Making the true "CP" ligand

Article abstract of DOI:10.1002/anie.200602499

Cyaphide attained: A high-yielding synthesis for the first stable terminal cyaphide complex [RuH(CP)(dppe)₂] (dppe = bis(1,2- diphenylphosphinoethane); see structure: P orange, Ru silver, C gray, H white) was developed from the silyl-substitute

Full text of DOI:10.1002/anie.200602499

Angewandte
Chemie
ꢀ
between two metals, the incorporation of C P into coordina-
tion polymers and new materials, and the basic synthetic
ꢀ
challenges of making C P have provided inspiration for
ꢁ ꢀ
decades. While phosphaalkynes (R C P) have been known
for some time,[1–3] the terminal M C P has only been
ꢁ ꢀ
reported as a transient species.[4,5] Other C-functionalized
X C P compounds (X = R3Si, R2N,[7,8] RO, [8] F,[9] Cl,[10]),
[6]
ꢁ ꢀ
ꢁ
[11]
anionic species [X C P] (X = R3B, RN,[8] O,[12] S[13]), and
ꢁ ꢀ
the cationic phosphonio phosphaalkyne [R3P C P]+,[14] have
ꢁ ꢀ
been synthesized, but the vast majority of reports deal with
tert-butyl,[15] adamantyl,[16] 2,4,6-trimethylphenyl[17] and 2,4,6-
tri-tert-butylphenyl[18] phosphaalkynes. Recently, we devised a
very simple method for accessing the kinetically stable
crystalline triphenylmethyl (trityl)-substituted phosphaal-
ꢀ
kyne, Ph3CC P (1), which allowed for the economical
metal-promoted synthesis of phosphorus heterocycles. How-
ever, our ultimate goal and incentive for synthesizing the
trityl-substituted phosphaalkyne was to find an adequate
leaving group that would lead us to cyaphide.[19]
Treatment of 1 or its complexes [MH(dppe)2(Ph3C-
C P)]OTf (M = Fe or Ru; dppe = bis(1,2-diphenylphosphi-
ꢀ
noethane); OTf = trifluoromethanesulfonate) with nucleo-
ꢁ
ꢀ
philes (Nu) did not furnish the desired cyaphide, C P , or its
complexes [Eq. (1); electrophile (E) = C].
Ph3EꢁCꢀP þ Nuꢁ ! Ph3EꢁNu þ CꢀPꢁ
ð1Þ
Therefore, we reasoned that silicon would be more
susceptible to nucleophilic attack (E = Si) and applied an
analogous synthetic procedure to access the higher homo-
ꢀ
logue Ph3SiC P (3).
Conversion of Ph3SiCH2Cl into the Grignard reagent
followed by treatment with PCl3 produced the silyl-substi-
tuted alkyl phosphonous dichloride, Ph3SiCH2PCl2 (2), in
over 87% yield (Scheme 1). Employing 2.2 equiv of DABCO
(1,8-diazabicyclo[2.2.2]octane) effected the dehydrohaloge-
nation reaction, and 2 was fully transformed into the
triphenylsilyl-substituted phosphaalkyne 3. The reaction
occurred at ambient temperature and in multiple solvents in
less than one hour. (In contrast, the reaction of DABCO with
ꢀ
C P Ligands
DOI: 10.1002/anie.200602499
Making the True “CP” Ligand**
ꢀ
Ph3CCH2PCl2 to furnish Ph3CC P required a 10-fold excess
Joseph G. Cordaro, Daniel Stein, Heinz Rüegger, and
Hansjörg Grützmacher*
of base, elevated temperatures, and proceeded best in
acetonitrile.[19]
)
While DABCO alone was effective for the conversion of 2
into 3, we saw rapid decomposition of this new phosphaal-
kyne, which was dependent on the solvent. Qualitatively, the
rate of decomposition of 3 followed the order Et2O ꢂ tolu-
ene < THF ! CH3CN. Suspecting that DABCO-HCl was
acting as a soluble source of chloride anion in the more
polar solvents that then attacked silicon to produce Ph3SiCl
and the CP anion, which is seemingly unstable under the
reaction conditions, we added AgOTf to the reaction mixture.
When a solution of 2 in toluene was pretreated with 2.2 equiv
of AgOTf for 5 min followed by addition of 2.2 equiv of
DABCO, complete conversion into 3 was observed in less
than one hour. Filtration through diatomaceous earth to
remove insoluble salts furnished indeed a clean solution of 3,
which only slowly decomposed. In the 31P NMRspectrum of
3, a deshielded signal at d = 111 ppm was observed. The a-
The quest for cyaphide, the phosphorus equivalent of cyanide,
has been a continuous struggle for many years. Potential
ꢀ
applications arising from the use of C P as a bridging ligand
[*] Dr. J. G. Cordaro, Dr. D. Stein, Dr. H. Rüegger,
Prof. Dr. H. Grützmacher
Laboratorium für Anorganische Chemie
Departement Chemie und Angewandte Biowissenschaften
ETH Zürich
8093 Zürich (Switzerland)
Fax: (+41)44-633-1418
E-mail: gruetzmacher@inorg.chem.ethz.ch
[**] We thank the ETH Zürich for financial support and Theo Zweifel for
helpful suggestions.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 6159 –6162
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6159

Communications
When 4 was treated with fluoride, no clean
reaction occurred. The observation of signals with
large P–F couplings in the 31P NMRspectrum of
the reaction mixture indicated that phosphorus
was attacked by fluoride. Reactions with alkox-
ides and hydroxide similarly resulted in nucleo-
philic attack at phosphorus, as indicated by the
31P NMRchemical shifts. However, when 4 was
treated with a slight excess of sodium phenoxide
(NaOPh) in THF, the immediate formation of an
intermediate was observed[23] which was then
converted into a new product 5 after 14 h. Product
5 showed a broad signal in the 31P NMRspectrum
at d = 165 ppm, which was weakly coupled to
another broad signal at d = 65.2 ppm. In the
1H NMRspectrum, the hydride appeared as a
sextet at d = ꢁ11.22 ppm with JP-H ꢂ 20 Hz. A 31P-
1H HMQC (heteronuclear multiple-quantum cor-
relation) experiment revealed cross-peaks
between the hydride and both phosphorus signals.
In a selective 1H{31P} NMRexperiment, the
Scheme 1. Synthesis of ruthenium cyaphide complex 5 from the silyl-substituted phosphaalkyne
3 (see text for details).
carbon atom, which gives a signal at d = 193.3 ppm (d, JC-P
=
16.4 Hz) in the 13C NMRspectrum, was also shifted signifi-
cantly downfield compared to other phosphaalkynes.[20]
Attempts to isolate 3 were unfortunately unsuccessful. If a
solution of 3 was concentrated, a red precipitate formed
which gave no detectable 31P NMRsignal upon redissolving in
more polar solvents. However, the lifetime of 3 in toluene or
diethyl ether (t1/2 ꢂ 1 day at 238C) was sufficiently long so that
31P and 13C NMRdata could be acquired and, more impor-
tantly, coordination of 3 to [RuH(dppe)2]OTf was successful.
Addition of a solution of 3 in toluene to 0.5 equiv of
[RuH(dppe)2]OTf in CH2Cl2 resulted in a color change
from dark red to yellow-orange. After workup, the colorless
1
ꢀ
h -coordinated complex [RuH(dppe)2(Ph3SiC P)]OTf (4)
was isolated in over 80% yield as the only product. In the
31P{1H} NMRspectrum, coordination of 3 to the ruthenium
cation was confirmed by the downfield-shifted signal at d =
143.8 ppm for the phosphaalkyne, which appeared as a
quintet (2JP-P = 27.8 Hz) as a result of coupling with the four
equivalent phosphorus atoms of the dppe ligands. The dppe
ligands were seen as a doublet at d = 60.1 ppm. A doublet of
Figure 1. Molecular structure of 4. The anion and hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at 30% probability.
Selected bond lengths [Š] and angles [8]: P1–C1 1.530(3), C1–Si1
1.825(3), Ru1–P1 2.2485(8), Ru1–P2 2.3811(7), Ru1–P3 2.3721(7),
Ru1–P4 2.3559(7), Ru1–P5 2.3694(7); P1-C1-Si1 165.5(2), C10-Si1-C20
114.4(2), C20-Si1-C30 109.0(2), C10-Si1-C30 110.2(2), P1-Ru1-P2
94.79(3), P1-Ru1-P3 94.12(3), P1-Ru1-P4 94.06(3), P1-Ru1-P5 98.34(3),
P2-Ru1-P3 82.96(3), P3-Ru1-P4 97.50(3), P4-Ru1-P5 82.63(3), P5-Ru1-
P2 95.00(3).
1
quintets in the H NMRspectrum at d = ꢁ8.13 ppm (trans-
2JH-P = 122.1 Hz, cis-2JH-P = 17.0 Hz) for the ruthenium hy-
dride was most diagnostic for the formation of 4. The signal
for the a-carbon atom of the coordinated phosphaalkyne was
observed at d = 175.1 ppm in the 13C NMRspectrum with a
significantly larger C–P coupling constant of JC-P = 71.4 Hz as
compared to 3.
31P NMRsignal at d = 165 ppm coupled with the hydride to
form a doublet with JP-H = 19.5 Hz, while the signal at d =
65.2 ppm coupled with the hydride to give a quintet with JP-
H = 20.5 Hz. Finally, a 13C-1H HMQC experiment with phos-
phorus decoupling centered at d = 65 ppm, detected a cross-
peak for a quaternary carbon atom at d = 287.1 ppm in the
13C dimension, which correlated with the hydride. Coupling
between the phosphorus and carbon centers corresponded to
1JC-P ꢂ 90 Hz. These spectroscopic data are fully consistent
The molecular structure of 4 is presented in Figure 1.[21]
The X-ray data for 4 are the only available data for any silyl-
ꢁ
substituted phosphaalkyne. The P C bond length (1.530 Š)
for 4 is not significantly different when compared to those in
ꢀ
the free phosphaalkyne Ph3CC P (1; 1.538 Š) and its iron
ꢀ
complex [Fe(dppe)2H(Ph3CC P)]OTf (1.535 Š). The devia-
tion from linearity seen in the P-C-Si angle (165.58) is likewise
not unusual and certainly has steric origins.[22]
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Angewandte
Chemie
with a {RuH(dppe)2} fragment carrying a CP ligand trans to
the hydride.
Experimental Section
All manipulations were performed under an inert atmosphere of
argon using standard Schlenk techniques. NaOPh was synthesized
from PhOH and Na in THF. The isolated salt was dried under vacuum
at 2008C. Selected analytical data are given below (see Supporting
Information for more details).
Crystals suitable for X-ray analysis were grown from a
THF solution layered with hexane. The results confirm the
NMRdata and demonstrate the first structurally character-
ized metal cyaphide complex [RuH(CP)(dppe)2] (5;
Figure 2). As expected, the CP ligand has rotated from
Ph3SiCH2PCl2 (2): A 250-mL Schlenk flask equipped with a stir
bar was charged with freshly ground Mg turnings (10 g, 0.411 mol)
and THF (50 mL). Meanwhile, Ph3SiCH2Cl[24] (13.0 g, 0.042 mol) was
dissolved in THF (50 mL) in a separate Schlenk flask. The alkyl
chloride was slowly added to the Mg turnings in 10 mL portions over
1 h, at which time the Grignard reaction was initiated. An additional
50 mL of THF was used to rinse the remaining alkyl chloride into the
reaction mixture. The Grignard reagent was mixed for 3 h and then
transferred via cannula to a solution of PCl3 (11 mL, 0.126 mol) in
THF (100 mL) cooled to ꢁ788C. The mixture was stirred and allowed
to warm to room temperature over 18 h. After removal of the volatile
materials, the gray solid was extracted with toluene (ca. 200 mL) and
filtered through diatomaceous earth on a medium-pore fitted glass
filter frit. The volatile materials were removed under reduced
pressure to furnish a white solid. Crystallization from THF/hexane
at ꢁ308C afforded analytically pure material (13.8 g, 87.6%); m.p.:
1
2
96–978C; H NMR(300 MHz, C D6, 238C): d = 2.60 ppm (d, JH-P
=
6
14.3 Hz, 2H); 13C{1H} NMR(75.5 MHz, C D6, 238C): d = 32.52 ppm
6
(d, JC-P = 64.6 Hz); 31P{1H} NMR(121.5 MHz, C 6D6, 238C): d =
Figure 2. Molecular structure of 5. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 30% probability. Selected bond
lengths [Š] and angles [8]: P1–C1 1.573(2), Ru1–C1 2.057(2), Ru1–P2
2.3342(5), Ru1–P3 2.3315(5), Ru1–P4 2.3222(5), Ru1–P5 2.3396(4);
Ru1-C1-P1 177.9(1), C1-Ru1-P2 92.48(5), C1-Ru1-P3 96.86(5), C1-Ru1-
P4 93.74(5), C1-Ru1-P5 94.30(5), P2-Ru1-P3 82.63(2), P3-Ru1-P4
94.60(2), P4-Ru1-P5 83.31(2), P5-Ru1-P2 98.25(2).
199.7 ppm.
ꢀ
Ph3SiC P (3): A 20-mL scintillation vial equipped with a stir bar
was charged with Ph3SiCH2PCl2 (2; 0.375 g, 1.00 mmol), AgOTf
(0.564 g, 2.20 mmol), and toluene (18 mL). The suspension was
protected from light and vigorously stirred. After 5 min, DABCO
(0.248 g, 2.20 mmol) was added to the vial and stirring was continued.
After an additional 60 min, the mixture was filtered through a pad of
diatomaceous earth on a medium-pore fitted glass filter frit eluting
with toluene (ca. 10 mL). The resulting yellow solution was used
immediately. 13C{1H} NMR(75.5 MHz, Et 2O, 23 8C): d = 193.3 ppm
(d, 1JC-P = 16.4 Hz); 31P{1H} NMR(121.5 MHz, Et 2O, 23 8C): d =
111.3 ppm.
being P-coordinated in the starting complex 4 to C-coordi-
nated in the product 5. With a CP bond length of 1.573(2) Š, 5
displays the longest reported uncoordinated C–P triple bond,
possibly arising from the ability of ruthenium to backbond to
the p* orbitals of the CP fragment.
ꢀ
[RuH(dppe)2(Ph3SiC P)]OTf (4):
A 250-mL Schlenk flask
equipped with a stir bar was charged with [RuH(dppe)2]OTf[25]
(0.524 g, 0.500 mmol) and CH2Cl2 (80 mL). The silyl-substituted
phosphaalkyne 3 (1.0 mmol, 2 equiv) synthesized above was added
via cannula transfer, resulting in a color change from red to light
orange. After 1 h, the solution was concentrated to approximately
40 mL under reduced pressure which resulted in copious amounts of
white precipitate. The suspension was mixed for 2 h then allowed to
settle. The mother liquor was removed via cannula filtration, and the
faint yellow solid was washed with diethyl ether (3 ” 10 mL). Excess
solvent was removed under reduced pressure to furnish a white solid
(0.576 g, 84.5%). Analytically pure material was obtained through
crystallization from THF/toluene; m.p.: 1858C (dec.); 1H NMR
An intense band at 1229 cmꢁ1 seen in both the Raman and
IRspectra was assigned to the CP stretching frequency by
comparison with the calculated value of 1336 cmꢁ1 (DFT
B3LYP/6-31G* for P,C,H; LANL2DZ for Ru). The CP
stretching frequency for alkyl-substituted phosphaalkynes is
typically found at higher energy (> 1500 cmꢁ1), while Willner
reported a value of 1468 cmꢁ1 for [(CF3)3B(CP)]ꢁ.[11] In the
solid state, 5 was only moderately air-sensitive. Crystals of 5
were extremely kinetically insoluble in [D8]THF. Although 5
was more soluble in CD2Cl2, a slow reaction was observed that
yielded a product identified as [RuHCl(dppe)2] by NMR
spectroscopy.
In conclusion, we have reported a rational synthesis to
access the first “true” M-CP complex. Using the triphenyl-
silyl-substituted phosphaalkyne 3, which functioned with
Ph3Si as a protecting group for the CP anion, we were able
to effectively deliver cyaphide to ruthenium upon deprotec-
tion of the silyl group with phenoxide. We hope that our
(300 MHz, [D8]THF, 238C): d = ꢁ8.13 ppm (dqnt, trans-2JP-H
=
122.1 Hz, cis-2JP-H = 17.0 Hz, 1H); 13C{1H} NMR(75.5 MHz,
[D8]THF, 238C): d = 175.1 ppm (d, 1JC-P = 71.4 Hz); 19F NMR
(188 MHz, [D8]THF, 238C): d = ꢁ78.4 ppm; 31P{1H} NMR
(121.5 MHz, [D8]THF, 238C): d = 143.8 ppm (qnt, 2JP-P = 27.8 Hz,
2
1P), 60.1 ppm (d, JP-P = 27.3 Hz, 4P).
ꢀ
[RuH(dppe)2(C P)] (5): A 20-mL vial equipped with a stir bar
was charged with 4 (0.270 g, 0.20 mmol) and THF (18 mL). The solid
dissolved after stirring for 5 min. The solution was then cooled to
ꢁ308C, and NaOPh (0.032 g, 0.276 mmol) in THF (2 mL) was added.
The solution was warmed to 238C over 30 min. After mixing for 14 h,
the yellow-orange solution was filtered through a pad of silica gel,
eluting with THF (5 mL). After removal of the volatile materials, the
solid was washed with CH3CN (2 ” 10 mL) then redissolved in THF
(ca. 15 mL) and filtered through diatomaceous earth in a Pasteur
filter pipette. Excess solvent was removed under reduced pressure to
furnish a yellow solid (0.135 g, 71.8%). This product was > 95% pure
ꢁ ꢀ
method for synthesizing Ph3Si C P will be generally appli-
cable and potentially lead to novel M-CP-M complexes.
Angew. Chem. Int. Ed. 2006, 45, 6159 –6162
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6161

Communications
by NMRspectroscopy. Crystallization from THF/hexane furnished
contribution of the hydrogen atoms, in their calculated positions,
was included in the refinement using a riding model. The
dichloromethane molecule in the crystal lattice is slightly
disordered. Structural data for 5: Pale yellow, air-sensitive
single crystals were obtained from a THF/hexane solution of 5 at
room temperature; C53H49P5Ru; monoclinic; space group P2(1)/
n; a = 10.8585(6), b = 24.019(1), c = 17.5981(9) Š; V=
4481.3(4) Š3; Z = 4; 1calcd = 1.396 Mgmꢁ3; crystal dimensions
0.30 ” 0.26 ” 0.26 mm3; Bruker SMART Apex diffractometer
with CCD area detector; MoKa radiation (0.71073 Š), 200 K,
analytically pure material, which was also suitable for X-ray analysis
(0.60 g, 31.9%); m.p. > 1988C (decomp); 1H NMR(300 MHz,
[D8]THF, 238C): d = ꢁ11.22 ppm (dqnt, cis-2JH-P = 20.7 Hz, trans-3JH-
P = 20.4 Hz, 1H); 13C{1H} NMR(75.5 MHz, [D 8]THF, 238C): d =
287.1 ppm (m); 31P{1H} NMR(121.5 MHz, [D 8]THF, 238C): d =
165.0 (br, 1P), 65.2 ppm (br, 4P); Raman: n˜ = 1885 (w, Ru-H), 1228
ꢀ
(vs, C P).
Received: June 21, 2006
Published online: August 28, 2006
2Vmax = 56.688; 33735 reflections, 11153 independent (Rint
=
0.0258); direct methods; refinement against full matrix (versus
F2) with SHELXTL (v6.12) and SHELXL-97; 536 parameters, 1
restraint (Ru-H), R1 = 0.0323 and wR2(all data) = 0.0888, max/
min residual electron density 0.862/ꢁ0.251 eŠꢁ3. All non-hydro-
gen atoms were refined anisotropically. The contribution of the
hydrogen atoms, in their calculated positions, was included in the
refinement using a riding model. CCDC 611173 (4) and 611174
(5) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Keywords: cyaphides · phosphaalkynes · P ligands · ruthenium ·
silanes
.
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Int. Ed. Engl. 1989, 28, 53.
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references therein.
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1980, 82, 185.
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[22] For related complexes with silylalkynyl ligands, see: a) K.
Venkatesan, T. Fox, H. W. Schmalle, H. Berke, Organometallics
2005, 24, 2834; b) B. Siggelkow, M. B. Meder, C. H. Galka, L. H.
Gade, Eur. J. Inorg. Chem. 2004, 3424.
[23] An intermediate species was observed by 31P NMRspectroscopy
at d = + 310 ppm suggesting reversible attack at P before
irreversible attack at Si (J. G. Cordaro, D. Stein, A. Ehlers, H.
Grützmacher, unpublished results).
[24] J. J. Eisch, C. C. S. Chiu, Heteroat. Chem. 1994, 5, 265.
[25] [RuH(dppe)2]OTf was made through anion metathesis of
[RuH(dppe)2]BF4 in THF using KOTf (1.2 mol equiv) followed
by filtration through Celite to remove the precipitated KBF4; for
[RuH(dppe)2]BF4, see: S. P. Nolan, T. R. Belderrain, R. H.
Grubbs, Organometallics 1997, 16, 5569.
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Allg. Chem. 1995, 621, 34.
[13] G. Becker, K. Hübler, Z. Anorg. Allg. Chem. 1994, 620, 405.
[14] U. Fleischer, H. Grützmacher, U. Krüger, J. Chem. Soc. Chem.
Commun. 1991, 302.
[15] G. Becker, G. Gresser, W. Uhl, Z. Naturforsch. B 1981, 36, 16.
[16] T. Allspach, M. Regitz, G. Becker, W. Becker, Synthesis 1986, 31.
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Synthesis 1998, 1305.
[18] G. Markl, H. Sejpka, Tetrahedron Lett. 1986, 27, 171.
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[20] M. Regitz, Chem. Rev. 1990, 90, 191.
[21] Structural data for 4: Colorless, air-sensitive single crystals of 4
used for X-ray analysis had the composition [RuH(dppe)2-
ꢀ
(Ph3SiC P)](BF4)0.94(Cl)0.06 and were obtained from the reaction
starting from [RuH(dppe)2]BF4 rather than the OTf salt and
recrystallized from CH2Cl2 layered with hexane at 238C;
C72H66B0.94Cl0.06F3.76P5RuSi; tetragonal; space group I4(1)/a;
a = 40.152(2), b = 40.152(2), c = 16.203(1) Š; V= 26121(3) Š3;
Z = 16; 1calcd = 1.393 Mgmꢁ3; crystal dimensions 0.46 ” 0.25 ”
0.14 mm3; Bruker SMART Apex diffractometer with CCD
area detector; MoKa radiation (0.71073 Š), 150 K, 2Vmax
=
56.668; 95351 reflections, 16245 independent (Rint = 0.0607);
direct methods; empirical absorption correction SADABS
(v2.03); refinement against full matrix (versus F2) with
SHELXTL (v6.12) and SHELXL-97; 787 parameters,
3
restraints (BF4/Cl, Ru-H), R1 = 0.0487 and wR2(all data) =
ꢁ3
0.1321, max/min residual electron density: 1.140/ꢁ1.329 eŠ
.
All non-hydrogen atoms were refined anisotropically. The
6162
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Angew. Chem. Int. Ed. 2006, 45, 6159 –6162