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Trimethylsilylmethylcalcium Iodide, an Easily Accessible GrignardType Reagent of a Heavy Alkaline Earth Metal Mathias Köhler, Alexander Koch, 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: The direct synthesis of iodomethyltrimethylsilane with calcium and magnesium in ether yields the corresponding ether adducts [(thf)4Ca(I)(CH2SiMe3)] (1a) and [(Et2O)2Mg(I)(CH2SiMe3)] (2a). The 1,4-dioxane method allows shifting the Schlenk equilibrium toward bis(trimethylsilylmethyl)magnesium 2b. After substitution of ligated thf ligands by tetrahydropyran (thp) crystalline [(thp)4Ca(I)(CH2SiMe3)] (1b) can be isolated. The Ca−C and Ca−I bond lengths of 252.7(3) and 319.11(3) pm represent characteristic values. Steric repulsion between ligated thp ligands and the rather bulky trimethylsilylmethyl group widens the Ca−C−Si angle to 131.19(14)°. NMR data and quantum chemical studies support that hyperconjugative effects might be operative, leading to a shortened Si−C bond of this Ca−C−Si fragment.

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order to avoid the formation of potassium trialkylcalciates.10 The transmetalation of organozinc derivatives yields the corresponding zincates11 as contact ion pairs with bridging alkyl groups; however, dialkylcalcium has not been isolated via this metal−metal exchange route. Other stabilized calcium methanide complexes such as benzyl derivatives have been prepared at the same time.12−15 Benzylcalcium compounds have been used for metalation reactions to prepare trimethylsilyl-substituted organometallics.16 Since alkylcalcium derivatives have a higher reactivity, especially a higher metalation potential, than arylcalcium derivatives and, hence, represent very interesting reagents for organic and organometallic chemistry, it is necessary to establish an efficient and reproducible synthesis of alkylcalcium compounds. On the basis of our experience of the preparation of aryl- and alkenylcalcium halides,1 the reduction of iodomethyltrimethylsilane with activated calcium was investigated. Generally, the direct synthesis offers the preferred method for the synthesis of organocalcium derivatives in terms of atom economy and costs. The trimethylsilyl group was chosen to ensure solubility and stability in ethereal solvents.

INTRODUCTION Organocalcium chemistry has remained in the shadow of the widely used Grignard reagents1,2 because these magnesiumbased organometallics are easily accessible or in some cases even commercially available.3 The first trials to prepare organocalcium compounds via direct synthesis date back more than a hundred years;4 however, early reports on the synthesis of dimethylcalcium could not be reproduced.5 Severe challenges hampered the development of calcium-based organometallics, such as the enormous discrepancy of the inertness of the metal and the extremely high reactivity of its organometallics. On the one hand, metal activation prior to use and extended reaction times are required; on the other hand, these factors lead to ether degradation and side reactions.1 The organometallic chemistry of calcium and, with limitations, that of its heavier congeners experienced a renaissance and vast interest after the determination of the molecular structure of [(diox)2Ca{CH(SiMe3)2}2] (diox = 1,4-dioxane) by Lappert and co-workers in 1991.6 This research group overcame the inertness of the metal by co-condensation of calcium and (Me3Si)2CHBr with tetrahydrofuran (THF) and obtained crystalline material after a solvent exchange to 1,4-dioxane. However, the synthesis of alkylcalcium halides via the cocondensation protocol does not represent a convenient route to obtain alkylcalcium halides on a large scale and with a short enough time scale for the development of subsequent organocalcium chemistry. Thereafter, the metathetical approach offered a more convenient access to calcium-based organometallics. Thus, coligand-free [Ca{C(SiMe3)3}2] with a bent C−Ca−C moiety was isolated, and immediate ether cleavage observed during dissolution in diethyl ether.7 This kind of complex can be stabilized by internal Lewis bases being part of the silyl side arms.8 Bis(trimethylsilyl)methyl derivatives of calcium, strontium, and barium were also prepared employing this strategy,9 but the stoichiometry has to be maintained in © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis. A convenient Grignard-type direct synthesis of alkylcalcium halides has not been developed yet, whereas a routine protocol allows the preparation of aryl- and alkenylcalcium halides with σ-bonds between calcium and sp2-hybridized carbon atom.1 This procedure requires activated calcium powder and ethereal solvents in order to guarantee sufficient solubility of the heavy Grignard reagent. In analogy, iodomethyltrimethylsilane was combined with activated calcium powder in tetrahydrofuran at −40 °C (Scheme 1). Received: November 16, 2015

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DOI: 10.1021/acs.organomet.5b00956 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

[D6]benzene. para-Xylene served as an internal standard for the degradation studies. The integration of the 1H NMR spectra permitted the monitoring of the decreasing concentration of 1b and the increasing amount of tetramethylsilane, giving a t50 value (time within the concentration decreased to half of the initial value)22 of approximately 1 week for this organocalcium reagent (Figure 1). This high stability of [(thp)4Ca(I)-

Scheme 1. Direct Synthesis of Trimethylsilylmethylcalcium Iodides

After 1 h the reaction mixture was warmed to 10 °C and stirred for another hour. Thereafter, residual calcium was removed by filtration. The conversion of approximately 50% was determined via acidimetric titration of an aliquot of the filtrate. Removal of the solvent and recrystallization from THF solution yielded very thin needles of [(thf)4Ca(I)(CH2SiMe3)] (1a), which were unsuitable for X-ray diffraction experiments. Due to this fact and based on earlier observations that tetrahydropyran (THP) can improve the crystallization behavior of organometallics,17,18 we exchanged the solvent and finally isolated crystalline [(thp)4Ca(I)(CH2SiMe3)] (1b). For comparison reasons we also reacted magnesium with iodomethyltrimethylsilane in diethyl ether, yielding [(Et2O)2Mg(I)(CH2SiMe3)] (2a) according to Scheme 2. An Figure 1. Degradation of [(thp)4Ca(I)(CH2SiMe3)] in THP (0.4 mL), C6D6 (0.2 mL), and para-xylene (10 μL).

Scheme 2. Synthesis of Bis(trimethylsilylmethyl)magnesium

(CH2SiMe3)] (1b) in THP is a prerequisite if this organocalcium reagent is to develop into a powerful reagent for group transfer and metalation reactions comparable to the lithium and magnesium congeners. The alkylcalcium complex 1b exhibits a durability in THP that is comparable to the t50 value of [(thf)4Ca(I)(Ph)] in tetrahydrofuran.22 The solvent exchange from THF to THP for phenylcalcium iodide and formation of [(thp)4Ca(I)(Ph)] led to significantly enhanced durability in this ether. In contrast to this finding, the corresponding tolylcalcium complex showed no enhanced stability.22 This fact also illustrates that a solvent exchange for the alkylcalcium iodide in order to extent the t50 values might be advantageous. Structure. The molecular structure and numbering scheme of 1b are depicted in Figure 2. The crystal of 1b contained 2% of [(thp)4CaI2], and the occupation factor could be refined freely. The calcium atom is in a distorted octahedral environment with the anionic ligands in trans position. The Ca1−O bond lengths vary in the narrow range between 238.93(13) and 241.14(12) pm and resemble characteristic values as also observed for other arylcalcium and calcium methanide complexes, for example in the complex [(thp)4Ca(I)(Ph)], where the values differ between 240.2(4) and 244.7(4) pm.18 In addition, the Ca1−I1 distance of 319.11(3) pm also adopts a characteristic value compared with arylcalcium compounds such as [(thp)4Ca(I)(Ph)], with a Ca−I bond length of 312.1(1) pm.18 The Ca1−C1 bond length of 252.7(3) pm is characteristic for unstrained calcium−carbon σ-bonds.1 Due to mainly steric repulsion between the trimethylsilyl group and neighboring thp ligands, the C1−Ca1−I1 bond angle of 170.53(7)° deviates from linearity, leading to a large C1− Ca1−O2 bond angle of 103.62(7)°. The ether molecule of O2 is pushed toward the iodide, and hence, small I1−Ca1−O2 and O2−Ca1−O4 bond angles of 84.28(3)° and 168.42(5)°, respectively, are observed.

acidimetric titration gave also a conversion of approximately 50%, which is comparable to the calcium compound 1a. In order to obtain crystalline material, 1,4-dioxane was added and the ether adduct of bis(trimethylsilylmethyl)magnesium 2b was isolated. Recrystallization of 2b failed due to the formation of oily substances from various donor solvents and solvent mixtures. The NMR parameters of 2b are in agreement with literature values as discussed later. Due to the fact that bis(trimethylsilylmethyl)magnesium has been employed routinely in preparative organic and organometallic chemistry, this complex was often synthesized in situ without isolation prior to subsequent transformations. Therefore, and in order to shed light on reaction mechanisms involving these reagents, solidstate structures have become of interest in recent years. Thus, Lappert and co-workers elucidated the dimeric nature of [(thf)Mg(CH2SiMe3)2]2 only quite recently.19 The direct synthesis of iodomethyltrimethylsilane with activated strontium in THF was accompanied by severe ether degradation reactions, and therefore the isolation of pure trimethylsilylmethylstrontium iodide failed. Stability of 1b in Ethereal Solvents. For the development of a successful organocalcium chemistry that is based on trimethylsilylmethylcalcium iodide a long-term durability in ethereal solvents is desirable. Advantageously, the calcium derivative 1b is a mononuclear and highly soluble reagent. Contrarily, the aggregation degree of LiCH2SiMe3 strongly depends on the donor strength of the accompanying ligands,20 and a hexameric structure has been elucidated for solvent-free [LiCH2SiMe3]6.21 For the investigation of the durability of [(thp)4Ca(I)(CH2SiMe3)] (1b) in tetrahydropyran, we dissolved this heavy Grignard reagent in 0.4 mL of THP and added 0.2 mL of B


Article

Organometallics

NMR Investigations. The decreased electronegativity of the alkaline earth metals slightly enhances the heteropolar character of the M−C bonds and can influence the degree of hyperconjugation and, hence, the NMR parameters. The 1H NMR parameters of the calcium and magnesium complexes 1b and 2b are rather similar, but the 13C NMR values exhibit significant differences. Whereas the SiMe3 groups show comparable shifts and coupling constants, the δ(13C) value of the methylene moiety of the calcium congener 1b is shifted toward lower field by 12.4 ppm. Compared with other calcium, magnesium, and lithium trimethylsilylmethanide complexes (see Table 2), the δ(1H) values for both the methylene protons and the protons of the trimethylsilyl groups as well as the δ(13C) values of the trimethylsilyl groups of 1b and 2b are within a typical range. This is also true for the chemical shifts of the 29Si nuclei. In contrast to this observation, the chemical shifts of the methylene carbon atoms in the 13C NMR spectra depend on the number of bound trimethylsilyl groups. An increasing number of bound trimethylsilyl groups leads to a downfield shift of this carbon resonance. In the case of the calcium complexes the δ(13C) value of 17.0 ppm for [(thf)2Ca{CH(SiMe3)2}2] is larger than the shift of +6.9 ppm for compound 1b and increases up to 20.3 ppm for [Ca{C(SiMe3)3}2]. The same trend can be observed in the case of trimethylsilylsubstituted magnesium methanide derivatives. The 1J(C,H) coupling constants of the complexes 1b and 2b are quite similar. In the case of the calcium complex 1b, the coupling constant for the methylene group is 101.5 Hz and for the methyl groups 115.5 Hz, whereas the corresponding magnesium complex 2b shows values of 104.2 and 116.1 Hz, respectively. The small 1J(C,H) coupling constants could hint toward a decreased s-orbital contribution for the C−H bonds of the methylene moiety, as could hyperconjugative effects with the anionic trimethylsilylmethyl group.25 However, the high quality of the structure determination shows no severe distortion of the Si−C−H bond angles (Si1−C1−H1A 107.6(12)° and Si1−C1−H1B 107.4(13)°) and the H1A− C1−H1B angle (106.0(17)°) as result of such effects. In order to shed light on the bonding situation in this complex, we performed quantum chemical calculations. Quantum Chemical Studies. Quantum chemical investigations were performed to unravel the electronic effects in 1b. To this end, the DFT method was chosen using the B3LYP functional in combination with the basis set 6-311++G** or 6311G**, implemented in the Gaussian09 program package. This procedure provides reliable geometries nearly independently of the chosen basis set (diffuse functions) in comparison to the X-ray structure of 1b (Table 3).

Figure 2. Molecular structure and numbering scheme of 1b. The ellipsoids represent a probability of 30%. Hydrogen atoms at C1 are drawn with arbitrary radii; all other H atoms are omitted for clarity reasons. Selected bond lengths (pm): Ca1−C1 252.7(3), Ca1−O1 241.14(12), Ca1−O2 238.93(12), Ca1−O3 239.63(12), Ca1−O4 240.09(12), Ca1−I1 319.11(3), Si1−C1 183.1(3), Si1−C2 188.1(2), Si1−C3 188.2(2), Si1−C4 188.3(2).

In contrast to the unknown trimethylsilylmethylcalcium complexes, the crystal structure of hexameric trimethylsilylmethyllithium has been known for 30 years.21 In this case deaggregation of [LiCH2SiMe3]6 can be induced only with strong Lewis bases such as N,N,N′,N′-tetramethylethylenediamine (tmeda) or N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdeta).20 A comparison of the characteristic bond lengths and angles of trimethylsilylmethanide complexes of calcium, lithium, and magnesium is shown in Table 1. The chosen dimeric magnesium derivative [(thf)Mg(CH2SiMe3)2]2 contains terminally bound and bridging trimethylsilylmethyl groups with significantly different structural parameters. In general, the trimethylsilyl-substituted methyl groups exhibit characteristic structural peculiarities, such as a short Cα−Si bond length compared with the CMe−Si distance, that are also found for the calcium complex 1b. In this compound the C1−Si1 bond length of 183.1(3) pm is approximately 5 pm smaller than the Si−C distances to the methyl groups (av Si− CMe 188.2 pm). This observation can at least partly be understood by attractive forces between the carbanionic site and the positively charged silicon atom. Also hyperconjugative effects may lead to this shortening of the bond length.23 However, especially striking within the calcium compound 1b is that steric repulsion between one thp ligand and the alkyl group leads to a very large Ca1−C1−Si1 bond angle of 131.19(14)°. In addition, electrostatic repulsion between the positively charged silicon and calcium atoms facilitates the large bond angle.

Table 1. Comparison of Selected Bond Lengths and Angles of Trimethylsilylmethyl Groups Bound to Calcium (1b), Magnesium, and Lithium (Bond Lengths [pm] and Angles [deg]) compound 1b [(diox)2Ca{CH(SiMe3)2}2] [Ca{C(SiMe3)3}2] [(thf)Mg(CH2SiMe3)2]2 [(tmeda)Li(CH2SiMe3)] [(pmdeta)Li(CH2SiMe3)] a

M−Cα 252.7 243.3 245.9 212.5,b 228.8c 226.5a 211.3

Cα−Si

CMe−Si

183.1 181.0 183a 182.9,b 184.8c 182.1a 179.7

a

188.2 185.3a 187a 188.2,b 186.7c 188.4a 188.9

M−Cα−Si

lit.

131.2 110.9, 118.7 95a 121.8,b 78.5,c 173.4c 126.7a 126.4

6 7 19 20 20

Average values. bAverage values for terminal group. cAverage values for bridging group. C


Article

Organometallics

Table 2. Comparison of Selected Chemical NMR Shifts δ [ppm] of the Trimethylsilyl-Substituted Methyl Groups at Calcium, Strontium, Magnesium, and Lithium M−CαHx

SiMe3

compound

1

δ( H)

13

δ( C)

1

δ( H)

δ(13C)

1b [(diox)2Ca{CH(SiMe3)2}2] [(thf)2Ca{CH(SiMe3)2}2] [Ca{C(SiMe3)3}2] [(thf)3Sr{CH(SiMe3)2}2] 2b [(thf)Mg(CH2SiMe3)2]2 [Mg{CH(SiMe3)2}2]∞ [(tmeda)Li(CH2SiMe3)]2 [(pmdeta)Li(CH2SiMe3)] [(pmdeta)Li{CH(SiMe3)2}]

−1.84 −1.81 −1.66

+6.9 +17.2 +17.0 +20.3 −13.6 −5.5 −4.19 +5.6 −5.7 −5.6 +1.9

−0.17 +0.31 +0.32 +0.24 +0.36 −0.09 +0.35 +0.16 +0.36 +0.48 +0.25

+5.4 +5.9 +4.2 +7.2 +6.3 +6.7 +4.4 +5.3 +6.0 +6.4 +7.6

−1.59 −1.75 −1.28 −1.57 −1.86 −1.57 −2.1

lit. 6 9 7 9 19 24 20 20 20

Table 3. Comparison of Calculated Quantum Chemical Geometries and Experimental Structure of 1b (Bond Lengths [pm], Angles [deg]) as Well as Major Charges q of the Anions and the Cation basis set

Ca1−C1 Ca1−I1 C1−Si1 C1−Ca1−I Ca1−C1−Si1 q([CH2SiMe3]) q(I) q(Ca)

B3LYP

B3LYP

exptl

6-311++G**

6-311G**

(X-ray)

254.0 323.9 185.1 176.0 132.7 −0.926 −0.881 +1.762

253.4 325.2 185.0 176.3 131.2 −0.926 −0.881 +1.762

252.7(3) 319.11(3) 183.1(3) 170.6(2) 131.19(14)

Figure 3. Hyperconjugation due to involved NBO orbitals in the anion (CH2SiMe3)− obtained from B3LYP/6-311G** calculations followed by NBO analysis.

elongation of the Si1−C3 bond. The hyperconjugative contribution E(2) to the Si−C bond energy represents a significant contribution on the order of more than 10% in relation to the reference value of 75 kcal mol−1 for the Si−C bond energy in tetramethylsilane.29 To conclude our investigations, we examined the carbon counterpart of 1b, the thp adduct of neopentylcalcium iodide of the type [(thp)4Ca(I)(CH2CMe3)], offering a similar electronic structure. Again, the geometry was optimized followed by an NBO 6.0 analysis. Despite the fact that the preparation of this neopentylcalcium halide has not been reported yet, a similar result to that for 1b was obtained, even of the interaction strength of the carbanionic lone pair with the σ*(C−C3) bond, the intrinsic stabilization of the negative charge in 1b (Table 5). For comparison purposes, also the anion (CH2CMe3)− (which is isoelectronic to tert-butylamine) was included in our studies. Here a strikingly different structure is predicted. Lower negative hyperconjugation results in a nearly ideal tetrahedral environment of C1 in contrast to the almost planar C1 atom in (CH2SiMe3)−. This finding suggests an enhanced nucleophilicity of the free neopentyl anion in comparison with the anionic trimethylsilylmethyl congener. However, for the calcium complexes a significantly smaller difference is predicted on the basis of these quantum chemical studies.

Furthermore, the interaction of the σ(M−C) bond (M = Mg, Ca, Sr, Ba with constant number of coordinated THP) with one of the σ*(Si−C) bonds (hyperconjugation) was investigated, and for comparison purposes, tetramethylsilane and the anion (CH2SiMe3)− (which is isoelectronic to H2N-SiMe3) were included in our studies. The importance of negative hyperconjugation is evident in silylamines with a planar nitrogen atom.26,27 Therefore, geometries were optimized with the previous methods as well as pseudopotentials for strontium (MWB28) and barium (MWB46) followed by NBO analysis using the NBO 6.0 program. The increase of the ion radius from magnesium to barium results in a longer as well as more ionic σ(M−C) bond, mainly consisting of carbon orbitals, localizing the negative charge at the α-carbon atom C1. The negative charge at this carbon atom is very similar for all alkaline earth metals (due to the interplay of diverse interactions in this molecule, this is not reflected in the NBO charges), allowing a significant charge transfer (CT) from the σ(M−C) to σ*(Si1−C3) bonds (Figure 3) with the maximum for the free anion (CH2SiMe3)− (Table 4). The occupancy of the σ*(Si−C3) bond, which is related to the magnitude of the CT, varies from 1% for SiMe4 over approximately 2−4% for the alkaline earth metal derivatives to 7.7% for the free trimethylsilylmethyl anion. The thereby related stabilization energy E(2) can be computed from secondorder perturbative theory as corrections to the zeroth-order natural Lewis structure, evaluating the magnitude of the negative hyperconjugation.28 As expected, E(2) increases in the same order as the occupation number as well as the

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CONCLUSION AND PERSPECTIVE A straightforward synthesis of an alkylcalcium iodide has been developed. This complex is highly soluble in common ethereal solvents and promises to further advance the Grignard-type chemistry of calcium. In THF and THP the corresponding D


Article

Organometallics

Table 4. Comparison of Structural Data of Complexes of the Type [(thp)4M(I)(CH2SiMe3)] (M = Mg, Ca, Sr, Ba), the (CH2SiMe3)− Anion, and Tetramethylsilane, Obtained by ab Initio Calculations (B3LYP) with the 6-311G** Basis Sets (Bond Lengths [pm], Angles [deg], E(2) [kcal mol−1]) SiMe4 M−C1 C1−Si1 Si1−C3 Si1−C2/C4 M−C1−Si1 H−C(1)−Si(1) q(C1)a q(Si)a q(M)a q(C3)a q(C2/C1)a occ. σ*(Si1−C3) E(2)b a

189.1 189.1 189.1 111.3 −1.058 1.576 −1.058 −1.058 0.020 1.46

(CH2SiMe3)−

Mg

Ca

Sr

Ba

221.9 186.0 190.8 190.5/190.8 136.5 104.4 −1.645 1.635 1.697 −1.063 −1.068 0.048 6.59

253.4 185.0 191.0 190.7/190.7 131.2 106.7 −1.622 1.602 1.762 −1.069 −1.067 0.064 9.55

269.9 184.6 191.1 190.8/190.9 131.2 107.5 −1.636 1.625 1.787 −1.065 −1.064 0.064 10.09

286.1 184.1 191.1 191.0/190.9 123.7 109.0 −1.632 1.617 1.809 −1.064 −1.064 0.068 11.33

177.6 195.6 192.7/192.8 118.5 −1.448 1.567 −1.073 −1.052 0.153 24.08

NBO charges of the respective atoms. bBond energy of the hyperconjugative contribution.

Table 5. Comparison of Structural Data of 1b and the (CH2SiMe3)− Anion with Their Carbon Congeners, Obtained by ab Initio Calculations with the 6-311G** Basis Sets (Bond Lengths [pm], Angles [deg], E(2) [kcal mol−1]) M-C1 Si1/C−C3 Si1/C−C2/C4 M−C1−Si1/C H−C1−Si1/C q(C1)a q(Si1/C)a q(M)a q(C3)a q(C2/C1)a occ. σ *(Si1−C3) E(2)b a

1b

[(thp)4Ca(I)(CH2CMe3)]

253.4 191.0 190.7/190.7 131.2 106.7 −1.622 1.602 1.762 −1.069 −1.067 0.064 9.55

254.9 154.7 154.1/154.1 127.9 106.5 −1.118 −0.024 1.749 −0.579 −0.575 0.038 7.67

(CH2SiMe3)−

(CH2CMe3)−

195.6 192.7/192.8

156.4 154.4/154.5

118.5 −1.448 1.567

107.6 −0.949 −0.034

−1.073 −1.052 0.153 24.08

−0.594 −0.573 0.067 13.7

NBO charges of the respective atoms. bBond energy of the hyperconjugative contribution.

mononuclear tetrakis(thf) and tetrakis(thp) complexes of calcium 1a and 1b are formed. The lack of aggregation equilibria as observed for trimethylsilylmethyllithium is advantageous for the development of a constructive organocalcium chemistry because the ethereal solvent has a negligible influence on the reactivity of the calcium-based organometallics. Increasing the electropositive character of the alkaline earth metals leads to an increasing low-field shift of the methylene resonances of the alkyl group in the 13C NMR spectra and decreasing 1J(C,H) coupling constants. The degree of hyperconjugation of the negative charge into σ*(Si−C) bonds within the trimethylsilylmethyl ligand also increases with the heteropolar nature of the M−C bond, leading to a significantly shortened Cα−Si bond of the methylene moiety. According to quantum chemical calculations, negative hyperconjugation leads to an increasing occupancy of the affected σ*(Si1−C3) bond of 2.4% for the Mg complex to 3.4% for the Ba derivative. The maximum value of 7.7% is calculated for the free trimethylsilylmethanide anion. In accordance with this finding, the hyperconjugative contribution to the Si1−C1 bond energy increases in the same order from 6.6 to 11.3 kcal mol−1 from the Mg to Ba complexes. The molecular structure of the thp adduct of trimethylsilylmethylcalcium iodide (1b) exhibits a significantly widened Ca−C−Si bond angle due to intramolecular steric repulsion between the trimethylsilyl group and

one ether ligand, also minimizing electrostatic repulsion between the alkaline earth metal cation and the positively charged silicon.

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under an inert argon atmosphere using standard Schlenk techniques. The solvents 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 yields given are not optimized. 1H, 13C, 13C{1H}, and 29 Si{1H} NMR spectra were recorded on Bruker AC 400 and AC 600 spectrometers. Chemical shifts are reported in parts per million relative to SiMe4 as an external standard. SiMe4 was formed as the hydrolysis product in very small amounts during preparation of the NMR sample, due to trace amounts of water adsorbed to the wall of the NMR tube. All substrates were purchased from Sigma-Aldrich, Merck, or Alfa Aesar and used without further purification. Calcium was activated via dissolving in liquid ammonia according to literature procedures.1b Synthesis of [(thp)4Ca(I)(CH2SiMe3)] (1b). Activated calcium (0.41 g, 10.23 mmol) was suspended in tetrahydrofuran (15 mL), and iodomethyltrimethylsilane (2.18 g, 10.21 mmol) was added at −40 °C. The resulting suspension was stirred for 1 h at this temperature. Afterward the suspension was warmed to 10 °C during another hour of stirring. Then unreacted calcium was removed by filtration using a Schlenk frit covered with diatomaceous earth, and the conversion E


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Organometallics

restraints, R1obs = 0.0225, wR2obs = 0.0452, R1all = 0.0252, wR2all = 0.0464, GOOF = 1.078, largest difference peak and hole: 0.556/− 0.510 e Å−3.

(55%) was determined by acidimetric titration of an aliquot of the resulting solution. Then the filtrate was reduced to dryness in vacuo, and the resulting bright yellow solid was taken up in 5 mL of THP. This procedure was repeated two times before the solid was dissolved in a mixture of THP (10 mL) and heptane (5 mL). After 3 days of storage of the yellow solution at −40 °C the formed slightly yellow crystals were isolated by filtration and dried in vacuo. Those crystals of the composition [Ca(CH2SiMe3)(I)(thp)4] (1b) were also suitable for X-ray diffraction experiments. Yield: 0.81 g (1.35 mmol, 13.2%). Anal. Calcd for C24H51CaIO4Si (598.74 g·mol−1): Ca 6.69. Found: Ca 6.47. Due to loss of ether ligands during handling and the extreme sensitivity toward air and moisture, combustion analysis gave no reliable data. 1H NMR ([D8]THF, 400 MHz): δ −1.84 (s, 2H, CH2Ca), −0.17 (s, 9H, CH3-Si), 1.50 (m, 16H, CH2 thp), 1.61 (m, 8H, CH2 thp), 3.55 (m, 16H, OCH2 thp). 13C{1H} NMR ([D8]THF, 100.6 MHz): δ 5.4 (3C, CH3-Si), 6.9 (1C, CH2-Ca), 24.4 (4C, CH2 thp), 27.6 (8C, CH2 thp), 69.0 (8C, OCH2 thp). 13C NMR ([D8]THF, 100.6 MHz): δ 5.4 (1JCH = 115.5 Hz, SiMe3), 6.9 (1JCH = 101.5 Hz, CH2), 24.4 (1JCH = 134.7 Hz), 27.6 (1JCH = 123.3 Hz), 69.0 (1JCH = 140.8 Hz). 29Si{1H} NMR ([D8]THF, 119.2 MHz): δ −3.5. Synthesis of [(Et2O)2Mg(CH2SiMe3)2] (2b). Magnesium (0.35 g, 14.36 mmol) was suspended in diethyl ether (15 mL), and iodomethyltrimethylsilane (2.83 g, 13.21 mmol) was added slowly within 1 h at ambient temperature. The resulting suspension was stirred for 1 h at this temperature. Afterward the suspension was refluxed for 1 h. Then unreacted magnesium was removed by filtration using a Schlenk frit covered with diatomaceous earth, and the conversion (50%) was determined by acidimetric titration of an aliquot of the resulting solution. Afterward 1,4-dioxane (0.96 mL, 10.83 mmol) was added to the stirred filtrate at ambient temperature and a white solid precipitated. The suspension was allowed to stand for 15 h, and then the precipitate was removed by filtration using a Schlenk frit covered with diatomaceous earth and the residue was washed with diethyl ether (2 × 5 mL). The resulting colorless solution was reduced to dryness in vacuo to yield [(Et2O)2Mg(CH2SiMe3)2] (2b) as a white solid that loses coordinated diethyl ether upon drying (crude product, 0.69 g, 1.98 mmol, 29.9%). Recrystallization of the crude product from Et2O, THF, THP, TMEDA, 1,4-dioxane, or nheptane did not result in the formation of crystalline products. 1 H NMR ([D8]THF, 600 MHz): δ −1.75 (s, 4H, CH2-Mg), −0.09 (s, 18H, CH3-Si), 1.14 (t, 12H, CH3 Et2O), 3.41 (q, 8H, OCH2 Et2O). 13 C{1H} NMR ([D8]THF, 100.6 MHz): δ −5.5 (2C, CH2-Mg), 6.7 (6C, CH3-Si), 17.8 (4C, CH3 Et2O), 68.4 (4C, OCH2 Et2O). 13C NMR ([D8]THF, 100.6 MHz): δ −5.5 (1JCH = 104.2 Hz, CH2), 6.7 (1JCH = 116.1 Hz, SiMe3), 17.8 (1JCH = 125.7 Hz, Et2O), 68.4 (1JCH = 138.8 Hz, Et2O). 29Si{1H} NMR ([D8]THF, 119.2 MHz): δ −3.3. X-ray Structure Determination of 1b. The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphite-monochromated Mo Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semiempirical basis using multiple scans.30−32 The structure was solved by direct methods (SHELXS)33 and refined by full-matrix leastsquares techniques against Fo2 (SHELXL-97).33 Compound 1b cocrystallized with [(thp)4CaI2]; for the final refinement its content was fixed to 2%. All hydrogen atoms were located by difference Fourier synthesis and refined isotropically. All non-hydrogen atoms were refined anisotropically.33 Crystallographic data as well as structure solution and refinement details are summarized in the Supporting Information. The programs XP (SIEMENS Analytical X-ray Instruments, Inc.)34 and POV-Ray35 were used for structure representations. Crystal Data for 1b: 0.98 (C24H51CaIO4Si)·0.02 (C20H40O4CaI2), M = 599.55 g mol−1, colorless prism, size 0.045 × 0.042 × 0.038 mm3, monoclinic, space group P21/c, a = 17.6823(3), b = 17.4313(3), c = 10.0658(2) Å, β = 100.715(1)°, V = 3048.44(10) Å3, T = −140 °C, Z = 4, ρcalcd = 1.305 g cm−3, μ(Mo Kα) = 12.82 cm−1, multiscan, transmin: 0.7095, transmax: 0.7456, F(000) = 1256, 21 354 reflections in h(−22/ 20), k(−22/21), l(−13/13), measured in the range 2.34° ≤ θ ≤ 27.44°, completeness θmax = 99.7%, 6928 independent reflections, Rint = 0.0249, 6483 reflections with Fo > 4σ(Fo), 494 parameters, 0

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00956. Crystallographic data deposited at the Cambridge Crystallographic Data Centre under CCDC-1030468 for 1b contain the supplementary crystallographic data excluding structure factors; this data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or via e-mail [email protected]. CIF files giving crystallographic data of the crystal structure determinations (CIF) NMR spectra and details for the quantum chemical studies (PDF) Cartesian coordinates of all calculated molecule structures (TXT)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 (0) 3641 9-48132. E-mail: [email protected] (M. Westerhausen). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support of the Fonds der Chemischen Industrie im Verband der Chemischen Industrie e.V. (FCI/VCI, Frankfurt/Main, Germany). A.K. thanks the Fonds der Chemischen Industrie im Verband der Chemischen Industrie e.V. for a generous Ph.D. stipend. Furthermore, we thank Dr. S. Krieck and P. Traber for their kind support.

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REFERENCES

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