A reactivity change of a strontium monohydroxide by umpolung to an acid

Article abstract of DOI:10.1021/ic800284u

Controlled hydrolysis of strontium amide LSrN(SiMe₃) ₂(thf) (L = CH(CMe2,6-i-Pr₂C₆H ₃N)₂) (1) gave an unprecedented example of a hydrocarbon-soluble strontium hydroxide, [LSr(thf)(μ-OH)₂Sr(thf) ₂L] (2). In compound 2, the tetrahydrofuran (THF) molecules can easily replaced by benzophenone and triphenylphosphine oxide to yield [LSr(μ-OH)(OCPh₂)]₂ (3) and [LSr(μ-OH)(OPPh ₃)]₂ (4) compounds. Among the two strontium atoms of 2, one is coordinated to a single THF molecule, while the other is coordinated to two THF molecules. Interestingly, strontium hydroxide complex 2 behaves as an acid in its reaction with Zr(NMe₂)₄ and results in a heterobimetallic oxide, [LSr(μ-O)Zr(NMe₂)₃]₂ (5). Compound 5 is dimeric in the solid state and contains a Sr ₂Zr₂O₂ core.

Full text of DOI:10.1021/ic800284u

Inorg. Chem. 2008, 47, 5971-5977
A Reactivity Change of a Strontium Monohydroxide by Umpolung to an
Acid
Sankaranarayanapillai Sarish, Sharanappa Nembenna, Selvarajan Nagendran, Herbert W. Roesky,*
Aritra Pal, Regine Herbst-Irmer, Arne Ringe, and Jörg Magull
Institut für Anorganische Chemie, UniVersität Göttingen, Tammannstrasse 4,
37077 Göttingen, Germany
Received February 15, 2008
Controlled hydrolysis of strontium amide LSrN(SiMe3)2(thf) (L ) CH(CMe2,6-i-Pr2C6H3N)2) (1) gave an unprecedented
example of a hydrocarbon-soluble strontium hydroxide, [LSr(thf)(µ-OH)2Sr(thf)2L] (2). In compound 2, the
tetrahydrofuran (THF) molecules can easily replaced by benzophenone and triphenylphosphine oxide to yield [LSr(µ-
OH)(OCPh2)]2 (3) and [LSr(µ-OH)(OPPh3)]2 (4) compounds. Among the two strontium atoms of 2, one is coordinated
to a single THF molecule, while the other is coordinated to two THF molecules. Interestingly, strontium hydroxide
complex 2 behaves as an acid in its reaction with Zr(NMe2)4 and results in a heterobimetallic oxide, [LSr(µ-
O)Zr(NMe2)3]2 (5). Compound 5 is dimeric in the solid state and contains a Sr2Zr2O2 core.
Introduction
heteropolymetallic oxides by utilizing the acidic character
of these hydroxide complexes.4 For example, the heterodi-
metallic compound LAl(Me)(µ-O)Zr(Me)Cp2 (L ) CH(C-
Me2,6-i-Pr2C6H3N)2) has been obtained by the reaction of
LAl(Me)OH with Cp2ZrMe2, and we have demonstrated that
it is a versatile catalyst in ethylene polymerization.5 Also,
the cubic silicon-titanium µ-oxo complex obtained by the
reaction of an aminosilanetriol with Ti(OEt)4 showed excel-
lent catalytic activity in the epoxidation of cyclohexene and
cyclooctene by t-butyl hydroperoxide.6 In view of this
importance, one may think of a possible extension of these
principles to group 2 elements. Nevertheless, two major
issues that need to be addressed are the synthesis of stable
and soluble group 2 hydroxides and the basicity of these
hydroxides. The preparation of group 2 hydroxides, mainly
those with heavier metals, is difficult (due to the large atomic
radii and high ionic character of these elements),7 and
The so-called water effect in organometallic compounds
has resulted in the formation of various interesting hydroxide
complexes.1 The reaction of these hydroxides with suitable
metal precursors generally leads to the formation of poly-
metallic oxides2 that are emerging as an important class of
compounds due to their potential application in catalysis.3
Therefore, we have prepared various p-block hydroxides and
have been successful in assembling novel heterobi- and
* To whom correspondence should be addressed. Email: hroesky@
gwdg.de.
(1) (a) Roesky, H. W.; Walawalkar, M. G.; Murugavel, R. Acc. Chem.
Res. 2001, 34, 201–211. (b) Roesky, H. W.; Singh, S.; Jancik, V.;
Chandrasekhar, V. Acc. Chem. Res. 2004, 37, 969–981.
(2) (a) See, for examples of mixed metal oxides containing alkaline earth
metals: Kennedy, A. R.; Mulvey, R. E.; Rowlings, R. B. J. Am. Chem.
Soc. 1998, 120, 7816–7824. (b) Kennedy, A. R.; Mulvey, R. E.;
Rowlings, R. B. Angew. Chem., Int. Ed. 1998, 37, 3180–3183.
(3) (a) Singh, S.; Roesky, H. W. Dalton Trans. 2007, 1360–1370. (b)
Gurubasavaraj, P. M.; Mandal, S. K.; Roesky, H. W.; Oswald, R. B.;
Pal, A.; Noltemeyer, M. Inorg. Chem. 2007, 46, 1056–1061.
(4) (a) Mandal, S. K.; Gurubasavaraj, P. M.; Roesky, H. W.; Schwab,
G.; Stalke, D.; Oswald, R. B.; Dolle, V. Inorg. Chem. 2007, 46, 10158–
10167. (b) Mandal, S. K.; Gurubasavaraj, P. M.; Roesky, H. W.;
Oswald, R. B.; Magull, J.; Ringe, A. Inorg. Chem. 2007, 46, 7594–
7600. (c) Gurubasavaraj, P. M.; Roesky, H. W.; Sharma, P. M. V.;
Oswald, R. B.; Dolle, V.; Pal, A. Organometallics 2007, 26, 3346–
3351. (d) Nembenna, S.; Roesky, H. W.; Mandal, S. K.; Oswald, R. B.;
Pal, A.; Herbst-Irmer, R.; Noltemeyer, M.; Schmidt, H.-G. J. Am.
Chem. Soc. 2006, 128, 13056–13057. (e) Chai, J.; Jancik, V.; Singh,
S.; Zhu, H.; He, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.;
Hosmane, N. S. J. Am. Chem. Soc. 2005, 127, 7521–7528. (f)
Winkhofer, N.; Voigt, A.; Dorn, H.; Roesky, H. W.; Steiner, A.; Stalke,
D.; Reller, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1352–1354.
(5) (a) Bai, G.; Singh, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-
G. J. Am. Chem. Soc. 2005, 127, 3449–3455. (b) Roesky, H. W.; Bai,
G.; Jancik, V.; Singh, S. European Patent, PCT/EP, 2005/002741,
2005; Int. Pat. Classification C07F17/00.
(6) Fujiwara, M.; Wessel, H.; Hyung-Suh, P.; Roesky, H. W. Tetrahedron
2002, 58, 239–243.
(7) (a) Hanusa, T. P. Polyhedron 1990, 9, 1345–1362. (b) Hanusa, T. P.
Coord. Chem. ReV. 2000, 210, 329–367. (c) Westerhausen, M. Angew.
Chem. 2001, 113, 3063–3065. (d) Westerhausen, M. Angew. Chem.,
Int. Ed. 2001, 40, 2975–2977. (e) Alexander, J. S.; Ruhlandt-Senge,
K. Eur. J. Inorg. Chem. 2002, 2761–2774. (f) There are some
O-centered cage compounds in the literature. See, for example:
Maudez, W.; Meuwly, M.; Fromm, K. M. Chem.sEur. J. 2007, 13,
8302–8316.
10.1021/ic800284u CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 13, 2008 5971
Published on Web 05/23/2008

Sarish et al.
consequently only a few group 2 hydroxide complexes are
reported. Two hydroxide complexes for magnesium8 and one
example for calcium9 are known. Apart from the magnesium
hydroxide complex {[TpAr,Me]Mg(µ-OH)}2 (stabilized by
tris(1-pyrazolyl)hydroborate ligand (TpAr,Me); Ar
)
p-tBuC6H4), the other two examples have been obtained by
exploiting the unique property of the ꢀ-diketiminato ligand,
L. All of these complexes are dimeric in the solid state and
contain coordinated tetrahydrofuran (THF) molecules except
{[TpAr,Me]Mg(µ-OH)}2.8a They can be exchanged with other
donors such as benzophenone.9 Surprisingly, no reactivity
study based on these complexes has been reported. Group 2
metals except Be react with water and form metal hydroxides
by the elimination of hydrogen gas. These metal hydroxides
2+
-
exist as M(aq) and OH(aq) ions in aqueous solution. The
basic strength increases within this group from Mg to Ba
and can be determined by pH measurements. This basic
knowledge informs us that the aforementioned group 2
hydroxide complexes might behave as a base. To shed light
into this issue and to find out the exact nature of the OH
functionality in these complexes, we prepared a novel
strontium hydroxide complex, [LSr(thf)(µ-OH)2Sr(thf)2L] (1),
by the controlled hydrolysis of strontium amide and carried
out a series of experiments on this hydroxide.
Herein, we report the first molecular hydrocarbon-soluble
strontium monohydroxide, 2, and a heterobimetallic com-
pound, [LSr(µ-O)Zr(NMe2)3]2 (5), obtained by the reaction
of strontium hydroxide 2 with Zr(NMe2)4, where compound
2 behaves as an acid instead of its expected reactivity as a
base. Such an umpolung is unprecedented in group 2
hydroxide chemistry.
Figure 1. Crystal structure of 1. All of the hydrogen atoms have been
omitted for clarity.
conformation with the strontium atom at a distance of 0.724
Å above the plane of the planar C3N2 framework. This can
be compared with the magnesium and calcium amides where
the respective metals are at a distance of 0.428 and 1.206
Å, respectively, above the plane of the planar C3N2 frame-
work.10,11 The Sr center is four-coordinate with two nitrogen
atoms of the ꢀ-diketiminate ligand, a nitrogen atom of the
amide moiety, and an oxygen atom of the THF. The
strontium atom has a distorted tetrahedral geometry, while
that around the nitrogen atom of the amide is trigonal-planar.
Controlled hydrolysis of amide 1 with a stoichiometric
amount of degassed water in THF at -60 °C gave strontium
hydroxide 2 as a colorless solid (55.2% yield; Scheme 1).
Colorless crystals suitable for X-ray structural analysis were
obtained either by keeping a dilute solution of 2 in THF at
-32 °C or by concentrating its THF solution at room
temperature.
Results and Discussion
The reaction of LH with 2 equiv of KN(SiMe3)2 in THF
was carried out for 5 h. Addition of this reaction mixture to
a slurry of SrI2 in THF at room temperature led to the
formation of the strontium amide, LSrN(SiMe3)2(thf) (1), as
colorless crystals (77.9% yield). Compound 1 is soluble in
a number of organic solvents. It has been well-characterized
by mass spectrometry, NMR spectroscopy (1H, 13C, and 29Si),
X-ray single-crystal structure analysis, and elemental analy-
Compound 2 is freely soluble in common organic solvents
such as benzene, toluene, and THF. Compound 2 was
characterized by NMR spectroscopy (1H and 13C), EI mass
spectrometry, and elemental and X-ray structural analysis.
The complete disappearance of the SiMe3 resonance (0.14
ppm) of 1 clearly indicates the formation of compound 2.
As expected, the resonances for the γ-CH (4.74 ppm) and
OH (-0.72 ppm) protons appear as singlets in the 1H NMR
spectrum. The absence of the molecular ion peak in the EI
mass spectrum of 2 shows its instability under these
conditions. The hydroxide stretching frequency of 2 appears
as a sharp absorption band (3677 cm-1) in the IR spectrum
and is comparable to those of the inorganic anhydrous
strontium dihydroxide (3618 cm-1).12 The strontium hy-
1
sis. The H and 29Si NMR spectra of compound 1 show a
singlet (0.14 ppm) for trimethylsilyl protons and a singlet
(-15.95 ppm) for the trimethylsilyl silicon atoms, respec-
tively. The molecular ion peak corresponding to 1 was not
observed in its electron impact (EI) mass spectrum. Colorless
crystals of 1 suitable for structural analysis were obtained
when a concentrated solution of 1 in n-hexane was allowed
to stand at room temperature for 12 h.
Compound 1 crystallizes in the monoclinic space group
P21/n with a molecule of THF coordinated to the metal
center. The structure of 1 (Figure 1) confirms the presence
of a six-membered C3N2Sr ring. This ring has an envelope
j
droxide crystallizes in the triclinic space group P1 with two
molecules of THF as colorless crystals. The structure of 2
(Figure 2) reveals its dimeric nature and shows the presence
of two six-membered C3N2Sr rings. These six-membered
(10) Dove, A. P.; Gibson, V. C.; Hormnirun, P.; Marshall, E. L.; Segal,
J. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 3088–
3097.
(8) (a) Ghosh, P.; Parkin, G. Inorg. Chem. 1996, 35, 1429–1430. (b)
Sa´nchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann,
M. Organometallics 2006, 25, 1012–1020.
(11) Chisholm, M. H.; Gallucci, J.; Phomphrai, C. K. Inorg. Chem. 2004,
43, 6717–6725.
(9) Ruspic, C.; Nembenna, S.; Hofmeister, A.; Magull, J.; Harder, S.;
Roesky, H. W. J. Am. Chem. Soc. 2006, 128, 15000–15004.
(12) Lutz, H. D.; Eckers, W.; Schneider, G.; Haeuseler, H. Spectrochim.
Acta 1981, 37A, 561–567.
5972 Inorganic Chemistry, Vol. 47, No. 13, 2008

A ReactiWity Change of a Strontium Monohydroxide
Scheme 1. Preparation of ꢀ-Diketiminate Supported Strontium Hydroxide Complex
Table 1. Crystallographic and Structure Refinement Data
4·0.5toluene +
0.5benzene
param
1
2·2THF
3·C6H6
5·toluene
empirical formula
fw
T (K)
C39H67N3OSi2Sr
737.76
133(2)
C78H124N4O7Sr2
1405.05
100(2)
C90H110N4O4Sr2
1487.06
100(2)
C
100.5H121N4O4P2Sr2
C77H126N10O2Sr2Zr2
1581.56
100(2)
1686.27
133(2)
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
11.5102(6)
18.3156(7)
20.0293(11)
90
91.165(4)°
90
4221.6(4)
4
1.161
triclinic
P1
triclinic
P1
monoclinic
P21/n
15.6034(6)
13.3159(4)
23.0252(8)
90
106.277(3)
90
4592.3(3)
2
triclinic
P1
j
j
j
12.958(3)
14.612(3)
22.946(3)
85.33(3)
73.94(3)
66.34(3)
3822.0(13)
2
12.829(1)
12.912(1)
15.205(1)
94.26(1)
110.54(1)
116.63(1)
2026.1(3)
1
12.296(1)
13.611(1)
13.597(1)
77.80(1)
65.73(1)
84.98(1)
2027.6(3)
1
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Fcalcd (mg/m3)
F(000)
1.221
1504
1.219
786
1.219
1780
1.295
830
1584
µ (Mo KR)
θ range for data
collcn (deg)
no. of reflcns
collcd/unique obsd
reflns [I > 2σ(I)]
data/restraints/params
refinement method
1.365
2.02-24.73
2.246
3.30-59.09
2.126
3.23-59.42
1.248
1.41-24.83
4.091
3.32-59.04
58275/7166 (Rint
0.0994)
)
77913/11259(Rint
0.0577)
)
18352/5471 (Rint
0.0573)
)
66801/7919 (Rint
0.0895)
)
20752/5672 (Rint )
0.0305)
7166/0/431
full-matrix
11259/525/ 961
full-matrix
5471/2/469
full-matrix
7919/0/518
full-matrix
5672/63/468
full-matrix
least-squares on F2
least-squares on F2
least-squares on F2
least-squares on F2
least-squares on F2
GOF on F2
1.000
1.047
1.093
0.981
1.047
R1, wR2 [I 2σ(I)]
R1, wR2 (all data)
largest diff peak/hole
(e Å3)
0.0387, 0.777
0.0599, 0.0839
0.453 and -0.266
0.0309, 0.0772
0.0374, 0.0820
0.568 and -0.513
0.0510, 0.1394
0.0636, 0.1503
1.652 and -0.778
0.0347, 0.0743
0.0519, 0.0790
0.635 and -0.327
0.0211, 0.0542
0.0219, 0.0546
0.386 and -0.290
rings are connected to each other by means of two µ-OH
groups, which results in the formation of a four-membered
Sr2O2ring. The six-membered rings possess an envelope
conformation and are perpendicular to each other (89.6°).
The four-membered Sr2O2 ring is planar and forms an angle
of 53.3° and 38.1° with the two six-membered rings.
Interestingly, the strontium atoms have an environment
that differs in the number of coordinated THF molecules.
Thus, one of the strontium atoms is penta-coordinate and
has distorted trigonal-bipyramidal geometry with two nitro-
gen atoms of the ꢀ-diketiminate ligand, two oxygen atoms
of the two hydroxyl groups, and an oxygen atom of the THF
molecule. The other strontium atom has a similar environ-
ment but contains one additional THF molecule in its
coordination sphere, which makes it hexa-coordinate with
distorted octahedral geometry. This observation is in contrast
to the magnesium and calcium hydroxides ([LMg(µ-
OH)(thf)]2 and [LCa(µ-OH)(thf)]2), where the alkaline earth
metal centers have the same coordination geometry. As
anticipated, the Sr-O bond distances in hydroxide 2
(2.402(2)av Å) are longer than the Mg-O (1.988(2) Å) and
Ca-O (2.225(6)av Å) distances found in the magnesium8b
and calcium congeners.9 The Sr-O-Sr and O-Sr-O bond
angles in 2 (103.83(9)av° and 76.17(7)av°) are comparable
with the Ca-O-Ca (103.49(11)°) and O-Ca-O (76.51(11)°)
bond angles present in [LCa(µ-OH)(thf)]2. Crystallographic
and structure refinement data are presented in Table 1.
We have tried the possibility for preparing a monomeric
strontium hydroxide by exploiting the ligand exchange
Figure 2. Crystal structure of 2·2THF. The two noncoordinate THF
molecules and all of the hydrogen atoms except those of hydroxyl groups
have been omitted for clarity.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5973

Sarish et al.
phenomenon observed in alkaline earth metal complexes.9
The addition of 2 equiv of benzophenone and triphenylphos-
phine oxide at room temperature to [LSr(thf)(µ-OH)2Sr-
(thf)2L] (2) in benzene resulted in the precipitation of [LSr(µ-
OH)(OCPh2)]2 (3) as reddish orange crystals and in toluene
led to the formation of [LSr(µ-OH)(OPPh3)]2 (4) as a
yellowish compound (78.3% yield) at room temperature,
respectively. Compound 4 is soluble in benzene and toluene.
Compound 4 shows a singlet (4.89 ppm) for the γ-CH proton
and another one for the hydroxyl group (-0.479 ppm). The
31P NMR spectrum contains only one singlet (29.5 ppm) for
the triphenylphosphine oxide. The SrO-H stretching fre-
quency of 3 (3676 cm-1) and 4 (3680 cm-1) matches with
that of 2 (3677 cm-1). The molecular ion peak corresponding
to 3 and 4 was not observed in the EI mass spectra.
they adopt distorted trigonal-bipyramidal geometry, with the
oxygen atom of a hydroxyl group and a nitrogen atom of
the ꢀ-diketiminato ligand occupying the apical positions. The
structure of 4 contains two enveloped C3N2Sr six-membered
rings and a planar Sr2O2 four-membered ring. The six-
membered rings are exactly parallel to each other and form
an angle of 43.95° with the four-membered ring. This angle
shows that the structure of 4 is more twisted than the structure
of the strontium hydroxide with coordinated benzophenone
molecules, where the same angle is 32.3°. The reason might
be the more bulky triphenylphosphine oxide ligand compared
to the benzophenone molecule.
A reactivity study of strontium hydroxide complex 2 with
non-cyclopentadienyl complexes of group 4 metal unveiled
its unprecedented mild acidic character. Interestingly, the
reaction of 2 with 2 equiv of Zr(NMe2)4 in toluene at -60
°C led to the intermolecular elimination of 2 equiv of Me2NH
and resulted in the µ-oxo-bridged heterobimetallic complex
[LSr(µ-O)Zr(NMe2)3]2 (5) as a colorless solid (69.1% yield;
Scheme 2). The special electronic and steric effect offered
by the ꢀ-diketiminate ligand may be the reason for the
umpolung of the hydroxide group in complex 2. Although,
we anticipate that the facile formation of the Zr-O-Sr bond
may be the driving force for the change of the polarity in
the hydroxide group of complex 2.
The strontium hydroxide 3 (Figure 3) with coordinate
benzophenone molecules crystallizes in the triclinic space
j
group P1 as orange-red diamond-shaped crystals containing
half of a molecule of benzene in the asymmetric unit. Similar
to the structure of 2, the structure of 3 contains three
heterocyclic rings, but their orientation differs significantly.
The enveloped six-membered C3N2Sr rings are exactly
parallel to each other, caused by the crystallographic inver-
sion center, and form an angle of 32.3° with the planar four-
membered Sr2O2 ring. Both of the strontium atoms are penta-
coordinate (with two nitrogen atoms of ligand L, two oxygen
atoms of the hydroxyl groups, and an oxygen atom of the
benzophenone molecule) and adopt a distorted square
pyramidal geometry.
Yellow crystals of 4 suitable for structural analysis were
obtained when a concentrated solution of 4 in toluene was
kept at -5 °C for 1 day. Compound 4 crystallizes in the
monoclinic space group P21/n with a disordered toluene
molecule. The structure of 4 (Figure 4) reveals the dimeric
nature of 4 and rules out the possibility of a monomeric
strontium hydroxide. Both of the strontium atoms are penta-
coordinate with two nitrogen atoms of the ꢀ-diketiminato
ligand, two oxygen atoms of the hydroxyl groups, and an
oxygen atom of the triphenylphosphine oxide. In addition,
Compound 5 is freely soluble in common organic solvents
such as benzene and toluene. Compound 5 was characterized
by NMR spectroscopy (1H and 13C), EI mass spectrometry,
and X-ray structural and elemental analyses. The complete
disappearance of the OH resonance and the stretching
frequency of 2 in the 1H NMR and IR spectra of 5,
1
respectively, clearly indicates its formation. The H NMR
spectrum shows a singlet for the γ-CH (4.72 ppm) and NMe2
(2.62 ppm) protons, respectively. No molecular ion peak was
observed in the EI mass spectrum of 5, and only fragment
ions are formed. Colorless crystals suitable for X-ray
structural analysis were obtained by keeping a concentrated
solution of 5 in toluene at -5 °C.
j
Compound 5 crystallizes in the triclinic space group P1
together with a disordered toluene molecule (Figure 5). The
structure reveals the dimeric nature of 5, and this dimerization
Figure 3. Crystal structure of 3·C6H6. The benzene molecule and all of
the hydrogen atoms except those of hydroxyl groups have been omitted
for clarity.
Figure 4. Crystal structure of 3·0.5toluene + 0.5benzene. The disordered
solvent molecules and all of the hydrogen atoms except those of hydroxyl
groups have been omitted for clarity.
5974 Inorganic Chemistry, Vol. 47, No. 13, 2008

A ReactiWity Change of a Strontium Monohydroxide
Scheme 2. Preparation of Heterobimetallic Compound [LSr(µ-O)Zr(NMe2)3]2
results in the formation of a planar four-membered Zr2O2
ring. The zirconium atoms are penta-coordinate (with three
nitrogen atoms of the dimethylamino groups and two oxygen
atoms) and adopt a distorted trigonal-bipyramidal geometry.
Among the three nitrogen atoms of the dimethylamino
groups, one nitrogen atom takes up the apical position and
the other two nitrogen atoms are arranged on equatorial
positions. One of the equatorial diaminomethyl nitrogen
atoms on each zirconium atom forms a coordinate bond with
the strontium atom, and consequently two puckered four-
membered SrONZr rings are assembled. Interestingly, both
the strontium atoms rest almost on the plane of the planar
Zr2O2 ring, and the nitrogen atoms that bridge the zirconium
and strontium atoms lie at a distance of 1.51 Å above and
below the Sr2Zr2O2 plane. The strontium atoms are tetra-
coordinate (with two nitrogen atoms of the ꢀ-diketiminato
ligand, a nitrogen atom of one of the dimethylamino groups,
and an oxygen atom) and create two six-membered C3N2Sr
rings due to the bidentate mode of binding offered by the
ꢀ-diketiminato ligands. In contrast to the enveloped C3N2Sr
rings present in the strontium amide LSrN(SiMe3)2(thf) and
strontium hydroxide 2, the C3N2Sr rings of 5 are nearly
planar. Additionally, the C3N2Sr rings of 5 are exactly parallel
to each other and are almost perpendicular (89.71°) to the
Sr2Zr2O2 plane. Selected bond distances and angles for 1-5
are given in Table 2.
Experimental Section
General Procedures. All manipulations were performed under
a dry and oxygen-free N2 atmosphere by using Schlenk and
glovebox techniques. SrI2 and KN(SiMe3)2 (95%), Ph2CO, Ph3PO
(98%), and Zr(NMe2)4 were purchased from Aldrich and used as
obtained. 1H and 13C NMR spectra were recorded on either a Bruker
Avance 300 or 500 NMR spectrometer. Chemical shifts are reported
in parts per million with reference to SiMe4 (external). All NMR
measurements were carried out at room temperature. IR spectra
were recorded on a Bio-Rad Digilab FTS7 spectrometer in the range
of 4000 to 350 cm-1 as nujol mulls. EI mass spectra were obtained
using a Finnigan MAT 8230 or a Varian MAT CH5 instrument
(70 eV). Melting points were measured in sealed glass tubes on a
Bu¨chi B-540 melting point apparatus and are uncorrected. Elemental
analyses were performed by the Analytisches Labor des Instituts
fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
Synthesis of 1. LH (2.931 g, 7.00 mmol) and KN(SiMe3)2 (2.933
g, 14.70 mmol) were dissolved in THF (60 mL) and stirred for 5 h
at room temperature. This clear solution was added to a suspension
of SrI2 (2.390 g, 7.00 mmol) in THF (60 mL) at room temperature.
It was stirred for another 18 h. After that, the solvent was removed,
and the residue was extracted with n-hexane (90 mL) and filtered.
Removal of the solvent from the filtrate in a vacuum gave
compound 1 as a pale yellow solid. The solid upon crystallization
from n-hexane at a low temperature gave an analytically pure
sample of 1. Yield: 4.02 g, 5.45 mmol, 77.9%. mp: 116-118 °C.
1H NMR (500 MHz, C6D6): δ 7.14-7.11 (m, 6 H, m-, p-Ar-H),
4.78 (s, 1H, γ-CH), 3.31 (m, 4H, O-CH2-CH2), 3.18 (sept, 4H,
CH(CH3)2), 1.69(s, 6H, CH3), 1.33-1.32 (d, 12H, CH(CH3)2),
125-1.24 (d, 12H, CH(CH3)2), 1.11 (m, 4H, O-CH2-CH2), 0.14
(s, 18H, SiMe3). 13C {1H} NMR(125.77 MHz, C6D6): δ 164.96,
147.27, 140.95, 124.31, 124.05, 92.49, 68.75, 28.42, 25.52, 25.05,
24.89, 24.51, 5.77. 29Si NMR (75 MHz, C6D6): δ -15. 95 (s, 2Si,
SiMe3). MS (70 eV): m/z (%) 505.16 (100) [M+-N(SiMe3)2-THF].
Anal. calcd for C39H67N3OSi2Sr (737.76): C, 63.49; H, 9.15; N,
5.70. Found: C, 62.45; H, 9.20; N, 5.61.
Synthesis of 2. Distilled and degassed water (24 µL, 1.36 mmol)
was added to a solution of LSrN(SiMe3)2(thf) (1.00 g, 1.36 mmol)
in THF (50 mL) at -60 °C. Then, it was taken to room temperature
and stirred for 1 h. Filtration followed by removal of the solvent
in a vacuum to give compound 2 as an off-white solid. It was
washed with a small amount of n-hexane and crystallized from THF
at -32 °C to exhibit compound 2 as colorless crystals. Yield: 0.47 g,
1
0.37 mmol, 55.2%. mp: 172-176 °C. H NMR (300.132 MHz,
C6D6): δ 7.15-7.06 (m, 12H, m-, p-Ar-H), 4.74 (s, 2H, γ-CH),
3.44 (m, 12H, O-CH2-CH2), 3.19 (sept, 8H, CH(CH3)2), 1.72 (s,
12H, CH3), 1.40 (m, 12H, O-CH2-CH2), 1.25-1.22 (d, 24H,
CH(CH3)2), 1.09-1.07 (d, 24H, CH(CH3)2), -0.72 (s, 2H, Sr-OH).
13C NMR (75.48 MHz, C6D6): δ 163.48, 147.81, 141.46, 123.48,
123.34, 93.15, 68. 25, 28.00, 25.55, 25.15, 24.50, 24.36. IR (nujol):
Figure 5. Crystal structure of 5·toluene. The toluene molecule and all of
the hydrogen atoms have been omitted for clarity. The dashed coordinate
bonds represent the weak interactions between the strontium atoms and the
-NMe2 groups.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5975

Sarish et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1-5
Compound 1
Compound 2
Sr1-N1, 2.554(2)
N1–Sr1–N2 74.13(7)
N2–Sr1-01 140.77(7)
Sr1-N2, 2.514(2)
N1–Sr1–N3 138.55(7)
N3–Sr1–01 95.43(7)
Sr1-N3, 2.446(2)
N2–Sr1–N3 118.24(7)
Sr1-O1, 2.536(2)
N1–Sr2-O1, 94.36(7)
Sr1-N4, 2.658(2)
Sr1-O5, 2.621(2)
Sr1-O1, 2.411(2)
Sr2-N1, 2.599(3)
Sr1-O2, 2.430(2)
Sr2-O1, 2.384(2)
Sr1-O4, 2.642(2)
Sr2-O2, 2.383(2)
Sr2-O80, 2.564(2)
N3-Sr1-N4, 70.75(8)
N2-Sr2-N1, 71.81(8)
O1-Sr2-N2, 128.16(8)
Sr1 · · ·Sr2, 3.7811(14)
O1-Sr1-O2, 75.48(7)
O1-Sr2-O2, 76.86(7)
O1-Sr1-N3, 146.21(8)
O2-Sr2-N2, 128.47(7)
O2-Sr1-N4, 150.47(8)
O2-Sr2-N1, 94.88(8)
Compound 3
Compound 4
Compound 5
Sr1-N1, 2.565(3)
Sr1-N2, 2.601(4)
Sr1 · · ·Sr1*, 3.7306(9)
O1-Sr1-O1*, 75.94(13)
Sr1-O1, 2.351(3)
Sr1-O1*, 2.381(3)
Sr1-O2, 2.531(3)
N1-Sr1-N2, 72.99(11)
O2-Sr1-N1, 99.92(10)
O1-Sr1-N1, 100.94(11)
O1*-Sr1-N2, 90.63(11)
Sr1-N1, 2.623(2)
Sr1-O2, 2.538(2)
N1-Sr1-N2, 72.18(6)
N2-Sr2-N1, 72.18(6)
Sr1-N2, 2.580(2)
P1-O2, 1.496(2)
O1-Sr1-O1*, 74.20(8)
O1-Sr1-O2, 130.33(7)
Sr1-O1, 2.368(2)
Sr1-O1*, 2.379(2)
Sr1 · · ·Sr1*, 3.786(1)
O1-Sr1-N2, 126.18(7)
Sr-O1-Sr*, 105.80(8)
O1*-Sr1-N1, 150.18(7)
Sr1-N1, 2.5421(17)
Zr1-O1, 2.1992(13)
N1-Sr1-N2, 72.17(5)
Zr1-O1-Zr1*, 103.67(6)
Sr1-N2, 2.5484(16)
Sr1-N4, 2.7865(17)
Sr1-O1, 2.3423(14)
O1-Sr1-N4, 68.11
Zr1-O1*, 1.9778(13)
O1-Sr1-N1, 115.88(5)
O1-Zr1-O1*, 76.33(6)
Zr1 · · ·Zr1*, 3.2870(4)
O1-Sr1-N2, 117.67(5)
Zr1-O1-Sr1, 96.63(5)
ν˜ 3677, 3053, 1917, 1624, 1549, 1510, 1429, 1408, 1379, 1314,
1254, 1225, 1166, 1100, 1042, 1018, 924, 890, 828, 783, 758, 724,
668, 618, 523, 437, 348 cm-1. MS (70 eV): m/z (%) 403 (100)
[L+-Me]. Anal. calcd for C70H108N4O5Sr2 (M ) 1260.87): C, 66.68;
H, 8.63; N 4.44. Found: C, 65.23; H, 8.57; N, 4.53.
Synthesis of 3. A solution of benzophenone (0.072 g, 0.397
mmol) in benzene (2 mL) was added to a colorless solution of
compound 2 (0.25 g, 0.198 mmol) in benzene at room temperature
(6 mL). Compound 3 was formed instantaneously and separated
out of the solution as orange-red crystals. Yield: 0.15 g, 0.109 mmol,
55.2%. mp: 147-149 °C (dec.). IR (Nujol): ν˜ 3676, 3059, 2278,
1667, 1641, 1626, 1596, 1576, 1551, 1508, 1465, 1364, 1321, 1286,
1255, 1227, 1175, 1167, 1099, 1078, 1056, 1018, 999, 946, 937,
925, 852, 827, 806, 793, 784, 769, 757, 729, 706, 640, 622, 532,
504, 434, 412, 352 cm-1. MS (70 eV) m/z (%): 403(100) [L+-Me].
Anal. calcd for C84H104N4O4Sr2 (M ) 1408.99): C, 71.60; H, 7.44;
N, 3.98. Found: C, 71.63; H, 7.49; N, 3.79.
Synthesis of 4. [LSr(thf)(µ-OH)2Sr(thf)2L] (0.35 g, 0.28 mmol)
and Ph3PO (0.156 g, 0.56 mmol) were dissolved in toluene (15
mL) and stirred for 0.5 h at room temperature. Removal of the
solvent from the filtrate in a vacuum gave compound 4 as a yellow
solid. The solid upon crystallization from toluene at -5 °C gave
an analytically pure sample of 4. Yield: 0.348 g, 0.217 mmol,
78.3%. mp: 189-191 °C. 1H NMR (300.132 MHz, C6D6): δ
7.69-7.66 (m, 30H, Ar-H), 7.13-7.01 (m, 12H, m-, p-Ar-H),
4.89 (s, 2H, γ-CH), 3.31 (sept, 8H, CH(CH3)2), 1.79 (s, 12H, CH3),
1.09-1.07 (d, 24H, CH(CH3)2), 0.98-0.97 (d, 24H, CH(CH3)2),
-0.479 (s, 2H, Sr-OH). 13C NMR (75.48 MHz, C6D6): δ 163.26,
149.45, 141.00, 132.87, 132.79, 132.13, 132.03, 125.64, 123.23,
122.89, 92.63, 27.77, 25.64, 25.08, 24.39. 31P NMR (75 MHz,
C6D6): δ 29.5 (s, 2P, OPPh3). IR (nujol): ν˜ 3680, 3049, 1965, 1909,
1683, 1624, 1591,1548, 1507, 1380, 1362, 1338, 1313, 1275, 1253,
1225, 1174, 1118, 1100, 1072, 1053, 1028, 1015, 998, 935, 923,
857, 827, 807, 794, 785, 751, 727, 693, 619, 598, 537, 512, 465,
436, 346 cm-1. MS (70 eV) m/z (%): 403 (100) [L+-Me]. Anal.
calcd for C94H114N4O4P2Sr2 (1601.12): C, 70.51; H, 7.18; N 3.50.
Found: C, 70.92; H, 7.42; N 3.26.
Synthesis of 5. A toluene solution of [LSr(thf)(µ-OH)2Sr(thf)2L]
(1) (0.5 g, 0.397 mmol) was added drop by drop to a solution of
Zr(NMe2)4 (0.212 g, 0.73 mmol) in toluene at -60 °C using a
cannula. After the addition was complete, the solution was brought
to room temperature and stirred for 1 day. Then, the solvent was
removed in Vacuo to get the crude sample of 5. Colorless crystals
were obtained when a toluene solution of 5 was kept at -5 °C for
3 days. Yield: 0.408 g, 0.274 mmol, 69.1%. mp: 252-254 °C. 1H
NMR (500.132 MHz, C6D6): δ 7.14-7.09 (m, 12H, Ar-H), 4.72
(s, 2H, γ-CH), 3.24 (sept, 8H, CH(CH3)2), 2.62 (s, 36H, N(CH3)2),
1.69 (s, 12H, CH3), 1.34-1.33 (d, 24H, CH(CH3)2), 1.25-1.23 (d,
24H, CH(CH3)2). 13C NMR (75.48 MHz, C6D6): δ 165.12, 147.56,
141.32, 124.34, 123.98, 91.77, 43. 22, 28.64, 25.25, 24.93, 24.45.
MS (70 eV) m/z (%): 505.2 (15) [M+-OZr(NMe2)3)], 403 (100)
[L+-Me]. Anal. calcd for C70H108N10O2Sr2Zr2 (M ) 1489.44): C,
56.45; H, 7.99. Found: C, 56.52; H, 7.59.
Crystallographic Details for Compounds 1-5. The data for
compounds 1 and 4 were collected on a STOE IPDS II instrument.
In the case of 2, the data were collected using a Bruker three-circle
diffractometer equipped with a SMART 6000 CCD detector on a
nonmerohedrally twinned crystal. There were three twin domains
with a refined fractional contribution of 0.5485(7):0.3979(7):
0.0536(7). The data for compounds 3 and 5 were also collected on
the same diffractometer used for 2. In the structure of 2, one
coordinated THF molecule and two noncoordinated THF molecules
are disordered. The solvent molecule toluene in both 4 and 5 was
found to be disordered. They were refined with distance restraints
and restraints for the anisotropic displacement parameters. All of
the structures were solved by direct methods with SHELXS-9713
and were refined with SHELXL-9714 on F2.
Conclusion
In summary, a novel strontium hydroxide, 2, was prepared
from strontium amide 1 and water. The reaction of strontium
hydroxide 2 with Zr(NMe2)4 gave the unprecedented het-
(13) Sheldrick, G. M. Acta Crystallogr, Sect. A 1990, 46, 467–473.
5976 Inorganic Chemistry, Vol. 47, No. 13, 2008

A ReactiWity Change of a Strontium Monohydroxide
erobimetallic oxide 5 and reveals for the first time the acidic
character of an alkaline earth metal hydroxide, 2. The stability
and good solubility of 5 has made it possible to prepare
hitherto unknown strontium oxide complexes. Compound 5
can also act as a precursor for polymetallic complexes in
view of its replaceable NMe2 groups. Such studies are
currently in progress in our laboratory.
Alexander von Humboldt foundation for a postdoctoral
research fellowship.
Supporting Information Available: X-ray data for 1-5 (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC800284U
Acknowledgment. Support of the Go¨ttinger Akademie der
(14) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Wissenschaften is highly acknowledged. S.N. thanks the
Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5977