β-diketiminate stabilized magnesium hydroxide, heterobimetallic, and halide complexes: Synthesis and X-ray structural studies

Article abstract of DOI:10.1002/zaac.201000348

The controlled hydrolysis of heteroleptic magnesium amide, LMgN(SiMe ₃)₂ (L = CH[C(Me)N(2,6-iPr₂C₆H ₃)]₂) with water afforded the corresponding hydroxide [LMg(OH)·THF]₂ as air and moisture sensitive compound. The presence of a sterically bulky β-diketiminate ligand prevents the self-condensation reaction of this hydroxide complex. Single crystal X-ray analysis shows that the hydroxide is dimeric in the solid state. Reaction of the magnesium amide or LMg(Me)·OEt₂ with LAlMe(OH) generates the heterobimetallic species containing the Mg-O-Al moiety. Additionally, the reaction of methylmagnesiumchloride with the free ligand leads to complex L′MgCl (L′ = CH[Et₂NCH₂CH₂N(CMe)] ₂). As revealed by the crystal structure, L′MgCl is a solvent free monomeric magnesium chloride complex that is analogues to the Grignard reagent.

Full text of DOI:10.1002/zaac.201000348

ARTICLE
DOI: 10.1002/zaac.201000348
β-Diketiminate Stabilized Magnesium Hydroxide, Heterobimetallic, and
Halide Complexes: Synthesis and X-ray Structural Studies
Sharanappa Nembenna,[a] Sanjay Singh,[a] Sakya S. Sen,[a] Herbert W. Roesky,*[a]
Holger Ott,[a] and Dietmar Stalke[a]
Keywords: Halides; Heterobimetallics; Hydroxides; Grignard reagents; Magnesium
Abstract. The controlled hydrolysis of heteroleptic magnesium amide, LMg(Me)·OEt2 with LAlMe(OH) generates the heterobimetallic spe-
LMgN(SiMe3)2 (L = CH[C(Me)N(2,6-iPr2C6H3)]2) with water af- cies containing the Mg–O–Al moiety. Additionally, the reaction of
forded the corresponding hydroxide [LMg(OH)·THF]2 as air and mois- methylmagnesiumchloride with the free ligand leads to complex
ture sensitive compound. The presence of a sterically bulky β-diketimi- L'MgCl (L' = CH[Et2NCH2CH2N(CMe)]2). As revealed by the crystal
nate ligand prevents the self-condensation reaction of this hydroxide structure, L'MgCl is a solvent free monomeric magnesium chloride
complex. Single crystal X-ray analysis shows that the hydroxide is complex that is analogues to the Grignard reagent.
dimeric in the solid state. Reaction of the magnesium amide or
droxide LGe(OH), which contains a hydroxyl group attached
Introduction
to a GeII atom.[8] These examples portray the evolution of the
In recent years, the β-diketiminate ligands have emerged as
potential spectator ligands, in view of their strong binding to
metals, their tunable steric and electronic effects, and diversity
in bonding modes.[1] Usage of this ligand has witnessed an
impressive growth in main group metal chemistry in the last
decade. A very important breakthrough, particularly in the al-
kaline earth metal chemistry, has been the synthesis of β-diket-
iminate stabilizied magnesium(I) complexes by Jones and co-
workers.[2] In this paper we are discussing a facile route for
the synthesis and characterization of β-diketiminate stabilized
magnesium hydroxide, heterobimetallic, and halide complexes.
There is a great interest in the synthesis and characterization
of novel main group hydroxide complexes due to their poten-
tial application as precursor for the synthesis of heterobi- and
heteropolymetallic compounds that can act as versatile cata-
lysts in organic transformations.[3, 4] In addition, the hydroxide
complexes can function as model compounds for the insoluble
or unstable metal hydroxides M(OH)x. In view of these appli-
cations, we have reported earlier on the syntheses and reaction
chemistry of various molecular hydroxide complexes such as
groups 13 and 14 hydroxide chemistry. Nevertheless, the orga-
nometallic hydroxide chemistry with respect to group 2 ele-
ments is still at its infancy. This is partly due to the higher
percentage of the ionic character in the M–OH (M = an alkaline
earth metal) bond and also due to a fast ligand exchange, al-
though very recently we have reported hydrocarbon soluble
heavier alkaline earth metal hydroxides [LCa(OH)·THF]2 and
[LSr(OH)·THF1.5]2.[9]
The magnesium hydroxide[10a] complex {[TpAr,Me]Mg(μ-
OH)}2 (Ar = p-tBuC6H4) stabilized by the tris(1-pyrazolyl)hy-
droborate (TpAr,Me) ligand was prepared by Parkin et al. An-
other
example
of
a
magnesium
hydroxide
[LMg(OH)·THF]2·4THF was obtained by Bochmann and co-
workers.[10b] The latter compound was isolated by serendipity,
when LMg(η1-C3H5)(THF) was kept at –26 °C for crystalliza-
tion and it lacks a direct synthetic route. In view of the increas-
ing importance of the heteroleptic complexes in polymeriza-
tion reactions,[11] we attempted for the synthesis of a
magnesium complex that contains both the halide and β-diket-
iminate ligand. In accordance with the theoretical studies on
RM–MR complexes[12] of group 2 elements, the β-diketiminate
stabilized group 2 metal halides could also be considered as
promising precursor to prepare compounds with low valent
group 2 elements containing a metal-metal bond.[2,13]
the N-bonded silanetriol[3,4] RSi(OH)3 (R
=
2,6-
iPr2C6H3NSiMe3), the AlIII and GaIII dihydroxides
LM'(OH)2,[5,6] and the AlIII and GaIII monohydroxide[7] by tai-
lor-made synthetic strategies and unveiled their reactivity. Re-
cently, our group also reported a unprecedented germylene hy-
Results and Discussion
* Prof. Dr. H. W. Roesky
Fax: +49-551-39-3373
Compound [LMg(OH)·THF]2 (1) was prepared by the reac-
tion of LMgN(SiMe3)2[14] with a stoichiometric amount of wa-
ter at –20 °C (Scheme 1). Compound 1 is a white solid soluble
in toluene, THF, and sparingly soluble in benzene. It was char-
E-Mail: hroesky@gwdg.de
[a] Institute of Inorganic Chemistry
Georg-August University
Tammannstrasse 4
37077 Göttingen, Germany
Z. Anorg. Allg. Chem. 2011, 637, 201–205
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
201

S. Nembenna, S. Singh, S. S. Sen, H. W. Roesky, H. Ott, D. Stalke
ARTICLE
acterized by IR, 1H NMR spectroscopy, EI mass spectrometry, agreement with the Mg–O bond lengths in compounds
elemental analysis, and X-ray structural analysis. In the IR L(Me)Al–O–Mg(THF)2–N(SiMe3)2
and
L(Me)Al–O–
spectrum a sharp absorption around 3741 cm–1 can be attrib- Mg(THF)2–O–Al(Me)L (1.854–2.085 Å).[15] The Mg(1)–
1
uted to the O–H stretching frequency. The H NMR spectrum O(1)–Al(1) bond angle (157.62(1)°) is similar to those found
of 1 exhibits a resonance at –0.46 ppm for MgO–H. The base in compounds L(Me)Al–O–Mg(THF)2–N(SiMe3)2 and
peak in the EI mass spectrum of 1 at m/z 916 corresponds to
molecular ion [M+]. The X-ray molecular structure of 1 is in
Table 1. Crystallographic data for the X-ray structural analyses of
agreement with that of Bochmann et al.[10b]
compounds 2 and 3.
parameter
2·THF·LAl(Me)OH 3·0.5 toluene
empirical formula
C93H138Al2MgN6O3 C20.50H39ClMg N4
formula weight
1466.36
100(2)
401.32
T /K
100(2)
crystal system
monoclinic
789418
C2/c
monoclinic
789419
P21/c
CCDC No
Space group
a /
20.462(2)
18.184(1)
47.396(3)
90
17.111(1)
7.133(1)
19.709(1)
90
b /
Scheme 1. Synthesis of β-diketiminate stabilized magnesium hydroxide 1.
c /
α /°
In our earlier report we have shown that soluble molecular
compounds with Mg–O–Al skeleton are achieved by the reac-
β /°
100.98(2)
90
110.70(2)
90
γ /°
V /Å3
17312(2)
8
2017.2(6)
4
tion of Mg[N(SiMe3)2]2 with aluminum monohydroxide.[15]
Z
[16]
This work was further extended by using LMgMe·OEt2
or
ρcalc /Mg·m–3
1.125
1.185
LMg–N(SiMe3)2.[14] The reaction of LAl(Me)OH[7b] with a
μ /mm–1
0.092
0.210
[16]
stoichiometric amount of LMgMe·OEt2
or LMg–
F(000)
6400
876
[14]
reflections collected
Independent reflection
data/restraints/ parameters
GooF
52737
36297
N(SiMe3)2
in toluene at reflux temperature results in the
15817
4273
formation of compound LMg–O–Al(Me)L (2) in excellent
yield (Scheme 2). Compound 2 is a colorless crystalline solid
that melts in the range 259–262 °C. It was characterized by 1H
NMR, IR spectroscopy, elemental analysis, EI mass spectrom-
15817/2/999
0.0481, 0.1149
0.0593, 0.1208
1.028
4273/0/266
0.0303, 0.0796
0.0329, 0.0812
1.049
R1, wR2[I > 2σ(I)]a, b)
R1, wR2 (all data)
largest diff peak, hole /e·Å–3 0.595, –0.348
0.575, –0.257
2 2 2 2 0.5
1
etry, and X-ray structural analysis. The H NMR spectrum of
2
a) R1 = Σ||Fo|–|Fc||/Σ|Fo|; b) wR2 = [Σw(Fo –Fc ) /Σw(Fo ) ]
2 exhibits the Al–Me resonance at –1.42 ppm. The backbone
protons of the ligand resonate at 4.82 and 4.92 ppm. Other
resonances are typical for the β-diketiminate ligand. The EI
mass spectrum of 2 reveals that the most intense peak appears
at m/z 901 and corresponds to the loss of one methyl group
from the molecular ion, [M+ – Me].
Scheme 2. Preparation of β-diketiminate magnesium-aluminum oxide
2.
X-ray quality crystals for 2·THF were obtained after several
attempts from a toluene/THF solution. The asymmetric unit
contains both 2·THF and LAl(Me)OH. Compound 2·THF and
LAl(Me)OH crystallize in the monoclinic space group C2/c
(Table 1, Figure 1 shows only 2·THF). The X-ray structural
analysis of 2·THF revealed that the magnesium and aluminum
atoms are connected by a bridging oxygen atom. Interestingly,
the magnesium and aluminum atoms are each arranged in a
C3N2M six-membered ring. The arrangement around the mag-
nesium, as well as the aluminum atom, is tetrahedrally dis-
Figure 1. Molecular crystal structure of compound LMg(THF)(μ–
O)Al(Me)L (2) depicted at 50 % probability level. All hydrogen atoms
are omitted for clarity. Selected bond lengths /Å and bond angles /°:
Mg(1)–O(1) 1.857(2), Mg(1)–N(3) 2.103(2), Mg(1)–N(4) 2.093(2),
Al(1)–O(1) 1.684(1), Al(1)–N(1) 1.977(2), Al(1)–N(2) 1.945(2);
Mg(1)–O(1)–Al(1) 157.62(1), N(1)–Al(1)–N(2) 93.56(1), N(3)–
torted. The Mg–O distances (1.851(1)–2.095(1) Å) are in Mg(1)–N(4) 90.99(1).
202 www.zaac.wiley-vch.de
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 201–205

β-Diketiminate Stabilized Mg Hydroxide, Heterobimetallic, and Halide Complexes
L(Me)Al–O–Mg(THF)2–O–Al(Me)L (Mg–O–Al av. bond an- nate and the coordination around the metal atom is distorted
gle 157.1°). The Al–C and Al–O distances are 1.982(2) and square pyramidal. The Mg–Cl bond length (2.360(1) Å) is
1.684(2) Å respectively, which are in good agreement with shorter than that found in LMgI·OEt2 (2.689(2) Å),[20] and
those observed in [OCMeCHCMeNAr]2AlMe (Ar = 2,6- [LFe(μ-Cl)2]Mg(THF)4 (2.520(1) Å).[21] The N(1)–Mg(1)–
iPr2C6H3) (Al–C 1.975(2) Å) and [HC(CMeNMe)2AlCl]2(μ-O) N(2) bond angle is compressed to 88.16(1)° due to the rigid
(Al–O 1.677(2) Å).[17,18]
ligand backbone, whereas the N(2)–Mg(1)–Cl(1) angle opens
A clean reaction of L'H[19] (L' = CH[Et2NCH2CH2N(CMe)]2) up to 109.47(1)°. The bond angle of N(2)–Mg(1)–N(1)
with MeMgCl in diethyl ether at –78 °C leads to the evolution (88.16(1)°) is more acute than that found in LMgI·OEt2
of methane gas, and the formation of compound L'MgCl (3) (93.1°).
(Scheme 3). Compound 3 is a colorless solid and melts at 158–
162 °C. It was characterized by 1H NMR spectroscopy, EI
Conclusion
mass spectrometry, elemental analysis, and X-ray crystal struc-
ture analysis. The 1H NMR spectrum of 3 exhibits a resonance
at 4.74 ppm for the γ-H proton. The other chemical shifts are
typical for the ligand L'. Moreover, the molecular ion of 3 was
detected at m/z = 354.
A facile route is described for the synthesis of a well-defined
magnesium
hydroxide
complex
of
the
formula
[LMg(OH)·THF]2 that is soluble in organic solvents. Access
to this hydroxide could also be of interest in sol-gel coatings,
and as polymerization catalyst. The heterobimetallic com-
pound LMg(THF)–O–Al(Me)L has been prepared by the reac-
tion of LMgN(SiMe3)2 or LMgMe·OEt2 with LAl(Me)OH, in
which the oxygen atom is linked to two six- membered MC3N2
(M = Mg, Al) rings. Furthermore, solvent free L'MgCl was
prepared. The reduction of L'MgCl is ongoing in our labora-
tory and will be published in due course.
Scheme 3. Preparation of β-diketiminate magnesiumchloride L'MgCl
(3).
Experimental Section
Single crystals of 3 suitable for X-ray structural analysis
were obtained from toluene with a small amount of THF solu-
tion. The compound crystallizes in the monoclinic space group
P21/c with one molecule in the asymmetric unit and half a
molecule of disordered toluene located on a crystallographic
inversion center (Figure 2, Table 1). The core of this structure
is MgN4Cl. The magnesium atom is surrounded by four nitro-
gen atoms of the monoanionic β-diketiminate ligand, and an
axial chlorine atom. So, the magnesium atom is pentacoordi-
All manipulations were performed under a dry and oxygen free atmos-
phere (N2) using standard Schlenk techniques or inside a MBraun MB
150-GI glove box. All solvents were distilled from Na/benzophenone
prior to use. Chemicals were purchased commercially and used as re-
1
ceived. H and 13C NMR spectra were recorded on a Bruker Avance
DRX 200 MHz instrument and referenced to the deuterated solvent in
case of the 1H and 13C NMR spectra. Elemental analyses were per-
formed by the Analytisches Labor des Instituts für Anorganische
Chemie der Universität Göttingen. EI-MS were measured on a Finni-
gan Mat 8230 or a Varian MAT CH5 instrument. Melting points were
measured in sealed glass tubes with a Büchi melting point B 450 in-
strument.
Preparation of Compound 1: Degassed and distilled water (46 μL,
2.55 mmol) was added to a solution of LMgN(SiMe3)2 (1.52 g,
2.53 mmol) in THF (30 mL) at –20 °C, and afterwards allowed to
warm to room temperature. After stirring at room temperature for
15 min, the slurry was dried in vacuo. The resulting solid of 1 was
washed with n-hexane (10 mL) and finally dried in vacuo. Yield:
(0.810 g, 1.528 mmol, 60 %). Mp: 336–340 °C. C58H84Mg2N4O2
(916): C 75.89, H 9.22, N 6.10; found: C 75.50, H 9.19, N 6.14 %.
1H NMR (200 MHz, C6D6, 25 °C, SiMe4): –0.46 (s, 2 H, Mg–OH),
0.88 (d, J = 6.9 Hz, 24 H, CH(CH3)2), 1.11 (d, J = 6.8 Hz, 24 H,
CH(CH3)2), 1.30 (m, 8 H, O–CH2–CH2), 1.54 (s, 12 H, CH3), 3.14
(sept, J = 6.8 Hz, 8 H, CH(CH3)2), 3.57 (m, 8 H, O–CH2–CH2), 4.81
(s, 2 H, γ-CH), 7.11 (s, 12 H, m-, p-Ar–H) ppm. 1H NMR (500 MHz,
[D8]THF, 25 °C, SiMe4): –1.41 (s, 2 H, Mg–OH), 0.89 (d, J = 6.9 Hz,
24 H, CH(CH3)2), 1.01 (d, J = 6.8 Hz, 24 H, CH(CH3)2), 1.43 (s, 12
H, CH3), 1.80 (m, 8 H, O–CH2–CH2), 2.97 (sept, 8 H, J = 6.9 Hz,
CH(CH3)2), 3.70 (m, 8 H, O–CH2–CH2), 4.63 (s, 2 H, γ-CH), 7.11 (s,
12 H, m-, p-Ar–H) ppm; 13C{1H} NMR (125.75 MHz, C6D6, 25 °C,
SiMe4): 19–23, 26, 28.2, 29.5, 96, 70, 127–145 ppm. EI-MS (70 eV):
Figure 2. Molecular crystal structure of compound L'MgCl (3) de-
picted at 50 % probability level. All hydrogen atoms are omitted for
clarity. Selected bond lengths /Å and bond angles /°: Mg(1)–N(1)
2.083(2), Mg(1)–N(2) 2.081(2), Mg(1)–N(3) 2.285(2), Mg(1)–N(4)
2.329(2), Mg(1)–Cl(1) 2.360(1); N(2)–Mg(1)–N(1) 88.16(1), N(2)–
Mg(1)–Cl(1) 109.47(1), N(2)–Mg(1)–N(4) 78.47(1), N(1)–Mg(1)–
N(3) 78.96(1), N(3)–Mg(1)–N(4) 97.51(1).
m/z (%): 916 (100) [M+]. IR (Nujol) ν = 3741 (MgO–H), 3669, 1624,
˜
1552, 1315, 1176, 1100, 1022, 931, 794, 759, 703 cm–1.
Z. Anorg. Allg. Chem. 2011, 201–205
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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203

S. Nembenna, S. Singh, S. S. Sen, H. W. Roesky, H. Ott, D. Stalke
ARTICLE
Preparation of Compound 2: LAl(Me)OH (0.47 g, 1.0 mmol) dis-
solved in toluene (20 mL) was added drop by drop at room tempera-
ture to a stirred solution of LMgN(SiMe3)2 (0.60 g, 1.0 mmol) or
LMgMe·OEt2 (0.530 g, 1.0 mmol) in toluene (20 mL). The solution
was set to reflux for 14 h. After removal of all the volatiles in vacuo
compound 2 remained as a solid. Yield (0.82 g, 89 %). Mp: 259–
262 °C. C59H85AlMgN4O (916): C 77.29, H 9.28, N 6.11; found: C
76.77, H 9.13, N 6.06 %. 1H NMR (200 MHz, C6D6, 25 °C, SiMe4): –
1.42 (s, 3 H, Al–CH3), 1.08 (d, 12 H, CH(CH3)2, 1.22 (d, 24 H,
CH(CH3)2, 1.31 (d, 12 H, CH(CH3)2, 1.52 (s, 6 H, CH3), 1.64 (s, 6 H,
CH3), 3.25 (sept, 6 H, CH(CH3)2), 3.75 (sept, 2 H, CH(CH3)2), 4.82
(s, 1 H, γ-CH), 4.92 (s, 1 H, γ-CH), 7.08–7.16 (m, 12 H, m-, p–Ar–
H) ppm; 13C{1H} NMR (125.75 MHz, C6D6, 25 °C, SiMe4): –3.3,
25.4, 25.5, 26.1, 26.2, 35.5, 43.2, 97.3, 141.3, 147.5. EI-MS (70 eV):
Acknowledgement
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG).
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Crystal Structure Determination
Shock cooled crystals were selected and mounted under nitrogen at-
mosphere using the X-TEMP.[22] The data for 2·THF was collected at
100(2) K using a INCOATEC Mo Microsource[23] with Quazar mirror
optics and APEX II detector with a D8 goniometer. The data of 3·0.5
toluene was measured using a Bruker TXS-Mo rotating anode with
Helios mirror optics and APEX II detector with a D8 goniometer. Both
diffractometers were equipped with a low-temperature device and used
Mo-Kα radiation, λ = 0.71073 Å. The data of 2·THF and 3·0.5toluene
were integrated with SAINT[24] and an empirical absorption (SAD-
ABS) was applied.[25] The structures were solved by direct methods
(SHELXS-97) and refined by full-matrix least-squares methods against
F2 (SHELXL-97).[26] All non-hydrogen-atoms were refined with aniso-
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Received: September 20, 2010
Published Online: November 19, 2010
Z. Anorg. Allg. Chem. 2011, 201–205
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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