Synthesis, Crystal Structures, and Solution Behavior of

Oct 25, 2012 - *Fax: +49 (0) 3641 948102. E-mail: [email protected]. Abstract. Abstract Image. The organomagnesium complexes [(thf)2Mg(μ-C5H10)]2 (1),...
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Synthesis, Crystal Structures, and Solution Behavior of Organomagnesium Derivatives of Alkane-1,4-diide as Well as -1,5diide Reinald Fischer, Regina Suxdorf, 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 organomagnesium complexes [(thf)2Mg(μC5H10)]2 (1), [(thf)2Mg(μ-C4H8)]∞ (2), and [(thf)2Mg(μ-(C(CH3)2C2H4C(CH3)2)]2 (3) were prepared via direct synthesis from magnesium turnings and appropriate dichloroalkanes in tetrahydrofuran (THF). The aggregation degree in the solid state depends on the nature of the alkanediide. The THF solution of 3 shows a temperature-dependent equilibrium. The reactions of MgCl2(thf)1.5 with 1,4-dilithiobutane yield the lithium magnesiates [Li(thf)4]2[Mg3(C4H8)4] (4) and [{(tmeda)Li}2Mg(C4H8)2] (5) depending on the applied stoichiometry. The addition reaction of Ph2Mg(diox) (1,4-dioxane = diox) with 1,4-dilithiobutane leads to the formation of the heteroleptic magnesiate [{(tmeda)Li}2MgPh2(C4H8)] (6), which shows in THF solution a ligand exchange (Schlenk-type) equilibrium with the homoleptic derivatives [{(thf)2Li}2Mg(C4H8)2] and [{(thf)2Li}2MgPh4].

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INTRODUCTION Dianions are widely used reagents with a broad field of application in organic syntheses; however, common procedures avoid isolation and characterization of these intermediate derivatives.1 As early as 1901 Tissier and Grignard reacted 1,2dibromoethane with magnesium turnings in order to produce a vicinal di-Grignard reagent.2 However, these reactions always proceeded with elimination of ethylene and formation of magnesium bromide. A few years later the reaction of 1,3dibromopropane with magnesium yielded mainly cyclopropane in a Wurtz-type coupling reaction, but as a side reaction 1,6hexanediyl-di(magnesium bromide) [hexamethylene di(magnesium bromide)] was transferred into suberic acid (octanedioic acid) via 2-fold addition of carbon dioxide and water.3 Generally, the direct synthesis of magnesium turnings with 1,ω-dihalogenated alkanes with more than three methylene units gave the corresponding di-Grignard reagents with high yields. However, the structures of these reagents remained unknown for a long time, and even in the recent textbook of Elschenbroich4 these compounds were formulated as magnesiacycles, which resulted from a Schlenk-type equilibrium from BrMg-(CH2)n+3-MgBr (n = 1, 2, 3) after addition of dioxane (eq 1). Bickelhaupt and co-workers

dimer equilibrium in tetrahydrofuran for magnesiacyclohexane (eq 2) and a second Schlenk-type equilibrium for pentam-

ethylene di(magnesium bromide) after addition of magnesium bromide (eq 1, n =2).6 In the solid state, a centrosymmetric dimer of magnesiacyclohexane, 1,7-dimagnesiacyclododecane (Mg−C 213(1) and 215(1) pm), with very large C−Mg−C angles of 141.5(3)° was observed.7 Bickelhaupt and co-workers explained the dimerization by entropic effects with the large C− Mg−C angle disfavoring small rings. On the contrary, magnesiacyclopentane (n = 1) was not detected in tetrahydrofuran solution, but this molecule completely dimerized to 1,6-dimagnesiacyclodecane.8 In addition a Schlenk-type equilibrium also converts this metallacycle into tetramethylene di(magnesium bromide) in the presence of MgBr2. Geminal di-Grignard derivatives are also accessible via a direct synthesis and have been known for a long time.5,9 In order to improve solubility and crystallization behavior on one hand and to reduce the reactivity on the other, bis(trimethylsilyl)dibromomethane was reacted with magnesium turnings, yielding (Me3Si)2C[Mg(thf)2Br]2 with Mg−C bond lengths of 210(4) and 214(4) pm.10 However, vicinal and 1,3dimagnesiated alkanes as well as their Schlenk-type counter-

intensively investigated these magnesium 1,ω-alkanediides in solution and the solid state.5 They proposed a monomer− © 2012 American Chemical Society

Received: September 13, 2012 Published: October 25, 2012 7579

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turnings according to the procedure of Bickelhaupt and coworkers.7 After removal of magnesium halide, single crystals of 1 were grown. Due to rather large estimated standard deviations of the structural parameters determined by Bickelhaupt and co-workers at +20 °C,7 we redetermined the crystal structure at −140 °C. These conditions allowed freely refining the positions of the hydrogen atoms. The molecular structure and numbering scheme of 1 are presented in Figure 1.

parts, magnesiacyclopropane and -cyclobutane, readily decompose to hydrocarbons and magnesium halide.5 Nevertheless, propane-1,3-diyl-di(magnesium bromide) could be prepared via the direct synthesis with a yield of approximately 30%.11 Quantum chemical investigations12 as well as reactions of magnesium with ethylene13 showed that there exists only a weak tendency to form magnesiacyclopropane, which can be regarded also as a π-complex of ethylene at a magnesium atom. Insertion of another equivalent of ethylene into one Mg−C bond expands the ring size by two carbon atoms, and magnesiacyclopentane is significantly favored.13 Di-Grignard reagents with sp2-hybridized carbon atoms in the backbone show different structural principles due to restricted flexibility. The thf complex of naphthalene-1,8diylmagnesium crystallized as a tetramer with the magnesium atoms forming a Mg4 tetrahedron with the organic dianions capping the Mg3 faces.14 In addition, dimagnesiated 1,4dihalogenohydrocarbons can be stabilized by ethylene and benzo units in the backbone. For example, ortho-bis(chloromethyl)benzene was reduced with magnesium, yielding o-C6H4(CH2MgCl)2; from a thf solution crystals of trimeric [(thf)2Mg(o-CH2-C6H4-CH2)]3 formed with the magnesium atoms bound to two thf ligands and to two anions, thus forming a 15-membered trimagnesiacycle (Mg−C 215.4(8) and 215.8(8) pm).15 Reduction of anthracene,16 1,4-dimethylanthracene,17 and 9,10-bis(trimethylsilyl)anthracene18 with magnesium led to the formation of monomeric complexes with fivemembered magnesiacycles.19 However, not only are benzo moieties able to stabilize di-Grignard compounds, but also 1,4bis(trimethylsilyl)but-2-ene-1,4-diylmagnesium crystallized as a monomeric tmeda complex (Mg−C 220.0(9) and 219.1(9) pm); however, the alkaline earth metal atom was located above the but-2-ene plane, enabling rather short contacts to the CC double bond (Mg···C 238.1(8) and 239.9(8) pm).20 Due to the fact that the structures of di-Grignard reagents of the type (XMg)2(CH2)x are limited in solution studies of Bickelhaupt and co-workers,5,6 we reinvestigated these complexes. Halide anions lead to more complicated situations because in addition to aggregation equilibria (eqs 3 and 4) a Schlenk-type equilibrium according to eq 5 is also operative. n(XMg)2 (CH 2)x ⇆ [(XMg)2 (CH 2)x ]n

(3)

nMg(CH 2)x ⇆ [Mg(CH 2)x ]n

(4)

n(XMg)2 (CH 2)x ⇆ n MgX 2 + n Mg(CH 2)x

(5)

Figure 1. Molecular structure and numbering scheme of [(thf)2Mg(μCH2)5]2 (1). The ellipsoids represent a probability of 40%; H atoms are neglected for clarity reasons (color code: C gray, Mg green, O red). Symmetry-related atoms (−x+1, −y, −z+2) are marked with the letter “A”. Selected bond lengths (pm): Mg1−C1 215.40(14), Mg1−C5A 215.98(14), Mg1−O1 209.18(11), Mg1−O2 208.59(11), C1−C2 153.58(19), C2−C3 153.17(19), C3−C4 152.91(18), C4−C5 153.27(19); angles (deg): C1−Mg1−C5A 143.19(6), C1−Mg1−O1 106.82(5), C1−Mg1−O2 98.09(5), C5A−Mg1−O1 101.10(5), C5A− Mg1−O2 104.92(5), O1−Mg1−O2 91.16(5), Mg1−C1−C2 121.58(9), Mg1A−C5-C4 121.81(9).

The Mg−C bond lengths show significantly more accurate values of 215.40(14) and 215.98(14) pm and a large C−Mg−C angle of 143.19(6)°. This large value leads to a squeezed O− Mg−O angle of only 91.16(5)°. In a similar procedure we synthesized tetramethylenemagnesium8 from 1,4-dichlorobutane and magnesium chips. The diGrignard compound was formed with a yield of 80%. After addition of 1,4-dioxane (1,4-dioxane = diox), magnesium chloride was removed by filtration of the hot solution. At ambient temperature crystals of 2 formed in the filtrate. Filtration at room temperature led to significantly smaller yields because the product precipitated together with the dioxane adducts of MgCl2. The molecular structure of 2 clearly shows the reason for the extreme sparing solubility of this compound. The molecular structure and numbering scheme of [(thf)2Mg(μ-C4H8)]∞ (2) are displayed in Figure 2. The bis(thf) adduct of tetramethylene magnesium 2 crystallized with a zigzag chain structure with Mg−C bond lengths of 216.0(3) and 216.2(3) pm. Again the distortion of the tetrahedral environment of the magnesium atom is obvious, and a widened C−Mg−C of 136.26(14)° and a small O−Mg− O angle of 92.54(10)° are observed. The 13C{1H} NMR spectrum was recorded in [D8]THF at +50 °C due to the low solubility at ambient temperature and showed only two sharp resonances at δ = 7.7 (α-C) and 35.1 (β-C). It was not possible to deduce if aggregation equilibria fast on the NMR time scale were operative or if the solid-state structure is maintained in THF solution, but the investigations of Bickelhaupt and coworkers8 suggest a breakup into magnesiacycles by measuring the degree of association in THF at three temperatures. An enhanced amount of MgCl2 in solution led to an increased solubility, most probably due to formation of terminal MgCl

In order to limit the diversity of possible species in solution, the solid-state halide-poor magnesium complexes were prepared and equilibriums according to eqs 3 and 5 can be excluded. This approach allows studying the magnesiacycles shown in eq 4. In order to investigate the nature of these diGrignard reagents we varied (i) the number of methylene units (x = 4 and 5), (ii) the steric hindrance at the α-carbon atoms with substituents, (iii) the molar ratio of Mg2+ ions to alkanediyl chains, and (iv) the cations by partial exchange of magnesium by lithium yielding magnesiates. The magnesium complexes were studied in solution with NMR experiments and in the solid state with X-ray diffraction methods in order to deduce aggregation behaviors.

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RESULTS AND DISCUSSION To begin with, we prepared [(thf)2Mg(μ-C5H10)]2 (1) via direct synthesis from 1,5-dichloropentane and magnesium 7580

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Figure 2. Cutout of the chain structure of [(thf)2Mg(μ-CH2)4]∞ (2) with numbering scheme. The ellipsoids represent a probability of 40%; H atoms are omitted for clarity reasons (color code see Figure 1). Symmetry-related atoms are marked with the letters “A” (−x, −y+2, −z+1) and “B” (−x, −y+2, −z). Selected bond lengths (pm): Mg1− C1 216.2(3), Mg1−C3 216.0(3), Mg1−O1 208.9(2), Mg1−O2 211.3(2), C1−C2 154.4(5), C2−C2A 153.3(6), C3−C4 153.4(4), C4−C4B 151.9(6); angles (deg): C1−Mg1−C3 136.26(14), C1− Mg1−O1 109.36(13), C1−Mg1−O2 104.61(12), C3−Mg1−O1 103.05(12), C3−Mg1−O2 102.41(11), O1−Mg1−O2 92.54(10), Mg1−C1−C2 116.7(2), Mg1−C3−C4 118.8(2).

Figure 3. Molecular structure and numbering scheme of [(thf)2Mg(μC(CH3)2(CH2)2C(CH3)2]2 (3_m1). The ellipsoids represent a probability of 40%, H atoms are not drawn for clarity reasons (color code see Figure 1). Symmetry-related atoms (−x+2, −y, −z+1) are marked with the letter “A”. Selected bond lengths (pm): Mg1−C1 219.09(18), Mg1−C4A 218.96(18), Mg1−O1 209.69(13), Mg1−O2 209.72(13), C1−C2 153.6(2), C1−C5 153.1(3), C1−C6 153.0(3), C2−C3 152.9(2), C3−C4 153.9(2), C4−C7 153.5(3), C4−C8 152.2(3); angles (deg): C1−Mg1−C4A 122.86(7), C1−Mg1−O1 111.75(6), C1−Mg1−O2 107.71(6), C4A−Mg1−O1 112.61(7), C4A−Mg1−O2 106.61(6), O1−Mg1−O2 89.76(6), Mg1−C1−C2 111.33(11), Mg1−C4−C3 107.59(11).

moieties in a Schlenk-type equilibrium and a breakup of the chain into smaller units according to eq 6. n/x[Mg(μ‐C4 H8)]x + MgCl2 ⇆ [(ClMg)2 (μ‐C4H8)n Mg n − 1]

(6)

In order to investigate the influence of bulky carbanions on the structures, 2,5-dimethyl-2,5-dichlorohexane was reacted with magnesium according to eq 7. The di-Grignard reagent formed

carbanions. In the 1H-DOSY NMR experiments the major component shows a 1.43 faster diffusion, supporting a monomer−dimer equilibrium (see Supporting Information). In principle, the formation of magnesium magnesiates would offer another possibility for this observation. However, these solvent-separated complexes usually require stabilization with very strong multidentate chelating Lewis bases.21,22 Alkylsubstituted magnesiate anions can more easily be obtained and isolated with more electropositive counter-cations such as alkali metals, especially lithium, and appropriate chelating bases.21,23 Due to the fact that magnesium magnesiates were not isolated from homometallic di-Grignard solutions, we intended their preparation with more electropositive lithium countercations via a metathetical approach. The reaction of MgCl2(thf)1.5 with ethereal 1,4-dilithiobutane solution in a molar ratio of 1:1.33 yielded a magnesiate of the composition [Li(thf)4]2[Mg3(C4H8)4] (4) after removal of precipitated LiCl according to eq 8. Structure and numbering scheme of this solvent-separated ion pair are displayed in Figure 4.

with a yield of 53%. After removal of magnesium chloride, the bis(thf) complex of 2,5-dimethylhexane-2,5-diylmagnesium (3) crystallized at −20 °C from the filtrate. A second recrystallization gave halide-free 1,1,6,6-tetrakis(thf)-2,2,5,5,7,7,10,10-octamethyl-1,6-dimagnesiacyclodecane (3) with two crystal shapes (small rods and hexagonal platelets). In one modification the dimeric molecule [(thf)2Mg{C(Me)2C2H4C(Me)2}]2 (3) adopts crystallographic C2 symmetry; in the other this symmetry is missing. Nevertheless, both molecular structures are very similar. Figure 3 shows the molecular structure and numbering scheme of the C2-symmetric molecule. The sterically crowded carbanionic sites enhance the repulsion between these units and the thf ligands, leading to significantly smaller C−Mg−C angles of 122.86(7)° in comparison to 1 and 2, but this value is still significantly larger than the tetrahedron angle. The bulkiness of the carbanions also causes a small O−Mg−O angle of only 89.76(6)°. In addition, the Mg−C bonds are elongated and exhibit values of 218.96(18) and 219.09(18) pm. Contrary to 1 and 2 (see Experimental Section), two signal sets with an intensity ratio of 1:4 were observed for 3 at ambient temperature. Increasing temperature favors the major component. Several explanations are conceivable. Different ring sizes and aggregation degrees (monomers and dimers) or different coordination numbers at Mg due to loss of thf ligands cause chemically different

3MgCl2 + 4Li 2C4 H8 + 8thf → [Li(thf)4 ]2 [Mg 3(C4H8)4 ] + 6LiCl

(8)

Figure 4. Stru ctural mo tif of solvent-separated [Li(thf)4]2[Mg3(C4H8)4] (4) (color code see Figure 1, Li purple). 7581

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The cations consist of lithium atoms that are coordinated tetrahedrally by four thf ligands. The doubly negative anion is a trinuclear magnesiate with four butane-1,4-diide anions. All magnesium atoms show distorted tetrahedral environments of carbon atoms and are connected via common edges. The opposite edges of the middle tetrahedron around Mg2 are also edges of the outer tetrahedrons around Mg1 and Mg3. Due to this arrangement, the Mg2-bound methylene fragments occupy bridging positions between two magnesium atoms, whereas the outer magnesium atoms are also bound to two additional terminal methylene units. The Mg−Ct bond lengths to the terminally bound carbon atoms adopt expected values between 213.7(8) and 216.6(10) pm. The Mg2−Cbr bonds to the bridging carbon atoms show values between 224.1(7) and 228.8(7) pm, whereas the distances between the outer alkaline earth metal atoms (Mg1 and Mg3) and the bridging carbon atoms are significantly enhanced and values larger than 231.9(7) pm are observed. This fact might be the consequence of a combination of repulsive forces between the carbanionic sites and ring strain of the magnesiacyclopentane moieties. In trinuclear [{Me2Mg(μ-Me)2}2Mg]2− similar bonding situations were observed.24 The average Mg−Ct bond length of 216.3(3) pm is smaller than the Mg−Cbr value of 223.4(3) pm for the middle magnesium center; the largest distances were found between the outer Mg atoms and the bridging methyl groups (av 235.5(3) pm).24 In dinuclear [Et2Mg(μ-Et)2MgEt2]2− the average Mg−Ct and Mg−Cbr bond lengths of 222.4(9) and 236.2(9) pm, respectively, differ significantly, too.25 The 1H and 13C{1H} NMR spectra show only two signals despite the fact that the trinuclear magnesiate contains four chemically different methylene groups. The solvolysis equilibrium according to eq 9 offers an explanation for the results of the NMR experiments.

Figure 5. Molecular structure and numbering scheme of the contact ion pair [{(tmeda)Li}2Mg(C4H8)2] (5). The ellipsoids represent a probability of 40%; H atoms are neglected for clarity reasons (color code: C gray, Li purple, Mg green, N blue, O red). The asymmetric unit contains two molecules A and B; due to far-reaching similarity, only molecule A is represented. Selected bond lengths of molecule A [molecule B] (pm): Mg1−C1 222.0(2) [223.4(2)], Mg1−C4 221.3(2) [222.6(2)], Mg1−C5 222.7(2) [223.6(2)], Mg1−C8 224.7(2) [222.5(2)], Li−C1 2.300(2) [2.301(2)], Li1−C8 2.292(2) [2.286(2)], Li2−C4 2.332(2) [2.275(2)], Li2−C5 2.264(2) [2.276(2)], C1−C2 154.4(3) [154.1(3)], C2−C3 153.3(3) [153.4(3)], C3−C4 154.4(3) [154.4(3)], C5−C6 154.2(3) [153.9(3)], C6−C7 153.4(3) [153.6(3)], C7−C8 155.3(3) [154.3(3)]; angles (deg): C1−Mg1−C4 90.33(8) [87.93(8)], C1− Mg1−C5 120.35(9) [128.81(9)], C1−Mg1−C8 113.29(8) [112.23(8)], C4−Mg1−C5 111.16(8) [111.43(8)], C4−Mg1−C8 136.28(9) [134.10(8)], C5−Mg1−C8 88.71(8) [88.06(8)].

content of tmeda, as for example in crystalline [Li(tmeda)2]+[(tmeda)Li(μ-Bz)2Mg(Bz)2]− (Bz being benzyl),27 were not observed under these reaction conditions with butane1,4-diide anions. A heteroleptic magnesiate was prepared via addition of an ethereal 1,4-dilithiobutane solution to a suspension of the 1,4dioxane complex of diphenylmagnesium according to eq 11. After addition of tmeda a clear reaction mixture resulted and [{(tmeda)Li}2{(Ph)2Mg(C4H8)}] (6) crystallized at 5 °C. Dissolution of crystalline 6 in [D8]THF showed two resonance sets in the NMR spectra with an intensity ratio of 2:3. The major component consists of the heterobimetallic complex 6, whereas the aliphatic resonances of the minor component can unambiguously be assigned to [{(tmeda)Li}2{Mg(C4H8)2}] (5), suggesting an equilibrium as shown in eq 12. The proposed tetraphenylmagnesiate anion was already isolated as a salt with a ligated sodium counterion.28

[Mg 3(C4 H8)4 ]2 − + nTHF ⇆ [Mg(thf)n ]2 + + 2[Mg(C4 H8)2 ]2 −

(9)

In order to shift the equilibrium toward the anion [Mg(C4H8)2]2−, MgCl2(thf)1.5 was reacted with ethereal 1,4dilithiobutane solution in a molar ratio of 1:2, as shown in eq 10. After removal of precipitated LiCl tetramethylethylenediamine (tmeda) was added. Layering of the filtrate with nheptane allowed the crystallization of the contact ion pair [{(tmeda)Li}2{Mg(C4H8)2}] (5). The 1H and 13C{1H} NMR data of 5 showed high-field-shifted resonances at δ = −0.95 and δ = −7.8, respectively, in comparison to [(thf)2Mg{μ(CH2)4}]∞ (2), in agreement with the expectation for enhanced shielding due to the negative charge.

(diox)MgPh2 + Li 2C4 H8 + 2tmeda ⇆ [{(tmeda)Li}2 {(Ph)2 Mg(C4 H8)}] + diox

MgCl2 + 2Li 2C4 H8 + 2tmeda (10)

2[{(tmeda)Li}2 {(Ph)2 Mg(C4 H8)}]

Molecular structure and numbering scheme of 5 are displayed in Figure 5. The constitution is rather similar to [{(tmeda)Li}2{Mg(μ-Me)4}] with methyl bridges between magnesium and lithium cations (Mg−Cbr bond lengths between 223.1(7) and 229.5(8) pm).26 In 5 slightly smaller Mg−C distances between 221.3(2) and 224.7(2) pm were found leading to nonbonding Li···Mg contacts of 255.9(3) and 256.4(3) pm. Intramolecular electrostatic repulsion between the carbanions leads to a lengthening of the Mg−C bonds in comparison to chain-like [(thf)2Mg{μ-(CH2)4}]∞ (2) with values of approximately 216 pm. Derivatives with a larger

⇆ [{(tmeda)Li}2 {Mg(C4 H8)2 }]

⇆ [{(tmeda)Li}2 {Mg(C4 H8)2 }] + 2LiCl

+ [{(tmeda)Li}2 {Mg(Ph4)}]

(11)

(12)

The molecular structure and numbering scheme of 6 are represented in Figure 6. The molecule contains a distorted tetrahedrally coordinated magnesium center with similar average Mg−CPh and Mg−Calkyl bond lengths of 222.33(15) and 222.70(16) pm, respectively. The tetracoordinate lithium cations bind to an ipso-carbon atom of the phenyl group, the αatom of the butane-1,4-diide anion, and the nitrogen donor sites of the chelating tmeda ligand. Despite the fact that all 7582

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allows ring formation also with the shorter alkane-1,4-diides and alkyl-substituted 1,6-dimagnesiacyclodecane rings are observed in the crystalline state. In tetrahydrofuran solution monomeric bis(thf)-2,2,5,5-tetramethylmagnesiacyclopentane is the major component, and a temperature-dependent monomer−dimer equilibrium is observed. Addition of two further carbanions (another butane-1,4-diide ligand or two phenyl substituents) to magnesium butane-1,4diide yielding the magnesiates ([Mg(C4H8)2]2− and [(Ph)2Mg(C4H8)]2−) enhances intramolecular electrostatic repulsion, and hence, elongation of the Mg−C bonds is observed. Furthermore, the fact that all four substituents are negatively charged also leads to a reduction of the C−Mg−C angles, and now, even five-membered magnesiacycles can be formed as in the homoleptic magnesiate anions [Mg(C4H8)2]2− and [Mg3(C4H8)4]2− of 5 and 4 as well as in heteroleptic [(Ph)2Mg(C4H8)]2− of 6. The bond lengths of these homoleptic magnesiates are comparable to those of the methyl-substituted magnesiates, with the ring strain leading to distortions mainly for the bond angles. Stabilization and crystallization of these magnesiates require electropositive counter-cations and strong (preferably chelating) Lewis bases yielding either solvent-separated ions (4) or contact ion pairs (5 and 6).

Figure 6. Molecular structure and numbering scheme of the contact ion pair [{(tmeda)Li}2{(Ph)2Mg(C4H8)}] (6). The ellipsoids represent a probability of 40%; H atoms are omitted for clarity reasons (color code see Figure 5). Selected bond lengths (pm): Mg1− C1 223.55(16), Mg1−C4 221.85(15), Mg1−C5 222.67(15), Mg1− C11 221.99(15), Li1−C1 228.5(3), Li1−C5 233.5(3), Li2−C4 227.2(3), Li2−C11 233.8(3), C1−C2 154.7(2), C2−C3 153.2(2), C3−C4 154.4(2); angles (deg): C1−Mg1−C4 88.51(6), C1−Mg1− C5 107.67(6), C1−Mg1−C11 118.59(6), C4−Mg1−C5 120.04(6), C4−Mg1−C11 108.86(6), C5−Mg1−C11 111.79(6), C1−Li1−C5 102.47(11), Li1−C1−Mg1 70.50(8), Li1−C5−Mg1 69.72(8), C4− Li2−C11 103.08(11), Li2−C4−Mg1 71.46(8), Li2−C11−Mg1 70.20(8).

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under anaerobic conditions in an argon atmosphere using standard Schlenk techniques. The solvents were dried according to common procedures and distilled in an argon atmosphere; deuterated solvents were dried over sodium, degassed, and saturated with argon. The yields given are not optimized. 1H and 13C{1H} NMR spectra were recorded on Bruker AC 200 MHz, AC 400, or AC 600 spectrometers. Solutions of thf adducts in [D8]THF showed ligand exchange reactions fast on the NMR time scale, and therefore the thf ligands show characteristic chemical shifts of free molecules. Chemical shifts are reported in parts per million. 2,5-Dimethyl-2,5-hexanediol was purchsed from Alpha Aesar. 1,5-Dichloropentane and 1,4-dichlorobutane were supplied by Merck, dried with CaH2, and distilled in an argon atmosphere. The 1,4-dioxane complex of diphenylmagnesium32 and MgCl2(thf)1.533 were prepared according to literature protocols. 1,4-Dilithium butane1,4-diide was prepared from lithium sand and 1,4-dichlorobutane in diethyl ether.34 2,5-Dimethyl-2,5-dichlorohexane35 was prepared via the reaction of 73.1 g (0.50 mol) of 2,5-dimethyl-2,5-hexanediol at 0 °C with 200 mL of concentrated hydrochloric acid, collected by filtration, and washed with water until a neutral pH value was achieved. The residual solid was dissolved in chloroform and dried with anhydrous Na2SO4. After removal of CHCl3 in vacuo, pure product was isolated [yield: 90.6 g of 2,5-dimethyl-2,5-dichlorohexane, 99%; mp 66 °C (lit. mp35 68 °C); 1H NMR (200.1 MHz, CDCl3) δ 1.54 (12H, s, CH3), 1.89 (4H, s, CH2); 13 C{1H} NMR (50.3 MHz, CDCl3) δ 32.4 (CH3), 41.0 (C), 51.9 (CH2), 70.0 (C−Cl)]. Synthesis of [(thf)2Mg(μ-C5H10)]2 (1). The synthesis was performed according to a literature procedure.7 1,5-Dichloropentane (7.0 g, 50.0 mmol) was dropped into a suspension of 3.0 g (112.4 mmol) of magnesium turnings in 50 mL of THF. During this process the reaction flask was gently heated. After the heat evolution decreased the reaction mixture was heated under reflux for 3 h. The reaction solution was filtered, and titration of an aliquot documented a yield of 70%. 1,4-Dioxane (11.5 g, 130.5 mmol) was added to the stirred filtrate, and a colorless precipitate formed. After removal of all solids the solution was stored at 5 °C, which led to formation of 3.19 g of colorless crystals (26.7%) that were suitable for an X-ray structure determination. 1H NMR (400.2 MHz, [D8]THF): δ −0.65 (4H, t, 3J = 8 Hz, CH2−Mg), 1.25 (2H, m, CH2), 1.58 (4H, tt, 3J = 8 Hz, CH2). 13 C{1H} NMR (100.6 MHz, [D8]THF): δ 9.3 (CH2−Mg), 31.9

Mg−C distances are very much alike, the Li−CPh (av 233.7(3) pm) and Li−Calkyl bond lengths (av 227.9(3) pm) differ significantly, by approximately 5 pm. Similar findings were published for [{(tmeda)Li}2{Mg(μ-Me)4}]26 and [{(tmeda)Li(μ-Ph)2}2{Mg(μ-Ph)2Mg}],29 whereas for [(tmeda)Li(μPh)]2 much shorter Li−Ci bonds were found.22,30,31 The ipso-carbon atoms C5 and C11 are in nearly trigonal-planar environments neglecting the contacts to the lithium atoms (angle sums of C5: 359.5°, C11: 357.4°). Whereas Mg1 binds to the sp2-hybrid orbitals of C5 and C11 (which contain the negative charges), the lithium atoms coordinate to the porbitals of these carbon atoms (which also participate in the aromatic π-systems of the phenyl groups). Due to electrostatic repulsion between the free electron pair at the ipso-carbon atom and the neighboring Ci−Co bonds, small Co−Ci−Co angles of 113.54(14)° are observed. Distortions lead to significantly different distal and proximal Mg1−Ci−Co bond angles. The Mg−Calkyl−Li bonds can be understood as threecenter−two-electron bonds, which are common for electropositive metals of the first three main groups. In conclusion, refined preparative procedures were developed for the synthesis of magnesium alkane-1,4-diides and -1,5diides. These derivatives crystallize from THF solutions as dimeric [(thf)2Mg(μ-C5H10)]2 (1) and [(thf)2Mg(μ-(C(CH3)2C2H4C(CH3)2)]2 (3) or polymeric [(thf)2Mg(μC4H8)]∞ (2). Electrostatic repulsion between the carbanions causes very large C−Mg−C angles, which hinder the formation of monomeric four- and five-membered magnesiacycles such as magnesiacyclopentane and -cyclohexane. If the α,ω-alkanediides consist of five methylene units, the formation of a dimeric molecule is feasible and can be crystallized. However, if this dianion consists of only four methylene fragments, the formation of dimers causes severe ring strain and a chain-like polymeric structure is preferred in the solid state. Reduction of the C−Mg−C angles by introducing bulky groups in α-position 7583

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(CH2), 47.7 (CH2). 13C NMR (150.9 MHz, [D8]THF): δ 8.5 (CH2− Mg, 1J(C,H) = 104.1 Hz), 31.1 (CH2, 1J(C,H) = 151.7 Hz), 46.9 (CH2, br). Synthesis of [(thf)2Mg(μ-C4H8)]∞ (2). 1,4-Dichlorobutane (6.4 g; 50.4 mmol) was added dropwise to a suspension of 3.0 g (112.4 mmol) of magnesium chips in 50 mL of THF; during this process reaction heat evolved. After complete addition the reaction solution was refluxed for an additional 3 h. All solids were removed by filtration, and an aliquot was titrated with N/10 H2SO4 against phenolphthalein indicator, giving a yield of 80% with respect to starting 1,4dichlorobutane. Then 7.2 g (81.7 mmol) of 1,4-dioxane was added, during which the solution was vigorously stirred, leading to a colorless precipitate. The reaction mixture was heated nearly to the boiling point of the solvent, and this hot mixture was filtered with a frit covered with diatomaceous earth. The filter cake was washed twice with 20 mL of hot THF. On the basis of the alkalinity of the unified filtrates the yield was 60%. Due to the fact that the solution still contained chloride, precipitation was repeated twice with 3.5 and 2 mL of 1,4-dioxane. From the final filtrate 4.35 g of colorless pyrophoric crystals (38%) was collected, which was nearly insoluble in THF at rt but soluble at reflux temperature. Elemental analysis (C12H24MgO2, 224.6): found Mg 10.86, calcd Mg 10.82. Alkalinity: found 445.2 mg of H2SO4/g (calcd 436.6 mg H2SO4/g). 1H NMR (200.1 MHz, 50 °C, [D8]THF): δ −0.63 (4H, t br, CH2Mg), 1.68 (4H, t br, CH2). 13 C{1H} NMR (50.3 MHz MHz, 50 °C, [D8]THF): δ 7.7 (CH2Mg), 35.1 (CH2). Synthesis of [(thf)2Mg(μ-(C(CH3)2C2H4C(CH3)2)]2 (3). A solution of 20.0 g (109 mmol) of 2,5-dichloro-2,5-dimethylhexane in 150 mL of THF was slowly dropped over 5 h at rt into a suspension of 9.0 g (370 mmol) of magnesium turnings in 100 mL of THF. It is important that during this reaction the temperature of the water cooling bath does not exceed 30 °C. After complete addition the reaction mixture was refluxed for an additional hour. Excess of magnesium was removed by filtration, an aliquot of the filtrate was titrated with 0.1 N sulfuric acid, and a yield of 53% was achieved. Thereafter, 25 mL (284 mmol) of 1,4-dioxane was added to the vigorously stirred filtrate. The colorless precipitate was removed by filtration, and the alkalinity of the filtrate was determined via titration with 0.1 N sulfuric acid (yield: 41%). At −20 °C colorless crystals of [(thf)2Mg(μ-C(CH3)2C2H4C(CH3)2]2 grew besides traces of a fine, colorless powder. The volume of the mother liquor was reduced to a few milliliters, and a second crop of crystals precipitated. The crystals were collected on a frit with large pores in order to separate the cloudy mother liquor from the crystalline material. Recrystallization from THF gave halide-free crystals that easily lose coordinated thf, leading to dull crystals. Therefore, the substance has to be stored in a freezer to minimize loss of coordinated thf. Yield: 10.08 g (35.9%). Elemental analysis (C32H64Mg2O4, 561.4): found Mg 8.67, calcd Mg 8.66. Alkalinity: found 448.7 mg H2SO4/g, calcd 449.3 mg H2SO4/g. 1H NMR (200.1 MHz, [D8]THF, rt): δ 0.86 (3H, s, CH3, dimer), 0.90 (12H, s, CH3, monomer), 0.98 (1H, s, CH2, dimer), 1.02 (4H, s, CH2, monomer). 13 C{1H} NMR (50.3 MHz, [D8]THF): δ 19.7 (C−Mg), 23.3 (C− Mg), 33.5 (CH3), 34.5 (CH3), 50.8 (CH2), 53.4 (CH2). 1H NMR (200.1 MHz, [D8]THF, 50 °C): δ 0.86 (0.6H, s, CH3), 0.90 (14.4H, s, CH3), 0.98 (0.2H, s, CH2), 1.02 (3.8H, s, CH2). Synthesis of [Li(thf)4]2[Mg3(C4H8)4] (4). A suspension of 1.52 g (7.51 mmol) of (thf)1.5MgCl2 in 20 mL of THF was cooled to −78 °C, and then, 19.5 mL of a 0.52 M solution of 1,4-dilithiobutane (10.14 mmol) in diethyl ether was added. During a slow warmup of the reaction mixture to 0 °C the substrates reacted and a LiCl-containing suspension formed. Precipitated LiCl was removed by filtration, and the solvent was distilled off the filtrate. The dry residue was dissolved in THF and layered with n-heptane. At −40 °C colorless crystals of 4 grew, and a small amount of an oil separated from this two-phase system. At rt the crystals melted, and therefore, exact determination of the yield and accomplishment of elemental analysis were challenging. 1 H NMR (200.1 MHz, rt, [D8]THF): δ −0.64 (8H, br, CH2Mg), 1.73 (8H, br, CH2). 13C{1H} NMR (50.3 MHz, rt, [D8]THF): δ 7.6 (CH2Mg), 34.0 (CH2).

Synthesis of [{(tmeda)Li}2Mg(C4H8)2] (5). Solid (thf)1.5MgCl2 (1.18 g, 5.83 mmol) was added to a 23.5 mL of a cooled (−78 °C) and stirred 0.52 M solution of 1,4-dilithiobutane (12.22 mmol) in diethyl ether. During the slow warmup procedure the starting materials reacted and a LiCl-containing suspension formed. The precipitate was removed by filtration, and 1.50 g (12,90 mmol) of TMEDA was added to the filtrate. A second filtration and storage at −40 °C led to the precipitation of 1.25 g of colorless crystals of 5 (56%), which were collected on a Schlenk frit and dried in vacuo. This compound is extremely sensitive toward air and moisture. Elemental analysis (C20H48Li2MgN4, 382.8): found Mg 6.42, calcd 6.35. 1H NMR (200.1 MHz, [D8]THF): δ −0.90 (8H, t br, CH2Mg), 1.68 (8H, br, CH2), 2.16 (12H, s, NCH3), 2.31 (4H, s, NCH2). 13C{1H} NMR (50.3 MHz, [D8]THF): δ 7.8 (CH2Mg), 34.1 (CH2), 46.1 (NCH3), 58.7 (NCH2). Synthesis of [{(tmeda)Li}2{(Ph)2Mg(C4H8)] (6). A stirred suspension of 1.70 g (6.37 mmol) of Ph2Mg(diox) in 10 mL of diethyl ether was cooled to 0 °C, and 12.5 mL (6.50 mmol) of a 0.52 M solution of 1,4-dilithiobutane was added at once. During this process the magnesium complex dissolved, and thereafter a colorless precipitate formed. This solid dissolved after addition of 1.65 g (14.2 mmol) of TMEDA. After filtration the filtrate was stored at 5 °C, leading to 2.27 g of large colorless crystals of 6 (74%), which were dried in vacuo to remove adherent solvent. Elemental analysis (C28H50Li2MgN4, 480.8): found Mg 5.08, calcd Mg 5.05. 1H NMR (400.1 MHz, [D8]THF): δ −0.89 (1.6H, t br, CH2Mg), −0.61 (2.4H, t br, CH2Mg), 1.69 (1.6H, t br, CH2), 2.00 (2.4H, t br, CH2), 2.12 (24H, s, NCH3), 2.28 (8H, s, NCH2), 6.8−7.0 (6H, br, Ph), 7.92 (4H, Ph). 13C{1H} NMR (100.6 MHz, [D8]THF): δ 7.8 (CH2Mg), 10.3 (CH2Mg), 34.1 (CH2), 34.7 (CH2), 46.1 (NCH3), 58.5 (NCH2), 122.9 (CH, br), 125.1 (CH), 125.9 (CH), 142.9 (CH, br), 143.9 (CH), 172.8 (i-C), 179 (i-C, br). 13C NMR (100.6 MHz, [D8]THF): δ 7.8 (t, 1J(1H13C) = 102.6 Hz, CH2Mg), 10.3 (t, 1J(1H13C) = 103.2 Hz, CH2Mg), 34.1 (t, 1J(1H13C) = 120.7 Hz, CH2), 34.7 (t, 1J(1H13C) = 120.7 Hz, CH2), 46.1 (q, 1J(1H13C) = 134.1 Hz, NCH3), 58.5 (t, 1 1 13 J( H C) = 131.6 Hz, NCH2), 122.9 (d, 1J(1H13C) ≈ 158 Hz, br, CH), 125.1 (d, 1J(1H13C) = 153.9 Hz, CH), 125.9 (d, 1J(1H13C) = 154.8 Hz, CH), 142.9 (d, br, CH), 143.9 (d, 1J(1H13C) = 153.9 Hz, CH), 172.8 (s, i-C), 179 (br, i-C). 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 but not for absorption effects.36,37 The structures were solved by direct methods (SHELXS38) and refined by full-matrix least-squares techniques against Fo2 (SHELXL9738). The hydrogen atoms of 1, 3_m1, and 6 were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. All nondisordered, non-hydrogen atoms were refined anisotropically.38 The crystals of 4 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 publish only 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 (see Supporting Information). XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for structure representations.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC-900495 for 1, CCDC900496 for 2, CCDC-900497 for 3_m1, CCDC-900498 for 3_m2, CCDC-900499 for 5, and CCDC-900500 for 6. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [e-mail: 7584

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(b) Holloway, C. E.; Melnik, M. J. Organomet. Chem. 1994, 465, 1− 63. (c) Bickelhaupt, F. In Grignard Reagents: New Developments; Richey, H. G., Ed.; Wiley: Chichester, 2000; Chapter 9, pp 299−328. (d) Jastrzebski, J. T. B. H.; Boersma, J.; van Koten, G. In The Chemistry of Organomagnesium Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, 2008; Chapter 1, pp 1−99. (22) Weiss, E. Angew. Chem. 1993, 105, 1565−1740; Angew. Chem., Int. Ed. Engl. 1993, 32, 1501−1523. (23) Baillie, S. E.; Clegg, W.; García-Á lvarez, P.; Hevia, E.; Kennedy, A. R.; Klett, J.; Russo, L. Organometallics 2012, 31, 5131−5142. (24) Viebrock, H.; Behrens, U.; Weiss, E. Angew. Chem. 1994, 106, 1364−1365; Angew. Chem., Int. Ed. Engl. 1994, 33, 1257−1259. (25) Squiller, E. P.; Whittle, R. R.; Richey, H. G. J. Am. Chem. Soc. 1985, 107, 432−435. (26) Greiser, T.; Kopf, J.; Thoennes, D.; Weiss, E. Chem. Ber. 1981, 114, 209−213. (27) Schubert, B.; Weiss, E. Chem. Ber. 1984, 117, 366−375. (28) Geissler, M.; Kopf, J.; Weiss, E. Chem. Ber. 1989, 122, 1395− 1402. (29) Thoennes, D.; Weiss, E. Chem. Ber. 1978, 111, 3726−3731. (30) Thoennes, D.; Weiss, E. Chem. Ber. 1978, 111, 3157−3161. (31) For reviews on organyllithium complexes see: (a) Setzer, W. N.; Schleyer, P. v. R. Adv. Organomet. Chem. 1985, 24, 353−451. (b) Stey, T.; Stalke, D. In The Chemistry of Organolithium Compounds; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, 2004; Chapter 2, pp 47−120. (c) Gossage, R. A.; Jastrzebski, J. T. B. H.; van Koten, G. Angew. Chem. 2005, 117, 1472−1478; Angew. Chem., Int. Ed. 2005, 44, 1448−1454. (d) Coles, M. P. Curr. Org. Chem. 2008, 12, 1220−1230. (e) Gessner, V. H.; Däschlein, C.; Strohmann, C. Chem.Eur. J. 2009, 15, 3320−3334. (32) Gärtner, M.; Fischer, R.; Langer, J.; Görls, H.; Walther, D.; Westerhausen, M. Inorg. Chem. 2007, 46, 5118−5124. (33) Heyn, B.; Hipler, B.; Kreisel, G.; Schreer, H.; Walther, D. Anorganische Synthesechemie: Ein Integriertes Praktikum, 2nd ed.; Springer: Berlin; 1986; pp 13−15. (34) Fischer, R.; Görls, H.; Westerhausen, M. Angew. Chem. 2009, 121, 10143−10146; Angew. Chem., Int. Ed. 2009, 48, 9958−9961. (35) Miller, S. A.; Bercaw, J. E. Organometallics 2004, 23, 1777−1789. (36) COLLECT, Data Collection Software; Nonius B.V.: The Netherlands, 1998. (37) Otwinowski, Z.; Minor, W. Processing of X-Ray Diffraction Data Collected in Oscillation Mode. In Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A; Carter, C. W.; Sweet, R. M., Eds.; Academic Press: New York, 1997; pp 307−326. (38) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.

[email protected]]. In addition, the table containing details of the crystal structure determinations, the NMR spectra of all reported complexes, and the DOSY experiment of 3 are available. 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 (0) 3641 948102. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Research Foundation (DFG, Bonn/ Germany) and the Friedrich Schiller University (Jena, Germany) for financial support. We are grateful to Dr. M. Friedrich for the measurement and interpretation of the DOSY experiments of complex 3.

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REFERENCES

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