Template synthesis of tungsten complexes with saturated N-heterocyclic carbene ligands

Article abstract of DOI:10.1039/b510996e

Tungsten complex 5 with a coordinated 2-azidoethyl isocyanide ligand reacts with PMe³ at the azido function to give a complex with a coordinated iminophosphorane which upon hydrolysis of the P=N bond yields a complex with an NH,NH-stabilized N-heterocyclic carbene ligand, 7; alkylation of the carbene ring nitrogen atoms gives a complex with an N,N′-dialkylated imidazolidin-2-ylidene ligand, 8. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510996e

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Template synthesis of tungsten complexes with saturated N-heterocyclic
carbene ligands{
F. Ekkehardt Hahn,* Volker Langenhahn and Tania Pape
Received (in Cambridge, UK) 3rd August 2005, Accepted 7th September 2005
First published as an Advance Article on the web 21st September 2005
DOI: 10.1039/b510996e
Tungsten complex 5 with a coordinated 2-azidoethyl isocyanide
Ligand 4 coordinates to photochemically generated [W(CO) -
5
ligand reacts with PMe at the azido function to give a complex
(THF)] to give the isocyanide complex 5{ (Scheme 2). Complex 5
was identified by NMR and by IR spectroscopy which showed
3
with a coordinated iminophosphorane which upon hydrolysis
of the PLN bond yields a complex with an NH,NH-stabilized
N-heterocyclic carbene ligand, 7; alkylation of the carbene ring
nitrogen atoms gives a complex with an N,N9-dialkylated
imidazolidin-2-ylidene ligand, 8.
21
the absorption for the CN stretching mode at n 5 2187 cm
.
The isocyanide ligand in 5 reacts at the azido function with
9
trimethylphosphine in a Staudinger-type reaction to give the
trimethylphosphorane complex 6, which was not isolated.
Hydrolysis of the trimethylphosphorane under acidic conditions
with traces of water in protic solvents (methanol) affords a
complex with the 2-aminoethyl isocyanide ligand which is not
stable and immediately undergoes an intramolecular cyclization to
give a complex with the NH,NH-stabilized carbene ligand 7{
(Scheme 2).
Nucleophilic attack at the carbon atom of a coordinated
isocyanide is one of the oldest methods for the preparation of
1
carbene complexes. Particularly protic nucleophiles like alcohols
2
and primary amines have been useful in this reaction. The use of
functionalized isocyanides containing both the isocyanide function
and the nucleophile in the same molecule gives access to complexes
with heterocyclic carbene ligands through an intramolecular
The formation of the carbene complex 7 was confirmed by the
observation of the resonance for the carbene carbon atom in the
1
3
1,2-addition across the CN triple bond.
C NMR spectrum at d 202.2 ppm and by the lack of absorptions
for the NMC and N groups in the IR spectrum. The IR spectrum
We have reported on the template controlled cyclization of
3
2-hydroxyphenyl isocyanide which was obtained from complexes
containing 2-(trimethylsiloxy)phenyl isocyanide via O–Si bond
2
cleavage. The synthesis of complexes containing benzannulated
NH,NH-stabilized N-heterocyclic carbene ligands was also
3
achieved in a template synthesis. This method constitutes
an alternative approach for the preparation of carbene com-
plexes compared to the direct reaction of stable benzannulated
4
5
carbene ligands or benzimidazolium salts with transition
metal complexes. Here we report on the cyclization reaction
of coordinated 2-azidoethyl isocyanide 4 which allows the
template synthesis of complexes with the widely used saturated
Scheme 1 Reagents and conditions: (i) NaN
3 2
(excess), H O, 70 uC, 5 h;
(ii) CH C(O)OCHO, THF, 25 uC, 5 h; (iii) pTol-SO Cl, solvent C H N,
3
2
9
7
25 uC.
6
imidazolidin-2-ylidenes.
2-Azidoethyl isocyanide 4 was synthesized by reaction of
2-bromoethylamine hydrobromide with sodium azide in water
followed by formylation of the primary amine and dehydration of
7
the obtained formamide using the method of Ugi or Casanova
(
Scheme 1). The isocyanide and the azido function in 4 were
21
identified by their absorptions in the IR spectrum at n 5 2152 cm
21
13
1
and n 5 2109 cm , respectively.{ In the C{ H} NMR spectrum
the resonances for the isocyanide carbon atom (d 160.3 ppm) and
for the carbon atom of the adjacent methylene group (d 41.2 ppm)
1
13
14
are observed as triplets due to J( C– N) coupling typical of
8
uncoordinated isocyanides.
Institut f u¨ r Anorganische und Analytische Chemie, Universit a¨ t M u¨ nster,
Wilhelm Klemm Straße 8, D-48149 M u¨ nster, Germany.
Scheme 2 Reagents and conditions: (i) [W(CO)
2 h; (ii) PMe , 2 N ; (iii) H O, HCl (catalytic amount), MeOH, 25 uC,
h; (iv) 1. KOtBu, DMF, 25 uC, 2 h, 2. CH CHCH –Br, DMF, 25 uC, 5 h
(for mono N-alkylation, repeat (iv) for second N-alkylation).
This journal is ß The Royal Society of Chemistry 2005
5
(THF)], THF, 25 uC,
E-mail: fehahn@uni-muenster.de; Fax: +49-251-833-3108
1
3
2
2
{
Electronic supplementary information (ESI) available: preparation of
–4, 5, 7–8, and details of the crystal structure solution for compounds 7
5
2
2
2
and 8. See http://dx.doi.org/10.1039/b510996e
5
390 | Chem. Commun., 2005, 5390–5392

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of 7 shows, however, a strong new absorption for the N–H
stretching mode. Fehlhammer et al. showed that 2-hydroxyethyl
of the carbene ligand in 7 generates an N-heterocyclic carbene
ligand of the imidazolidin-2-ylidene type. The method described
here could, for example, lead to new Grubbs-type catalysts
by generating the carbene ligand from an isocyanide at Ru(II)
instead of substituting a ligand at Ru(II) for an N-heterocyclic
carbene ligand.
isocyanide coordinated to the W(CO) complex fragment does not
5
cyclize to the NH,O-stabilized carbene ligand which we attribute to
1
0
the lower nucleophilicity of the hydroxyl group. However, both
2,11
2
,3
2
-aminophenyl isocyanide and 2-hydroxyphenyl isocyanide
readily cyclize to give ylidenes when coordinated to the W(CO)
5
complex fragment.
Notes and references
The acidity of the NH protons in complex 7 allows the synthesis
of a complex with an N-alkyl functionalized carbene ligand.
Stepwise or simultaneous deprotonation of 7 and subsequent
reaction with allyl bromide gives complex 8{ with an N,N9-
diallylimidazolidin-2-ylidene ligand (Scheme 2). The C NMR
spectrum of complex 8 exhibits a resonance for the carbene carbon
atom at d 207.7 ppm, slightly downfield compared to this
resonance in the complex with the NH,NH-substituted carbene
ligand 7.
1
{
Spectroscopic data for compounds 4–8. 4: H NMR (300 MHz, THF-d
8
):
): d 160.3 (t,
), 41.2 (t, JCN 5 7.0 Hz, CH –NC); IR
(benzene): n 2152 (s, CN), 2109 (s, N ). 5: H NMR (300 MHz, CDCl ): d
13
1
2 8
d 3.60 (m, 4H, CH ); C{ H} NMR (75.4 MHz, THF-d
1
J
1
CN 5 4.5 Hz, CN), 50.0 (CH
2
–N
3
2
1
3
3
1
3
3
3
3.89 (t, JHH 5 6 Hz, 2H, CH
2
–N
C{ H} NMR (75.4 MHz, CDCl
), 50.0 (CH –N ), 44.3 (CH
), 2069 (s, CO), 1923 (s, br, CO); MS (EI): m/z 420 (22, [M] ),
3
), 3.65 (t, JHH 5 6 Hz, 2H, CH
): d 196.3 (COtrans), 194.4 (COcis), 147.3
–NC); IR (KBr): n 2187 (s, CN),
2
–CN);
13
1
3
(CN–CH
2
2
3
2
+
2104 (s, N
3
+
+
+
392 (5, [M 2 CO] ), 364 (2, [M 2 2CO] ), 336 (18, [M 2 3CO] ), 308 (31,
+
+
1
[
7
M 2 4CO] ), 280 (54, [M 2 5CO] ). 7: H NMR (400 MHz, THF-d
.53 (s, 2H, NH), 3.32 (s, 4H, CH
6
): d
); C{ H} NMR (75.4 MHz, THF-d ):
d 202.2 (NCN), 200.4 (COtrans), 198.8 (COcis), 45.0 (CH ); IR (KBr): n 3475
s, NH), 2063 (s, CO), 1888 (s, CO); MS (EI): m/z 394 (70, [M] ), 366 (36,
1
3
1
2
6
The molecular structures of 7 and 8 were determined by
X-ray diffraction (Fig. 1).§ The structure analyses confirmed the
formation of the carbene complexes. The W–C1 separation in 7
2
+
(
+
+
+
[M 2 CO] ), 338 (24, [M 2 2CO] ), 310 (100, [M 2 3CO] ), 282 (91, [M 2
+
+
1
4CO] ), 254 (60, [M 2 5CO] ). 8: H NMR (400 MHz, THF-d
6
): d 5.81
), 5.24–5.21 (m, 4H, CH
), 3.51 (s, 4H, N–CH –CH –N);
): d 207.7 (NCN), 201.6 (COtrans),
–CHLCH
), 118.9 (CH ), 56.5 (N–
2
–N); MS (EI): m/z 474 (26, [M] ), 446 (18,
(2.221(5) s) compares well to the equivalent distance in the
3
(
ddt, 2H, JHH 5 16, 10, 4 Hz, CH
CHLCH ), 4.36 (m, 4H, CH –CHLCH
C{ H} NMR (100.6 MHz, THF-d
198.9 (COcis), 134.5 (CH
–CHLCH
CH –CH), 49.0 (N–CH –CH
[M 2 CO] ), 390 (23, [M 2 3CO] ), 362 (100, [M 2 4CO] ).
Crystal data for compounds 7 and 8. 7: C W, M 5 394.00,
2
–CHLCH
2
2
–
complex with an NH,NH-stabilized benzimidazolin-2-ylidene
2
2
2
2
2
3a
ligand (2.203(4) s). A significant lengthening of the W–C
distance is observed upon N,N9-alkylation for both the N,N9-
diallylimidazolidin-2-ylidene (2.266(3) s in 8) and the N,N9-
diallylbenzimidazolin-2-ylidene (2.256(3) s) ligand.
distances fall in the range observed for the W(CO) complex with
13
1
8
2
2
2
2
+
2
2
+
+
+
3
a
These
§
8 6 2 5
H N O
5
T 5 153(2) K, l 5 0.71073 s, triclinic, P-1, Z 5 2, a 5 6.6670(10),
b 5 8.5579(12), c 5 10.1724(15) s, a 5 94.486(3), b 5 106.524(3),
c 5 100.679(3)u, V 5 541.48(14) s , 6155 measured reflections, 3109 unique
reflections (Rint 5 0.0370), R 5 0.0304, wR 5 0.0741 for 2977 contributing
reflections [I ¢ 2s(I)], refinement against |F | with anisotropic thermal
parameters for all non-hydrogen atoms and hydrogen atoms on calculated
12
the unsaturated N,N9-diethylimidazolin-2-ylidene (2.275(8) s).
A significantly shorter W–C(carbene) separation was observed
for the W(CO) complex with the benzoxazolin-2-ylidene ligand
3
5
2
11
(2.198(5) s).
Here we have described an alternative route leading to com-
positions. 8: C14
14 2 5
H N O W, M 5 474.12, T 5 153(2) K, l 5 0.71073 s,
monoclinic, P2 /n, Z 5 4, a 5 11.181(5), b 5 10.629(5), c 5 13.731(6) s,
1
plexes with N-heterocyclic carbene ligands of the imidazolidin-2-
ylidene type. Previously such complexes were obtained by cleavage
of electron rich enetetramines or from imidazolidinium salts by
reaction with suitable transition metal precursors. In contrast to
this, we present a method to generate the N-heterocyclic carbene
ligand at a suitable template metal center starting from a
coordinated b-functionalized alkyl isocyanide. This method
offers some advantages. For example, it gives access to complex
3
b 5 105.519(8)u, V 5 1572.3(12) s , 17607 measured reflections, 4579
unique reflections (Rint 5 0.0473), R 5 0.0262, wR 5 0.0606 for 4084
2
contributing reflections [I ¢ 2s(I)], refinement against |F | with anisotropic
thermal parameters for all non-hydrogen atoms and hydrogen atoms on
calculated positions. CCDC 280981 (7) and 280982 (8). See http://
dx.doi.org/10.1039/b510996e for crystallographic data in CIF or other
electronic format.
1 (a) L. Tschugajeff, M. Skanawy-Grigorjewa and A. Posnjak, Z. Anorg.
Allg. Chem., 1925, 148, 37; (b) W. M. Butler, J. H. Enemark, J. Parks
and A. L. Balch, Inorg. Chem., 1973, 12, 451.
7
with an NH,NH-stabilized imidazolidinylidene, a ligand not
stable or available in the free state. Alkylation of the NH-functions
2
3
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Fig. 1 Thermal ellipsoid plots showing the molecular structures of
complexes 7 (left) and 8 (right). Hydrogen atoms have been omitted for
clarity. Selected bond distances [s] and bond angles [u] for 7 [8]: W–C1
6
7
(a) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18; (b)
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2.221(5) [2.266(3)], C1–N1 1.326(6) [1.362(4)], C1–N2 1.330(6) [1.349(4)];
N1–C1–N2 106.6(4) [106.8(2)].
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8
9
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3
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392 | Chem. Commun., 2005, 5390–5392
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