Structure–Solubility Relationship of 1,4‐Dioxane Complexes of Di(hydrocarbyl)magnesium

n‐Alkylmagnesium–dioxane adducts

We first studied the effect of the chain length of alkyl groups on the properties and molecular structures of [R₂Mg(dx)_n] after precipitation and removal of the magnesium halides. The simplest derivative is dimethylmagnesium.16 Isolation of this compound succeeded by the precipitation of magnesium bromide with 1,4‐dioxane in commercially available solutions of methylmagnesium bromide in THF. After removal of the solvents, the sparingly soluble residue was recrystallized from a mixture of toluene and 1,4‐dioxane to yield colorless and highly pyrophoric crystals of the coordination polymer [Me₂Mg(μ‐dx)]_∞ (1′‐Me) depicted in Figure 1.

The crystal structure consists of parallel zigzag chains of MgMe₂ units bridged by dioxane ligands with chair conformations. The dx ligands are arranged in a face‐to‐face manner. This arrangement leads to a zigzag chain with a larger amplitude than observed for the coordination polymers of the type [R₂Mg(μ‐dx)]_∞ with R=Et, Ph, Cy, and iPr. The Mg⋅⋅⋅Mg distance of 889.14(3) pm between the first and third MgMe₂ moieties in 1′‐Me is significantly smaller than in other magnesium congeners of this type with values of approximately 1150–1240 pm.17 In all these complexes the magnesium‐bound dx ligands with chair conformations are arranged back‐to‐back (Scheme 4). The nonbonding Mg⋅⋅⋅Mg distance of 690.5(2) pm between neighboring magnesium atoms is determined by the dx ligands and is very similar for all derivatives with a strand structure.18 Due to the larger spacing of the MgR₂ units with dioxane ligands arranged back‐to‐back, this is the favored structure for larger R groups.

Complex 1′‐Me is only very sparingly soluble in diethyl ether, but readily dissolves in THF and is soluble in warm 1,4‐dioxane. From a pure dioxane solution, again strand‐like 1′‐Me crystallized and a complex containing more dioxane was not observed. The NMR spectra in [D₈]THF solution show the resonances for the magnesium‐bound methyl groups at δ(¹H)=−1.82 ppm and δ(¹³C)=−16.5 ppm (¹ J _C,H=105.6 Hz). In this solvent, the strand structure is deaggregated by the substitution of dx ligands by THF Lewis bases.

The addition of 1,4‐dioxane to an ethereal solution of ethylmagnesium bromide and removal of the precipitated magnesium bromides yielded the known strand structure [Et₂Mg(μ‐dx)]_∞ (1′‐Et).8a This compound is only soluble in diethyl ether or benzene if dioxane has been added. The strand structure [Et₂Mg(μ‐dx)]_∞ crystallized again from a solvent mixture of dioxane and diethyl ether (ratio of 5:1).

The dioxane adduct of di(n‐propyl)magnesium is soluble in diethyl ether and this complex crystallized again within several days from a 2.0 m solution of dioxane and diethyl ether (ratio of dx and Et₂O of approx. 1:1) at −40 °C to yield colorless crystals of [nPr₂Mg(μ‐dx)]_∞ (1′‐nPr), as shown in Figure 2.

The crystal structure of 1′‐nPr also consists of parallel zigzag chains of (nPr)₂Mg moieties bridged by 1,4‐dioxane bases. However, the dioxane ligands show a face‐to‐back arrangement, as depicted in Scheme 4. In comparison with 1′‐Me, this orientation of the dx ligands leads to an elongation of the distance between the first and third magnesium atoms to 1080.96(8) pm. The nonbonding Mg1⋅⋅⋅Mg1A distance between neighboring magnesium atoms is 698.7(1) pm.

Contrary to the shorter n‐alkyl complexes, (nBu)₂Mg did not crystallize from a 4.0 m ethereal solution at −40 °C. At −78 °C and upon layering with n‐pentane, an amorphous solid with the composition [nBu₂Mg(dx)] precipitated. The longer alkyl groups significantly enhances the solubility in ethereal solvents, thereby causing a deterioration in its crystallization behavior. Probably, the longer alkyl groups do not fit as well between the chains and smaller degrees of aggregation are realized.

1,4‐Dioxane adducts of (Me_3−xH_xC)₂Mg

We next investigated the effect of the degree of substitution on the molecular structures of the dx adducts of (Me_3−xH_xC)₂Mg (x=3: Me; 2: Et; 1: iPr; 0: tBu). The simplest magnesium complex with secondary alkyl groups is di(isopropyl)magnesium, [iPr₂Mg(μ‐dx)]_∞ (1′‐iPr), which has already been studied by Blasberg et al.8c Another complex with a larger dx content was accessible during crystallization in a more dioxane‐rich solution of isopropylmagnesium chloride; the molecular structure of this derivative, [{iPr₂Mg(dx)}₂(μ‐dx)] (1.5‐iPr), is depicted in Figure 3.

The structure is of type 1.5 and the Mg−C and Mg−O bond lengths are of the same order of magnitude as those observed for the strand structure.8c Contrary to this finding, the bond angles show significant differences. Thus, the C−Mg−C bond angles of 128.7(1) and 130.4(1)° are larger than the value of 122.19(9)° in the strand structure, and the O−Mg−O angles of 99.90(7) and 99.68(7)° are more acute in comparison with the O−Mg−O bond angle of 104.37(7)° in [iPr₂Mg(μ‐dx)]_∞ (1′‐iPr). Thus, for the complexes with bulky isopropyl groups, two structures, 1′ and 1.5, are observed depending on the dioxane concentration in the mother liquor, whereas for the n‐propyl congeners the coordination polymer 1′ with exclusively bridging dioxane ligands seems to be favored.

To further increase the bulkiness of the alkyl group, we studied the tert‐butylmagnesium complex. After precipitation and removal of [MgCl₂(dx)₂] from an ethereal solution of tert‐butylmagnesium chloride we crystallized [tBu₂Mg(η¹‐dx)₂] (2‐tBu). The structure of this compound is depicted in Figure 4.

This complex is the fifth derivative of type 2. This structure type forms with sterically demanding groups and in the presence of a moderate excess of 1,4‐dioxane during crystallization. The Mg−C bond lengths of dialkylmagnesium increase in the order primary (213–215 pm) < secondary (214–216 pm) < tertiary alkyl groups (216–218 pm), whereas the C−Mg−C bond angles show no clear trend.

The crystalline 2‐tBu partially loses ligated dioxane during the drying process in vacuo. We did not isolate this dioxane‐poor congener, but structures of tert‐butylmagnesium complexes with dioxane ligands have already been observed in the heteroleptic [L₂{Mg(tBu)}₄(μ‐dx)₂], in which L is a bidentate bridging ligand, and in the trinuclear compound [(thf)Mg(tBu)₂(μ‐dx)Mg(tBu)LMg(tBu)].19

Structures of [Ar₂Mg(dx)_n] with increasing hindrance at the ortho‐substituted phenyl groups

The coordination polymer [Ph₂Mg(μ‐dx)]_∞ (1′‐Ph) is insoluble in toluene. The addition of 1,4‐dioxane to this suspension led to a clear solution. Cooling of this solution again yielded the starting [Ph₂Mg(μ‐dx)]_∞ (1′‐Ph). Dissolution of these crystals in pure dioxane gave another complex that is highly soluble in dx. The clear crystals of this complex turned dull after isolation and removal of the mother liquor. The molecular structure of this dioxane adduct, [Ph₂Mg(dx)₃] (3‐Ph), is depicted in Figure 5.

The τ parameter, which is an indicator of the geometry of pentacoordinate metal complexes, can be calculated according to the equation τ=(β−α)/60° (in which β is the largest and α the second largest bond angle at the metal center) with τ=1 for an ideal trigonal bipyramid and τ=0 for a square pyramid.20 For 3‐Ph, a τ value of 0.66 was elucidated, which is much closer to a trigonal‐bipyramidal coordination sphere with O1 and O5 in apical positions (O1−Mg1−O5 164.68(4)°). Deviation from a linear arrangement is enforced by the steric repulsion between these dioxane ligands and the phenyl groups. The equatorial plane contains the ipso‐carbon atoms C1 and C7 and the Lewis base O3 (angle sum=359.73(5)°). The higher coordination number of the magnesium atom in 3‐Ph leads to elongated Mg−C and Mg−O bonds (Mg−C 217.19(14) and 216.01(13) pm, Mg−O_axial 224.51(11) and 228.99(11) pm, Mg−O_equatorial 209.10(10) pm) in comparison with the strand structure [Ph₂Mg(μ‐dx)]_∞ (1′‐Ph; Mg−C 213.5(2) pm, Mg−O 208.1(2) and 206.1(2) pm) with tetracoordinate metal atoms.

The type 2 diphenylmagnesium complex [Ph₂Mg(dx)₂] is unknown even though Bickelhaupt and co‐workers were able to prepare the corresponding THF adduct [Ph₂Mg(thf)₂].2a

Ortho substitution with methyl groups increases the steric requirements of the aryl groups and a coordination number of 5 could not be achieved. Consequently, addition of dioxane to an ethereal solution of o‐tolylmagnesium bromide and removal of [MgBr₂(dx)₂] allowed the crystallization and isolation of [(oTol)₂Mg(dx)₂] (2‐oTol) but not a strand‐like structure of type 1′ as found for diphenylmagnesium. The molecular structure of 2‐oTol is depicted in Figure 6.

Whereas the magnesium atom in the phenyl derivative 3‐Ph has a coordination number of 5, the bulkier oTol group only allows tetracoordinate magnesium centers. The Mg−C bond lengths in the distorted tetrahedral complexes of type 2 increase with increasing steric pressure in the order phenyl (213.5(2) pm)<o‐tolyl (214.9(2)–215.6(2) pm)<2,4,6‐trimethylphenyl (mesityl, 216.7(2) pm).7 The C−Mg−C bond angles vary from 117.55(9)° (Mes) to 123.96(8)° (Ph) to 124.39(9)° (oTol).

The complex [(oTol)₂Mg(dx)₂] (2‐oTol) is soluble in warm toluene. From this solution, tetracoordinate type 1.5 magnesium centers were also accessible. The molecular structure of [{(oTol)₂Mg(dx)}₂(μ‐dx)] (1.5‐oTol) is shown in Figure 7. The o‐methyl substituents are disordered but crystallization as a type 1.5 complex was verified unequivocally. The two magnesium atoms are in distorted tetrahedral environments with a bridging dx ligand.

Dimesitylmagnesium (bis(2,4,6‐trimethylphenyl)magnesium) crystallized as [Mes₂Mg(dx)₂] (2‐Mes) from an ethereal solution of mesitylmagnesium bromide after addition of 1,4‐dioxane and removal of magnesium bromide.7 Furthermore, the structure of 2‐Mes is comparable to the structures of the thf adducts of dimesityl‐ ([Mes₂Mg(thf)₂]) and bis(2,4,6‐triisopropylphenyl)magnesium ([Trip₂Mg(thf)₂]).21 Contrary to this finding, bis[2,4,6‐tri(tert‐butyl)phenyl]magnesium forms no stable adducts with tetrahydrofuran.15b

1,4‐Dioxane adducts of coordination polymers with layer structure

After the investigation of di(hydrocarbyl)magnesium complexes with sp³‐ (alkyl) and sp²‐hybridized (aryl) carbon atoms, we also studied congeners with an sp‐hybridized carbon atom. Compounds of the type [(R‐C≡C)₂Mg(dx)_n] (n=1.5 (SiMe₃) and 2 (Ph)) are nearly insoluble in diethyl ether, dioxane, and toluene, but soluble in THF. From such a solution, [(Ph‐C≡C)₂Mg(thf)₄] was isolated with a distorted octahedral environment of the magnesium center.22

For solubility reasons we prepared an alkynyl complex by the deprotonation of trimethylsilylacetylene with [nPr₂Mg(μ‐dx)]_∞ (1′‐nPr) in excess dioxane to yield a compound with the composition [(Me₃Si‐C≡C)₂Mg(dx)_1.5]. This complex is soluble in THF but only very sparingly soluble in dioxane and insoluble in hydrocarbons, which is quite unique for a complex of type 1.5. The ¹³C{¹H} NMR spectrum shows two characteristic resonances at δ=112.1 and 158.5 ppm for the ethynyl fragment. To grow single crystals of this complex we stored a mixture of [nBu₂Mg(μ‐dx)] (1‐nBu) and trimethylsilylacetylene in 1,4‐dioxane at 5 °C. This procedure allowed the slow crystallization of [(Me₃Si‐C≡C)₂Mg(dx)_1.5]_∞ (1.5′‐C₂SiMe₃), but the crystal quality was very poor. Therefore, we could only elucidate a structural motif. The structure is depicted in Figure 8.

Despite poor crystal quality and a substandard data set, the structural motif was deduced unequivocally and revealed the solid‐state structure. The pentacoordinate magnesium atoms are embedded in trigonal‐bipyramidal environments with the alkynyl groups in apical positions. The 1,4‐dioxane ligands with chair conformations occupy equatorial positions and act as bridging ligands between magnesium atoms, leading to a layer structure. As a consequence of this packing, the magnesium atoms form a layer structure of regular six‐membered rings. The alkynyl ligands are bound above and below the layer and establish borders between two layers. Okuda and co‐workers23 determined the structure of [(all)₂Mg(thf)(μ‐dx)]_∞ (all=allyl); the slim allyl groups here would have also allowed the formation of a layer structure, but two‐dimensional (2D) binding is prevented by the presence of the thf ligands instead of dx molecules. Therefore, a strand structure is observed in the crystalline state. A 2D network was found for the heterobimetallic compound [{M(all)₃Mg(all)₂}₂(μ‐dx)₅]_∞ with M=Y and La.24 In [(Me₃Si‐C≡C)₂Mg(thf)₄], all the dx ligands are substituted by thf ligands leading to the breakup of the coordination polymer to yield discrete molecular complexes with hexacoordinate magnesium atoms.25

The metalation of phenylacetylene with bis(dioxane)di(tert‐butyl)magnesium (2‐tBu) in diethyl ether yielded quantitatively [(Ph‐C≡C)₂Mg(dx)₂]_∞ (2′‐C₂Ph), which is nearly insoluble in 1,4‐dioxane. The NMR spectra reveal that two dioxane molecules are bound to the magnesium center. The crystal structure of [(Ph‐C≡C)₂Mg(dx)₂]_∞ (2′‐C₂Ph) verifies that the magnesium atoms are embedded in octahedral environments forming a 2D network. The structure is depicted in Figure 9. The phenylethynyl groups are bound above and below the layer formed by the magnesium atoms and the bridging ether molecules.

Finally, we also investigated the 1,4‐dioxane adducts of magnesium chloride and bromide, which precipitated during the addition of dioxane to solutions of Grignard reagents. Extremely sparingly soluble [MgBr₂(dx)₂]_∞ (2′‐Br) was tempered in warm tetrahydrofuran leading to a single crystalline solid. This observation is in agreement with the expectation that THF and dx exhibit comparable basicity leading to very low concentrations of soluble [MgBr₂(thf)₄], which has been characterized previously.26, 27 This procedure allowed us to grow single crystals of sufficient quality for X‐ray crystal‐structure determination. Homologous [MgCl₂(dx)₂]_∞ (2′‐Cl) exhibited comparable crystallization properties.

The molecular structure of the coordination polymer [MgBr₂(dx)₂]_∞ (2′‐Br) with a layer structure is depicted in Figure 10; the analogous structure of [MgCl₂(dx)₂]_∞ (2′‐Cl) is presented in the Supporting Information. The magnesium centers in 2′‐Br are embedded in octahedral environments with trans‐arranged halide ions. The environments of the magnesium centers are very similar to that in the molecular tetrahydrofuran adduct [MgBr₂(thf)₄] with average Mg−O and Mg−Br bond lengths of 216 and 262.5 pm, respectively.26, 27 In the coordination polymer [Mg(μ‐Br)₂(thf)₂]_∞, which has been crystallized from a mixture of dichloromethane and hexane, the bromine atoms occupy bridging positions leading to a coordination polymer with rather similar Mg−O and Mg−Br distances of 212.6 and 263.3 pm, respectively.27

The Mg(dx) networks of the layer structures of [(Me₃Si‐C≡C)₂Mg(dx)_1.5]_∞ (1.5′‐C₂SiMe₃) and [MgBr₂(dx)₂]_∞ (2′‐Br) are compared in Figure 11. The groups R are positioned above and below the magnesium atoms, leading to penta‐ and hexacoordinated metal centers, respectively. The steric requirements of the R groups in 1.5′‐C₂SiMe₃ lead to a honeycomb structure, whereas the smaller bromine atoms in 2′‐Br lead to a tessellated structure. The directing influence exerted by the steric demand of the groups R is evident in Figure 11, in which the same Mg⋅⋅⋅Mg distances are indicated by the solid lines (symbolizing the bridging dx ligands).

Substitution of dx in [MgBr₂(dx)₂]_∞ (2′‐Br)

Due to the fact that [MgBr₂(dx)₂]_∞ (2′‐Br) is nearly insoluble in common organic solvents, several attempts were undertaken to recrystallize this coordination polymer. Crystallization of 2′‐Br from a solvent mixture of DMF and dioxane led to the precipitation of [{(dmf)₆Mg}Br₂] (A). The molecular structure contains a centrosymmetric cation with a hexacoordinate metal center and clearly separated bromide ions (Mg⋅⋅⋅Br 527.7(3) and 678.2(3) pm, see the Supporting Information). Traces of water also led to the precipitation of a few crystals of the more soluble [{trans‐(H₂O)₂Mg(dmf)₄}Br₂]_∞ (B, see the Supporting Information). In this latter centrosymmetric molecule, the magnesium atom is in an octahedral environment, coordinated to the oxygen donors of the dmf ligands (Mg−O 202.86(10) and 208.26(10) pm) and water (Mg−O 209.89(11) pm). The bromide ions are again separated from the cations (Mg⋅⋅⋅Br 480.8 and 493.8 pm), but are bound at the periphery through hydrogen bridges to the water ligands (Br⋅⋅⋅H 248.6 and 252.1 pm). This mode of coordination leads to the formation of a coordination polymer with a strand structure. Extraction of [MgBr₂(dx)₂]_∞ (2′‐Br) with hot 1,2‐dimethoxyethane (dme) only allowed the isolation of the already known [cis‐Br₂Mg(dme)₂] (C).28 Tridentate diglyme also forms very stable molecular complexes with MgBr₂, namely [(diglyme)Mg(Br)(μ‐Br)]₂ (D) and [(diglyme)(thf)MgBr₂] (E) with distorted octahedrally coordinated metal centers.29

General

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. The magnesium contents of the compounds were determined by complexometric titrations with Eriochrom Black T.30 The alkalinities of the solid diorganylmagnesium compounds were determined after hydrolysis of a specific amount in ice/water by acidimetric titration with 0.1 n H₂SO₄ against phenolphthalein. ¹H, ¹³C{¹H}, and ²⁹Si{¹H} NMR spectra were recorded on a Bruker AC 400 spectrometer at given temperatures. Chemical shifts are reported in parts per million (ppm, δ scale) relative to the residual signal of the solvent.31 Due to fast [D₈]THF/dx exchange reactions in solution, only resonances of noncoordinated dioxane were detected in the NMR spectra.

Trimethylsilylacetylene (98 %) and tert‐butyl chloride (95–98 %) were supplied by ABCR GmbH, 1.0 m phenylmagnesium bromide solution in THF by Merck, o‐bromotoluene (98 %) by Lancaster, isopropyl chloride by Fluka, n‐butyl chloride by Riedel de Haën, n‐propyl bromide by Acros, and 3.0 m methylmagnesium chloride solution in THF by Sigma–Aldrich.

Synthesis and characterization

Synthesis of [(CH₃)₂Mg(μ‐dx)]_∞ (1′‐Me): A solution of commercial 3.0 m methylmagnesium chloride in THF (17 mL, 51 mmol) was diluted with THF (50 mL). The slow addition of 1,4‐dioxane (18 mL, 204 mmol) led to the precipitation of [MgCl₂(dx)₂]. The reaction mixture was stored overnight at room temperature and then filtered through a frit covered with diatomaceous earth. A yield of 87 % was determined by titration of an aliquot of the filtrate with 0.1 n HCl. All volatiles were removed in vacuo and the residue evacuated until dryness. The residue was dissolved in dioxane (30 mL) and this suspension was heated at reflux for 10 min. Then, the hot solution was filtered through a Schlenk frit covered with diatomaceous earth. During cooling of the filtrate colorless crystals precipitated. These crystals were collected on a frit, washed with diethyl ether, and dried in vacuo. Yield: 1.3 g of 1′‐Me (35.8 % relative to CH₃MgCl). During hydrolysis of this compound with ice/water, inflammation occurred. Therefore, a specific amount of this compound was dissolved in THF and this solution was carefully hydrolyzed to quantitatively determine the metal content. Alkalinity: calcd: 688.3 mg H₂SO₄ g⁻¹; found: 691.8 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=−1.82 (s, 6 H; Mg‐CH₃), 3.54 ppm (s, 8 H; dx); ¹³C NMR (100.6 MHz, [D₈]THF): δ=−16.5 (q, ¹ J _CH=105.6 Hz, Mg‐CH₃), 67.9 ppm (t, ¹ J _CH=142.3 Hz, O‐CH₂, dx); elemental analysis calcd (%) for C₆H₁₄MgO₂ (142.5): Mg 17.06; found: Mg 16.98.

Synthesis of [(n‐C₃H₇)₂Mg(μ‐dx)]_∞ (1′‐nPr): A solution of n‐propylmagnesium bromide was prepared from magnesium turnings (3.0 g, 123.4 mmol) and n‐propyl bromide (12.4 g, 100.8 mmol) in diethyl ether (100 mL; yield: 82 %). 1,4‐Dioxane (16 mL, 182 mmol) was then added dropwise to this reaction mixture. During this highly exothermic procedure a colorless precipitate formed. After resting overnight the precipitate was removed by means of a Schlenk frit covered with diatomaceous earth. An aliquot of the filtrate was titrated with 0.1 m HCl (yield: 64 %). The volume of the solution was reduced to a fifth of the original volume and stored in a refrigerator at −40 °C. The colorless crystals of 1′‐nPr precipitated from this mother liquor were washed with very cold diethyl ether and dried in vacuo. Yield: 1.1 g of 1′‐nPr (11.0 % relative to n‐propyl bromide); alkalinity: calcd: 493.9 mg H₂SO₄ g⁻¹; found: 486.5 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=−0.64 (t, J=7.8 Hz, 4 H; CH₂Mg), 0.87 (t, J=7.2 Hz, 6 H; CH₃), 1.54 (m, 4 H; CH₂), 3.55 ppm (s, 8 H; dx); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=12.2 (CH₂Mg), 23.7 (CH₃), 24.3 (CH₂), 67.8 ppm (dx); elemental analysis calcd (%) for C₁₀H₂₂MgO₂ (198.6): Mg 12.24; found: Mg 12.07.

Synthesis of [(n‐C₄H₉)₂Mg(dx)] (1‐nBu): A solution of n‐butylmagnesium chloride was prepared in diethyl ether (200 mL) from magnesium turnings (6.0 g, 0.247 mol) and n‐butyl chloride (19.5 g, 0.211 mol; yield: 82 %). 1,4‐Dioxane (22 mL, 0.25 mol) was then added dropwise to this solution. During this highly exothermic procedure a colorless solid precipitated. After standing overnight the precipitate was removed by means of a Schlenk frit covered with diatomaceous earth. An aliquot of the filtrate was titrated with 0.1 m HCl (yield: 59 %). All volatiles were removed and the residue dried in vacuo. Then diethyl ether (10 mL) was added and this solution was stored at −78 °C. A colorless amorphous precipitate formed that was collected and dried in vacuo. Yield: 2.4 g of 1‐nBu (10.3 % relative to n‐butyl chloride); alkalinity: calcd: 432.7 mg H₂SO₄ g⁻¹; found: 422.9 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=−0.70 (t, J=8.0 Hz, 4 H; CH₂Mg), 0.80 (t, J=7.3 Hz, 6 H; CH₃), 1.19 (m, 4 H; γ‐CH₂), 1.47 (m, 4 H; β‐CH₂), 3.54 ppm (s, 8 H; dx); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=8.2 (CH₂Mg), 14.7 (CH₃), 32.4 (CH₂), 34.0 (CH₂), 67.8 ppm (dx); elemental analysis calcd (%) for C₁₂H₂₆MgO₂ (226.6): Mg 10.73; found: Mg 10.49.

Synthesis of [{(iPr)₂Mg(η¹‐dx)}₂ ( μ‐dx)] (1.5‐iPr): A 1.5 m isopropylmagnesium chloride solution was prepared from magnesium turnings (6.0 g, 0.247 mol) and isopropyl chloride (24.0 g, 0.195 mol) in diethyl ether (100 mL; yield: 55 %). 1,4‐Dioxane (28 mL, 0.318 mol) was then slowly added at room temperature. During this procedure a colorless solid precipitated. This suspension was stored overnight. Then, the precipitate was removed by means of a Schlenk frit covered with diatomaceous earth and washed with diethyl ether. The filtrate contained the product (yield: 34 %). At −20 °C, [{(iPr)₂Mg(dx)}₂(μ‐dx)] precipitated as colorless, air‐ and moisture‐sensitive crystals. Yield: 7.8 g of 1.5‐iPr (33 % relative to isopropyl chloride); alkalinity: calcd: 404.2 mg H₂SO₄ g⁻¹; found: 396.9 mg. ¹H NMR (400.1 MHz, [D₈]THF): δ=−0.40 (sept, J=7.8 Hz, 4 H; Mg‐CH), 1.28 (d, J=7.8 Hz, 24 H; CH₃), 3.61 ppm (s, 24 H; dx); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=10.0 (Mg‐CH), 26.3 (CH₃), 67.8 ppm (dx); elemental analysis calcd (%) for C₂₄H₅₂Mg₂O₆ (485.3): Mg 10.02; found: Mg 9.83.

Synthesis of [{(oTol)₂Mg(η¹‐dx)}₂(μ‐dx)] (1.5‐oTol): 2‐oTol (1.8 g, 4.70 mol; see below for the preparation of 2‐oTol) was suspended in toluene (10 mL) and the mixture stirred and warmed until a clear solution formed. Then, the stirring was finished and the solution kept in a warm water bath. Colorless crystals of the product precipitated overnight. The crystals were collected on a Schlenk frit, washed with diethyl ether, and dried in vacuo. Yield: 1.40 g of colorless crystals of 1.5‐oTol (88 % with respect to the initially used 2‐oTol). The compound is highly soluble in THF but sparingly soluble in toluene and diethyl ether. Alkalinity: calcd: 289.5 mg H₂SO₄ g⁻¹; found: 278.9 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=2.38 (s, 12 H; CH₃), 3.54 (s, 24 H; dx), 6.78–6.79 (t, 4 H; 4‐H), 6.80–6.832 (td, 4 H; 5‐H), 6.84–6.90 (dd, 4 H; 3‐H), 7.53–7.54 ppm (dd, 4 H; 6‐H); ¹³C{¹H} NMR (150.9 MHz, [D₈]THF): δ=28.4 (CH₃), 67.8 (dx), 123.3 (C‐4), 124.9 (C‐5), 126.5 (C‐3), 140.7 (C‐6), 147.8 (C‐2‐C), 169.2 ppm (C‐1); elemental analysis calcd (%) for C₄₀H₅₂Mg₂O₆ (677.4): calcd: Mg 7.18; found: Mg 7.09.

Synthesis of [(tBu)₂Mg(η¹‐dx)₂] (2‐tBu): 1,4‐Dioxane (30 mL, 0.341 mol) was slowly added to a 1.08 m tert‐butylmagnesium chloride solution (100 mL) in diethyl ether. During this procedure a colorless solid precipitated. The suspension was stored overnight at room temperature Then, the precipitate was removed by means of a Schlenk frit covered with diatomaceous earth and washed with diethyl ether. Acidimetric titration of an aliquot gave a yield of 33 %. The filtrate was concentrated till crystallization started and then stored in a refrigerator. This suspension was mixed with heptane (20 mL) and the precipitate collected on a frit and washed with heptane. The solid was briefly dried in vacuo because it lost ligated dioxane when exposed to a vacuum. Yield: 9.0 g of colorless, air‐ and moisture‐sensitive crystals of 2‐tBu (21.5 % relative to the initially used tert‐butyl chloride); alkalinity: calcd: 311.6 mg H₂SO₄ g⁻¹; found: 318.0 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=0.86 (s, 18 H; CH₃), 3.54 ppm (s, 10 H; dx); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=15.7 (C‐Mg), 35.8 (CH₃), 67.8 ppm (dx); elemental analysis calcd (%) for C₁₆H₃₄MgO₄ (314.7): Mg 7.72; found: 7.88. Single crystals were grown from a solvent mixture of diethyl ether and 1,4‐dioxane.

Synthesis of [(oTol)₂Mg(η¹‐dx)₂] (2‐oTol): Magnesium turnings (3.0 g, 123 mmol) were suspended in diethyl ether (100 mL). Thereafter, o‐tolyl bromide (17.09 g, 99 mmol) was slowly added to the mixture in portions of 2 mL. The reaction was initiated by the addition of some drops of 1,2‐dibromoethane. After the reaction seemed to cease, the mixture was heated at reflux for 1 h. Then, the solution was filtered through a Schlenk frit (filled with anhydrous diatomaceous earth; yield: 67 %, as calculated from titration of an aliquot with 0.1 n H₂SO₄). The filtrate was cooled to room temperature and then dioxane (13 mL, 122 mmol) was slowly added in 2 mL portions. The reaction mixture was kept overnight at room temperature and precipitated [MgBr₂(dx)₂] was removed by filtration through a Schlenk frit (titrated yield: 17.4 %). Pure 2‐oTol crystallized from the filtrate at 5 °C. Yield: 2.91 g of colorless crystals of 2‐o‐Tol (15.2 % with respect to the initially used o‐tolyl bromide); alkalinity: calcd: 256.2 mg H₂SO₄ g⁻¹; found: 250.8 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=2.38 (s, 6 H; CH₃), 3.54 (s, 16 H; dx), 6.78–6.79 (t, 2 H; 4‐H), 6.80–6.832 (td, 2 H; 5‐H), 6.84–6.90 (dd, 2 H; 3‐H), 7.53–7.54 ppm (dd, 2 H; 6‐H); ¹³C{¹H} NMR (150.9 MHz, [D₈]THF): δ=28.4 (CH₃), 67.8 (dx), 123.3 (C‐4), 124.9 (C‐5), 126.5 (C‐3), 140.7 (C‐6), 147.8 (C‐2), 169.2 ppm (C‐1); elemental analysis calcd (%) for C₂₂H₃₀MgO₄ (382.8): Mg 6.35; found: Mg 6.22.

Synthesis of [Ph₂Mg(η¹‐dx)₃] (3‐Ph): A commercially available 1.0 m PhMgBr solution in THF (50 mL, 50 mmol) and dioxane (50 mL) were combined and the mixture heated at reflux. During this procedure a colorless solid precipitated, which was removed by means of a Schlenk frit. All volatiles of the filtrate were removed in vacuo. The dry residue was dissolved in boiling dioxane (50 mL) and filtered through a Schlenk frit covered with diatomaceous earth. Then, toluene (5 mL) was added to the filtrate and the volume reduced until crystallization started. At room temperature the majority of the colorless compound crystallized. These crystals were collected on a Schlenk frit, washed with cold diethyl ether, and briefly dried in vacuo. Yield: 2.1 g of 3‐Ph (19 % relative to initially used phenylmagnesium bromide solution); alkalinity: calcd: 221.5 mg H₂SO₄ g⁻¹; found: 224.9 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=3.55 (s, 24 H; dx), 6.86–6.90 (m, 2 H; p‐H), 6.94–6.99 (m, 4 H; m‐H), 7.67–7.70 ppm (m, 4 H; o‐H); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=67.8 (dx), 124.4 (C‐p), 126.3 (C‐m), 141.3 (C‐o), 170.3 ppm (C‐i); elemental analysis calcd (%) for C₂₄H₃₄MgO₆ (442.8): Mg 5.49; found: Mg 5.57.

Synthesis of [(Me₃Si‐C≡C)₂Mg(μ‐dx)_1.5]_∞ (1.5′‐C₂SiMe₃): Trimethylsilylacetylene (4.3 g, 43.78 mmol) and dioxane (4.0 mL, 45.45 mmol) were added at room temperature to a 0.87 m solution of 1′‐nPr in diethyl ether (50 mL, 43.5 mmol). Shortly thereafter, a microcrystalline solid of 1.5′‐C₂SiMe₃ formed. This precipitate was collected on a Schlenk frit, thoroughly washed with diethyl ether, and dried in vacuum. Yield: 5.30 g of 1.5′‐C₂SiMe₃ (69 % relative to the initially used trimethylsilylacetylene); alkalinity: calcd: 279.5 mg of H₂SO₄ g⁻¹; found: 287.8 mg. The crystals slowly lost dioxane at room temperature once isolated and therefore slowly became dull leading to enhanced magnesium values and alkalinities. ¹H NMR (400.1 MHz, [D₈]THF): δ=−0.04 (s, 18 H; SiMe₃; ²⁹Si satellites ² J _HSi=6.5 Hz), 3.56 ppm (s, 12 H; dx); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=1.8 (SiMe₃), 67.8 (dx), 112.1 (≡C‐Si), 158.4 ppm (≡C‐Mg); ²⁹Si{¹H} NMR (79.5 MHz, [D₈]THF): δ=−29.2 ppm; elemental analysis calcd (%) for C₃₂H₆₀Mg₂O₆Si₄ (701.7): Mg 6.92; found: Mg 7.11. Single crystals of 1.5′‐C₂SiMe₃ were obtained when trimethylsilylacetylene (1.2 g, 12.2 mmol) and dioxane (1.0 mL, 11.4 mmol) were added to a 0.36 m solution of [(n‐C₄H₉)₂Mg(dx)] (12.6 mmol) in diethyl ether (35 mL) at −20 °C and stored overnight in a refrigerator at 5 °C.

Synthesis of [(Ph‐C≡C)₂Mg(dx)₂]_∞ (2′‐C₂Ph): A freshly prepared 0.365 m solution of [(nBu)₂Mg(dx)] in diethyl ether (90 mL; see above) was cooled to 0 °C. Phenylacetylene (6.4 g, 62.7 mmol) was added dropwise to this solution with stirring. Evolution of a colorless gas was observed as the drops of phenylacetylene hit the solution. After the addition of half of the Ph‐C≡C‐H a colorless precipitate formed. After complete addition of phenylacetylene the reaction mixture was stirred at room temperature for an additional hour. The precipitate was collected on a Schlenk frit, washed with diethyl ether, and dried in vacuo. Yield: 12.13 g (96 % relative to the initially used phenylacetylene) of 2′‐C₂Ph; alkalinity: calcd: 243.3 mg H₂SO₄ g⁻¹; found: 243.5 mg; ¹H NMR (400.1 MHz, [D₈]THF): δ=3.58 (s, 16 H; dx), 6.91 (m, 2 H; p‐H), 7.08 (m, 4 H; m‐H), 7.20 ppm (m, 4 H; o‐H); ¹³C{¹H} NMR (100.6 MHz, [D₈]THF): δ=67.9 (dx), 109.2 (≡C‐), 124.9 (C‐p), 128.2 (C‐m), 130.6 (C‐i), 131.3 (C‐o), 131.8 ppm (≡C‐Mg); elemental analysis calcd (%) for C₂₄H₂₆MgO₄ (402.7): Mg 5.96; found: Mg 6.03. Single crystals of 2′‐C₂Ph were grown by heating a suspension of the microcrystalline substance (0.66 g, 1.6 mmol) in a mixture of THF (10 mL) and dioxane (4.5 mL, 51.1 mmol) at 65 °C for 72 h.

Crystallography

Crystallization of [MgCl₂(μ‐dx)₂]_∞ (2′‐Cl): A 3.0 m solution of methylmagnesium chloride (3.0 mL, 9.0 mmol) in THF was diluted with THF (25 mL). 1,4‐Dioxane (2 mL, 22.7 mmol) was added to this solution. The clear solution was then heated at 65 °C in a 270 mL Schlenk tube to yield a white amorphous precipitate after a few minutes. After 1 week at this temperature, the precipitate transformed into colorless crystals of 2′‐Cl of different size.

Crystallization of [MgBr₂(μ‐dx)₂]_∞ (2′‐Br): A 0.9 m solution of n‐propylmagnesium bromide (3.0 mL, 2.7 mmol) in THF was diluted with THF (25 mL). Then, 1,4‐dioxane (0.9 mL, 10.2 mmol) was added. The clear solution was heated at 65 °C. During this procedure a colorless precipitate of 2′‐Br slowly formed. This suspension was tempered for 1 week at this temperature. During this time, the precipitate aged and turned into a crystalline solid with crystals of different size.

Crystal structure determinations: The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphite‐monochromated Mo_Kα irradiation. The data were corrected for Lorentzian and polarization effects; absorption was taken into account on a semi‐empirical basis using multiple scans.32, 33, 34 The structures were solved by direct methods (SHELXS)35 and refined by full‐matrix least‐squares techniques against F _o ² (SHELXL‐97).36 The hydrogen atoms of the compounds 1′‐Me, 1′‐nPr, 2‐tBu, and B were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. The crystal of 2′‐C₂Ph was a non‐merohedral twin. The twin law was determined by PLATON37 as (0.003 0.997 0.000) (1.003 −0.003 0.000) (0.000 0.000 −1.000). The contribution of the main component was refined to 0.845(1). All non‐disordered, non‐hydrogen atoms were refined anisotropically.36 The crystals of 1.5′‐C₂SiMe₃ were extremely thin and/or of low quality, resulting in a substandard data set; however, the structure is of sufficient quality to show connectivity and geometry despite the high final R value. We only publish here the conformation of the molecule and the crystallographic data. We will not deposit the data at the Cambridge Crystallographic Data Centre. The crystallographic data as well as structure solution and refinement details are summarized in Table S1 in the Supporting Information). XP38 and POV‐Ray39 software were used for structure representations.

CCDC https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/chem.201903120 contain the supplementary crystallographic data for this paper. These data are provided free of charge by http://www.ccdc.cam.ac.uk/.