Formation of six-membered palladacycles from phenanthroline Pd(II) bisacetate precursors and phenylisocyanate Dedicated to Professor John A. Osborn.

Article abstract of DOI:10.1016/j.crci.2013.10.024

Phenylisocyanate reacts with palladium(II) bis-acetate phenanthroline complexes to give six-membered palladacycles in nearly quantitative yields. In this new reaction, the acetate ligands act as decarbonylating agents toward the isocyanate functionality by possibly forming the isolated palladacycles via an intramolecular rearrangement.

Full text of DOI:10.1016/j.crci.2013.10.024

C. R. Chimie 17 (2014) 521–525
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Preliminary communication/Communication
Formation of six-membered palladacycles from
phenanthroline Pd(II) bisacetate precursors and
phenylisocyanate
a
a
b
a,
*
´ ´
Solenne Moulin , Olivier Pellerin , Loı¨c Toupet , Frederic Paul
a Institut des sciences chimiques de Rennes (UMR CNRS 6226), Universite´ de Rennes-1, campus de Beaulieu, 35042 Rennes cedex, France
b Institut de physique de Rennes (UMR 6251 CNRS), Universite´ de Rennes-1, campus de Beaulieu, 35042 Rennes cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Phenylisocyanate reacts with palladium(II) bis-acetate phenanthroline complexes to give
six-membered palladacycles in nearly quantitative yields. In this new reaction, the acetate
ligands act as decarbonylating agents toward the isocyanate functionality by possibly
forming the isolated palladacycles via an intramolecular rearrangement.
Received 16 September 2013
Accepted after revision 31 October 2013
Available online 24 March 2014
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Dedicated to Professor John A. Osborn.
Keywords:
Isocyanate
Dimine Pd(II) complex
Palladacycle
Catalytic carbonylation of nitroaromatics
´
´
R E S U M E
´
´
´
L’isocyanate de phenyle reagit avec des complexes phenanthroline de palladium(II) bis-
´
Mots cles :
´
`
ˆ
acetate pour former quasi quantitativement des palladacycles a six chaınons. Dans cette
Isocyanate
nouvelle re´action, le ligand ace´tate joue possiblement le roˆle d’agent de´carbonylant vis-a`-
Complexes de diamine Pd(II)
Palladacycle
´
´
vis de l’isocyanate en formant les palladacycles isoles par un rearrangement intramo-
Carbonylation catalytique des
nitroaromatiques
´
leculaire.
ß 2013 Acade´mie des sciences. Publie´ par Elsevier Masson SAS. Tous droits re´serve´s.
1. Introduction
mation about the possible deactivation pathways of
catalytic processes involving isocyanate as a reactant or a
product [8–10]. In this regard, we have previously shown
that diimine-Pd(0) precursors catalytically trimerize aryli-
socyanates to form triarylisocyanurates (1, Scheme 1) [11],
while some inactive palladacycles like 2, 3 or 4a [9,12],
whichpossiblyresultfromdeactivationoftheactivespecies,
are eventually also formed.
To date, the reactivity of isocyanates in the coordination
sphere of various group-VIII metal centers is still a field only
partially explored [1–3]. Such reactions can provide a very
valuable access to various types of heterocycles and are thus
especially attractive with respect to atom-economy con-
siderations [4–7]. Moreover, they often bring some infor-
In the process of exploring the scope of this reaction, we
have now investigated the reaction of PhNCO with various
Pd(II)-phenanthroline precursors, like Pd(N–N)(OAc)2
complexes (5a-c), where N–N represents 1,10-phenan-
throline (o-phen), 3,4,7,8-tetra-methylphenanthroline
*
Corresponding author.
E-mail addresses: loic.toupet@univ-rennes1.fr (L. Toupet),
frederic.paul@univ-rennes1.fr (F. Paul).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.10.024

522
S. Moulin et al. / C. R. Chimie 17 (2014) 521–525
Scheme 1. Cyclotrimer and palladacycles formed from diimine-Pd(0) complexes.
(tmphen), and 2,9-dimethylphenanthroline (dmphen),
respectively. In a manner similar to the reaction with
Pd(0) precursors, a reaction yielding the six-membered
palladacycles 4a–c took place, providing thereby an easy
access to these interesting compounds.
2. Results and discussion
Scheme 2. Reaction between 5a and PhNCO.
When reacted with one equivalent of phenylisocyanate
in dichloromethane at 20 8C, the bis-acetato diimine Pd(II)
complex 5a [13,14] forms the known 6-membered
palladacycle 4a [9,12] in ca. 30% yield after two days. In
presence of an excess of PhNCO (20 equiv), this reaction
becomes nearly quantitative, allowing isolation of ca. 95%
of 4a after the same time. This reaction can also be
performed at higher temperature in an aromatic solvent
like toluene, 4a being then quantitatively isolated after
16 h (Scheme 2).
Scheme 3. Reaction between 5c and PhNCO.
This reaction can also be performed with the analogue
of 5a possessing a tmphen (5b) ligand instead of phen to
give the corresponding palladacycle 4b in good yields. This
new palladacycle was characterized by high-resolution
mass spectrometry (HRMS), infrared and 1H NMR spectro-
scopies. In contrast, this reaction is not observed with
other Pd(II) precursors such as Pd(o-phen)Cl2, Pd(OAc)2 or
from an equimolar Pd(acac)2/o-phen mixture, pointing at
the need to have a carboxylate anion and a diimine ligand
simultaneously present in the medium for this reaction to
proceed. Notably, we have checked that this reaction takes
place with similar yield from a carefully dried complex 5a
in anhydrous toluene, suggesting that traces of water are
not involved in its mechanism.1
We then turned our attention toward the Pd(II)
precursor 5c, presenting the more bulky dmphen ligand
in place of o-phen [14], and with this sterically crowded
Pd(II) complex, the reaction takes also place and a nearly
complete conversion of 5c into 4c is observed in
dichloromethane at ambient temperature after two days
(Scheme 3). The new palladacycle 4c is the 2,9-dimethy-
lated analogue of 4a. This complex was fully characterized,
allowing notably the observation of the two strong
carbonyl nCO stretching modes near 1645 and 1625 cm–1
diagnostic of the three carbonyl groups of this metallacycle
[9]. Attempts to perform this reaction at 80 8C in toluene or
other aromatic solvents were, however, not successful with
partial decomposition of 5c taking place.
,
The steric pressure exerted by the two methyl groups is
revealed by the X-ray structure presently obtained for
2
˚
5cÁOEt2ÁOH2 (Fig. 1). It results in a bending of ca. 30 of the
phenanthroline core with the mean plane defined by the
square planar coordination sphere around the Pd(II) atom.
These crystallographic data are in line with those
previously obtained for this and the related complex 5a,
and clearly indicate that 5a–c are mononuclear species
[14,15].
This reaction presents some analogy with the trimer-
ization reaction of PhNCO catalyzed by ‘‘(N–N)Pd0’’ species
previously studied by some of us [11]. However, the
isocyanurate cyclotrimer 1 was never isolated as a side-
product during any reaction of 5a with PhNCO, suggesting
that ‘‘(o-phen)Pd0’’ is presently not transiently formed in
2
This solvated complex presents nearly identical crystallographic
parameters than two different solvates of 5c previously reported.[14]
1
We have stated that Pd(II) complexes 5a–c often give rise to quite
Crystallographic data after refinement without solvate(s): PdC18H18N2O4,
˚
stable solvates with water molecules. A water-free sample of 5a could
however be obtained after reacting this complex with acetic anhydride in
nitrobenzene (12 h, 80 8C) followed by recrystallization with Et2O from
nitrobenzene.
M = 524.88, monoclinic, space group C 2/c, a = 18.3528(5) A, b = 18.1223(6)
3
˚
˚
b = 119.459(1) , U = 3882.6(2) A , Z = 4, T = 120(2) K, Dc
˚
˚
A, c = 13.4071(4) A,
= 1.481 g cm–3. The final wR(F2) was 0.073 (all data). CCDC deposition No.:
CCDC 221834.

S. Moulin et al. / C. R. Chimie 17 (2014) 521–525
523
Scheme 4. Reaction mechanism proposed for the formation of 4a–c.
the medium. Furthermore, it had been previously shown
that the catalytic trimerization of PhNCO was not taking
place with the sterically crowded 2,9-dimethylphenan-
throline ligand. When this ligand was used, decomposition
of the catalyst into metallic palladium was observed [11].
In contrast, the formation of palladacycle 4c takes readily
place at ambient temperatures with the complex 5c. Thus,
it is quite likely that the mechanistic pathway for the
formation of 4a–c does not involve an elusive ‘‘(N–N)Pd0’’
intermediate. Actually, the reaction is observed only from
dimine Pd(II) precursors having acetate ligands. A possible
reaction mechanism should therefore involve the Pd(II)
diimine center and the acetate ligand(s).
acting as the nucleophile. This makes sense given the fact
thatsimpleacetatesaltssuchasKOAc, ina manner similarto
many other inorganic salts [17], are known to catalyze the
cyclotrimerization of arylisocyanates [18]. Moreover, their
activity as catalysts in this reaction has been shown to be
strongly influenced by the nature of the associated cation
[19]. However, this anionic oligomerization process would
not ultimately lead to the cyclotrimer 1, but to the
palladacycle 4a, due to the presence of the second acetate
ligand on the nearby Pd(II) center. Indeed, the final steps
would involve an intramolecular rearrangement taking
place by elimination of acetic anhydride and carbon dioxide
to form the metallacyclic core by ring closure. Such an
acetate-based intramolecular rearrangement resemble the
one proposed by Moiseev to rationalize the reduction of
Pd(II) acetate clusters in presence of CO [20], except that the
‘‘CO’’ molecule is presently replaced by PhNCO. Then,
depending on the number of inserted isocyanate molecules
after ring closure, either 3a–c (n = 1) or 4a–c (n = 2) would be
generated in the medium. Since we have previously clearly
established that strained palladacycles such as 3a react
readily with arylisocyanate to form 4a [9], this mechanism
rationalizes the selective formation of the 6-membered
palladacycles 4a–c in the presence of an excess of PhNCO.3
Also, based on literature reports [14,22], the acetate ligand
should be more labile in 5c than in 5a. If the first insertion of
isocyanate in the Pd–OAc bond in the former complex is rate
determining, the reaction might take place as fast with 5c
than with 5a, in line with our preliminary observations.
A tentative mechanistic proposal for this transformation
is given in Scheme 4. The first step would involve the formal
insertion of PhNCO into the Pd–O bond of the carboxylate
ligand, possibly on a [Pd(N–N)(OAc)]+ intermediate, fol-
lowed by further insertion of PhNCO into the resulting Pd–N
bond (n = 2 or 3). These steps would be in line with the well-
established insertion chemistry for this kind of cationic
species[16]. Itisindeed wellknownthatonsuchcomplexes,
the carboxylate ligands are labile and can be exchanged for
other ligands [9,14]. Alternatively, this reaction sequence
might be seen as
a
[Pd(N–N)(OAc)]+-assisted anionic
oligomerization of PhNCO by the acetate ion, the latter
3. Conclusions
In conclusion, we report here a novel and efficient
reaction toward the six-membered palladacycles 4a–c
from air-stable phenanthroline Pd(II) acetate complexes
and phenylisocyanate. While revealing that the particular
complexes 5a–c are reactive toward arylisocyanates, this
contribution clearly shows that other synthetic routes than
these previously evidenced from [(N–N)Pd0] intermediates
are possible to obtain palladacycles such as 2–4. Thus, such
species might also be directly formed from Pd(II) catalyst
3
Fig. 1. ORTEP plot of the palladacycle of 5cÁOEt2ÁOH2. Thermal ellipsoids
With n = 0, a Pd-imido diimine intermediate would be formed rather
are at the 50% probability level (see supporting information for details).
than a metallacycle. While such kind of complex has recently been
claimed to play a determining role in the catalytic carbonylation of
nitroaromatics [21], we did not consider the possibility of its inter-
mediacy here. However, based on the available data [1,9], such a complex
should also transform quantitatively into 4a–c in presence of excess
PhNCO.
˚
Selected bond lengths (A) and angles (deg): Pd1–O1 1.998(3), Pd1–O2
3.015, Pd1–O3 2.008(3), Pd1–O4 2.963, Pd1–N1 2.021(4), Pd1–N2
2.033(4), C15–O1 1.280(8), C15–O2 1.259(9), C17–O3 1.290(7), C17–
O4 1.219(7), N1–Pd1–N2 81.64(16), O1–Pd1–O3 85.74(16), O1–Pd1–N1
94.58(16), O3–Pd1–N2 96.89(16).

524
S. Moulin et al. / C. R. Chimie 17 (2014) 521–525
precursors when carboxylate ligands are present in the
medium. This observation is particularly important in the
context of Pd-catalyzed reductive carbonylations of
nitroaromatics, because (Pd(II)/diimine/carboxylic acid)
catalytic systems are among the most active ones [10] and
because the formation of 2–4 and related palladacycles
[23] has been proposed to be at the origin of the rapid
deactivation of these catalytic systems [8,9], especially
when these reactions are performed in the absence of
alcohol or amine able to trap the isocyanate formed [24].
The scope and reaction mechanism of this new reaction is
currently under investigation.
followed by addition of an excess of diethylether. The
suspension was then decanted and washed with several
fractions of diethylether, yielding 39 mg of a brown–
orange solid after vacuum-drying. This solid is identified as
palladacycle 4b (54%). Color: brown–orange. HRMS
(positive ESI, CH2Cl2/MeOH, m/Z) z 672.1578 [M+H]+, m/
z calculated for [C36H32N5O2Pd] 672.1591. FT-IR (KBr, n,
cm–1) 1642 (s, C=O), 1623 (s, C=O). 1H NMR (200 MHz,
3
CDCl3,
d
, ppm) 8.53 (d, JHH = 8 Hz, 4H, HPh), 8.05 (s, 24H,
3
Hphen), 7.88 (s, 2H, Hphen), 7.53 (t, JHH = 8 Hz, 4H, HPh),
7.36–6.98 (m, 7H, HPh), 2.68 (s, 6H, CH3/phen), 2.20 (s, 6H,
CH3/phen).
4. Experimental
Pd(dmphen){N(C6H5)C(O)N(C6H5C(O)N(C6H5)} (4c).
4.1. General
To 100 mg of Pd(dmphen)(OAc)2 (0.23 mmol) dissolved in
15 mL of CH2Cl2 were added 550 mg of phenylisocyanate
(ca. 20 equiv). The orange solution was stirred during two
days at ambient temperature. The solution was filtrated
and diethylether was added to precipitate the title
complex as an orange solid. It was collected, washed
with several fractions of diethylether and dried in vacuo,
yielding 130 mg of the title palladacycle (88%). Color:
orange. Dec. Pt: 180 Æ 10 8C. HRMS (positive ESI, CH2Cl2, m/
Z) 644.1291 [M+H]+, m/z calculated for [C34H28N5O2Pd]
644.1278. Anal. calculated for C34H27N5O2PdÁH2O: C, 61.68;
H, 4.42; N, 10.58; found: C, 61.67; H, 4.48; N, 10.52. FT–IR
(KBr/Nujol, n, cm–1) 1645 (s, C=O), 1625 (s, C=O). 1H NMR
The solvents were dried and distilled prior to use and the
reaction was performed under inert (Ar) atmospheres in
deaerated solvents. Unless otherwise specified, all reagents
were purchased from commercial suppliers and used
without further purification. Infrared spectra were recorded
on a Bruker IFS28 spectrometer (400–4000 cm–1). 1H NMR
spectra were obtained using Bruker SY 200 (200 MHz) or SY
400 (400 MHz) Fourier Transform spectrometers. Chemical
shifts are given in parts per million relative to tetramethyl-
silane (TMS) for 1H spectra. Mass spectral studies and
elemental analyses were entrusted to the corresponding
´
services of the Centre regional de mesures physiques de l’Ouest
3
(200 MHz, CD2Cl2, d, ppm) 8.26 (d, JHH = 8 Hz, 2H, Hphen),
(CRMPO). The complexes 5a–c were synthesized according
to previously reported procedures [13,14] (see also support-
ing information).
3
8.01 (d, JHH = 7 Hz, 4H, HPh), 7.90 (s, 2H, Hphen), 7.69 (d,
3JHH = 8 Hz, 2H, Hphen), 7.49 (t, 3JHH = 8 Hz, 2H, HPh), 7.37 (t,
3
3JHH = 8 Hz, 4H, HPh), 7.26 (d, JHH = 8 Hz, 2H, HPh), 6.89 (t,
3JHH = 7 Hz, 4H, HPh), 6.69 (t, 3JHH = 7 Hz, 1 H, HPh), 2.72 (s, 6H,
CH3/phen).
4.2. Reactions of Pd(N–N)(OAc)2 and PhNCO
4.3. X-ray crystallography
Pd(phen){N(C6H5)C(O)N(C6H5C(O)N(C6H5)} (4a).
Diffraction data frames for 5c were collected using a
CCD Saphire 3 Xcalibur apparatus using the graphite
˚
a radiation (l = 0.71073 A). The
cell parameters were obtained with Denzo and Scalepack
To 100 mg of Pd(phen)(OAc)2 (0.24 mmol) were added
600 mg of phenyl isocyanate (20 equiv) dissolved in 15 mL
of toluene. The pale-yellow suspension was subsequently
heated at 80 8C under stirring. After 16 h, an abundant
orange-yellow suspension had formed in the reaction
medium. The medium was cooled and 10 mL of ethanol
were added to neutralize the excess of isocyanate. The
suspension was then decanted and washed with several
fractions of diethylether, yielding 140 mg of a yellow-
orange solid after subsequent vacuum-drying. This solid is
identified as palladacycle 4a by comparison with authentic
samples (95%).
monochromatized Mo K
˚
[25], with 10 frames (psi rotation: 1 per frame). The data
collection leads to 4235 independent reflections, from
which 3338 with I > 2.0
s(I) [26]. The structures were
solved by direct methods using SIR97 program [27], and
then refined with full-matrix least-square methods based
on F2 (SHELX-97) [28]. The contribution of the disordered
solvents (i.e. one molecule of diethylether and one
molecule of water) to the calculated structure factors
was removed using the SQUEEZE option in PLATON.
Atomic scattering factors were taken from the Interna-
tional tables for X-ray crystallography (1992) [29]. Ortep
views were realized with PLATON98 [30].
Pd(tmphen){N(C H )C(O)N(C H C(O)N(C H )} (4b).
6
5
6
5
6 5
Acknowledgements
To 50 mg of Pd(tmphen)(OAc)2 (0.11 mmol) were added
630 mg of phenyl isocyanate (50 equiv) dissolved in 15 mL
of toluene. The pale-yellow suspension was subsequently
heated at 80 8C under stirring. After 16 h, the dark-red
reaction medium was cooled and 1 mL of EtOH was added,
ˆ
We thank the CNRS for financial support and ‘‘Rhone-
Poulenc Recherches’’ for a PhD grant (F.P.) when this work
was initiated under the supervision of J.A. Osborn.

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