Magnesiacycloalkanes with Different Ring Sizes - ACS Publications

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Magnesiacycloalkanes with Different Ring Sizes Reinald Fischer,† Helmar Görls,† Jens Langer,†,‡ Marcel Enke,† and Matthias Westerhausen*,† †

Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, D-07743 Jena, Germany Department of Chemistry and Pharmacy, Friedrich Alexander University Erlangen-Nürnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: Di-Grignard compounds are prepared via the direct synthesis of magnesium with Br2C{Si(CH3)3}2, α,ωdihalides of XCH2YCH2X (X: Cl, Br; Y: Si(CH3)2, (CH2)n with n = 4, 5, 6), and with ClC(CH3)2(CH2)n(CH3)2CCl (n = 3, 4) in tetrahydrofuran (THF) or diethyl ether. After removal of the sparingly soluble magnesium halides after addition of 1,4dioxane (dx) the corresponding magnesium alkanediides remain in solution. Only the geminal di-Grignard complex (BrMg)2(μC{Si(CH 3) 3 }2) forms the corresponding dioxane adduct [{(dx)2MgBr}2(μ-C{Si(CH3)3}2)] (1). The magnesium alkanediides can be recrystallized from THF and characterized with Xray crystallography and NMR spectroscopy. Preferably dinuclear rings of the types [(thf)2Mg(μ-CH2-Y-CH2)}2] (Y: Si(CH3)2 (2), (CH2)6 (5)) and [({thf}2Mg{μ-C(CH3)2(CH2)4C(CH3)2})2] (7) crystallize. From a 1 M solution the strand-like coordination polymer [({thf}2Mg{μ-(CH2)6})∞] (3) precipitates. Only [(thf)2Mg{C(CH3)2(CH2)3(CH3)2C}] (6) crystallizes as a mononuclear compound. The NMR spectra of 2, 3, 5, and 6 show only one set of resonances for the alkanediyl moieties in [D8]THF solutions. The NMR spectra of 7 showed also at 50 °C two signal sets with the intensity ratios depending on the measurement temperature. Complex 7 shows a temperature-dependent monomer−dimer equilibrium in THF solution.

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INTRODUCTION The synthesis of simple bis-magnesiated alkyl reagents starting from dihalogeno compounds attracted Grignard’s interest already shortly after his initial investigations of magnesiumbased organometallics.1 These primary studies that were performed in collaboration with Tissier failed due to the disadvantageous choice of the vicinal dihalide 1,2-dibromoethane as well as the homologous 1,3-dibromopropane in the reduction reaction with magnesium, yielding magnesium bromide and ethene and cyclopropane, respectively. Similar experiments of other research groups2 verified these results. In contrast to this finding Blaise and Houillon3 prepared such an α,ω-di-Grignard reagent from 1,9-dibromononane for the first time in 1904. Subsequent investigations verified that these Grignard reactions were successful with high yields only if the halogen atoms were separated by at least four carbon atoms.4 Shorter carbon chains reacted in these reduction reactions with magnesium by elimination of the aforementioned hydrocarbons. The Schlenk equilibrium interconverted the diGrignard compounds in ethereal solvents (diethyl ether, THF) into cyclic magnesiacycles, as depicted in Scheme 1. Initial investigations on the structures of halide-free α,ωalkanediides of magnesium in solution and the crystalline state were performed by Bickelhaupt et al., who could show via stationary isothermal distillation that these derivatives prefer dinuclearity in solution (n = 1, Scheme 1).5 Only in very © XXXX American Chemical Society

Scheme 1. Schlenk Equilibrium Interconverting the DiGrignard Complexes [{(thf)2MgBr}2{μ-(CH2)5}] into Magnesiacycles and Magnesium Bromide

diluted samples were mononuclear species detected in some cases. Furthermore, Bickelhaupt and co-workers were able to determine the solid-state structure of the 1,5-pentanediide congener that crystallized as the dinuclear centrosymmetric thf adduct of 1,7-dimagnesiacyclododecane [({thf} 2 Mg{μ(CH2)5})2] (n = 1).6 The large C−Mg−C bond angle of Received: December 16, 2015

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performed in diethyl ether, leading to solutions of these derivatives with moderate yields of 42% and 50%, respectively. In order to isolate halide-free pure magnesium complexes, the magnesium halides were precipitated via addition of 1,4dioxane (dx), shifting the Schlenk-type equilibrium toward the homoleptic derivatives,12 and separated by filtration. Surprisingly, this proven method13,14 failed for the geminal diGrignard reagent (BrMg)2(μ-C{Si(CH3)3}2). The precipitate contained not only the bromide but also a major amount of the alkanediide anions. A small amount of the germinal di-Grignard reagent was isolated as its 1,4-dioxane adduct [{Br(dx)2Mg}2(μ-C{Si(CH3)3}2)] (1) (Figure 1).

141.5(3)° was a conceivable reason for the dinuclearity of this derivative because smaller six-membered magnesiacycles would cause severe ring strain. Complexes with (partially) unsaturated bridging organyl ligands often prefer other structures.7 We could show that an analogous dinuclear magnesium compound is also realized if the tertiary 2,5-dimethyl-2,5dichlorohexane is used as a substrate in the Grignard reaction.8 However, in [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2] the C−Mg−C bond angle of 122.86(7)° between the magnesiumbound alkyl chains is significantly smaller.8b Nevertheless, this bond angle is still large enough to prevent the formation of a mononuclear magnesiacyclopentane, but larger rings should be accessible with tertiary alkanediide moieties. In contrast to these findings the analogous complex [({thf} 2 Mg{μ(CH2)4})∞] with a primary alkanediide linker forms a polymer strand structure with C−Mg−C bond angles of 136.26(14)°.8b Due to the fact that only magnesium-based derivatives with medium-sized alkanediide chain lengths (C4−C5) were studied in the solid state, we became interested in the structures of such derivatives with shorter and longer α,ω-alkanediide units in order to elucidate the relationship between structure and chain length. Therefore, we intended to crystallize and study such magnesium complexes.

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RESULTS AND DISCUSSION As starting materials for the synthesis of di-Grignard reagents we employed primary α,ω-dibromides of the type BrCH2(CH2)nCH2Br (n = 4, 5, 6) and tertiary dichlorides of the type ClC(CH3)2(CH2)n(CH3)2CCl (n = 3, 4). The reaction of these substrates with excess magnesium turnings in THF gave solutions of the corresponding primary and tertiary diGrignard complexes with good (78−90%) to moderate (35− 42%) yields (Scheme 2). This protocol is not suitable for the Scheme 2. Synthesis of the Di-Grignard Reagents in Ethereal Solvents

Figure 1. Molecular structure and numbering scheme of [{Br(dx)2Mg}2(μ-C{Si(CH3)3}2)] (1). The ellipsoids represent a probability of 30%, and H atoms are omitted for the sake of clarity. Selected bond lengths (Å): Mg(1)−C(1) 2.125(5), Mg(1)−Br(1) 2.5151(18), Mg(1)−O(1) 2.083(4), Mg(1)−O(3) 2.061(4), Mg(2)−C(1) 2.134(5), Mg(2)−Br(2) 2.5085(18), Mg(2)−O(5) 2.089(4), Mg(2)−O(7) 2.048(4); bond angles (deg): Mg(1)−C(1)−Mg(2) 113.5(2), Si(1)−C(1)−Si(2) 113.2(3); nonbonding distance (Å): Mg(1)···Mg(2) 3.562(2).

preparation of analogous derivatives with short alkanediide fragments (C2−C3).9 A different strategy employing the mercury derivatives according to Costa and Whitesides10 was rejected due to toxicity reasons. Instead of this route we chose the silyl-substituted derivative ClCH2Si(CH3)2CH2Cl in order to produce a stable 1,3-dimagnesium reagent. In addition, we integrated also the dibromide Br2C{Si(CH3)3}2, which was already used by Bickelhaupt and co-workers to synthesize a stable geminal dimagnesium complex, in our investigations.11 The synthesis of the silyl-substituted di-Grignard reagent was

Complex 1 is isomorphous to the thf adduct [{Br(thf)2Mg}2(μ-C{Si(CH3) 3}2)], which was characterized earlier by Bickelhaupt and co-workers.11 This complex was isolated after extraction of the sparingly soluble residue with THF, and the structure was determined at −90 °C in order to obtain a data set of high quality (see SI). The bond lengths and angles of the central units of 1 and [{Br(thf)2Mg}2(μ-C{Si(CH3)3}2)] are very similar, and differences are insignificant. In the NMR spectra of 1 in [D8]THF solution singlets were observed for 1,4-dioxane, verifying that this ligand was quantitatively exchanged by [D8]thf molecules. Under these conditions the 13C resonance of the dianionic carbon atom was observed at δ = 2.3 ppm. The 29Si NMR resonance was detected at δ = −14.4 ppm, and hence, a significant high-field shift was observed in comparison to tetramethylsilane. Whereas the addition of 1,4-dioxane did not yield the germinal halide-free Grignard reagent via shift of the Schlenk equilibrium toward the homoleptic species, this dioxane method was successful for the other studied derivatives without any difficulties (Scheme 3). From halide-free solutions, we were able to isolate the desired compounds as thf solvates in crystalline form, and further purification succeeded via recrystallization from saturated THF solutions. This study allowed us to expand the knowledge on structurally characterized α,ω-alkanediide derivatives to shorter B

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are in distorted tetrahedral coordination spheres of two thf ligands and two carbanions of the alkanediides. The bond angle C(1)−Mg−C(6) at magnesium of 132.01(11)° (molecule A) and 132.95(12)° (molecule B) is widened in comparison to the tetrahedral angle due to electrostatic repulsion between the anionic ligands. Similar considerations are also valid for other derivatives of this compound class. In the 1H NMR spectrum of compound 2 two resonances were observed for the dianionic ligands with 29Si satellites (I = 1/2, 4.7% abundance) at δ = −1.82 ppm (CH2−Mg) and −0.21 ppm. In the 13C NMR spectrum the corresponding signals appear at δ = −2.4 ppm (t, 1JCH = 102.6 Hz) and δ = 9.2 ppm (q, 1JCH = 114.5 Hz). NMR spectra of 2, recorded at −50 °C, are identical to those measured at room temperature. We interpret this finding as the fact that the structure of the solid state is maintained in solution. In order to investigate the influence of the length of the alkanediides on the nuclearity of the ring structures, we prepared the oligonuclear magnesium derivatives 3−5 (see Scheme 3) with hexanediyl, heptanediyl, and octanediyl units with yields of 68.8%, 64.9%, and 42.8%. The 1H NMR and 13 C{1H} NMR spectra of 3−5 in [D8]THF solution at room temperature showed only one signal set. The resonances of the α-methylene groups were observed in the 1H NMR spectra as triplets at δ = −0.67 (3), −0.66 (4), and −0.68 ppm (5). In the 13 C{1H} NMR spectra the magnesium-bound carbon atoms exhibited chemical shifts of 9.1 (3), 8.1 (4), and 8.6 ppm (5). These values correspond well to those of the literature-known derivatives [({thf}2Mg{μ-(CH2)5})2] (1H: −0.65 ppm, 13C: 8.5 ppm)6 and [({thf}2Mg{μ-(CH2)4})∞] (1H: −0.63 ppm, 13C: 7.7 ppm).8b The investigations of Bickelhaupt and co-workers with respect to the structures of such compounds in diluted THF solutions suggest that dinuclear magnesium complexes are favored for carbon chain lengths of 4−9 carbon atoms5 (C7 and C8 chains were not studied explicitly), as also found for derivative 2. For compound 3, aggregation determinations in such solutions verify the existence of the dinuclear species. However, in concentrated solutions higher nuclearity seems to be possible because from these solutions the complex [({thf}2Mg{μ-(CH2)6})∞] (3) crystallized at room temperature (Figure 3). Attempts to detect oligonuclear aggregates via two-dimensional DOSY NMR experiments gave only resonances of a single compound in solution that can be assigned to the dinuclear derivative. The solid-state structure of 3 exhibits an aliphatic zigzag chain with every seventh methylene group being substituted by a Mg(thf)2 unit. The magnesium atoms represent corners of such parts. A similar chain structure was observed for crystalline [({thf}2Mg{μ-(CH2)4})∞], also.8b Expectedly, the Mg−C bond lengths of 3 and [({thf}2Mg{μ-(CH2)4})∞] are nearly identical and exhibit average values of 2.165 and 2.161 Å, respectively. The C−Mg−C bond angle of 129.97(7)° of the strand structure of 3 is smaller than the value of the homologous butanediide-bridged compound (136.26(14)°)8b and of the dinuclear complex 2 (132.01(11)° and 132.95(12)°). Attempts to crystallize compound 3 from diluted THF solution in order to crystallize the dinuclear species to compare both of the aggregates have failed as of yet. Furthermore, the homologous compound [({thf}2M{μ-(CH2)7})n] (4) could not be obtained as a single-crystalline substance, and the elucidation of the constitution in the solid state was impossible. In contrast to these experiments [({thf}2M{μ-(CH2)8})2] (5) yielded colorless crystals that were extremely sensitive toward air and

Scheme 3. Synthesis of Aliphatic Organomagnesiacycles via Precipitation of the Sparingly Soluble Magnesium Halides Starting from Di-Grignard Reagents

as well as to longer alkanediide chains. The synthesis and characterization of [({thf}2Mg{μ-(CH2)2Si(CH3)2})2] (2) sheds light on the chemistry of these scarcely investigated 1,3-dimagnesium α,ω-alkanediides.4 For the first time the X-ray structure of 2 provides insight into the structural behavior of such derivatives. This complex 2 crystallizes similar to the already known [({thf}2Mg{μ-(CH2)5})2] as a dinuclear complex (Figure 2).6,8b

Figure 2. Molecular structure and numbering scheme of [({thf}2Mg{μ-(CH2)2Si(CH3)2})2] (2). Only molecule A of two molecules is depicted. The hydrogen atoms are neglected for clarity reasons. Selected bond lengths (Å) for molecule A (values for molecule B are given in square brackets): Mg(1A)−C(1A) 2.141(3) [2.139(3)], Mg(1A)−C(6A) 2.145(3) [2.153(3)], Mg(1A)−O(1A) 2.080(2) [2.081(2)], Mg(1A)−O(2A) 2.086(2) [2.085(2)], Mg(2A)−C(2A) 2.138(3) [2.136(3)], Mg(2A)−C(5A) 2.147(3) [2.144(3)], Mg(2A)− O(3A) 2.075(2) [2.089(2)], Mg(2A)−O(4A) 2.089(2) [2.098(2)]; bond angles (deg): C(1A)−Mg(1A)−C(6A) 132.01(11) [132.95(12)], O(1A)−Mg(1A)−O(2A) 94.84(9) [94.59(10)], C(2A)−Mg(2A)−C(5A) 130.85(11) [132.69(12)], O(3A)−Mg(2A)− O(4A) 94.22(9) [94.91(9)]; nonbonding distances (Å): Mg(1A)··· Mg(2A) 4.010(2) [3.940(2)].

The asymmetric unit contains two molecules, A and B, that show only very small differences in the structural parameters. The central unit consists of a substituted cyclooctane ring in “saddle” or also boat−boat configuration15 with the 1,5positions consisting of Si(CH3)2 fragments and the 3,7positions of Mg(thf)2 moieties (Figure 2). All heteroatoms of the cycle are in a coplanar arrangement. The magnesium atoms C

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significantly larger ring than all known derivatives, the structural parameters resemble characteristic values. Complex 5 exhibits the smallest C−Mg−C bond angle, with a value of 129.07(12)°, in comparison to the other studied magnesium complexes with primary alkanediide ligands. Nevertheless, this angle is only slightly smaller than the corresponding values of the cyclic derivative 2 (132.01(11)° and 132.95(12)°) or of the strand structure of 3 (129.93(7)°); all these angles are significantly widened in relation to the tetrahedral angle. Due to the large ring size, the nonbonding distance between the magnesium atoms in 5 of 9.822(2) Å is larger than in the other investigated magnesiamacrocycles (2: 4.010(2) Å (molecule A); [{(thf)2Mg{μ-(CH2)5}}2]: 5.6103(7) Å;6 [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2]: 5.0492(7) Å)8b and exceeds the values that are realized in structurally related tin compounds16 with a similar (M{μ-(CH 2 ) 8 }) 2 subunit ([(ClSn) 2 {μ(CH2)8}3]: 7.566(2);16b [(PhSn)2{μ-(CH2)8}3]: 8.454(6) Å16a). The smallest intermolecular Mg···Mg distance of neighboring strands in the crystal of 5 shows a value of 6.889(2) Å and is even smaller than the smallest intramolecular transannular Mg(1)···Mg(1A) distance. Complex 5 can be isolated from an approximately 1 M THF solution with another modification, which is significantly less soluble in THF than the aforementioned modification. This solid was obtained by layering of a concentrated solution with diethyl ether or n-heptane, leading to a slow solidification of the precipitating oil. We assume that this modification resembles the polymeric strand structure of 5 (compare with compound 3). The poor crystal quality hampered a reliable X-ray structure determination. The existence of a mononuclear form of 5 could neither in the solid state nor in solution be verified by X-ray diffraction studies or NMR spectroscopy. Considering our findings and also the results of the Bickelhaupt group, it can be concluded that for mononuclear derivatives the six-membered rings represent the most stable ring size. Only for this ring size was it possible to detect the mononuclear derivative besides the dominating dinuclear cycles such as 2 and 5. In larger ring systems, the strain seems to rise again, as is also true for cycloalkanes. In order to isolate a mononuclear magnesiacycloalkane, tertiary alkanediide ligands seemed to be promising candidates because the only known derivative with tertiary ligands of this kindthe dinuclear complex [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2]exhibits a rather small bond angle of 122.86(7)° between the α-C atoms of the alkanediide chains at magnesium.8b This decreased angle should reduce the ring strain in such a mononuclear magnesiacyclohexane in comparison to the derivative with primary alkanediide moieties and should shift the equilibrium in solution toward the mononuclear species. The starting material for the synthesis of this complex is 2,6dimethyl-2,6-dichloroheptane, which was prepared via a twostep procedure starting from glutaric acid dimethyl ester (for details and characterization data see the SI). Reduction of this substrate with magnesium and subsequent removal of magnesium chloride with the dioxane method yields the corresponding diorganyl derivative 6, which is highly soluble in hydrocarbons and was recrystallized from n-heptane. The NMR spectra of this complex exhibit only one set of resonances for the anionic alkanediide ligand. In the 1H NMR spectrum only three signals at δ = 0.67 (CH3), 0.96, and 1.21 (CH2) were detected. In addition, the 13C{1H} NMR spectrum showed four resonances for the organyl groups besides the

Figure 3. Part of the strand structure (top and bottom) and the numbering scheme (top) of [({thf}2Mg{μ-(CH2)6})∞] (3). The ellipsoids represent a probability of 30%; H atoms are neglected for clarity reasons. Selected bond lengths (Å): Mg(1)−O(1) 2.0869(11), Mg(1)−O(2) 2.0663 (2), Mg(1)−C(1) 2.1575(17), Mg(1)−C(4) 2.1720(16); bond angles (deg): O(2)−Mg(1)−O(1) 90.51(5), C(4)− Mg(1)−C(1) 129.93(7), O(2)−Mg(1)−C(1) 105.05(5), O(2)− Mg(1)−C(4) 110.35(6); nonbonding distance (Å): Mg(1)···Mg(1A) 8.034(2).

moisture. The molecular structure and numbering scheme of 5 is depicted in Figure 4. Complex 5 crystallizes as a centrosymmetric cyclooctadecane with the first and tenth methylene moieties being substituted by Mg(thf)2 units. Despite the fact that derivative 5 forms a

Figure 4. Molecular structure and numbering scheme of [({thf}2Mg{μ-(CH2)8})2] (5). The ellipsoids represent a probability of 30%; H atoms are neglected for clarity reasons. Symmetry-related atoms are marked with the letter “A”. Selected bond lengths (Å): Mg(1)−O(1) 2.070(2), Mg(1)−O(2) 2.082(2), Mg(1)−C(1) 2.157(3), Mg(1)− C(8A) 2.154(3); bond angles (deg): O(2)−Mg(1)−O(1) 96.23(10), C(1)−Mg(1)−C(8A) 129.07(12), O(2)−Mg(1)−C(1) 102.89(9), O(2)−Mg(1)−C(8A) 105.98(9); nonbonding distance (Å): Mg(1)···Mg(1A) 9.822(2). D

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Organometallics signals of the thf molecules. The chemical shift of the magnesium-bound carbon atoms shows a value of δ = 21.4. This shift lies between the values of δ = 19.7 and 23.3, which were assigned to the mono- and dinuclear species in solution of related [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2].8 Therefore, it was impossible to clarify the aggregation degree despite the fact that a uniform compound exists in solution. An X-ray structure determination of single crystals of 6 proved that the mononuclear magnesium derivative [(thf)2Mg{C(CH3)2(CH2)3C(CH3)2}] crystallized. The structural motif of 6 is depicted in Figure 5.

Scheme 4. Equilibrium Interconverting the Mono- and Dinuclear Species of 7 in THF Solution

(CH2)4C(CH3)2})2] (7) crystallized, as was verified by an Xray structure determination (Figure 6).

Figure 5. Structural motif of [(thf)2Mg{C(CH3)2(CH2)3C(CH3)2}] (6). The H atoms are omitted for clarity reasons, and the atoms are drawn with arbitrary radii.

Figure 6. Molecular structure and numbering scheme of [({thf}2Mg{μ-C(CH3)2(CH2)4C(CH3)2})2] (7). The ellipsoids represent a probability of 30%; H atoms are omitted for the sake of clarity. Selected bond lengths (Å): Mg(1)−C(1) 2.1923(13), Mg(1)−C(6A) 2.1938(14), Mg(1)−O(1) 2.0940(11), Mg(1)−O(2) 2.0786(10); bond angles (deg): C(1)−Mg(1)−C(6A) 123.44(5), O(1)−Mg(1)− O(2) 89.21(4), O(1)−Mg(1)−C(1) 107.56(5), O(1)−Mg(1)− C(6A) 106.99(5); nonbonding distance (Å): Mg(1)···Mg(1A) 7.293(2).

The magnesium atom of 6 is located in a distorted tetrahedral environment and binds to two thf ligands and to two carbanions of the substituted pentanediyl group. The C− Mg−C bond angle of approximately 110° is close to the tetrahedral angle. The poor quality of the single crystals and the large R values prohibit a detailed discussion of the molecular structure. The magnesiacyclohexane complex 6 represents the first derivative that could be isolated as a mononuclear compound in the solid state. This success initiated further investigations to clarify if such mononuclear structures can also be obtained with increasing ring size of the magnesia cycles. Therefore, the homologous 2,6-dimethyl-2,6-dichlorooctane was reduced with magnesium and thereafter transformed into the halide-free diorganyl complex. In contrast to complex 6, the isolated product 7 is only sparingly soluble in THF at room temperature. This fact forced us to record the NMR spectra at elevated temperatures (50 °C). These spectra showed two sets of resonances for the alkanediide moiety, contrary to the NMR spectra of 6. The intensity ratio of these two sets was approximately 1:2. The 1H DOSY NMR experiment verified that the major component under these conditions was the compound with the smaller molar mass. We conclude that in solution two species are observed that are related in a temperature-dependent equilibrium according to Scheme 4. At 50 °C the equilibrium favors the mononuclear complex, and a molar ratio of 4:1 was observed. At lower temperature the equilibrium shifts to the dinuclear species, and at room temperature the less polar complex [({thf}2Mg{μ-C(CH3)2-

The basic structure of this centrosymmetric derivative is a cyclotetradecane with the first and eighth methylene moiety being exchanged by a Mg(thf)2 unit, and all H atoms of the second, seventh, ninth, and 14th methylene fragments are substituted by methyl groups. As already observed for complex [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2],8b the C(1)− Mg(1)−C(6A) bond angle of 123.44(5)° in derivative 7 is significantly smaller than found in comparable dinuclear complexes with primary alkanediide groups (see, for example, 2, 5, and [({thf}2Mg{μ-(CH2)5})2]6) due to the bulky carbanionic sites. In strand structures of polynuclear complexes such as the related hexanediyl compound 3 or [({thf}2Mg{μ(CH2)4})∞]8b larger C−Mg−C bond angles of 129.93(7)° and 136.26(14)°, respectively, were observed, also.

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CONCLUSION Organomagnesium compounds with primary α,ω-alkanediide ligands with 3−8 carbon atoms prefer dinuclear aggregates in solution and the solid state. The thus formed dimagnesiacycloalkanes exhibit ring sizes of 8 (compound [({thf}2Mg{μ(CH2)2Si(CH3)2})2] (2)) to 18 atoms (derivative [({thf}2Mg{μ-(CH2)8})2] (5)). These two derivatives represent the smallest and largest structurally authenticated derivatives of this type. Furthermore, these complexes show the tendency to E

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of 50% according to a procedure of Bickelhaupt et al.18 from 16.0 g of bis(trimetylsilyl)dibromomethane (50.3 mmol) and 3.0 g of Mg turnings (123.4 mmol). To this reaction mixture was added dropwise 16.0 mL of 1,4-dioxane (182 mmol), and the resulting mixture was stirred at room temperature (rt) overnight. After removal of all colorless precipitates an aliquot of the filtrate was hydrolyzed with 0.1 N sulfuric acid and titrated against phenolphthalein (yield: 13%). The volume of the filtrate was reduced to half of the original volume, and the residue was stored at −20 °C. The formed colorless crystals were collected with a Schlenk frit, washed with cold diethyl ether, and dried in vacuo. Yield: 0.7 g of 1 (1.9% with respect to {(CH3)3Si}2CBr2). 1H NMR (600.1 MHz, [D8]THF): δ 0.08 (18H, s, Si(CH3)3), 3.56 (32H, s, dx). 13C{1H} NMR (150.9 MHz, [D8]THF): δ 2.3 (C(MgBr)2), 9.4 (1JCSi = 45.0 Hz from 29Si satellites, Si(CH3)3), 29Si{1H} NMR (119.2 MHz, [D8]THF): δ −14.4 (Si(CH3)3). The colorless precipitate was dissolved in 25 mL of THF and warmed to approximately 50 °C, and the remaining solids were removed by filtration. In the filtrate [(thf)2MgBr2]19 crystallized at −20 °C, and the solution was decanted from this crystalline material. The mother liquor was layered with 10 mL of n-heptane. Then colorless crystals of [{Br(thf)2Mg}2(μ-C{Si(CH3)3}2)] grew at −20 °C and were collected, washed with cold THF, and very briefly dried in vacuo. Yield: 2.7 g of [{Br(thf)2Mg}2(μ-C{Si(CH3)3}2)] (8.2% with respect to (Me3Si)2CBr2). These crystals were suitable for X-ray diffraction studies. Synthesis of [({thf}2Mg{μ-(CH2)2Si(CH3)2})2] (2). A three-necked flask was equipped with a condenser and a dropping funnel and cooled in a water bath. Magnesium turnings (5.0 g, 205.8 mmol) and 70 mL of diethyl ether were placed in this flask. Over the dropping funnel, 10.0 g (63.6 mmol) of bis(chloromethyl)dimethylsilane was added slowly to the suspension. A colorless precipitate formed immediately. After complete addition the reaction mixture was refluxed for 3 h. The precipitate was collected on a Schlenk frit and dissolved in 80 mL of THF. An aliquot of the solution was hydrolyzed with 0.1 N sulfuric acid and titrated against phenolphthalein (yield: 42%).20 In order to precipitate magnesium chloride, 18.0 mL of 1,4-dioxane (205 mmol) was added and stirred overnight. The solid was removed and washed with THF. The volatiles of the combined filtrates were removed, and the residue was dried in vacuo. Thereafter, this residue was redissolved in THF and filtered. The clear filtrate was layered with heptane. At −20 °C colorless crystals of 2 formed. Yield: 2.44 g of 2 (15% with respect to the dichloride). Anal. Calcd for C24H52Mg2O4Si2 (M: 509.4): Mg 9.54. Found: Mg 9.60. Alkalinity: calcd 385.0 mg of H2SO4/g, found 380.0 mg of H2SO4/g. 1H NMR (400.1 MHz, [D8]THF): δ −1.82 (8H, s, d, 2JHSi = 3.6 Hz, CH2−Mg), −0.21 (12H, s, d, 2JHSi = 2.8 Hz, CH3−Si). 13C{1H} NMR (100.6 MHz, [D8]THF): δ −2.4 (1JCSi = 47.6 Hz, CH2−Mg), 9.2 (1JCSi = 40.6 Hz, CH3−Si). 13C NMR (100.6 MHz, [D8]THF): δ −2,4 (t, 1JCH = 102.6 Hz, CH2− Mg), 9.2 (q, 1JCH = 114.5 Hz, CH3−Si). Synthesis of [({thf}2Mg{μ-(CH2)6})∞] (3). Magnesium turnings (3.0 g, 112.4 mmol) and 70 mL of THF were placed in a three-necked flask and cooled with a water bath to rt. Into this suspension, 12.2 g of 1,6-dibromohexane (50.0 mmol) was slowly added dropwise within 3 h. After complete addition the reaction mixture was stirred at rt for an additional 3 h. Thereafter, all solids were removed by filtration. An aliquot of the filtrate was titrated with 0.1 N sulfuric acid against phenolphthalein (yield: 84%). 1,4-Dioxane (14 mL, 159 mmol) was added to the stirred solution, leading to the formation of a colorless solid. After waiting overnight, the solid that contains the majority of the bromide was removed and washed with THF (residual yield: 76%). The filtrate was dried in vacuo, and the residue dissolved in 20 mL of warm THF. At rt this THF solution was layered with 5 mL of nheptane. Within 3 d crooked colorless crystals formed. Yield: 8.70 g of 3 (68.8% with respect to initially used dibromide). Anal. Calcd for C14H28MgO2 (M: 252.7): Mg 9.62. Found: Mg 9.39. Alkalinity: calcd 388.1 mg of H2SO4/g, found 368.9 mg of H2SO4/g. 1H NMR (400.2 MHz, [D8]THF): −0.67 (4H, t, J = 6.9 Hz, Mg-CH2), 1.22 (4H, m, CH2), 1.53 (4H, br, CH2). 13C{1H} NMR (100.6 MHz, [D8]THF): 9.1 (Mg−CH2), 31.8 (CH2), 31.2 (CH2), 40.5 (CH2).

form coordination polymers in solution via ring opening and aggregation of the magnesiacycles. Due to the fact that these strand structures are only sparingly soluble, they can be isolated in some cases. Thus, isolation and structural characterization succeeded for the 1,6-hexanediyl derivative [({thf}2Mg{μ(CH2)6})∞] (3). In contrast to this observation it was impossible to favor the formation of mononuclear magnesiacycloalkanes with primary alkanediide anions and their isolation. As Bickelhaupt and co-workers demonstrated earlier, such a derivative can only be obtained in significant amounts in solution with a pentanediyl spacer in equilibrium with larger aggregates. These results support that six-membered cycles are favored for magnesiacycloalkanes, as is also true for simple cycloalkanes. This finding is surprising because severe distortions of the magnesiacycles are observed due to the large C−Mg−C bond angles of more than 129°. The formal substitution of one CH2 group in cyclohexane by a (thf)2Mg fragment should therefore enhance the ring strain significantly and shift the optimal ring size toward larger cycles. Obviously this is not the case. In fact, the use of tertiary alkanediide ligands that stabilize smaller C−Mg−C bond angles allows the isolation of singlecrystalline tetramethyl-substituted magnesiacyclohexane [(thf)2Mg{C(CH3)2(CH2)3C(CH3)2)}] (6). This species represents also the major component in solution, and a dinuclear derivative cannot be detected. The fact that this ring size represents the preferred number of ring atoms is also verified by the comparison with the homologous compounds [({thf}2Mg{μ-C(CH3)2(CH2)2C(CH3)2})2] and [({thf}2Mg{μ-C(CH3)2(CH2)4C(CH3)2})2] (7). Despite the fact that these derivatives exist predominantly as mononuclear species in solution at 50 °C, a second set of resonances can be detected (contrary to the NMR spectra of 6). This second set of resonances is assigned to the dinuclear complexes that crystallize upon cooling of the reaction mixtures. Nevertheless, such magnesium derivatives with tertiary alkanediide ligands show an enhanced tendency to form the mononuclear species in solution in comparison to compounds with primary α,ωalkanediide anions.

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under an anaerobic argon atmosphere using standard Schlenk techniques. The solvents were dried over KOH and subsequently distilled over sodium/benzophenone in 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. Due to the use of [D8]THF solvent for the NMR experiments and a fast exchange of ligated thf molecules by deuterated congeners, the resonances of coordinated thf ligands were not detected. Chemical shifts are reported in parts per million relative to SiMe4 as an external standard. The determination of the magnesium content and the alkalinity of the organomagnesium compounds was performed by complexometric titration (EDTA, Eriochrom BlackT as indicator) as well as with an acid−base titration after protolysis. Bis(chloromethyl)dimethylsilane was purchased from abcr AG & Co KG, 1,8-dibromooctane and 1,6-dibromohexane were from Riedel de Haën, and 1,7-dibromoheptane was from Sigma-Aldrich. Bis(trimetylsilyl)dibromomethane17 and [{BrMg(thf)2}2C{Si(CH3)3}2]11 were synthesized according to literature protocols. The preparation of 2,6-dichloro-2,6-dimethylheptane and 2,7-dichloro-2,7-dimethyloctane are given in the SI. Formation of [{Br(dx)2Mg}2{μ-C(Si(CH3)3)2}] (1). A solution of (BrMg)2{μ-C(Si(CH3)3)2} in diethyl ether was prepared with a yield F

DOI: 10.1021/acs.organomet.5b01010 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Synthesis of [({thf}2Mg{(CH2)7})n] (4). 1,7-Dibromoheptane (5.0 g, 19.3 mmol) was continuously added to a suspension of 1.16 g of magnesium turnings (47.8 mmol) in 30 mL of THF within 2 h. After complete addition the reaction mixture was stirred for an additional 3 h at rt. Then an aliquot of the solution was hydrolyzed with 0.1 N sulfuric acid and titrated against phenolphthalein (yield: 78%). Thereafter, 14 mL of 1,4-dioxane (159 mmol) was added. The formed suspension was stirred for another hour and stored overnight at rt. Then the colorless precipitate and excess magnesium were removed with a frit covered with diatomaceous earth and washed with THF. At −20 °C a colorless compound crystallized from the combined filtrate, which was recrystallized from THF. Yield: 3.34 g of colorless, dull crystals of 4 (64.9% with respect to 1,7dibromoheptane). Anal. Calcd for C15H30MgO2 (M: 266.7): Mg 9.11. Found: Mg 9.02. Alkalinity: calcd 367.7 mg of H2SO4/g, found 346.8 mg of H2SO4/g. 1H NMR (400.1 MHz, [D8]THF): δ −0.66 (4H, t, J = 6.9 Hz, Mg−CH2), 1.29 (6H, m, CH2), 1.59 (4H, br, CH2). 13 C{1H} NMR (100.6 MHz, [D8]THF): δ 8.1 (Mg−CH2), 30.7 (CH2), 31.2 (CH2), 39.3 (CH2). Synthesis of [({thf}2Mg{μ-(CH2)8})2] (5). 1,8-Dibromooctane (12.24 g, 45.0 mmol) was dropped into a suspension of 3.0 g of magnesium turnings (112.4 mmol) in 90 mL of THF, whereupon the reaction mixture was cooled with a water bath. After complete addition, the reaction solution was refluxed for 3 h and then filtered with a Schlenk frit. Titration of an aliquot of the clear filtrate with 0.1 N sulfuric acid gave a yield of 90%. 1,4-Dioxane (14 mL, 159 mmol) was added to the filtrate, and a colorless solid precipitated, which was removed after several hours. The filtrate was reduced to half of the original volume and stored at −40 °C afterward. Under these conditions colorless crystals formed. Yield: 5.60 g of 5 (42.8% with respect to initially used dibromide). Anal. Calcd for C32H64Mg2O4 (M: 581.4): Mg 8.36. Found: Mg 8.45. Alkalinity: calcd 337.3 mg of H2SO4/g, found 337.3 mg of H2SO4/g. 1H NMR (400.2 MHz, [D8]THF): δ −0.68 (8H, t, 3J = 6 Hz, CH2−Mg), 1.22 (16H, m, CH2), 1.58 (8H, m, CH2). 13C{1H} NMR (100.6 MHz, [D8]THF): δ 8.6 (CH2−Mg), 31.5 (CH2), 31.6 (CH2), 40.3 (CH2). Synthesis of [(thf)2Mg{C(CH3)2(CH2)3C(CH3)2)}] (6). A solution of 12.0 g of 2,6-dichloro-2,6-dimethylheptane (60 mmol) in 30 mL of THF was added to a suspension of 3.9 g of magnesium turnings (160 mmol) in 60 mL of THF at rt over a period of 5 h. During this time, the temperature of the cooling bath has to be maintained below 30 °C. After complete addition the reaction mixture was refluxed for an additional hour. Excess magnesium was removed, and an aliquot titrated with 0.1 N sulfuric acid against phenolphthalein (yield: 33%). Then 15 mL of 1,4-dioxane (170 mmol) was added to the filtrate, and the solution was stored overnight at rt. The colorless precipitate was removed with a Schlenk frit, and titration of the filtrate gave a final yield of 30%. All volatiles were removed from the filtrate in vacuo, and the residue was dissolved in n-heptane with a small amount of THF. At −40 °C colorless 6 crystallized. This compound is extremely soluble in THF and very soluble in n-heptane. Yield: 2.24 g of colorless crystals of 6 (12.7% with respect to initially used dichloride). Anal. Calcd for C17H34MgO2 (M: 294.7): Mg 8.24. Found: Mg 8.17. Alkalinity: calcd 332.7 mg of H2SO4/g, found 332.8 mg of H2SO4/g. 1H NMR (600.1 MHz, [D8]THF): δ 0.67 (12H, s, CH3), 0.96 (4H, m, CH2), 1.21 (2H, m, CH2). 13C{1H} NMR (150.9 MHz, [D8]THF): δ 21.4 (2 × C− Mg), 26.9 (1 × CH2), 33.8 (4 × CH3), 55.7 (2 × CH2). Synthesis of [({thf}2Mg{μ-C(CH3)2(CH2)4C(CH3)2})2] (7). A solution of 6.4 g of 2,7-dichloro-2,7-dimethyloctane (30.3 mmol) in 30 mL of THF was added to a suspension of 2.43 g of magnesium turnings (100 mmol) in 30 mL of THF over a period of 3 h. The temperature of the water bath has to be kept below 30 °C. After complete addition, the reaction mixture was refluxed for another hour. Excess magnesium was removed by filtration, and an aliquot of the clear filtrate was titrated with 0.1 N sulfuric acid against phenolphthalein (yield: 41%). Then 8 mL of 1,4-dioxane (110 mmol) was added to the stirred solution. The colorless precipitate was removed at approximately 60 °C with a Schlenk frit. Titration of an aliquot with 0.1 N sulfuric acid against phenolphthalein gave a final yield of 35%. At rt colorless crystals precipitated, which were collected

on a frit, washed with THF, and dried in vacuo. Yield: 1.44 g of colorless crystals of 7 (15.6% with respect to initially used dichloride). Anal. Calcd for C36H72Mg2O4 (M: 617.5): Mg 7.87. Found: Mg 7.99. Alkalinity: calcd 317.9 mg of H2SO4/g, found 315.2 mg of H2SO4/g. 1 H NMR (600.1 MHz, [D8]THF, 50 °C): The resonances of the mononuclear and dinuclear species were detected in a molar ratio of 4:1 besides traces of 2,7-dimethyloctane from hydrolysis. Mononuclear complex: δ 0.85 (12H, s, CH3), 1.03 (4H, br, CH2), 1.26 (4H, m, CH2); dinuclear compound: δ 0.88 (24H, s, CH3), 1,07 (8H, br, CH2) 1.41 (8H, qi, J = 2.7 Hz, CH2); 2,7-dimethyloctane: δ 0.87 (d, J = 6.6 Hz, CH3), 1.19 (m, CH2), 1.28 (m, CH2), 1.53 (hept, J = 6.6 Hz, CH). 13 C{1H} NMR (100.6 MHz, [D8]THF, 50 °C): mononuclear compound: δ 20.6 (C−Mg), 28.9 (CH2), 32.9 (CH3), 47.8 (CH2); dinuclear derivative: δ 22.1 (C−Mg), 32.8 (CH3), 34.0 (CH2), 51.8 (CH2); 2,7-dimethyloctane: δ 21.9 (CH3), 27.5 (CH2), 27.9 (CH), 39.0 (CH2). Structure Determinations. The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphitemonochromated Mo Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semiempirical basis using multiple scans.21−23 The structures were solved by direct methods (SHELXS24) and refined by full-matrix leastsquares techniques against Fo2 (SHELXL-9724). All hydrogen atoms bound to the compounds 3 and [{(thf)2BrMg}2(μ-C{Si(CH3)3}2)] plus the hydrogen atoms of the alkyl ligand of 5 and 7 were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. All non-hydrogen atoms were refined anisotropically.24 The crystal of [{(thf)2BrMg}2(μ-C{Si(CH3)3}2)] contained large voids, filled with disordered solvent molecules. The size of the voids is 212 Å3/unit cell. Their contribution to the structure factors was secured by back-Fourier transformation using the SQUEEZE routine of the program PLATON,25 resulting in 16 electrons/unit cell. The crystals of 6 were extremely thin and of low quality, resulting in a substandard data set; however, the structure is sufficient to show connectivity and geometry despite the high final R value. We will only publish the conformation of the molecule and the crystallographic data. We will not deposit the data in the Cambridge Crystallographic Data Centre. Crystallographic data as well as structure solution and refinement details are summarized in Table S1 (SI). XP (SIEMENS Analytical Xray Instruments, Inc.)26 and POV-Ray27 were used for structure representations.

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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.5b01010. In addition, crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC-1439336 for 1, CCDC-1439337 for 2, CCDC-1439338 for 3, CCDC-1439339 for 5, CCDC-1439340 for 7, CCDC1439341 for 2,6-dichloro-2,6-dimethylheptane, and CCDC1439342 for [{Br(thf)2Mg}2(μ-C{Si(CH3)3}2)]. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail: deposit@ccdc. cam.ac.uk). Synthesis of starting materials, representation of molecular structures, NMR spectra, and crystallographic and refinement data (PDF) Crystallographic data (CIF) G

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Chichester, 2000. (d) Krieck, S.; Westerhausen, M. In The Lightest Metals: Science and Technology from Lithium to Calcium (Encyclopedia of Inorganic and Bioinorganic Chemistry); Hanusa, T. P., Ed.; Wiley: Chichester, 2015; pp 213−229. (14) For selected examples see also: (e) Seidel, W.; Buerger, I. Z. Anorg. Allg. Chem. 1978, 447, 195−198. (f) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264−3268. (g) Langer, J.; Krieck, S.; Fischer, R.; Görls, H.; Walther, D.; Westerhausen, M. Organometallics 2009, 28, 5814−5820. (h) Blasberg, F.; Bolte, M.; Wagner, M.; Lerner, H.-W. Organometallics 2012, 31, 1001−1005. (15) Petasis, N. A.; Patane, M. A. Tetrahedron 1992, 48, 5757−5821. (16) (a) Newcomb, M.; Blanda, M. T.; Azuma, Y.; Delord, T. J. J. Chem. Soc., Chem. Commun. 1984, 1159−1160. (b) Newcomb, M.; Horner, J. H.; Blanda, M. T.; Squattrito, P. J. J. Am. Chem. Soc. 1989, 111, 6294−6301. (c) Blanda, M. T.; Horner, J. H.; Newcomb, M. J. Org. Chem. 1989, 54, 4626−4636. (17) Wiberg, N.; Link, M.; Fischer, G. Chem. Ber. 1989, 122, 409− 418. (18) Hogenbirk, M.; van Eikema Hommes, N. J. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Klumpp, G. W. Tetrahedron Lett. 1989, 30, 6195−6198. (19) Sarma, R.; Ramirez, F.; Mc Keever, B.; Chaw, Y. F.; Marecek, J. F.; Niermann, D.; Mc Caffrey, T. M. J. Am. Chem. Soc. 1977, 99, 5289−5295. (20) Klosin, J.; Kruper, W. J., Jr.; Nickias, P. N.; Roof, G. R.; Sato, J. PCT patent WO 01/42315 A1 (14.06. 2001), classification: C08F/ 643. (21) Hooft, R. COLLECT, Data Collection Software; Nonius B.V.: The Netherlands, 1998. (22) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C. W.; Sweet, R. M., Eds.; Academic Press: New York, 1997; Macromolecular Crystallography, Part A, Vol. 276, pp 307−326. (23) SADABS 2.10; Bruker-AXS Inc.: Madison, WI, USA, 2002. (24) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (25) Spek, A. L. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, D65, 148−155. (26) XP; Siemens Analytical X-ray Instruments Inc.: Madison, WI, USA, 1994. (27) POV-Ray; Persistence of Vision Raytracer: Victoria, Australia, 2007.

AUTHOR INFORMATION

Corresponding Author

*Fax (M. Westerhausen): +49 (0) 3641 948132. E-mail: m. [email protected]. Author Contributions

The manuscript was written through contributions of all authors, and all authors have given approval to the final version of the manuscript. 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). We thank Ms. Regina Suxdorf for the preparation of di-Grignard reagents.

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REFERENCES

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DOI: 10.1021/acs.organomet.5b01010 Organometallics XXXX, XXX, XXX−XXX