Experimental and computational study of the bonding properties of mixed bis-ylides of phosphorus and sulfur

Article abstract of DOI:10.1021/ic900765x

The reactivity of the known ylide-sulfonium salt [Ph₃P=CHC(O) CH₂SMe2]Br 1 and the new ylide-sulfide [Ph₃P=CHC(O)CH ₂SMe] 2 toward Pd^II complexes has been studied. Compound 1 reacts with PdCl₂

Full text of DOI:10.1021/ic900765x

Inorg. Chem. 2009, 48, 6823–6834 6823
DOI: 10.1021/ic900765x
Experimental and Computational Study of the Bonding Properties of Mixed
Bis-Ylides of Phosphorus and Sulfur
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,†
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ꢀ
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Elena Serrano, Rafael Navarro, Tatiana Soler, Jorge J. Carbo,* Agustı Lledos, and Esteban P. Urriolabeitia*
†
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ꢀ
Departamento de Compuestos Organometalicos, Instituto de Ciencia de Materiales de Aragon, CSIC-
‡
ꢀ
ꢀ
Universidad de Zaragoza, E-50009 Zaragoza, Spain, Servicios Tecnicos de Investigacion, Facultad de Ciencias
Fase II, 03690 San Vicente de Raspeig, Alicante, Spain, Departament de Quımica Fısica i Inorganica,
§
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Universitat Rovira i Virgili, 43007 Tarragona, Spain, and Departament de Quımica, Edifici C.n., Universitat
ꢁ
Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain
Received April 21, 2009
The reactivity of the known ylide-sulfonium salt [Ph3PdCHC(O)CH2SMe2]Br 1 and the new ylide-sulfide [Ph3PdCHC-
(O)CH2SMe] 2 toward PdII complexes has been studied. Compound 1 reacts with PdCl2(NCMe)2 and NEt3 to give cis-
[PdCl2[Ph3PCHC(O)CHSMe2-κ-C,C]] 3, which is obtained selectively as the meso diastereoisomer (RS/SR). The
reactivity of 3 has been studied, and shows the stability of the bis-ylide unit. However, reflux in NCMe of [PdCl(PPh3)-
[Ph3PCHC(O)CHSMe2-κ-C,C]]ClO4 6 promotes orthopalladation and affords [PdCl(PPh3)(C6H4-2-PPh2CHC(O)-
CH2SMe2-κ-C,C)]ClO4 12, which is characterized by X-ray methods. Density functional theory (DFT) and Bader’s
Atoms in Molecules (AIM) studies on S-ylides, mixed P-S bis-ylides, and the corresponding Pd complexes have
been performed. Free S-ylides show strong conformational preferences, which lies with the establishment of a set of
cooperative intramolecular interactions of weak strength: the 1,4 S O interactions and the 1,6-C-H O H-bonds
between the protons of the methyl substituents and the carbonyl oxygen was fully characterized for the first time. For
3 3 3 3 3 3
free mixed P-S bis-ylides, an additional 1,4-P O intramolecular interaction of moderate strength was character-
3 3 3
ized. These interactions play a key role in determining the preferred conformations, which then are transferred to the
complexes, explaining the observed diastereoselectivity in complex 3. The ylide-sulfide 2 reacts with PdCl2(NCMe)2
and NEt3 affording [Pd(Cl)[Ph3PCHC(O)CHSMe]]2 9, which in turn reacts with PPh3 giving [Pd(Cl)(PPh3)[Ph3PCHC-
(O)CHSMe-κ-C,C]] 10. The X-ray structure of 10 shows the anion [Ph3PCHC(O)CHSMe]- acting as a C,C-chelate.
The bonding in 10 is produced with complete diastereoselectivity but, instead of the expected meso form, the d,l pair
(RR/SS) is formed. This inversion is observed for the first time.
Introduction
of them being comprehensively reviewed.1 The variety of
research lines covers from structural studies, such as the
different bonding modes of the ylides as ligands,2,3 or
particular reactivities such as their addition to multiple
bonds,4,5 to practical processes6-9 some of them of industrial
interest.7 Multidentate ylides are specially attractive because
of the additional stabilization provided by the chelating
effect, and different strategies have been developed: classical
C,X (X=heteroatom) chelates9,10 orthometalated species11
or multiylides12-14 are some approximations developed in
this area. Among these interesting species, the synthesis and
bonding properties of bis-ylides has been one of the most
studied topics in our group,11f-11h,12h,13b-13f because of two
noteworthy reactions shown in Figure 1. The first one is that,
in spite of the presence of two chiral centers in the free
bis-ylide, only one diastereoisomer (the meso form) is
obtained.11f-11h,12h,13b-13f The second one is the isomeriza-
tion of the chelating bis-ylides in the [Pd(Ph3PCHC(O)-
CHEZn)] units (EZn=PR3, NC5H5) into the orthometalated
ligands [Pd(C6H4-2-PPh2CHC(O)CH2EZn] through arylic
The behavior of the ylides as ligands toward transition
metals is a well established research area.1-14 During the last
40 years many different subjects have been developed, most
*To whom correspondence should be addressed. E-mail: j.carbo@urv.cat
(J.J.C.), esteban@unizar.es (E.P.U.). Fax: þ34976761187 (E.P.U.).
(1) For general reviews: (a) Johnson, A. W. In Ylides and Imines of
Phosphorus; John Wiley&Sons: New York, 1993; Chapter 14.(b) Kolodiazhnyi,
O. I. In Phosphorus Ylides; Wiley-VCH: Weinheim, Germany, 1999. (c)
Schmidbaur, H. Acc. Chem. Res. 1975, 8, 62. (d) Schmidbaur, H. Angew.
Chem., Int. Ed. Engl. 1983, 22, 907. (e) Weber, L. Angew. Chem., Int. Ed. Engl.
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Cristau, H. J. Chem. Rev. 1994, 94, 1299. (i) Belluco, U.; Michelin, R. A.;
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ꢀ
r
2009 American Chemical Society
Published on Web 06/17/2009
pubs.acs.org/IC

6824 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
Figure 2. Conformational preferences on P, As and N bis-ylides.
intramolecular interaction is the one mainly responsible for
the conformational preferences,13b-13e while in the case of
N-ylides a hydrogen bond of moderate strength plays the
same role.13d,13e We have reported very recently some exam-
ples of sulfur bis-ylides. Once again, we have observed the
meso form of the bonded bis-ylide, both in the chelating and
in the bridging modes.13f
This paper describes the coordinating properties of mixed
bis-ylides of phosphorus and sulfur of the type [Ph3PCHC-
(O)CHSMe2] and those of the ylide-sulfide [Ph3PdCHC-
(O)CH2SMe] which, in spite of their similarity, show a quite
different behavior. Moreover, the study of the ligands
Figure 1. Remarkable reactivity of bis-ylides.
CH bond activation followed by an intramolecular acid-
base reaction.11g,11h,13e
With respect to the diastereoselective bonding of the ylides,
we have shown that bis-ylides of phosphorus,13b,13c arsenic,
and nitrogen13d,13e show the same selectivity, since in all cases
the meso form is obtained (Figure 2). This is due to the
presence of strong conformational preferences in free ligands
favoring the cisoid form. However, although the experimen-
tal fact is the same (obtention of the meso form), the detailed
analysis of the two systems reveals subtle differences. The
study of these systems using density functional theory (DFT)
methods and Bader’s analysis of the charge density shows
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Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6825
Scheme 1. Synthesis of the Ylide-Sulfonium Salt 1 and the Ylide-
Sulfide 2
the resulting bis-ylide to the Pd center. The ammonium
salt remains in the alcoholic solution. Complex 3 was very
insoluble in the most common deuterated solvents; this
prevented its characterization in solution. Because of this
fact, it has been transformed into the more soluble
acetylacetonate derivative.22 One-pot reaction of 3 with
AgClO4 and Tlacac (1:1:1 molar ratio; acac=acetylace-
tonate) in CH2Cl2 affords 4, which could be isolated
from the insoluble AgCl and TlCl byproducts and prop-
erly characterized (Scheme 2). Attempts to isolate the
(11) For selected examples of orthometallated ylides, see: (a) Illingsworth
M. L.; Teagle, J. A.; Burmeister, J. L.; Fultz, W. C.; Rheingold, A. L.
[Me2SCHC(O)R], [H3PCHC(O)CHSMe2], and their corres-
ponding complexes by means of DFT methods and Bader’s
analysis of charge density allows the characterization of the
ultimate source of the conformational preferences observed,
providing further understanding of the chemical behavior of
bis-ylides.
ꢀ
Organometallics 1983, 2, 1364. (b) Vicente, J.; Chicote, M. T.; Fernandez-
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Results and Discussion
ꢀ
L. R.; Fernandez, S.; Larraz, C.; Llusar, R.; Navarro, R.; Urriolabeitia, E. P.
1. Synthesis of the Organic Precursors. The cationic
ylide-sulfonium salt [Ph3PdCHC(O)CH2SMe2]Br 1 has
been prepared previously,15 but no data concerning its
characterization have been given in the original report.
Our synthetic method involves the reaction of [Ph3PCHC-
(O)CH2Br]16,17 with an excess of SMe2 (1:40 molar ratio)
in MeOH as solvent (Scheme 1) at the reflux temperature.
The high polarity of the solvent allows an adequate
stabilization of the cationic species, which can be isolated
in good yield (74%). Interestingly, the ylide-sulfide
[Ph3PdCHC(O)CH2SMe] 2 can be prepared following
the same procedure but using dry tetrahydrofuran (THF)
instead of methanol, this pathway implying the elimina-
tion of MeBr during the reaction. The demethylation of
sulfonium salts to give the corresponding sulfides is a
known process.18-20 In our case this process is probably
promoted by the presence of the bromide anion acting as
a nucleophile.
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Compounds 1 and 2 show correct elemental analyses
and mass spectra, and 2 behaves as non-electrolyte in
acetone solution (ΛM=12.7 Ω-1cm2mol-1 for a solution
10-4 M).21 The IR spectra of 1 and 2 show strong bands at
about 1540 cm-1, in keeping with the presence of the
ylide-delocalized carbonyl group.1j The 1H NMR spectra
show the expected peaks; therefore, signals at 4.78 (CH2S),
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Mazany, A. M. Organometallics 1984, 3, 1310. ( j) Fackler, J. P., Jr.;
2
4.14 (CHP, JPH = 21.8 Hz), and 3.16 (SMe2) ppm are
observed for 1, while the corresponding peaks for 2 appear
at 3.17, 4.04 (2JPH = 23 Hz), and 2.13 ppm. The strong
shielding of the signals assigned to the CH2SMe unit with
respect to the CH2SMe2 moiety is in keeping with the
neutral character of the former. Signals observed in the 31P-
{1H} and 13C{1H} NMR spectra are also in good agree-
ment with the structure shown in Scheme 1. The reactivity
of 1 and 2 has given quite different results. Because of this
reason, they will be treated in different sections.
2. Reactivity of Ylide-Sulfonium Salt 1. Synthesis of
Bis-Ylide Complexes. The ylide-sulfonium salt 1 reacts
cleanly with NEt3 and PdCl2(NCMe)2 (1:1:1 molar ratio)
in MeOH to give the mononuclear derivative cis-
[PdCl2[Ph3PCHC(O)CHSMe2-κ-C,C]] 3, as represented
in Scheme 2. The reaction occurs with deprotonation of
the methylene adjacent to the sulfonium unit, formation
of the corresponding sulfur ylide, and C,C-chelation of
ꢀ
Trczinska-Bancroft, B. Organometallics 1985, 4, 1891. (k) Uson, R.; Laguna,
ꢀ
A.; Laguna, M.; Tarton, M. T.; Jones, P. G. J. Chem. Soc. Chem. Commun.
ꢀ
ꢀ
1988, 740. (l) Uson, R.; Laguna, A.; Laguna, M.; Jimenez, J.; Jones, P. G.
Angew. Chem., Int. Ed. Engl. 1991, 30, 198. (m) King, C.; Heinrich, D. D.;
Wang, J. C.; Fackler, J. P., Jr.; Garzon, G. J. Am. Chem. Soc. 1989, 111, 2300.
(15) Hercouet, A.; Le Corre, M. Tetrahedron Lett. 1976, 17, 825.
(16) Hudson, R. F.; Chopard, P. A. J. Org. Chem. 1963, 28, 2446.
(17) Hercouet, A.; Le Corre, M. Tetrahedron 1977, 33, 33–37.
~
(18) Giner, J.; Vinas, C.; Teixidor, F.; Hursthouse, M. B.; Light, M. E.
Dalton Trans. 2004, 2059.
(19) Nenaidenko, V. G.; Balenkova, E. S. Russ. J. Org. Chem. 2003, 39,
291.
(20) Clark, J. S. Nitrogen, Oxygen and Sulfur Ylide Chemistry; Oxford
University Press: New York, 2002.
(21) Geary, W. J. Coord. Chem. Rev. 1971, 7, 81.
ꢀ
(22) Fornies, J.; Martı
´
nez, F.; Navarro, R.; Urriolabeitia, E. P.
J. Organomet. Chem. 1995, 495, 185.

6826 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
Scheme 2. (i) PdCl2(NCMe)2, NEt3, MeOH, -[HNEt3]Br; (ii) AgClO4,
-AgCl, Tlacac, -TlCl; (iii) HCl, -Hacac; (iv) PPh3; (v) NCMe/Δa
obtained as a unique diastereoisomer. Definitive proof of
the stereochemistry of this complex is the presence of an
intense NOE at the signal at 4.32 ppm when the resonance
at 3.63 ppm is irradiated (and viceversa), implying that
both protons are at the same side of the molecular plane
and that the bis-ylide is coordinated in the meso form.
These results are highly relevant since they point to a
coordinating behavior of the sulfur ylides similar to that
described previously,13f and also similar to that reported
for N- and P-ylides:1j,1k in all cases the meso form is
obtained, and it seems logical to propose the existence of
conformational preferences in S-ylides, cooperative with
those coexisting in P-ylides.
Complex 5 showed a limited solubility in dmso-d6, but
it could be characterized in solution as the mixture of two
geometric isomers (pseudocis and pseudotrans) derived
from the relative arrangement of the ylidic fragments
SMe2 and PPh3. A pseudocis arrangement is produced
when identical fragments are on the same side with
respect to the Pd-Pd axis, while if they are on opposites
sides, we have a pseudotrans arrangement. This situation
is very similar to that observed in related complexes with
bis-ylides derived from phosphonium and pyridinium
salts.13e Complex 6 shows a set of spectroscopic data
consistent with the presence of two geometric isomers
(6.7/1 molar ratio), arising from the coordination of the
incoming PPh3 ligand trans to the CHPPh3 fragment or
trans to the CHSMe2 fragment. Since the CHSMe2 group
has a lower steric demand compared with the CHPPh3
moiety, it seems logical to assume that most of the PPh3
ligand is bonded cis to the former, as represented in
Scheme 2, this arrangement being less hindered and more
stable. This arrangement has already been observed in
mixed bis-ylides derived from pyridinium salts.13e There-
fore, the minor isomer would be the most hindered, in
which the PPh3 ligand is cis to the CHPPh3 unit. The
analysis of the NMR spectra of 6 gives proof of these
hypotheses. For instance, the signal due to the CHP
proton in the major isomer (1H NMR) appears as a
a Only the major isomer is shown for 6 and 12.
expected dinuclear halide bridging 5, as a result of the
reaction of 3 with AgClO4 (1:1 molar ratio), failed
because of the difficulties found in separating 5 from
the silver chloride. An alternative procedure to obtain the
pure complex 5 is the reaction of 4 with a methanolic
solution of HCl (1:1 molar ratio; Scheme 2). The reaction
occurs with protonation of the acac group, liberated as
acetylacetone, bonding of the chloride ligand in the
vacant coordination sites, and dimerization of the result-
ing fragments. Complex 5 shows the expected reactivity of
a dinuclear halide bridging and, therefore, it reacts with
PPh3 (1:2 molar ratio) giving the mononuclear derivative
6 (Scheme 2). All compounds show correct elemental
analyses and mass spectra.
3
virtual triplet (2JPH ≈ JPH) while that due to the CHS
The IR spectra of 3-6 show an intense absorption in
the range 1602-1630 cm-1. This band is shifted to
higher energies with respect to the ylide-sulfonium salt
1, and falls in the usual range found for other C,C-
chelates.12h,13d-13f Both facts agree with the coordina-
proton appears as a singlet. Moreover, the signal due to
the CHP carbon atom in the major isomer (13C NMR)
appears as a doublet of doublets with a large value of the
2
coupling constant JPC (65.8 Hz) while in the minor
isomer the observed value is only 9.7 Hz.
1
tion of the two C atoms. The H NMR spectrum of 4
In summary, the coordination of the bis-ylide
[Ph3PCHC(O)CHSMe2], derived from deprotonation of
1, to Pd(II) centers is produced in the meso form. This fact
implies that the bonding of stabilized sulfur ylides is also
governed by conformational preferences, similarly to
other reports dealing with P- and N-ylides. Moreover,
the observed behavior shows that the conformational
preferences are cooperative and not mutually exclusive.
3. Theoretical Analysis of Conformational Preferences
in Sulfur Ylides. We have previously studied the bonding
properties and conformational preferences of P-, As- and
N-ylides by DFT methods, and Bader’s Atoms in Mole-
cules (AIM) theory,23 showing that the selectivity ob-
served in their coordination modes can be related to the
reveals a static situation in the NMR time scale, which
does not change with the temperature, and shows a
single set of signals which can be assigned to the presence
of the acac ligand and to the [Ph3PCHC(O)CHSMe2]
ligand. The C,C-bonding of the bis-ylide can be inferred
from the hybridization change Csp2-Csp3 for the P ylidic
moiety,1j,1k which in turn implies: (i) the decrease of the
2JPH value observed in the signal at 3.63 ppm (4.8 Hz),
compared with the homologous in 1 (21.8 Hz); (ii) the low
field shift of the phosphorus signal (24.75 ppm) with
respect to 1 (15.46 ppm); (iii) the high field shift of the
resonances due to the ylidic C atoms (26.22 ppm, CHP;
43.28 ppm, CHS) with respect to their corresponding
values in 1. Moreover, the diastereotopic character of the
methyl groups of the SMe2 unit is in good agreement with
the presence of the CHS unit. These facts, together with the
observation of a single set of signals, means that 4 is
(23) (a) Bader, R. F. W. Atoms in Molecules: A Quantum Theory;
Clarendon Press: Oxford, U.K., 1990. (b) Bader, R. F. W. Chem. Rev.
1992, 92, 893.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6827
the dihedral angles CSCRCβ are 52.1° and -52.7°,
showing the two methyl substituents of sulfur in a
nearly symmetric arrangement with respect to the ylidic
plane formed by the S-CR-Cβ-O atoms. Both methyl
substituents point toward the carbonyl oxygen, whereas
the sulfur lone pair should be located opposite to the
carbonyl group (see Figure 4). This structure is the most
frequently encountered among the 53 X-ray structures
mentioned before, regardless if they have one or two
stabilizing substituents. Regarding the possible intra-
molecular interactions, relevant parameters of 13csym
are the S
O distance, 3.029 A, shorter than the sum of
O
3 3 3
the van der Waals radii (3.32 A)28 and the CH
3 3 3
Figure 3. Different rotations on Me2SCHC(O)Me species 13.
distances, 2.371 A and 2.358 A, typical of weak hydro-
gen bonds.29 The rotation of about 120° around S-CR
bond from structure 13csym led to a new minimum
13casym, which reflects the arrangement of the SMe2
group in the X-ray structures of the previously
reported coordinated sulfur bis-ylide13f and of 12 (see
below). In the new rotamer, one of the methyl groups
is in anti disposition with respect to the carbonyl
establishment of different intramolecular interactions.13
Thus, we have characterized 1,4-E O electrostatic type
interactions between the carbonyl oxygen and the E hetero-
3 3 3
atoms P and As; and 1,6-C-H OdC hydrogen bond
3 3 3
interactions in pyridinium ylides. Prompted by these pre-
vious results, we have studied the free S-ylide type ligands
and their coordinated species using the same methodo-
logy.24 Before performing the DFT calculations, we first
conducted a detailed search in the Cambridge Crystal-
lographic Data Center.25 The search lead 53 entries for a
general structure [R2SC(R0)C(O)R00], most of which are
doubly stabilized ylides; that is, compounds having two
keto and/or ester stabilizing substituents [R2SC(C(O)R0)-
C(O)R00] (R0 and/or R00 = alkoxy). In all of these struc-
tures26 the C(O)R0 or C(O)R00 groups are almost coplanar;
and at least one of the oxygens points to the S atom, being
the dihedral angle SCRCβO closeto0°. Thismeansthatthe
free sulfur ylides show a strong preference for the cisoid
conformation which is maintained even in the correspond-
ing available coordination complexes.27
group (Figure 4), showing the shortest S
O distance
3 3 3
(2.791 A) among all the characterized isomers. Remark-
ably, the oxygen points to the sulfur atom following the
approximation line of a nucleophile to a sulfonium
cation; and the resulting arrangement may be consid-
ered as an incipient intramolecular nucleophilic attack
of the oxygen to the sulfonium group.30 However,
although the S
O distance in 13casym is shorter than
3 3 3
in 13csym, the energy of the former is 6.5 kcal mol-1
higher. The computed energy barrier associated with
the S-CR rotation to transform 13csym into 13casym
amounts to 12.5 kcal mol-1. Finally for rotation around
the S-CR bond, we found an additional minimum
13canti, which corresponds to the eclipsed arrangement
of the CdO bond and the lone pair at the S atom
(Figure 4). This structure shows a relatively long
Initially, we considered the simple model sulfur ylide
Me2SCHC(O)Me (13) to analyze in detail the possible
conformational preferences and intramolecular interactions
of free ligands. Moreover, because of the characteristics of
the SMe2 group, we have analyzed the conformations
resulting from two different rotations: (i) rotation around
S-CR bond within the observed cisoid conformation (SP1,
see Figure 3), and (ii) rotation around the CR-Cβ bond
to interconvert the cisoid and transoid forms (SP2, see
Figure 3). The relaxed energy scan of the rotations around
S-CR and CR-Cβ bonds can be found in the Supporting
Information section.
First, the rotation of the SMe2 group around the S-
CR bond for compound 13 led to the localization of
three different cisoid-type minima 13csym, 13casym,
and 13canti, their relative energies being 0.0, þ6.5,
and þ6.5 kcal mol-1, respectively. Figure 4 shows their
geometrical structures. In the global minimum 13csym
S
O distance (2.956 A), which could be explained
3 3 3
by the mutual repulsion of the lone pairs at S and O
atoms. Interestingly, on going from 13casym to 13canti
the relaxed scan of the S-CR bond rotation led to a very
flat potential energy curve, being the two structures
degenerate in energy. To explain the energy invariabil-
ity upon rotation, the increase of the repulsion between
the lone pairs of sulfur and oxygen atoms in 13canti
should be balanced by the release of the steric strain in
13casym (see bellow). The value of 12.5 kcal mol-1 for
the rotation barrier indicates that all rotamers could
be accessible under experimental conditions through
rotation around the S-CR bond.
Second, we have analyzed the rotation around CR-Cβ
bond leading to transoid isomers. Assuming accessible
rotation around S-CR bond, one should analyze CR-Cβ
bond rotation from all possible cisoid species. However,
we have not found significant differences in geometries
and energies for the different paths, and we will base our
discussion on the lowest energy species. For S-ylide 13,
calculations show that the cisoid conformation is 9.2 kcal
mol-1 lower in energy than the transoid one, and that the
(24) Calculations were carried out with the Gaussian03 series of programs
using the B3LYP-functional. The basis set for Pd atom was LANL2DZ. For
the atoms C, O, P, S, and Cl we used the 6-31G(d) basis set, for the H rest of
atoms the 6-31G basis was used. The inclusion of diffuse functions and
polarization functions at the H atoms (basis set 6-31 gþþ(d,p)) was tested
and no significant changes in relative energies were observed. See Supporting
Information for further details.
(25) Allen, F. H. Acta Crystallogr. Sect. B 2002, B58, 380.
(26) Selected references (20) of the CCDC search can be found as
Supporting Information
(27) Matsubayashi, G.; Kawafune, I.; Tanaka, T.; Nishigaki, S.; Nakatsu,
K. J. Organomet. Chem. 1980, 187, 113.
(28) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(29) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48.
(30) Briton, D.; Dunitz, J. D. Helv. Chim. Acta 1980, 63, 1068.

6828 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
Figure 4. Relevant computed minimum structures for Me2SCHC(O)Me 13 in cisoid conformation.
rotational barrier is relatively high (26.2 kcal mol-1).
Table 1. Topological Properties at the Bond Critical Points of C-H
O
3 3 3
Intramolecular Interactions of the S-Ylide 13; N-Ylide [H5C5N-C(H)C(O)-
These values agree with the strong cisoid conformational
preference found experimentally for ketostabilized sulfur
ylides. The results show very similar trends to those
observed for model P and N ylides,13b,13d and they could
be explained in an analogous way: the cisoid form is
favored because of the interaction of the ketonic oxygen
with the SMe2 fragment, while the high rotational barrier
reflects the loss of resonance and of intramolecular inter-
action at the TS.
Once we have computed the energies of the different
isomers, the nature of the eventual intramolecular inter-
actions and their influence on the observed conforma-
tional preferences remains to be clarified. To asses these
questions, we have applied the Bader’s atoms in molecules
(AIM) theory.24 For the lowest energy conformation
13csym, we confirmed the existence of two intramolecular
CH3]; and MDAE.a
2
d
d
d
e
d(A) F(r)c r F(r)c ΔrH ΔrA ΔrH þΔrA Δgeoþelþlap
N-ylideb 1.957 0.030 0.099 0.463 0.298
13csym 2.358 0.014 0.047 0.248 0.108
2.371 0.013 0.045 0.242 0.098
0.761
0.356
0.340
0.385
0.106
0.073
0.077
0.062
MDAEb 2.337 0.014 0.048 0.248 0.137
a 1-Methoxy-2-(dimethylamino)ethane, MDAE. b Values taken from
2
ref 11. c Electron charge density, F(r), and Laplacian, r F(r), in au.
d Penetration of the hydrogen atom, ΔrH, penetration of the acceptor
atom ΔrA, mutual penetration ΔrH þ ΔrA, in A. e Modified Grabowski’s
complex parameter Δgeoþelþlap (ref 35).
atom (ΔrH > ΔrA) (see Table 1).34 Thus, the 1,6-C-
H
ambiguously assigned to true hydrogen bonds.
O interactions characterized for S-ylides can be un-
3 3 3
To estimate the strength of the intramolecular hydrogen
bonds, we have used a procedure based also on AIM
properties. Grabowski has proposed a complex parameter
(Δgeoþelþlap) based on structural data and topological para-
meters for describing hydrogen bond strength.35 We have
already used the Grabowski’s parameter for evaluating
intramolecular hydrogen bond strength in pyridinium yli-
des.13d Here, we compare the computed Δgeoþelþlap values
for S-ylides with those of previous study (Table 1), assuming
that AIM results are system independent.24 For N-ylide
[H5C5N-C(H)-C(O)-CH3] and the reference compound
1-methoxy-2-(dimethylamino)ethane (MDAE)36 the calcu-
lated Δgeoþelþlap parameters were 0.106 and 0.062, respec-
tively, whereas the estimated intramolecular interaction
energies were 10 and 2 kcal mol-1, respectively. In the case
of S-ylide 13csym, the Δgeoþelþlap values (0.073 and 0.077)
are closer to the MDAE values than to the N-ylide ones,
i.e., closer to weak than to moderate hydrogen bonding.
However, for 13csym, Δgeoþelþlap values are somewhat
larger than for MDAE, suggesting that the intramolecular
interaction could be also somewhat stronger, about 3-
4 kcal mol-1 for each hydrogen bond. Obviously, this is a
rough approximation, but is still meaningful for our pur-
poses. Thus, most of the energy difference between the cisoid
1,6-C-H O interactions by the topological analysis of
3 3 3
the electron density. These interactions were characterized
by the corresponding bond critical points (bcp’s) in the
electronic charge density F(r). On the other hand, no bcp
was found between the negatively charged carbonylic
oxygen and the positively charged S atom. These results
suggest that the methyl substituents on the sulfonium
group are able to form hydrogen bonds with the carbonyl
oxygen, similarly to pyridinium ylides.13d,13e In addition,
on the basis of AIM theory a set of criteria have been
proposed to characterize the hydrogen bonds.31-33 Fol-
lowing the procedure used for pyridinium ylides,13d we
have considered a subset of four criteria that comprise the
local topological properties of F(r). Table 1 summarizes the
results of this analysis and compares them with previous
studies. The interactions between the SMe2 fragment and
the carbonyl oxygen fulfill all four criteria: (i) a bond
critical point is found between the donor hydrogens and
the acceptor oxygen, and the bond path links the two pairs
of atoms, (ii) the electron density F(r) at the bcp’s are low
and lie within the typical range of 0.002-0.040 au, (iii) the
2
Laplacian of the charge density r F(r) at the bcp’s are
positive and lie within the typical range of 0.015-0.150 au,
and (iv) there is a positive mutal penetration between
hydrogen and the acceptor atom (ΔrH þΔrA >0), and
the hydrogen atom is more penetrated than the acceptor
structure 13csym and the transoid structure (9.2 kcal mol-1
)
could be explained from the formation of two intramolecular
hydrogen bonds (6-8 kcal mol-1).
Finally, we analyze the possible intramolecular interac-
tions in cisoid rotamer 13casym, in which the SMe2 group
(31) Popelier, P. L. A. J. Phys. Chem. A 1998, 102, 1873.
(32) Koch, U.; Popelier, P. L. A. J. Phys. Chem. A 1995, 99, 9747.
ꢀ
(33) Pacios, L. F.; Gomez, P. C. J. Comput. Chem. 2001, 22, 702.
(34) The penetration was evaluated as the difference between the non-
bonded van der Waals type radius and the bonding radius defined as the
distance between the corresponding bcp.
(35) Grabowski, S. J. J. Phys. Chem. A 2001, 105, 10739.
(36) Matsura, H.; Yoshida, H.; Hieda, M.; Yamanaka, S.; Harada, T.;
Shin-Ya, K.; Ohmo, K. J. Am. Chem. Soc. 2003, 125, 13910.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6829
has the same arrangement as in the X-ray determined
coordination complexes. This structure is 6.5 kcal mol-1
higher in energy than the global minimum 13csym, but still
2.7 kcal mol-1 more stable than the transoid isomer. The
calculated S O distance (2.791 A) is the shortest among
the series of isomers, whereas the closest CH O distance
3 3 3
3 3 3
is relatively long (2.977 A), precluding hydrogen bonding.
Thus, it is reasonable to think that in 13casym there is some
kind of weak intramolecular interaction between the car-
bonylic oxygen and the sulfur atoms responsible for its
stabilization with respect to the transoid isomer. Never-
theless, and in spite of an extensive research, it was not
possible to localize any bcp between the oxygen and the
sulfur atoms of structure 13casym. This is quite surprising
because 1,5-S O intramolecular interactions on R,R0-
3 3 3
bithiophene derivatives were characterized by Bader’s
analysis at interatomic distances ranging from 2.88 to
2.91 A,37,38 still longer than that for 13casym (2.791 A).
To back up our methodology and to allow direct compar-
ison with S-ylides, we have performed Bader’s AIM
analysis on the 2,20-bithiophene containing an OMe sub-
stituent in β position at the same computational level of
this work. The computed S O distance is 2.909 A,
3 3 3
Figure 5. Contour maps of electron charge density F(r) at the S-OdC
plane for 13casym (a), the monosubstituted R,R0-bithiophene (b), and
13casym at distorted S-CR-Cβ bond angles of 103° (c) and 100° (d). The
solid squares represent the bcp’s and the circles the rcp’s.
showing an excellent agreement with the previous reported
distance (2.906 A).38 Contrary to S-ylides, we have located
a bcp between the oxygen and sulfur atoms and the
corresponding rcp for bithiophene structure, evidencing
the intramolecular interaction. Figure 5 shows the contour
maps of the charge density F(r) for the S-ylide 13casym
(Figure 5a) and the calculated bithiophene structure
(Figure 5b). To understand the observed differences on
charge density distribution patterns between 1,4-S
and 1,5-S O contacts, one should consider the spatial
O
3 3 3
3 3 3
arrangement of the charge density in the determination of
non-bonded intramolecular interactions.3i The calculated
value of the C-O S angle in the S-ylide 13casym is
3 3 3
significantly smaller (68.6°) than that calculated for bithio-
phene structure (88.0°). Thus, in the case of bithiophene,
the lone pair of the oxygen atom is better oriented for
establishing an electrostatic interaction with the positively
Figure 6. Potential energy curves of the S-CR-Cβ bond angle distor-
tion for cisoid 13casym and transoid 13t compounds. Relative energies in
kcal mol-1 respect to the lowest energy rotamer 13csym.
charged S atom, explaining why 1,5-S O intramolecular
interactions could be characterized through Bader’s AIM
theory.
3 3 3
are present, the energy rises continuously as we distort the
SCRCβ angle from its equilibrium value 123.8°. In the case
of the cisoid structure 13casym, the energy curve is shifted
down in energy. The energy difference between the distor-
tion energy curves of transoid and cisoid isomers can be
attributed to the formation of the stabilizing nonbonding
interactions between the SMe2 fragment and the car-
bonylic oxygen in the cisoid structure. Note also that
starting at a SCRCβ value of 124°, the smaller the SCRCβ
value is the larger the energy differences between the
distorted cisoid and transoid structures because of the
reinforcement of the stabilizing intramolecular interac-
Taking a closer look to the calculated geometry of
13casym, we could observe another noteworthy structural
feature. The S-CR-Cβ bond angle is 112.4°, notably
smaller than the expected 120°, and than that found in
the transoidstructure(123.8°), where no S O interaction
3 3 3
is possible. The bond angle distortion suggests the forma-
tion of an interaction between the S and O atoms to
counterbalance the increase of steric stress. In fact, we
observed an analogous phenomenon for P-ylides, in which
the 1,4-P O interaction was characterized at a P-CR-
3 3 3
tions upon shortening of S O distance. In 13casym,
Cβ bond angle value of 100.9°. Thus, we have analyzed in
more detail the energy curve associated with SCRCβ angle
distortion in the cisoid rotamer 13casym, and compared it
with that for transoid isomer (Figure 6). For the transoid
isomer, where no stabilizing intramolecular interactions
3 3 3
the balance between distortion and intramolecular inter-
actions leads to a displacement of the minimum to nar-
rower SCRCβ angle (112.4°), and to a very flat potential
energy surface, in which the variation of SCRCβ angle from
101° to 121° has an energy cost of only 0.2 kcal mol-1
.
Interestingly, we could locate bcp’s between the S and
ꢀ
ꢀ
ꢀ
(37) Rethore, C.; Madalan, A.; Fourmigue, M.; Canadell, E.; Lopes, E.
O atoms at narrow SCRCβ angle values ranging from 100°
ꢀ
B.; Almeida, M.; Clerac, R.; Avarvari, N. New. J. Chem. 2007, 31, 1468.
to 103° (S O distance 2.46 -2.53 A) for 13casym. On
(38) Raos, G.; Famulari, A.; Meille, S.; Gallazzi, M. C.; Allegra, G.
J. Phys. Chem. A 2004, 108, 691.
3 3 3
going from SCRCβ=100° to 103° the distance between the

6830 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
Figure 7. Computed structures for mixed bis-ylide Me2SCHC(O)CHPH3 and its corresponding complexes.
bcp and the corresponding rcp diminishes from 0.24 to
0.07 A. This means that, in the direction perpendicular to
the S-O bond path, a maximum of charge density
corresponding to the bcp becomes very close to a mini-
mum corresponding to the rcp, and finally, both critical
points collapse (Figure 5: df c f a). Therefore, for SCR-
Cβ angles larger than 103° it was not possible to char-
4. Theoretical Analysis of Conformational Preferences
and Coordination Properties of Mixed Bis-Ylides of Phos-
phorus and Sulfur. Once we have analyzed in detail the
origin of conformational preferences on free S-ylides, the
next step in our discussion is centered on the mixed P-S
bis-ylide ligands [Me2SCHC(O)CHPH3] (14) and on their
corresponding palladium complexes [(Me2SCHC(O)-
CHPH3)PdCl2] 15. Figure 7 shows the computed struc-
tures, and geometric parameters are provided in the
Supporting Information, Table S2. As expected the ci-
soid-cisoid conformation of free ligand 14cc is the most
stable rearrangement, being the P-cisoid-S-transoid
(14cp) and the P-transoid-S-cisoid (14cs) structures less
stable by 8.1 and 10.3 kcal mol-1, respectively. We could
not find a minimum for the transoid-transoid isomer
since all the attempts to optimize such geometry ended
in the 14cs structure. The most stable isomer 14cc is
rigorously planar, and it shows the presence of a 1,4-
acterize the 1,4-S O interactions by Bader’s AIM
3 3 3
theory in cisoid isomers. Note that during the last years
the role of bond paths and bcp’s of F(r) as a direct
interpretation of chemical interactions has been a matter
of debate in the chemical community.39 These arguments,
in conjunction with the geometric and energetic evidence
stated above, indicate that in cisoid isomers a 1,4-S
O
3 3 3
type interaction can also take place, despite the fact that it
is not being manifested by a bcp. Moreover, we can
estimate the S O interaction strength through compar-
3 3 3
ison of distortion energy curves in Figure 6. For an SCRCβ
angle 112.4° (value in 13casym), the estimated stabilizing
P
O interaction (d(P O) = 2.381 A, a(PCRCβ) =
3 3 3
3 3 3
O interaction energy is about 5.6 kcal mol-1, balan-
103.7°, and bcp between the P and O atoms).13b,13c,13e
The sulfonium half in 14cc shows the same types of
interactions between the SMe2 fragment and the carbonyl
oxygen (d(H O)=2.369 A, d(S O)=3.030 A, and
S
3 3 3
cing the energy required for an SCRCβ angle distortion
(estimated in about 2.8 kcal mol-1 from the 13t distortion
curve) and explaining the computed energetic preference
of 13casym with respect to transoid isomers. Following
the same procedure for the SCRCβ angle of 121° (value in
13csym), the distortion-interaction balance results in an
overall stabilization of about 2.5 kcal mol-1, which can be
3 3 3
3 3 3
bcp’s between the H’s and O atoms) as in S-ylide
13csym. On going from 14cc to 14cp, two 1,6-C-H
OdC hydrogen bonds and one 1,4-S O weak-type
3 3 3
3 3 3
intramolecular interactions disappear explaining the
energy raising of 8.1 kcal mol-1. This provides further
evidence of the stabilizing interaction between SMe2
fragment and carbonyl oxygen. Moreover, it shows that
the intramolecular interactions characterized in the two
halves of the mixed P-S bis-ylides are cooperative lead-
ing to the conformational preference of the cisoid-cisoid
isomer.
attributed exclusively to a weak stabilizing S O contact
since the closest CH O distance is 4.42 A. Note also
that if we add the contribution of this weak S O contact
3 3 3
3 3 3
3 3 3
to the value estimated for the formation of two weak
hydrogen bonds (6-8 kcal mol-1), then we better explain
the energy difference between 13csym and the transoid
isomer (9.2 kcal mol-1).
In summary, ketostabilized sulfur ylides show confor-
mational preferences similar to those reported for N- and
P-ylides, which favor the cisoid form. Theoretical analysis
allowed identifying and evaluating two types of weak
stabilizing non-bonded intramolecular interactions be-
tween the SMe2 fragment and carbonyl oxygen: the 1,6-
CH O hydrogen bonds and 1,4-S O interaction,
which are the ultimate reason of the conformational
preferences. Moreover, both types of interactions can
occur simultaneously in a cooperative way.
The coordination of ligands 14cc, 14cp, and 14cs to the
PdCl2 group results in the formation of the C,C-chelates
15cc, 15cp, and 15cs, respectively (Figure 7). Not surpris-
ingly, the meso form 15cc is the most stable isomer, in very
good agreement with the experimental data (synthesis of
complexes 3-6), being the d,l forms 15cp and 15cs desta-
bilized by 6.7 and 6.6 kcal mol-1, respectively. The desta-
bilization of the d,l forms with respect to the meso form is
smaller than in the free ligands probably because of the
elongation of intramolecular contacts upon coordination,
this fact promoting an attenuation of the interactions.
Anyway, there is a clear transfer of the conformational
preferences from the free ligand to the complex, and it is
possible to explain the observed stereoselectivity of the
3 3 3
3 3 3
::
(39) See for example: (a) Grimme, S.; Muck-Lichtenfeld, C.; Erker, G.;
Kehr, G.; Wang, H.; Beckers, H.; Willner, H. Angew. Chem., Int. Ed. 2009,
48, 2592. (b) Cerpa, E.; Krapp, A.; Vela, A.; Merino, G. Chem.;Eur. J.
2008, 14, 10232 and references therein.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6831
bonding of the bis-ylides from their structure in free
form. An additional question to be addressed is the
orientation of the SMe2 groups with respect to the
carbonyl group. The X-ray structures of the previously
reported coordinated sulfur bis-ylide bridging two Pd
atoms13f and that reported in this work (12, see below)
showed an asymmetric arrangement of methyl substi-
tuents. On the other hand, in the calculated lowest
energy rotamer of complex 15cc, the methyl groups
are nearly symmetric forming two hydrogen bonds.
Nevertheless, the rotamer with the asymmetric (X-ray-
like) orientation of sulfonium unit (15cc0) is only
1.1 kcal mol-1 higher in energy. Therefore, and assum-
ing a low S-C R rotational barrier, both orientations of
the sulfonium fragment should be energetically acces-
sible, and subtle effects should modulate their energetic
Scheme 3. (i) PdCl2(NCMe)2, MeOH; (ii) AgClO4, -AgCl; Tlacac,
-TlCl; (iii) PdCl2(NCMe)2; NEt3, MeOH, -[HNEt3]Cl; (iv) PPh3;
(v) Tlacac, -TlCl
balance. This would explain why previously reported
13f
X-ray structures,
asymmetric orientation of the sulfonium moiety.
or those in this work, show an
5. Reactivity of Ylide-Sulfide 2. Synthesis of Ylide-
Methanide Complexes. The ylide-sulfide [Ph3PCHC(O)-
CH2SMe] 2 possess at least two strong donor atoms, the
sulfur and the ylidic carbon. Then it is not surprising that
the reaction of 2 with [PdCl2(NCMe)2] (1:1 molar ratio)
occurs with displacement of both nitrile ligands and C,S-
chelation of 2, giving cis-[PdCl2[Ph3PCHC(O)CH2SMe-
κ-C,S]] 7. Complex 7 reacts with AgClO4 and Tlacac
(1:1:1 molar ratio) to give [Pd(acac-O,O0)[Ph3PCHC(O)-
CH2SMe-κ-C,S]]ClO4 8 (Scheme 3). The C-bonding of
the ylidic carbon in 7 and 8 is inferred from the compar-
ison of their IR and NMR parameters with those of 2, in
the same way than reported for 1-6: the increase of the
carbonyl stretch (IR), the decrease of the values of the
coupling constants 2JPH and 1JPC, the low field shift of the
31P signal, and the shielding of the ylidic carbon signal in
the 13C NMR spectra. The low field shift of the methyl
resonances in the NMR spectra (SCH3 in 1H and SCH3 in
13C) strongly suggest the S-bonding. In addition, the S-
coordination of 2 introducesa second stereogenic center in
7 and 8, which are obtained as mixtures of diastereoi-
somers (see Supporting Information). Both the C bond-
ing of the ylide, as well as the S bonding of the sulfide,
should be configurationally stable in the NMR time scale,
as it seems clear from the shape of the signals. In these
conditions, the 2D NOESY spectrum of 8 allows the
determination of the stereochemistry of each diastereoi-
somer. Obviously, each diastereoisomer is obtained as the
racemic mixture, and the product, as a whole, is not
optically active. For the minor isomer, a clear NOE
between the ylidic proton (5.99 ppm) and the methyl
protons of the SMe group (2.79 ppm) is observed, showing
that both units are on the same side of the molecular plane
(Supporting Information). A strong NOE between the
ylidic proton (5.99 ppm) and one of the methylenic pro-
tons (4.23 ppm) is also observed. The configurations
shown in the Supporting Information for the minor iso-
mer of 8 (RCSS/SCRS) agree with these observations. In
addition, the major isomer shows a NOE effect
between the ylidic proton (5.60 ppm) and one of the
methylenic protons (4.37 ppm) and an additional NOE
between the other methylenic proton (2.49 ppm) and the
methyl protons (2.41 ppm), showing that the former and
the latter are on different sides of the molecular plane.
More interestingly, 2 reacts with PdCl2(NCMe)2 in
presence of NEt3 (1:1:1 molar ratio) to give 9, a com-
plex of stoichiometry [PdCl[Ph3PCHC(O)CHSMe]] (see
Scheme 3). The reaction occurs through deprotonation of
the methylene unit in 2, which results in the synthesis of
the ylide-methanide anion [Ph3PCHC(O)CHSMe]-.
The dimeric nature of 9 was inferred from its MALDIþ
spectrum, which shows a strong peak at 975 amu, with the
correct position and isotopic distribution for the stoichi-
ometry [Pd2[Ph3PCHC(O)CHSMe]2Cl]þ. Attempts to det-
ermine the molecularweightinCHCl3 solution gaveerratic
results, probably because of its low solubility in this
solvent. While the C,C-bonding of the ylide-methanide
seems clear from the IR data (strong absorption at
1593 cm-1), the nature of the bridging ligands can not
be determined unambiguously from the spectroscopic
parameters. The chemical shift of the SMe protons in 9
(2.44 ppm) is shifted downfield with respect to 2 (2.13 ppm)
and falls in the same region as those found for S-bonded
derivatives as 7 (2.42 and 2.50 ppm) or 8 (2.41 ppm),
suggesting S-bonding. On the other hand, the IR spectrum
of 9 shows absorptions in the low energy region at 251, 243,
239, and 237 cm-1, this observation being in agreement
with either a bridging chloride structure40a or a terminal
Pd-Cl bond trans to an atom with large trans influence,40b
as it could be the methanide carbon. In spite of this
ambiguity, and according with its dimeric nature, [PdCl-
[Ph3PCHC(O)CHSMe]]2 9 reacts with strong donors and
with chelating ligands, allowing a proper characterization
of the bonding mode of the ylide-methanide ligand.
The reaction of a suspension of 9 with PPh3 (1:2 molar
ratio) gives the mononuclear [Pd(Cl)(PPh3)[Ph3PCHC-
(O)CHSMe-κ-C,C]] 10, while treatment of 9 with Tlacac
ꢀ
ꢀ
(40) (a) Uson, R.; Fornies, J.; Martınez, F.; Tomas, M.; Reoyo, I.
´
Organometallics 1983, 2, 1386. (b) Nonoyama, M. Trans. Met. Chem.
1987, 12, 1–3.

6832 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
gives [Pd(acac-O,O0)[Ph3PCHC(O)CHSMe-κ-C,C]] 11
(Scheme 3). The structure of 10 has been determined by
X-ray diffraction methods, and shows some amazing
features. A drawing of complex 10 is shown in Figure 8,
selected bond distances (A) and angles (deg) are collected
in Table 2, and the most relevant parameters concerning
data collection and structure solutions and refinement are
given in the Supporting Information, Table S1. The Pd
atom is located in a distorted square-planar environ-
ment, surrounded by the ylidic carbon C(37), the metha-
nide carbon C(39), the chlorine atom Cl(1), and the P
atom of the phosphine ligand P(1). This fact confirms the
chelating bonding mode of the ylide-methanide ligand,
the nonbonding of the sulfur atom, and the cis arrange-
ment of the bulky phosphine and the CH(SMe) fragment.
The four membered ring comprising the chelating ylide-
methanide [68.33(13)°] is the main one responsible of the
distortion of the square-planar environment. The most
noteworthy features are the cisoid arrangement of the C-
(37)-P(2) and C(38)-O(1) bonds, reflected in the dis-
tance P(2)-O(1)=3.11 A and in the dihedral angle O(1)-
C(38)-C(37)-P(2)=-3.2°, and the alterned disposition
of the C(38)-O(1) and C(39)-S(1) bonds. That is, the
ylidic moiety involving the phosphorus atom behaves as
expected, adopting the same distribution of bonds as
observed previously, while the methanide unit coordi-
nates showing the opposite conformation to that shown
by sulfur ylides. The absolute configurations of the
Figure 8. Thermal ellipsoid plot of complex 10. Non-hydrogen atoms
are drawn at the 50% probability level.
of a C,C-chelating four membered ring. The C-bonding
of the ylide can be inferred from the NMR data using the
same keyfeatures thanin the preceding section: (i) decrease
of the values of the 2JPH and 1JPC coupling constants with
respect to2; (ii)low field shift of the 31P signal; and (iii)high
field shift of the signal of the 13C ylidic carbon. The trans
coordination of the PPh3 ligand with respect to the phos-
phorus ylide moiety is also evident from the shape
and coupling constant values of the CHP signals in the
1H (3JPH=7.0 Hz) and 13C (2JPC=48.1 Hz) NMR spectra.
The nonbonding of the SMe group in 10 can also detected
bonded carbons shown in Figure 8 are RC39 and RC37
,
1
in the H NMR spectrum, since the chemical shift of
although both enantiomers are present in the crystal
which, as a whole, is racemic (RCRC/SCSC). Therefore,
the simple change from the sulfur ylide to the sulfide
reverses the selectivity of the coordination and, instead of
the expected cisoid form, the transoid conformation is
obtained.
the SMe protons is slightly shifted to high field in 10
(1.96 ppm) with respect to 2 (2.13 ppm), while a clear low
field shift has been observed when the SMe group is
bonded, for instance, in 8 (2.41 or 2.79 ppm). Unfortu-
nately, NOESY 1D experiments are not informative in this
case, since irradiation of the CHP or CHS protons produce
NOE effects only in their respective neighboring protons
(PPh3 and SMe), but the absence of NOE between the
CHP and CHS nuclei can not beconsidered a proof of their
relative transoid disposition.43 The comparison of the IR
and NMR data of 11 with those reported for 10 (see
Supporting Information) allows to propose a chelating
structure with the nonbonded SMe fragment, as repre-
sented in Scheme 3.
In summary, the ylide-sulfide ligand [Ph3PCHC(O)-
CH2SMe] coordinates to the Pd as a chelate κ2-C,S, and
can be easily deprotonated at the methylene group to give
the ylide-methanide anion [Ph3PCHC(O)CHSMe]-.
This anion can behave as chelate κ2-C,C ligand, and
shows also strong conformational preferences but oppo-
site to those found on bis-ylides: while the latter favor the
RS/SR configurations (meso form), the former gives
exclusively the RR/SS configurations (d,l pair). We have
attempted a similar theoretical analysis, searching for the
presence of conformational preferences, but the situation
is less clear in the anionic species than in the neutral ones.
It seems sensible to assume that the interactions involving
the SMe fragment should be weaker in [H3PCHC(O)-
CHSMe]- than in [H3PCHC(O)CHSMe2] because of the
change of the net charge: the protons of the CHSMe are
The Pd(1)-C(37) bond distance [2.181(3) A] falls in the
usual range of distances found for C-bonded ylides trans
to a phosphine ligand,11g while the Pd(1)-C(39) bond
distance [2.086(3) A] is slightly longer that those found in
palladium complexes with related methanide-sulfide
ligands [ranging from 2.035(3) to 2.044(4) A].41 Probably
the difference arises from the different substituents of the
sulfide and from the different bonding mode. The Pd-Cl
and Pd-P bond distances (Table 2) are typical for this
type of bonds.42 Concerning the ylide-methanide skele-
ton, the bond distances C(38)-C(37) [1.489(5) A] and C-
(38)-C(39) [1.491(4) A] reflect charge delocalization and
are identical, within experimental error, to those found in
bonded bis-ylides.12h,13e,13f This fact is also reflected in
the C(38)-O(1) bond distance [1.237(4) A]. On the other
hand, the S-C and P-C bond distances are also similar
to those found in related complexes,12h,13e,13f,41 while
other internal parameters are as expected.
The interpretation of the spectroscopic parameters of
10 (IR and NMR) agrees with its crystal structure and
suggests a static behavior in solution. The IR shows a
strong band at 1590 cm-1, according with the formation
(41) (a) Basato, M.; Bertani, C.; Zecca, M.; Grassi, A.; Valle, G.
J. Organomet. Chem. 1999, 575, 163. (b) Basato, M.; Grassi, A.; Valle, G.
Organometallics 1995, 14, 4439.
(42) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(43) Claridge, T. D. W. High Resolution NMR Techniques in Organic
Chemistry, 2nd ed.; Elsevier: Oxford, U.K., 2009.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6833
Table 2. Selected Bond Distances (A) and Angles (deg) for 10
through its spectroscopic data and the determination of
the X-ray structure.
Pd(1)-C(39)
Pd(1)-P(1)
C(38)-O(1)
C(38)-C(39)
S(1)-C(40)
2.086(3)
2.297(9)
1.237(4)
1.491(4)
1.820(5)
Pd(1)-C(37)
Pd(1)-Cl(1)
C(38)-C(37)
C(39)-S(1)
P(2)-C(37)
2.181(3)
2.399(8)
1.489(5)
1.795(3)
1.773(3)
The IR of 12 shows a very strong absorption at
1630 cm-1, shifted to higher energies with respect to 6
(1602 cm-1), and in good agreement with a higher keto
character. This increase has already been observed when
chelating P- and N-ylides undergo orthometalation
processes.11g,11h,13e The NMR spectra show the presence
of two sets of signals with a molar ratio (6.7/1) identical to
that observed in the starting compound 6. This means
that 12 is obtained as two geometric isomers, in which the
phosphine is bonded either trans to the aryl ring or trans
to the ylidic carbon. Even if the PPh3 ligand is reluctant
to coordinate trans to an aryl ligand, a phenomenon
known as transphobia53,54 and based on the antisymbiotic
effect,55 there are some reports in which this situation is
postulated or crystallographically characterized.56-62 On
these grounds, we have assigned the PPh3 trans to Cylide
structure to the major isomer because this should be the
most favorable arrangement. These spectra also show
unambiguously the presence of the orthometalated unit.
The 1H NMR spectrum shows clearly four signals in the
7.0-6.5 ppm region, assigned to the C6H4 unit, an AB
spin system around 4.5-5.5 ppm, assigned to the diaster-
eotopic protons of the CH2S unit, and singlets due to the
ylidic protons, while the 13C{1H} NMR spectrum shows
six well spread peaks due to the palladated ring.
C(37)-Pd(1)-C(39) 68.33(13)
C(37)-Pd(1)-Cl(1) 101.35(9)
C(39)-Pd(1)-Cl(1)
C(39)-Pd(1)-P(1)
O(1)-C(38)-C(39)
169.36(10) C(37)-Pd(1)-P(1)
167.60(9)
90.72(3)
99.73(10)
107.2(3)
P(1)-Pd(1)-Cl(1)
O(1)-C(38)-C(37) 126.4(3)
C(39)-C(38)-C(37) 107.2(3)
C(38)-C(39)-Pd(1) 83.88(18)
C(39)-S(1)-C(40)
P(2)-C(37)-Pd(1)
C(38)-C(39)-S(1)
S(1)-C(39)-Pd(1)
P(2)-C(37)-C(38)
116.2(2)
107.22(16)
118.1(2)
100.3(2)
123.13(17)
less positive, the own S atom also, and there is an
additional lone pair over the S atom interacting with the
O atom. Probably, all these facts militate against the
establishment of conformational preferences on the sul-
fur side. It is interesting to note that those in the phos-
phorus side are maintained because of the cationic
character of the CHPR3 group. Clearly, this area needs
further development.
6. Orthometalation of Bis-Ylides. The enormous im-
portance of the CH bond activation processes nowadays is
doubtless because of their implications in the functionaliza-
tion of organic substrates.44,45 We have studiedthe selective
orthometalation of iminophosphoranes and ylides, and
related species,46-52 and we have also reported that di-
nuclear complexes containing chelating bis-ylides are
adequate precursors for the synthesis of orthopalladated
complexes through CH activation (Figure 1).1j,1k,11g,13e
These syntheses have been achieved by either thermal
induction or ligand addition, the steric effects being the
driving force in the latter cases. We have attempted the
orthopalladation of mixed phosphorus-sulfur bis-ylides
by thermal treatment of dimer 5 (NCMe, reflux, 24 h), but
no reaction was observed since the starting compound was
recovered. However, refluxing of 6 in NCMe for 24 h
affords cleanly 12 (Scheme 2) as a mixture of two geometric
isomers (6.7/1 molar ratio). For complex 5, the pattern of
reactivity is similar to that found with mixed phospho-
nium-pyridinium ylides,13e and this lack of reactivity
probably rests on the weak steric intramolecular interac-
tions between the bulky groups of the molecule. We have
also attempted the orthometalation of the ylide-metha-
nide derivatives 9-11, but in all studied cases decomposi-
tion to Pd0 was observed. Probably the neutral character of
these complexes provides a less electrophilic Pd center,
therefore less prone to promote CH bond activations
compared with 6. Complex 12 has been characterized
The crystal structure of 12 CHCl3 has been determined
3
by X-ray diffraction methods. A drawing of the orga-
nometallic cation of 12 is shown in Figure 9, selected bond
distances (A) and angles (deg) are collected in Table 3,
and the most relevant parameters concerning data collec-
tion and structure solutions and refinement are given in
the Supporting Information, Table S1. The Pd atom is
located in a distorted square-planar environment, sur-
rounded by the orthopalladated carbon C(1), the ylidic
carbon C(37), the chlorine Cl(1), and the phosphorus P-
(2). This fact confirms the metalation of the phenyl ring
through CH bond activation and the formation of the
sulfonium unit. The environment of the palladium center
is less distorted than in 10, since the metallacycle com-
prises a five membered ring. The phosphine ligand is
bonded trans to the ylidic carbon C(37), and the P(1)-
C(37) and S(1)-C(39) bonds are cisoid with respect to the
C(38)-O(1) bond. These facts imply short P O and
3 3 3
S
O intramolecular contacts [3.039 A and 2.891 A,
3 3 3
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6834 Inorganic Chemistry, Vol. 48, No. 14, 2009
Serrano et al.
have been prepared and fully characterized. Their depro-
tonation generates the bis-ylide [Ph3PCHC(O)CHSMe2]
and the ylide-methanide [Ph3PCHC(O)CHSMe]- for
which three different bonding modes have been character-
ized: neutral κ2-C,C for the bis-ylide, neutral κ2-C,S for
the ylide-sulfide, and anionic κ2-C,C for the ylide-metha-
nide. The coordination mode of the neutral κ2-C,C occur
with complete diastereoselectivity for the meso form (RS/
SR). The DFT studies (B3LYP level) on model systems of
sulfur ylides [Me2SCHC(O)R] show strong conforma-
tional preferences favoring the cisoid forms. In conjunc-
tion with Bader analysis of electron density, it was also
possible to characterize the intimate nature of the phenom-
enon originating these preferences. The main factor
responsible for the preference of the cisoid form (9.2 kcal
mol-1) is a set of cooperative weak-type intramolecular
interactions including two 1,6-C-H
O hydrogen bonds
3 3 3
Figure 9. Thermal ellipsoid plot of the cationic fragment of complex 12.
Non-hydrogen atoms are drawn at the 50% probability level. Phenyl
groups of the PPh3 ligand (except Cipso) have been omitted for clarity.
between the protons of the methyl substituents (around 3-
4 kcal mol-1 each) and one 1,4-S
O interaction (around
3 3 3
2.5 kcal mol-1). These interactions are also operative in the
sulfonium half of mixed P-S bis-ylides [Me2SCHC(O)-
Table 3. Selected Bond Distances (A) and Angles (deg) for 12 CHCl3
3
CHPH3], whereas a 1,4-P
O interaction is present in the
3 3 3
Pd(1)-C(1)
Pd(1)-P(2)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
C(6)-P(1)
C(37)-C(38)
C(38)-C(39)
S(1)-C(40)
1.989(3)
2.2906(11) Pd(1)-Cl(1)
Pd(1)-C(37)
2.187(3)
2.3979(11)
1.401(4)
1.370(5)
1.387(4)
1.776(3)
1.223(3)
1.803(3)
1.775(3)
phosphonium half, favoring the cisoid-cisoid structures
and showing that the conformational preferences in the
two halves are cooperative. Thus, the observed adoption of
the meso form in the bonded mixed P-S bis-ylides is the
direct result of the coordination of the cisoid-cisoid form
of the free ligand, showing that the conformational pre-
ferences are preserved during the bonding process. On the
other hand, the coordination modes of anionic κ2-C,C
occur with complete diastereoselectivity but displaying the
d,l form (RR/SS), which constitute a unique case since the
reversed of the meso form is observed in the formation of
bis-ylides.
1.388(4)
1.380(4)
1.382(4)
1.784(3)
1.433(4)
1.529(4)
1.781(3)
C(1)-C(6)
C(3)-C(4)
C(5)-C(6)
P(1)-C(37)
C(38)-O(1)
C(39)-S(1)
S(1)-C(41)
C(1)-Pd(1)-C(37)
C(37)-Pd(1)-P(2)
C(37)-Pd(1)-Cl(1)
C(2)-C(1)-Pd(1)
C(37)-P(1)-C(6)
C(38)-C(37)-Pd(1) 106.1(2)
O(1)-C(38)-C(37) 125.7(3)
C(37)-C(38)-C(39) 115.9(3)
85.88(12)
167.80(9)
91.54(8)
125.2(2)
102.86(15) C(38)-C(37)-P(1)
P(1)-C(37)-Pd(1)
C(1)-Pd(1)-P(2)
C(1)-Pd(1)-Cl(1)
P(2)-Pd(1)-Cl(1)
C(6)-C(1)-Pd(1)
95.67(9)
170.98(9)
88.68(4)
117.6(2)
118.0(2)
96.05(13)
O(1)-C(38)-C(39) 118.4(3)
C(38)-C(39)-S(1)
Acknowledgment. Funding by the Ministerio de Ciencia
ꢀ
108.0(2)
100.43(16)
C(41)-S(1)-C(40)
C(40)-S(1)-C(39)
100.48(17) C(41)-S(1)-C(39)
101.80(15)
e Innovacion (MICINN) (Spain, Projects CTQ2008-
01784, CTQ2008-06549-C02-01/BQU, and CTQ2008-
06866-C02-01) is gratefully acknowledged. E.S. thanks
respectively] and dihedral angles close to zero in the case
of the phosphonium unit [P(1)-C(37)-C(38)-O(1) =
-7.52°, while O(1)-C(38)-C(39)-S(1)=-39.17°]. The
Pd(1)-C(37) bond distance [2.187(3) A] is identical (with-
in experimental error) to that found in 10 and to those
found on related ylides bonded to Pd(II) trans to a
phosphine ligand.11g The Pd(1)-C(1) bond distance
[1.989(3) A] is also identical to that found in related
orthometalated complexes [1.999(8) A].11g Other internal
parameters are as expected.
ꢀ
the Diputacion General de Aragon (Spain) for a Ph.D.
research grant.
ꢀ
Supporting Information Available: Complete experimental
section with all preparative data and references, table of crys-
tallographic parameters of data collection and refinement, table
of energetic and geometric parameters for bisylides [H3PCHC-
(O)CHSMe2] (14) and the corresponding C,C-chelating com-
plexes (15), potential energy profiles for [Me2SCHC(O)Me],
Cartesian coordinates of computed cisoid (c), transition state
(TS) and transoid (t) structures of ylide [Me2SCHC(O)Me] (13),
bisylides (14) and the corresponding complexes (15). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusions
Two new species, the ylide-sulfonium salt [Ph3PCHC(O)-
CH2SMe2]þ and the ylide-sulfide [Ph3PCHC(O)CH2SMe],