SilylâHydrosilane Exchange at a Magnesium Triphenylsilyl Complex

Article Cite This: Inorg. Chem. 2017, 56, 14979−14990

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Silyl−Hydrosilane Exchange at a Magnesium Triphenylsilyl Complex Supported by a Cyclen-Derived NNNN-Type Macrocyclic Ligand Lara E. Lemmerz, Valeri Leich, Daniel Martin, Thomas P. Spaniol, and Jun Okuda* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany

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S Supporting Information *

ABSTRACT: The magnesium triphenylsilyl complex [(Me3TACD)Mg(SiPh3)] (2) was synthesized from magnesium bis(triphenylsilyl) [Mg(SiPh3)2(THF)2]·THF (1; THF = tetrahydrofuran) and the NNNN-type macrocyclic amidotriamine proligand (Me3TACD)H ((Me3TACD)H = Me3[12]aneN4 = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). Treating 2 with AlR3 (R = Me, Et) gave the magnesium triphenylsilyl complexes with “blocked” amido function [(Me3TACD·AlR3)Mg(SiPh3)] (3a: R = Me; 3b: R = Et). Instead of forming a Mg−H bond upon reaction with dihydrogen or hydrosilanes, 2 and 3a,b underwent rapid silyl−silane exchange with hydrosilanes. Treating the ethyl complex [(Me3TACD·AlEt3)MgEt] with H3SiPh gave [(Me3TACD·AlEt3)MgH] (4), albeit not in a reproducible manner. The silyl−hydrosilane exchange allows access to other magnesium silyls of the type [(Me3TACD)Mg(SiR′3)] (5a: SiR′3 = SiH2Ph; 5b: SiR′3 = SiHPh2) and [(Me3TACD·AlR3)Mg(SiR′3)] (6a: SiR′3 = SiH2Ph, R = Me; 6b: SiR′3 = SiH2Ph, R = Et; 7a: SiR′3 = SiHPh2, R = Me; 7b: SiR′3 = SiHPh2, R = Et). The reaction of 2 with H2SiMePh in THF at room temperature resulted in an equilibrium (Keq ≈ 1). Protonolysis of 2 with Brønsted acids (HX) 2,5-di-tertbutylphenol, phenylacetylene, acetophenone, aniline, and triethylammonium chloride each gave a compound [(Me3TACD)Mg(X)] with the conjugated base coordinated at the magnesium along with HSiPh3. The magnesium silyls 1, 2, and 7b as well as the magnesium hydride 4 contain a distorted square-pyramidal magnesium center according to single-crystal X-ray diffraction.

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INTRODUCTION In contrast to Grignard reagents and related organomagnesium compounds,1,2 only few magnesium silyl compounds are known. Chelating ligands allowed the isolation of structurally characterized magnesium silyl compounds [Mg(SiMe3)2(L)] (L: DME = 1,2-dimethoxyethane (A); TMEDA = N,N,N′,N′tetramethylethylenediamine; TMDAP = 1,3-bis(N,Ndimethylamino)propane).3−6 Sterically bulky silyl groups led to the synthesis of bis(silyl)magnesium compounds, such as [Mg{Si(SiMe3)3}2(TMEDA)],7 [Mg{Si(SiMe3)3}2(THF)2] (B; THF = tetrahydrofuran),8 [Mg(SitBu3)2(THF)2],9 [Mg{Si(SiMe3) 2Me}2(L)2] (L = THF; 1,4-dioxane),7 [Mg{Si(SiMe3)2Ph}2(THF)2],7 and [{Fe[C5H4{Si(SiMe3)2}]2}Mg(THF)2].10 In addition, magnesium mono(silyl)s such as [(ToM)Mg{Si(SiRMe2)3}] (C) (ToM = tris(4,4-dimethyl-2oxazolinyl)phenylborate; R = H; Me),11 [(DIPPnacnac)Mg(SiMe 2 Ph)] (D) (DIPPnacnac = HC[C(Me)N(2,6- iPrC6H3)]2),12 and [(Me3TACD·AliBu3)Mg(SiH2Ph)]13,14 as well as mixed magnesium silyl halides7−9,15−20 and magnesium silyl alkyl compounds7,21 became known (Chart 1). Recently we found that triphenylsilyl compounds of electropositive metals such as potassium,22−24 calcium,25−27 and zinc28 are readily accessible and that the calcium triphenylsilyl compounds are suitable for the synthesis of molecular hydrides. We wondered whether the magnesium− silicon bonds would undergo σ-bond metathesis with dihydrogen more readily than with magnesium alkyls as it © 2017 American Chemical Society

Chart 1. Examples of Structurally Characterized Magnesium Silyl Complexes

was observed for calcium−silicon bonds compared to calcium alkyls.26,27,29 Molecular magnesium hydrides are commonly obtained by σ-bond metathesis of magnesium alkyls with hydrosilanes (in most cases H3SiPh) as the hydride source.30,31 In contrast, [(Me3TACD·AliBu3)MgiBu] ((Me3TACD)H = Me3[12]aneN4 = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododeReceived: September 5, 2017 Published: December 1, 2017 14979

DOI: 10.1021/acs.inorgchem.7b02233 Inorg. Chem. 2017, 56, 14979−14990

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Inorganic Chemistry cane) reacted with H3SiPh to give the magnesium silyl [(Me3TACD·AliBu3)Mg(SiH2Ph)].13,14 The terminal magnesium hydride [(Me3TACD·AliBu3)MgH], prepared by the dissociation of [(Me3TACD·AliBu3)Mg(HAliBu3)],13 reacted with H3SiPh in the same way. The polarity of the Si−H bond appears to be reversed in these reactions, with H3SiPh formally acting as a Brønsted acid, as the negatively charged alkyl carbon or hydride are formally protonated. Here we present a systematic reactivity study of a magnesium triphenylsilyl complex toward dihydrogen, hydrosilanes, and weak Brønsted acids.

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RESULTS AND DISCUSSION Triphenylsilyl Complexes. Magnesium bis(triphenylsilyl) [Mg(SiPh3)2(THF)2]·THF (1) was prepared by salt metathesis of anhydrous magnesium diiodide with potassium triphenylsilyl [K(THF)SiPh3]32,33 in THF (Scheme 1). Compound 1 is soluble in THF and benzene but insoluble in aliphatic hydrocarbons. It was characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction.

Figure 1. Molecular structure of [Mg(SiPh3)2(THF)2]·THF (1). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms and the THF molecule within the crystal lattice are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mg1− Si1 2.6737(15), Mg1−O1 2.026(3), Si1−Mg1−O1 111.69(9), Si1− Mg1−O1′ 105.92(9), Si1−Mg1−Si1′ 123.72(9).

162 °C and is stable in THF at 60 °C for four weeks without significant decomposition. Complex 2 was characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction. The 29Si NMR spectrum shows a resonance at δ −12.2 ppm, which is similar to that of 1. Single crystals of 2 were obtained by layering n-hexane on a benzene solution at rt. [(Me3TACD)Mg(SiPh3)] (2) is monomeric, and the magnesium center is coordinated by one triphenylsilyl group and four nitrogen atoms of the monoanionic Me3TACD ligand (Figure 2). The angular

Scheme 1. Synthesis of [Mg(SiPh3)2(THF)2]·THF (1) and [(Me3TACD)Mg(SiPh3)] (2)

The 29Si NMR spectrum shows one signal at δ −12.8 ppm. Single crystals of [Mg(SiPh3)2(THF)2]·THF (1) were obtained from a THF/n-pentane solution upon cooling to −30 °C and analyzed by X-ray diffraction (Figure 1). Complex 1 crystallizes with one additional equivalent of THF as a mono(THF) solvate. The magnesium atom is situated on a crystallographic C2 axis and is coordinated by two silyl ligands as well as by two THF molecules. The angles Si1−Mg1−O1 of 111.69(9)°, Si1− Mg1−O1′ of 105.92(9)°, and Si1−Mg1−Si1′ of 123.72(9)° indicate tetrahedral coordination geometry at the magnesium atom. The Mg1−Si1 bond length of 2.6737(15) Å compares to those in [Mg(SiMe3)Br(TMEDA)]2 (2.603(4) Å),15 [Mg{Si(SiMe 3 ) 3 } 2 (THF) 2 ] (2.682(2) Å), 8 [Mg{Si(SiMe 3 ) 3 } 2 (TMEDA)] (2.708(3) Å), 7 and [Mg{Si(SiMe 3 ) 3 }Me(TMEDA)] (2.6414(9) Å).21 [(Me3TACD)Mg(SiPh3)] (2) was synthesized from [Mg(SiPh3)2(THF)2]·THF (1) and (Me3TACD)H in benzene at room temperature (rt) and was isolated as a colorless powder in 83% yield (Scheme 1). Complete removal of the byproduct triphenylsilane was only possible in benzene after a reaction time of 2 h. Triphenylsilane could not be fully separated by washing with n-pentane, if THF was used as the solvent or if the reaction time was not long enough. [(Me3TACD)Mg(SiPh3)] (2) is insoluble in aliphatic hydrocarbons but soluble in benzene and THF; it melts in the temperature range of 156−

Figure 2. Molecular structure of [(Me3TACD)Mg(SiPh3)] (2). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mg1−Si1 2.679(3), Mg1−N1 1.985(5), Mg1−N2 2.267(3), Mg1−N3 2.181(4), N1−Mg1−N3 112.18(17), N2−Mg1− N2′ 144.10(10).

structural parameter τ, describing the coordination geometry of five-coordinate metal complexes, has the value of 0.32 and indicates distorted square pyramidal geometry at the magnesium center.34 The atoms Mg1, Si1, N1, and N3 are located within a crystallographic mirror plane. The Mg1−Si1 distance of 2.679(3) Å is almost identical to that in 1 with 2.6737(15) Å, similar to that in [(Me3TACD·AliBu3)Mg(SiH2Ph)] (2.640(1) Å)13 and lies in the range of known bis(silyl)magnesium complexes.7,8 The distance between the magnesium center and the amido nitrogen N1 of 1.985(5) Å is significantly shorter than the Mg−Namido bond length in 14980


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Inorganic Chemistry [(Me3TACD·AliBu3)Mg(SiH2Ph)] (2.160(2) Å).13 The introduction of a trialkylaluminum to the Me3TACD ligand leads to an elongation of the Mg−Namido bond. The N1−Mg−N3 angle of 112.18(17)° differs from the angle between N2−Mg− N2′ (144.10(10)°). Treating [(Me3TACD)Mg(SiPh3)] (2) with 1 equiv of AlR3 (R = Me, Et) in benzene at rt gave colorless [(Me3TACD· AlR3)Mg(SiPh3)] (3a: R = Me; 3b: R = Et) in high yield (Scheme 2). It is noteworthy that the strong Lewis acids AlR3 did not attack the presumably highly nucleophilic SiPh3 group.

Single crystals of 4 were obtained from a benzene solution at rt. The crystallographic data showed that the unit cell contains two crystallographically independent, monomeric [(Me3TACD·AlEt3)MgH] units (Figure 3) and noncoordinated benzene.

Scheme 2. Synthesis of [(Me3TACD·AlR3)Mg(SiPh3)]a

a

Figure 3. Molecular structure of [(Me3TACD·AlEt3)MgH] (4). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms, except for the hydride H1, are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mg1−H1 1.71(2); Mg1− N1 2.1746(17); Mg1−N2 2.2310(18); Mg1−N3 2.2822(17); Mg1− N4 2.2216(18); Al1−N1 1.9576(17); N1−Mg1−N3 133.11(7); N2− Mg1−N4 132.61(7).

3a R = Me; 3b: R = Et.

The coordination geometry of the magnesium center in [(Me3TACD·AlEt3)MgH] (4) can be described as slightly distorted square pyramidal (τ value 0.01). Four nitrogen atoms of the Me3TACD ligand are forming the base, and the apical position is occupied by the terminal hydride. The structure is comparable to that of [(Me3TACD·AliBu3)MgH].13 The Mg− H distance of 1.71(2) Å compares to that in the other examples featuring a terminal Mg−H bond.13,39−41 The presence of aluminum alkyl blocks the amido nitrogen of the Me3TACD ligand in 4 and prevents cluster formation.42 Whereas H3SiPh reacted with [(Me3TACD·AlEt3)MgEt] to form 4, treatment with [(Me3TACD·AliBu3)MgiBu] gave [(Me3TACD·AliBu3)Mg(SiH2Ph)].13 Silyl−Hydrosilane Exchange. To gain further insight into the behavior of Si−H bonds toward Me3TACD stabilized magnesium silyl, [(Me3TACD)Mg(SiPh3)] (2) was treated with different hydrosilanes. 2 underwent silyl−silane exchange with hydrosilanes leading to HSiPh 3 and magnesium compounds with different silyl groups. This silyl−hydrosilane exchange can be classified as a σ-bond metathesis,43 in which the Si−H bond shows positively polarized hydrogen. Thus, the silyl−hydrosilane exchange can also be seen as Brønsted acid− base reaction. A radical pathway cannot be excluded, because phenyl-substituted silanes (Si−H bond dissociation energies 351−368 kJ/mol) are known to form silyl radicals.44,45 A silyl− hydrosilane exchange has so far been reported for a molybdenum phenylsilyl hydride complex [({2,6-iPr-C6H4}N)Mo(H)(SiH2Ph)(PMe3)3]46 and for [CpCp*M{Si(SiMe3)3}Cl] (M = Zr, Hf).47−49 As expected from the pKa value of 34.8 in dimethyl sulfoxide for HSiPh3, 2 reacted with the relatively stronger Brønsted acids H3SiPh (pKa: 31.6)50 and H2SiPh2 (pKa: 33.9)50 in THF at rt to give [(Me 3 TACD)Mg(SiH 2 Ph)] (5a) and [(Me3TACD)Mg(SiHPh2)] (5b), respectively (Scheme 4). The complexes 5a and 5b were isolated in ∼70% yield and fully characterized. The 1H NMR spectra of 5a and 5b in benzene-d6 revealed signals for the silicon-bound protons at δ 4.43 and 5.21 ppm. The 29Si{1H} NMR spectrum of 5a showed a signal at δ −77.3 ppm, while the corresponding resonance for 5b was detected at δ −29.8 ppm. The former lies in the same range as the 29Si{1H} NMR signal of [(Me3TACD·AliBu3)Mg(SiH2Ph)]

The complexes 3a and 3b are insoluble in aliphatic hydrocarbons but soluble in THF. Complex 3b is also slightly soluble in benzene. 3a melts in the temperature range of 154− 160 °C, whereas 3b melts in the temperature range of 184−187 °C. In solution, 3a and 3b completely decomposed at 60 °C after 24 h. In the 1H NMR spectra, the characteristic signal for the methyl groups of the AlMe3 unit in 3a appears at δ −0.96 ppm, and the signals for the ethyl groups of AlEt3 in 3b appear at δ 0.45 (quartet) and 1.72 ppm (triplet). A Hydride Complex. When a solution of 2 in THF-d8 was heated at 80 °C for 5 d in the presence of dihydrogen (1 bar), formation of triphenylsilane (50% based on 2) was observed by 1 H NMR spectroscopy. No signal for a Mg−H species was identified in the 1H NMR spectrum. The AlR3 adducts 3a and 3b decomposed at rt in the presence of 1 bar dihydrogen after 24 h. In THF-d8, 10 mol % of 2 was found to hydrogenate 1,1diphenylethylene in the presence of 1 bar dihydrogen to give 1,1-diphenylethane (DPE) with 70% conversion over a period of 10 d at 80 °C. So far, there are no magnesium and only a few calcium and strontium catalysts known to be active in the hydrogenation of DPE.27,29,35 The formation of a group 2 metal hydride as an intermediate is postulated. The hydride transfer step from the group 2 metal hydride to the substrate has the highest energy barrier in the case of magnesium, and as a result, the hydrogenation of DPE catalyzed by the magnesium catalyst is less favored.36 Applying the common “silane method”,29−31 the magnesium hydride complex [(Me3TACD·AlEt3)MgH] (4) was obtained from [(Me3TACD·AlEt3)MgEt]37,38 and H3SiPh (Scheme 3). The synthesis of complex 4 was rather irreproducible for reasons unknown to date. Scheme 3. Synthesis of [(Me3TACD·AlEt3)MgH] (4)

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Scheme 4. Synthesis of [(Me3TACD)Mg(SiH2Ph)] (5a) and [(Me3TACD)Mg(SiHPh2)] (5b) and Their AlR3 Adducts [(Me3TACD·AlR3)Mg(SiH2Ph)] (6a: R = Me; 6b: R = Et) and [(Me3TACD·AlR3)Mg(SiHPh2)] (7a: R = Me; 7b: R = Et)

(δ −75.7 ppm).13 No reaction of 2 with the weaker Brønsted acids such as HSiMe2Ph (pKa: 41.2)50 and HSiEt3 was observed. [(Me3TACD·AlR3)Mg(SiPh3)] (R = Me: 3a; R = Et: 3b) also reacted with H3SiPh in THF-d8 at rt to give [(Me3TACD· AlR3)Mg(SiH2Ph)] (R = Me: 6a; R = Et: 6b), as identified by 1 H NMR spectroscopy. In contrast to complex 2, the compounds 3a and 3b showed no reaction with H2SiPh2. As previously described, the introduction of a trialkylaluminum to the ligand results in a longer Mg−Namido bond. Consequently, the Mg−Namido bond is weakened, and the electron density at the nitrogen is decreased, which increases the covalency of the Mg−Si bond. The Mg−Si bond seems to be strengthened, as it can only be cleaved by the most acidic H3SiPh among the hydrosilanes used in this work. The slightly less acidic but sterically bulkier H2SiPh2 may not be reactive enough to cleave the Mg−Si bond to induce a silyl−hydrosilane exchange. 6a and 6b can also be generated by reacting [(Me3TACD)Mg(SiH2Ph)] (5a) with AlR3 (R = Me, Et) in THF. 6a and 6b are isolated in high yield and fully characterized. [(Me3TACD· AlR3)Mg(SiHPh2)] (R = Me: 7a; R = Et: 7b) is also obtained from [(Me3TACD)Mg(SiHPh2)] (5b) using the same route (Scheme 4). The 1H NMR spectra of 6a and 7a showed signals for the Me3TACD ligand as well as for the AlEt3 unit, which are comparable to those of 3a. The 1H NMR signals for the Me3TACD ligand as well as for the AlEt3 unit of 6b and 7b are comparable to those of 3b. Signals for the silicon-bound protons are observed at δ 3.67 ppm for 6a and 6b and at δ 4.66 ppm for 7a and 7b, respectively. The 29Si NMR spectra of 6a and 6b showed signals at δ −73.2 and −74.1 ppm, similar to those in 5a and [(Me3TACD·AliBu3)Mg(SiH2Ph)].13 The 29 Si{1H} NMR signals at δ −28.0 and −29.0 ppm for 7a and 7b are in the same range as the 29Si{1H} NMR signal of 5b. Single crystals of 7b were obtained by layering n-pentane on a THF solution at −30 °C. The monomeric structure of [(Me 3TACD·AlEt 3)Mg(SiHPh2)] (7b) shows the magnesium center coordinated by the diphenylsilyl group and the four nitrogen atoms of the monoanionic Me3TACD ligand in a distorted square pyramidal coordination geometry with a τ value of 0.02 (Figure 4). The Mg1−Si1 distance of 2.6772(6) Å is almost identical to the Mg1−Si1 bond length of 1 and 2 and is comparable with the Mg−Si distance in [(Me 3 TACD·Al i Bu 3 )Mg(SiH 2 Ph)] (2.640(1) Å).13 The Mg1−N1 bond length of 2.1615(15) Å

Figure 4. Molecular structure of [(Me3TACD·AlEt3)Mg(SiHPh2)] (7b). Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms, except for the silicon bound hydrogen atom, are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mg1− Si1 2.6772(6), Mg1−N1 2.1615(15), Mg1−N2 2.2715(13), Mg1−N3 2.2456(14), Mg1−N4A 2.251(2), N1−Al1 1.9648(14), N1−Mg1−N3 131.75(6), N2−Mg1−N4A 132.84(6).

is comparable to the Mg−Namido distance in 4 and in [(Me3TACD·AliBu3)Mg(SiH2Ph)] (2.160(2) Å)13 but significantly longer than the Mg−Namido distance in 2 (1.985(5) Å). The Al−Namido distance of 7b (1.9648(14) Å) is in the same range as the Al−Namido distance in 4 and in [(Me3TACD· AliBu3)Mg(SiH2Ph)].13 Equilibrium Reaction of 2 with H 2SiMePh and HSiMePh2. The silyl−hydrosilane exchange reaction of [(Me3TACD)Mg(SiPh3)] (2) with H2SiMePh or HSiMePh2 (pKa: 37.9)50 of comparable pKa value as HSiPh3 (pKa: 34.8)50 gave an equilibrium mixture that contained [(Me3TACD)Mg(SiHMePh)] (5c) (51%) or [(Me3TACD)Mg(SiMePh2)] (5d) (20%) along with HSiPh3, as determined by 1H NMR spectroscopy (Scheme 5). The equilibrium reaction of 2 with H2SiMePh was studied by NMR spectroscopy using a THF-d8 solution of 2 (0.03 mmol; 0.6 mL). For several starting ratios of 2 to H2SiMePh (1:1; 1:2; 1:4, 1:6, and 1:8), the equilibrium concentrations of 2, [(Me3TACD)Mg(SiHMePh)] (5c), and HSiPh3 were determined by 1H NMR spectroscopy (see Supporting Information). The amounts of 2, 5c, HSiPh3, and the equilibrium constant Keq were determined for each case (Table 1). 14982


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Inorganic Chemistry Scheme 5. Equilibrium Reactions of [(Me3TACD)Mg(SiPh3)] (2) with H2SiMePh and HSiMePh2

Table 1. Initial Concentration Ratio of [(Me3TACD)Mg(SiPh3)] (2) to H2SiMePh as well as the Relative Amount of 2, [(Me3TACD)Mg(SiHMePh)] (5c), and HSiPh3 in the Equilibrium concentration ratio of [2]:[H2SiMePh]

amount of 2 [%]

amount of 5c [%]

amount of HSiPh3 [%]

Keq

1:1 1:2 1:4 1:6 1:8

55 35 20 12 8.8

51 70 80 84 86

49 67 77 84 85

0.90(3) 1.02(1) 0.95(1) 1.19(1) 1.15(2)

Figure 6. van’t Hoff plot for the equilibrium of 2 with H2SiMePh.

(SiHMePh)] (5c), [(Me3TACD)Mg(SiDMePh)] (5c-D), HSiPh3, and DSiPh3 (Scheme 6). Scheme 6. Equilibration between 2, H2SiMePh, and D2SiMePh to give HDSiMePh, [(Me3TACD)Mg(SiHMePh)] (5c), [(Me3TACD)Mg(SiDMePh)] (5c-D), HSiPh3, and DSiPh3

With a higher initial ratio of 2 to H2SiMePh of 1:8, 5c and HSiPh3 were formed with 86 and 85%; with only 9% of 2 remaining (Figure 5). The equilibrium constant Keq remains within the range of 0.90−1.19. To study the temperature dependence of the equilibrium reaction between 2 and H2SiMePh (1:1), the equilibrium concentrations of 2, 5c, and HSiPh3 were determined after 24 h at 23, 35, 45, and 55 °C by 1H NMR spectroscopy in THF-d8 (for detailed description see Supporting Information). The values for the reaction enthalpy and entropy of ΔH0 = −2.28 ± 0.22 kJ·mol−1 and ΔS0 = −8.6 ± 0.7 J·mol−1·K−1 were determined by plotting the equilibrium constant Keq as a function of temperature in a van’t Hoff plot (Figure 6) and turned out to be small. Treating 2 with an equimolar ratio of H2SiMePh and D2SiMePh led within 5 min to a mixture containing 2, H 2 SiMePh, D 2 SiMePh, HDSiMePh, [(Me 3 TACD)Mg-

Figure 5. Amounts of 2 (blue), [(Me3TACD)Mg(SiHMePh)] (5c) (red), and HSiPh3 (green) depending on the equivalent amounts of H2SiMePh. 14983


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Inorganic Chemistry The reaction mixture was analyzed by 29Si{1H}DEPT-135 NMR spectroscopy (Figure 7). This method shows only silicon

silane HDSiMePh unequivocally indicates the existence of an equilibrium. Protonolysis. As expected, [(Me3TACD)Mg(SiPh3)] (2) was protonolyzed by the stronger Brønsted acids water, alcohols, phenylacetylene, acetophenone, and amines (pKa ≪ 34.8) to give HSiPh3 with the conjugate base coordinated at the magnesium center (Scheme 7).43 These protonolytic reactions follow a pattern similar to that of the hydride [(Me3TACD· AliBu3)MgH].13,14 A solution of 2 in THF-d8 reacted with 1 equiv of water (pKa: 31.4)51,52 or ethanol (pKa: 29.8)51,52 to form colorless precipitates of Mg(OH)2 and Mg(OEt)2 along with HSiPh3 and (Me3TACD)H, as determined by 1H NMR spectroscopy. In contrast, treating 2 with 1 equiv of 2,6-di-tert-butylphenol (pKa: 16.9)52 in benzene gave [(Me3TACD)Mg(O-2,6-tBu2C6H3)] (8) in 49% yield. The 1H NMR spectrum of 8 in THFd8 showed characteristic signals for the coordinated Me3TACD ligand as well as signals for the tert-butyl groups of the phenoxy group, similar to those of the magnesium diaryloxide [Mg(O2,6-tBu2-C6H3)2(TMEDA)].53 2 reacted with phenylacetylene (pKa: 28.7)54,55 and acetophenone (pKa: 24.7)52 in benzene at rt to give the acetylide [(Me3TACD)Mg(CCPh)] (9) and enolate [(Me3TACD)Mg(OC(Ph)CH2)] (10) in yields of 71 and 77%, respectively. The 13C{1H} NMR spectrum of 9 in benzene-d6 showed a signal for the acetylide carbon at δ 130.1 ppm, a value similar to that reported for other magnesium phenylacetylide complexes.13,14 The 1H NMR spectrum of 10 in benzene-d6 showed two multiplets for the enolate protons at δ 4.41 and 4.89 ppm, comparable to resonances of other magnesium enolate complexes.56 The reaction of 2 with 1,1,3,3-tetramethylsilazine (pKa: 30)57 in THF-d8 at rt gave the known compound [(Me3TACD)Mg{N(SiMe3)2}]58 with full conversion after 5 min and HSiPh3 as a byproduct, as indicated by 1H NMR spectroscopy. Likewise, 2 reacted with aniline (pKa: 30.6)52,59,60 to give [(Me3TACD)Mg(NHPh)] (11) in 72% yield. The 1H NMR spectrum of 11

Figure 7. 29Si{1H}DEPT-135 NMR spectrum of the equilibrium mixture of 2 and an equimolar amount of H2SiMePh and D2SiMePh at 23 °C in THF-d8.

atoms with at least one proton bonded such as in H2SiMePh, 5c, HSiPh3, and HDSiMePh. Other silicon atoms such as in 2, D2SiMePh, and DSiPh3 are not detected. A positive signal allows the identification of silicon centers with one proton and distinguishes them from those with two protons, since the latter lead to negative signals. The 29Si{1H}DEPT-135 NMR spectrum showed a positive singlet for HSiPh3 at δ −18.0 ppm, a negative singlet for H2SiMePh at δ −36.2 ppm, and a positive singlet for 5c at δ −45.4 ppm. Furthermore, HDSiMePh was recognized by a triplet at δ −36.5 ppm due to the coupling of the proton bonded to silicon with the deuterium. The presence of the mixed isotopomeric hydro-

Scheme 7. Reactivity of [(Me3TACD)Mg(SiPh3)] (2) with OH, NH, and CH Acidic Compounds

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Inorganic Chemistry showed a signal for the NH proton at δ 2.82 ppm. This value is similar to that reported for other magnesium amides such as [MgnBu{NH(2,6-Me2-C6H3)}(TMEDA)] (δ 2.70 ppm),61 [Mg(NHPh){N(SiMe3)2}(THF)]2 (δ 3.62 ppm),62 and [Mg{NH(2,4,6-Me3-C6H2)}2{OP(NMe2)3}2] (δ 3.05 ppm).63 Finally, 2 underwent protonolysis with [NEt3H]Cl (pKa ≈ 10) to give [(Me3TACD)MgCl] (12) in 68% yield along with triphenylsilane and triethylamine. The 1H NMR spectrum of 12 revealed the characteristic signals for a Me3TACD ligand coordinated to the magnesium center. However, [(Me3TACD)MgCl] (12) did not react with MH (M = Li, Na, K), M(HBPh3) (M = Li, K), or LiAlH4 to produce selectively any magnesium hydride species.

electropositive metals also critically depends on the polarity of the individual metal−silicon bond.26

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EXPERIMENTAL SECTION

General Considerations. All operations were performed under inert atmosphere of dry argon using standard Schlenk techniques or glovebox techniques. Solvents (THF, n-pentane, n-hexane, toluene) were purified using an MB SPS-800 solvent purification system or distilled under argon from sodium/benzophenone ketyl prior to use. Deuterated solvents (THF-d8, benzene-d6) were distilled under argon from sodium/benzophenone ketyl prior to use. The starting materials 1,1,1-trimethyl-2,2,2-triphenyldisilane,67 (Me3TACD)H,68 and [K(SiPh3)(THF)]32,33 were prepared according to literature procedures. NMR spectra were recorded on a Bruker Avance II 400 or a Bruker Avance III HD 400 spectrometer at 23 °C in J. Young-type NMR tubes. Chemical shifts (δ in ppm) in the 1H, 13C{1H}, and 29Si{1H} NMR spectra were referenced to the residual proton signals of the deuterated solvents and reported relative to tetramethylsilane. The resonances in the 1H and 13C{1H} NMR spectra were assigned on the basis of two-dimensional NMR experiments (COSY, HSQC, HMBC). Combustion analyses were performed with an Elementar Vario EL. The low carbon content for 5b and 8−12 may be ascribed to incomplete combustion due to possible formation of incombustible magnesium carbide or magnesium carbonate. Similar problems are reported for calcium complexes in the literature.69 Metal contents were determined by metal titrations. Titration of magnesium was performed according to the published procedure.70 Magnesium contents of samples that also contained aluminum were determined by inductively coupled plasma mass spectrometry using a Spectro ICP Spectroflame D instrument. A defined amount of sample was dissolved in 8 mL of 40% hydrofluoric acid, 2 mL of concentrated sulfuric acid, and 40 mL of water. Aluminum contents were determined by metal titration following the following procedure: 30−40 mg of the product was dissolved in 20 mL of THF, and ethylenediaminetetraacetic acid (EDTA) disodium salt (20 mL) and 1 M hydrochloric acid (2 mL) were added. The solution was boiled for 10 min on a water bath. After the solution was cooled to rt, methanol (20 mL), acetate buffer solution (10 mL; 2 M NaAc solution and 2 M HAc in 1:1 ratio), and 0.5 mL of freshly prepared dithiazone solution (0.025 weight-% in methanol) were added. The mixture was titrated with a 0.1 M solution of zinc sulfate until the equivalence point from green-blue to pink was observed.70 [Mg(SiPh3)2(THF)2]·THF (1). A solution of [K(THF)SiPh3] (371 mg, 1.0 mmol) in THF (5 mL) was added to a suspension of anhydrous magnesium diiodide (139 mg, 0.5 mmol) in THF (5 mL). The reaction mixture was stirred for 3 h. The colorless precipitate was filtered off, and the solvent was removed under reduced pressure. After it was washed with n-pentane (3 × 2 mL) and dried in vacuum, [Mg(SiPh3)2(THF)2]·THF (1) (345 mg, 0.45 mmol) was obtained in 91% yield as a pale yellow powder. Single crystals of [Mg(SiPh3)2(THF)2]·THF (1) suitable for X-ray diffraction were grown from THF/n-pentane at −30 °C within 12 h. 1 H NMR (THF-d8, 400.1 MHz): δ 1.78 (m, 12H, THF), 3.62 (m, 12H, THF), 7.08−7.11 (m, 18H, meta/para-Ph), 7.38−7.41 (m, 12H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8, 100.6 MHz): δ 26.45 (THF), 68.30 (THF), 127.03 (para-Ph), 127.95 (meta-Ph), 137.20 (ortho-Ph), 147.45 (ipso-Ph) ppm. 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −13.6 (MgSi) ppm. 1H NMR (benzene-d6, 400.1 MHz): δ 1.11 (m, 12H, THF), 3.43 (m, 12H, THF), 7.17−7.21 (m, 6H, paraPh), 7.23−7.27 (m, 12H, meta-Ph), 7.75−7.78 (m, 12H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 25.60 (THF), 69.75 (THF), 127.41 (para-Ph), 128.24 (meta-Ph), 137.23 (ortho-Ph), 147.15 (ipso-Ph) ppm. 29Si{1H} NMR (benzene-d6, 79.5 MHz): δ −12.8 (MgSi) ppm. Anal. Calcd for C48H54O3MgSi2 (759.43 g mol−1): C, 75.92; H, 7.17; Mg, 3.20. Found: C, 75.15; H, 6.42; Mg, 3.15%. [(Me3TACD)Mg(SiPh3)] (2). A solution of (Me3TACD)H (118 mg; 0.55 mmol) in benzene (3 mL) was added to a solution of [Mg(SiPh3)2(THF)2]·THF (420 mg; 0.55 mmol) in benzene (7 mL). The color of the reaction mixture turned yellow. Stirring for 3 h

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CONCLUSION The magnesium ethyl compound [(Me3TACD·AlEt3)MgEt] reacted with phenylsilane to give the terminal hydride [(Me3TACD·AlEt3)MgH] (4) (Scheme 8a), albeit not in a Scheme 8. Reaction of Mg−C and Mg−Si Bonds with H−Si Bonds

reproducible manner for reasons unknown. Apparently this σbond metathesis-type reaction is kinetically controlled by the nucleophilic alkyl group attacking the electrophilic silicon center. As phenylsilane is relatively acidic (pKa: 31.6),50 one could also conceive a (thermodynamically controlled) protonolysis of the alkyl to give silyl compounds, as indeed observed.38 The triphenylsilyl complex 2 did not give any hydride with dihydrogen (pKa: 35)64 or with phenylsilane (Scheme 8b,c). Whereas magnesium−silyl bonds should be relatively covalent (bond dissociation energies for Mg−H: 299 kJ/mol65 vs Mg−Si: 490 kJ/mol66), the polarity of the Si−H bond in hydrosilanes can change. The silyl ligand in complex 2 showed silyl−silane exchange with a hydrosilane that has a pKa value close to that of Ph3SiH to result in an equilibrium mixture. No effect of AlR3 addition was observed. In the silyl− hydrosilane exchange of [CpCp*M{Si(SiMe3)3}Cl]47−49 (M = Zr, Hf) studied in great detail, the equilibrium was dominated by steric factors. At this stage, we cannot exclude a radical mechanism, but σ-bond metathesis involving silanes acting as Brønsted acids could be operative (Scheme 8d). Therefore, the Brønsted acidity and nucleophilicity of silicon (toward magnesium) in the incoming hydrosilane appears to determine the outcome of the reactivity of 2. The facile hydrogenolysis of the Ca−Si bond in [(Me3TACD)Ca(SiPh3)] to give [(Me3TACD)3Ca3H2]+(SiPh3)− suggests that the reactivity of triphenylsilyl compounds of 14985


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Inorganic Chemistry

(NCH3), 48.26 (CH2), 54.55 (CH2), 54.76 (CH2), 57.30 (CH2), 126.73 (para-Ph), 127.71 (meta-Ph), 137.67 (ortho-Ph), 149.12 (ipsoPh). 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −14.5 (MgSi) ppm. 27Al NMR (THF-d8, 100 MHz): δ 171.32 (AlEt3) ppm. Anal. Calcd for C35H55N4AlMgSi (611.22 g·mol−1): C, 68.78; H, 9.07; N, 9.17; Mg, 3.98; Al, 4.41. Found: C, 68.44; H, 8.62; N, 8.42; Mg, 3.62; Al, 4.05%. [(Me3TACD·AlEt3)MgH] (4). A solution of H3SiPh (28 mg, 0.26 mmol) in benzene (0.5 mL) was added to a solution of [(Me3TACD· AlEt3)MgEt]38 (50 mg, 0.13 mmol) in benzene (3 mL). The reaction mixture was stirred for 15 h at 50 °C. The crystalline precipitate was filtered, washed with n-pentane (3 × 5 mL), and dried under vacuum to afford [(Me3TACD·AlEt3)MgH] (4) (37 mg, 0.10 mmol) in 66% yield as microcrystals. 1 H NMR (THF-d8; 400.1 MHz): δ −0.20 (q, 3JHH = 8.0 Hz, 6H, Al(CH2CH3)3), 1.04 (t, 3JHH = 8.0 Hz, 9H, Al(CH2CH3)3), 2.43 (s, 9H, NCH3), 2.55−2.46 (m, 8H, CH2), 2.87−2.72 (m, 6H, CH2), 3.05−3.01 (m, 2H, CH2), 3.56 (br s, 1H, MgH), 7.30 (s, 6H, C6H6) ppm. 13C{1H} NMR (THF-d8; 100.6 MHz): δ 2.1 (Al(CH2CH3)3), 12.0 (Al(CH2CH3)3), 43.5 (CH3), 45.1 (CH3), 48.4 (CH2), 54.1 (CH2), 54.3 (CH2), 56.6 (CH2), 129.2 (C6H6) ppm. 27Al NMR (THFd8, 100 MHz): δ 161.08 (br s, AlEt3) ppm. [(Me3TACD)Mg(SiH2Ph)] (5a). A solution of H3SiPh (27 mg; 31 μL; 0.25 mmol) in benzene (1 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (124 mg; 0.25 mmol) in benzene (2 mL), and the reaction mixture was stirred for 20 min at room temperature. The solvent was removed under reduced pressure, and the yellow oil was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The yellow oil was dried under vacuum to give [(Me3TACD)Mg(SiH2Ph)] (5a) (59 mg; 0.17 mmol) in 68% yield. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.42−1.53 (m, 4H, CH2), 1.74 (s, 3H, NCH3), 1.80−1.84 (m, 2H, CH2), 2.04 (s, 6H, NCH3), 2.10−2.18 (m, 4H, CH2), 2.83−2.97 (m, 4H, CH2), 3.32−3.35 (m, 2H, CH2), 4.43 (s, 1JSiH = 128 Hz, 1H, SiH2Ph), 7.17−7.19 (m, 1H, para-Ph), 7.26−7.30 (m, 2H, meta-Ph), 8.04−8.06 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.30 (NCH3), 46.92 (NCH3), 50.60 (CH2), 52.88 (CH2), 54.95 (CH2), 62.95 (CH2), 126.09 (para-Ph), 127.80 (meta-Ph), 137.23 (ortho-Ph), 146.33 (ipsoPh). 29Si{1H} NMR (benzene-d6, 79.5 MHz): δ −77.3 (MgSi) ppm. [(Me3TACD)Mg(SiHPh2)] (5b). A solution of H2SiPh2 (96 mg; 96 μL; 0.52 mmol) in THF (1 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (258 mg; 0.52 mmol) in THF (2 mL), and the reaction mixture was stirred for 20 min at room temperature. The solvent was removed under reduced pressure, and the solid residue was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The solid was dried under vacuum to give [(Me3TACD)Mg(SiHPh2)] (5b) (148 mg; 0.35 mmol) as a bright yellow powder in 67% yield. 1 H NMR (THF-d8; 400.1 MHz): δ 2.17−2.23 (m, 2H, CH2), 2.20 (s, 6H, NCH3), 2.30 (s, 3H, NCH3), 2.35−2.43 (m, 4H, CH2), 2.51− 2.57 (m, 2H, CH2), 2.68−2.75 (m, 2H, CH2), 2.81−2.87 (m, 2H, CH2), 2.92−3.03 (m, 4H, CH2), 4.64 (s, 1H, SiHPh2), 6.98−7.02 (m, 2H, para-Ph), 7.06−7.09 (m, 4H, meta-Ph), 7.56−7.58 (m, 4H, orthoPh) ppm. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.43−1.57 (m, 4H, CH2), 1.74 (s, 3H, NCH3), 1.80−1.86 (m, 2H, CH2), 1.96 (s, 6H, NCH3), 2.13−2.20 (m, 4H, CH2), 2.81−2.88 (m, 2H, CH2), 2.96−3.03 (m, 2H, CH2), 3.33−3.36 (m, 2H, CH2), 5.21 (s, 1JSiH = 129 Hz, 1H, SiHPh2), 7.14−7.18 (m, 2H, para-Ph), 7.25−7.29 (m, 4H, meta-Ph), 7.99−8.01 (m, 4H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.38 (NCH3), 46.95 (NCH3), 50.64 (CH2), 52.97 (CH2), 55.03 (CH2), 63.07 (CH2), 126.56 (para-Ph), 127.92 (meta-Ph), 137.22 (ortho-Ph), 148.35 (ipso-Ph). 29Si{1H} NMR (benzene-d6, 79.5 MHz): δ −29.8 (MgSi) ppm. Anal. Calcd for C23H36N4MgSi (420.96 g·mol−1): C, 65.62; H, 8.62; N, 13.31; Mg, 5.77. Found: C, 64.45; H, 8.45; N, 12.55; Mg, 5.88%. [(Me3TACD·AlMe3)Mg(SiH2Ph)] (6a). A solution of H3SiPh (27 mg; 31 μL; 0.25 mmol) in THF (1 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (124 mg; 0.25 mmol) in THF (2 mL), and the reaction mixture was stirred for 1 h at room temperature. To the reaction solution, a solution of AlMe3 (18 mg; 24 μL; 0.25 mmol) in THF (1 mL) was added, and the reaction mixture was stirred for 1 h

at room temperature resulted in a light brown solution. The solvent was removed under reduced pressure, and the solid residue was washed with n-pentane (3 × 5 mL) to completely separate HSiPh3. The solid was dried under vacuum, and [(Me3TACD)Mg(SiPh3)] (2) (227 mg; 0.45 mmol) was obtained as a colorless powder in 83% yield. Single crystals suitable for X-ray diffraction were grown from benzene/ n-hexane at room temperature over a period of 48 h. 1 H NMR (THF-d8; 400.1 MHz): δ 2.05 (s, 6H, NCH3), 2.14−2.21 (m, 2H, CH2), 2.26 (s, 3H, NCH3), 2.33−2.40 (m, 4H, CH2), 2.52− 2.58 (m, 2H, CH2), 2.68−2.75 (m, 2H, CH2), 2.81−2.87 (m, 2H, CH2), 2.94−3.08 (m, 4H, CH2), 7.03−7.06 (m, 3H, para-Ph), 7.10− 7.13 (m, 6H, meta-Ph), 7.55−7.57 (m, 6H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8; 100.6 MHz): δ 43.95 (NCH3), 47.23 (NCH3), 51.59 (CH2), 53.55 (CH2), 56.05 (CH2), 63.55 (CH2), 126.65 (para-Ph), 127.73 (meta-Ph), 137.35 (ortho-Ph), 149.54 (ipso-Ph) ppm. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.37−1.52 (m, 4H, CH2), 1.66 (s, 6H, NCH3), 1.78−1.83 (m, 2H, CH2), 1.85 (s, 3H, NCH3), 2.12−2.17 (m, 4H, CH2), 2.83−2.89 (m, 2H, CH2), 3.05−3.12 (m, 2H, CH2), 3.42−3.46 (m, 2H, CH2), 7.17−7.20 (m, 3H, para-Ph), 7.28−7.32 (m, 6H, meta-Ph), 7.97−7.99 (m, 6H, ortho-Ph), ppm. 13 C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.95 (NCH3), 47.23 (NCH3), 51.59 (CH2), 53.55 (CH2), 56.05 (CH2), 63.55 (CH2), 126.65 (para-Ph), 127.73 (meta-Ph), 137.35 (ortho-Ph), 149.54 (ipsoPh) ppm. 29Si{1H} NMR (benzene-d6, 79.5 MHz): δ −12.2 (MgSi) ppm. Anal. Calcd for C29H40N4MgSi (497.05 g·mol−1): C, 70.08; H, 8.11; N, 11.27; Mg, 4.89. Found: C, 69.86; H, 9.04; N, 11.54; Mg, 4.71%. [(Me3TACD·AlMe3)Mg(SiPh3)] (3a). A solution of AlMe3 (16 mg; 0.22 mmol) in benzene (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (109 mg; 0.22 mmol) in benzene (2 mL). The reaction mixture was stirred for 1 h at room temperature, until a colorless precipitate formed. The precipitate was filtered, washed with n-pentane (3 × 5 mL), and dried under vacuum. [(Me3TACD·AlMe3)Mg(SiPh3)] (3a) (105 mg; 0.18 mmol) was obtained as a colorless powder in 84% yield. Complete decomposition occurred over six weeks at −30 °C in the solid state. 1 H NMR (THF-d8, 400.1 MHz): δ −0.96 (s, 9H, Al(CH3)3), 2.11 (s, 3H, NCH3), 2.24 (s, 6H, NCH3), 2.36−2.50 (m, 6H, CH2), 2.56− 2.62 (m, 2H, CH2), 2.71−2.83 (m, 6H, CH2), 3.02−3.07 (m, 2H, CH2), 7.03−7.06 (m, 3H, para-Ph), 7.09−7.13 (m, 6H, meta-Ph), 7.55−7.57 (m, 6H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8, 100.6 MHz): δ −5.70 (br, Al(CH3)3), 44.85 (NCH3), 46.43 (NCH3), 48.23 (CH2), 54.55 (CH2), 54.89 (CH2), 57.29 (CH2), 126.77 (para-Ph), 127.70 (meta-Ph), 137.75 (ortho-Ph), 148.97 (ipso-Ph). 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −14.7 (MgSi) ppm. 27Al NMR (THF-d8, 100 MHz): δ 157.55 (AlMe3) ppm. Anal. Calcd for C32H49N4AlMgSi (569.14 g·mol−1): C, 67.53; H, 8.68; N, 9.84; Mg, 4.27; Al, 4.74. Found: 67.55; H, 8.54; N, 9.77; Mg, 3.85; Al, 4.38%. [(Me3TACD·AlEt3)Mg(SiPh3)] (3b). A solution of AlEt3 (28.5 mg; 0.25 mmol) in benzene (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (124 mg; 0.25 mmol) in benzene (2 mL). The reaction mixture was stirred for 1 h at room temperature, until a colorless precipitate formed. The precipitate was filtered, washed with n-pentane (3 × 5 mL), and dried under vacuum. [(Me3TACD·AlEt3)Mg(SiPh3)] (3b) (132 mg; 0.21 mmol) was obtained as a colorless powder in 86% yield. Complete decomposition occurred over six weeks at −30 °C in the solid state. 1 H NMR (benzene-d6, 400.1 MHz): δ 0.45 (q, 3JHH = 8.03 Hz, 6H, Al(CH2CH3)3), 1.27−1.31 (m, 2H, CH2), 1.40−1.53 (m, 6H, CH2), 1.56 (s, 3H, NCH3), 1.72 (t, 3JHH = 8.03 Hz, 9H, Al(CH2CH3)3), 1.80−1.83 (m, 2H, CH2), 1.83 (s, 6H, NCH3), 1.96−2.09 (m, 4H, CH2), 2.99−3.05 (m, 2H, CH2), 7.17−7.21 (m, 3H, para-Ph), 7.32− 7.36 (m, 6H, meta-Ph), 7.87−7.89 (m, 6H, ortho-Ph) ppm. 1 H NMR (THF-d8, 400.1 MHz): δ −0.14 (q, 3JHH = 8.03 Hz, 6H, Al(CH2CH3)3), 0.99 (t, 3JHH = 8.03 Hz, 9H, Al(CH2CH3)3), 2.06 (s, 3H, Me), 2.26 (s, 6H, NCH3), 2.34−2.48 (m, 4H, CH2), 2.50−2.53 (m, 4H, CH2), 2.72−2.80 (m, 6H, CH2), 3.01−3.06 (m, 2H, CH2), 7.03−7.07 (m, 3H, para-Ph), 7.10−7.14 (m, 6H, meta-Ph), 7.54−7.57 (m, 6H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8, 100.6 MHz): δ 3.16 (Al(CH2CH3)3), 11.90 (Al(CH2CH3)3), 44.93 (NCH3), 46.55 14986


Article

Inorganic Chemistry

was removed under reduced pressure, and the oily residue was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The solid was dried under vacuum and recrystallized from THF and n-hexane at −30 °C to afford [(Me3TACD·AlEt3)Mg(SiHPh2)] (7b) (113 mg; 0.21 mmol) as colorless crystals in 84% yield. Single crystals suitable for X-ray diffraction were grown from THF/n-hexane at −30 °C over a period of 24 h. 1 H NMR (THF-d8, 400.1 MHz): δ −0.16 (q, 6H, 3JHH = 8.03 Hz, Al(CH2CH3)3), 1.04 (t, 9H, 3JHH = 8.03 Hz, Al(CH2CH3)3), 2.24 (s, 6H, NCH3), 2.37−2.56 (m, 8H, CH2), 2.42 (s, 3H, NCH3), 2.69−2.81 (m, 6H, CH2), 3.00−3.05 (m, 2H, CH2), 4.65 (s, 1JSiH = 137 Hz, 1H, SiH2Ph), 6.98−7.01 (m, 1H, para-Ph), 7.06−7.10 (m, 2H, meta-Ph), 7.63−7.65 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8, 100.6 MHz): δ 2.68 (Al(CH2CH3)3), 12.10 (Al(CH2CH3)3), 44.25 (NCH3), 46.17 (NCH3), 48.03 (CH2), 54.18 (CH2), 54.37 (CH2), 56.73 (CH2), 125.25 (para-Ph), 127.64 (meta-Ph), 136.82 (ortho-Ph), 148.21 (ipsoPh) ppm. 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −29.0 (MgSi) ppm. 27 Al NMR (THF-d8, 100 MHz): δ 180.82 (AlEt3) ppm. Anal. Calcd for C29H51N4AlMgSi (535.13 g·mol−1): C, 65.09; H, 9.61; N, 10.47; Mg, 4.54; Al, 5.04. Found: C, 65.15; H 9.17; N, 10.75; Mg, 3.89; Al 4.95%. [(Me3TACD)Mg(O-2,6-tBu2-C6H3)] (8). A solution of 2,6-di-tertbutylphenol (62 mg; 0.30 mmol) in benzene (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (149 mg; 0.30 mmol) in benzene (2 mL), and the reaction mixture was stirred for 30 min at room temperature. The solvent was removed under reduced pressure, and the solid residue was washed with n-pentane (3 × 3 mL) to completely separate HSiPh3. The solid was dried under vacuum to give [(Me3TACD)Mg(O-2,6-tBu2-C6H3)] (8) (65 mg; 0.15 mmol) as a colorless powder in 49% yield. 1 H NMR (THF-d8, 400.1 MHz): δ 1.46 (s, 18H, CCH3), 2.35 (s, 6H, NCH3), 2.37−2.38 (m, 2H, CH2), 2.40−2.46 (m, 4H, CH2), 2.62 (s, 3H, NCH3), 2.64−2.70 (m, 2H, CH2), 2.81−2.89 (m, 4H, CH2), 3.04−3.08 (m, 2H, CH2), 3.12−3.19 (m, 2H, CH2), 6.17 (t, 3JHH = 7.53 Hz, 1H, para-Ph), 6.90 (d, 3JHH = 7.53 Hz, 2H, meta-Ph) ppm. 13 C{1H} NMR (THF-d8, 100.6 MHz): δ 31.61 (CCH3), 46.51 (NCH3), 46.90 (NCH3), 51.99 (CH2), 53.43 (CH2), 56.42 (CH2), 63.47 (CH2), 112.59 (para-Ph), 124,58 (br, meta-Ph), 137.1 (CCH3, identified by HMBC), 164.71 (ipso-Ph) ppm. Anal. Calcd for C25H46N4OMg (442.98 g·mol−1): C, 67.79; H, 10.47; N, 12.65; Mg, 5.49. Found: C, 68.26; H, 9.98; N, 11.07; Mg, 5.03%. [(Me3TACD)Mg(CCPh)] (9). A solution of phenylacetylene (20 mg; 0.20 mmol) in benzene (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (99 mg; 0.20 mmol) in benzene (2 mL), and the reaction mixture was stirred for 30 min at room temperature. The solvent was removed under reduced pressure, and the solid residue was washed with n-pentane (3 × 3 mL) to completely separate HSiPh3. The solid was dried under vacuum to give [(Me3TACD)Mg(CCPh)] (9) (48 mg; 0.14 mmol) as a colorless powder in 71% yield. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.52−1.58 (m, 2H, CH2), 1.61−1.67 (m, 2H, CH2), 1.88−1.94 (m, 2H, CH2), 1.98 (s, 3H, NCH3), 2.19.2.24 (m, 2H, CH2), 2.26 (s, 6H, NCH3), 2.25−2.29 (m, 2H, CH2), 2.89−3.05 (m, 4H, CH2), 3.39−3.42 (m, 2H, CH2), 6.97− 7.01 (m, 1H, para-Ph), 7.10−7.14 (m, 2H, meta-Ph), 7.77−7.79 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.37 (NCH3), 46.79 (NCH3), 50.82 (CH2), 53.05 (CH2), 54.87 (CH2), 62.99 (CH2), 110.35 (CCPh), 125.29 (ipso-Ph), 125.62 (para-Ph), 128.56 (meta-Ph), 130.11 (CCPh), 132.44 (ortho-Ph) ppm. Anal. Calcd for C19H30N4Mg (338.78 g·mol−1): C, 67.36; H, 8.93; N, 16.54; Mg, 7.17. Found: C, 66.33; H, 8.72; N, 16.43; Mg, 6.56%. [(Me3TACD)Mg(OCCH2Ph)] (10). A solution of acetophenone (30 mg; 0.25 mmol) in benzene (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (124 mg; 0.25 mmol) in benzene (2 mL), and the reaction mixture was stirred for 1 h at room temperature. The solvent was removed under reduced pressure, and the brown oily residue was washed with n-pentane (3 × 3 mL) to completely separate HSiPh3. The solid was dried under vacuum to give [(Me3TACD)Mg(OCCH2Ph)] (10) (69 mg; 0.19 mmol) as a colorless powder in 77% yield.

at room temperature. The solvent was removed under reduced pressure, and the oily residue was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The solid was dried under vacuum and recrystallized from THF/n-pentane at −30 °C to give [(Me3TACD· AlMe3)Mg(SiH2Ph)] (6a) (85 mg; 0.20 mmol) as colorless microcrystals in 82% yield. 1 H NMR (THF-d8, 400.1 MHz): δ −0.98 (s, 9H, Al(CH3)3), 2.35 (s, 6H, NCH3), 2.38 (s, 3H, NCH3), 2.42−2.53 (m, 6H, CH2), 2.60− 2.63 (m, 2H, CH2), 2.72−2.85 (m, 6H, CH2), 3.04−3.11 (m, 2H, CH2), 3.67 (s, 1JSiH = 134 Hz, 1H, SiH2Ph), 6.91−6.95 (m, 1H, paraPh), 6.98−7.02 (m, 2H, meta-Ph), 7.49−7.51 (m, 2H, ortho-Ph) ppm. 13 C{1H} NMR (THF-d8, 100.6 MHz): δ −6.56 (Al(CH3)3), 43.82 (NCH3), 45.94 (NCH3), 47.91 (CH2), 54.04 (CH2), 54.34 (CH2), 56.65 (CH2), 125.64 (para-Ph), 127.49 (meta-Ph), 136.62 (ortho-Ph), 146.85 (ipso-Ph) ppm. 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −73.2 (MgSi) ppm. 27Al NMR (THF-d8, 100 MHz): δ 165.40 (AlMe3) ppm. Anal. Calcd for C20H41N4AlMgSi (416.95 g·mol−1): C, 57.61; H, 9.91; N, 13.44; Mg, 5.83; Al, 6.47. Found: C, 57.65; H, 9.76; N, 13.39; Mg, 5.29; Al 6.32%. [(Me3TACD·AlEt3)Mg(SiH2Ph)] (6b). A solution of H3SiPh (27 mg; 31 μL; 0.25 mmol) in THF (1 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (2) (124 mg; 0.25 mmol) in THF (2 mL), and the reaction mixture was stirred for 1 h at room temperature. To the reaction solution, a solution of AlEt3 (28 mg; 34 μL; 0.25 mmol) in THF (1 mL) was added, and the reaction mixture was stirred for 1 h at room temperature. The solvent was removed under reduced pressure, and the oily residue was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The solid was dried under vacuum and recrystallized from THF/n-pentane at −30 °C to give [(Me3TACD· AlEt3)Mg(SiH2Ph)] (6b) (104 mg; 0.23 mmol) as colorless microcrystals in 91% yield. 1 H NMR (THF-d8, 400.1 MHz): δ −0.16 (q, 6H, 3JHH = 8.03 Hz, Al(CH2CH3)3), 1.07 (t, 9H, 3JHH = 8.03 Hz, Al(CH2CH3)3), 2.35 (s, 6H, NCH3), 2.39 (s, 3H, NCH3), 2.45−2.53 (m, 8H, CH2), 2.71−2.84 (m, 6H, CH2), 3.02−3.08 (m, 2H, CH2), 3.67 (s, 1JSiH = 134 Hz, 1H, SiH2Ph), 6.91−6.95 (m, 1H, para-Ph), 6.99−7.02 (m, 2H, meta-Ph), 7.49−7.51 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (THF-d8, 100.6 MHz): δ 2.58 (Al(CH2CH3)3), 12.13 (Al(CH2CH3)3), 43.87 (NCH3), 45.91 (NCH3), 48.00 (CH2), 54.07 (CH2), 54.27 (CH2), 56.65 (CH2), 125.65 (para-Ph), 127.50 (meta-Ph), 136.60 (ortho-Ph), 146.80 (ipsoPh) ppm. 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −74.1 (MgSi) ppm. 27 Al NMR (THF-d8, 100 MHz): δ 171.63 (AlEt3) ppm. Anal. Calcd for C23H47N4SiMgAl (459.03 g·mol−1): C, 60.18; H, 10.32; N, 12.21; Mg, 5.29; Al, 5.88. Found: C, 61.30; H 10.18; N 12.22; Mg, 5.01, Al 5.52%. [(Me3TACD·AlMe3)Mg(SiHPh2)] (7a). A solution of AlMe3 (18 mg; 24 μL, 0.25 mmol) was added to a solution of [(Me3TACD)Mg(SiHPh2)] (5b) (105 mg; 0.25 mmol) in THF (2 mL), and the reaction mixture was stirred for 1 h at room temperature. The solvent was removed under reduced pressure, and the oily residue was washed with n-pentane (3 × 3 mL) to separate HSiPh3. The solid was dried under vacuum and recrystallized from THF/n-hexane at −30 °C to afford colorless crystals of [(Me3TACD·AlMe3)Mg(SiHPh2)] (7a) (95 mg; 0.19 mmol) in 77% yield. 1 H NMR (THF-d8, 400.1 MHz): δ −1.00 (s, 9H, Al(CH3)3), 2.23 (s, 6H, NCH3), 2.36−2.49 (m, 6H, CH2), 2.43 (s, 3H, NCH3), 2.54− 2.60 (m, 2H, CH2), 2.68−2.82 (m, 6H, CH2), 3.02−3.08 (m, 2H, CH2), 4.65 (s, 1JSiH = 137 Hz, 1H, SiHPh2), 6.97−7.01 (m, 1H, paraPh), 7.05−7.08 (m, 2H, meta-Ph), 7.63−7.65 (m, 2H, ortho-Ph) ppm. 13 C{1H} NMR (THF-d8, 100.6 MHz): δ −6.25 (Al(CH3)3), 44.19 (NCH3), 46.10 (NCH3), 47.95 (CH2), 54.14 (CH2), 54.42 (CH2), 56.80 (CH2), 126.24 (para-Ph), 127.64 (meta-Ph), 136.84 (ortho-Ph), 146.26 (ipso-Ph). 29Si{1H} NMR (THF-d8, 79.5 MHz): δ −28.0 (MgSi) ppm. 27Al NMR (THF-d8, 100 MHz): δ 177.59 (AlMe3) ppm. Anal. Calcd for C26H45N4AlMgSi (493.05 g·mol−1): C, 63.34; H, 9.20; N, 11.36; Mg, 4.93; Al, 5.47. Found: C, 64.00; H 9.31; N, 11.05; Mg, 4.00, Al 5.31%. [(Me3TACD·AlEt3)Mg(SiHPh2)] (7b). A solution of AlEt3 (28 mg; 34 μL; 0.25 mmol) was added to a solution of [(Me3TACD)Mg(SiHPh2)] (5b) (105 mg; 0.25 mmol) in THF (2 mL), and the reaction mixture was stirred for 1 h at room temperature. The solvent 14987


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H NMR (benzene-d6, 400.1 MHz): δ 1.46−1.62 (m, 4H, CH2), 1.83−1.89 (m, 2H, CH2), 1.92 (s, 3H, NCH3), 2.12 (s, 6H, NCH3), 2.16−2.26 (m, 4H, CH2), 2.87−2.94 (m, 2H, CH2), 2.99−3.06 (m, 2H, CH2), 3.40−3.44 (m, 2H, CH2), 4.41 (s, 1H, CCHH), 4.89 (s, 1H, CCHH), 7.18−7.21 (m, 1H, para-Ph), 7.33−7.37 (m, 2H, meta-Ph), 8.23−8.25 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.44 (NCH3), 46.51 (NCH3), 50.69 (CH2), 52.81 (CH2), 54.71 (CH2), 63.21 (CH2), 79.33 (CCHH), 126.66 (meta-Ph), 126.95 (para-Ph), 128.15 (ortho-Ph, identified by HSQC due to overlap with benzene-d6 resonances), 145.36 (ipso-Ph) 165.50 (CCHH) ppm. Anal. Calcd for C19H32N4OMg (356.80 g· mol−1): C, 63.96; H, 9.04; N, 15.70; Mg, 6.81. Found: C, 62.83; H, 8.68; N, 15.13; Mg, 6.24%. [(Me3TACD)Mg(NHPh)] (11). A solution of aniline (20.5 mg; 20 μL; 0.22 mmol) in THF (2 mL) was added to a solution of [(Me3TACD)Mg(SiPh3)] (109 mg; 0.22 mmol) in THF (2 mL). The reaction mixture was stirred for 30 min at room temperature. The solvent was removed under reduced pressure, and the solid residue was washed with n-pentane (3 × 5 mL) to completely separate triphenylsilane. The solid was dried under vacuum to give [(Me3TACD)Mg(NHPh)] (11) (52 mg; 0.12 mmol) as a colorless powder in 83% yield. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.43−1.54 (m, 4H, CH2), 1.64 (s, 3H, NCH3), 1.83−1.88 (m, 2H, CH2), 2.02 (s, 6H, NCH3), 2.16−2.26 (m, 4H, CH2), 2.82 (br, s, 1H, NHPh), 2.84−2.91 (m, 2H, CH2), 2.99−3.06 (m, 2H, CH2), 3.38−3.42 (m, 2H, CH2), 6.57−6.61 (m, 1H, para-Ph), 6.91−6.93 (m, 2H, meta-Ph), 7.29−7.32 (m, 2H, ortho-Ph) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.02 (NCH3), 46.25 (NCH3), 50.57 (CH2), 52.85 (CH2), 54.84 (CH2), 62.99 (CH2), 110.36 (para-Ph), 116.31 (meta-Ph), 130.09 (ortho-Ph), 162.64 (ipso-Ph) ppm. Anal. Calcd for C17H31N5Mg (329.78 g·mol−1): C, 61.92; H, 9.48; N, 21.24; Mg, 7.37. Found: C, 61.88; H, 8.97; N, 20.12; Mg, 6.96%. [(Me3TACD)MgCl] (12). A mixture of [NEt3H]Cl (74 mg; 0.54 mmol) and [(Me3TACD)Mg(SiPh3)] (266 mg; 0.54 mmol) in THF (6 mL) was stirred for 2 h at room temperature. The reaction mixture was filtered, and the solvent was removed from the filtrate under reduced pressure. The solid residue was washed with n-pentane (3 × 5 mL) to completely separate HSiPh3. [(Me3TACD)MgCl] (12) (101 mg; 0.37 mmol) was obtained as a colorless powder in 68% yield. 1 H NMR (benzene-d6, 400.1 MHz): δ 1.48−1.54 (m, 2H, CH2), 1.57−1.63 (m, 2H, CH2), 1.83−1.89 (m, 2H, CH2), 1.91 (s, 3H, NCH3), 2.14−2.19 (m, 2H, CH2), 2.21 (s, 6H, NCH3), 2.21−2.24 (m, 2H, CH2), 2.83−2.90 (m, 2H, CH2), 2.93−3.00 (m, 2H, CH2), 3.32− 3.36 (m, 2H, CH2) ppm. 13C{1H} NMR (benzene-d6, 100.6 MHz): δ 43.56 (NCH3), 47.03 (NCH3), 50.66 (CH2), 52.36 (CH2), 54.61 (CH2), 62.98 (CH2) ppm. Anal. Calcd for C11H25N4ClMg (273.10 g· mol−1): C, 48.38; H, 9.23; N, 20.52; Mg, 8.90. Found: C, 48.15; H, 8.77; N, 19.52; Mg, 8.42%. 1

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AUTHOR INFORMATION

Corresponding Author

* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany. Fax: +49 241 80 92644. E-mail: [email protected]. ORCID

Jun Okuda: 0000-0002-1636-5464 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (Cluster of Excellence “Tailor Made Fuels from Biomass”) for financial support and Dr. D. Mukherjee for helpful discussions.

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REFERENCES

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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.inorgchem.7b02233. 1 H, 13C{1H}, 27Al, and 29Si{1H} NMR spectra of the complexes 1 to 12 and X-ray crystallographic details for 1, 2, 4, and 7b as well as [(Me3TACD·AlEt3)MgEt] and [(Me3TACD·AlMe3)MgMe] (PDF) Accession Codes

CCDC 1565209−1565214 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 14988


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