Well-defined hydrocarbon soluble strontium fluoride and chloride complexes of composition [LSr(thf)(μ-F)2Sr(thf)2L] and [LSr(thf)(μ-Cl)2Sr(thf)2L]

Article abstract of DOI:10.1039/b822148k

Reaction of LSrN(SiMe₃)₂(thf) 1 (L = CH(CMe-2,6-i-Pr₂C₆H₃N)₂) with Me₃SnF and LAlCl(Me), respectively, gave the first example of a strontium mono fluoride complex [LSr(thf)(μ-F)<

Full text of DOI:10.1039/b822148k

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Well-defined hydrocarbon soluble strontium fluoride and chloride
complexes of composition [LSr(thf)(l-F)2Sr(thf)2L] and
[LSr(thf)(l-Cl)2Sr(thf)2L]wz
Sankaranarayana Pillai Sarish, Herbert W. Roesky,* Michael John, Arne Ringe
¨
and Jorg Magull
Received (in Cambridge, UK) 10th December 2008, Accepted 17th February 2009
First published as an Advance Article on the web 11th March 2009
DOI: 10.1039/b822148k
Reaction of LSrN(SiMe3)2(thf)
1 (L = CH(CMe-2,6-i-
Pr2C6H3N)2) with Me3SnF and LAlCl(Me), respectively, gave
the first example of a strontium mono fluoride complex
[LSr(thf)(m-F)2Sr(thf)2L] 2 and the corresponding chloride
derivative [LSr(thf)(m-Cl)2Sr(thf)2L] 3.
There is a great deal of interest in the synthesis and
characterization of novel group 2 halide complexes of the type
RMX (M = an alkaline earth metal) due to their potential
applications in synthetic chemistry and material science.1
In view of these applications, various halide complexes of
alkaline earth metals have been synthesized and structurally
characterized.2 Nevertheless, the organometallic halide chemistry
with respect to heavier group 2 elements is still in its infancy.
This is due to the percentage of ionic character in the
M–X bond increasing from magnesium to strontium and also
due to a fast ligand exchange.3 In recent times, a calcium
fluoride and a chloride were prepared as stable species by
exploiting the unique electronic and steric effect offered by the
b-diketiminato ligand L (L = CH(CMe-2,6-i-Pr2C6H3N)2).1c,4
Another calcium fluoride was also reported by Hill and
co-workers.5 But the strontium analogs of its lighter congener
were missing because of the non-availability of a suitable
precursor until recently. For example a well-defined molecular
strontium hydroxide was obtained by the controlled hydrolysis
of a strontium amide. The realization of the strontium hydroxide
initiated the study of the Sr–OH bond present in these complexes
in hydrocarbon solvents.6 Consequently it is anticipated that the
synthesis of the well-defined halide complexes of strontium
enables the investigation of the nature of the Sr–X bond.
In view of these, herein, we report the first example of a
hydrocarbon soluble molecular strontium mono fluoride
[LSr(thf)(m-F)2Sr(thf)2L] 2 and a mono chloride [LSr(thf)-
(m-Cl)2Sr(thf)2L] 3 obtained from the reaction of strontium
amide LSrN(SiMe3)2(thf) 16 with Me3SnF and LAlCl(Me),
respectively.
Scheme 1 Preparation of the strontium fluoride 2 and chloride 3
complex.
The reaction of LSrN(SiMe3)2(thf) 1 and Me3SnF in THF
at room temperature for 15 h led to the formation of the
strontium fluoride 2 as colorless crystals (Scheme 1)7a while 1
and LAlCl(Me) in THF at room temperature yielded
strontium chloride 3 (Scheme 1).7b The formation of 3
proceeds under elimination of HN(SiMe3)2 and generation
of L1AlMe(thf) (L1 = CH[C(CH2)](CMe)(2,6-i-Pr2C6H3N)2)
4.7c So far we have not been able to obtain single crystals of 4.
However to support the composition of 4 indirectly we treated
4 with water to generate LAlOH(Me), which was compared
with an original sample.8 Compounds 2 and 3 are soluble in
toluene, benzene, and THF, respectively. They have been well
characterized by mass spectrometry, NMR spectroscopy
[1H, 13C and 19F (for 2)], single crystal X-ray diffraction,
and elemental analysis.
The 1H and 19F NMR spectra of compound 2 show a singlet
(4.51 ppm) for the g-protons and a singlet (ꢀ59.97 ppm) for
the fluorine atoms, respectively.
The complete disappearance of the SiMe3 resonance (0.14 ppm)
1
of 1 in the H NMR spectra of 2 and 3 clearly indicates the
elimination of Me3SnN(SiMe3)2 and HN(SiMe3)2, respectively.
Compounds 2 and 3 are very sensitive to air and moisture
and in non-coordinating hydrocarbon solvents both have a
tendency slowly to rearrange to form L2Sr.9 The molecular ion
peak corresponding to 2 and 3 was not observed in the EI mass
spectra.
Institut fur Anorganische Chemie, Universitat Gottingen,
¨
¨
E-mail: hroesky@gwdg.de; Fax: +49 551 39 3373
w This paper is dedicated to Professor Martin Jansen on the occasion
of his 65th birthday.
z Electronic supplementary information (ESI) available: General
experimental details and X-ray crystallographic details for compounds
2 and 3. CCDC 708838 and 708839. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b822148k
¨
¨
Tammannstrasse 4, 37077 Gottingen, Germany.
Single crystals of 2 and 3 suitable for structural analysis
were obtained when a concentrated solution of 2 and 3,
respectively, in a mixture of THF–toluene was stored at
ꢀ5 1C in a freezer. Compounds 2 and 3 crystallize in the
ꢀ
triclinic P1 and monoclinic C2/c space group, respectively.y
The structures of 2 and 3 (Fig. 1 and 2) reveal the dimeric
nature of the complexes and contain two six-membered
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2390 | Chem. Commun., 2009, 2390–2392

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results in a hexa-coordinate environment. This makes 2 and
3
different from the magnesium and calcium analogs
2d
1c,4
{[LMg(m-F)(thf)]2 and [LCa(m-X)(thf)]2
(X = F or Cl)},
where both of the alkaline earth metal centers have the same
coordination geometry. As expected the average Sr–F
(2.348(1)av A) and Sr–Cl (2.891(1)av A) bond distances are
longer than those of the corresponding calcium [2.180(2)av
and 2.680(1)av A] analogs.
A
In conclusion we have shown the facile synthesis of
well-defined strontium fluoride 2 and chloride 3 by utilizing
strontium amide 1, and the stability and good solubility of 2
and 3 have provided a chance to study the nature of the M–X
bond present in these complexes.
Support of the Deutsche Forschungsgemeinschaft is highly
acknowledged.
Fig. 1 Crystal structure of 2ꢂ1.5C7H8. Selected bond distances (A)
and angles (1): Sr(1)–N(1) 2.605(2), Sr(1)–F(1) 2.397(1), Sr(1)–F(2)
2.345(1), Sr(1)–O(1) 2.581(2), Sr(1)–O(2) 2.581(2), Sr(2)–F(1) 2.317(1),
Sr(2)–F(2) 2.333(1), Sr(2)–O(3) 2.554(2), Sr(1)–Sr(2) 3.739(1);
Sr(1)–F(1)–Sr(2) 104.94(5), Sr(1)–F(2)–Sr(2) 106.09(5), F(1)–Sr(1)–F(2)
73.53(5), F(1)–Sr(2)–F(2) 75.23(5). All of the hydrogen atoms and
toluene molecules have been omitted for clarity.
Notes and references
y Crystal data for
Mr 1403.03, triclinic, space group P1,
b = 14.3939(4), c = 24.1189(6) A, a = 75.519(2)1, b = 84.178(2)1,
2
and 3. 2ꢂ1.5C7H8: C80.5H118F2N4O3Sr2,
ꢀ
a = 12.5946(3),
=
g = 64.647(2)1, V = 3825.68(17) A3, Z = 2, rcalcd = 1.218 Mg mꢀ3
,
F(000) = 1494, T = 133(2) K, m(Mo-Ka) = 1.447 mmꢀ1, 58 191
reflections measured, 15 932 independent (Rint = 0.0499). The final
refinement converged to R1 = 0.0440 for I 4 2s(I), wR2 = 0.0922 for
all data. 3ꢂ0.5C7H8: C73.50H110Cl2N4O3Sr2, Mr = 1343.80, monoclinic,
space group C2/c, a = 53.0398(12), b = 12.2780(3), c = 22.2041(5) A,
b = 98.945(2)1, V = 14 284.0(6) A3, Z = 8, rcalcd = 1.250 Mg mꢀ3
,
F(000) = 5704, T = 133(2) K, m(Mo-Ka) = 1.616 mmꢀ1, 65 549
reflections measured, 14 872 independent (Rint = 0.0953). The final
refinement converged to R1 = 0.0590 for I 4 2s(I), wR2 = 0.0960 for
all data.
1 (a) N. N. Greenwood and A. Earnshaw, Chemistry of the Elements,
Pergamon Press, Oxford, 1st edn, 1984, pp. 150–151; (b) S. P. Green,
C. Jones and A. Stasch, Science, 2008, 318, 1754–1757;
(c) S. Nembenna, H. W. Roesky, S. Nagendran, A. Hofmeister,
J. Magull, P.-J. Wilbrandt and M. Hahn, Angew. Chem., Int. Ed.,
2007, 46, 2512–2514.
Fig. 2 Crystal structure of 3ꢂ0.5C7H8. Selected bond distances (A)
and angles (1): Sr(1)–N(1) 2.618(3), Sr(1)–Cl(1) 2.954(1), Sr(1)–Cl(2) 2.
898(1), Sr(1)–O(1A) 2.584(7), Sr(1)–O(2) 2.577(2), Sr(2)–Cl(1)
2.890(1), Sr(2)–Cl(2) 2.821(1), Sr(2)–O(3) 2.599(2), Sr(1)–Sr(2)
4.469(1); Sr(1)–Cl(1)–Sr(2) 99.76(3), Sr(1)–Cl(2)–Sr(2) 102.78(3),
Cl(1)–Sr(1)–Cl(2) 77.58(2), Cl(1)–Sr(2)–Cl(2) 79.87(3). All of the
hydrogen atoms and the toluene molecule have been omitted for clarity.
2 (a) M. Westerhausen, M. H. Digeser, C. Guckel, H. Noth, J. Knizek
¨
¨
and W. Ponikwar, Organometallics, 1999, 18, 2491–2496;
(b) J. Prust, K. Most, I. Muller, E. Alexopoulos, A. Stasch,
I. Uson and H. W. Roesky, Z. Anorg. Allg. Chem., 2001, 627,
´
¨
2032–2037; (c) J. M. Smith, R. J. Lachicotte and P. L. Holland,
Chem. Commun., 2001, 1542–1543; (d) H. Hao, H. W. Roesky,
Y. Ding, C. Cui, M. Schormann, H.-G. Schmidt, M. Noltemeyer
and B. Zemva, J. Fluorine Chem., 2002, 115, 143–147;
(e) H. Sitzmann, F. Weber, M. D. Walter and G. Wolmershauser,
¨
Organometallics, 2003, 22, 1931–1936; (f) H. M. El-Kaderi,
M. J. Heeg and C. H. Winter, Polyhedron, 2006, 25, 224–234.
3 (a) T. P. Hanusa, Polyhedron, 1990, 9, 1345–1362; (b) T. P. Hanusa,
Coord. Chem. Rev., 2000, 210, 329–367; (c) M. Westerhausen,
Angew. Chem., Int. Ed., 2001, 40, 2975–2977; (d) J. S. Alexander
and K. Ruhlandt-Senge, Eur. J. Inorg. Chem., 2002, 2761–2774.
4 C. Ruspic and S. Harder, Inorg. Chem., 2007, 46, 10426–10433.
5 A. G. M. Barrett, M. R. Crimmin, M. S. Hill, P. B. Hitchcock and
P. A. Procopiou, Angew. Chem., Int. Ed., 2007, 46, 6339–6342.
6 S. Sarish, S. Nembenna, S. Nagendran, H. W. Roesky, A. Pal,
R. Herbst-Irmer, A. Ringe and J. Magull, Inorg. Chem., 2008, 47,
5971–5977.
C3N2Sr rings. These six-membered rings are connected to each
other by two m-F or m-Cl atoms, which result in the formation
of a four-membered Sr2F2 ring in 2 and a corresponding
Sr2Cl2 ring in 3. The six-membered rings are not planar and
are almost perpendicular to each other in 2 [81.31] but exhibit
an angle of 55.51 in 3. The four-membered Sr2Cl2 and Sr2F2
rings are nearly planar and form angles with the two
six-membered rings of 35.841 and 66.481 in compound 2,
and 67.671 and 82.911 in compound 3.
Interestingly, like the strontium hydroxide [LSr(thf)(m-OH)2-
Sr(thf)2L] the strontium atoms in 2 and 3 have different
environments due to the number of coordinated THF
molecules. Thus, one of the strontium atoms is penta-coordinate
and has distorted trigonal bipyramidal geometry with two
nitrogen atoms of the b-diketiminate ligand, one oxygen atom
of the THF molecule and two fluorine or chlorine atoms. The
other strontium atom in 2 and 3, respectively, has an
additional THF molecule in its coordination sphere that
7 (a) Synthesis of 2: a solution of LSrN(SiMe3)2(thf) 16 (2.213 g,
3.00 mmol) in THF (25 mL) was added to a slurry of Me3SnF
(0.548 g, 3.00 mmol) in THF (40 mL) at room temperature and stirred
for 15 h. The solvent was removed in vacuum and the residue was
dissolved in a mixture of THF–toluene, concentrated and stored for
crystallization at ꢀ5 1C in a freezer for 2 days. Compound 2
was obtained as colorless crystals. Yield (0.526 g, 0.416 mmol,
27.7%); mp 169–170 1C. 1H NMR (500 MHz, THF-d8):
d = 6.97–6.86 (m, 12H, m-, p-Ar-H); 4.51 (s, 2H, g-CH), 3.08
(sept, 8H, CH(CH3)2), 1.49 (s, 12H, CH3), 1.09–1.08 (d, 24H,
CH(CH3)2), 1.06–1.05 (d, 24H, CH(CH3)2). 13C NMR (125.77 MHz,
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THF-d8): d = 163.99, 149.47, 142.14, 123.73, 123.50, 93.35, 28.34,
25.38, 24.83, 24.73 ppm. 19F NMR (188.3 MHz, THF-d8):
d = ꢀ59. 97 ppm (s, 2F, SrF). MS (70 eV): m/z (%): 403.2 (100)
[L+ ꢀ Me]. Anal. calc. for C70H106F2N4O3Sr2 (1264.85): C, 66.5; H,
8.5; N 4.4. Found: C, 65.6; H, 8.4; N 4.5%; (b) Synthesis of 3:
a solution of LAlCl(Me) (1.485 g, 3 mmol) in THF (25 mL) was
added to a clear solution of LSrN(SiMe3)2(thf) 16 (2.213 g, 3 mmol)
in THF (40 mL) at room temperature under stirring and after the
completion of addition the stirring was continued for 1 day. All
volatiles were removed in vacuum and the residue was dissolved in
THF–toluene mixture. The solution was concentrated and stored for
crystallization at ꢀ5 1C in a freezer. Compound 3 was obtained
as pale yellow crystals. Yield: (0.974 g, 0.751 mmol, 49.59%);
mp 155–157 1C. 1H NMR (500.13 MHz, C6D6): d = 7.13–7.10
(m, 12 H, m-, p-Ar-H), 4.77 (s, 2H, g-CH), 3.53 (m, 12H, O-CH2-CH2),
3.26 (sept, 8H, CH(CH3)2), 1.67 (s, 12H, CH3), 1.33 (m, 12H,
O-CH2-CH2), 1.25–1.24 (d, 48H, CH(CH3)2). 13C {1H}
NMR(125.77 MHz, C6D6): d = 164.33, 147.36, 141.95, 124.02,
123.73, 93.88, 68.72, 28.15, 25.40, 25.37, 24.71 ppm. MS (70 eV):
m/z (%): 403 (100) [L+ ꢀ Me]. Anal. calc. for C70H106Cl2N4O3Sr2,
(1297.76): C, 64.8; H, 8.2; N, 4.3. Found: C, 64.3; H, 8.2; N, 4.2%;
(c) Isolation of 4: after the separation of compound 3 as crystals, the
solvent was removed and the residue was dissolved in toluene–
n-pentane mixture, filtered, dried and washed with a minimum
amount of n-pentane. Again dried in vacuum to yield compound
4
as a
pale yellow solid. 1H NMR (500.131 MHz, C6D6):
d = 7.28–7.26, 7.19–7.18, 7.09–7.08 (m, 6 H, m-, p-Ar-H), 5.38
(s, 1H, g-CH), 4.05 (sept, 1H, CH(CH3)2), 3.91 (sept, 1H,
CH(CH3)2), 3.90 (s, 1H, NCCH2), 3.84 (m, 2H, O-CH2-CH2),
3.77 (m, 2H, O-CH2-CH2), 3.33 (sept, 1H, CH(CH3)2), 3.21 (sept,
1H, CH(CH3)2), 3.18 (s, 1H, NCCH2),1.59–1.58 (d, 3H, CH(CH3)2),
1.58 (s, 3H, CH3), 1.52–1.50 (d, 3H, CH(CH3)2), 1.44–1.41 (3d
(merged together), 9H, CH(CH3)2), 1.20 (m, 4H, O-CH2-CH2),
1.18–1.16 (d, 3H, CH(CH3)2), 1.07–1.06 (d, 3H, CH(CH3)2),
1.00–0.99 (d, 3H, CH(CH3)2), ꢀ0.99 (s, 3H, AlMe). 13C {1H}
NMR (125.77 MHz, C6D6): d = 155.21, 148.66, 147. 59, 147.01,
145.47, 145.46, 143.14, 143.01, 125.71, 125.51, 124.67, 124.45,
123.74, 123.14, 104.88, 81.30, 72.48, 29.03, 28.55, 27.69, 27.15,
26.53, 26.09, 26.06, 25.75, 25.30, 25.10, 24.65, 24.63, 24.37,
23.34, ꢀ15.13 ppm. Anal. calc. for C34H51AlN2O, (530.76):
N, 5.3. Found: N, 4.8%.
8 G. Bai, S. Singh, H. W. Roesky, M. Noltemeyer and H.-G. Schmidt,
J. Am. Chem. Soc., 2005, 127, 3449–3455.
9 S. Harder, Organometallics, 2002, 21, 3782–3787.
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2392 | Chem. Commun., 2009, 2390–2392