A bis(thiophosphinoyl)methanediide palladium complex: Coordinated dianion or nucleophilic carbene complex?

Article abstract of DOI:10.1002/anie.200461392

(Chemical equation presented). A question of identity: A palladium complex featuring a carbenic S,C,S-based pincer ligand is reported (see scheme). DFT calculations show that, unlike classical carbene complexes, coordination occurs through donation from a d orbital at the metal into the empty n_p orbital of the carbene fragment, with the n_σ orbital essentially acting as a nonbonding orbital. The complex is shown to have nucleophilic character.

Full text of DOI:10.1002/anie.200461392

Communications
Carbene Complexes
sensitive towards moisture gives rise to a singlet in the
31P NMR spectrum at d = 20.6 ppm (121.5 MHz,toluene,
258C,85 % H 3PO4 as external standard) and was used without
purification for further reactions. The reaction of 2 with
[(PPh3)2PdCl2] yielded a burgundy red solution and a red
precipitate. The 31P NMR spectrum of the new species is
highly diagnostic. Indeed the new complex 3 gives rise to an
AX2 spin system (d(A) = 21.5 ppm; d(X2) = + 39.8 ppm,
CD2Cl2,298 K) revealing a symmetrical structure featuring
three phosphorus atoms (Scheme 1).
A Bis(thiophosphinoyl)methanediide Palladium
Complex: Coordinated Dianion or Nucleophilic
Carbene Complex?**
Thibault Cantat, Nicolas MØzailles, Louis Ricard,
Yves Jean,* and Pascal Le Floch*
Transition-metal carbene complexes are one of the most
widely studied organometallic species because of their high
activity in numerous catalytic processes.[1] Most efforts were
recently focused on the synthesis of isolable free carbene
ligands.[2] However,new and rapid synthetic strategies allow-
ing the generation of new carbenes in the coordination sphere
of metals from easily available and cheap precursors have yet
to be developed. In this perspective,phosphorus-substituted
methane derivatives have attracted the attention of many
groups as potential precursors.[3,4] Besides the particular
electronic properties of phosphorus,which markedly differ
from that of their nitrogen counterparts,the interest of such
derivatives also resides in the possibility of using phosphorus
as an anchor for other heteroatoms through oxidation of
its lone pair. This approach was elegantly demonstrated
by Cavell and co-workers who successfully developed
the chemistry of bis(iminophosphorano)methanediide com-
plexes.[5]
As part of a program aimed at exploring the use of
thiophosphinoyl ligands in coordination chemistry and catal-
ysis[6] we recently explored the possibility of generating
analogous carbene complexes from the bis(diphenylthiophos-
phinoyl)methane ligand. This work was also motivated by the
tendency of thiophosphinoyl ligands to favor coordination of
electron-rich metal centers. Furthermore,sulfur ligands were
rarely employed as ancillary ligands in carbene chemistry.[7]
Herein,we report a new type of pincer ligand featuring a
formal “carbenic” atom and two ancillary sulfide ligands.
The new dianion 2 was readily synthesized in quantitative
yield through a double deprotonation of the methylene group
in 1 using MeLi in toluene at low temperature (Scheme 1)
following a similar procedure to that used for the deproto-
nation of bis(diphenyl-N-trimethylsilylphosphinimino)-meth-
ane.[8] Formation of 2 was evidenced by vigorous evolution of
methane at room temperature and the formation,over two
hours,of a yellow turbid mixture. Dianion 2 which is highly
Scheme 1.
After work-up aimed at eliminating the LiCl salt formed
and the free PPh3 ligand,complex 3 was fully characterized by
NMR spectroscopic techniques (1H and 13C) and elemental
analyses. Though no 13C NMR signal could be recorded for
the carbenic carbon atom (as was reported by Cavell and co-
workers for some carbene complexes[5g,h]),the absence of
methylenic protons was confirmed by both H and 13C NMR
1
spectroscopy. Fortunately,X-ray-quality red crystals of 3 were
obtained either from a concentrated CH2Cl2 solution or by
diffusion of hexanes into a CH2Cl2 solution of the complex
(Figure 1).[9] Complex 3 was found to be remarkably resistant
to moisture and it crystallizes with a molecule of CH2Cl2 a
situation which already points towards a weak nucleophilic
character. The shortest distances between the solvent and the
C1 and Pd atoms are 5.0 and 5.9 Š,respectively.
[*] T. Cantat, Dr. N. MØzailles, Dr. L. Ricard, Prof. Y. Jean,
Prof. P. Le Floch
Laboratoire HØtØroØlØments et Coordination
UMR CNRS 7653 (DCPH)
DØpartement de Chimie
Ecole Polytechnique
91128 Palaiseau cedex (France)
Fax: (+33)1-6933-3990
E-mail: yves.jean@poly.polytechnique.fr
lefloch@poly.polytechnique.fr
Figure 1. View of one molecule of 3 (thermal ellipsoids set at 50%
probability). The hydrogen atoms and cocrystallized CH2Cl2 are
omitted for clarity. Selected bond lengths [Š] and angles [8]: Pd1-C1
2.113(2), C1-P1 1.689(2), C1-P2 1.690(2), Pd1-S1 2.3741(6), Pd1-S2
2.3677(6), Pd1-P3 2.3030(6), P1-S1 2.0451(7), P2-S2 2.0424(8);
P1-C1-P2 139.2(1), C1-Pd1-S1 80.63(6), S1-Pd1-P3 96.45(2), P3-Pd1-S2
103.78(2), S2-Pd1-C1 80.91(6), C1-Pd1-P3 174.61(6), S1-Pd1-S2
158.42(2).
[**] This work was supported by the CNRS and the Ecole Polytechnique.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461392
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Angewandte
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The structure of 3 definitely establishes the presence of a
central tricoordinate carbon atom that is bound to the Pd
center and the two PPh2S groups. However,the relatively long
structure is to consider that the complex results from the
interaction between a d10 PdL3 fragment and a neutral
carbene (Figure 3): an occupied MO of the metal fragment
interacts with the vacant np orbital,leading to a nearly
perpendicular orientation of the carbene ligand (distorted
“open book” conformation).[5e] A four-electron interaction
develops between the ns and dxz orbitals.
ꢀ
C1 Pd separation (2.113(2) Š) rules out the formation of a
true double bond (2.005 Š).[7] This bond is shorter than a
ꢀ
classical single Pd C bond (about 2.15 Š). Moreover,even
though the overall geometry around the palladium center is
ꢀ
square planar,the angle between the Pd C1 bond and the
plane defined by the “carbene moiety” measures 102.08. Thus
in 3,the metal center seems to be located in a quasiperpen-
dicular plane to the carbene fragment. Similar distortions,but
to a lesser extent (angles varying between 0 and 37.78),were
also observed in some of Cavellꢀs complexes.[5e] A striking
ꢀ
feature is the very short P C1 bonds of 1.689(2) and 1.690(2),
which are similar to bond lengths found in carbodiphosphor-
anes and their complexes.[10] These short bonds very likely
result from negative hyperconjugation from carbon to
phosphorus s* orbitals. How can this geometry be rational-
ized? ONIOM calculations were carried out on the real
system using the Gaussian03 set of programs[11] (all phenyl
groups were calculated at the MM level and the other atoms
at the MQ level of theory).[12] Theoretical data are very
similar to those given by X-ray crystallography with the
Figure 3. A) Usual bonding mode in carbene complexes, B) electronic
situation in 3 assuming a d10 PdL3 fragment.
ꢀ
exception of the P S bond lengths which were found to be
slightly longer.
Or,one can assume the two electrons of the s bond to be
associated to the carbenic ligand as is usual for electron
counting in transition-metal complexes. Then,the whole
complex can be described as a d8 [PdL3]2+ metal fragment
interacting with a dianionic carbenic center. A further
interpretation is found by using the isolobal analogy. The T-
The electronic structure of 3 can be rationalized by
looking at the molecular orbital (MOs) which involve the np
and ns nonbonding orbitals on the carbenic center.[13] Owing
to the almost perpendicular orientation of the carbene ligand,
the sigma bonding MO results from the interaction between
the np orbital and a metal-centered orbital directed along the
Pd C axis. This doubly occupied MO characterizes a s Pd C
bond since its antibonding counterpart (the LUMO of the
complex) is vacant (Figure 2,right). The n orbital interacts
shape d10 PdL3 fragment is isolobal to CH3 , so that complex 3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
is analogous to the CH3 CH2 ion (Figure 4) with a single
ꢀ
C C bond and a lone pair on the pyramidalized methylene
moiety.[14]
s
Figure 4. By the isolobal analogy complex 3 is analogous to the
ꢀ
ꢀ
CH3 CH2 ion.
The nucleophilic character of the carbenic atom was
confirmed by NBO calculations (qC = ꢀ1.39 and qPd = + 0.37).
To demonstrate the presence of a reactive lone pair on this
“carbenic” carbon atom,complex 3 was treated with various
electrophiles. For example,the reaction with MeI,which was
carried out in CH2Cl2 at room temperature,exclusively takes
place on the carbon atom. The cationic complex 4 formed was
isolated in a quantitative yield (Scheme 2).[15]
This new complex was fully characterized by NMR
spectroscopy,elemental analysis,and X-ray crystallography.
Orange crystals of 4 were grown by a slow diffusion of
hexanes into a dichloromethane solution of the complex (see
Supporting Information).[9] Interestingly,we note that the
Figure 2. HOMO and LUMO of the theoretical structure of 3 as given
by DFT calculations.
with the dxz orbital through a p-type overlap. Note that both
the bonding and the antibonding combinations are occupied,
the latter being the HOMO of the complex (Figure 2,left).
Therefore the p interaction does not lead to any p bonding
character between the Pd and C1 centers. The bonding picture
which emerges from this six-electron four-orbital analysis is
that there is: 1) a single (s) metal–carbon bond involving the
np orbital on the carbenic center and 2) two nonbonding
electron pairs on Pd and C1,the electron pair on the carbenic
center is located approximately in the plane of the carbene (ns
orbital). A way to understand this particular electronic
ꢀ
C Pd bond in 4 (2.146(2) Š) is only slightly elongated with
regard to 3 thus confirming the pronounced single-bond
ꢀ
character of the C Pd bond in 3 (2.113(2) Š). Theoretical
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6383

Communications
Keywords: carbenes · density functional calculations ·
.
palladium · phosphorus · sulfides
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Scheme 2.
[2] D. Bourissou,O. Guerret,F. P. Gabbai,G. Bertrand, Chem. Rev.
2000, 100,39 – 91.
calculations using the ONIOM method (same level of theory
[3] See for example: a) J. Ruiz,M. E. G. Mosquera,G. Garcia,E.
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Le Floch, Chem. Commun. 2004,1274 – 1275.
than in 3) yielded a structure which is very close to that
experimentally observed. NBO calculations indicate that the
palladium-bound carbon atom still bears a substantial neg-
ative charge (qC = ꢀ1.02 and qPd = + 0.36).
[4] Phosphoniumphosphanylcarbene ions [(R2P)(R2PH)CD]+ exist as
free species,see: M. Soleilhavoup,A. Baceiredo,O. Treutler,R.
Ahlrichs,M. Nieger,G. Bertrand, J. Am. Chem. Soc. 1992, 114,
10959 – 10961.
In conclusion,we have developed an easy synthetic access
to a new type of nucleophilic “carbene” complex featuring
two pendant thiophosphinoyl ancillary ligands. Theoretical
calculations indicate that these new species can be either
regarded as a coordinated dianion with a strong delocaliza-
[5] a) R. G. Cavell,R. P. Kamalesh Babu,A. Kasani,R. McDonald,
J. Am. Chem. Soc. 1999, 121,5805 – 5806; b) R. P. Kamalesh
Babu,R. McDonald,S. A. Drecker,M. Klobukowski,R. G.
Cavell, Organometallics 1999, 18,4226 – 4229; c) R. P. Kama-
lesh Babu,R. McDonald,R. G. Cavell, Organometallics 2000,
ꢀ
tion of the charge onto the two P C bonds,or as a
nucleophilic carbene complex.
19,3462 – 3465; d) K. Aparana,M. Ferguson,R. G. Cavell,
Am. Chem. Soc. 2000, 122,726 – 727; e) R. G. Cavell,R.
J.
Experimental Section
Kamalesh Babu,K. Aparana, J. Organomet. Chem. 2001, 617–
618,158 – 169; f) K. Aparana,R. P. Kamalesh Babu,R. McDo-
nald,R. G. Cavell, Angew. Chem. 2001, 113,4535 – 4537; Angew.
Chem. Int. Ed. 2001, 40,4400 – 4402; g) N. D. Jones,G. Lin,R. A.
Gossage,R. McDonald,R. C. Cavell, Organometallics, 2003, 22,
2832 – 2841; h) G. Lin,N. D. Jones,A. Gossage,R. McDonald,
R. G. Cavell, Angew. Chem. 2003, 115,4188 – 4191; Angew.
Chem. Int. Ed. 2003, 42,4054 – 4057.
All experiments were carried out under dry argon or nitrogen
atmosphere using distilled and degassed solvents.
3: Two equivalents of MeLi (0.84 mL,1.6 m in diethyl ether,
1.34 mmol) were added to a solution of 1 (300 mg,0.67 mmol) in
toluene (5 mL) at ꢀ788C. The mixture was warmed to room
temperature and stirred for 2 h leading to the formation of a yellow
suspension of 2. Then [PdCl2(PPh3)2] (470 mg,0.67 mmol) was added
in one portion at room temperature. The resulting mixture immedi-
ately turned red and 31P NMR spectroscopy showed the reaction to be
complete by indicating the presence of complex 3 and PPh3. Complex
3 and LiCl are poorly soluble in toluene and were isolated by
centrifugation. Pure complex 3 was finally obtained in 88% yield
[6] a) M. Doux,C. Bouet,N. MØzailles,L. Ricard,P. Le Floch,
Organometallics 2002, 21,2785 – 2788; b) M. Doux,N. MØzailles,
M. Melaïmi,L. Ricard,P. Le Floch,
Chem. Commun. 2002,
1566 – 1567; c) M. Doux,N. MØzailles,L. Ricard,P. Le Floch,
Eur. J. Inorg. Chem. 2003,3878 – 3894; d) M. Doux,N. MØzailles,
L. Ricard,P. Le Floch, Organometallics 2003, 22,4624 – 4626;
e) M. Doux,L. Ricard,F. Mathey,P. Le Floch,N. MØzailles, Eur.
J. Inorg. Chem. 2003,687 – 698.
(480 mg,0.59 mmol) after dissolution in CH Cl2 (6 mL) followed by
2
filtration to remove LiCl and evaporation of the solvent. Selected
data: 1H NMR (300 MHz,CD 2Cl2,25 8C): d = 7.13–7.66 ppm (m,
35H; H of phenyl); 31P{1H} NMR (121.5 MHz,CD Cl2,25 8C,85%
2
[7] N. Matsumura,J.-I. Kawano,N. Fukunishi,H. Inoue,
J. Am.
H3PO4 as external standard): d = 21.5 (t, 3J(P,P) = 14.6 Hz; PPh3),
Chem. Soc. 1995, 117,3623.
[8] a) A. Kasani,R. P. Kamalesh Babu,R. McDonald,R. G. Cavell,
3
39.8 ppm (d, J(P,P) = 14.6 Hz; PPh2S); 13C{1H} NMR (75.465 MHz,
CD2Cl2,25 8C,CD 2Cl2 d = 53.73 ppm as internal reference): d =
125.4–139.1 ppm (m; C of phenyl),C-Pd not observed; elemental
analysis (%) calcd for C43H35P3PdS2: C 63.35,H 4.33,found: C 62.97,
H 4.05.
Angew. Chem. 1999, 111,1580 – 1582; Angew. Chem. Int. Ed.
1999, 38,1483 – 1484; b) C. M. Ong,D. W. Stephan,
Chem. Soc. 1999, 121,2939 – 2940.
J. Am.
[9] Crystal data for 3 (C43H35P3PdS2·1/2(CH2Cl2})): space group
P21/c, a = 9.0270(10), b = 36.1930(10), c = 12.7440(10) Š, b =
4: Complex 4 was obtained by adding MeI (27 mL,0.43 mmol) to
a solution of 3 (350 mg,0.43 mmol) in CH 2Cl2 (10 mL) at room
temperature. The mixture was stirred for five minutes and then taken
to dryness. Complex 4 was thus isolated in a quantitative yield (100%,
410 mg,0.43 mmol). Selected data: 1H NMR (300 MHz,CD 2Cl2,
258C): d = 1.86 (dt, 3J(H,P) = 16.2 Hz, 4J(H,P) = 8.6 Hz,3H; CH 3),
7.00–8.04 ppm (m,35H; H of phenyl); 31P{1H} NMR (121.5 MHz,
CD2Cl2,25 8C,85% H 3PO4 as external standard): d = 22.8 (t, 3J(P,P) =
15.8 Hz; PPh3),62.3 ppm (d, 3J(P,P) = 15.8 Hz; PPh2S); 13C{1H} NMR
(75.465 MHz,CD 2Cl2,25 8C,CD 2Cl2 d = 53.73 ppm as internal
reference): d = 20.7 (pq, J(C,P) = 5.0 Hz; CH3),125.6 (dt, 1J(C,P) =
109.3620(10)8, V= 3928.2(5) Š3, Z = 4, 1calcd = 1.450 gcmꢀ3
,
;
F(000) = 1748,Mo
radiation (l = 0.71073 Š), m = 0.800 cmꢀ1
Ka
crystal dimensions 0.20 ” 0.10 ” 0.08 mm. Data collection was
performed on a Nonius KappaCCD single crystal diffractometer
at T= 150 K. Crystal structure was solved with SIR97,[16] refine-
ment against F2 (SHELXL97[17]
) with anisotropic thermal
parameters for all non-hydrogen atoms,calculated hydrogen
positions with riding isotropic thermal parameters. hkl ranges:
ꢀ6 12; ꢀ46 50; ꢀ17 17,22119 reflections collected,11087 unique
(Rint = 0.0445),8922 data with I > 2s(I),444 parameters refined,
GOF(F2) = 1.013,final R indices (R1 = ꢀ j Fo j ꢀ j Fc j j /ꢀ j Fo j ,
2
48.0 Hz, J(C,P) = 8.7 Hz; C-Pd),127.7–135.4 ppm (m; C of phenyl);
wR2 = [ꢀw(F2oꢀF2c)2/ꢀ w(Fo2)2]1/2
, R1 = 0.0403,w R2 = 0.1161,
elemental analysis (%) calcd for C44H38IP3PdS2: C 55.21,H 4.00;
found: C 55.10,H 3.87.
max/min residual electron density 0.973(0.097)/ꢀ1.215(0.097)
eŠ3. Crystal data for 4 are given in the Supporting Information.
CCDC-245083 (3) and CCDC-245084 (4) contains the supple-
mentary crystallographic data for this paper. These data can be
Received: July 22,2004
Revised: August 27,2004
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Chemie
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12 Union Road,Cambridge CB21EZ,UK; fax: ( + 44)1223-336-
033; or deposit@ccdc.cam.ac.uk).
function of exponent 1.472[21] was used for the Pd atom. The
same combination of basis set was chosen for calculation of the
theoretical structure of 4 but the 6-31 + G* basis set was used the
H atoms of the methyl group.
[10] See for example: a) A. T. Vincent,P. J. Wheatley, J. Chem. Soc.
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[13] Molecular orbitals of 3 were calculated at the MQ (B3PW91)
level on the ONIOM structure using the 6-311 + G** basis set
for the carbenic atom,S,and P atoms and the 6-31G* for all
phenyl groups (C and H atoms). Natural bond order (NBO)
calculations were performed at the same level of theory for 3.[22]
For the theoretical structure of 4,the 6-31 + G* basis set was
used for calculating the H atoms of the methyl group.
[14] An X-ray crystal analysis of a pyramidalized carbanion has been
recently reported: F. Breher,J. Grunenberg,S. C. Lawrence,P.
Mountford,H. Rꢁegger, Angew. Chem. 2004, 116,2575 – 2578;
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Organometallics
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Knox,H. P. Hratchian,J. B. Cross,C. Adamo,J. Jaramillo,R.
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Voth,P. Salvador,J. J. Dannenberg,V. G. Zakrzewski,S.
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A. D. Rabuck,K. Raghavachari,J. B. Foresman,J. V. Ortiz,Q.
Cui,A. G. Baboul,S. Clifford,J. Cioslowski,B. B. Stefanov,G.
Liu,A. Liashenko,P. Piskorz,I. Komaromi,R. L. Martin,D. J.
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Challacombe,P. M. W. Gill,B. Johnson,W. Chen,M. W. Wong,
[15] Complexes with an H atom replacing the methyl group have
already been synthesized by trapping reactions of bis(imino-
phosphorano)methanide anions with transition-metals frag-
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Ormsby,P. J. Maddox,P. Brꢂs,M. Bochmann,
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[16] A. Altomare,M. C. Burla,M. Camalli,G. Cascarano,C.
Giacovazzo,A. Guagliardi,A. G. G. Moliterni,G. Polidori,R.
Spagna,SIR97,an integrated package of computer programs for
the solution and refinement of crystal structures using single
crystal data, 1999.
C. Gonzalez,J. A. Pople,Gaussian,Inc.,Pittsburgh,PA,
b) The authors wish to thank the IDRIS center (University
Paris-Sud XI Orsay,Institut du dØveloppement et des ressources
en Informatiques Scientifique) for computational facilities.
[12] Theoretical calculations were carried out using the ONIOM
(B3PW91[18]/UFF[19]) method implemented in the Gaussian03
package. All the phenyl groups were considered as the second
shell and were calculated using the UFF force field.[18] The
carbenic carbon atom,P,S,and Pd atoms were calculated using
the B3PW91 functional. A 6-31 + G* basis set was used for H
and the 6-311 + G** was used for the carbenic atom,S and P
atoms. The quasirelativistic small-core ECP basis set (441/2111/
31) developed by Hay–Wadt[20] augmented with a f polarization
2003;
[17] M. Sheldrick,SHELXL-97,Universitꢃt Gꢄttingen,Gꢄttingen,
Germany, 1998.
[18] a) A. D. J. Becke, J. Chem. Phys. 1993, 98,5648 – 5662; b) J. P.
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[19] A. K. RappØ,C. J. Casewitt,K. S. Colwell,W. A. Goddard,
W. M. Skiff, J. Am. Chem. Soc. 1992, 114,10024 – 10035.
[20] P. J. Hay,W. R. Wadt, J. Chem. Phys. 1985, 82,299 – 310.
[21] A. W. Ehlers,M. Bꢄhme,S. Dapprich,A. Gobbi,A. Hꢄllwarth,
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