Reactivity of a Molecular Magnesium Hydride Featuring a Terminal

Dec 1, 2016 - Although the rare terminal Mg−H bond is regarded as highly polar, the molecular magnesium hydride [Mg(Me3TACD·AliBu3)H], supported by...
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Reactivity of a Molecular Magnesium Hydride Featuring a Terminal Magnesium−Hydrogen Bond Silvia Schnitzler, Thomas P. Spaniol, and Jun Okuda* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany S Supporting Information *

ABSTRACT: The reactivity of the molecular magnesium hydride [Mg(Me3TACD·AliBu3)H] (1) featuring a terminal magnesium−hydrogen bond and an NNNN-type macrocyclic ligand, Me3TACD ((Me3TACD)H = Me3[12]aneN4 = 1,4,7trimethyl-1,4,7,10-tetraazacyclododecane), can be grouped into protonolysis, oxidation, hydrometalation, (insertion), and hydride abstraction. Protonolysis of 1 with weak Brønsted acids HX such as terminal acetylenes, amines, silanols, and silanes gave the corresponding derivatives [Mg(Me3TACD·AliBu3)X] (X = CCPh, 3; HN(3,5-Me2-C6H3), 4; OSiMe3, 5; OSiPh3, 6; Cl, 7; Br, 8). Single-crystal X-ray diffraction of anilide 4 showed a square-pyramidal coordination geometry for magnesium. No correlation with the pKa values of the acids was detected. Oxidation of 1 with elemental iodine gave the iodide [Mg(Me3TACD· AliBu3)I] (9), and oxidation with nitrous oxide afforded the μ-oxo-bridged compound [{Mg(Me3TACD·AliBu3)}2(μ-O)] (10) with a linear Mg−O−Mg core, as characterized by single-crystal X-ray diffraction. The Mg−H bond reacted with benzaldehyde, benzophenone, fluorenone, and CO2 under insertion but not with the olefins 1,1,2-triphenylethylene, tert-butylethylene, and cyclopentene. The unstable formate, prepared also by salt metathesis of iodide 9 with potassium formate, revealed κO,κO′ coordination in the solid state. Hydride abstraction with triphenylborane gave the ion pair [Mg(Me3TACD·AliBu3)(thf)][HBPh3] (16), which catalyzed the hydroboration of polar substrates by pinacolborane.

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INTRODUCTION The availability, low cost, and nontoxicity of magnesium compounds render their catalytic applications attractive. Recently, several magnesium-catalyzed hydroaminations, hydroborations, hydrosilylations, coupling reactions, and C−H bond activations have been reported.1 Magnesium hydrides are implied as reactive intermediates generated in the initial step of a catalytic cycle.2 Isolation and characterization of molecularly defined magnesium hydrides, in particular mononuclear compounds, remain difficult. This is due to their inherent thermodynamic instability, as MgH2 with an ionic lattice is highly stable (ΔH = 2709 kJ mol−1), and also to the occurrence of the Schlenk equilibrium. The dimeric magnesium hydride [CH{C(Me)NAr}2Mg(μ-H)]2 (Ar = 2,6-iPr2C6H3) with a bulky nacnac-type ligand reported by Jones et al.3 catalyzes hydroborations and hydrosilylations.2b,e As a convenient precursor for the magnesium hydride, the corresponding alkyl compound [CH{C(Me)NAr}2MgnBu] was used in several catalytical applications.2a,d,f,i,3 To date, only three other © XXXX American Chemical Society

compounds with a terminal magnesium−hydrogen bond are known, and their reactivity has not been studied systematically.4 We recently reported the synthesis of the molecular magnesium hydride [Mg(Me3TACD·AliBu3)H] (1) featuring a terminal magnesium−hydrogen bond. As an ancillary ligand, the NNNN-type macrocyclic ligand Me3TACD ((Me3TACD) H = Me3[12]aneN4 = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) blocked by AliBu3 at the Lewis basic amido function was used (Scheme 1).5 This ligand modification was necessary to prevent the formation of larger clusters.6 The compound was structurally characterized by X-ray diffraction, and the Mg−H bond length of 1.76(4) Å agrees well with the sum of the covalent radii (1.72(7) Å) and with the result of DFT calculations.5 Initial studies revealed that despite the high polarity of the magnesium−hydrogen bond, 1 reacts rather selectively with polar substrates both as a Brønsted base and as Received: October 15, 2016

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DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Molecular Structure and HOMO of [Mg(Me3TACD·AliBu3)H] (1)5

a Lewis acid. A pronounced steric influence appeared to limit the substrate scope.5 Here we describe the reactivity of 1 toward several substrates and its potential as a useful precursor for various magnesium complexes with the ligand (Me3TACD·AliBu3). The reactions are grouped into four classes: protonolysis, oxidation, hydrometalation (insertion), and hydride abstraction. Hydroboration catalysis of polar substrates by the ion pair [Mg(Me3TACD· AliBu3)(thf)][HBPh3] has also been demonstrated.7

Analogous to the previously reported reaction with trimethylsilylacetylene to give [Mg(Me3TACD·AliBu3)(C CSiMe3)] (2),5 reaction of 1 with phenylacetylene (pKa = 28.7)9 gave [Mg(Me3TACD·AliBu3)(CCPh)] (3) in 86% yield. The 13C{1H} NMR spectrum in THF-d8 showed the signal for the acetylide carbon at 130.2 ppm, consistent with expected values.5,10 With respect to the reactivity against primary and secondary amines, the steric effect of the amine substrate rather than its NH acidity apparently influences the selectivity of the aminolysis. Complex 1 was inert toward the secondary amines bis(trimethylsilyl)amine (pKa = 25.8)11 and diisopropylamine (pKa = 18.8).12 In contrast, reaction of 1 with diphenylamine (pKa = 6.0)12 or pyrrolidine (pKa = 19.6)12 or with aniline (pKa = 10.6),12 2,4,6-trimethylaniline, or 2,6-diisopropylaniline led to intractable product mixtures. Only the addition of 3,5dimethylaniline (pKa = 10.5)12 to a THF solution of 1 did selectively give the complex [Mg(Me3TACD·AliBu3){HN(3,5Me2-C6H3)}] (4) in 84% isolated yield. The compound was soluble in THF and aromatic hydrocarbons but decomposed in solution after about 12 h at 25 °C. The 1H NMR spectrum of 4 in THF-d8 showed the characteristic signals for the Me3TACD ligand and the isobutyl and 3,5-dimethylphenyl groups. The NH resonance was not assigned because of overlap with other signals from the Me3TACD ligand. It was expected to appear at 2.6−3.6 ppm, as reported for similar complexes such as [Mg n Bu{HN(2,6-Me 2 -C 6 H 3 )}(TMEDA)] (TMEDA = N,N,N′,N′-tetramethylethylenediamine) (2.70 ppm),13 [Mg(HNPh){N(SiMe3)2}(thf)]2 (3.62 ppm),14 and [Mg{HN(2,4,6-Me3-C6H2)}2{OP(NMe2)3}2] (3.05 ppm).15 Crystal structure analysis confirmed the coordination of [HN(3,5Me2-C6H3)]− at the magnesium center. Single crystals grown from toluene at −35 °C were suitable for X-ray diffraction. Complex 4 crystallized in the triclinic space group P1̅ with three independent molecules of 4 in the unit cell. The magnesium center shows a square-pyramidal coordination geometry formed by the amido function of [HN(3,5-Me2C6H3)]− (N1) and four nitrogen atoms of the Me3TACD ligand (N2−N5) (Figure 1). The amido atom N2 of the Me3TACD ligand is also attached to a tetrahedrally coordinated aluminum center (Al1). The bond lengths Mg1−N1 (1.998(2) Å), Mg2−N6 (1.997(2) Å), and Mg3−N11 (2.001(2) Å) are consistent with the expected values.13,16 In the context of hydrosilylation catalysis, we wanted to see whether the anilide converted back to the hydride 1 or to the phenylsilyl complex isolated previously. 5 Complex 4 did not react with triethylsilane. With phenylsilane, 4 gave an intractable product mixture. The stoichiometric reactions of 1 with alcohols such as triphenylmethanol (pKa = 17.0)17 and phenols such as 2,6-ditert-butylphenol (pKa = 16.9)9 in THF-d8 were complete within 5 min at 25 °C, but the 1H NMR spectra in THF-d8 indicated the formation of product mixtures. Apparently, both the

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RESULTS AND DISCUSSION Protonolysis. The terminal magnesium hydride 1 reacted with weak Brønsted acids, resulting in the formation of dihydrogen and coordination of the conjugate base to the metal center (Scheme 2). A side reaction involving protonation of the isobutyl groups at the aluminum center gave isobutane. Reactions of 1 with terminal acetylenes, amines, silanols, and silanes may also be mechanistically classified as σ-bond metathesis.1b,8

Scheme 2. Protonolysis of 1

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DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

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(Scheme 3). Excess iodine attacked the AliBu3 group of the ligand to form side products. Scheme 3. Oxidation of 1 with Elemental Iodine

The reaction of 1 with excess nitrous oxide in benzene at 25 °C gave the μ-oxo compound [{Mg(Me3TACD·AliBu3)}2(μO)] (10). It is conceivable that the hydride ligand in 1 could react to give the magnesium hydroxide [Mg(Me3TACD· AliBu3)OH] (11) with the formation of dinitrogen (Scheme 4). Similar behavior was reported for the germanium hydride complex [CH{C(Me)NAr}2GeH] (Ar = 2,6-iPr2C6H3)20 and the hafnium hydride complex [Cp*2HfH(Ph)] (Cp*= η5C5Me5).21 Isolation of magnesium hydroxide 11 failed because of fast condensation in solution to give 10 (Scheme 4). The isolated compound 10 was stable for several months in the solid state at −30 °C and for 2 weeks in THF or in aromatic solvents at 25 °C. Compound 10 is unreactive toward excess N2O or dihydrogen. Single crystals grown from a THF/n-pentane mixture were suitable for X-ray diffraction. Complex 10 crystallized in the orthorhombic space group P21212 with crystallographic C2 symmetry. The oxygen atom O1 is located on a twofold axis of Wyckoff position 2a and bridges the two monomeric magnesium units (Figure 2). Magnesium is coordinated in a square-pyramidal environment by oxygen atom O1 and the nitrogen atoms of the Me3TACD ligand. The amido function N1 is blocked as in 4. The Mg1−O1 bond length of 1.8380(6) Å is slightly shorter than that in the previously reported complex [CH{C(Me)NAr} 2 Mg(thf)(μ-O)] (Mg1−O1, 1.8080(5) Å).22 The Mg1−O1−Mg1* moiety is nearly linear, forming an angle of 179.27(14)°. Hydrometalation. Insertion of unsaturated substrates into the magnesium−hydrogen bond can be regarded as hydrometalation or as an addition of a magnesium−hydrogen bond.1a This fundamental organometallic reaction plays an important role in catalysis of group 2 metal centers.1a,2i,23 The reaction with pyridine as an example of a hydrometalation of 1 resulted in dearomatization to give the thermodynamically preferred 1,4-dihydropyridinato derivative [Mg(Me3TACD·AliBu3)(C5H6N)].5 The molecular calcium hydride [CH{C(Me)NAr}2CaH(thf)]224 reacted with 1,1-diphenylethylene via insertion of the carbon−carbon double bond. Olefins were expected to insert into the magnesium−hydrogen bond (hydromagnesiation), but 1 did not react with ethylene, styrene, 1,1,2triphenylethylene, tert-butylethylene, or cyclopentene in THFd8 at 60 °C. Only 1,1-diphenylethylene was completely consumed by 1 within 30 min in THF-d8 at 25 °C. The 1H NMR spectrum in THF-d8 suggested the formation of two regioisomers, [Mg(Me3TACD·AliBu3)(CPh2CH3)] (A) and [Mg(Me3TACD·AliBu3)(CH2CHPh2)] (B) (Scheme 5). Separation and isolation of these insertion products has failed to date. The 1H NMR spectrum in THF-d8 at 25 °C showed two sets of signals for the Me3TACD ligand and the aluminumbonded isobutyl groups in the ratio of A:B = 3:1. Apart from

Figure 1. Molecular structure of [Mg(Me3TACD·AliBu3){HN(3,5Me2-C6H3)}] (4) in the solid state. Displacement parameters are shown at 50% probability; hydrogen atoms except H1 have been omitted for clarity. Selected bond distances (Å): Mg1−N1 1.998(2), Mg1−N2 2.165(2), Mg1−N3 2.217(2), Mg1−N4 2.274(2), Mg1−N5 2.264(2), Al1−N2 1.976(2), Mg2−N6 1.997(2), Mg2−N7 2.173(2), Mg2−N8 2.233(2), Mg2−N9 2.334(2), Mg2−N10 2.215(2), Al2−N7 1.985(2), Mg3−N11 2.001(2), Mg3−N12 2.166(2), Mg3−N13 2.246(2), Mg3−N14 2.287(2), Mg3−N15 2.196(2), Al3−N12 1.978(2).

hydride ligand and the aluminum-bound isobutyl groups reacted with the alcohols, leading to the formation of dihydrogen and isobutane. In contrast, 1 reacted with trimethylsilanol and triphenylsilanol (pKa = 16.6)17 in THF at 25 °C to give the magnesium siloxides [Mg(Me3TACD· AliBu3)(OSiR3)] (R = Me, 5; R = Ph, 6), respectively. The 29Si NMR spectra in THF-d8 showed one signal for the Si−O resonances at −111.59 (5) and −112.41 (6) ppm. These are significantly shifted toward high field compared to the silicon resonances in the complexes [Mg{(μ-OSiPh3)2Mg(OSiPh3)}2] (−15.30 and −26.30 ppm), [Mg(OSiPh3)2(thf)]2 (−17.20 and −29.30 ppm), and [Mg(OSiPh3)2(py)]2 (−30.50 ppm).18 Compounds 5 and 6 did not react with phenylsilane or dihydrogen. Treatment of 1 with triethylammonium chloride and bromide in THF at ambient temperature gave [Mg(Me3TACD·AliBu3)Cl] (7) and [Mg(Me3TACD·AliBu3)Br] (8), respectively, with concomitant formation of dihydrogen and triethylamine. Colorless crystals were isolated in good yield (77% for 7; 72% for 8) and characterized by NMR spectroscopy in solution and by elemental analysis. The 1H NMR spectra in THF-d8 at 25 °C showed only the characteristic signals for the Me3TACD ligand in the range of 2.45−3.14 ppm and one set of signals for the isobutyl groups. As previously reported,5 the reactivity of 1 toward silanes was influenced by both Brønsted acidity and steric effects. Reaction of 1 with phenylsilane (pKa = 31.6)19 under forced conditions gave the phenylsilylmagnesium complex [Mg(Me3TACD· AliBu3)(SiH2Ph)],5 which was characterized by NMR spectroscopy and X-ray diffraction. In contrast, 1 did not react with triphenylsilane (pKa = 34.8)19 or dimethylphenylsilane (pKa = 41.2).5,19 Oxidation. Hydride 1 was oxidized by elemental iodine and nitrous oxide. Stoichiometric reaction of 1 with a THF solution of elemental iodine led to the formation of dihydrogen and gave [Mg(Me3TACD·AliBu3)I] (9) in 75% isolated yield. 9 was characterized by NMR spectroscopy and elemental analysis C

DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 4. Oxidation of 1 with N2O

the signals for the phenyl protons at 7.05−7.11 and 7.15−7.22 ppm for A and B, respectively, the spectrum exhibited two overlapping signals at 1.60 ppm (A) and 1.59 ppm (B) that can be assigned to the methyl group of A and the CHPh resonance of B, consistent with expected values.25 The corresponding carbon signals were observed at 28.3 ppm (A) and 28.0 ppm (B) in the 13C{1H} NMR spectra (see the Supporting Information). The remaining proton and carbon signals of the magnesium-bound alkyl groups could not be assigned. Treatment of 1 with aldehydes and ketones gave magnesium alkoxide complexes (Scheme 5). Hydride 1 reduced benzaldehyde in THF at 25 °C to give [Mg(Me3TACD·AliBu3)(OCH2Ph)] (12) as colorless crystals in 74% isolated yield. The 1H NMR spectrum in THF-d8 exhibited one singlet for the OCH2Ph protons at 5.07 ppm. Likewise, hydride 1 reacted with benzophenone and fluorenone to give colorless [Mg(Me3TACD·AliBu3)(OCHPh2)] (13) and greenish (possibly as a result of traces of ketyl radical) [Mg(Me3TACD· AliBu3){OCH(C6H4)2}] (14), each in 83% yield. Their 1H NMR spectra showed the OCH proton resonance at 6.08 ppm (13) and 5.95 ppm (14) in THF-d8.

Figure 2. Molecular structure of [{Mg(Me3TACD·AliBu3)}2(μ-O)] (10) in the solid state. Displacement parameters are shown at 50% probability; hydrogen atoms have been omitted for clarity. Symmetry operation: *, 1 − x, 1 − y, z. Selected bond distances (Å) and angles (deg): Mg1−O1 1.8380(6), Mg1−N1 2.2973(18), Mg1−N2 2.2546(19), Mg1−N3 2.3425(19), Mg1−N4 2.298(2), Al1−N1 1.9848(18), Mg1−O1−Mg1* 179.27(14).

Scheme 5. Hydrometalation of 1

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DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

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Mg1−O2 (2.121(2) Å) bond distances as well as the Mg2− O3 (2.124(2) Å) and Mg2−O4 (2.129(2) Å) bond distances are similar, but they are significantly longer than those of the dimeric formate species [(C6F5)3BOC(H)OC(H){(Me)CNAr}2Mg(O2CH)]2 reported by Hill and co-workers28 (1.979(4) and 1.935(3) Å). The double bond is delocalized over the O1−C1−O2 moiety and accordingly to the O3− C25−O4 unit. Consequently, the oxygen−carbon bond lengths C1−O1 (1.258(4) Å) and C1−O2 (1.255(4) Å) as well as C25−O3 (1.262(4) Å) and C25−O4 (1.255(4) Å) are also similar, but they are significantly longer than those in [(C 6 F 5 ) 3 BOC(H)OC(H){(Me)CNAr} 2 Mg(O 2 CH)] 2 (1.218(6) and 1.203(6) Å).28 Hydride Abstraction. Addition of triphenylborane to a THF solution of 1 resulted in hydride abstraction to give [Mg(Me3TACD·AliBu3)(thf)][HBPh3] (16) as a colorless powder in 75% yield (Scheme 7). Tris(pentafluorophenyl)-

The reaction of 1 with acetophenone or cyclohexanone in THF-d8 was complete at 25 °C within 15 min. However, the addition products were not stable, and enolization occurred with the formation of dihydrogen.26 Separation of the resulting product mixture failed. The reduction of ketones with the dimeric calcium hydride complex [CH{C(Me)NAr}2CaH(thf)]2 also gave product mixtures.27 The hydrometalation of 1 with CO2 was previously reported to give the formate complex [Mg(Me3TACD·AliBu3)(O2CH)] (15), which could not be isolated since it decomposed rapidly in THF solution, probably because of a small excess of CO2.5 This compound could be isolated from salt metathesis of [Mg(Me 3 TACD·Al i Bu 3 )I] (9) with potassium formate (Scheme 6). The quantitative reaction is completely selective Scheme 6. Synthesis of Formate Complex 15

Scheme 7. Reaction of 1 with BH3(thf)5 and BPh3

at 25 °C after 4 h, but tedious crystallization and isolation account for the moderate isolated yield of 42%. The compound is soluble in THF but decomposes in solution after about 5 days at 25 °C. The formate carbon signal at 174.5 ppm is identical to that recorded for the product of the reaction of 1 and 13CO2.5 Single crystals grown from a THF/n-pentane mixture at −35 °C were suitable for X-ray diffraction. Complex 15 crystallized in the orthorhombic space group Pbca with two crystallographically independent molecules. The magnesium center is coordinated by four nitrogen atoms of the Me3TACD ligand (N1−N4) and both formate oxygen atoms (Figure 3). The latter adopts a rare κO,κO′ bonding mode. Thus, the magnesium center exhibits a distorted trigonal-prismatic coordination geometry. The Mg1−O1 (2.120(2) Å) and

borane, pinacolborane, and tris(trimethylsilylmethyl)borane did not cleanly react with 1, and intractable product mixtures formed. BH3(thf) reacted with 1 to give the tetrahydroborate [Mg(Me3TACD·AliBu3)(BH4)]5 as the result of a Lewis acid− base reaction. The magnesium hydridoborate complex 16 could be stored for about 3 weeks at −35 °C but decomposed under reduced pressure. Single crystals grown from a THF/n-pentane mixture at −35 °C were suitable for X-ray diffraction. Complex 16 crystallized in the triclinic space group P1̅. The asymmetric unit contains a separated ion pair of the tetrahedrally coordinated [HBPh3]− anion and the cationic magnesium center [Mg(Me3TACD·AliBu3)(thf)]+ (Figure 4). The Me3TACD ligand and one THF molecule coordinate the magnesium atom in a square-pyramidal environment. The amido function N1 of the Me3TACD ligand is also blocked by the tetrahedrally coordinated aluminum center Al1. The hydride atom H1 of the [HBPh3]− anion was located from difference maps, and its positional parameters were refined. The H1···Mg1 distance of 6.25(3) Å and the arrangement of the molecules in the crystal lattice do not indicate an interaction between H1 and Mg1. The H1−B1 bond length of 1.19(3) Å is consistent with expected values.29 The NMR spectroscopic data of 16 agree with the structure in the solid state. The 1H NMR spectrum in THF-d8 exhibited a broad 1:1:1:1 quartet at 3.51 ppm (1JBH = 74 Hz) for the BH

Figure 3. Molecular structure of [Mg(Me3TACD·AliBu3)(OCHO)] (15) in the solid state. Displacement parameters are shown at 50% probability; hydrogen atoms except H1 have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mg1−N1 2.186(2), Mg1−N2 2.285(2), Mg1−N3 2.257(3), Mg1−N4 2.262(3), Mg1−O1 2.120(2), Mg1−O2 2.121(2), Al1−N1 1.971(2), C1−O1 1.258(4), C1−O2 1.255(4), Mg2−N5 2.195(2), Mg2−N6 2.247(2), Mg2−N7 2.283(3), Mg2−N8 2.278(3), Mg2−O3 2.124(2), Mg2−O4 2.129(2), Al2−N5 1.972(2), C25−O3 1.262(4), C25−O4 1.255(4), O1−C1− O2 122.6(3), Mg1−N1−Al1 114.89(11), O3−C25−O4 122.1(3), Mg2−N5−Al2 115.53(11). E

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Table 1. Hydroboration and Deoxygenation of Various Substrates Catalyzed by 16

Figure 4. Molecular structure of [Mg(Me3TACD·AliBu3)(thf)][HBPh3] (16) in the solid state. Displacement parameters are shown at 50% probability; hydrogen atoms except H1 have been omitted for clarity. Selected bond distances (Å) and angles (deg): Mg1−N1 2.122(2), Mg1−N2 2.214(2), Mg1−N3 2.185(2), Mg1−N4 2.230(2), Mg1−O1 2.0089(17), Al1−N1 1.991(2), B1−H1 1.19(3), Mg1−N1−Al1 115.98(10).

a n(HBpin) = 0.27 mmol. bn(substrate) = 0.27 mmol. cn(substrate) = 0.14 mmol. dCatalyst loading =1.0 mol % Mg. e0.5 mL of solvent (THF:THF-d8 = 2:3); fTime required for complete consumption of the substrate to give a quantitative yield of the product, as determined by 1H NMR spectroscopy.

resonance and two multiplets at 1.77 and 3.62 ppm for the free THF resonances. The 11B NMR spectrum showed a doublet at −7.91 ppm (1JBH = 74 Hz). Compared with the corresponding signals of the bridged magnesium hydridoborate complex [ToMMgHB(C6F5)3] (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) at −18.2 ppm, the resonances were slightly shifted to lower field.30 Hydridoborates of lighter s-block metals are common stoichiometric reducing agents for polar substrates such as organic carbonyl compounds.31 Recently, the group 1 hydridotriphenylborates [(L)M][HBPh3] (L = Me6TREN = tris{2-(dimethylamino)ethyl}amine; M = Li, Na, K) were shown to catalyze the hydroboration of carbonyls and CO2 chemoselectively.7 The ionic bis(hydridotriphenylborate)magnesium complex [Mg(thf)6][HBPh3] was also shown to be active in hydroboration of various substrates and deoxygenation of sulfoxides.32 Complex 16 catalyzed the hydroboration of benzophenone, pyridine, tert-butylnitrile, and N-benzylideneaniline as well as the deoxygenation of N,N-dimethylacetamide using pinacolborane (HBpin) (Table 1). Compared with [Mg(thf)6][HBPh3], complex 16 required longer reaction times for full conversion of the substrates under similar reaction conditions.32 The dearomatization of pyridine gave the 1,4-insertion product regioselectively.

The hydride was oxidized with elemental iodine and nitrous oxide to give the iodide and μ-oxo species, respectively. In contrast to the small Lewis acid BH3, which gave the tetrahydroborate in THF,5 less Lewis acidic but sterically bulkier BPh 3 led to hydride abstraction to give the hydridotriphenylborate anion.7 This compound catalyzed hydroboration, albeit with significantly lower activity than [Mg(thf)6][HBPh3],32 in agreement with the oversaturated, sterically encumbered 10-electron configuration at the magnesium center.32 Despite the highly polar Mg−H bond,5 the hydride ligand reacts both as a base toward Brønsted acids and as a nucleophile toward polar unsaturated substrates. The strong steric effect of the bulky amidotriamine ligand (Me3TACD· AliBu3) apparently results in a constrained environment at the magnesium center. The bulky amide-blocking, potentially reactive AliBu3 group, however, limits the substrate scope.

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EXPERIMENTAL SECTION

General Considerations. All of the manipulations were performed under an argon atmosphere using standard Schlenk techniques or glovebox techniques. Solvents (THF, n-pentane) were purified using an MB SPS-800 solvent purification system or distilled under argon (benzene, THF-d8, C6D6) from sodium/benzophenone ketyl prior to use. The starting material [Mg(Me3TACD·AliBu3)H] (1) was prepared according to literature procedures.5 All other chemicals were commercially available and used after appropriate purification. NMR spectra were recorded on a Bruker Avance II 400 or a Bruker Avance III HD 400 spectrometer at 25 °C in J. Young-type NMR tubes. The chemical shifts (δ, in ppm) in the 1H NMR spectra were referenced to the residual proton signals of the deuterated solvents and reported relative to tetramethylsilane. IR spectra were measured as KBr pellets using an AVATAR 360 FT-IR spectrometer. The following abbreviations are used for IR spectra: w (weak), m (medium), s (strong). Combustion analyses were performed with an elementar vario EL machine. For 3−6, 10, 13, 14, and 16, the analytical results were outside acceptable limits despite repeated recrystallizations. The problem of low carbon content is probably due to incomplete combustion as a result of metal carbide formation during combustion. Similar problems are known in the literature.35,36

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CONCLUSION Despite the sterically demanding ligand sphere at a fivecoordinate magnesium center, hydride 1 remains highly reactive toward a variety of polar reagents. Protonolysis with Brønsted acids was observed, but the pKa values do not correlate with the outcome of the reaction. The steric demand of the protic substrate appears to be critical, at least for the reaction with amines. The presence of (somewhat labile) AliBu3 in the molecule obviously complicates the reaction considerably. While nonpolar double bonds do not undergo insertion (i.e., hydromagnesiation),25,33 polar substrates such as aldehydes and ketones smoothly reacted with 1 to give the expected alkoxides. CO2 gave the formate, which exhibits a rather unusual κO,κO′ coordination.34 This probably reflects stronger Lewis acidity of the five-coordinate magnesium. Pyridine gave the thermodynamically favored 1,4-hydropyridinato complex selectively.2f,5 F

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Article

Inorganic Chemistry

(m, 3H, AlCH2CH(CH3)2), 0.92 (d, 3 JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.08 (d, 3JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2), −0.08 (s, 9H, Si(CH3)3). 13C{1H} NMR (100 MHz, THF-d8): δ 56.1 (CH 2 (Me 3 TACD)), 54.1 (CH 2 (Me 3 TACD)), 54.0 (CH 2 (Me3TACD)), 47.7 (CH2 (Me3TACD)), 45.6 (CH3N (Me3TACD)), 43.6 (CH3N (Me3TACD)), 29.9 (CH3 (AlCH2CH(CH3)2)), 28.6 (CH (AlCH2CH(CH3)2)), 5.1 (CH3 (Si(CH3)3)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 161.9. 29Si{1H} NMR (80 MHz, THF-d8): δ −11.86 (SiMe3). Anal. Calcd for C26H61N4AlMgOSi: C 59.46, H 11.71, N 10.67, Al 5.14, Mg 4.63. Found: C 56.80, H 11.98, N 10.04, Al 3.98, Mg 4.47. [Mg(Me3TACD·AliBu3)(OSiPh3)] (6). Solid triphenylsilanol (0.127 g, 0.46 mmol) was added to a solution of 1 (0.200 g, 0.46 mmol) in 5 mL of THF. The reaction mixture was stirred at 25 °C for 0.5 h and then filtered, and all of the volatiles were removed in vacuo. The residue was washed with n-pentane and dried under reduced pressure to give [Mg(Me3TACD·AliBu3)(Ph3SiO)] (6) as a colorless solid. Yield: 0.285 g (0.40 mmol, 87%). 1H NMR (400 MHz, THF-d8): δ 7.68− 7.70 (m, 6H, o-CH (C6H5)), 7.20−7.23 (m, 9H, m-CH (C6H5), p-CH (C6H5)), 3.10−3.15 (m, 2H, CH2 (Me3TACD)), 2.68−2.77 (m, 6H, CH2 (Me3TACD)), 2.47−2.57 (m, 4H, CH2 (Me3TACD)), 2.25− 2.39 (m, 4H, CH2 (Me3TACD)), 2.30 (s, 6H, CH3N (Me3TACD)), 1.99 (s, 3H, CH 3 N (Me 3 TACD)), 1.86−1.96 (m, 3H, AlCH2CH(CH3)2), 0.90 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), 0.01 (d, 3JHH = 6.9 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100 MHz, THF-d8): δ 144.01 (C (C6H5)), 136.6 (o-CH (C6H5)), 128.6 (p-CH (C6H5)), 127.8 (m-CH (C6H5)), 56.5 (CH2 (Me3TACD)), 54.3 (CH2 (Me3TACD)), 54.1 (CH2 (Me3TACD)), 47.8 (CH2 (Me 3 TACD)), 45.9 (CH 3 N (Me 3 TACD)), 44.0 (CH 3 N (Me 3 TACD)), 29.9 (CH 3 (AlCH 2 CH(CH 3 ) 2 )), 28.6 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 154.0. 29Si{1H} NMR (80 MHz, THF-d8): δ −30.80 (SiPh3). Anal. Calcd for C41H67N4AlMgOSi: C 69.22, H 9.49, N 7.88, Al 3.79, Mg 3.42. Found: C 62.89, H 10.48, N 5.52, Al 2.75, Mg 3.13. [Mg(Me3TACD·AliBu3)Cl] (7). Solid triethylammonium chloride (0.047 g, 0.34 mmol) was added to a solution of 1 (0.150 g, 0.34 mmol) in 5 mL of THF. The reaction mixture was stirred at 25 °C for 2 h and then filtered. All of the volatiles were removed in vacuo, and the residue was washed with n-pentane and dried under reduced pressure. [Mg(Me3TACD·AliBu3)Cl] (7) was obtained as a colorless solid. Yield: 0.124 g (0.26 mmol, 77%). 1H NMR (400 MHz, THFd8): δ 3.06−3.12 (m, 2H, CH2 (Me3TACD)), 2.75−2.89 (m, 6H, CH2 (Me3TACD)), 2.45−2.59 (m, 8H, CH2 (Me3TACD)), 2.51 (s, 6H, CH3N (Me3TACD)), 2.49 (s, 3H, CH3N (Me3TACD)), 1.83−1.92 (m, 3H, AlCH2CH(CH3)2), 0.92 (d, 3 JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.07 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 56.3 (CH2 (Me3TACD)), 54.1 (CH2 (Me3 TACD)), 47.7 (CH2 (Me 3 TACD)), 45.8 (CH3 N (Me3TACD)), 43.8 (CH3N (Me3TACD)), 29.8 (CH3 (AlCH2CH(CH3)2)), 28.4 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 160.6. Anal. Calcd for C23H52N4AlClMg: C 58.60, H 11.12, N 11.89, Al 5.72, Mg 5.16. Found: C 59.24, H 10.76, N 12.16, Al 4.60, Mg 5.87. [Mg(Me3TACD·AliBu3)Br] (8). Solid triethylammonium bromide (0.021 g, 0.11 mmol) was added to a solution of 1 (0.050 g, 0.11 mmol) in 3 mL of THF. The reaction mixture was stirred at 25 °C for 20 min and then filtered. All of the volatiles were removed in vacuo, and the residue was washed with n-pentane and dried under reduced pressure. [Mg(Me3TACD·AliBu3)Br] (8) was obtained as a colorless solid. Yield: 0.042 g (0.08 mmol, 72%). 1H NMR (400 MHz, THFd8): δ 3.07−3.14 (m, 2H, CH2 (Me3TACD)), 2.75−2.90 (m, 6H, CH2 (Me3TACD)), 2.45−2.62 (m, 8H, CH2 (Me3TACD)), 2.53 (s, 6H, CH3N (Me3TACD)), 2.52 (s, 3H, CH3N (Me3TACD)), 1.83−1.93 (m, 3H, AlCH2CH(CH3)2), 0.92 (d, 3 JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.05 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 56.2 (CH2 (Me3TACD)), 54.0

For compound 16, the CHN elemental analysis was additionally affected by loss of coordinated solvent molecules. Metal contents 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. For 3, 5, 6, 7, 9, 15, and 16, the aluminum contents were outside the acceptable limits despite repeated recrystallizations, possibly because of incomplete dissolution of the aluminum-containing sample. [Mg(Me3TACD·AliBu3)(CCPh)] (3). Neat phenylacetylene (0.023 g, 0.23 mmol) was added to a solution of 1 (0.100 g, 0.23 mmol) in 2 mL of THF. The reaction mixture was stirred at 25 °C for 5 min and filtered. All of the volatiles were removed in vacuo, and the residue was washed with n-pentane and dried under reduced pressure. [Mg(Me3TACD·AliBu3)(CCPh)] (3) was obtained as a colorless solid. Yield: 0.105 g (0.20 mmol, 86%). 1H NMR (400 MHz, THF-d8): δ 7.24−7.27 (m, 2H, o-CH (C6H5)), 7.04−7.09 (m, 2H, m-CH (C6H5)), 6.96−7.00 (m, 1H, p-CH (C 6 H 5 )), 3.08−3.14 (m, 2H, CH 2 (Me3TACD)), 2.72−2.86 (m, 6H, CH2 (Me3TACD)), 2.44−2.72 (m, 8H, CH2 (Me3TACD)), 2.61 (s, 3H, CH3N (Me3TACD)), 2.56 (s, 6H, CH3N (Me3TACD)), 1.88−1.98 (m, 3H, AlCH2CH(CH3)2), 0.94 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), 0.01 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100 MHz, THF-d8): δ 132.2 (o-CH (C6H5)), 130.2 (CCPh), 128.1 (m-CH (C6H5)), 125.2 (p-CH (C6H5)), 123.9 (C (C6H5)), 110.0 (CCPh), 56.3 (CH2 (Me3TACD)), 54.2 (CH2 (Me3TACD)), 54.0 (CH2 (Me3TACD)), 48.0 (CH2 (Me3TACD)), 45.5 (CH3N (Me3TACD)), 43.6 (CH3N (Me 3 TACD)), 29.7 (CH 3 (AlCH 2 CH(CH 3 ) 2 )), 28.3 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 160.8. Anal. Calcd for C31H57N4AlMg: C 69.32, H 10.70, N 10.43, Al 5.02, Mg 4.53. Found: C 66.06, H 11.76, N 9.95, Al 4.00, Mg 4.48. [Mg(Me3TACD·AliBu3){HN(3,5-Me2-C6H3)}] (4). Neat 3,5-dimethylaniline (0.013 g, 0.11 mmol) was added to a solution of 1 (0.050 g, 0.11 mmol) in 2 mL of THF. The reaction mixture was stirred at 25 °C for 5 min and filtered. Addition of n-pentane resulted in a precipitate, which was isolated, washed with n-pentane, and dried under reduced pressure. [Mg(Me3TACD·AliBu3){HN(3,5-Me2C6H3)}] (4) was obtained as a colorless solid. Yield: 0.053 g (0.10 mmol, 84%). Recrystallization from toluene at −30 °C gave colorless crystals suitable for X-ray diffraction.37 1H NMR (400 MHz, THF-d8): δ 5.86 (br s, 2H, o-CH (C6H3)), 5.62 (br s, 1H, p-CH (C6H3)), 3.07− 3.13 (m, 2H, CH 2 (Me 3 TACD)), 2.75−2.91 (m, 6H, CH 2 (Me3TACD)), 2.45−2.62 (m, 8H, CH2 (Me3TACD)), 2.56 (s, 3H, CH3N (Me3TACD)), 2.54 (s, 6H, CH3N (Me3TACD)), 2.00 (s, 6H, CH3), 1.81−1.90 (m, 3H, AlCH2CH(CH3)2), 0.89 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.10 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2); the proton signal for NH could not be reliably assigned because of overlap with the Me3TACD ligand signals. 13C{ 1H} NMR (100 MHz, THF-d8): δ 161.4 (C (C6H3)), 137.4 (C (C6H3)), 115.1 (o-CH (C6H3)), 112.2 (p-CH (C6H3)), 56.9 (CH2 (Me3TACD)), 54.4 (CH 2 (Me 3 TACD)), 54.3 (CH 2 (Me 3 TACD)), 48.1 (CH 2 (Me 3 TACD)), 45.1 (CH 3 N (Me 3 TACD)), 44.3 (CH 3 N (Me 3 TACD)), 29.7 (CH 3 (AlCH 2 CH(CH 3 ) 2 )), 28.4 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 156.9. Anal. Calcd for C31H62N5AlMg: C 66.95, H 11.24, N 12.59. Found: C 63.07, H 11.63, N 10.91. [Mg(Me3TACD·AliBu3)(OSiMe3)] (5). Neat trimethylsilanol (0.041 g, 0.46 mmol) was added to a solution of 1 (0.200 g, 0.46 mmol) in 5 mL of THF. The reaction mixture was stirred at 25 °C for 12 h and then filtered, and all of the volatiles were removed in vacuo. The residue was washed with n-pentane and dried under reduced pressure to give [Mg(Me3TACD·AliBu3)(OSiMe3)] (5) as a colorless solid. Yield: 0.115 g (0.22 mmol, 48%). 1H NMR (400 MHz, THF-d8): δ 3.05− 3.11 (m, 2H, CH 2 (Me 3 TACD)), 2.70−2.84 (m, 6H, CH 2 (Me3TACD)), 2.48−2.55 (m, 8H, CH2 (Me3TACD)), 2.46 (s, 6H, CH3N (Me3TACD)), 2.44 (s, 3H, CH3N (Me3TACD)), 1.84−1.94 G

DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(m, 3H, AlCH2CH(CH3)2), 0.92 (d, 3 JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.03 (d, 3JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 153.1 (C (C6H5)), 127.9 (mCH (C6H5)), 127.0 (o-CH (C6H5)), 125.1 (p-CH (C6H5)), 68.8 (CH (CH2O)), 56.2 (CH2 (Me3TACD)), 54.2 (CH2 (Me3TACD)), 54.0 (CH2 (Me3 TACD)), 47.9 (CH2 (Me 3 TACD)), 45.1 (CH3 N (Me3TACD)), 43.6 (CH3N (Me3TACD)), 29.8 (CH3 (AlCH2CH(CH3)2)), 28.6 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 153.6. Anal. Calcd for C30H59N4AlMgO: C 66.34, H 10.95, N 10.32, Al 4.97, Mg 4.48. Found: C 65.69, H 10.89, N 10.03, Al 4.42, Mg 5.02. [Mg(Me3TACD·AliBu3)(OCHPh2)] (13). Solid benzophenone (0.089 g, 0.49 mmol) was added to a solution of 1 (0.213 g, 0.49 mmol) in 5 mL of THF. The reaction mixture was stirred at 25 °C for 0.5 h and then filtered, and all of the volatiles were removed in vacuo. The residue was washed with n-pentane and dried under reduced pressure to give [Mg(Me3TACD·AliBu3)(OCHPh2)] (13) as a colorless solid. Yield: 0.251 g (0.41 mmol, 83%). 1H NMR (400 MHz, THF-d8): δ 7.46 (d, 3JHH = 7.2 Hz, 4H, o-CH (C6H5)), 7.08 (t, 3JHH = 7.6 Hz, 4H, m-CH (C6H5)), 6.93 (d, 3JHH = 7.3 Hz, 2H, p-CH (C6H5)), 6.08 (s, 1H, CHO), 3.03−3.10 (m, 2H, CH2 (Me3TACD)), 2.68−2.81 (m, 6H, CH2 (Me3TACD)), 2.35−2.50 (m, 8H, CH2 (Me3TACD)), 2.53 (s, 3H, CH3N (Me3TACD)), 2.22 (s, 6H, CH3N (Me3TACD)), 1.84−1.93 (m, 3H, AlCH2CH(CH3)2), 0.90 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.06 (d, 3JHH = 6.9 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 154.9 (C (C6H5)), 127.9 (oCH (C6H5)), 127.9 (m-CH (C6H5)), 125.5 (p-CH (C6H5)), 79.4 (CH (CHO)), 56.2 (CH2 (Me3TACD)), 54.1 (CH2 (Me3TACD)), 54.0 (CH2 (Me3 TACD)), 47.7 (CH2 (Me 3 TACD)), 45.6 (CH3 N (Me3TACD)), 43.7 (CH3N (Me3TACD)), 29.8 (CH3 (AlCH2CH(CH3)2)), 28.7 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 163.7. Anal. Calcd for C36H63N4AlMgO: C 69.83, H 10.26, N 9.05, Al 4.36, Mg 3.93. Found: C 68.64, H 10.48, N 9.12, Al 3.81, Mg 4.09. [Mg(Me3TACD·AliBu3){OCH(C6H4)2}] (14). Solid fluorenone (0.082 g, 0.46 mmol) was added to a solution of 1 (0.200 g, 0.46 mmol) in 5 mL of THF. The greenish reaction mixture was stirred at 25 °C for 0.5 h and then filtered, and all of the volatiles were removed in vacuo. The residue was washed with n-pentane and dried under reduced pressure to give [Mg(Me3TACD·AliBu3){(C6H4)2CHO}] (14) as a greenish solid. Yield: 0.235 g (0.38 mmol, 83%). 1H NMR (400 MHz, THFd8): δ 7.70−7.74 (m, 2H, CH (C6H4)), 7.51−7.55 (m, 2H, CH (C6H4)), 7.12−7.16 (m, 4H, CH (C6H4)), 5.95 (s, 1H, CHO), 3.14− 3.20 (m, 2H, CH 2 (Me 3 TACD)), 2.47−2.74 (m, 8H, CH 2 (Me3TACD)), 2.53 (s, 9H, CH3N (Me3TACD)), 2.21−2.35 (m, 4H, CH2 (Me3TACD)), 1.94−2.03 (m, 3H, AlCH2CH(CH3)2), 1.00 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), 0.13 (d, 3JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100 MHz, THF-d8): δ 156.0 (C (C6H4)), 140.3 (C (C6H4)), 127.2 (CH (C6H4)), 127.1 (CH (C6H4)), 126.3 (CH (C6H4)), 119.7 (CH (C6H4)), 79.2 (CH (CHO)), 56.7 (CH2 (Me3TACD)), 54.3 (CH2 (Me3TACD)), 53.9 (CH2 (Me3 TACD)), 48.3 (CH2 (Me 3 TACD)), 44.9 (CH3 N (Me3TACD)), 44.5 (CH3N (Me3TACD)), 29.9 (CH3 (AlCH2CH(CH3)2)), 28.6 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 154.1. Anal. Calcd for C36H61N4AlMgO: C 70.06, H 9.96, N 9.08, Al 4.37, Mg 3.94. Found: C 68.92, H 9.74, N 8.69, Al 4.58, Mg 3.71. [Mg(Me3TACD·AliBu3)(OCHO)] (15). Solid potassium formate (0.015 g, 0.18 mmol) was added to a solution of 9 (0.100 g, 0.18 mmol) in 5 mL of THF. The reaction mixture was stirred at 25 °C for 4 h and then filtered, and all of the volatiles were removed in vacuo. The residue was dissolved in 0.5 mL of THF. Addition of n-pentane (10.0 mL) and cooling at −30 °C for 24 h gave a precipitate, which was isolated and washed with n-pentane. [Mg(Me3TACD·AliBu3)(OCHO)] (15) was obtained as a colorless solid. Yield: 0.036 g (0.07 mmol, 42%). Recrystallization from THF/n-pentane at −30 °C gave colorless crystals suitable for X-ray diffraction.37 1H NMR (400 MHz,

(CH 2 (Me 3 TACD)), 53.9 (CH 2 (Me 3 TACD)), 47.6 (CH 2 (Me 3 TACD)), 46.5 (CH 3 N (Me 3 TACD)), 44.0 (CH 3 N (Me 3 TACD)), 29.6 (CH 3 (AlCH 2 CH(CH 3 ) 2 )), 28.3 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 159.3. Anal. Calcd for C23H52N4AlBrMg: C 53.55, H 10.16, N 10.86, Al 5.23, Mg 4.71. Found: C 53.24, H 10.01, N 10.76, Al 4.49, Mg 4.82. [Mg(Me3TACD·AliBu3)I] (9). A THF solution (5 mL) of elemental iodine (0.044 g, 0.17 mmol) was slowly added to a solution of 1 (0.150 g, 0.34 mmol) in 3 mL of THF at 25 °C. The reaction mixture was stirred at 25 °C for 5 min and then filtered. All of the volatiles were removed in vacuo, and the residue was washed with n-pentane and dried under reduced pressure. [Mg(Me3TACD·AliBu3)I] (9) was obtained as a colorless solid. Yield: 0.143 g (0.25 mmol, 75%). 1H NMR (400 MHz, THF-d8): δ 3.08−3.14 (m, 2H, CH2 (Me3TACD)), 2.76−2.93 (m, 6H, CH2 (Me3TACD)), 2.48−2.56 (m, 8H, CH2 (Me3TACD)), 2.57 (s, 3H, CH3N (Me3TACD)), 2.56 (s, 6H, CH3N (Me3TACD)), 1.84−1.94 (m, 3H, AlCH2CH(CH3)2), 0.92 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.03 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100 MHz, THF-d8): δ 56.3 (CH2 (Me3TACD)), 54.1 (CH2 (Me3TACD)), 54.0 (CH2 (Me3TACD)), 48.0 (CH2 (Me3TACD)), 47.7 (CH3N (Me3TACD)), 44.6 (CH3N (Me 3 TACD)), 29.7 (CH 3 (AlCH 2 CH(CH 3 ) 2 )), 28.4 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 162.5. Anal. Calcd for C23H52N4AlIMg: C 49.08, H 9.31, N 9.95, Al 4.79, Mg 4.32. Found: C 49.34, H 9.92, N 10.25, Al 3.79, Mg 4.43. [{Mg(Me3TACD·AliBu3)}2(μ-O)] (10). A degassed suspension of 1 (0.130 g, 0.30 mmol) in 3 mL of benzene was charged with N2O (1 bar) and stirred at 25 °C for 3 min. All of the volatiles were removed in vacuo, and the residue was dissolved in benzene. After filtration of the solution, all of the volatiles were removed in vacuo, and the residue was washed with n-pentane and dried under reduced pressure. [{Mg(Me3TACD·AliBu3)}2(μ-O)] (10) was obtained as a colorless solid. Yield: 0.093 g (0.11 mmol, 71%). Recrystallization from THF/npentane at 25 °C gave colorless crystals suitable for X-ray diffraction.37 1 H NMR (400 MHz, THF-d 8 ): δ 3.04−3.10 (m, 4H, CH 2 (Me3TACD)), 2.72−2.80 (m, 8H, CH2 (Me3TACD)), 2.68 (s, 6H, CH3N (Me3TACD)), 2.38−2.66 (m, 20H, CH2 (Me3TACD)), 2.56 (s, 12H, CH3N (Me3TACD)), 1.82−1.92 (m, 6H, AlCH2CH(CH3)2), 0.93 (d, 3JHH = 6.5 Hz, 18H, AlCH2CH(CH3)2), −0.12 (d, 3JHH = 6.9 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100 MHz, THF-d8): δ 56.4 (CH2 (Me3TACD)), 55.1 (CH2 (Me3TACD)), 54.5 (CH2 (Me3TACD)), 48.2 (CH2 (Me3TACD)), 48.1 (CH3N (Me3TACD)), 45.4 (CH3N (Me3TACD)), 29.7 (CH3 (AlCH2CH(CH3)2)), 28.4 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. IR (KBr) cm−1: 3674 (w), 2941 (s), 2851 (s), 2599 (w), 1461 (s), 1419 (w), 1370 (m), 1357 (s), 1297 (s), 1266 (w), 1201 (w), 1167 (s), 1135 (m), 1124 (m), 1107 (m), 1085 (s), 1074 (s), 1062 (s), 1036 (m), 1025 (m), 1014 (m), 972 (s), 938 (w), 905 (s), 815 (s), 776 (m), 757 (m), 746 (m), 680 (s), 628 (s), 580 (s), 563 (m), 495 (s), 446 (m), 428 (m). Anal. Calcd for C46H104N8Al2Mg2O: C 62.22, H 11.81, N 12.62, Al 6.08, Mg 5.47. Found: C 61.05, H 12.06, N 12.56, Al 6.54, Mg 5.62. [Mg(Me3TACD·AliBu3)(OCH2Ph)] (12). Neat benzaldehyde (0.009 g, 0.09 mmol) was added to a solution of 1 (0.040 g, 0.09 mmol) in 1 mL of THF. Diffusion of n-pentane (2 mL) into the reaction mixture followed by cooling to −30 °C formed a crystalline precipitate. Removal of the mother liquor, washing with cool n-pentane, and drying under reduced pressure gave [Mg(Me 3TACD·AliBu3)(OCH2Ph)] (12) as a colorless solid. Yield: 0.036 g (0.07 mmol, 74%). 1H NMR (400 MHz, THF-d8): δ 7.37−7.39 (d, 3JHH = 7.0 Hz, 2H, o-CH (C6H5)), 6.97−7.10 (t, 3JHH = 7.6 Hz, 2H, m-CH (C6H5)), 6.93−6.95 (t, 3JHH = 7.3 Hz, 1H, p-CH (C6H5)), 5.07 (s, 2H, CH2O), 3.07−3.13 (m, 2H, CH2 (Me3TACD)), 2.71−2.82 (m, 6H, CH2 (Me3TACD)), 2.47−2.56 (m, 8H, CH2 (Me3TACD)), 2.48 (s, 6H, CH3N (Me3TACD)), 2.45 (s, 3H, CH3N (Me3TACD)), 1.85−1.95 H

DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry THF-d8): δ 8.25 (br s, 1H, OCHO), 3.20−3.26 (m, 2H, CH2 (Me3TACD)), 2.76−2.86 (m, 6H, CH2 (Me3TACD)), 2.51−2.62 (m, 6H, CH2 (Me3TACD)), 2.41−2.46 (m, 2H, CH2 (Me3TACD)), 2.43 (s, 6H, CH3N (Me3TACD)), 2.36 (s, 3H, CH3N (Me3TACD)), 1.78−1.88 (m, 3H, AlCH2CH(CH3)2), 0.91 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.16 (d, 3JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 174.5 (OCHO (C6H5)), 59.3 (CH 2 (Me 3 TACD)), 55.7 (CH 2 (Me 3 TACD)), 54.5 (CH 2 (Me3TACD)), 51.0 (CH2 (Me3TACD)), 44.7 (CH3N (Me3TACD)), 44.3 (CH3N (Me3TACD)), 29.8 (CH3 (AlCH2CH(CH3)2)), 28.4 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 27Al NMR (100 MHz, THF-d8): δ 160.3. Because of the instability, no meaningful IR spectra could be recorded. Anal. Calcd for C24H53N4AlMgO2: Al 5.61, Mg 5.05. Found: Al 3.91, Mg 4.78. [Mg(Me3TACD·AliBu3)(thf)][HBPh3] (16). Solid triphenylborane (0.028 g, 0.11 mmol) was added to a solution of 1 (0.050 g, 0.11 mmol) in 2 mL of THF. The reaction mixture was stirred at 25 °C for 5 min and then filtered. Addition of n-pentane resulted in a precipitate, which was isolated and washed with n-pentane. [Mg(Me3TACD· AliBu3)(thf)][HBPh3] (16) was obtained as a colorless solid. Yield: 0.065 g (0.09 mmol, 75%). Recrystallization from THF/n-pentane at −30 °C gave colorless crystals suitable for X-ray diffraction.37 1H NMR (400 MHz, THF-d8): δ 7.32 (m, 6H, o-CH (C6H5)), 6.88−6.92 (m, 6H, m-CH (C6H5)), 6.72−6.75 (m, 3H, p-CH (C6H5)), 3.51 (q, 1 JHB = 74 Hz, 1H, HB), 2.95−3.02 (m, 2H, CH2 (Me3TACD)), 2.64− 2.70 (m, 2H, CH 2 (Me 3 TACD)), 2.54−2.58 (m, 4H, CH 2 (Me3TACD)), 2.26−2.48 (m, 8H, CH2 (Me3TACD)), 2.39 (s, 6H, CH3N (Me3TACD)), 2.15 (s, 3H, CH3N (Me3TACD)), 1.80−1.88 (m, 3H, AlCH2CH(CH3)2 ), 0.92 (d, 3JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), −0.25 (d, 3JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2). 13 C{1H} NMR (100 MHz, THF-d8): δ 165.5 (q, 1JCB = 49.3 Hz, C (C6H5)), 136.6 (o-CH (C6H5)), 126.4 (m-CH (C6H5)), 122.2 (p-CH (C6H5)), 56.55 (CH2 (Me3TACD)), 53.8 (CH2 (Me3TACD)), 53.6 (CH2 (Me 3TACD)), 47.4 (CH2 (Me 3TACD)), 44.5 (CH3N (Me3TACD)), 43.9 (CH3N (Me3TACD)), 29.4 (CH3 (AlCH2CH(CH3)2)), 28.1 (CH (AlCH2CH(CH3)2)); the carbon signal for CH2 (AlCH2CH(CH3)2) could not be reliably assigned because of overlap with the solvent signals. 11B NMR (128 MHz, THF-d8): δ −7.9 (d, 1 JHB = 74 Hz, HB). Anal. Calcd for C45H76N4AlBMgO: C 71.95, H 10.20, N 7.46, Al 3.59, Mg 3.24. Found: C 63.13, H 11.10, N 7.05, Al 2.87, Mg 3.49. Typical NMR-Scale Catalysis Reaction.32 A J. Young tube was charged with substrate (0.27 or 0.14 mmol), pinacolborane (0.035 g, 0.27 mmol), and 0.5 mL of solvent (2:3 THF/THF-d8). The catalyst was loaded by adding 0.2 mL of THF stock solution of 16 (c = 14 mmol·L−1). The reaction progress was monitored by 1H NMR spectroscopy.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the the Deutsche Forschungsgemeinschaft (Cluster of Excellence “Tailor Made Fuels from Biomass”) for financial support.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02509. 1 H, 13C{1H}, 11B, 27Al, and 29Si{1H} NMR spectra of complexes 3−16 and X-ray crystallographic details for 4, 10, 15, and 16 (PDF) Crystallographic data for 4, 10, 15, and 16 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax: +49 241 80 92644. E-mail: [email protected]. ORCID

Jun Okuda: 0000-0002-1636-5464 I

DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02509 Inorg. Chem. XXXX, XXX, XXX−XXX