Reduction of nitrosoarene ligands in binuclear palladium(II) complexes

Article abstract of DOI:10.1023/A:1013015425154

Reduction of the binuclear Pd¹¹ complexes Pd₂(OCOR)₂(o-CH₂C₆H₄-NO)₂ (1) and Pd₂(OCOR)₂(o-PhN-C₆H₄-NO)₂ (2) (where R = Me, C

Full text of DOI:10.1023/A:1013015425154

Russian Chemical Bulletin, International Edition, Vol. 50, No. 9, pp. 1689—1692, September, 2001
1689
Reduction of nitrosoarene ligands in binuclear palladium(II) complexes
S. T. Orlova, D. N. Kazyul´kin, L. K. Shubochkin, D. I. Shishkin, and T. A. Stromnova«
N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 954 1279. E-mail: strom@igic.RAS.ru
Reduction of the binuclear PdII complexes Pd2(OCOR)2(o-CH2C6H4—NO)2 (1) and
Pd2(OCOR)2(o-PhN—C6H4—NO)2 (2) (where R = Me, CF3, But, or Ph) by sodium
borohydride, an ethanolic solution of KOH, or molecular hydrogen was examined. The first
stage of reduction was demonstrated to afford metallic palladium and aromatic amines, viz.,
o-toluidine o-Me—C6H4—NH2 from complex 1 and aniline Ph—NH2 from complex 2. The
reactions with molecular hydrogen involve deeper stages to yield cyclic ketones (o-methyl-
cyclohexanone and cyclohexanone) and then cycloalkanes (methylcyclohexane and cyclohex-
ane, respectively). The latter reactions are accompanied by elimination of N2. The mechanism
of reduction of complexes 1 and 2 with molecular hydrogen was proposed.
Key words: palladium, reduction, nitrosoarenes, aromatic amines.
The reactivities of palladium complexes containing
nitrosoarene ligands attract interest primarily because
these complexes are formed and undergo subsequent
conversions (primarily, reductive) in industrially impor-
tant processes involving reduction of nitroarenes cata-
lyzed by transition metal compounds.1—5
The data on the reactivities of nitrosoarene palla-
dium complexes, which are of considerable interest for
both the coordination chemistry of transition metals
and metal-complex catalysis, are scarce.
Thermolysis of complexes 1 (R = CF3, Me, or Ph)
in organic solvents (benzene, toluene, or m-xylene) at
temperatures higher than 150 °C afforded metallic Pd,
toluidine Me—C6H4—NH2, and acetic acid or benzoic
acid, respectively.
Complexes 1 and 2 were reduced to metallic palla-
dium by sodium borohydride, an ethanolic solution of
KOH, or molecular hydrogen even at ∼20 °C.
The reactions of complexes 1 (R = Me, CF3, or Ph)
with NaBH4 afforded the corresponding carboxylic acid
or its salt, PdII was reduced to Pd0, and the metallated
nitroso ligand was reduced to o-toluidine in ∼90% yield
(Scheme 1).
With the aim of studying the reactivities of com-
pounds of this type, we examined the behavior of the
6
complexes Pd2(OCOR)2(CH2C6H4NO)2 (1)
Pd2(OCOR)2(PhNC6H4NO)2 (2)
and
7,8
(where R = Me,
Complexes 1 (R = CF3) were reduced to metallic Pd
and o-toluidine also under the action of an ethanolic
CF3, But, or Ph), which we have synthesized previously,
under the conditions of thermal decomposition and in
reductive media.
T/°C
∆m/mg
Results and Discussion
450
TGA
DTG
DTA
0
Binuclear complexes 1 and 2, in spite of particular
structural differences (1 involves the Me-metallated
o-nitrosotoluene molecule, whereas 2 includes the phe-
nyl-o-nitrosophenylamide ligand), contain the nitroso
group, which is bound to the phenyl ring and is coordi-
nated to the Pd atom through the Pd←N donor-accep-
tor bond. It is the behavior of the coordinated nitroso
group in reductive media that is of prime interest.
The differential thermal analysis (DTA/TGA) dem-
onstrated that decomposition of the complexes in the
absence of solvents (dry thermolysis) under an Ar atmo-
sphere started at 160 °C for complex 1 and at 195 °C for
complex 2 and was accompanied by a substantial exo-
thermic effect and sharp weight loss within a narrow
temperature range (162—220 °C for 1) (Fig. 1). In this
case, palladium is reduced to Pd0.
400
350
300
250
200
150
100
50
2
4
162°
6
8
220°
220°
10
12
14
16
162°
Fig. 1. DTA/TGA data for complex 1 (weighted sample 23.8 mg,
total mass loss 14.5 mg (60.9%), resudue 9.3 mg (39.1%)).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1609—1612, September, 2001.
1066-5285/01/5009-1689 $25.00 ©ꢀ2001 Plenum Publishing Corporation

1690
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Orlova et al.
Scheme 1
forded initially metallic Pd and the corresponding aro-
matic amines (toluidine and aniline in 10—30% yields
with respect to the starting complex) (Scheme 3).
R
R
C
C
O
O
O
Scheme 3
O
CH2
Pd
Pd
H2C
NaBH4
NH2
N
H2
N
ꢀꢁ
Pd0 + RCOOH +
O
O
Me
ꢁ
or
NH2
Me
Pd0 + RCOOH +
NH2
H2
ꢀ
Pd0 + RCOOH +
solution of KOH. In this case, the amount of o-tolui-
dine that formed was substantially lower than stoichio-
metric. Apparently, the starting organic substrates and
those formed in the course of the reaction readily
underwent condensation in a strongly alkaline medium
to yield products, which were difficult to identify. The
carboxylate groups present in the starting complexes
were found as the KOOCR salts.
The reactions of complexes 2 (R = Me, CF3, or
Ph) with NaBH4 also afforded metallic Pd and
the corresponding carboxylic acid or its salt. The
reactions gave rise to aniline instead of the ex-
pected products of reduction of the amide-containing
nitroso ligand (N-(o-nitrosophenyl)-N-phenylamine or
N-(o-aniline)-N-phenylamine). The yield of aniline was
∼40% with respect to the starting complex (Scheme 2).
However, the latter reactions involved deeper stages.
Upon subsequent hydrogenation of complex 1 (Fig. 2),
the amount of toluidine decreased slowly and methyl-
cyclohexanone (after 4—5 h) and then methylcyclo-
hexane (after 45—50 h) were found in the reaction
mixture; N2 was detected in the gaseous phase (≤15% of
the stoichiometric amount assuming that one N2 mol-
ecule is formed from two nitroso groups). The total
amount of methylcyclohexanone and methylcyclohexane
was at most 15—20% with respect to the stoichiometry.
We believe that the resulting toluidine was only partially
consumed for the formation of methylcyclohexanone,
which was subsequently reduced to methylcyclohexane.
The reactions of complexes 2 with H2 proceeded
analogously. The concentration of aniline first increased
and then decreased in parallel with the increase in the
concentrations of cyclohexanone and cyclohexane. In
the gaseous phase, N2 was also detected. The formation
of methylcyclohexane upon hydrogenation of methyl-
cyclohexanone and the generation of cyclohexane from
cyclohexanone in the presence of palladium as a catalyst
are readily explicable. However, the production of
Scheme 2
R
R
C
C
O
Ph
O
O
O
N
Pd
Ph
Pd
N
NaBH4
N
Products/mmol per mg-at Pd
N
O
O
0.25
0.20
NH2
Pd0 + RCOOH +
0.15
1
2
Apparently, the reactions occurred with the cleavage
of the PhN—C bond in the amide ligand accompanied
by reduction both of the nitroso group and the
phenylnitrene fragment to give aniline. Subsequent con-
versions of aniline, like those of toluidine in the above-
considered reactions, in a reductive medium afforded
products, which we failed to identify.
0.10
0.05
3
0
10
20
30
40
t/h
Fig. 2. Variation in time of the products of reaction of complex 1
with H2: 1 — toluidine, 2 — methylcyclohexanone, 3 — methyl-
cyclohexane.
The reactions of complexes 1 and 2 with H2 under
mild conditions, like the reactions with NaBH4, af-

Reduction of palladium nitrosoarene complexes
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1691
sphere; the weights of the samples were 20—25 mg; the rate of
heating was 10 deg min–1). Gas-chromatographic analysis of
gaseous reaction products was carried out on an LKhM-80
instrument (molecular sieves and Polysorb; the gas mixtures
were analyzed for N2, O2, NO, and Ar). The liquid phases were
analyzed by GLC on a 3700 chromatograph (an OV-101 col-
umn for analysis of cyclic hydrocarbons, cyclic ketones, and
high-boiling aromatic amines; a REOPLEX column for analy-
sis of low-boiling highly polar organic products). GLC-mass
spectrometric analysis of organic products was carried out on
an Automass instrument (Delsi Nermag, France; columns with
PEG-20M phases for analysis of low-boiling highly po-
lar organic products and SE-30 silicon for analysis of
cyclic hydrocarbons, cyclic ketones, and high-boiling aromatic
amines).
Thermolysis of the complexes was carried out in tempera-
ture-controlled tubes at 180 °C for 4—6 h. Complex 1 or 2
(0.25 mmol) was placed in a tube 6—7 mm in diameter and
20—25 cm in length. Then a solvent (benzene, toluene, or
m-xylene; 2 mL) was added, the tube was evacuated, and the
solvent was frozen in a Dewar vessel filled with liquid nitrogen.
The tube was filled with argon (the operation was repeated
three times to completely remove oxygen), sealed, heated to
∼20 °C, placed in a thermostat, and heated to the reaction
temperature. After completion of the reaction, the tube was
cooled and opened. The liquid phase was analyzed by GLC-mass
spectrometry and GLC.
methylcyclohexanone from toluidine as well as of cyclo-
hexanone from aniline is unexpected. It can be sug-
gested that either H2O molecules or carboxylate groups
serve as a source of O atoms in the formation of cyclic
ketones. The ability of the coordinated carboxylate groups
to act as donors of O atoms has been noted previously
in a number of studies9—12 on the chemistry of carboxy-
late palladium compounds.
The available data on hydrogenation of complexes 1
and 2 suggest that the formation of cyclic ketones, viz.,
methylcyclohexanone from toluidine and cyclohexanone
from aniline, accompanied by elimination of N2 can
proceed through several successive stages. First, aro-
matic amine is reduced (analogously to the Birch reduc-
tion13,14) to unsaturated cyclic amine (an unstable in-
termediate), which readily undergoes rearrangement into
imine (imine-enamine tautomerism).
NH2
NH2
NH
The resulting imine is subjected to hydrolysis (or
acidolysis) with H2O or carboxylic acid to give the
corresponding cyclic ketone and ammonia. It seems
likely that hydrolysis (or acidolysis) is favored by the
decrease in the basicity of the imine through its coordi-
nation to Pd+2 as well as by the involvement of ammo-
nia that formed into an amino complex of palladium.
Molecular nitrogen is generated, apparently, through
intrasphere oxidation of ammonia (or amine), which is
coordinated to palladium, resulting in reduction of
Pd to Pd0.
Reduction of the complexes with sodium borohydride was
carried out in tubes 6 mm in diameter and ∼50 mm in height.
The complex (∼30 mg, 0.1 mmol) was placed in a pre-weighed
dry tube, and NaBH4 (1.0 mmol) was added. Then tetrahydro-
furan (∼0.25 mL) was rapidly added and the upper part of the
tube was closed with a porous stopper. The reaction proceeded
vigorously and was accompanied by the formation of metallic
Pd and elimination of H2. After completion of the reaction
(the gas evolution ceased and the solvent turned color-
less), the solution was analyzed by GLC and GLC-mass spec-
trometry.
Reaction of Pd2(CF3COO)2(CH2C6H4NO)2 (2, R = CF3)
with an ethanolic solution of KOH was carried out according to
the following procedure: the complex (0.1 mmol, 35 mg) was
placed in a round-bottom flask and an ethanolic solution of
KOH (10 mL, 2.5 mol L–1) was added. The system was
evacuated and purged with argon. The reaction mixture was
stirred at 20 °C until the solution turned colorless (∼5 min).
Then the solution was filtered off from metallic Pd through a
porous filter, which was preliminarily dried to a constant
weight. The filtrate (a pale-yellow solution) was studied by
GLC and GLC-mass spectrometry. The filter containing me-
tallic Pd was brought to a constant weight at ≤100 °C (to
prevent the formation of palladium oxide) and weighed. Metal-
lic Pd was obtained in a yield of ∼10 mg (0.1 mg-at. of Pd).
Studies of the reactions of complexes 1 and 2 with H2 were
carried out in a two-neck flask equipped with a magnetic stirred
and a sampler for gas and liquid phases at 20 °C under the gas
pressure of 1 atm. Complex 1 or 2 (0.25 mmol) was placed in a
flask, a solvent (benzene or toluene; 2 mL) was added, the
system was evacuated, H2 was fed to the mixture, and the
stirrer was turned on. The gaseous and liquid reaction products
were analyzed in the course of the experiments during several
weeks by GLC and GLC-mass spectrometry.
Experimental
Organic solvents were purified according to standard proce-
dures.16
Palladium nitroso complexes Pd2(OCOR)2(CH2C6H4NO)2
(1) and Pd2(OCOR)2(PhNC6H4NO)2 (2) (R = Me, CF3, But,
or Ph) were prepared according to procedures reported previ-
ously7,8 from carbonyl carboxylate palladium complexes and
the corresponding nitrosoaromatic compounds.
Nitrosoaromatic compounds o-R´C6H4NO (R´ = H or Me)
were prepared according to a general procedure15 by oxidation
of phenylhydroxylamine with sodium bichromate in aqueous
H2SO4 (R´ = H) or by reduction of the corresponding nitro
compound (R´ = Me). The resulting nitrosoarenes were iso-
lated from the reaction mixture by azeotropic distillation with a
water vapor under atmospheric or reduced pressure. The puri-
ties of the compounds were monitored by TLC on Silufol plates
and based on the melting points. Taking into account that the
nitroso group in nitrosoaromatic compounds is readily oxidized
by atmospheric oxygen, nitrosoarenes were stored over a short
period in Schlenk tubes under Ar in a refrigerator and all
reactions involving these compounds were carried out under an
inert atmosphere.
We thank A. E. Gekhman for help in analyzing the
reaction solutions by GLC-mass spectrometry and for
valuable advice. We also acknowledge G. V. Ostrovskaya
and L. I. Boganova for assistance in syntheses.
DTA/TGA analysis was performed on an OD-102 MOM
derivatograph (Hungary) (plate platinum supports; an Ar atmo-

1692
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Orlova et al.
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9. T. A. Stromnova, N. Yu. Tikhonova, L. K. Shubochkin,
and I. I. Moiseev, Koord. Khim., 1993, 19, 460 [Russ. J.
Coord. Chem., 1993, 19 (Engl. Transl.)].
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
32519).
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Received April 28, 2001;
in revised form June 25, 2001