Investigation of the reactivity of palladium(0) complexes with nitroso compounds: Relevance to the palladium-phenanthroline-catalysed carbonylation reactions of nitroarenes

Article abstract of DOI:10.1016/S0022-328X(99)00263-6

The electron transfer reaction between palladium(0) complexes and RNO compounds afforded different palladium species depending on the aromatic or aliphatic nature of R. When R=Ph a paramagnetic palladium complex 1 was isolated, whereas if R=Bu^t the palladium enolate complex 2 was the unexpected reaction product. Complex 1 reacted with methanol and CO to yield Pd(phen){C(O)OCH₃}₂ 3, which was characterised by single-crystal X-ray structure determination. Compound 3 is a probable intermediate in the reductive carbonylation reaction of organic nitro compounds catalysed by palladium complexes. Nitrobenzene is in fact carbonylated to PhNHCO₂Me, by using 3 as a very efficient catalyst.

Full text of DOI:10.1016/S0022-328X(99)00263-6

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Journal of Organometallic Chemistry 586 (1999) 190–195
Investigation of the reactivity of palladium(0) complexes with
nitroso compounds: relevance to the palladiumꢀphenanthroline-
catalysed carbonylation reactions of nitroarenes
Emma Gallo a, Fabio Ragaini a, Sergio Cenini a,*, Francesco Demartin b
a Dipartimento di Chimica Inorganica, Metallorganica e Analitica and CNR Center, 6ia G. Venezian 21-20133, Milan, Italy
b Dipartimento di Chimica Strutturale e Stereochimica Inorganica and CNR Center, 6ia G. Venezian 21-20133, Milan, Italy
Received 12 March 1999; received in revised form 5 May 1999
Abstract
The electron transfer reaction between palladium(0) complexes and RNO compounds afforded different palladium species
depending on the aromatic or aliphatic nature of R. When R=Ph a paramagnetic palladium complex 1 was isolated, whereas if
R=But the palladium enolate complex 2 was the unexpected reaction product. Complex 1 reacted with methanol and CO to yield
Pd(phen){C(O)OCH3}2 3, which was characterised by single-crystal X-ray structure determination. Compound 3 is a probable
intermediate in the reductive carbonylation reaction of organic nitro compounds catalysed by palladium complexes. Nitrobenzene
is in fact carbonylated to PhNHCO2Me, by using 3 as a very efficient catalyst. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Palladium; Phenanthroline; Carbonylation reactions; Nitroarenes; X-ray structure
complexes [5]. Therefore we reasoned that the investiga-
tion of the reactivity of Pd(0) complexes towards ni-
troso compounds might shed some light on the
mechanism of these reactions.
1. Introduction
The reductive carbonylation of nitro compounds is a
field of great interest and much effort has been devoted
to clarify the mechanisms of several catalytic cycles [1].
Palladium phenanthroline complexes [2,3,20] are the
most active catalysts for this class of reactions. A
probable intermediate in the catalytic cycle, the metalla-
cycle (NꢀN)P¸dC¹(¹O¹)O¹¹N¹(A¹r¹)Cº(O), has been obtained
from the reaction between Pd(phen)(OAc)2 (phen=
1,10-phenanthroline) and nitrobenzene, under CO pres-
sure [3], and recently characterised by X-ray structure
determination [4]. It is believed that the formation of
this intermediate is the result of a transient palla-
dium(0) species but neither this last complex, nor any
of the intermediates proposed for the reaction leading
to the isolated metallacycles, could even be observed by
using nitroarenes as reagents. The reductive carbonyla-
tion of nitro compounds occurs through a deoxygena-
tion reaction and the formation of nitrosoareneꢀmetal
2. Results and discussion
First we reacted Pd(phen)(pbq) (pbq=p-benzo-
quinone) [6], a source of ‘Pd(0)(phen)’, with different
amounts of nitrosobenzene and in different solvents
(THF, diglyme, methanol). Invariably an insoluble
green paramagnetic product 1, was obtained. Complex
1
was the only isolated product even when
Pd(phen)(pbq) was replaced by Pd(phen)(dba) (dba=
dibenzylidene acetone) as starting material. The last
complex was generated in situ by reaction of
Pd2(dba)3·CHCl3 with 1,10-phenanthroline [7] before
the addition of PhNO. Based on elemental analysis, the
composition Pd2(phen)2(PhNO)3 can be proposed for 1,
but its insolubility also suggests it to be a polymer.
In addition to that, in the FAB+ mass spectrum
* Corresponding author. Tel.: +39-2-2364512; fax: +39-2-
2362748.
a
parent peak at
M
+ =500 attributable to
E-mail address: scenini@csmtbo.mi.cnr.it (S. Cenini)
Pd(phen)(PhNO)2, and peaks corresponding to the loss
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00263-6

E. Gallo et al. / Journal of Organometallic Chemistry 586 (1999) 190–195
191
of one and two PhNO groups were observed. The IR
spectrum of complex 1 showed a sharp peak at 848
cm−1 that may be attributed to the w(NꢀO) [8] of the
h2-bonded nitroso ligand. Complex 1 showed a broad
unresolved EPR signal centred at 3208 G with g=
2.0001. The EPR spectrum did not present any hy-
perfine structure, although this may in part be due to
the fact that we had to perform the analysis in the solid
state, given the high insolubility of 1, and at room
temperature (r.t.). In any case, these magnetic data
suggest an electron transfer from palladium(0) to the
nitroso ligand to yield a Pd-anionic radical nitroso
complex. However, the analytical and spectroscopic
data did not allow us to identify the exact structure of
1 and unfortunately, the high insolubility of the
product prevented a structural characterisation.
cent’ ligand [10] and the same is true for nitroarenes.
The electron transfer from rhodium(-I) [11] and ruthe-
nium(0) [12] complexes to nitro compounds has been
followed in situ by EPR but the final organometallic
products of these reactions are diamagnetic complexes.
On the other hand the reaction between Pd(phen)L
(L=pbq, dba) and nitrosobenzene allowed for the
isolation of complex 1 as a stable paramagnetic
compound.
To synthesise a compound more soluble than com-
plex 1, to have more chances for a structural character-
isation, PhNO was replaced by ButNO. However, no
reaction was observed between Pd(phen)(pbq) and
ButNO even after several days at r.t. On the other
hand, a reaction occurred when ButNO was added to
an acetone suspension of Pd2(dba)3·CHCl3 also con-
taining 1,10-phenanthroline, under the same conditions
in which the use of PhNO afforded 1. Unexpectedly, a
different product 2 was obtained for which the struc-
ture shown in Scheme 1 can be proposed. If no nitroso
compound was added, Pd(dba)(phen) was isolated in an
analytically pure form [7].
Electron transfer reactions between transition metal
complexes and nitroso compounds are quite common
[9] due to the spin trapping capacity of this ‘non-inno-
The structure of 2 is supported by the analytical and
spectroscopic data. The peaks at 1633 and 1622 cm−1
,
observed in the IR spectrum, had been attributed to the
1
carbonyl groups of the enolate fragment. The H-NMR
spectrum showed the well-known pattern of the 1,10-
phenanthroline ligand, with a different chemical shift
for the ortho protons of the nitrogen atoms, and two
singlets for the ꢀCH2ꢀ and ꢀCH3ꢀ groups of the enolate
1
ligand in the correct integrated ratio. The H-resonance
Scheme 1.
of the methylene group was observed at l 3.11 and this
resonance indicates that the enolate is C-bound [13].
Moreover the diamagnetic behaviour of the palladi-
um(I) complex 2 requires the presence of a metalꢀmetal
bond.
The structure shown in Scheme 1 is also consistent
with the 13C-NMR APT (attached proton test), the
13C-NMR off-resonance and the two-dimensional hete-
rocorrelate inverse spectroscopy (HMQC) experiments
(Fig. 1). The 15N-NMR INEPT (insensitive nuclei en-
hancement by polarisation transfer) experiment showed
only the 1,10-phenanthroline peak at −159.9 ppm
(nitromethane was used as reference) indicating that no
nitrogen atom derived from ButNO is coordinated to
the metal centre.
The parent peak in the FAB+ mass spectrum is due
to the Pd(phen)(CH2COCH3)2 fragment and peaks indi-
cating loss of one or two ꢀCH2COCH3 fragments were
also present, but no peaks corresponding to the loss of
ButNO derived fragments were detected. The molecular
peak could not be observed.
Finally, the elemental analysis of complex 2 was
consistent with the proposed structure, but also showed
the presence of one CHCl3 molecule for dimeric unit.
The presence of CHCl3 in the compound was also
Fig. 1. A two-dimensional heterocorrelate inverse spectroscopy spec-
trum (HMQC) of complex 2 with: A, 1H-NMR spectrum; B, 13C-
NMR APT spectrum (Attached Proton Test).
1
confirmed by a H-NMR spectrum of 2 in (CD3)2CO.

192
E. Gallo et al. / Journal of Organometallic Chemistry 586 (1999) 190–195
Fig. 2. ORTEP drawing of complex 3. Thermal ellipsoids are drawn at 30% probability.
The experimental procedures to synthesise complex 1
the ones used in catalysis. The complex Pd-
(phen){C(O)OCH3}2 3 (Fig. 2) was obtained as yellow
crystals together with several organic products deriving
from the reduction of nitrosobenzene [azoxybenzene
(70%), methyl phenylcarbamate (18%), aniline (6%),
azobenzene (2%), diphenylurea (2%)]. A small amount of
nitrobenzene (2%), deriving from the disproportionation
of nitrosobenzene [14], was also observed.
Complex 3 could also be synthesised either directly
from the reaction of Pd(phen)(pbq) with PhNO under CO
pressure in methanol or according to a slight modification
of the published procedure for the synthesis of Pd(2,2%-
bipy){C(O)OCH3}2 [15] (we used NaOCH3 instead of
LiOCH3 to obtain 3 as a yellow powder in 70% yield).
Very recently Romano and co-workers [16] determined
the structure of 3 by ab initio X-ray powder diffraction
methods. However, we were able to grow crystals of this
material suitable for X-ray diffraction and here we report
the single-crystal X-ray structure determination for this
compound. The molecular structure of 3 with the atom
labelling scheme adopted is shown in Fig. 2.
and 2 were identical, therefore the different pathways for
the two reactions must be due to the different nature of
the nitroso compounds. In both cases the first step of the
reaction appears to be an electron transfer from the metal
center to a nitroso ligand to yield an anionic radical
nitroso species and a palladium(I) complex. The anionic
radical phenyl nitroso collapses with the palla-
diumꢀphenanthroline fragment to generate the paramag-
netic nitroso palladium complex 1, whereas the anionic
t-butyl nitroso radical is responsible for the deprotona-
tion of the acetone molecule. The resulting enol species
coordinates to the palladium center to afford the palla-
dium enolate complex 2 (Scheme 1).
Compound 1 is unlikely to be formed during the
catalytic carbonylation of nitrobenzene catalysed by
palladiumꢀphenanthroline complexes, but mono- or di-
nuclear complexes in which the extent of reduction of
nitrobenzene is the same as the one found in 1 may be
formed. To investigate this aspect, we reacted 1 with
CH3OH under CO pressure under conditions similar to

E. Gallo et al. / Journal of Organometallic Chemistry 586 (1999) 190–195
193
Pd(phen){C(O)OCH3}2 3 was reacted with aniline in
CH3OH and under CO atmosphere diphenylurea was
obtained with methyl phenyl carbamate in a 95:5 ratio
(by GC and HPLC analysis), thus supporting an active
role of complex 3 in the catalytic reaction (Scheme 2).
To further support this active role of 3 we compared
the results of a ‘classic’ catalytic reaction, employing
[Pd(phen)2](BF4)2 as catalyst [20], with one in which 3
was used instead. Both of the reactions were performed
in the presence of benzoic acid as promoter [21]. The
conversion of nitrobenzene was even higher in the case
of 3 (27.5% vs. 21.7%) while the selectivities were
similar in the two cases.
Selected interatomic distances and angles are re-
ported in Table 1. A comparison of our data with those
obtained from X-ray powder diffraction [16] shows that
there is no significant difference in the geometrical
parameters of the molecule obtained with the two tech-
niques. The values are also very close to those of the
analogous bonds and angles observed in Pd(2,2%-
bipy){C(O)OCH3}2 [15].
In their studies on the reductive carbonylation of
nitroarenes catalysed by Ru(CO)3(dppe) (dppe=1,2-
bis(diphenylphosphino)ethane), Gladfelter and co-
workers have demonstrated that this ruthenium
complex reacts with nitroarenes to initially afford the
nitrosoarene complex Ru(CO)2(dppe)(h2-ArNO) [5]. In
the presence of methanol and CO, this complex affords
Ru(dppe)(CO)2{C(O)OCH3}2 [17] and aniline. The ani-
line then attacks the carbomethoxy complex generating
an arylisocyanate molecule that is then trapped by
excess aniline to generate a diarylurea [18]. Only at a
later stage of the catalytic reaction the diarylurea reacts
with methanol to afford a methyl arylcarbamate and
one equivalent of aniline.
During our studies on the mechanism of the palla-
diumꢀphenanthroline-catalysed carbonylation of ni-
troarenes, we discovered that the reaction proceeds
through at least three independent pathways, two of
which pass through the aniline as an intermediate [19].
By analogy with ruthenium chemistry it may be pro-
posed that complex 3 is an intermediate in one (or
both) of the palladium-based catalytic cycles passing
through the aniline as an intermediate. Indeed, when
3. Conclusions
The aim of this work was to investigate the reaction
of palladium(0) complexes with nitroso compounds to
improve our knowledge about the mechanism of the
catalytic reductive carbonylation of nitro compounds.
The isolation of a paramagnetic complex 1 suggests
that the first step of the interaction between Pd(0) and
the nitroso species is an electron transfer reaction.
Moreover complex 3 had already been proposed to be
an intermediate in the catalytic cycle [21], and the
isolation of this palladium carbomethoxy complex from
the reaction of a palladium(0) complex with PhNO
under CO pressure in methanol supports this proposal.
Complex 3 is currently under deeper investigation for
confirming its importance as intermediate in the reduc-
tive carbonylation of nitro compounds.
The different reactivity of ButNO with respect to the
one of PhNO led to the isolation of 2, which is by itself
an interesting compound, although it is surely not an
intermediate in the carbonylation reactions of organic
nitro compounds. The insertion into the PdꢀC bond of
unsaturated molecules might demonstrate a palladium-
assisted transfer of the enolate moiety to substrates
which would be unlikely to react in a metal-free reac-
tion [22]. On the other hand, it has already been shown
[23] that enolate complexes of palladium(II) react with
organic electrophilic species to form new CꢀC bonds.
Table 1
,
Selected interatomic distances (A) and angles (°) for 3
,
Bond length (A)
PdꢀN(1)
PdꢀC(15)
O(1)ꢀC(15)
O(2)ꢀC(15)
O(2)ꢀC(16)
2.145(4)
1.975(5)
1.206(6)
1.356(6)
1.457(7)
PdꢀN(12)
PdꢀC(17)
O(3)ꢀC(17)
O(4)ꢀC(17)
O(4)ꢀC(18)
2.117(4)
1.991(5)
1.178(7)
1.351(7)
1.442(8)
Bond angles (°)
N(1)ꢀPdꢀN(12)
N(12)ꢀPdꢀC(17)
N(1)ꢀPdꢀC(17)
78.4(2)
95.0(2)
172.3(2)
N(1)ꢀPdꢀC(15)
C(15)ꢀPdꢀC(17)
N(12)ꢀPdꢀC(15)
97.2(2)
89.9(2)
171.8(2)
4. Experimental
4.1. General comments
Unless otherwise specified all reactions were carried
out under a purified dinitrogen atmosphere using
Schlenk techniques. Solvents were dried and distilled
before use by standard methods. Gas chromatographic
analyses were performed on a Perkin–Elmer 8420 capil-
lary gas chromatograph equipped with a RTX 5 amine
column. HPLC analyses were performed on a Hewlett–
Scheme 2.

194
E. Gallo et al. / Journal of Organometallic Chemistry 586 (1999) 190–195
Packard 1050 instrument equipped with a Purospher®
RP-18 e (5 mm) column. Infrared spectra were recorded
on a Bio Rad FTS-7 FT-IR spectrometer, and NMR
spectra using a Brucker 200-AC or a Brucker 300-AC
instrument. Elemental analyses and mass spectra were
recorded in the analytical laboratories of Milan Univer-
sity. EPR spectra were recorded on a Varian E-109
spectrometer operating at x-band frequencies.
2H, J=5.1 Hz); 8.45 (m, 4H); 7.96 (s, 4H); 7.88 (m,
4H); 3.11 (s, 4H); 2.44 (s, 6H). 13C-NMR (CDCl3, 75
MHz, 298 K) l, ppm: 214.1 (CO), 152.6 (CH), 149.8
(CH), 138.3 (CH), 138.2 (CH), 130.3(C), 129.9 (C),
127.6 (CH), 127.5 (CH), 126 (CH), 125.5 (CH), 31
(CH3), 25.6 (CH2). IR (nujol, wmax cm−1) 1633 (s)
(CꢁO), 1622 (s) (CꢁO), 1513 (m), 1377 (m), 1232 (m),
836 (m), 714 (s).
4.2. Reaction of Pd(phen)(pbq) with PhNO
4.5. Preparation of Pd(phen){C(O)OCH3}2 3
PhNO (206 mg, 1.92 mmol) was added to a suspen-
sion of Pd(phen)(pbq) (189 mg, 0.48 mmol) in THF (20
ml). The resulting red suspension was stirred at r.t. for
48 h to yield a green microcrystalline solid that was
collected by filtration, washed with Et2O (10 ml), to
eliminate the excess PhNO, and dried in vacuo (70%).
Found C, 56.82; H, 3.69; N, 10.85. [C42H31N7O3Pd2]n
requires C, 56.39; H, 3.49; N, 10.96%. IR (nujol,
wmax cm−1) 1515 (m), 1427 (m), 1310 (m), 1283 (s), 1206
(m), 1108 (w), 1070 (w), 848 (s), 756 (s), 716 (s), 685 (s),
671 (s).
Complex 1 (100 mg, 0.11 mmol) was suspended in
CH3OH (10 ml) and the resulting green suspension was
placed in a glass liner inside an autoclave. The auto-
clave was frozen at dry ice temperature, evacuated and
filled with dinitrogen three times, then CO (30 bar) was
added at r.t., and the autoclave was heated at 80°C for
4 h. The reaction mixture was filtered to eliminate a
small quantity of metallic palladium and stored in a
refrigerator for 24 h. The precipitated yellow solid was
collected by filtration and dried in vacuo (28%). Recrys-
tallisation from CH3OH gave crystals suitable for X-ray
analysis. The data are in agreement with those reported
in the literature [16]. The organic products were iden-
tified and quantified by GC-MS and GC analyses.
4.3. Reaction of Pd2(dba)3·CHCl3 with PhNO
1,10-Phenanthroline (105 mg, 0.58 mmol) was added
to a suspension of Pd2(dba).3CHCl3 (200 mg, 0.19
mmol) in degassed acetone (50 ml). The resulting pur-
ple suspension was stirred at r.t. for 3 h during which
time the mixture turned yellow and then PhNO (207
mg, 1.9 mmol) was added. The resulting red suspension
was stirred at r.t. for 24 h to yield a green microcrys-
talline solid that was collected by filtration, washed
with Et2O (10 ml), to eliminate the excess PhNO, and
dried in vacuo (63%). The elemental analysis and spec-
troscopic data are in accord with those reported in
Section 4.2.
4.6. Reaction of Pd(phen){C(O)OCH3}2 3 with PhNH2
PhNH2 (0.045 ml, 0.5 mmol) was added to a suspen-
sion of Pd(phen){C(O)OCH3}2 3 (10 mg, 0.025 mmol)
in methanol (7 ml) under CO atmosphere (1 bar) and at
r.t. Palladium black formation started immediately and
the suspension was stirred at r.t. for 12 h. The resulting
suspension was analysed by GC and HPLC analysis
and the organic products observed were PhN-
HCONHPh and PhNHCOOCH3 in a 95:5 ratio.
4.7. Catalytic reduction and carbonylation of
4.4. Preparation of [Pd(phen)(C3H5O)]2·CHCl3 2
nitrobenzene to methyl phenylcarbamate
1,10-Phenanthroline (96 mg, 0.53 mmol) was added
to a suspension of Pd2(dba)3·CHCl3 (250 mg, 0.24
mmol) in degassed acetone (30 ml). The purple suspen-
sion was stirred at r.t. for 3 h during which time the
mixture turned yellow and then ButNO (106 mg, 0.60
mmol) was added. The suspension was stirred at r.t. for
24 h, then the solvent was evaporated to dryness and
CH2Cl2 (40 ml) was added to the residue. The resulting
yellow solution was filtered through celite to remove a
trace amount of palladium black, evaporated to dryness
and the yellow residue washed with Et2O (15 ml) to
eliminated dibenzylidene acetone. The yellow solid was
collected by filtration and dried in vacuo (67%). Found
C, 46.53; H, 3.31; N, 6.84 [C31H27Cl3N4O2Pd2] requires
C, 46.27; H, 3.38; N, 6.97%. 1H-NMR (CDCl3, 300
MHz, 298 K) l, ppm: 9.75 (d, 2H, J=5.6 Hz); 9.45 (d,
The experimental procedure to perform the catalytic
reactions was identical to that previously described
[24] using the following reagents amounts:
Pd(phen){C(O)OCH3}2 3 or [Pd(phen)2](BF4)2 [20]
6.6×10−3
mmol,
molar
ratios
Pd:PhNO2:
PhNH2:phen:benzoic acid=1:2500:66.6:33.3:227. The
reactions were run in methanol (15 ml)+2,2%-
dimethoxypropane (0.5 ml) at 170°C, under 60 bar CO
for 1 h. The products were analysed by gas chromatog-
raphy.
4.8. Crystal structure of Pd(phen){C(O)OCH3}2 3
Crystal data for 3: yellow prismatic crystal 0.08×
0.10×0.18 mm3, C16H14N2O4Pd, M=404.70, graphite-
,
monochromatized Mo–Ka radiation (u=0.71073 A),

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,
,
b=16.264(6), c=22.816(12) A, U=2986(4) A , Z=8,
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Acknowledgements
The authors would like to thank Dr Laura Santagos-
tini for help in recording and discussing EPR spectra
and Dr Chiara Cazzaniga for performing the catalytic
reactions. The authors also thank MURST (EX 40%)
for financial support.
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