Solution Stability of Organocalcium Compounds in Ethereal Media

Article pubs.acs.org/Organometallics

Solution Stability of Organocalcium Compounds in Ethereal Media Mathias Köhler, Jens Langer, Helmar Görls, and Matthias Westerhausen* Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, D-07743 Jena, Germany S Supporting Information *

ABSTRACT: Organocalcium compounds (post-Grignard reagents) of the type [Ca(R)(X)(L)n] are very reactive and able to degrade ethers with α- and β-deprotonation as possible first reaction step. In this systematic study, the durability of phenyl-, α-naphthyl-, and 1,2-dihydronaphth-4-ylcalcium derivatives in cyclic ethers such as tetrahydrofuran (THF), tetrahydropyran (THP), and α-methyltetrahydrofuran (Me-THF) is investigated. The temperature, solvent, and the nature of the second anionic ligand X [iodide, bis(trimethylsilyl)amide, α-naphthyl] significantly influence the durability of their ethereal solutions.

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even though calcium itself is also able to cleave ethers.14 With the development of a refined methodology for the synthesis of arylcalcium compounds, first reports on the stability of welldefined derivatives in ethereal media occurred.15−20 However, the influence of diverse factors on the lifetimes of organocalcium compounds was not studied systematically, and the varying conditions applied make a comparison of different derivatives challenging. Therefore, herein an attempt is made to identify the major factors influencing the stability of arylcalcium compounds in ethereal solvents under inclusion of previous results, for instance on the stability of tolylcalcium iodide, 20 αnaphthylcalcium iodide,19 1,2-dihydronaphth-4-ylcalcium iodide,19 di(α-naphthyl)calcium,18 and the phenylcalcium cation.17 Especially the durability in cyclic ethers such as THF is of importance because these solvents were commonly used for the preparation of the majority of the organocalcium complexes.21 In Figure 1, the general structure of aryl (and alkenylcalcium) derivatives is depicted and the parameters that likely influence its stability are listed. For quantification of the durability of post-Grignard solutions we have chosen the time t50 until 50% of the initially organocalcium compound is degraded. For this purpose and to minimize side effects from preparative procedures (such as from remaining metal particles) these post-Grignard reagents were purified by recrystallization from the appropriate solvent prior to our studies. The starting point of our investigations was the root compound [Ca(Ph)(I)(thf)4].

INTRODUCTION Refined procedures for the preparation of organocalcium compounds such as allyl,1 benzyl,2 aryl,3 and methanide calcium complexes2f,4 were developed in the last years. These advances have already led to a growing importance of organocalcium chemistry and make these derivatives generally accessible for use in organic syntheses, catalysis, or metalation reactions.5 In this context, the elaboration of a reliable protocol for the efficient synthesis of arylcalcium halides in analogy with the well-known synthesis of Grignard reagents was crucial to keep up with the commonly available and already widely used organomagnesium (Grignard reagents) and lithium compounds. Another prerequisite for a broader application remains the knowledge about the factors that influence the long-term stability of these compounds in various organic solvents. It is obvious that the high polarity of the calcium carbon bond and the resulting high reactivity of calcium-based organometallics significantly limit their durability in ethereal media, as observed for related lithium (and magnesium) compounds. Here, first studies on ether cleavage reactions date back more than 60 years,6−8 and more intense investigations followed as the interest in these organometallics increased and widespread applications were developed. Depending on the nature of the utilized organolithium or organomagnesium derivatives, steric shielding, nucleophilicity, coligands, reaction temperature, and solvent were identified as major factors influencing the durability of these compounds.9,10 Substitution of magnesium by the heavier homologous alkaline earth metal significantly enhances the reactivity of the post-Grignard reagents.11 Therefore, ether degradation reactions were already observed during early attempts to prepare organocalcium compounds.12,13 However, these early investigations were not based on purified organocalcium compounds but on solutions prepared from calcium and organyl halides © 2014 American Chemical Society

Received: July 3, 2014 Published: November 6, 2014 6381

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Figure 1. General structure of aryl and alkenylcalcium derivatives, with respect to the influencing factors of ether degradation.

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RESULTS AND DISCUSSION The experiments that built the basis of this investigation are summarized in Table 1. The tetrakis(thf) adduct of phenylcalcium iodide was synthesized according to a standard procedure reported earlier.22 Recrystallized material was used for the degradation experiments in THF, which were monitored by 1H NMR spectroscopy. As an internal standard in this and most of the following experiments, a definite amount of cyclohexane was added (in a few cases other substances were used as internal standard; see Table 1), and integration of the well-separated resonances of the o-CH groups of phenylcalcium iodide at δ 7.69 (see Figure 2) in relation to the standard was used to quantify the loss of the concentration of this organocalcium complex. During measurements, the sample was stored in a temperature-conditioned room at 22 ± 1 °C. The initial experiment was performed with a 0.13 M solution of [Ca(Ph)(I)(thf)4] in a mixture of THF and benzene-d6 in the ratio of 2:1 (Table 1, entry 1). Expectedly, the resonance of benzene at δ 7.16 increased during the experiment, whereas the intensity of the other aryl signals in the aromatic region decreased. Half of the initial phenylcalcium iodide was consumed after t50 = 8 ± 1 days (see Table 1, entry 1). This

Figure 2. Time-dependent 1H NMR spectra (aromatic region), measured at 400 MHz, of a 0.13 M solution of [Ca(Ph)(I)(thf)4] in THF/benzene-d6 (2:1) at 22 °C (* = benzene).

value, which will serve as a benchmark for further experiments, is in good agreement with the one reported for the closely related derivative [Ca(Tol)(I)(thf)4] under comparable conditions (8 days, see Table 1, entry 12).20 In contrast, the lithium derivative [(thp)2Li2(μ-Tol)(μ-Br)] decomposes twice as fast.20 The simultaneous formation of ethene (δH = 5.4) during the degradation of [Ca(Ph)(I)(thf)4] suggests that α-deprotonation and subsequent [3+2] cycloreversion of THF is the predominantly occurring degradation mechanism under the applied conditions, in agreement with earlier reports; see Scheme 1.8,20 Repetition of this experiment, using THF-d8 as ligand and solvent, resulted in a drastic decrease of the t50 value and 69% of the initial phenylcalcium iodide was still present after 35 days (Table 1, entry 2), while this degree of decomposition was already reached after 3.9 days in case of nondeuterated THF. For the related strontium derivative [Sr(Ph)(I)(thf)5] a t50 of

Table 1. NMR Experiments for the Investigation of Ether Degradation of Compounds of the Type [Ca(R)(X)(L)n] and Related Compounds entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

compounda [Ca(Ph)(I)(thf)4] [Ca(Ph)(I)(thf-d8)4] [Ca(Ph)(I)(thf)4] [Ca(Ph)(I)(thf)4] [Ca(Ph)(Hmds)(thf)3] [Ca(Naph)(I)(thf)4] [Ca(Naph)(Hmds)(thf)3] [Ca(Naph)2(thf)4] [Ca(1,2-Dihydronaph)(I)(thf)4] [Ca(Ph)(I)(thp)4] [Ca(Tol)(I)(thp)4] [Ca(Tol)(I)(thf)4] [Ca(Naph)(I)(Me-thf)4] [Ca(Ph)(dme)3]I [Sr(Ph)(I)(thf)5] [Ca(Hmds)2(thf)2]

solvents (2:1) THF/benzene-d6b THF-d8/benzene-d6b THF/benzene-d6b THF/benzene-d6b THF/benzene-d6b THF/benzene-d6b THF/benzene-d6b THF/benzene-d6b THF/benzene-d6b THP/benzene-d6c THP/benzene-d6d THF/benzene-d6d Me-THF/benzene-d6b DME/benzene-d6e THF-d8 THF/benzene-d6c

T (°C)

c (mol/L)

t50 (d)f

22 22 50 −7 22 22 22 22 22 22 22 22 22 22 22 22

0.13 0.08 0.13 0.14 0.12i 0.09 0.12 0.10 0.13 0.07 0.09 0.09 0.09 0.05 n.a. 0.12

8 ≫35g 0.2 ≫118h 2.5i 42 5 2.0 2.5 23 8 8 0.6 0.3 4 n.a.j

ref

19 18 19 20 20 17 23

Ph, phenyl; Tol, 4-methylphenyl; Naph, α-naphthyl; 1,2-Dihydronaph, 1,2-dihydronaphth-4-yl; Hmds, bis(trimethylsilyl)amido; thf, tetrahydrofuran; thp, tetrahydropyran; Me-thf, α-methyltetrahydrofuran; dme, dimethoxyethane. bInternal standard: cyclohexane. cInternal standard: p-xylene. dInternal standard: benzene. eInternal standard: mesitylene, ratio of the solvents 3:1. fDuration until 50% of the primary organocalcium species is degraded. g69% of [Ca(Ph)(I)(thf-d8)4] is still present after 35 days. hNot available because these organocalcium compounds are rather stable in THF; 78% of [Ca(Ph)(I)(thf)4] is still present after 118 days. iTwo organocalcium species were observed at the beginning of the experiment; the given duration refers to the total concentration of both species. jNo detectable degradation after 14 days. a

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Besides the above-mentioned hydrolysis products and impurities, the products of the solvent degradation themselves also led to deviation from the often-expected pseudo-first-order reaction rates, most likely by a gradual change of the reaction mechanism. As a result, the decomposition is slower than expected during later stages of these reactions. While the influence of the concentration on the decay of phenylcalcium iodide in THF remains uncertain, temperature changes had a pronounced effect. The reaction was studied at 50 °C (Table 1, entry 3), 22 °C (Table 1. entry 1), and −7 °C (Table 1, entry 4). As expected at 50 °C the degradation is accelerated by a factor of approximately 40 in comparison to the process at 22 °C. Cooling of the solution to −7 °C significantly elongates the lifetime of arylcalcium iodide in this ether, and after 118 days 78% of these primary organocalcium species are still detectable (see Figure 3). In the last case, a significant part of the degradation probably occurred during the transfer from the freezer, where the sample was stored, to the NMR spectrometer.

Scheme 1. Major Cleavage Reaction Pathways of THF and Me-THF in the Presence of Strongly Basic s-Block Organometallics

4 days in THF-d8 was determined (Table 1, entry 15), which suggests a higher reactivity of the strontium derivative.23 However, the use of nondeuterated THF as ligand and perdeuterated THF as solvent will result in formation of [Sr(Ph)(I)(thf-d8)5−x(thf)x] in solution. Consequently, the comparability of these experiments is limited. However, the observation of such a pronounced primary isotopic effect24 in the case of the calcium derivative suggests that proton abstraction from THF is the rate-determining step in the degradation of [Ca(Ph)(I)(thf)4]. Additionally, the slow but constant formation of a secondary phenylcalcium species was observed during the degradation experiment with THF-d8. This compound, which shows a signal for the o-CH protons at δ 8.60, indicative of a bridging phenyl group,18,25,26 eventually became the dominating phenylcalcium species (>100 days). Although similar species were occasionally observed as minor impurities in the decomposition experiments of phenylcalcium iodide in THF, too, their overall concentration remained low and almost unchanged during those experiments. As it is known from organolithium chemistry, the half-life times of these compounds in ethereal solvents can depend on the concentration of the organometallic compound, and stabilization at low concentrations was observed in certain cases.27 Given the similarities between aryllithium and arylcalcium derivatives (NMR shifts of the ipso C in the 13C NMR spectrum, polarity of the M−C bond as a result of similar electronegativities of the metals) as well as of the occurring degradation mechanisms, the initial concentration might also influence the stability in the case of phenylcalcium iodide. During initial experiments performed with 0.13 and 0.03 M solutions of [Ca(Ph)(I)(thf)4] no significant differences in t50 values were found. Additional attempts over a wider concentration range were hampered by the limited solubility of [Ca(Ph)(I)(thf)4] (higher concentrations) under the applied conditions and problems arising from trace amounts of water (e.g., absorbed to the wall of the NMR tube) present during sample preparation at much lower concentrations. It was found that formed hydrolysis products (as well as impurities present in the arylcalcium compounds) influence the t50 values and led to slower decomposition. Due to the mentioned difficulties, no explicit influence of the starting concentration on the stability and the t50 value could be verified.

Figure 3. Decay of the primary organocalcium species of [Ca(Ph)(I)(thf)4] solutions in THF/benzene-d6 (2:1) at different temperatures.

The observed durability of phenylcalcium iodide in THF at the lower temperature underlines that storage of these organometallics is possible over a long period of time without extensive degradation. Therefore, storage of such solutions at −20 °C or below is recommended. Since the temperature has such an enormous influence on the t50 values, the samples used in all following experiments were stored and measured at 22 °C to ensure the comparability. The second anionic ligand besides the aryl group also seemed to affect the stability of arylcalcium derivatives significantly. However, contradictory reports in the literature initiated a reinvestigation.16,28 On one hand, a qualitative stabilization was reported as the result of an exchange of iodide in [Ca(Ph)(I)(thf)4] by the strongly basic bis(trimethylsilyl)amido anion (Hmds), yielding [Ca(Ph)(Hmds)(thf)3] with a pentacoordinate calcium center.28 On the other hand, substitution of the iodide by an Hmds anion in closely related naphthylcalcium iodide gave the opposite effect.16 However, repetition of the experiment with [Ca(Ph)(Hmds)(thf)3] (entry 5 of Table 1) showed an increased decomposition rate relative to [Ca(Ph)(I)(thf)4], in contrast to the earlier finding. 6383

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Another peculiarity observed during the degradation experiments of [Ca(Ph)(Hmds)(thf)3] was the initial observation of a second phenylcalcium species in solution, although the crystalline product used for this experiment was prepared according to the published procedure.28 Most noticeable, a second resonance of the ortho phenyl protons at 8.09 ppm, a typical value for protons of bridging phenyl groups,18,25,26 besides the expected signal at 7.68 ppm was detected. This second signal set disappears within the first 5 h of the experiment nearly completely, whereas the integral of the signal at 7.68 ppm increases drastically (Figures 4 and 5). The

Obviously, the incorporation of a second highly basic anionic ligand led to a dramatic reduction of the stability of the complex under the applied conditions. Formation of the various calcium bis(trimethylsilyl)amido species as well as the delayed and substoichiometric formation of benzene (in late stages of the reaction up to 60% of the expected amount; see Figure 6) clearly indicates that protophilic attack at a THF ligand is not the only route for the decay of [Ca(Ph)(Hmds)(thf)3].

Figure 4. Time-dependent 1H NMR spectra (part of aromatic region), measured at 400 MHz, of a 0.12 M solution of [Ca(Ph)(Hmds)(thf)3] in THF/benzene-d6 (2:1) at 22 °C.

Figure 6. Time-dependent trend of the concentration of benzene and the primary organocalcium species of a 0.12 M solution of [Ca(Ph)(Hmds)(thf)3] in THF/benzene-d6 (2:1) at 22 °C.

Extension of the investigation to [Ca(Ph)2(thf)4], which contains a second, even more basic (relative to I− and (Me3Si)2N−) phenyl group, is hampered by very broad signals obtained for this compound in the 1H NMR spectrum at 22 °C, which preclude accurate integration of the signals.18,25 Therefore, the investigation was extended to the related naphthylcalcium derivatives, which might allow a verification of the result obtained for the Hmds-modified system and additionally permit experiments toward the effect of ring annelation on the overall stability of the calcium derivatives. Since such an annelation leads to a stabilization of the naphthyl anion due to polarization effects and consequently to a lower basicity relative to the phenyl anion,29 an increased durability of naphthylcalcium derivatives in comparison to their phenyl counterparts was expected; see Figure 7. Indeed, it takes 42 days (in contrast to 8 days for the phenyl system) until half of [Ca(Naph)(I)(thf)4] (Table 1, entry 6) is decomposed in a 0.09 M solution in THF/benzene-d6 (2:1) at 22 °C. As already described for the corresponding phenyl derivative, an exchange of the iodide anion by Hmds shortens this time span dramatically and [Ca(Naph)(Hmds)(thf)3] (Table 1, entry 7) decomposed more rapidly. However, the determined duration of 5 days until consumption of half of the initial amount of [Ca(Naph)(Hmds)(thf)3] is still twice as long as in the case of the phenyl derivative [Ca(Ph)(Hmds)(thf)3] (2.5 days). An even shorter t50 of 2.0 days was determined for [Ca(Naph)2(thf)4] in a THF/benzene-d6 mixture under identical conditions (Table 1, entry 8). In contrast [Ca(Hmds)2(thf)2] shows no detectable degradation in THF/ benzene-d6 at 22 °C within 2 weeks (Table 1, entry 16). The time-dependent decay of the concentration of the naphthylcalcium derivatives is depicted in Figure 8.

Figure 5. Time-dependent decay of the concentration of Ca-bound phenyl species in a 0.12 M THF/benzene-d6 (2:1) solution of [Ca(Ph)(Hmds)(thf)3] at 22 °C.

chemical shifts in the resulting spectrum were in agreement with the reported values of [Ca(Ph)(Hmds)(thf)3]. Although the initial appearance of this second signal set is a clear indicator for the presence of a second species, testing of several crystals from the crop under investigation by X-ray diffraction analysis gave only the well-known cell parameters of [Ca(Ph)(Hmds)(thf)3].28 In order to make the results of the decomposition of [Ca(Ph)(Hmds)(thf)3] comparable to the decomposition of the other organocalcium complexes, the sum of the concentrations of both initially observed organocalcium species was used as starting concentration. This led to a t50 of 2.5 days. Similar results were obtained when the point in time related to the maximal concentration of the compound with the signal at δ 7.68 was defined as the starting point and only the decomposition of this compound was followed (t50 = 2.2 days). 6384

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the absence of the well-known signals of [Ca(Naph)2(thf)4]25 in the NMR spectra of [Ca(Naph)(Hmds)(thf)3]. The comparison of naphthylcalcium and phenylcalcium derivatives showed that expansion of the aromatic system by ring annulation enhanced the stability of the organocalcium compounds. That the disruption of the aromatic system leads to the opposite effect is revealed by comparison of the stabilities of [Ca(Naph)(I)(thf) 4] and the related alkenylcalcium derivative [Ca(1,2-Dihydronaph)(I)(thf)4] in THF (Table 1, entry 9). In both systems the organyl groups have similar steric demands at the calcium center, but while the metalated sp2hybridized carbon in the first compound is part of an aromatic ring, it belongs to an isolated double bond in the latter. This leads to a dramatic reduction of the stability in THF. Whereas half of [Ca(Naph)(I)(thf)4] is still present after 42 days, half of [Ca(1,2-Dihydronaph)(I)(thf)4] is already decomposed after 2.5 days; see Figure 7. Besides temperature and counterion, the coordinating solvent utilized to stabilize the arylcalcium compounds has a significant influence on their stability. THF is most commonly used for the preparation and the subsequent use of arylcalcium compounds.15,21 Taking the listed t50 (Table 1) of organocalcium compounds in this solvent into account, which are measured in days, this solvent is probably suitable for most of the applications yet to develop. Only reactions using the more reactive diarylcalcium derivatives or taking place at elevated temperatures might require the use of solvents that are more resistant toward such organocalcium derivatives. Additionally, the identification of such a solvent in which these organometallics are indefinitely stable would increase their utility and might result, in the long-term, in their commercial availability. The chemistry of organolithium and organomagnesium reagents serves as an inspiration in this context, but the similar preparation of arylcalcium compounds in hydrocarbons was unsuccessful, limiting the choice to several ethereal solvents. Among them, tetrahydropyran (THP) and α-methyltetrahydrofuran (Me-THF) seemed promising since they led to an increased durability of related organolithium compounds.32 The use of THP indeed decelerated degradation of phenylcalcium iodide by a factor of 3 (23 days; Table 1, entry 10) until half of the initial concentration of [Ca(Ph)(I)(thp)4] (0.07 mol/L) was reached; see Figure 10. In the case of the closely related complex [Ca(Tol)(I)(thp)4] (Table 1, entry 11) this positive effect was much less pronounced and only a slight stabilization was achieved.20 When only the decay of the primary species is taken into account, actually no difference is noticeable. However, the observation of additional broad signals in the aromatic region indicates that the loss of primary [Ca(Tol)(I)(thp)4] is partially due to the formation of secondary species that contain a significant amount of calcium-bound tolyl groups. In contrast to THP, the use of Me-THF as ligand and solvent led to a dramatic decrease of the stability of the arylcalcium solutions. Attempts to synthesize phenylcalcium iodide from iodobenzene and elemental calcium in Me-THF led to solutions that showed only 30% of the expected alkalinity due to extensive decomposition of the envisioned product under the applied standard reaction conditions (1 h at 0 °C, 5 h at ambient temperature). The isolation of a well-defined product from these reaction mixtures failed. However, under optimized conditions the synthesis of the more stable naphthylcalcium derivative [Ca(Naph)(I)(Me-thf)4] was possible (see Experimental Section). The isolated crystalline

Figure 7. Decay of organocalcium derivatives of the type [Ca(R)(I)(thf)4] (R = Ph, Tol, Naph, 1,2-Dihydronaph) in THF/benzene-d6 solutions at 22 °C.

Figure 8. Time-dependent decay of organocalcium derivatives of the type [Ca(Naph)(X)(thf)n] (X = I, Hmds, Naph) in THF/benzene-d6 (2:1) solutions at 22 °C.

These experiments clearly verify that the basicity of the aryl anion alone does not determine the stability of arylcalcium derivatives (although it has a significant influence), and the second anionic ligand has a strong impact, too, and can accelerate the solvent degradation as observed in the case of the Hmds systems. This finding is remarkable because the calciumbound bis(trimethylsilyl)amido anion is expected to be several orders of magnitude less basic than the calcium-bound phenyl anion. While experimental values for the basicity of these calcium-bound groups are not available, a comparison of the pKa values of the corresponding acids benzene (pKa = 37)30 and bis(trimethylsilyl)amine (pKa;THF = 29.5)31 supports this assumption. The stability of [Ca(Hmds)2(thf)2]32 in THF (see Table 1, entry 16) shows that the calcium-bound bis(trimethylsilyl)amido anion itself is not suitable to cleave THF at a reasonable rate under the applied conditions. Therefore, the acceleration observed in the case of [Ca(Naph)(Hmds)(thf)3] might be the result of cooperative effects between the two present anions. Alternatively, the difference in reactivity observed between [Ca(Naph)(Hmds)(thf)3] and [Ca(Naph)(I)(thf)4] merely reflects the different amount of [Ca(Naph)2(thf)4] present due to the Schlenk equilibrium, assuming that this is the solely active species in the cleavage of THF. However, an argument against the latter interpretation is 6385

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investigated naphthylcalcium iodide seems to be a mononuclear species in the solid state as well as in THF and Me-THF solution. Hence, no stabilization due to aggregation is observed. On the contrary, the methyl group in Me-THF is easily accessible and opens a new decomposition pathway via βelimination, which seems to be rapid and becomes dominant as the absence of ethene and propene indicates. 1,2-Dimethoxyethane (dme) also represents a disadvantageous solvent. As ligand, dme induced the formation of ionic [Ca(Ph)(dme)3]I and led to its rapid decomposition when used as solvent (Table 1, entry 14). Within 8 h already half of the initial phenylcalcium compound decomposed in this case; see Figure 9.17 This prompt decomposition is based on the well-known base lability of dme.27

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CONCLUSION AND PERSPECTIVE The syntheses of well-defined arylcalcium compounds and related derivatives often require ethereal solvents, and therefore we focused our studies on cyclic monodentate ethers such as THF and THP since they are the most commonly used solvents. The durability studies of phenylcalcium iodides and related compounds in cyclic ethers gave t50 measured in days rather than hours or minutes and therefore verified the suitability of these post-Grignard reagents for a fruitful organic and organometallic chemistry, comparable to organolithium and classic Grignard reagents, even at ambient temperature. The comparison of different organocalcium derivatives of the type [Ca(R)(I)(thf)4] shows an increasing durability in the row R = 1,2-dihydronaphth-4-yl < phenyl/tolyl < α-naphthyl,leading to a significant increase of the durability of their tetrahydrofuran solutions. The temperature dependence of the decay was also investigated. The obtained data suggest that a decrease in temperature to −20 °C and below will make solutions of common arylcalcium halides in THF almost infinitely storable. Strained ethers should exhibit a higher tendency to be cleaved than unstrained molecules. However, it has been shown that the presence of easily accessible β-hydrogen atoms of the ether have a much stronger effect on t50 values, since rapid βelimination becomes the dominant decomposition pathway in this case. Consequently, the durability of post-Grignard solutions grows in the row α-methyltetrahydrofuran < tetrahydrofuran < tetrahydropyran with the remarkable observation that α-methylated tetrahydrofuran is not able to shield the reactive calcium center but is cleaved much faster than its unsubstituted congener most likely due to β-H elimination. The reactivity of the calcium-bound aryl group is strongly influenced by the second anionic ligand. For these investigations we have chosen the more stable α-naphthylcalcium derivatives and altered the second anionic ligand in its thf complex. The durability increased in the row α-naphthyl < bis(trimethylsilylamido) < iodide with half of di(α-naphthyl) calcium degrading within 2 days. The stability of [Ca(Naph)(X)(thf)4] is twice as large for X = N(SiMe3)2 and even approximately 20 times larger for X = I. Although this effect seems to be related to the basicity of the counterion, it is much more pronounced than expected and points to cooperative effects between the two basic moieties. In summary, we demonstrate that the durability and, hence, reactivity of organocalcium compounds can be tuned by diverse factors. Durabilities between a few hours and many weeks allow

Figure 9. Decay of the primary organocalcium species of complexes of the type [Ca(Ph)(I)(L)4] (L = thf, thf-d8, thp) in a solution of L/ benzene-d6 (2:1), as well as the decay of [Ca(Ph)(dme)3]I in a solution of DME/benzene-d6 (3:1) at 22 °C.

Figure 10. Decay of the primary organocalcium species of complexes of the type [Ca(Naph)(I)(L)4] (L = thf, Me-thf) in a solution of L/ benzene-d6 (2:1) at 22 °C.

substance contains four Me-THF ligands per calcium center as judged by NMR spectroscopy. A structural motif of [Ca(Naph)(I)(Me-thf)4] was accessible by X-ray analysis of the crystals (see Supporting Information), but suffers from a heavy disorder of all four Me-THF units as a result of the use of racemic Me-THF. Investigation of the stability of this compound in a solvent mixture of Me-THF and benzene-d6 (2:1) revealed a rapid decomposition of the product (t50 = 0.6 days; Table 1, entry 13), while the THF-ligated system is rather stable in the presence of THF (Table 1, entry 6); see Figure 10. The reactivity of arylcalcium compounds toward Me-THF is in marked contrast to organolithium derivatives,33 but can be rationalized if the different tendency of these organometallics to form higher aggregates is taken into account. It is likely that the use of Me-THF in the case of the lithium derivative led to a shift of the equilibrium from highly reactive monomeric species to less reactive dimeric or tetrameric species,34 which results in an overall stabilization relative to THF-solvated systems where monomeric species play an important role in solution.34a Such an equilibrium, although also principally possible, was not observed in the case of simple arylcalcium halides, and the 6386

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Crystal data for the structural motif of [Ca(Naph)(I)(Me-thf)4]: colorless prism, size 0.059 × 0.048 × 0.052 mm3, orthorhombic, space group Pnma, a = 17.6889(4), b = 13.7527(2), c = 12.6503(3) Å, V = 3077.44(11) Å3, T = −140(2) °C, Z = 4, ρcalcd. = 1.257 g·cm−3, μ(Mo Kα) = 12.31 cm−1, F(000) = 1200, 16 095 reflections in h(−22/17), k(−16/17), l(−16/14), measured in the range 2.19° ≤ θ ≤ 27.46°, completeness θmax = 99.5%, 3655 independent reflections, 3177 reflections with Fo > 4σ(Fo), largest difference peak and hole 1.585/− 1.107 e Å−3. Preparation of NMR Samples for Decomposition Experiments. An amount of the organocalcium complex was weighed into a valved NMR tube using standard Schlenk techniques. Afterwards, benzene-d6 (0.2 mL) and the appropriate ether (0.4 mL) were added by syringe. At last, the internal standard (10 μL), given in Table 1, was added by a syringe, and the tube was sealed under an argon atmosphere. The tubes for the measurements at 22 °C were stored in a temperature-conditioned room at 22 ± 1 °C. The sample for the investigation at −7 °C was stored in a freezer at this temperature. A dry ice/ethanol bath (−7 °C) was used for the transport of this sample to the NMR spectrometer. The sample for the high-temperature measurement (50 °C) was prepared at ambient temperature. Afterwards, it was placed in the NMR spectrometer, already adjusted to 50 °C. This temperature was kept throughout the whole experiment. The initial 1H NMR measurements took place directly after the sample preparation. The 1H NMR spectra were remeasured at intervals as given in Figure 3 and Figures 5−10. Chemical shifts were reported relative to the residual signal of benzene-d6 (δ = 7.15).

developing sophisticated applications for these post-Grignard reagents. In some cases, these reagents offer a reactivity different from organolithium and classic Grignard reagents due to the fact that calcium represents a metal with properties between typical s-block metals (highly ionic character, strong nucleophiles, salt-like compounds) and early transition metals (highly Lewis acidic character, d-orbital participation, catalytic activity).35

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under an inert atmosphere (argon or nitrogen) using standard Schlenk techniques. The solvents THF, THP, and Me-THF were dried over KOH and subsequently distilled over sodium/benzophenone under an argon atmosphere prior to use. Deuterated solvents were dried over sodium, degassed, and saturated with argon. The yield given is not optimized. 1H and 13C{1H} NMR spectra were recorded on Bruker AC 400 and AC 600 spectrometers. Chemical shifts are reported in parts per million. Cyclohexane and p-xylene were used as internal standards for NMR experiments and were dried over CaH2, degassed, and distilled under an argon atmosphere prior to use. The diatomaceous earth was purchased from Merck (Kieselguhr-purified and calcined GR for analysis Reag. Ph. Eur.). The utilized calcium was received from Alfa Aesar (Calcium shot, redistilled, 1 cm (0.4 in) and down, 99.5% metals basis). Activation of the calcium was performed by dissolution of calcium in liquid ammonia and subsequent drying under reduced pressure according to literature procedures.5b,15 [Ca(Ph)(I)(thf) 4 ], 22 [Ca(Ph)(I)(thp) 4 ], 36 [Ca(Naph)(I)(thf) 4 ], 15 [Ca(Naph)2(thf)4],25 [Ca(Ph)(Hmds)(thf)3],28 [Ca(Naph)(Hmds)(thf)3],16 and [Ca(Hmds)2(thf)2]32 were prepared according to published protocols. All substrates were purchased from SigmaAldrich, Merck, or Alfa Aesar and used without further purification. [Ca(Ph)(I)(thf-d8)4] was prepared by repeated recrystallization of [Ca(Ph)(I)(thf)4] from thf-d8. The calcium content was determined by complexometric titration of a hydrolyzed aliquot with 0.05 M EDTA using Eriochrome BlackT as indicator.37 X-ray crystal analyses of [Ca(Naph)(I)(Me-thf)4] were performed on a Nonius KappaCCD diffractometer, using graphitemonochromated Mo Kα radiation. The data were corrected for Lorentz and polarization effects, but not for absorption.38,39 The structure was solved by direct methods (SHELXS40) and refined by full-matrix least-squares techniques against Fo2 (SHELXL-9740). Synthesis of [Ca(α-Naph)(I)(Me-thf)4]. α-Iodonaphthalene (3.04 g, 12.0 mmol) was added slowly at −20 °C to a suspension of activated calcium (0.6 g, 15.0 mmol) in 20 mL of α-methyltetrahydrofuran. The suspension was shaken for 5 h at 0 °C. Then the resulting reaction mixture was filtered through a Schlenk frit, covered with diatomaceous earth. The residue on the filter was dried in vacuo and suspended in αmethyltetrahydrofuran (20 mL), and the suspension was stirred for 0.5 h at ambient temperature. The solution obtained after filtration was combined with the mother liquor. A conversion of over 90% was determined by acidimetric consumption of an aliquot of the combined solutions. After 1 day of storage of the dark brown solution at −40 °C the colorless precipitate was collected on a Schlenk frit and dried in vacuo. Yield: 4.2 g (6.6 mmol, 55.3%). Anal. Calcd (C30H47CaIO4, 638.68): Ca 6.28. Found: Ca 6.41. 1H NMR (THF-d8, 600 MHz): δ 1.14 (d, 12H, CH3 Me-THF), 1.33 (m, 4H, CHH′ Me-THF), 1.75− 1.89 (m, 8H, CH2 Me-THF), 1.90−1.97 (m, 4H, CHH′ Me-THF), 3.58 (m, 4H, OCHH′ Me-THF), 3.78 (m, 4H, OCHH′ Me-THF), 3.83 (m, 4H, OCH Me-THF), 7.05 (m, 1H, CH naphthyl), 7.12 (m, 1H, CH naphthyl), 7.16 (m, 1H, CH naphthyl), 7.29 (m, 1H, CH naphthyl), 7.54 (m, 1H, CH naphthyl), 7.87 (m, 1H, CH naphthyl), 7.94 (m, 1H, CH naphthyl). 13C{1H} NMR (THF-d8, 100.6 MHz): δ 21.3 (4C, CH3 Me-THF), 26.6 (4C, CHH′ Me-THF), 33.9 (4C, CH2 Me-THF), 68.0 (4C, OCHH′ Me-THF), 75.5 (4C, OCH Me-THF), 122.2 (1C, CH naphthyl), 123.0 (1C, CH naphthyl), 123.1 (1C, CH naphthyl), 124.6 (1C, CH naphthyl), 128.8 (1C, CH naphthyl), 134.1 (1C, C naphthyl), 136.9 (1C, CH naphthyl), 138.0 (1C, CH naphthyl), 146.2 (1C, C naphthyl) 195.4 (1C, C-Ca naphthyl).

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra of [Ca(α-Naph)(I)(Me-thf)4] and the decomposition experiments as well as plots in units of concentration and a figure of the structural motif of [Ca(α-Naph)(I)(Methf)4] are depicted. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 3641 948132. E-mail: [email protected]. Homepage: http://www.lsac1.uni-jena.de/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. is grateful to the Fonds der Chemischen Industrie (Frankfurt/Main, Germany) for a generous Ph.D. fellowship. We thank B. Rambach and Dr. M. Friedrich for the NMR measurements. We also appreciate the financial support of the Verband der Chemischen Industrie (VCI/FCI, Frankfurt/ Main, Germany). Infrastructure of our institute was partly provided by the EU (European Regional Development Fund, EFRE).

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REFERENCES

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