Soluble molecular dimers of CaO and SrO stabilized by a lewis acid

Article abstract of DOI:10.1002/anie.200904054

Soluble oxides: The reaction between a βdiketiminate aluminum methyl hydroxide and [M{N(SiMe₃)₂}₂(thf)₂] (M=Ca, Sr) results in alkaline-earth-metal oxides such as [{LAl(Me)(μ-O) Ca(thf)}₂] (see core s

Full text of DOI:10.1002/anie.200904054

Communications
DOI: 10.1002/anie.200904054
Alkaline-Earth-Metal Oxides
Soluble Molecular Dimers of CaO and SrO Stabilized by a Lewis
Acid**
Sankaranarayana Pillai Sarish, Sharanappa Nembenna, Herbert W. Roesky,* Holger Ott,
Aritra Pal, Dietmar Stalke, Sudipta Dutta, and Swapan K. Pati
Dedicated to Professor Alan H. Cowley on the occasion of his 75th birthday
Oxides of alkaline earth metals, especially calcium oxide,
have been known since ancient times. The heavier alkaline-
earth-metal oxides, such as calcium oxide (commonly known
as quick lime) and strontium oxide, are prepared by heating
the corresponding carbonates. Calcination of CaCO3 appears
to have been carried out in the Stone Age to judge by the
remains of kilns that have been found. The Romans achieved
a high degree of expertise in its use as a building material.
Strontium oxide is mainly important as a source of other
strontium salts. These oxides have a broad range of applica-
tion in material science and technology. For instance, SrO
finds application in cathode ray tubes, as it can absorb
dangerous UV light. Strontium oxide aluminate doped with
suitable metals can act as a photoluminescent phosphor. CaO
and SrO are also catalysts for the Tishchenko reaction.[1]
However, these inorganic compounds are insoluble in organic
solvents, and making them available to organometallic
chemistry is a synthetic challenge owing to their high lattice
energy (CaO 816 kcalmolÀ1, SrO 769 kcalmolÀ1). For this
reason, and also because of their high melting points (CaO
27078C, SrO 24308C),[2] they are not useful as precursors for
organometallic compounds. A well-defined synthetic strategy
is needed to obtain these types of complexes. Organometallic
oxides can act as catalysts and can serve as model compounds
for the fixation of catalytically active species on oxide surfaces
and can thus help to clarify structure–activity relationships.[3]
Such knowledge would help with the design of good hetero-
geneous catalysts, which are always preferred by industry.
Thus, we became interested in trapping or incorporating
molecular inorganic oxides into an organometallic or organic
matrix. These lipophilic complexes can be studied using
characterization techniques available for organometallic
complexes. A suitable synthesis of these complexes starts
with their hydroxide complexes as precursors. The unique
property of b-diketiminate ligands (herein, L = CH(CMe-2,6-
iPr2C6H3N)2) to stabilize unusual coordination sites has been
successfully applied in the syntheses of various well-defined
hydroxide complexes of main-group metals such as alumi-
num,[4] gallium,[5] and germanium.[6] Soluble heavy alkaline-
earth-metal hydroxide complexes of calcium ([{LCa(m-OH)-
(thf)}2][7] and strontium [LSr(thf)(m-OH)2Sr(thf)2L][8]) have
been reported. Using the strontium hydroxide [LSr(thf)(m-
OH)2Sr(thf)2L] as a precursor, we were able to assemble the
heterobimetallic oxide [{LSr(m-O)Zr(NMe2)3}2] containing
strontium and zirconium.[8] The previously prepared calcium
and strontium hydroxides, however, turned out to be useless
precursors for the preparation of soluble CaO and SrO
compounds. Therefore, we decided to use [LAlMe(OH)]
(1)[4b] as a source for oxygen, because we observed that
[LAlMe(Cl)] (2) in the presence of the Lewis base [LSrN-
(SiMe3)2(thf)] (3) is converted to [L1Al(Me)(thf)] (4) (L1 =
CH{C(CH2)}(CMe)(2,6-iPr2C6H3N)2) upon deprotonation of
a methyl group of L.[9] Subsequently, we treated 1 with
[Ca{N(SiMe3)2}2(thf)2] (5) and [Sr{N(SiMe3)2}2(thf)2] (6) to
yield [{L1Al(Me)(m-O)Ca(thf)}2] (7) and [{L1Al(Me)(m-O)Sr-
(thf)}2] (8).
Herein, we report the synthesis, characterization and
theoretical investigation of the first soluble molecular CaO
and SrO complexes (7 and 8, respectively). Compound 7 was
obtained by the reaction of 1 with 5 in a molar ratio of 2:2 in
THF/n-hexane (1:1) at 08C, and 8 was prepared by reaction of
1 and 6 in the same ratio in THF at room temperature
(Scheme 1). [L1Al(Me)] is formed by deprotonation of 1 with
5 or 6 with concomitant formation of HN(SiMe3)2. Simulta-
neously, the oxygen atom of 1 is transferred to the alkaline
[*] Dipl.-Chem. S. P. Sarish, Dr. S. Nembenna, Prof. Dr. H. W. Roesky,
Dr. H. Ott, Dr. A. Pal, Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie, Universitꢁt Gꢂttingen
Tammannstrasse 4, 37077 Gꢂttingen (Germany)
Fax: (+49)551-39-3373
E-mail: hroesky@gwdg.de
S. Dutta, Prof. Dr. S. K. Pati
Theoretical Sciences Unit and New Chemistry Unit, JNCASR
Bangalore, 560064 (India)
[**] Support of the Deutsche Forschungsgemeinschaft is highly
acknowledged. S.D. acknowledges the CSIR for a research fellow-
ship and S.K.P. acknowledges the CSIR and DST, Govt. of India for a
research grant.
Supporting information for this article, including synthetic details,
physical data, and theoretical calculations, is available on the WWW
under http://dx.doi.org/10.1002/anie.200904054.
Scheme 1. Preparation of calcium and strontium oxide complexes.
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Chemie
earth metal to form CaO and SrO cores. [L1Al(Me)] acts as a
Lewis acid and coordinates to the oxygen atoms in 7 and 8.
This type of deprotonation is the key step for generating 7 and
8.
Compound 7 is a colorless solid and 8 is a pale greenish-
yellow solid. Both are soluble in benzene, toluene, and THF.
Both 7 and 8 were characterized by NMR spectroscopy, EI
mass spectrometry, elemental analysis, and X-ray structural
analysis. The 1H NMR spectrum of 8 showed broad signals at
room temperature. Therefore, the 1H NMR spectroscopy
experiment was conducted at low temperature to give a better
Figure 2. Crystal structure of [{L1Al(Me)(m-O)Sr(thf)}2] (8); anisotropic
À
resolution of the resonances. The Al Me protons of 7 and 8
displacement parameters are shown at a 50% probability level.
Selected bond lengths [ꢃ] and angles [8] of one half of a formula unit
in the asymmetric unit: Al1–O1 1.729(3), Al1–N1 1.895(4), Al1–N2
1.906(4), Sr1–O1 2.391(3), Sr1–O2 2.324(3), Sr1–O3 2.567(3), Sr1–C3
2.787(4), C3–C5 1.348(7); O1-Sr1-O2 81.15(11), O1-Al1-N2 105.91(17),
N1-Al1-N2 96.96(17), Sr1-O1-Sr2 98.89(12), Sr1-O2-Sr3 98.66(12).
Noncoordinating THF molecules, substituents on the nitrogen atoms,
and all hydrogen atoms (except those at C5 and C35) are omitted for
clarity.
resonate at d = À1.05 and À1.06 ppm, respectively. The
signals at d = 1.58 (7) and 1.73 ppm (8) and at d = 3.98, 3.45
(7) and 3.36, 2.79 ppm (8) can be attributed to the CH3 and
CH2 groups on the L1 backbone. The IR spectra of 7 and 8
show bands at 1619 and 1624 cmÀ1, which can be assigned to
=
the C C bond. No molecular ion peaks were observed in the
mass spectra for compounds 7 and 8, but fragment ions were
detected.
Compounds 7 and 8 crystallize in the triclinic space group
P1 and the monoclinic space group Cc, respectively, both with
indicative of a single and a double bond, respectively. The
bond lengths within L1 are in the range of those known in the
¯
À
À
two molecules in the asymmetric unit (Figure 1 and
Figure 2).[10] The structures reveal the dimeric nature of
both compounds resulting in the formation of almost planar
four-membered Ca2O2 and Sr2O2 rings, which are connected
to two terminal L1AlMe units. The calcium and strontium
atoms are tetracoordinate (coordinated by one oxygen atom
of thf, one interaction to the ligand backbone, and two oxygen
literature, except for the two N C bonds (N1 C1 1.355(4),
À
À
À
N2 C3 1.395(4) (7) and N1 C1 1.354(6), N2 C3 1.410(6) ꢀ
(8)), which differ slightly from each other.[11] The two Ca O
and Sr O bond lengths in the M2O2 core are different (on
average 2.175, 2.259 (7) and 2.323, 2.394 ꢀ (8)). The longer
bonds are similar to those found in [{LCa(m-OH)(thf)}2]
À
À
(2.225(6)av ꢀ)[7]
and
[LSr(thf)(m-OH)2Sr(thf)2L]
atoms of the Al O group), and both adopt a distorted square-
(2.402(2)av ꢀ).[8]
À
planar geometry. The aluminum atoms are also tetracoordi-
nate (coordinated by two nitrogen atoms of L1, the methyl
carbon atom, and an oxygen atom) and generate two six-
membered C3N2Al rings owing to the bidentate nature of L1.
The coordination polyhedra of the aluminum atoms can best
For a detailed understanding of the electronic structure
and bonding properties of 7 and 8, we performed ab initio
density functional theory calculations as implemented in the
Gaussian 03 package.[12] We have adopted a hybrid B3LYP[13]
exchange and a correlation functional with the LANL2DZ[14]
basis set. The molecular structures from the X-ray diffraction
experiments served as the starting point for the calculations.
Since the hydrogen atom positions from the X-ray structural
determination are not comparable to the gas phase, we
relaxed all the hydrogen atoms. The calculations for natural
atomic orbitals (NAO) and natural bond order (NBO) were
performed to investigate the charge localization, bonding, and
hybridization characteristics of the structures under consid-
eration.
À
be described as highly distorted tetrahedra. The terminal C C
À
À
bond lengths in the ligand backbone (C1 C4 1.510(5), C3 C5
À
À
1.369(5) (7) and C1 C4 1.516(7), C3 C5 1.348(7) ꢀ (8)) are
We find that although the aluminum atoms of individual
rings in both 7 and 8 are surrounded by two nitrogen atoms of
the bidentate ligand L1, one methyl carbon atom, and one
oxygen atom in a slightly distorted tetrahedral fashion, the
bonding characteristics of Al show a mixture of covalent and
ionic characters with more contribution from the ionic part.
This situation is evident from the localization of charges on
the individual atoms (see the Supporting Information). The
Wiberg bond order analysis also shows a diminished cova-
lency of the bonds (Supporting Information). This ionic
character can further be argued from the molecular orbital
wave functions in Figure 3, which show the absence of
delocalization over these bonds. However, the Ca2O2 or
Sr2O2 rings in between two L1Al moieties show complete ionic
character with approximately + 1.9 e charge on each of the
Figure 1. Core structure of [{L1Al(Me)(m-O)Ca(thf)}2] (7); anisotropic
displacement parameters are shown at the 50% probability level.
Selected bond lengths [ꢃ] and angles [8] of one half of a formula unit
in the asymmetric unit: Al1–O1 1.740(2), Al1–N1 1.898(3), Al1–N2
1.900(3), Al1–C30 1.959(4), Ca1–O1 2.255(2), Ca1–O2 2.179(2), Ca1–
O3 2.415(3), Ca1–C3 2.680(3), C3–C5 1.369(5); O1-Ca1-O2 82.47(9),
O1-Al1-N2 104.24(12), N1-Al1-N2 98.14(13), Ca1-O1-Ca2 97.69(9),
Ca1-O2-Ca2 97.26(9). Noncoordinating n-hexane molecules, substitu-
ents on the nitrogen atoms, and all hydrogen atoms (except those at
C5 and C35) are omitted for clarity
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www.angewandte.org
8741

Communications
Keywords: alkaline earth metals · calcium · hydroxides · oxides ·
.
strontium
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Figure 3. a) HOMOÀ1, b) HOMO, c) LUMO, and d) LUMO+1 of 7
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ꢀ
1246.90, 0.10 ꢂ 0.08 ꢂ 0.06 mm, triclinic, space group P1, a =
17.771(1), b = 17.815(1), c = 25.285(2) ꢀ, a = 103.393(2), b =
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=
1.114 MgmÀ3, m(MoKa) = 0.224 mmÀ1, T= 100(2) K, 2qmax = 508,
94970 reflections measured, 26248 independent (Rint = 0.0593),
R1 = 0.067 (I > 2s(I)), wR2 = 0.184 (all data), max. res. density
peaks: 1.11 to À0.58 eꢀÀ3. b) Crystal data for 8·1.7THF:
C75H116Al2N4O6Sr2, Mr = 1391.31, 0.2 ꢂ 0.1 ꢂ 0.1 mm, monoclinic,
space group Cc, a = 25.915(1), b = 25.999(1), c = 23.384(1) ꢀ,
b = 94.443(1)8, V= 15708.5(7) ꢀ3, Z = 8, 1calcd = 1.177 MgmÀ3, m
(CuKa) = 2.377 mmÀ1, T= 100(2) K, 2qmax = 1208, 81493 reflec-
tions measured, 21355 independent (Rint = 0.0909), R1 = 0.041
(I > 2s(I)), wR2 = 0.112 (all data), max. res. density peaks: 1.20
to À0.55 eꢀÀ3. CCDC 739370 (7), 739371 (8) contain the
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graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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À
and double-bond character in the terminal C C bonds in the
ligand backbones are consistent with the Wiberg bond order
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the symmetric ligands on either side, the highest occupied
molecular orbital (HOMO) becomes almost degenerate with
the HOMOÀ1, as does the lowest unoccupied molecular
orbital (LUMO) with the LUMO + 1, as can be seen in
Figure 3.
In summary, the reaction of aluminum methyl hydroxide 1
with [M{N(SiMe3)2}2(thf)2] (M = Ca, Sr) gave the unprece-
dented heterobimetallic oxides 7 and 8 and reveals alkaline-
earth-metal oxide in an organometallic matrix. Formation of
these two complexes is accompanied by the deprotonation of
one methyl group in the ligand backbone. The stability and
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electron cloud of the methylene groups of the ligand back-
bone come within the coordination sphere of Ca2+/Sr2+
cations to stabilize the Ca2O2/Sr2O2 clusters in the organic
matrix through Coulombic attraction.
[12] Gaussian03 (RevisionC.02): M. J. Frisch et al., see the Support-
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Received: July 22, 2009
Published online: October 8, 2009
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Angew. Chem. Int. Ed. 2009, 48, 8740 –8742