Metal-assisted, reversible phosphinyl phosphination of the carbon-nitrogen triple bond in a nitrile

Article abstract of DOI:10.1002/anie.200604622

(Chemical Equation Presented) Split P's: Phosphinyl-substituted N-heterocyclic phosphines react by unprecedented metal-assisted addition of the P-P bond to a nitrile triple bond to give complexes of hybrid bisphosphines. Surprisingly, the addition is reve

Full text of DOI:10.1002/anie.200604622

Angewandte
Chemie
DOI: 10.1002/anie.200604622
À
P P Insertions
Metal-Assisted, Reversible Phosphinyl Phosphination of the Carbon–
Nitrogen Triple Bond in a Nitrile
Sebastian Burck, Dietrich Gudat,* and Martin Nieger
Bidentate ligands have a long standing in organometallic
chemistry, coordination chemistry, and catalysis. For their
synthesis, additions to alkenes that allow simultaneous
introduction of two donor groups to an organic backbone
have recently gained attention. The largest progress has been
made in the field of O,O and N,N ligands, for which protocols
for stereo- and even enantioselective dihydroxylation[1] or
diamination of olefins[2] have been worked out. Viable
approaches to P,P-donor ligands, which are likewise of great
significance, by diphosphination of organic precursors have
been found in the double metathesis of 1,2-disubstituted
olefins,[3] or in the addition of the P P bonds of diphosphines
À
to alkenes[4] or alkynes.[5,6]
We have recently described the N-heterocyclic phos-
phines 1[4] (Scheme 1), which are distinguished by exception-
À
ally reactive P P bonds and undergo phosphinyl phosphina-
tion of alkenes to yield hybrid bisphosphines that have two
donor sites with different electronic properties. Specimen of
this type have received great interest as ligands in catalysis.[7]
We have now discovered an unprecedented metal-assisted
addition of 1 to the triple bond of a nitrile, which offers a
surprisingly simple access to complexes of hybrid 1,2-bisphos-
phines. Quite interestingly, the addition is reversible and
permits controlled conversion of the formed complexes into
Scheme 1. PR2 =2,3,4,5-tetraethylphospholyl, R’=Mes (1a, 3a, 5);
R=Ph, R’=2,6-Me2C6H3 (1b, 3b, 6, 7). Reagents and conditions:
a)1 equiv [W(CO) 4(cod)] (2), MeCN, 208C (for 1a)or 50 8C (for 1b);
b)185 8C; c)1 equiv [W(CO) 3(MeCN)3] (4) , 208C, toluene; d)1 equiv
ethyl propiolate, toluene, 0–208C; cod=cycloocta-1,5-diene,
Mes=2,4,6-Me3C6H2.
À
species that arise formally from metal insertion into the P P
bond of 1.
While exploring the coordination properties of 1a, we
treated an acetonitrile solution of the diphosphine with the
tungsten complex 2. A color change from orange to green
indicated the formation of a new product, which was isolated
by crystallization and identified as a bisphosphine complex by
observation of an AX pattern with 183W satellites on both
signals in the 31P{1H} NMR spectrum (see the Experimental
Section). The anticipated presence of a chelate complex of 1
was, however, ruled out as the 1H and 13C NMR spectra gave
evidence for a CH3C fragment that could not be accounted
for. Unambiguous structural assignment was finally feasible
from a single-crystal X-ray diffraction study,[8] which showed
the presence of the chelate complex 3a, which features a
À
hybrid 1,2-bisphosphine ligand arising from addition of the P
P bond of 1a to the triple bond of acetonitrile (Scheme 1).
The crystals of 3a (Figure 1) contain discrete complexes
with a distorted octahedral coordination at the tungsten atom
that results mainly from the small bite angle of the chelate
ligand (P2-W1-P1 79.33(4)8). The bond lengths in the
diazaphospholene and phosphole rings differ by less than
0.02 Š from those in free 1a.[4] The chelate ring is nearly
planar (deviations from mean plane less than 0.07 Š). The
[*] Dr. S. Burck, Prof. Dr. D. Gudat
Institut für Anorganische Chemie
Universität Stuttgart
À
À
bonds W1 P2 (2.450(1) Š) and W1 P1 (2.459(1) Š) are very
similar and shorter than known bond lengths in {bis(phosphi-
ne)W(CO)4} complexes ((2.50 Æ 0.02) Š[9]), regardless of the
different environment of the donor atoms (N3 vs. C3
Pfaffenwaldring 55, 70550 Stuttgart (Germany)
Fax : (+49)711-685-64241
E-mail: gudat@iac.uni-stuttgart.de
Homepage: http://www.iac.uni-stuttgart.de/akgudat
À
substituent pattern). The bonds N1 P1 (1.766(4) Š) and
À
À
À
C1 P2 (1.888(5) Š) are notably longer than the P N and P C
bonds in the diazaphospholene and phosphole rings. All these
Dr. M. Nieger
Laboratory of Inorganic Chemistry
University of Helsinki
À
features point to the strengthening of the W P bonds by
substantial metal-to-ligand back donation, which occurs at the
P.O. Box 55, 00014 University of Helsinki (Finland)
À
À
expense of weakening of the N1 P1 and C1 P2 bonds by
hyperconjugation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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geometry at the P1 atom (sum of bond angles 359.7(3)8)
À
and the very short P1 W1 bond (2.218(1) Š) are typical for
phosphenium complexes; the value of the latter is close to the
shortest bond lengths in known tungsten phosphenium
complexes (2.18–2.34 Š)[11] and suggests a high degree of
phosphorus–metal p bonding.
By analogy to 1a, reaction of diphosphine 1b with 2 in
acetonitrile at 508C afforded the spectroscopically detectable
complex 3b, as well as by-products arising from hydrolysis of
1b.[4] Thermolysis of the crude product mixture resulted again
in elimination of acetonitrile from 3b and formation of the
phosphido–phosphenium complex 6, which was isolated by
crystallization. The presence of a dinuclear complex followed
from the AA’XX’-type splitting of the 31P NMR signals and
was confirmed by a single-crystal X-ray diffraction study,[8]
which revealed the presence of a centrosymmetric dimer with
mer arrangement of CO ligands and mutual trans orientation
of m2-phosphido and phosphenium ligands at each metal
center (Figure 3). The deviation from regular octahedral
Figure 1. Molecular structure of 3a in the solid state (H atoms
omitted; 50% probability thermal ellipsoids). Selected bond lengths
[Š] and angle [8]: W1-P2 2.450(1), W1-P1 2.459(1), N1-C1 1.270(6), N1-
P1 1.766(4), C1-P2 1.888(5), P1-N5 1.688(4), P1-N2 1.692(4), N2-C3
1.418(6), C3-C4 1.327(7), C4-N5 1.417(6), P2-C24 1.803(5), P2-C27
1.807(5), C24-C25 1.354(7), C25-C26 1.466(7), C26-C27 1.365(6); P2-
W1-P1 79.33(4).
Solid 3a is moderately air- and moisture-stable and was
found to melt above 1858C under specific conversion into the
dark-red phosphenium–phospholide complex 5. This species
was likewise accessible from 1a and a dilute solution of
[W(CO)3(MeCN)3] (4) and was identified by analytical and
spectroscopic data and a single-crystal X-ray diffraction study
(Figure 2).[8] The h5-bound phospholyl ligand in this half-
sandwich complex exhibits intraligand and metal–ligand bond
lengths that are similar to those of the only known compa-
rable complex, [h5-(Me2C4H2P)W(CO)3I].[10] The planar
Figure 3. Molecular structure of 6 in the solid state (H atoms omitted;
50% probability thermal ellipsoids). Selected bond lengths [Š] and
angles [8]: W1-P1 2.251(2), W1-P2’ 2.577(2), W1-P2 2.596(2), P1-N2
1.684(5), P1-N5 1.692(5), N2-C3 1.386(7), C3-C4 1.327(9), C4-N5
1.392(7); P2’-W1-P2 76.9(1), N2-P1-N5 88.5(2), N2-P1-W1 134.4(2),
N5-P1-W1 135.7(2), W1’-P2-W1 101.7(1).
coordination owes chiefly to angle distortions in the W2P2 ring
(P2-W1-P2’ 76.9(2), P1-W1-P2 107.3(2)8) that are presumably
caused by steric interference of the phosphido and phosphe-
À
nium ligands. The terminal P1 W1 bonds (2.251 Š) are
longer than those in 3b, but are comparable to the bonds in
other tungsten phosphenium complexes (2.18–2.34 Š).[11] The
À
bridging W1 P2/2’ bonds (2.577(2)/2.596(2) Š) match those
in [{Ph2PW(CO)4}2]2À,[12] and the large difference in bond
lengths between terminal and bridging phosphorus atoms is in
accord with the presence of formal single and double bonds.
Although the presence of singly and doubly bonded R2P
ligands at one metal atom has precedence,[13] the deviation of
bond lengths between these types of ligands in 6 is unsur-
passed and points to a special bonding situation which can be
Figure 2. Molecular structure of 5 in the solid state (H atoms omitted;
50% probability thermal ellipsoids). Selected bond lengths [Š] and
angles [8]: W1-P1 2.218(1), W1-C25 2.362(4), W1-C26 2.371(3), W1-C24
2.381(4), W1-C27 2.398(3), W1-P2 2.560(1), W1-centroid(C4P)
2.014(2), P1-N5 1.681(3), P1-N2 1.684(3), N2-C3 1.405(4), C3-C4
1.333(4), C4-N5 1.391(4); N5-P1-N2 88.7(1), N5-P1-W1 134.0(1), N2-
P1-W1 137.0(1).
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5: a) Milligram amounts were prepared by heating 3a for 1 min to
described by considering the ligand properties of the R2P
groups. In previously known complexes, both singly and
doubly bonded ligands are electron-rich phosphido species
that act as simple s-donor or s/p-donor ligands to a metal
center in high oxidation state. In 6, the bridging R2P moieties
behave as pure s donors, but the terminal phosphenium units
represent s-donor/p-acceptor ligands that interact with a
metal center in low oxidation state, thus creating a similar
bonding situation as in a Fischer-type carbene complex.
Investigations of mechanistic details in the formation of
3a showed that 1a and acetonitrile did not react in the
absence of complex 2, and 1a and 2 did not react in inert
solvents in the absence of acetonitrile. These findings lead us
to conclude that the formation of 3a is initiated by replace-
ment of the diolefin ligand in 2 by nitriles. Since metal
coordination of a nitrile facilitates addition of nucleophiles,[14]
the next step is presumably formation of a carbon–phospho-
rus bond between the R2P moiety of 1a and the nitrile carbon
atom. The completion of the reaction must then involve
migration of the phosphenium moiety to the nitrogen atom by
1868C. b) Complex 4 (0.76 g, 2 mmol) in toluene (70 mL) was added
dropwise to a solution of 1a (1.36 g, 2 mmol) in toluene (30 mL). The
mixture was stirred for 4 h and volatiles were evaporated in vacuum.
The residue was dissolved in hexane (20 mL) and filtered. Crystal-
lization at À208C afforded orange crystals, which were collected by
filtration and dried in vacuum to yield 1.15 g (76%) of 5. M.p. 1438C;
2
31P{1H} NMR (C6D6, 303 K, 101.2 MHz): d = 182.9 (d, JPP = 11.4 Hz,
1JPW = 728 Hz), À17.8 ppm (d, 2JPP = 11.4 Hz, 1JPW = 7.6 Hz).
6: A solution of 1b (0.48 g, 1 mmol) and 2 (0.41 g, 1 mmol) in
MeCN (25 mL) was stirred for 6 h at 508C. Volatiles were evaporated
in vacuum. A 31P NMR measurement revealed formation of 3b (d =
151.2 (d, 2JPP = 42.5 Hz, 1JPW = 312 Hz), 86.4 ppm (d, 2JPP = 42.5 Hz,
1JPW = 256 Hz)) as well as products arising from the hydrolysis of 1b.
A portion of the crude product (409 mg) was melted in vacuum until
gas evolution ceased. The solid was allowed to cool to room
temperature and extracted with hexane (5 mL). Recrystallization of
the residue at 48C from THF/toluene afforded red crystals, which
were filtered off and dried in vacuum to give 295 mg (88%) of 6.
M.p. 3488C; 31P{1H} NMR (C6D6, 303 K, 101.2 MHz): d = 166.3,
À98.6 ppm (AA’XX’ pattern,
JAA’ = 80 Hz, JXX’ = 15 Hz, JAX =
100 Hz, JAX’ = À19 Hz).
Comprehensive anatytical and spectroscopic data of 3a and 5–7
À
are available in the Supporting Information.
P P bond cleavage and formation of two new phosphorus–
metal bonds. This sequence is obviously facilitated by a higher
Received: November 13, 2006
Published online: March 14, 2007
À
degree of P P bond weakening in 1a. The formation of 5
from 1a and 4 may either involve 3a as an intermediate or
À
proceed by direct “nonoxidative” addition of the P P bond to
Keywords: chelates · insertion · nucleophilic addition ·
phosphane ligands · phosphenium ligands
À
the metal center; analogous insertions into P C bonds have
.
precedence for phosphenium–carbene adducts.[15]
In view of the reactivity of 1a,b one would expect that
these compounds also undergo phosphinyl phosphination of
electron-poor alkynes.[6] We found this to be true for 1b,
which reacted with ethyl propiolate at room temperature
even in the absence of an activating metal in a regio- and Z-
stereoselective addition to afford the bisphosphine 7 (see the
Supporting Information). Surprisingly, no such reaction was
observed for 1a, and it is still unclear if this lack of reaction
can be attributed to the lower nucleophilicity of a phospholyl
substituent as compared to a Ph2P substituent or to an
increased degree of steric hindrance.
[1] H. C. Kolb, M. S. VanNieuwenhze, K. B. Sharpless, Chem. Rev.
1994, 94, 2483; K. B. Sharpless, Angew. Chem. 2002, 114, 2126;
Angew. Chem. Int. Ed. 2002, 41, 2024.
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Zayas, H. Jiao, W. Baumann, A. Spannenberg, A. Monsees, T. H.
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2006, 12, 5001.
[4] S. Burck, D. Gudat, M. Nieger, Angew. Chem. 2004, 116, 4905;
Angew. Chem. Int. Ed. 2004, 43, 4801.
In summary, we have demonstrated the first transition-
metal-assisted addition of diphosphines to a nitrile, which
gives direct access to complexes of hybrid 1,2-bisphosphines.
Retroaddition at high temperature or direct “nonoxidative”
addition[15] of a diphosphine to a metal center yielded
complexes featuring a combination of phospholide-donor
and phosphenium-acceptor ligands which offer interesting
prospects for further reactions. The extension of this chemis-
try to the synthesis of complexes of other metals and new
ligands for catalysis is currently under investigation.
[5] W. R. Cullen, D. S. Dawson, Can. J. Chem. 1967, 45, 2887; A.
Sato, H. Yorimitsu, K. Oshima, Angew. Chem. 2005, 117, 1722;
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[6] During writing of this article, the addition of diphosphines to
electron-poor alkynes was reported: D. L. Dodds, M. F. Haddow,
A. G. Orpen, P. G. Pringle, G. Woodward, Organometallics 2006,
25, 5937.
[7] Selected references: N. Sakai, S. Mano, K. Nozaki, H. Takaya, J.
Am. Chem. Soc. 1993, 115, 7033; K. Nozaki, N. Sakai, T. Nanno,
T. Higashijima, S. Mano, T. Horiuchi, H. Takaya, J. Am. Chem.
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128, 7198.
[8] Crystal structures: Nonius Kappa-CCD diffractometer, T=
123(2) K, MoKa radiation, empirical absorption correction,
SHELX97[16] for structure solution (Patterson methods (3a)
and direct methods (5, 6)) and refinement (full matrix, least
squares refined against F2). The positions of the hydrogen atoms
were refined with a riding model. Empirical absorption correc-
tions were applied. 3a: C38H47N3O4P2W, Mr = 855.6, crystal size
0.50 ” 0.20 ” 0.15 mm3, orthorhombic, space group Pbca
(No. 61), a = 14.5762(1), b = 20.5667(2), c = 25.2197(3) Š, V=
7560.5(1) Š3, Z = 8, m = 3.18 mmÀ1, F(000) = 3456, qmax = 258,
99751 reflexes, 6662 independent reflexes (Rint = 0.068), R1 =
Experimental Section
3a: Comlpex 2 (0.81 g, 2 mmol) in MeCN (30 mL) was added
dropwise to 1a (1.36 g, 2 mmol) in MeCN (30 mL). The mixture was
stirred for 4 h and volatiles were evaporated in vacuum. The residue
was dissolved in hexane (20 mL) and filtered. Crystallization at
À208C afforded green crystals, which were collected by filtration and
dried in vacuum to yield 1.23 g (63%) of 3a. M.p. 1868C;
31P{1H} NMR (C6D6, 303 K, 101.2 MHz): d = 149.5 (d, JPP = 32.5 Hz,
2
1JPW = 315 Hz), 93.6 ppm (d, JPP = 32.5 Hz, 1JPW = 240 Hz).
0.031
(for
I > 2s(I)),
wR2 = 0.082
(all
data).
5:
2
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Communications
C34H44N2O2P2W·C6H6, Mr = 836.6, crystal size 0.15 ” 0.10 ”
[10] S. Holand, F. Mathey, J. Fischer, Polyhedron 1986, 5, 1413.
0.05 mm3, monoclinic, space group C2/c (No. 15), a =
29.3068(5), b = 15.0260(3), c = 18.9921(4) Š, b = 113.689(2)8,
[11] Result of a search in the CSD database for [LnM PR2].
=
[12] S.-G. Shyu, M. Calligaris, G. Nardin, A. Wojcicki, J. Am. Chem.
Soc. 1987, 109, 3617.
V= 7658.7(3) Š3, Z = 8, m = 3.14 mmÀ1, F(000) = 3392, qmax
=
27.58, 34046 reflexes, 8605 independent reflexes (Rint = 0.056),
R1 = 0.030 (for I > 2s(I)), wR2 = 0.069 (all data). 6:
C66H60N4O6P4W2·3C7H8, Mr = 1773.2, crystal size 0.32 ” 0.16 ”
0.08 mm3, monoclinic, space group P21/n (No. 14), a =
20.5351(5), b = 16.0361(4), c = 24.9648(4) Š, b = 110.389(2)8,
[13] R. T. Baker, P. J. Krusic, T. H. Tulip, J. C. Calabrese, S. S.
Wreford, J. Am. Chem. Soc. 1983, 105, 6763; R. T. Baker, J. F.
Whitney, S. S. Wreford, Organometallics 1983, 2, 1049; R. T.
Baker, J. C. Calabrese, T. E. Glassman, Organometallics 1988, 7,
1889.
[14] V. Y. Kukushkin, A. J. L. Pombeiro, Chem. Rev. 2002, 102, 1771.
[15] N. J. Hardman, M. B. Abrams, M.A. Pribisko, T. M. Gilbert,
R. L. Martin, G. J. Kubas, R. T. Baker, Angew. Chem. 2004, 116,
1989; Angew. Chem. Int. Ed. 2004, 43, 1955.
V= 7705.9(3) Š3, Z = 4, m = 3.12 mmÀ1, F(000) = 3560, qmax
=
258, 44022 reflexes, 13557 independent reflexes (Rint = 0.074),
R1 = 0.043 (for I > 2s(I)), wR2 = 0.091 (all data). CCDC-632090
(3a), CCDC-632091 (5), and CCDC-632092 (6) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[16] G. M. Sheldrick, SHELX-97, Program for the Solution and
Refinement of Crystal Structures, University of Göttingen,
Göttignen, Germany, 1998.
[9] Result of
a search in the CSD database for {(bisphos-
phine)W(CO)4} complexes.
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