Alkoxymagnesium iodide complexes

Article abstract of DOI:10.1002/ejic.201100370

The complexes R(Ph)MeCOMg(Et₂O)I [1, R = Me (a), Ph (b)] were obtained by the reaction of R(Ph)C=O with MeMgI in diethyl ether in good yields. Crystallisation of 1 from acetonitrile resulted in single crystals of di- and trinuclear complexes I₂(μ-OCPhMe₂)₂Mg(MeCN) ₄ (2) and {2[I(MeCN)Mg(μ-OCPh₂Me)₂Mg(MeCN)I] (3) × [I(MeCN)Mg(μ-OCPh₂Me)₂Mg(μ-OCPh ₂Me)₂Mg(MeCN)I] (4) × 2CH₂Cl₂} in which the metal centres are linked by alkoxy substituents. The thermal decomposition of the complexes 1 was studied by successive detection of the EI mass spectra of diethyl ether, the olefins 5, the tertiary alcohols 6, and the formal olefin dimers 7. The complexes R¹(Ph)MeCOMg(Et₂O)I [1, R¹ = Me (a), Ph (b)] are obtained by the reaction of R ¹(Ph)C=O with MeMgI in diethyl ether in good yields. Crystallisation of 1 from acetonitrile results in single crystals of di- and trinuclear complexes (2, 3 and 4) in which the metal centres are linked by alkoxy substituents. The thermal decomposition of the complexes 1 is monitored by mass spectral detection of the decomposition products. Copyright

Full text of DOI:10.1002/ejic.201100370

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100370
Alkoxymagnesium Iodide Complexes
Peter Haiss,[a] Annette Kuhn,[b] Norbert Kuhn,*[c] Cäcilia Maichle-Mößmer,[c]
Stefan Laufer,*[b] Manfred Steimann,[c] and Klaus-Peter Zeller*[a]
Dedicated to Professor Michael Hanack on the occasion of his 80th birthday
Keywords: Magnesium / Metallacycles / Grignard reaction / X-ray diffraction / Mass spectrometry
The complexes R(Ph)MeCOMg(Et2O)I [1, R = Me (a), Ph (b)]
were obtained by the reaction of R(Ph)C=O with MeMgI in
diethyl ether in good yields. Crystallisation of 1 from acetoni-
Me)2Mg(μ-OCPh2Me)2Mg(MeCN)I] (4)ϫ2CH2Cl2} in which
the metal centres are linked by alkoxy substituents. The ther-
mal decomposition of the complexes 1 was studied by suc-
trile resulted in single crystals of di- and trinuclear com- cessive detection of the EI mass spectra of diethyl ether, the
plexes I2(μ-OCPhMe2)2Mg(MeCN)4 (2) and {2[I(MeCN)Mg-
(μ-OCPh2Me)2Mg(MeCN)I] (3) ϫ [I(MeCN)Mg(μ-OCPh2-
olefins 5, the tertiary alcohols 6, and the formal olefin dimers
7.
vent adducts R1(Ph)MeCOMg(Et2O)I [1; R1 = Me (a), Ph
(b)] in almost quantitative yields.[8] NMR spectroscopic
data from CDCl3 solutions show a duplicate set of signals
for 1a (Me) and 1b (Ph), which indicates a rigid structure
presumably as dimers with trans orientation of the terminal
substituents owing to the prochiral nature of the organic
substituent R1(Ph)MeC.[9] Unfortunately, single crystals of
1 could not be obtained as a consequence from their limited
stability in dichloromethane and chloroform (Scheme 1).
Introduction
There has been considerable interest in metal alkoxides
and their derivatives in the past decades concerning their
structural chemistry and application.[1] The polymeric
structures of Mg(OR)2 compounds[2] are broken by co-li-
gands, preferentially containing nitrogen or oxygen donor
functions, and numerous crystal-structure investigations
have been reported thereon.[3,4] Though the Grignard anal-
ogous alkoxymagnesium halides,[5] as well as their organo-
metallic neighbours,[6] play an important role in organic
synthesis, structures of the parent type RO–Mg–X stabi-
lized by solvent molecules are almost completely absent
from the literature.[7] We have therefore been interested in
the structural chemistry and thermal properties of alkoxy-
magnesium iodide complexes.
Results and Discussion
The reaction of acetophenone and benzophenone with
CH3MgI in diethyl ether precipitates the alkoxides as sol-
[a] Institut für Organische Chemie der Eberhard-Karls-Universität
Tübingen,
Auf der Morgenstelle 18, 72076 Tübingen, Germany
Fax: +49-7071-29-5076
E-mail: kpz@uni-tuebingen.de
[b] Pharmazeutisches Institut der Eberhard-Karls-Universität
Tübingen,
Auf der Morgenstelle 8, 72076 Tübingen, Germany
Fax: +49-7071-5037
E-mail: stefan.laufer@uni-tuebingen.de
[c] Institut für Anorganische Chemie der Eberhard-Karls-
Universität Tübingen,
Auf der Morgenstelle 18, 72076 Tübingen, Germany
Fax: +49-7071-29-5224
E-mail: norbert.kuhn@uni-tuebingen.de
Scheme 1.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Alkoxymagnesium Iodide Complexes
The thermal decomposition of magnesium alkoxides has
been used as a technical route to obtain pure MgO.[10]
Analogously, we obtained the corresponding olefins in ad-
dition to their formal dimers on heating the diethyl ether
complexes 1 up to 150 °C in vacuo.[11,12] Introduction of the
complexes 1 in the high vacuum of the EI source of a mass
spectrometer and stepwise heating of the direct insertion
probe (35 up to 150 °C) resulted in the time-dependent for-
mation of decomposition products (Scheme 2). The com-
plexes 1, after removal of the diethyl ether ligands, decom-
pose into the olefins 5 and the tertiary alcohols 6. With
increasing temperature, the alcohols yield additional
amounts of the olefins 5. By further rise of the temperature,
the mass spectra of the olefins diminish and are replaced
by those of the dimeric products 7.
Figure 1. View of the molecule C26H34I2Mg2N4O2 (2) (hydrogen
atoms omitted for clarity) in the crystal. Selected bond lengths [Å]
and angles [°]: Mg(1)–O(1) 1.9382(19), Mg(1)–O(2) 1.9388(19),
Mg(1)–I(1) 2.7396(9), Mg(1)–I(2) 2.7724(9), Mg(2)–N(30) 2.226(3),
Mg(2)–N(40) 2.207(3), Mg(2)–N(50) 2.236(3), Mg(2)–N(60)
2.231(3), O(1)–C(11) 1.425(3), O(2)–C(21) 1.423(3), Mg(1)···Mg(2)
2.9478(12); O(1)–Mg(1)–O(2) 85.24(8), O(1)–Mg(2)–O(2) 81.36(7),
Mg(1)–O(1)–Mg(2) 96.46(8), O(1)–Mg(2)–O(2) 96.43(8); in-
terplanar angles [°]: Mg(1)O(1)Mg(2)//Mg(1)O(2)Mg(2) 8.2, O(1)
Mg(1)O(2)//O(1)Mg(2)O(2) 7.3, O(1)Mg(1)O(2)//I(1)Mg(1)I(2)
90.7.
Scheme 2.
From acetonitrile solutions of 1, products of varying
compositions [R(Ph)MeCOMg(MeCN)xI (x = 1–3)] were
obtained, which could not be purified by recrystallisation
from dichloromethane or acetonitrile on a preparative scale.
However, small amounts of single crystals could be isolated.
Their crystal structure analyses now reveal the presence of
di- and trinuclear complexes in which the metal atoms are
linked by alkoxy bridges exclusively (Figures 1, 2, and 3).[13]
Whereas 1a forms a dimer of the composition I2Mg(μ-
OCPhMe2)2Mg(MeCN)4 with “unsymmetrical” coordina-
tion spheres around the metal centres (2), the unit cell of
the crystals obtained from 1b contains both 2 equiv. of a
dimer [I(MeCN)Mg(μ-OCPh2Me)2Mg(MeCN)I] of the cis
type (3) and
a trinuclear complex [I(MeCN)Mg(μ-
OCPh2Me)2Mg(μ-OCPh2Me)2Mg(MeCN)I] (4)[14] in which
the central magnesium atom Mg(50) lies on a twofold crys-
tallographic rotation axis and is linked to its metal neigh-
bours by alkoxy bridges including additional 2 equiv. of
dichloromethane. Thus, overall the crystal has the compo-
sition (Ph2MeOC)8Mg7I6(MeCN)6·2CH2Cl2. All bond
lengths and angles inside the alkoxy substituents are in the
expected range.[15] For individual bond lengths and angles
around the metal centres see Figures 1, 2, and 3.
Figure 2. View of the molecule C32H32I2Mg2N2O2 (3) in the crystal
of 23·4·2CH2Cl2 (hydrogen atoms and solvent molecules omitted
for clarity). Selected bond lengths [Å] and angles [°]: Mg(5)–O(21)
1.956(6), Mg(5)–O(36) 1.947(6), Mg(6)–O(21) 1.942(6), Mg(6)–
O(36) 1.946(6), Mg(5)–I(1) 2.665(3), Mg(6)–I(2) 2.657(3), Mg(5)–
N(15) 2.086(8), Mg(6)–N(18) 2.081(9), O(21)–C(22) 1.425(10),
O(36)–C(7) 1.417(11), Mg(5)···Mg(6) 2.885(4); O(21)–Mg(5)–O(36)
84.1(2), O(21)–Mg(6)–O(36) 84.5(2), Mg(5)–O(21)–Mg(6) 95.5(3),
Mg(5)–O(36)–Mg(6) 95.6(3); interplanar angles [°]: Mg(5)O(21)
Mg(6)//Mg(5)O(36)Mg(6) 5.0, O(21)Mg(5)O(36)//O(21)Mg(6)
O(36) 4.5.
Eur. J. Inorg. Chem. 2011, 3284–3287
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N. Kuhn, K.-P. Zeller et al.
SHORT COMMUNICATION
Scheme 3.
Conclusions
Our results confirm the strong tendency of alkoxymagne-
sium derivatives to form oxygen-bridged oligomers even in
the presence of halide substituents. Excess of donor solvent
molecules, however, can split the Mg2O2 units as figured by
the presence of solvated magnesium iodide, and novel – in
part trinuclear – magnesium alkoxide derivatives in which
the oxygen bridges are reformed. Though a definite proof
is not present, our results are best interpreted in terms of a
Schlenk-type equilibrium including oligomeric alkoxides
and monomeric dihalides.
Thermal decomposition of alkoxymagnesium iodides,
monitored by EI mass spectrometry, proves these complexes
to be a valuable tool for the construction of organic moie-
ties derived from the alkoxide substituents. We are continu-
ing our investigations in this direction and will report on
our results in due course.
Figure 3. View of the molecule C60H58I2Mg3N2O4 (4) in the crystal
of C126H126Cl4I6Mg7N6O8 (23·4·2CH2Cl2) (hydrogen atoms and
solvent molecules omitted for clarity). Selected bond lengths [Å]
and angles [°]: Mg{(RO)2MgI(MeCN)}2: Mg(50)–O(51) 2.001(5),
Mg(50)–O(66) 1.972(6), Mg(80)–O(51) 1.952(6), Mg(80)–O(66)
1.959(6), Mg(80)–I(3) 2.709(3), Mg(80)–N(81) 2.100(10), O(51)–
C(52) 1.445(10), O(66)–C(67) 1.434(11), Mg(50)···Mg(80) 2.936(3);
O(51)–Mg(50)–O(66) 82.2(2), O(51)–Mg(50)–O(51A) 122.3(3),
O(66)–Mg(50)–O(66A) 127.5(4), O(51)–Mg(50)–O(66A) 124.2(2),
O(51)–Mg(80)–O(66) 96.6(3), Mg(50)–O(51)–Mg(80) 95.9(3),
Mg(50)–O(66)–Mg(80) 96.6(3); interplanar angles [°]: O(51)Mg(50)
O(66)//O(51A)Mg(50)O(66A) 90.5, O(51)Mg(50)O(66)//O(51)
Mg(80)O(66) 11.9, Mg(50)O(51)Mg(80)//Mg(50)O(66)Mg(80) 13.3,
O(51)Mg(80)O(66)//I(3)Mg(80)N(81) 96.5.
Experimental Section
Synthesis: [Mg(MeCN)6]I2 was obtained from iodine and excess
magnesium in acetonitrile. The filtered solution was used without
further workup. Mg[OCMe(Ph)R]2 was prepared from stoichiomet-
ric amounts of Mg(nBu)2 and the corresponding alcohol in n-hept-
ane. The complexes 1 were prepared in almost quantitative yields
from the reaction of the corresponding ketone with 2 equiv. of
MeMgI in diethyl ether (12 h, room temp.) followed by washing
of the resulting precipitate with diethyl ether. Crystals of 2 and
23·4·2CH2Cl2 were obtained by slow diffusion of diethyl ether into
About 80 years ago, Schlenk proposed his famous equi-
librium to explain the properties of Grignard reagents.[16]
Later on, his idea was confirmed by 25Mg NMR spec-
troscopy[17] and by 28Mg isotope experiments.[18] From the
composition of the single crystals mentioned above, the
presence of a Schlenk-type equilibrium of alkoxymagne-
sium iodides in acetonitrile, which includes solvated magne-
sium iodide and alkoxide, may be suggested. The structures
of 2, 3 and 4 demonstrate the presence of oligonuclear spe-
cies in which the metal centres are linked by alkoxy bridges.
Thus, equilibria can explain both the results of solution and
solid-state characterisation of the acetonitrile complexes.
Therefore, we investigated solutions of 1 by 25Mg NMR
spectroscopy. In [D2]dichloromethane, broad signals at δ ≈
40 ppm [vs. Mg(H2O)62+] were detected, whereas in [D3]-
acetonitrile broad signals at δ ≈ 0 ppm appeared in addition
to a sharp signal at δ = –17 ppm, which could be assigned
1
dichloromethane/acetonitrile (1:10) solutions of 1a and 1b. 1a: H
NMR (CDCl3): δ = 0.88 (t, 3J = 7 Hz, CH2Me), 1.51, 1.58 (2 s,
PhCMe2), 3.43 (q, CH2Me), 6.88–7.12 (m, Ph) ppm. 13C NMR
(CDCl3): δ = 13.9 (CH2Me), 33.7, 34.7 (PhCMe2), 65.6 (CH2CH3),
71.9 (PhCMe2), 125.6, 126.8, 128.4, 149.9 (Ph) ppm. C13H21IMgO2
1
(360.52): calcd. I 35.20, Mg 6.74; found I 35.32, Mg 6.78. 1b: H
NMR (CDCl3): δ = 0.72 (t, 3J = 7 Hz, CH2Me), 2.01 (s, Ph2CMe),
3.3 (q, CH2Me), 6.97–7.38 (m, Ph) ppm. 13C NMR (CDCl3): δ =
13.9 (CH2Me), 33.7, 34.7 (Ph2CMe), 65.6 (CH2Me), 71.9
(Ph2CMe), 125.6, 126.8, 128.4, 149.9 (Ph) ppm. C18H23IMgO2
(422.59): calcd. I 30.03, Mg 5.75; found I 30.00, Mg 6.19.
Crystallography: Single-crystal data for C26H34I2Mg2N4O2 (2): M
= 737, monoclinic space group P21/c, a = 14.407(1), b = 11.459(1),
c = 20.623(1) Å, β = 107.01(1)°, V = 3255.7(4) Å3, Z = 4, Dcalcd.
=
1.504 gcm–3, μ = 1.997 mm–1, Mo-Kα, λ = 0.71073, T = 173(2) K,
R(F2 Ͼ2σ) = 0.0317, R(F2, all data) = 0.0599, goodness-of-fit =
1.166 for all 6646 unique data (45648 measured, 5863 refined,
2+
to the Mg(MeCN)6 dication. Very similar spectra were
obtained from mixtures of MgI2 and Mg[OCR(Me)Ph]2 in
acetonitrile, which implies decomposition of the complexes
1 in donor solvents possibly due to equilibria of the Schlenk
type (Scheme 3), though we could not prove their presence
by temperature-dependent NMR spectroscopy.
Rint
=
0.09030, 2Θ
Ͻ
52.74°). Single-crystal data for
C126H126Cl4I6Mg7N6O8 (23·4·2CH2Cl2): M = 2928, monoclinic
space group C2/c, a = 23.179(1), b = 21.205(1), c = 28.970(2) Å, β
= 112.16(1)°, V = 13187(1) Å3, Z = 4, Dcalcd. = 1.471 gcm–3, μ =
1.582 mm–1, Mo-Kα, λ = 0.71073, T = 173(2) K, R(F2 Ͼ2σ) =
3286
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Alkoxymagnesium Iodide Complexes
0.0832, R(F2, all data) = 0.1405, goodness-of-fit = 1.145 for all
10614 unique data (69326 measured, 7118 refined, Rint = 0.1980,
2Θ Ͻ 48.82°). CCDC-788536 (2), CCDC -788535 (23·4·2CH2Cl2)
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: April 6, 2011
Published Online: June 27, 2011
Eur. J. Inorg. Chem. 2011, 3284–3287
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3287