Soluble molecular compounds with the Mg-O-Al structural motif: A model approach for the fixation of organometallics on a MgO surface

Article abstract of DOI:10.1021/ja064460p

We report a facile route to the molecular compounds with the Mg-O-Al structural motif. The reaction of Mg[N(SiMe₃)₂]₂ (1) with a stoichiometric amount of LAlOH(Me) (2) [L = CH{(CMe)(2,6-iPr₂C₆H₃N)}₂] in THF/n-hexane at 0 °C results in the formation of the heterobimetallic compound (Me₃Si)₂NMg(THF)₂-O-Al(Me)L (3) in high yield. The similar reaction of 1 equiv of Mg[N(SiMe₃)₂]₂ and 2 equiv of LAlOH(Me) results in the formation of trimetallic compound L(Me)Al-O-Mg(THF)₂-O-Al(Me)L (4). Structural analyses of 3 and 4 have been carried out, revealing the presence of the Mg-O-Al motif. A tentative assignment of the Mg-O-Al vibrations has been made and was supported by calculations. Copyright

Full text of DOI:10.1021/ja064460p

Published on Web 09/20/2006
Soluble Molecular Compounds with the Mg-O-Al Structural Motif: A Model
Approach for the Fixation of Organometallics on a MgO Surface
Sharanappa Nembenna,† Herbert W. Roesky,*,† Swadhin K. Mandal,† Rainer B. Oswald,‡ Aritra Pal,†
Regine Herbst-Irmer,† Mathias Noltemeyer,† and Hans-Georg Schmidt†
Institute of Inorganic Chemistry, UniVersity of Goettingen, Tammannstrasse 4, D-37077 Goettingen, Germany, and
Institute of Physical Chemistry, UniVersity of Goettingen, Tammannstrasse 6, D-37077 Goettingen, Germany
Received June 23, 2006; E-mail: hroesky@gwdg.de
Spinel is a very attractive and historically important gemstone
and mineral. Spinel of composition MgAl2O4 is found in nature,
and it is prepared by reacting aluminum oxide with MgO at high
temperatures. The structure of MgAl2O4 consists of cubic closest
packed oxide ions. A multitude of spinels using other elements has
been reported.1-3 These compounds have a variety of applications;
for example, hot pressed MgAl2O4 is used as an optical window,
and the ferrite spinels are an important family of magnetic materials.
All spinels have in common that they are high melting inorganic
solids and insoluble in organic solvents. Moreover, it was shown
that SiO2 and MgCl2 function as supports for metallocene-
methylaluminoxane (MAO) catalysts.4 The magnesium-supported
system exhibits activity 2-fold higher in ethylene polymerization
than that with the silica analogues. An explanation of this
phenomenon has not been given.5,6 Previous quantum chemical
Figure 1. Molecular structure of 3; thermal ellipsoids set at 50% probability.
All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and
bond angles [°]: Mg(1)-O(1) 1.850(2), Mg(1)-O(30) 2.094(2), Mg(1)-
O(40) 2.070(2), Mg(1)-N(3) 2.019(2), O(1)-Al(1) 1.682(2), Al(1)-C(6)
calculations have shown that adsorbed rhenium subcarbonyls on
MgO surfaces form strong Re-O adsorption bonds, justifying that
the inert MgO surface is able to anchor organometallic fragments.7
Recently, an alkoxy-bridged Mg-O(R)fAl compound has been
reported.8 However, the stability and bonding situation of this
system is quite different from that of an oxide-bridged Mg-O-Al
motif.
1.975(2); Mg(1)-O(1)-Al(1) 157.1(1), O(40)-Mg(1)-O(30) 88.4(1),
O(1)-Al(1)-C(6) 120.6(1), Si(1)-N(3)-Si(2) 125.1(1).
Scheme 1
Scheme 2
Herein, we report the molecular compounds containing the Mg-
O-Al skeleton. The reaction of Mg[N(SiMe3)2]2 (1)9,10 with a
stoichiometric amount of LAlOH(Me) [L ) CH{(CMe)(2,6-
iPr2C6H3N)}2; 2]11 in THF/n-hexane (1:1) at 0 °C results in the
elimination of HN(SiMe3)2 and the formation of the heterobimetallic
compound 3 in high yield (Scheme 1). Compound 3 is soluble in
a wide range of common organic solvents. It was characterized by
1H, 13C, 29Si NMR and IR spectroscopy, EI mass spectrometry,
1
elemental analysis, and X-ray structural analysis. The H NMR
spectrum of 3 exhibits a resonance at -0.36 ppm for Al-Me, which
by comparison with that of 2 (-0.88 ppm) clearly shows a
downfield shift. The expected resonance for SiMe3 is observed as
a singlet (0.15 ppm). The 29Si NMR shows a signal at -9.8 ppm
which is shifted downfield compared to that of 1 (-12.6 ppm).12
No molecular ion of 3 was detected in the EI mass spectrum; only
small fragment ions are found.
Crystals of 313 suitable for X-ray structural analysis were obtained
from a THF/n-pentane solution at 0 °C. Compound 3 crystallizes
in the monoclinic space group P21/n. The molecular structure is
shown in Figure 1. The X-ray structural analysis of 3 revealed that
the magnesium is bonded through a bridging oxygen atom to an
aluminum atom. The geometry around the magnesium, as well as
that of the aluminum, is tetrahedrally distorted. The Mg-O bond
distances (1.850-2.095 Å) are consistent with the Mg-O bond
lengths compiled by Holloway and Melnik for compounds of
magnesium with the coordination number 4 at the magnesium
(1.819-2.219 Å).14 The Mg(1)-O(1)-Al(1) bond angle (157.1-
(1)°) is much wider compared to that in the alkoxy-bridged [Al3-
Mg3(µ3-O)(THFFO)4Cl4(Me)5(THF)] (THFFO ) 2-tetrahydrofur-
furoxide) compound (Al-µ-O(alkoxide)-Mg (128.2(2)°)).8 The Al-C
and Al-O bond distances are 1.975(2) and 1.682(2) Å, respectively,
which are in good agreement with those observed in
[OCMeCHCMeNAr]2AlMe (Ar ) 2,6-iPr2C6H3) (Al-C ) 1.975-
(2) Å) and [HC(CMeNMe)2AlCl]2(µ-O) (Al-O ) 1.677(2) Å).15,16
Compound 4 (shown in Scheme 2, also see Supporting Informa-
tion) is soluble in hexane, benzene, toluene, and THF. It was
characterized by 1H, 13C, 27Al NMR and IR spectroscopy, EI mass
spectrometry, elemental analysis, and X-ray structural analysis. The
1H NMR spectrum of 4 shows a single resonance for the Al-Me
(-0.66 ppm). The 27Al NMR for 4 was silent, obviously due to
the quadrupole moment of the aluminum atom.
† Institute of Inorganic Chemistry.
‡ Institute of Physical Chemistry.
9
13056
J. AM. CHEM. SOC. 2006, 128, 13056-13057
10.1021/ja064460p CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
In summary, we report a facile route to the molecular compounds
with the Mg-O-Al structural motif. Compound 3 can be consid-
ered to function as a precursor for the preparation of heterotrime-
tallic systems. Furthermore, compounds 3 and 4 might be useful
precursors for spin-coated spinel surfaces and function as a
molecular model for surface fixation of the organometallic species.
A tentative assignment of the Mg-O-Al vibrations has been made
and was supported by calculations.
Acknowledgment. This work was supported by the Goettinger
Akademie der Wissenschaften. S.K.M. thanks the Alexander von
Humboldt Stiftung for a research fellowship.
Supporting Information Available: Crystallographic (CIF) and
experimental details, complete ref 19, and details of the calculation.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. Molecular structure of 4; thermal ellipsoids at the 50% probability
level. All hydrogen atoms are omitted for clarity. Selected bond lengths
[Å] and bond angles [°]: Al(1)-O(1) 1.671(2), O(1)-Mg(1) 1.873(2), Mg-
(1)-O(2) 1.858(2), O(3)-Mg(1) 2.043(2), O(4)-Mg(1) 2.089(2), O(2)-
Al(2) 1.671(2), Al(1)-C(6) 1.971(3), Al(2)-C(36) 1.980(3); Al(1)-O(1)-
Mg(1) 150.2(1), Al(2)-O(2)-Mg(1) 159.6(1), O(2)-Mg(1)-O(1) 142.4(1),
O(3)-Mg(1)-O(4) 99.8(1).
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19.870(3) Å, â ) 101.66(2)°, V ) 4900.5(1) Å3, Z ) 4, Fcalcd ) 1.091 M
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