Reactivity of a Molecular Magnesium Hydride Featuring a Terminal Magnesium-Hydrogen Bond

Article abstract of DOI:10.1021/acs.inorgchem.6b02509

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 = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane), can be grouped into protonolysis, oxidation, hydrometalation, (insertion), and hydride abstraction. Protonolysis of 1 with weak Br?nsted 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 pK_a 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 ?°O,?°O′ 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.

Full text of DOI:10.1021/acs.inorgchem.6b02509

Article
pubs.acs.org/IC
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 = 1,4,7-
trimethyl-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.
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-tetraazacyclodo-
decane) 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
INTRODUCTION
■
The availability, low cost, and nontoxicity of magnesium
compounds render their catalytic applications attractive.
Recently, several magnesium-catalyzed hydroaminations, hy-
droborations, 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
Received: October 15, 2016
© XXXX American Chemical Society
A
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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
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
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, hydro-
metalation (insertion), and hydride abstraction. Hydroboration
catalysis of polar substrates by the ion pair [Mg(Me3TACD·
AliBu3)(thf)][HBPh3] has also been demonstrated.7
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 3,5-
dimethylaniline (pKa = 10.5)12 to a THF solution of 1 did
selectively give the complex [Mg(Me3TACD·AliBu3){HN(3,5-
Me2-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
[MgnBu{HN(2,6-Me2-C6H3)}(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(3,5-
Me2-C6H3)]− at the magnesium center. Single crystals grown
from toluene at −35 °C were suitable for X-ray diffraction.
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
Complex 4 crystallized in the triclinic space group P1 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-Me2-
C6H3)]− (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-di-
tert-butylphenol (pKa = 16.9)9 in THF-d8 were complete within
1
5 min at 25 °C, but the H NMR spectra in THF-d8 indicated
the formation of product mixtures. Apparently, both the
B
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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(μ-
O)] (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*= η5-
C5Me5).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}2Mg(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)°.
Figure 1. Molecular structure of [Mg(Me3TACD·AliBu3){HN(3,5-
Me2-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.
Hydrometalation. Insertion of unsaturated substrates into
the magnesium−hydrogen bond can be regarded as hydro-
metalation 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
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
1
spectroscopy in solution and by elemental analysis. The H
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 spectros-
copy 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
The molecular calcium hydride [CH{C(Me)NAr}2CaH-
24
(thf)]2 reacted with 1,1-diphenylethylene via insertion of
the carbon−carbon double bond. Olefins were expected to
insert into the magnesium−hydrogen bond (hydromagnesia-
tion), but 1 did not react with ethylene, styrene, 1,1,2-
triphenylethylene, tert-butylethylene, or cyclopentene in THF-
d8 at 60 °C. Only 1,1-diphenylethylene was completely
1
consumed by 1 within 30 min in THF-d8 at 25 °C. The H
NMR spectrum in THF-d8 suggested the formation of two
regioisomers, [Mg(Me3TACD·AliBu3)(CPh2CH3)] (A) and
[Mg(Me3TACD·AliBu3)(CH2CHPh2)] (B) (Scheme 5). Sep-
aration and isolation of these insertion products has failed to
1
date. The H NMR spectrum in THF-d8 at 25 °C showed two
sets of signals for the Me3TACD ligand and the aluminum-
bonded isobutyl groups in the ratio of A:B = 3:1. Apart from
C
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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 benzalde-
hyde 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·
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).
1
AliBu3){OCH(C6H4)2}] (14), each in 83% yield. Their H
NMR spectra showed the OCH proton resonance at 6.08 ppm
(13) and 5.95 ppm (14) in THF-d8.
Scheme 5. Hydrometalation of 1
D
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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(Me3TACD·AliBu3)I] (9) with potassium formate
(Scheme 6). The quantitative reaction is completely selective
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
[(C6F5)3BOC(H)OC(H){(Me)CNAr}2Mg(O2CH)]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)-
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 crystallo-
graphically 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 P1. 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
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).
The NMR spectroscopic data of 16 agree with the structure
1
in the solid state. The H NMR spectrum in THF-d8 exhibited
a broad 1:1:1:1 quartet at 3.51 ppm (1JBH = 74 Hz) for the BH
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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
b
c
n(HBpin) = 0.27 mmol. n(substrate) = 0.27 mmol. n(substrate) =
d
e
0.14 mmol. Catalyst loading =1.0 mol % Mg. 0.5 mL of solvent
(THF:THF-d8 = 2:3); Time required for complete consumption of
f
the substrate to give a quantitative yield of the product, as determined
by H 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)mag-
nesium 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-benzylidenea-
niline 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.
1
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 BPh3 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.
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
CONCLUSION
■
Despite the sterically demanding ligand sphere at a five-
coordinate 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 thermody-
namically favored 1,4-hydropyridinato complex selectively.2f,5
1
NMR tubes. The chemical shifts (δ, in ppm) in the H 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
F
DOI: 10.1021/acs.inorgchem.6b02509
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Inorganic Chemistry
Article
3
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.
(m, 3H, AlCH2CH(CH3)2), 0.92 (d, JHH = 6.4 Hz, 18H,
3
AlCH2CH(CH3)2), −0.08 (d, JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2),
−0.08 (s, 9H, Si(CH3)3). 13C{1H} NMR (100 MHz, THF-d8): δ 56.1
(CH2 (Me3TACD)), 54.1 (CH2 (Me3TACD)), 54.0 (CH2
(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)(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.
[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)(Ph31SiO)] (6) as a colorless solid. Yield:
0.285 g (0.40 mmol, 87%). H 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, CH3N (Me3TACD)), 1.86−1.96 (m, 3H,
1
Yield: 0.105 g (0.20 mmol, 86%). H 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 (C6H5)), 3.08−3.14 (m, 2H, CH2
(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),
3
3
0.94 (d, JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), 0.01 (d, JHH = 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
(Me3TACD)), 29.7 (CH3 (AlCH2CH(CH3)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.
3
AlCH2CH(CH3)2), 0.90 (d, JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2),
0.01 (d, JHH = 6.9 Hz, 6H, AlCH2CH(CH3)2). 13C{1H} NMR (100
3
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
(Me3TACD)), 45.9 (CH3N (Me3TACD)), 44.0 (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 154.0. 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){HN(3,5-Me2-C6H3)}] (4). Neat 3,5-dimethy-
laniline (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-Me2-
C6H3)}] (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, CH2 (Me3TACD)), 2.75−2.91 (m, 6H, CH2
(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,
[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
1
solid. Yield: 0.124 g (0.26 mmol, 77%). H NMR (400 MHz, THF-
d8): 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
3
CH3), 1.81−1.90 (m, 3H, AlCH2CH(CH3)2), 0.89 (d, JHH = 6.4 Hz,
3
3
(m, 3H, AlCH2CH(CH3)2), 0.92 (d, JHH = 6.4 Hz, 18H,
18H, AlCH2CH(CH3)2), −0.10 (d, JHH = 6.7 Hz, 6H, AlCH2CH-
3
AlCH2CH(CH3)2), −0.07 (d, JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2).
(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
(CH2 (Me3TACD)), 54.3 (CH2 (Me3TACD)), 48.1 (CH2
(Me3TACD)), 45.1 (CH3N (Me3TACD)), 44.3 (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. 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.
13C{1H} NMR (100 MHz, THF-d8): δ 56.3 (CH2 (Me3TACD)), 54.1
(CH2 (Me3TACD)), 47.7 (CH2 (Me3TACD)), 45.8 (CH3N
(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
[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:
1
solid. Yield: 0.042 g (0.08 mmol, 72%). H NMR (400 MHz, THF-
d8): 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
1
0.115 g (0.22 mmol, 48%). H NMR (400 MHz, THF-d8): δ 3.05−
3
(m, 3H, AlCH2CH(CH3)2), 0.92 (d, JHH = 6.4 Hz, 18H,
3.11 (m, 2H, CH2 (Me3TACD)), 2.70−2.84 (m, 6H, CH2
(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
3
AlCH2CH(CH3)2), −0.05 (d, JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2).
13C{1H} NMR (100 MHz, THF-d8): δ 56.2 (CH2 (Me3TACD)), 54.0
G
DOI: 10.1021/acs.inorgchem.6b02509
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3
(CH2 (Me3TACD)), 53.9 (CH2 (Me3TACD)), 47.6 (CH2
(Me3TACD)), 46.5 (CH3N (Me3TACD)), 44.0 (CH3N
(Me3TACD)), 29.6 (CH3 (AlCH2CH(CH3)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.
(m, 3H, AlCH2CH(CH3)2), 0.92 (d, JHH = 6.4 Hz, 18H,
3
AlCH2CH(CH3)2), −0.03 (d, JHH = 6.8 Hz, 6H, AlCH2CH(CH3)2).
13C{1H} NMR (100 MHz, THF-d8): δ 153.1 (C (C6H5)), 127.9 (m-
CH (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 (Me3TACD)), 47.9 (CH2 (Me3TACD)), 45.1 (CH3N
(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.
[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
1
obtained as a colorless solid. Yield: 0.143 g (0.25 mmol, 75%). H
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
=
1
Yield: 0.251 g (0.41 mmol, 83%). H NMR (400 MHz, THF-d8): δ
3
7.46 (d, 3JHH = 7.2 Hz, 4H, o-CH (C6H5)), 7.08 (t, 3JHH = 7.6 Hz, 4H,
6.4 Hz, 18H, AlCH2CH(CH3)2), −0.03 (d, JHH = 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
(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. 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.
3
m-CH (C6H5)), 6.93 (d, JHH = 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, JHH = 6.4 Hz, 18H,
AlCH2CH(CH3)2), −0.06 (d, JHH = 6.9 Hz, 6H, AlCH2CH(CH3)2).
3
3
13C{1H} NMR (100 MHz, THF-d8): δ 154.9 (C (C6H5)), 127.9 (o-
CH (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 (Me3TACD)), 47.7 (CH2 (Me3TACD)), 45.6 (CH3N
(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)}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/n-
pentane at 25 °C gave colorless crystals suitable for X-ray diffraction.37
1H NMR (400 MHz, THF-d8): δ 3.04−3.10 (m, 4H, CH2
(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){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)21CHO}] (14) as a greenish
solid. Yield: 0.235 g (0.38 mmol, 83%). H NMR (400 MHz, THF-
d8): 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, CH2 (Me3TACD)), 2.47−2.74 (m, 8H, CH2
(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, JHH = 6.4 Hz, 18H, AlCH2CH(CH3)2), 0.13 (d, JHH = 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 (Me3TACD)), 48.3 (CH2 (Me3TACD)), 44.9 (CH3N
(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.
3
3
[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(Me3TACD·AliBu3)-
(OCH2Ph)] (12) as a colorless solid. Yield: 0.036 g (0.07 mmol,
[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,
1
74%). H 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

Inorganic Chemistry
Article
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)),
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
3
■
1.78−1.88 (m, 3H, AlCH2CH(CH3)2), 0.91 (d, JHH = 6.4 Hz, 18H,
3
We thank the the Deutsche Forschungsgemeinschaft (Cluster
of Excellence “Tailor Made Fuels from Biomass”) for financial
support.
AlCH2CH(CH3)2), −0.16 (d, JHH = 6.7 Hz, 6H, AlCH2CH(CH3)2).
13C{1H} NMR (100 MHz, THF-d8): δ 174.5 (OCHO (C6H5)), 59.3
(CH2 (Me3TACD)), 55.7 (CH2 (Me3TACD)), 54.5 (CH2
(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.
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(m, 3H, AlCH2CH(CH3)2), 0.92 (d, JHH = 6.4 Hz, 18H,
3
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13C{1H} NMR (100 MHz, THF-d8): δ 165.5 (q, JCB = 49.3 Hz, C
(C6H5)), 136.6 (o-CH (C6H5)), 126.4 (m-CH (C6H5)), 122.2 (p-CH
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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.inorg-
chem.6b02509.
1H, 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)
(6) (a) Martin, D.; Beckerle, K.; Schnitzler, S.; Spaniol, T. P.; Maron,
L.; Okuda, J. Discrete Magnesium Hydride Aggregates: A Cationic
Mg13H18 Cluster Stabilized by NNNN-Type Macrocycles. Angew.
Chem., Int. Ed. 2015, 54, 4115−4118. (b) Martin, D. Doctoral
Dissertation, RWTH Aachen University, Aachen, Germany, 2015.
(7) Mukherjee, D.; Osseili, H.; Spaniol, T. P.; Okuda, J. Alkali Metal
Hydridotriphenylborates [(L)M][HBPh3] (M = Li, Na, K): Chemo-
selective Catalysts for Carbonyl and CO2 Hydroboration. J. Am. Chem.
Soc. 2016, 138, 10790−10793.
Crystallographic data for 4, 10, 15, and 16 (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +49 241 80 92644. E-mail: jun.okuda@ac.rwth-aachen.de.
ORCID
(8) Waterman, R. σ-Bond Metathesis: A 30-Year Retrospective.
Organometallics 2013, 32, 7249−7263.
Jun Okuda: 0000-0002-1636-5464
I
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