m/z 303, Figure 1c). The CID spectra shown in Figure 1 are the most complex of the metal carboxylate ions that we have observed to date.6,9,10 They highlight that the crown ether auxiliary ligand readily undergoes fragmentation in competition with the desired decarboxylation pathway. Indeed, high-resolution FT-ICR mass spectra show that in each system the ion assigned to the decarboxylation pathway (eq 2) was actually comprised of a combination of two distinct, isobaric ions (Figure 1d and Supplementary Figure S1). The first is due to loss of CO2, while the second arises from loss

of C2H4O. This second pathway most likely results from ring contraction of the crown ether ligand and is observed for all three crowns with varying relative intensities. Decarboxylation was by far the preferred route for [CH3CO2Mg(12-crown-4)]+ (Figure 1d); however for [CH3CO2Mg(18-crown-6)]+ fragmentation of the crown ring through loss of C2H4O was more abundant (Supplementary Figure S1b). [CH3CO2Mg(15-crown-5)]+ was intermediate between these (Supplementary Figure S1a). In order to unravel the various competing fragmentation reactions, we have carried out additional CID experiments on the isotopically labeled complexes [CH313CO2Mg(3X-crown-X)]+, and the results of these experiments are shown in Figure 2. These experiments allow the primary fragmentation reactions, which involve ring contraction in which the acetate ligand remains bound to the Mg center (eq 7), to be readily distinguished from the isobaric decarboxylation reactions (eq 8). An examination of Figure 2 also reveals that primary (eq 9) and secondary reactions (eq 10) in which acetic acid is lost are readily distinguished from other secondary reactions. ½CH3 13 CO2 MgðC2 H4 OÞn þ f ½CH3 13 CO2 MgðC2 H4 OÞn
1 þ

ð7Þ

þ C2 H4 O

f ½CH3 MgðC2 H4 OÞn þ þ

13

CO2

f ½MgðC2 H4 OÞn
Hþ þ CH3 13 CO2 H

ð8Þ ð9Þ

½MgðC2 H4 OÞn
Hþ f ½MgðC2 H4 OÞn
1 þ OHþ þ C2 H2

ð10Þ 4300

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Figure 2. LTQ mass spectra associated with the decarboxylation of the 13 C-acetate labeled magnesium complexes [CH313CO2Mg(3X-crownX)]+: (a) CID where X = 4 (m/z 260); (b) CID where X = 5 (m/z 304); (c) CID where X = 6 (m/z 348). The mass-selected precursor ion is marked with an * in each case, and peaks corresponding to the desired decarboxylation are marked with an #. Color codes refer to proposed structures shown in Scheme 2, which are further described in the text.

Collisional activation of each of the crown ether complexes [CH3CO2Mg(3X-crown-X)]+ (X = 4
6) produced many common fragment ions that are highlighted in the mass spectra shown in Figures 1 and 2. Potential structures for these ions are given in Scheme 2. Although a detailed investigation of the mechanisms of each of the fragmentation reactions is beyond the scope of this work, it is worth drawing some analogies between the fragmentation reactions observed here and those previously reported in the gas17d and condensed30 phases for other alkaline earth metal crown complexes. Loss of C2H4O (eq 7) is a common gas-phase fragmentation reaction of crown ethers that has been previously reported to occur for the magnesium hydroxide crown complexes [HOMg(3X-crown-X)]+ (where X = 4
6).17d Loss of acetic acid (eq 9) is directly related to water loss from the magnesium hydroxide crown complexes [HOMg(3X-crown-X)]+ (where X = 4
6).17d This reaction requires removal of a proton from the crown auxiliary ligand.

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Previous gas-phase17d and condensed-phase studies30 have suggested a mechanism that leads to ring-opening of the crown to form a new coordinated acylic polyether ligand consisting of an alkoxide and a vinyl ether end group (to give rise to the series of ions at m/z 287, 243, and 199, structure set (A) in Scheme 2). These ions readily undergo loss of C2H2 (eq 10), affording products that contain polyether ligands end-capped by alcohol and alkoxide groups (to give rise to the series of ions at m/z 261, 217, 173, and 129, structure set (B) in Scheme 2). In each system examined, ions from this class formed the most abundant fragments observed upon CID. Finally, addition of background water present in the ion trap gives rise to the series of magnesium hydroxides ligated by neutral acyclic ethers capped by two hydroxyl groups (m/z 191 and 147, structure set (C) in Scheme 2). The product ions formed from loss of C2H4O from [CH3CO2Mg(18-crown-6)]+ and [CH3CO2Mg(15-crown-5)]+ (eq 4) were subjected to CID. These gave identical spectra (data not shown) to those of the crown ether ions [CH3CO2Mg(15crown-5)]+ and [CH3CO2Mg(12-crown-4)]+ generated directly via ESI. This suggests that C2H4O loss involves a ring contraction reaction to produce a new complex with a smaller, intact macrocyclic crown ether ligand. In order to establish which of the crown ether complexes [CH3CO2Mg(3X-crown-X)]+ most easily undergoes decarboxylation, energy-resolved CID experiments were carried out in order to provide qualitative information of the energy thresholds for decarboxylation. Figure 3 shows the relative change in the abundance of organomagnesium product ion [CH3Mg(3Xcrown-X)]+ as a function of CID energy for all three systems (X = 4
6). The [CH3Mg(12-crown-4)]+ ion is produced in significantly greater abundance and at an earlier onset than the analogous products [CH3Mg(15-crown-5)]+ and [CH3Mg(18crown-6)]+, suggesting that the desired decarboxylation occurs most easily for this system. The CID threshold curves for the [CH3Mg(15-crown-5)]+ and [CH3Mg(18-crown-6)]+ product ions are similar, suggesting that these decarboxylation reactions have similar barriers. DFT Calculations on Decarboxylation. DFT calculations relevant to the synthesis of [CH3Mg(3X-crown-X)]+ are summarized in Figure 4. The decarboxylation reactions of magnesium crown ethers follow similar reaction pathways to those previously studied for various magnesium carboxylate complexes.6a,9e,g,10c They are endothermic overall and proceed via a four-centered transition state with partially broken Mg
O and C
C bonds and a partially formed Mg
C bond. The DFT barrier heights for decarboxylation are 2.59 eV for [CH3CO2Mg(12-crown-4)]+, 2.72 eV for [CH3CO2Mg(15-crown-5)]+, and 2.71 eV for [CH3CO2Mg(18-crown-6)]+. Thus decarboxylation is expected to occur more readily for [CH3CO2Mg(12-crown-4)]+. Transition-state theory calculations have been performed on the B3LYP energy surfaces in order to determine decarboxylation rate constants for the three magnesium crown ethers as a function of temperature in the high-pressure limit (note that this analysis assumes that in each case the postreaction complex instantaneously dissociates to the methyl magnesium crown ether cation and CO2). For [CH3CO2Mg(12-crown-4)]+ we find k [s
1] = 1.06  1013 exp(
30710/T[K]), whereas for [CH3CO2Mg(15-crown-5)]+ we obtain k [s
1] = 7.23  1012 exp(
32161/T[K]) and for [CH3CO2Mg(18-crown-6)]+ we find k [s
1] = 2.80  1013 exp(
32132/T[K]). Our results indicate an increase in activation energy of around 3 kcal mol
1 from the 12-crown-4 to 15-crown-5 and 18-crown-6 cations, 4301

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Scheme 2. Potential Structures for the Product Cations Formed in Competition with Decarboxylation: (A) Products Arising from Loss of Acetic Acid; (B) Homologous Series of Magnesium Deprotonated Polyethylene Glycol Complexes; (C) Magnesium Hydroxide Polyethylene Glycol Complexes

Figure 3. Energy-resolved CID of the isotopically labeled [CH313CO2Mg(3X-crown-X)]+ performed on the LCQ spectrometer. The thresholds for the decarboxylation channel (eq 8) are shown. Data were obtained by calculating the sum of the integrated areas for peaks due to decarboxylation [CH3Mg(3X-crown-X)]+ and their associated products arising from subsequent hydrolysis, [HOMg(3X-crown-X)]+. These were then normalized as a percentage of total ion abundance.

which is expected to largely explain the threshold CID measurements for the onset of [CH3Mg(3X-crown-X)]+ (cf. Figure 3). The product [CH3Mg(12-crown-4)]+ begins to appear at lower CID energies, consistent with [CH3CO2Mg(12-crown-4)]+ having the lowest activation energy for decarboxylation. In contrast, [CH3CO2Mg(15-crown-5)]+ and [CH3CO2Mg(18-crown-6)]+

show very similar decarboxylation profiles as the CID energy is varied, which is consistent with the nearly identical activation energies calculated from the DFT potential energy diagrams. Although a comprehensive analysis of the decomposition reactions of these magnesium acetate crown ether cations is outside the scope of the present study, it is important to note that the kinetics of competing pathways observed in the CID experiments shown in Figure 2 will impact upon the decarboxylation CID products. For example, the barrier heights for C2H4O loss from the crown ether (eq 7) are expected to decrease as the crown ether increases in size, due to reduced ring strain. Furthermore, because decarboxylation is unlikely to be the lowest-energy pathway in the dissociation of the cations (given that there are more abundant CID products), RRKM effects will also decrease the reaction rate, where reactions with lower reaction thresholds will deplete states that would have otherwise contributed to decarboxylation. This effect will be compounded in the larger structures—where decarboxylation is found to be less preferred—due to randomization of internal energy among a significantly increased number of internal degrees of freedom (see the Supporting Information Figure S2 for a comparison of the density of states for the magnesium acetate crown ether cations). 4302

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Figure 4. DFT (B3LYP/6-31+G(d))-calculated potential energy diagram associated with the decarboxylation of [CH3CO2Mg(3X-crown-X)]+. Energies for the decarboxylation of (a) X = 4, (b) X = 5, and (c) X = 6. Data labels refer to structures and energies presented in panel (d), which shows the structures and relative energies of key reactants, intermediates, and products. In each case the crown ether ring hydrogen atoms have been omitted for clarity. Electronic energies and Cartesian coordinates for all structures are provided in the Supporting Information.

DFT-Calculated Structures of the Organomagnesium Cations [CH3Mg(3X-crown-X)]+. At this point it is worth discussing the coordination modes of the crown ether ligands to the magnesium center in the organomagnesium complexes and comparing these to previous X-ray structures.16c,26 For the smallest organomagnesium complex optimized via DFT calculations, [CH3Mg(12-crown-4)]+ (1c of Figure 4d), the Mg atom is coordinated by all four crown ether oxygen atoms and lies approximately 1.1 Å above the plane of the crown ether ring with the Mg
Me bond roughly perpendicular to the plane formed by the four O atoms. The maximum and minimum Mg
O bond lengths are 2.23 and 2.11 Å, respectively, with an average of 2.17 Å, which is slightly shorter than that reported for the Mg(12-crown-4)22+ complex.26a DFT calculations resulted in a similar structure for the next crown ether complex, [CH3Mg(15-crown-5)]+ (2c of Figure 4d). In this case, however, the Mg atom lies closer to the plane of the crown ring (approximately 0.7 Å above the O atom plane). The metal is coordinated by all five crown ether oxygen atoms with an average Mg
O bond length of 2.29 Å and maxima and minima of 2.32 and 2.23 Å, respectively. In 2008 Jaenschke and co-workers reported the structure of the crown ether complex [CH3Mg(15-crown-5)]+ as an ion pair with the noncoordinating cyclopentadienyl anion counterion.16c This structure shows the crown ring to be slightly distorted from the symmetrical, planar conformation and has an average Mg
O bond length of 2.22 Å.

However when the structure was subjected to a DFT calculation, the resultant optimized structure proved higher in energy than our de novo generated structure. Guino-o and co-workers reported the structures of Mg(15-crown-5)2+ crystallized with two 4-tertbutylphenylacetylide ligands.26c Here the 15-crown-5 ring adopted a planar geometry with an average Mg
O bond length of 2.23 Å, which is, again, shorter than those in our structure. When this structure was modified to the desired [CH3Mg(15crown-5)]+ complex and subjected to a DFT calculation, the resultant complex was once again higher in energy than our de novo structure. Our de novo structure for [CH3Mg(18-crown-6)]+ shows the 18-crown-6 ligand acting as a tetradentate ligand (3c of Figure 4d) with an average bonding Mg
O length of 2.27 Å with maxima and minima of 2.37 and 2.21 Å, respectively. Two oxygen atoms were noncoordinating, sitting 3.33 and 4.16 Å away from the central Mg2+. Crystal structures have been reported for 18-crown-6 complexed to Mg2+ and many other metals showing denticities ranging from 3 to 6.26d
f Thus [CH3Mg(18-crown-6)]+ was optimized using the ligand geometry and metal position of the complexes in which the crown ether ligand binds in a tetradentate fashion ([AlCl2(18-crown-6)]+26e), pentadentate fashion ([Li(OH2)(18-crown-6)]+26f), and hexadentate fashion ([Et2Mg(18-crown-6)]+ 26d). The structure derived from [Li(OH2)(18-crown-6)]+ showed a slightly lower energy than our de novo structure (by 0.14 eV). [CH3CO2Mg(18-crown-6)]+ 4303

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Figure 5. LCQ experiments comparing the rates of reaction of the organomagnesium cations [CH3Mg(3X-crown-X)]+ with water. [CH3Mg(3Xcrown-X)]+ were formed via CID of the isotopically labeled precursor ions [CH313CO2Mg(3X-crown-X)]+ (Figure 2). Reactant ions are marked with an *, and reaction times are as stated in each case: (a) X = 4, (b) X = 5, and (c) X = 6. Panel (d) shows a graph of ln(relative ion count) as a function of reaction time. Observed reactivity order is (most reactive) X = 4 > X = 6 > X = 5 (least reactive).

and [HOMg(18-crown-6)]+ calculated using this crown geometry differed negligibly in energy when compared to our structure despite the Mg2+ being coordinated by an extra oxygen donor. (2) Hydrolysis Reactions of [CH3Mg(3X-crown-X)]+. Experimental Kinetic Measurements. The hydrolysis reactions of [CH3Mg(3X-crown-X)]+ proceed via addition/elimination reactions (cf. eq 6) to give magnesium hydroxides (eq 11) in a manner similar to those reported previously.10c An examination of Figure 5a
c reveals a qualitative reactivity order of (most reactive) [CH3Mg(12-crown-4)]+ > [CH3Mg(18-crown-6)]+ > [CH3Mg(15-crown-5)]+ (least reactive). These relative reactivity orders were confirmed by measuring the rates with background water (data not shown) using both the LTQ (comparative rates were measured on four separate days) and LCQ mass spectrometers (absolute rates were measured on three separate days). ½CH3 Mgð3X-crown-XÞþ

þ H2 O f ½HOMgð3X-crown-XÞþ þ CH4

ð11Þ

The absolute rate constants and reaction efficiencies for these hydrolysis reactions were determined from three independent measurements (carried out on different days) of the rate of hydrolysis of the relevant ions measured in the LCQ spectrometer via the direct infusion of known amounts of water. The amounts of adventitious background water present in the ion trap were measured on the day of the experiment and taken into account.9g The results for these studies are presented in Figure 5 and Table 1. The observed hydrolysis reaction rates of methyl magnesium crown ether complexes follow the order (most reactive) [CH3Mg(12-crown-4)]+ > [CH3Mg(18-crown-6)]+ > [CH3Mg(15crown-5)]+ (least reactive). The measured reaction efficiencies for the hydrolysis reactions were 115% for [CH3Mg(12-crown-4)]+, 0.2% for [CH3Mg(15-crown-5)]+, and 0.7% for [CH3Mg(18crown-6)]+ (Table 1). This indicates that for the X = 4 system, hydrolysis reactions with water are limited only by the rate of collision between reacting species. In contrast, the hydrolysis reactions of [CH3Mg(18-crown-6)]+ and [CH3Mg(15-crown-5)]+

are significantly slower, with less than one collision in one hundred resulting in a productive reaction. DFT Calculated Potential Energy Diagrams Associated with Hydrolysis of [CH3Mg(3X-crown-X)]+. The potential energy diagrams for the hydrolysis of the methyl magnesium crown ether ions [CH3Mg(3X-crown-X)]+ are shown in Figure 6, and all are both exothermic overall and proceed via a typical doublewell surface.33 Each hydrolysis reaction involves the initial formation of a prereaction complex between water and the methyl magnesium crown ether cation. The activated reaction adduct then passes over a four-centered transition state that involves breaking the Mg
C and O
H bonds and forming of the Mg
O and C
H bonds. The product complex between [HOMg(3X-crown-X)]+ and CH4 is sparingly stable and will quantitatively dissociate to yield the isolated products. The barrier height for hydrolysis of each cation, relative to the prereaction complex, increases from 0.16 eV to 0.41 eV to 0.60 eV as the crown ether is enlarged from 12-crown-4 through 15-crown-5 to 18-crown-6. This ordering of barrier heights is consistent with [CH3Mg(12-crown-4)]+ being the most reactive species, but does not explain the greater reactivity of the 18crown-6 compound vs 15-crown-5. The increasing barrier heights are attributed to increasing crown ether solvation of the magnesium center, which decreases its reactivity. Another important difference between the varied hydrolysis reactions is illustrated in Figure 6; we observe that the initial complex with water is much more stable for the large [CH3Mg(18-crown-6)]+ cation. It appears that the larger ether structure can sterically accommodate both Mg and H2O, and this reduces the overall barrier height to below the reactant energies, making it similar to that for the most-reactive [CH3Mg(12-crown-4)]+ system. The effect of this significantly increased well-depth is explored further below, via RRKM theory rate constant calculations. Theoretical Hydrolysis Kinetics. From the potential energy diagrams generated above we have calculated purely theoretical rate constants for hydrolysis of the methyl magnesium crown ether cations. For [CH3Mg(12-crown-4)]+ the theoretical rate constant is 1.14  10
9 cm3 molecule
1 s
1, corresponding to a reaction efficiency of 65.5%. This value agrees with the 4304

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Table 1. Reaction Rate Constants (k)a and Reaction Efficiencies (j)b for the Hydrolysis of [CH3Mg(3X-crown-X)]+, from Experiment, RRKM Theory, and Collision Theoryc experimental k

ion

1.99  10
9

[CH3Mg(12-crown-4)]+ +

[CH3Mg(15-crown-5)]

+

[CH3Mg(18-crown-6)] a c


1
1

3.97  10


12

1.49  10


11

theoretical j

k

115%

1.14  10
9

j
16

1.72  10

0.20%


10

2.17  10

0.70%

collision rate kADO

65.5%

1.74  10
9

0.00001%

1.72  10
9

9.5%

2.27  10
9

Units of cm molecule s . As in our previous study, errors are conservatively estimated as (25%. Reaction efficiency (j) = k/kADO  100. Calculated using the theory of Chesnavich et al.31 The calculation was performed using the program COLRATE.32 3

b

6a

Figure 6. DFT (B3LYP/6-31+G(d))-calculated potential energy diagrams associated with the hydrolysis of [CH3Mg(3X-crown-X)]+. Energies for the hydrolysis of (a) X = 4, (b) X = 5, and (c) X = 6. Data labels refer to structures and energies presented in panel (d), which shows structures and relative energies of key reactants, intermediates, and products. In each case the crown ether ring hydrogen atoms have been omitted for clarity. Electronic energies and Cartesian coordinates for all structures are provided in the Supporting Information.

experimental rate constant to within around a factor of 2. The experimental and theoretical rate constants can be brought into agreement by reducing the barrier height by 0.13 eV (∼98% efficiency). With the [CH3Mg(15-crown-5)]+ crown ether, the RRKM calculations predict a very low reaction efficiency of 0.00001% (k = 1.72  10
16 cm3 molecule
1 s
1) due to the positive overall reaction barrier. Again, the theoretical rate constant is smaller than the experimental value. Agreement between the two can be achieved, however, if the barrier is in this case decreased by around 0.2 eV (∼0.2% efficiency). For [CH3Mg(18-crown-6)]+ + H2O the calculated rate constant is 2.17  10
10 cm3 molecule
1 s
1, with a reaction efficiency of 9.5%. This result demonstrates the large reduction in reaction rate (close to an order of magnitude) achieved by increasing the well-depth

for the initial complex with water, all else being equal. In this final instance, increasing the barrier height by only 0.7 eV provides good agreement between theory and experiment (∼1% efficiency). For all three systems, the adjustments required in the calculated barrier heights are within the generally accepted error of the B3LYP functional and demonstrate that the experimentally determined hydrolysis rates can be explained by (i) decreasing Mg reactivity with increasing solvation and (ii) increased Mg
H2O complex strength with increasing size of the crown ether ligand.

’ CONCLUSIONS Here we have successfully formed the organomagnesium crown ether cations [CH3Mg(3X-crown-X)]+ for X = 4
6 for 4305

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Organometallics the first time in the gas phase via decarboxylation of [CH3CO2Mg(3X-crown-X)]+. Decarboxylation is in competition with a range of fragmentation reactions involving the crown ether ring. DFT calculations show that all three crown ether complexes undergo decarboxylation via a four-centered transition state, with decarboxylation of [CH3Mg(12-crown-4)]+ being the most facile, consistent with energy-resolved CID experiments. This contrasts with our previous study on the decarboxylation of the acetate ligand in the betaine magnesium complexes [CH3CO2MgO2CCH2X(CH3)2]+ (where X = NCH3 and S), where we found that the activation energy for decarboxylation of the acetate ligand is uninfluenced by the neutral betaine ligand.10c Thus the crown ether ligand plays a role in tuning the energetics associated with these decarboxylation reactions. The crown ether ligand also plays a key role in the hydrolysis reactions of the organomagnesium cations, which follow the reactivity order (most reactive) [CH3Mg(12-crown-4)]+ > [CH3Mg(18-crown-6)]+ > [CH3Mg(15-crown-5)]+ (least reactive). DFT calculations support the experimental observations and highlight that (i) these hydrolysis reactions proceed via the same addition/elimination pathway found for a range of other organomagnesium ions; (ii) the crown ether ligand plays a key role in the reactivity of these complexes by tuning both the solvation of the CH3Mg+ core and the strength of the Mg
H2O bond in the precomplex. [CH3Mg(12-crown-4)]+ is the most reactive of all the organomagnesium anions RMgL2
and cations [RMgL]+ whose hydrolysis reactions have been studied to date. Since most of these studies have focused on R = CH3, work is underway to develop an intrinsic basicity scale for a wide range of R groups by examining the hydrolysis of the organomagnesium anions RMgCl2
.

’ ASSOCIATED CONTENT

bS

Supporting Information. Complete citation for ref 21 along with high-resolution mass spectra, density of states for the magnesium acetate crown ether cations, and Cartesian coordinates and energies (Hartrees) for species relevant to each of the reaction pathways described in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: +613 9347-5180. Tel: +61 3 8344-2452. E-mail: rohair@ unimelb.edu.au.

’ ACKNOWLEDGMENT We thank the ARC for financial support via grants DP110103844 (R.A.J.O. and G.N.K.) and DP1096134 (G.N.K.). The Victorian Partnership for Advanced Computing (VPAC) is acknowledged for access to the High Performance Computing Facility. ’ REFERENCES (1) Pesci, L. Atti Accad. Nazionale Lincei 1901, 10, 362–363. For an English abstract, see: J. Chem. Soc., Abstr. 1901, 80 (I), 576. (2) For older reviews on the use of decarboxylation reactions to produce organometallics in the condensed phase see: (a) Deacon, G. B. Organomet. Chem. Rev. A 1970, 5, 355. (b) Deacon, G. B.; Faulks, S. J.; Pain, G. N. Adv. Organomet. Chem. 1986, 25, 237.

ARTICLE

(3) For reviews see: (a) Goossen, L. J.; Goossen, K.; Rodriguez, N.; Blanchot, M.; Linder, C.; Zimmermann, B. Pure Appl. Chem. 2008, 80, 1725. (b) Goossen, Rodriguez, N.; Goossen, K. Angew. Chem., Int. Ed. 2008, 47, 3100. (4) Nilsson, M. Acta Chem. Scand. 1966, 20, 423–426. (5) (a) Yin, L.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2009, 131, 9610–9611. (b) Lu, P. F.; Sanchez, C.; Cornella, J.; Larrosa, I. Org. Lett. 2009, 11, 5710–5713. (c) Goossen, L. J.; Linder, C.; Rodriguez, N.; Lange, P. P.; Fromm, A. Chem. Commun. 2009, 46, 7173–7175. (d) Shang, R.; Fu, Y.; Wang, Y.; Xu, Q.; Yu, H. Z.; Liu, L. Angew. Chem., Int. Ed. 2009, 48, 9350–9354. (6) (a) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (b) Khairallah, G. N.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 254, 145. (7) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (8) 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, 2010; Chapter 6, pp 199
227; ISBN: 978-3-527-32351-7. (9) (a) O’Hair, R. A. J. Chem. Commun. 2002, 20. (b) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761. (c) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255
256, 45. (d) Rijs, N. J.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069. (e) Thum, C. C. L.; Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118. (f) Rijs, N. J.; O’Hair, R. A. J. Organometallics 2009, 29, 2684. (g) Khairallah, G. N.; Thum, C.; O’Hair, R. A. J. Organometallics 2009, 28, 5002. (h) Rijs, N. J.; Yates, B. F.; O’Hair, R. A. J. Chem.—Eur. J. 2010, 16, 2674. (i) Rijs, N. J.; O’Hair, R. A. J. Organometallics 2010, 29, 2282. (j) Rijs, N. J.; Sanvido, G. B.; Khairallah, G. N.; O’Hair, R. A. J. Dalton Trans. 2010, 39, 8655. (10) (a) O’Hair, R. A. J.; Waters, T.; Cao, B. Angew. Chem., Int. Ed. 2007, 46, 7048. (b) Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. 2009, 2832. (c) Khairallah, G. N.; Yoo, J. H.; O’Hair, R. A. J. Organometallics 2010, 29, 1238. (11) For reviews of organocuprate reactivity from a theoretical, mechanistic perspective see: (a) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3751–3771. (b) Mori, S.; Nakamura, E. In Modern Organocopper Chemistry; Wiley-VCH: Weinheim, Germany, 2002; Chapter 10, pp 315
346. (c) Nakamura, E.; Yoshikai, N. The Chemistry of Organocopper Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, UK, 2009; Chapter 1, pp 1
21. (12) The formulation RMgL is a gross simplification of Grignard reagents and does not take into account the range of equilibria and the role of solvent molecules coordinated to magnesium. For an excellent review that brings together a wide range of spectrospcopic, physical, and theoretical data to explain the structures of Grignard reagents see: Ertel, T. S.; Bertagnolli, H. In Grignard Reagents: New Developments; Richey, H. G., Jr., Ed.; Wiley: Chichester, 2000; Chapter 10. (13) These charged Grignard complexes have been implicated in the conductivity of Grignard reagents arising from the equilibria shown in eqs 3
5 as well as the enhanced reactivity caused by salt effects: Loupy, A.; Tchoubar, B. Salt Effects in Organic and Organometallic Chemistry; VCH: Weinheim, 1992; Chapter 7, pp 250
253. (14) For a review on the role of Grignard reagents as bases see: Kosar, W. In Handbook of Grignard Reagents; Silverman, G. S., Rakita, P. E., Eds.; Dekker: New York, 1996; Chapter 23, pp 441
453. (15) The organomagnesium cation cis-[CH3Mg(thf)(dme)2]+, which has a CH3Mg+ core solvated by two dimethoxy ethane (dme) and one tetrahydrofuran (thf) ether molecules, has been synthesized in the condensed phase, and its crystal structure has been determined:  Vestergren, M.; Eriksson, J.; Hakansson, M. Chem.—Eur. J. 2003, 9, 4678. (16) (a) Tang, H.; Richey, H. G. Organometallics 1996, 15, 4891. (b) Tang, H.; Parvez, M.; Richey, H. G. Organometallics 2000, 19, 4810. (c) Jaenschke, A.; Paap, J.; Behrens, U. Z. Anorg. Allg. Chem. 2008, 634, 461. 4306

dx.doi.org/10.1021/om200359w |Organometallics 2011, 30, 4297–4307

Organometallics

ARTICLE

(17) For reviews on the gas-phase chemistry of metal crown complexes see: (a) Brodbelt, J. S.; Liou, C.-C. Pure Appl. Chem. 1993, 65, 409. (b) Armentrout, P. B. Int. J. Mass Spectrom. 1999, 193, 227. (c) Schalley, C. A. Mass Spectrom. Rev. 2001, 20, 253. For a paper on the formation and fragmentation reactions of [HOMg(12-crown-4)]+ see: (d) Wu, H.-F.; Brodbelt, J. S. J. Am. Chem. Soc. 1994, 116, 6418. (18) (a) Dishong, D. M.; Gokel, G. W. J. Org. Chem. 1982, 47, 147. (b) Kuz’mina, L. A.; Sharnin, V. A.; Golikov, A. N. Russ. J. Gen. Chem. 2009, 79, 2591. (19) (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. (20) Colorado, A.; Brodbelt, J. J. Am. Soc. Mass Spectrom. 1996, 7, 1116. (21) Frisch, M. J.; et al. Gaussian_03; Gaussian, Inc.: Pittsburgh, PA, 2003. (22) (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. (23) (a) Patterson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (b) Clark, T.; Chandrasekhar, J.; Spitzmagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (24) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (25) (a) Al-Jallal, N. A.; Al-Kahtani, A. A.; El-Azhary, A. A. J. Phys. Chem. A 2005, 109, 3694. (b) El-Azhary, A. A.; Al-Kahtanl, A. A. J. Phys. Chem. A 2004, 108, 9601. (26) (a) Wesemann, L.; Trinkaus, M.; Englert, U.; Muller, J. Organometallics 1999, 18, 4654. (b) Siegler, M. A.; Hao, X.; Parkin, S.; Brock, C. P. Acta Crystallogr. 2008, B64, 738. (c) Guino-o, M. A.; Baker, E.; Ruhlandt-Senge, K. J. Coord. Chem. 2008, 61, 125. (d) Pajerski, A. D.; BergStresser, G. L.; Parvez, M.; Richey, H. G., Jr. J. Am. Chem. Soc. 1988, 110, 4844. (e) Atwood, J. L.; Elgamal, H.; Robinson, G. H.; Bott, S. G.; Weeks, J. A.; Hunter, W. E. J. Incl. Phenom. 1984, 2, 367. (f) Dimos E., Wilde B., Olbrich F. Private communication with the Cambridge Crystal Database, deposition number: CCDC 276102. (27) For inorganic magnesium crown structures determined via X-ray crystallography see: (a) Wei, Y. Y.; Tinant, B.; Declercq, J.-P.; Van Meerssche, M.; Dale, J. Acta Crystallogr. 1988, C44, 73–77. (b) Wei, Y. Y.; Tinant, B.; Declercq., J.-P.; Van Meerssche, M.; Dale, J. Acta Crystallogr. 1988, C44, 77. (c) Owen, J. D. J. Chem. Soc., Dalton Trans. 1978, 1418–1423. For a general review on the coordination of crown ethers to alkali metal cations see: (d) Steed, J. W. Coord. Chem. Rev. 2001, 215, 171. (28) Smith, G. P.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 489. (29) (a) Barker, J. R. with contributors N. F. Ortiz, J. M. Preses, L. L. Lohr, A. Maranzana, P. J. Stimac, T. L. Nguyen, and T. J. D. Kumar; MultiWell-2011.1 Software; University of Michigan: Ann Arbor, MI, 2011; http://aoss.engin.umich.edu/multiwell/. (b) Barker, J. R. Int. J. Chem. Kinet. 2001, 33, 232. (c) Barker, J. R. Int. J. Chem. Kinet. 2009, 41, 748. (d) Nguyen, T. L.; Barker, J. R. J. Phys. Chem. A 2010, 114, 3718. (30) (a) Bradley, D. C.; Hursthouse, M. B.; Ibrahim, A. A.; Abdul Malik, K. M.; Motevalli, M.; Moseler, R.; Powell, H.; Runnacles, J. D.; Sullivan, A. C. Polyhedron 1990, 9, 2959. (b) Alexander, J. S.; RuhlandtSenge, K. Angew. Chem., Int. Ed. 2001, 40, 2658. (c) Alexander, J. S.; Karin Ruhlandt-Senge, K.; Hope, H. Organometallics 2003, 22, 4933. (31) Chesnavich, W. J.; Su, T.; Bowers, M. T. J. Chem. Phys. 1980, 72, 2641. (32) Lim, K. F. Quantum Chemistry Program Exchange 1994, 14, 3. (33) Brauman, J. L. J. Mass Spectrom. 1995, 30, 1649.

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