Influence of the ligand backbone in pincer complexes: Indenediide-, indolyl-, and indenyl-based SCS palladium complexes

Article abstract of DOI:10.1021/om400464j

A new pincer ligand featuring an indolyl backbone was prepared and coordinated to palladium. The properties of related indenediide-, indenyl-, and indolyl-based complexes have been compared by spectroscopic and structural means. Modulation of the ligand backbone was shown to significantly influence the electron density at Pd. Its impact on catalytic performance has been illustrated in the Pd-catalyzed imine allylation. The higher the electron density at Pd, the more active the SCS pincer complex.

Full text of DOI:10.1021/om400464j

Article
pubs.acs.org/Organometallics
Influence of the Ligand Backbone in Pincer Complexes: Indenediide‑,
Indolyl‑, and Indenyl-Based SCS Palladium Complexes
Jerome Lisena,† Julien Monot,† Sonia Mallet-Ladeira,‡ Blanca Martin-Vaca,*,† and Didier Bourissou*,†
́
̂
†Universite
́
de Toulouse, UPS, LHFA, 118 Route de Narbonne, F-31062 Toulouse, France, and CNRS, LHFA UMR 5069, F-31062
Toulouse, France
‡Universite
́
de Toulouse, UPS, Institut de Chimie de Toulouse, FR2599, 118 Route de Narbonne, F-31062 Toulouse, France
S
* Supporting Information
ABSTRACT: A new pincer ligand featuring an indolyl
backbone was prepared and coordinated to palladium. The
properties of related indenediide-, indenyl-, and indolyl-based
complexes have been compared by spectroscopic and
structural means. Modulation of the ligand backbone was
shown to significantly influence the electron density at Pd. Its
impact on catalytic performance has been illustrated in the Pd-
catalyzed imine allylation. The higher the electron density at
Pd, the more active the SCS pincer complex.
he stereoelectronic properties of transition metal
benzannelated five-membered ring) and only differ in the
nature of the ligand backbone. As an extension of our work on
indenediide and indenyl systems [Y = C− and C(CH3)],5 a new
pro-ligand deriving from indole (Y = N) has been prepared and
coordinated to Pd. The nature of the ligand backbone has been
shown by NMR, IR, and X-ray diffraction analyses to
substantially influence the properties of complexes III, in
particular the electron density at Pd. Catalytic tests on imine
allylation have also been carried out, and the electron-rich Pd
indenediide complex surpassed the indenyl- and indolyl-based
systems.
T
complexes can be finely tuned by modulating the
surrounding ligands, and this plays a fundamental role in
organometallic chemistry. The spectacular developments
achieved over the past decade with pincer complexes nicely
illustrate how the properties and reactivity of a metal can be
altered and adjusted through ligand modifications.1 Here,
complexes of types I and II (Chart 1) are noteworthy examples,
Chart 1. Schematic Representation of Pincer Complexes I−
III
RESULTS AND DISCUSSION
■
The new pro-ligand 1 was prepared in one pot from indole.
The first diphenylphosphino group was introduced by metal-
ation of C3 with MeMgBr followed by reaction with Ph2PCl,6
while the second one was introduced at N using NEt3 as base
and again Ph2PCl. After oxidation with S8 and workup, 1 was
isolated in 64% overall yield as a white solid. Coordination to
palladium was envisioned by direct C−H activation, as readily
achieved upon reaction of the related indene-based pro-ligands
with [Pd(RCN)2Cl2] or [Pd(cod)Cl2].5a,b However, no
reaction took place in the case of 1, even after prolonged
heating. To favor the coordination of the two pending sulfur
atoms, the use of cationic Pd precursors was then considered.4a
Gratifyingly, pretreatment of [Pd(CH3CN)2Cl2] with two
equivalents of AgBF4 allowed a smooth reaction to occur
with 1. Complex 2 was thereby obtained in 57% yield as a red-
orange powder (Scheme 1).
and an impressive body of work has been carried out to develop
and evaluate new DXD ligand frameworks.2,3 Not surprisingly,
these variations mainly involve the central donor atom and the
lateral donating groups (the donor atom, its substituents, and
the linker toward the central moiety). Modulations of the
pincer ligand without directly affecting the coordination sites
are comparatively rare and usually rely on varying the
substitution pattern of the remote backbone.4 Such variations
offer the opportunity to fine-tune the electronic properties of
the metal while retaining identical structural and steric features.
In this context, we report here a comparative study of SCS
palladium pincer complexes of type III. They all feature the
same framework (two thiophosphinoyle side arms and a central
Received: May 24, 2013
Published: July 18, 2013
© 2013 American Chemical Society
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coordinated PPh3 (δ 18.3 ppm, pseudo-t, JPP = 41 Hz). In our
series of Pd complexes, a very slight but progressive upfield shift
of the 31P NMR resonance is noted (δ 18.9 and 18.7 ppm for
the indenyl-based complexes,5b δ 18.3 ppm for the indolyl-
based complex, δ 17.6 ppm for the indenediide-based one5b).
The small magnitude of this shift does not allow drawing
definite conclusions on the electronic properties of the three
ligands.8 Complex 4 has been analyzed by X-ray diffraction
(Figure 1),9 and its geometric features have been compared
with those of the related indenyl and indenediide complexes.
The rigid chelating SCS coordination induces quasi invariant
Pd−C2 bond distances [2.009(4) Å with indenyl, 2.005(3) Å
with indolyl, and 1.997(9) Å with indenediide]. The Pd−PPh3
bond distance also lies within a narrow range, with a slight
increase from the indenyl [2.3593(10) Å] and indolyl
[2.3587(8) Å] complexes to the indenediide one [2.380(2)
Å].5b These data suggest very little, if any, variation of the trans
influence in the ligand series (indenyl, indolyl, indenediide).10
To precisely assess and compare the electronic properties of
the three SCS Pd pincer complexes, we then turned to the
corresponding carbonyl complexes (Scheme 2). Infrared
Scheme 1. Coordination to Pd of the Indole-Based Pincer
Ligand 1
The coordination of the two thiophosphinoyl side arms is
apparent from the downfield shift of the corresponding 31P
NMR resonance signals (δ 73.3 and 48.3 ppm for complex 2, vs
59.5 and 29.9 ppm for the pro-ligand 1). In addition, metalation
1
of C2 is supported by the disappearance of the associated H
NMR signal (δ 6.59 ppm for 1) and the appearance of a Cq
signal at 178.3 ppm in the 13C NMR spectrum. The
coordination sphere of Pd is completed by an acetonitrile
molecule, and the respective NMR signals are clearly identified
at δ 1H 2.27 and δ 13C 3.7 ppm. To unequivocally confirm the
molecular structure of 2, crystals were grown by slow
evaporation of a dichloromethane solution at room temper-
ature, and an X-ray diffraction study was carried out (Figure 1).
Scheme 2. Synthesis and IR Data of the Pd Carbonyl
Complexes 6−8
analysis of carbonyl complexes is widely used to probe the
electronic properties of ligands, giving an overall estimate of σ-
donation and π-back-donation.11 Such studies most frequently
involve Rh and Ni, which readily form carbonyl complexes, but
more rarely Pd, whose carbonyl complexes tend to be labile due
to weak Pd to CO back-donation.12 The Pd carbonyl
complexes 6 and 7 deriving from the indenyl and indolyl
pincer ligands, respectively, were prepared from the corre-
sponding chloro complexes. The chloride at Pd was abstracted
with AgBF4, and the reaction vessel was then pressurized with
CO. According to 31P NMR monitoring, the reaction requires 5
bar of CO to reach completion. The CO ligand is too labile to
enable isolation,13 but the Pd carbonyl complexes were
unambiguously authenticated by NMR spectroscopy (a
diagnostic P-coupled 13C NMR signal is found at δ ∼176
ppm). The corresponding indenediide complex 8 cannot be
prepared following the same route.14 Instead of the chloro
precursor, the trimeric complex {Pd[Ind(Ph2PS)2]}3 featur-
ing labile sulfur bridges was used as precursor, and complex 8
was quantitatively formed under 5 bar of CO. Complexes 6−8
stand as rare examples of Pd carbonyl pincer complexes.15
Their FT-IR spectra were recorded under a CO atmosphere,
and the corresponding νCO bands were found at 2161 cm−1 for
the indenyl complex 6, 2151 cm−1 for the indolyl complex 7,
and 2121 cm−1 for the indenediide complex 8. The variation of
Figure 1. Ellipsoid drawing (30% probability) of the molecular
structure of 2 (left) and 4 (right). For clarity, the tetrafluoroborate
counteranion, lattice solvent molecules, and hydrogen atoms are
omitted, and the phenyl groups at phosphorus are simplified.
The Pd center is in a square-planar environment, with
acetonitrile in trans position to C2 [PdN 2.077(3) Å, NC
1.129(4) Å, and PdNC 174.2(3)°].7 The SP(indolyl)PS ligand
framework is essentially planar and only slightly tilted with
respect to the coordination plane around Pd.
Displacement of acetonitrile at Pd was then studied as a
means to vary the coligand in trans position to the indolyl
moiety. Accordingly, treatment of 2 with sodium chloride in
tetrahydrofuran at 50 °C yielded the chloro complex 3, which
could not be obtained directly from 1 and [Pd(CH3CN)2Cl2].
Upon exchange of CH3CN for Cl at Pd, the 31P NMR signals
remain essentially unchanged (δ 74.0 and 50.6 ppm), but the
13C NMR signal for C2 shifts to higher field by about 10 ppm
(δ 167.4 ppm). Triphenylphosphine also reacts cleanly with 2
in dichloromethane at room temperature to readily afford
complex 4. The 31P NMR spectrum displays an AMX pattern,
as expected for two inequivalent thiophosphinoyle side arms (δ
75.2 ppm, d, JPP = 41 Hz and δ 52.9 ppm, d, JPP = 41 Hz) and a
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the CO stretching frequency parallels that of the 31P NMR
chemical shift and Pd−P distance in the PPh3 complexes and
clearly indicates an increase of the electron density at Pd in the
series indenyl ≤ indolyl < indenediide. This is consistent with
the electronic properties of the ligand backbone, whose central
fragment can be formally described as a vinyl, an enamine, and
an allyl anion, respectively. Despite the structural similarity of
the three complexes, the variation is relatively large, with a shift
of the νCO band by up to 40 cm−1.16
The comparison of the indenyl, indolyl, and indenediide
complexes described herein shows that slight variations of the
ligand backbone can have a significant influence electronically,
while retaining very similar structural and steric features. This is
an attractive strategy to finely tune the properties of pincer
complexes and a complementary approach to the modulation of
the donor moieties, as more commonly employed.
with the indenyl and indolyl complexes 3 and 5. These results
show that the ligand backbone of the SCS pincer Pd complexes
can indeed impact significantly the catalytic properties. The
higher catalytic activity of complex 9 may be attributed to the
higher electron density induced at Pd by the indenediide
backbone (as substantiated by the CO stretching frequencies),
increasing the nucleophilic character of the η1-allyl intermediate
complex and favoring its attack on the tosyl imine.19,20 Finally,
it is interesting to note that complex 9 compares well in activity
with the PCP chloro Pd complexes {PdCl[1,3-(Ph2PCH2)2-
phenyl]} and {PdCl[(1,8-(Ph2P)2-anthracenyl]},20 despite the
presence of weaker donating side arms (thiophosphinoyle vs
phosphine).
In conclusion, SCS indolyl-based Pd pincer complexes have
been prepared, and their properties have been compared with
those of the previously described indenediide and indenyl
complexes. The catalytic activity of the three related complexes
has been evaluated in the Pd-catalyzed imine allylation. The
most active catalyst is the one featuring the most electron-rich
Pd atom, as determined by IR analysis of the respective
carbonyl complexes. These results illustrate how the modu-
lation of the remote backbone of pincer ligands can impact the
electronic and catalytic properties of the ensuing complexes.
To illustrate how such backbone modulations can influence
catalytic properties, preliminary tests were performed in the Pd-
catalyzed allylation of imines.17 Recently, Szabo
́
et al. have
shown that PCP pincer ligands are well suited for this Pd-
catalyzed transformation, and a catalytic cycle involving an η1-
allyl complex as key intermediate was proposed.18 Our study
consisted in comparing the activities of the indenyl, indolyl, and
indenediide chloro Pd complexes in the reaction of phenyl
tosylimine with allyltributylstannane (Table 1). Initial tests
EXPERIMENTAL SECTION
■
Preparation of {Pd(CH3CN)[Indolyl(Ph2PS)2]}BF4 (2). A
suspension of AgBF4 (360 mg, 1.85 mmol) in dry CH3CN (7 mL)
was added to a solution of [PdCl2(CH3CN)2] (236 mg, 0.91 mmol) in
dry CH3CN (3 mL) at rt in the dark. A white precipitate was
immediately formed. After 1 h stirring, the yellow solution obtained
was filtrated and transferred on a solution of 1 (500 mg, 0.91 mmol) in
dry CH2Cl2 (10 mL). The mixture was stirred overnight, and a beige
precipitate was rapidly formed in an orange solution. Solvents were
evaporated, and the product was extracted with CH2Cl2 (4 × 15 mL).
Finally, the cationic complex was precipitated and washed with
pentane (about 100 mL). A red-orange solid was isolated (380 mg,
Table 1. Catalytic Allylation of Phenyl Tosylimine with the
a
SCS Pincer Pd Complexes 3, 5, and 9
1
57%). Mp = 256−258 °C (dec); H NMR (400 MHz, CDCl3) δ
7.87−7.82 (m, 10H, PPh2), 7.73−7.64 (m, 6H, PPh2), 7.64−7.58 (m,
4H, PPh2), 7.18−7.10 (m, 2H, Hbenzo), 7.07−7.02 (m, 1H, Hbenzo),
6.75−70 (m, 1H, H5/8), 2.27 (bs, 3H, CH3CN); 31P{1H} NMR (202
MHz, CDCl3) δ 73.3 (s, P1), 48.3 (s, P2); 13C NMR (125 MHz,
CDCl3) δ 178.3−178.2 (m, C2), 141.3 (dd, J(P,C) = 10.4, 2.9 Hz, C4),
136.2−136.0 (m, CparaPhP), 135.1−134.9 (m, CparaPhP), 132.9 (d,
J(P,C) = 12.9 Hz, CmetaPhP), 132.4 (d, J(P,C) = 12.2 Hz, CmetaPhP),
131.0 (dd, J(P,C) = 14.1, 5.0 Hz, C9), 130.3 (d, J(P,C) = 14.1 Hz,
CorthoPhP), 130.2 (d, J(P,C) = 14.0 Hz, CorthoPhP), 125.4 (d, J(P,C) =
84.6 Hz, CipsoPhP), 124.7 (C8), 124.4 (C6),123.3 (d, J(P,C) = 95.7 Hz,
CipsoPhP), 121.5 (CN), 118.8 (C5), 112.2 (C7), 3.7 (CH3CN);
HRMS calcd for [M]+ = C34H27N2P2S2106Pd+ 695.01202; found
695.0126. Anal. Calcd for C34H27BF4N2P2PdS2: C, 52.16; H, 3.48; N,
3.58. Found: C, 52.34; H, 3.72; N, 3.24.
Preparation of {PdCl[Indolyl(Ph2PS)2]} (3). Dry CH2Cl2 (5
mL) was added to a mixture of [PdCl2(PhCN)2] (141 mg, 0.55 mmol)
and AgBF4 (212.5 mg, 1.10 mmol). After 5 min in the dark, a solution
of 1 (300 mg, 0.55 mmol) in dry CH2Cl2 (10 mL) was added. The
reaction mixture was then stirred overnight at rt. The white precipitate
was eliminated by filtration. Then, solvents were evaporated, and THF
(20 mL) and NaCl (300 mg, 5.5 mmol) were added. The suspension
was heated at 50 °C for 48 h. A brown precipitate appeared. The
reaction mixture was concentrated (ca. 10 mL), and pentane (60 mL)
was added in order to precipitate the product. The brown solid was
filtered, and the complex was extracted with CH2Cl2 (4 × 20 mL).
After evaporation, a clear brown solid was isolated (210 mg, 55%). Mp
= 183 °C (dec); 1H NMR (400 MHz, CD2Cl2) δ 7.91−7.80 (m, 10H,
HPPh), 7.72−7.64 (m, 6H, HPPh), 7.62−7.57 (m, 4H, HPPh), 7.17−7.16
(m, 1H, H5), 7.13−7.09 (m, 1H, H8), 7.00−6.96 (m, 1H, H6), 6.74−
6.72 (m, 1H, H7); 31P {1H} NMR (161 MHz, CD2Cl2) δ 74.05 (s, P1),
b
c
run cat. solvent time (h) temp (°C) conversion (%) yield (%)
1
2
3
4
5
6
7
8
9
9
9
5
3
9
5
3
THF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
21
21
21
21
21
5
60
60
40
40
40
40
40
40
72
94
88
63
41
70
37
39
63
88
83
51
33
61
26
27
5
5
a
Reactions performed with 0.45 mmol of PhCHNTs, 0.54 mmol of
b
1
nBu3Sn(allyl) at 5% mol catalyst loading. Determined by H NMR.
c
Isolated yields.
were carried out at 60 °C in THF and DMF, the usual solvents
of choice, with the indenediide complex 9 (runs 1 and 2). The
reaction performed significantly better in DMF (88% vs 63%
isolated yield after 21 h at 60 °C). The reaction temperature
was then decreased to 40 °C (run 3), and the allyl tosyl amine
10 was obtained in only slightly lower yield (83% after 21 h).
Under the same reaction conditions (DMF, 40 °C, 21 h), the
indolyl and indenyl chloro complexes 3 and 5 showed
substantially lower activities, affording 10 in 33% and 51%
yields, respectively (runs 4 and 5). When the reactions were
stopped after 5 h (runs 6−8), the allyl tosyl amine was obtained
in 61% yield with the indenediide complex 9 vs 26% and 27%
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50.60 (s, P2); 13C NMR (100 MHz, CD2Cl2) δ 167.4−167.3 (m, C2),
142.3−142.2 (m, C4), 134.9 (d, J(P,C) = 3.2 Hz, CparaPPh), 133.3 (d,
J(P,C) = 3.1 Hz, CparaPPh), 132.5 (d, J(P,C) = 12.5 Hz, CmetaPPh),
132.1 (C9), 132.1 (d, J(P,C) = 11.7 Hz, CmetaPPh), 129.8 (d, J(P,C) =
14.0 Hz, CorthoPPh), 129.3 (d, J(P,C) = 13.1 Hz, CorthoPPh), 128.5 (d,
J(P,C) = 86.3 Hz, CipsoPPh), 126.0 (d, J(P,C) = 95.7 Hz, CipsoPPh),
124.5 (C8), 123.7 (C6), 121.2−119.9 (m, C3), 118.9 (C5), 112.6 (C7);
HRMS calcd for [M − Cl]+ = C32H24NP2S2106Pd+ 649.9881; found
649.9904. Anal. Calcd for C32H24ClNP2PdS2: C, 55.66; H, 3.50; N,
2.03. Found: C, 55.84; H, 3.72; N, 1.91.
ACKNOWLEDGMENTS
■
This work was supported financially by the Centre National de
la Recherche Scientifique (CNRS) and the Universite Paul
́
Sabatier (UPS).
REFERENCES
■
(1) (a) The Chemistry of Pincer Ligands; Morales−Morales, D.;
Jensen, C. M., Eds.; Elsevier: Oxford, 2007. (b) Albrecht, M.; van
Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750.
Preparation of {Pd(PPh3)[Indolyl(Ph2PS)2]}BF4 (4). Dry
CH2Cl2 (5 mL) was added to a mixture of [PdCl2(PhCN)2] (35.4
mg, 0.13 mmol) and AgBF4 (53 mg, 0.26 mmol). After 5 min stirring
at rt in the dark, a solution of 1 (75 mg, 0.13 mmol) in dry CH2Cl2
(10 mL) was added. Then, the mixture was stirred overnight at rt. The
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decompose upon workup and was thus characterized without
purification. Suitable crystals for X-ray diffraction were obtained after
concentration, filtration, and slow diffusion of a pentane/CH2Cl2
mixture. 1H NMR (500 MHz, CD2Cl2) δ 7.85−7.33 (m, 32H,
phenyl), 7.32−7.28 (m, 1H, H5), 7.26−7.23 (m, 2H, HPh and H8),
7.17−7.11 (m, 3H, HPh and H6), 6.82 (dd, 1H, J(P,H) = 8.3 Hz, 2.3
Hz, H7); 31P{1H} NMR (202 MHz, CD2Cl2) δ 75.22 (d, J = 41.5 Hz,
P1), 52.85 (d, J = 41.3 Hz, P2), 18.34 (t, J = 41.5 Hz, P3); 13C NMR
(125 MHz, CD2Cl2) δ 177.1 (ddd, J(P,C) = 144.8 Hz, 46.4 Hz, 21.6
Hz, C2), 142.2−142.1 (m, C4), 135.5−124.7 (m, CPhenyl), 132.0−131.9
(m, C9), 124.4 (C8), 124.1 (C6), 121.4 (dd, J(P,C) = 125.3 Hz, 11.9
Hz, C3 ), 119.1 (C5 ), 112.3 (C7 ). Anal. Calcd for
C50H39BF4NP3PdS2 ·2CH2Cl2: C, 53.20; H, 3.69; N, 1.19. Found: C,
53.05; H, 3.52; N, 1.24.
Representative Example of Preparation of a Carbonyl
Complex, {Pd(CO)[Indolyl(Ph2PS)2]}BF4 (6). A suspension of
AgBF4 (8 mg, 0.04 mmol) in CD2Cl2 (0.8 mL) was transferred over 4c
(25 mg, 0.032 mmol) in a Shlenk in order to generate the cationic
species. The mixture was stirred 1 h at rt in the dark. A yellow solution
was obtained with a white precipitate. The solution was then filtered in
a pressure NMR tube, and CO (5 bar) was added. A full conversion
was observed by 31P NMR. IR (CD2Cl2): νCO = 2151 cm−1; 1H NMR
(500 MHz, CD2Cl2) δ 7.94−7.83 (m, 8H, PPh2), 7.81−7.78 (m, 2H,
PPh2), 7.76−7.72 (m, 6H, PPh2), 7.68−7.65 (m, 4H, PPh2), 7.29−
7.24 (m, 2H, Hbenzo), 7.18−7.15 (m, 1H, Hbenzo), 6.84−6.82 (m, 1H,
Hbenzo); 31P{1H} NMR (202 MHz, CD2Cl2) δ 76.7 (s, P1), 52.3 (s,
P2); 13C NMR (125 MHz, CD2Cl2) δ 176.4 (t, J(P,C) = 13 Hz, CCO);
166.8 (dd, J(P,C) = 38 Hz, 17 Hz, C2), 141.5 (dd, J(P,C) = 10 Hz, 4
Hz, C4/9), 136.1 (d, J(P,C) = 3 Hz, CparaPhP), 134.4 (d, J(P,C) = 3 Hz,
CparaPhP), 132.7 (d, J(P,C) = 13 Hz, CmetaPhP), 132.3 (d, J(P,C) = 12
Hz, CmetaPhP), 131.2 (dd, J(P,C) = 14 Hz, 5 Hz, C4/9), 130.3 (d,
J(P,C) = 14 Hz, CorthoPhP), 129.8 (d, J(P,C) = 14 Hz, CorthoPhP),
125.9 (d, J(P,C) = 88 Hz, CipsoPhP), 125.2 (Cbenzo), 125.0 (Cbenzo),
123.9 (d, J(P,C) = 97 Hz, CipsoPhP), 119.3 (Cbenzo), 112.2 (Cbenzo).
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Ladeira, S.; Martin-Vaca, B.; Bourissou, D. Organometallics 2011, 30,
6416. (d) Nebra, N.; Saffon, N.; Maron, L.; Martin-Vaca, B.;
Bourissou, D. Inorg. Chem. 2011, 50, 6378. (e) Nebra, N.; Ladeira,
S.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Chem.Eur. J. 2012, 18,
8474.
(6) For the selective functionalization of C3 using Grignard reagents,
see for example: (a) Reinecke, M. G.; Sebastian, J. F.; Johnson, H. W.,
Jr.; Pyun, C. J. Org. Chem. 1972, 37, 3066. (b) Ikunaka, M.; Kato, S.;
Sugimori, D.; Yamada, Y. Org. Process Res. Dev. 2007, 11, 73.
(7) These geometric features are similar to those reported for related
complexes. See for example: Evans, D. R.; Huang, M.; Segenish, W.
M.; Fettinger, J. C.; Williams, T. L. Organometallics 2002, 21, 893.
(8) 31P NMR has been occasionally used as a probe to compare the
electronic properties of ligands in trans position; see Hohman, W. H.;
Kountz, D. J.; Meek, D. W. Inorg. Chem. 1986, 25, 616. However, as
chemical shifts can be affected by other factors, conclusions are
difficult to draw.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and characterization data, including
crystallographic data for 2 (CCDC 931614) and 4 (CCDC
931615). This material is available free of charge via the
Internet at http://pubs.acs.org.
(9) See Supporting Information for details.
(10) Trans influence can be at the origin of important differences in
reactivity, as recently demonstrated for PCP palladium hydride pincer
complexes; see: Gerber, R.; Fox, T.; Frech, C. M. Chem.Eur. J. 2010,
16, 6771.
AUTHOR INFORMATION
■
(11) Tollman, C. A. Chem. Rev. 1977, 77, 813.
Corresponding Author
(12) Kundig, E. P.; McIntosh, D.; Moskovits, M.; Ozin, G. A. J. Am.
̈
Chem. Soc. 1973, 95, 7234.
*E-mail: dbouriss@chimie.ups-tlse.fr (D. Bourissou); bmv@
chimie.ups-tlse.fr (B. Martin-Vaca). Fax: 33 (0)5 61 55 82 04.
Tel: 33 (0)5 61 55 68 03.
(13) Under vacuum, CO rapidly dissociates and complexes 6−8 give
back the corresponding Pd precursors. Pressurization with CO
regenerates the carbonyl complexes.
Notes
(14) Treatment of the chloro indenediide palladate with silver salts
The authors declare no competing financial interest.
does not induce chloride abstraction but rather leads to bimetallic
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Organometallics
Article
complexes. This is reminiscent of what we observed with other
electrophilic metal fragments.5d,e
(15) (a) Yun, J.-G.; Seul, J. M.; Lee, K. D.; Kim, S.; Park, S. Bull.
Korean Chem. Soc. 1996, 17, 311. (b) Bolliger, J. L.; Blacque, O.; Frech,
C. M. Angew. Chem., Int. Ed. 2007, 46, 6514. (c) Bolliger, J. L.;
Blacque, O.; Frech, C. M. Chem.Eur. J. 2008, 14, 7969. (d) Gerber,
R.; Blacque, O.; Frech, C. M. Chem. Cat. Chem. 2009, 1, 393.
(e) Zucca, A.; Petretto, G. L.; Cabras, M. L.; Stoccoro, S.; Cinellu, M.
A.; Manassero, M.; Minghetti, G. J. Org. Chem. 2009, 694, 3753.
(16) Frech et al. have compared the electronic properties of POCOP
and P(NH)C(NH)P Pd pincer complexes using the same method-
ology, and the νCO band was found to shift by 27 cm−1.14
c
(17) Selander, N.; Szabo,
(18) (a) Solin, N.; Kjellgren, J.; Szabo,
2003, 42, 3656. (b) Solin, N.; Kjellgren, J.; Szabo,
Soc. 2004, 126, 7026. (c) Wallner, O. A.; Szabo,
K. J. Chem.Eur. J.
2006, 12, 6976.
(19) Szabo et al. have noticed that a proper balance is necessary for
́
K. J. Chem. Rev. 2011, 111, 2048.
K. J. Angew. Chem., Int. Ed.
K. J. J. Am. Chem.
́
́
́
́
pincer complexes to catalyze efficiently this transformation, because
the formation and reaction of the η1-allyl complex have opposite
electron demands.17
(20) At this point, it is difficult to compare the catalytic efficiency of
the indolyl and indenyl complexes because of stability issues. The two
systems give identical results after 5 h (within margin error), but when
the reactions are pursued over 21 h, a substantially higher yield is
obtained with the indenyl over the indolyl complex (63% vs 41%). The
indolyl complex 3 tends to decompose over time under the catalytic
conditions (as apparent from the formation of black insoluble
material).
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