Reactivity of palladium(II) complexes supported on carbon toward molecular hydrogen

Article abstract of DOI:10.1134/S1070328411050022

The reactivity of Pd(II) complexes supported on carbon toward H₂ was studied. For the carboxylate complexes Pd(RCOO)₂ (R = Me, Me₃C, F₃C), it decreases upon decrease in the basicity of the acid RCO₂H. The reactivity of Pd(II) η³-allyl complexes increases with increase in the Mulliken charges on the C atom of the allyl ligand connected to the substituent R. The results are in line with the heterocyclic mechanism of H-H bond activation in the hydrogen molecule and can be used for optimization of the composition of the initial compounds for the preparation of palladium catalysts.

Full text of DOI:10.1134/S1070328411050022

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2011, Vol. 37, No. 6, pp. 460–462. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A. S. Berenblyum, H.A. AlꢀWadhaf, O.N. Shishilov, E.M. Evstigneeva, M.H. ElꢀHussien, V.R. Flid, 2011, published in Koordinatsionnaya Khimiya, 2011,
Vol. 37, No. 6, pp. 458–460.
Reactivity of Palladium(II) Complexes Supported
on Carbon toward Molecular Hydrogen
,
A. S. Berenblyuma *, H. A. AlꢀWadhafa, O. N. Shishilovb, E. M. Evstigneevab,
M. H. ElꢀHussiena, and V. R. Flida
a Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571 Russia
bKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 117907 Russia
*Eꢀmail: ayb@go.ru
Received October 15, 2010
Abstract—The reactivity of Pd(II) complexes supported on carbon toward H2 was studied. For the carboxyꢀ
late complexes Pd(RCOO)2 (R = Me, Me3C, F3C), it decreases upon decrease in the basicity of the acid
η
RCO2H. The reactivity of Pd(II)
3ꢀallyl complexes increases with increase in the Mulliken charges on the
C atom of the allyl ligand connected to the substituent R. The results are in line with the heterocyclic mechꢀ
anism of H–H bond activation in the hydrogen molecule and can be used for optimization of the composition
of the initial compounds for the preparation of palladium catalysts.
DOI: 10.1134/S1070328411050022
The reactivity of palladium(II) coordination comꢀ complexes were synthesized by the following proceꢀ
pounds in solutions with respect to H2 was studied by
Berenblyum et al. [1, 2]. The interest in this reaction is
due to a number of reasons. First, it can serve as a
model reaction for studying the activation of H2 in the
palladium coordination sphere [3]. Second, the
reduction of Pd(II) complexes with molecular hydroꢀ
gen results in the formation of catalytically active palꢀ
ladium clusters [4].
Almost no studies dealing with the reactivity of
supported palladium(II) complexes toward H2 were
reported. Meanwhile, these data are needed for the
selection of the optimal precursors for the synthesis of
modern heterogeneous highly efficient catalysts conꢀ
taining metal nanoparticles [5].
dures: bis[(
η
3ꢀallyl)palladium chloride] and bis[(1ꢀ
methylꢀ
η
3ꢀallyl)palladium chloride] were prepared by
the reduction of allyl chloride and crotyl chloride,
respectively, under carbon monoxide atmosphere in a
wet methanol solution of sodium tetrachloropalladate
[8]; bis[(1ꢀphenylꢀ
3ꢀallyl)palladium chloride] was
prepared by the reaction of cinnamic alcohol with
sodium tetrachloropalladate in acidified aqueous
methanol solution [9]; bis[(1ꢀmethoxymethylꢀ
3ꢀ
allyl)palladium chloride] was obtained from sodium
tetrachloropalladate and 1,3ꢀbutadiene in a dry methꢀ
anol solution [10]; bis[(1ꢀformylꢀ
3ꢀallyl)palladium
η
η
η
chloride] was synthesized from 1ꢀtrimethylsilyloxyꢀ
1,3ꢀbutadiene and bis(acetonitrile)palladium dichloꢀ
ride in dry benzene [11]. The compounds were identiꢀ
The purpose of this work is to study the reaction of
Pd(II) complexes with H2 on the surface of a carbon
support and to establish the relationship between the
ligand nature and the activity of complexes in this
reaction.
1
fied by H and 13C NMR spectroscopy [12]. The
method of deposition of palladium complexes on carꢀ
bon support was reported [13].
The reaction of the supported palladium comꢀ
plexes with H2 was studied as follows. A weighed porꢀ
tion (0.1 g) of the substance was placed in a tubular
reactor and H2 was passed through. The acid evolved
in the reaction was carried away by the H2 and then N2
flow and absorbed by an aqueous solution of excess
alkali, which was titrated by hydrochloric acid after the
experiment. The detailed description of the facility,
procedure of reduction of samples, and acid titration
was reported previously [13]. The error of measureꢀ
EXPERIMENTAL
Highꢀpurity grade palladium acetate was used as
received. Special purity grade acetone, grade A hydroꢀ
gen gas (99.99% purity), and special purity grade
nitrogen gas (first quality, 99.999% purity) were used.
Carbon Vulcan XCꢀ72 was used as the support. Pallaꢀ
dium pivalate Pd3(Me3CCO2)2 [6] and palladium trifꢀ
luoroacetate [Pd(F3CCO2)2 2/3PhH]n [7] were preꢀ
⋅
pared by replacement of the acetate groups in pallaꢀ
dium acetate on treatment with a 2ꢀ to 3ꢀfold excess of
the appropriate carboxylic acid. The palladium allyl ments did not exceede 7% (relative) [13].
460

REACTIVITY OF PALLADIUM(II) COMPLEXES
461
Table 1. Dependence of the reactivity of palladium carboxyꢀ
lates toward hydrogen on the nature of the carboxylate anion
RESULTS AND DISCUSSION
The reaction of palladium complexes with H2
occurs according to the following general scheme:
Compound
pKa
Y, %
LmPdX2 + H2
Pd + 2HX + qN
,
(1)
Pd3(Me3CCO2)6
Pd3(MeCO2)6
Pd3(F3CCO2)6
5.03
4.75
0.588
86
77
62
where X is the anion of a monobasic acid, L are
ligands, are the products of their transformation.
N
The reaction was monitored by determining the
amount of evolved acid with time. Recently it was
shown [14] that the amount of the formed acid, for
example, acetic acid is well correlated with the data of
Xꢀray photoelectron spectra (XPS) obtained for
reduction of palladium complexes with molecular
hydrogen according to reaction (1).
Table 2. Reactivity of palladium(II) allyl complexes and
charge on the C(1) atom in the allylic ligand
R
Charge on C(1), a.u.
Y, %
First, we studied the reaction of carbonꢀsupported
palladium carboxylates with H2. As a measure of reacꢀ
tivity, the Y value, namely, the percentage of acetic
acid formed of the theoretically possible amount, was
used. The palladium content in the samples was 9.1%.
Me
Ph
–0.197
–0.217
–0.220
–0.301
55
67
87
CH2OMe
CHO
~100
The reduction was carried out at 120 С, the reaction
°
time being 3 h. The results are presented in Table 1.
the series of complexes of the same type whose propꢀ
erties (structure, nature of other ligands, their mutual
influence, etc.) are rather similar, except for the basicꢀ
ity of the studied ligand.
For this purpose, we studied the reactivity of comꢀ
plexes with the general formula
The reactivity of the complexes was found to
decrease with a decrease in the basicity of the acid
whose anion participates in the reaction as ligand
X [15]. This is in line with heterolytic mechanism of
H–H bond cleavage in the H2 molecule, which was
proposed for palladium complexes in solution [3]. The
mechanism includes the formation of a fourꢀcenter
transition state in which the Pd atom captures a
hydride ion, while the proton is bound by the nucleoꢀ
philic ligand X
H
R
C(1)
C(3)
Cl
H
C(2)
H
+
δ
Pd
–
δ
Pd
X
δ
H– H+
δ
H
2
Obviously, the higher the basicity of this ligand, the
easier the attainment of the transition state. In other
words, one can expect that the stronger the acid, the
lower the reactivity of the complex that incorporates
its anion. The same trend was observed previously in
solutions of palladium phosphine complexes [1–3];
this is also true for supported palladium complexes.
100
80
60
40
20
0
2
1
As was to be expected, the introduction of an allylic
ligand in palladium acetate increased the reactivity of
the synthesized coordination compound (figure). The
obtained results agree with heterolytic mechanism of
H2 activation as it is obvious that the Cꢀligand (allyl) is
more basic than the Oꢀligand (carboxylate). Apparꢀ
ently, the rateꢀlimiting step is the formation of the palꢀ
ladium hydride complex Pd(H)X, which rapidly
decomposes to acid and palladium metal. Hence, the
effect of the nature of the allylic ligand on the reactivꢀ
ity of palladium complexes with respect to H2 can be
studied by measuring the amount of evolved acid.
50
100
150
200
T, °C
The effect of the basicity of
3ꢀallyl ligands was
η
studied using allyl chloride complexes with various
substituents. These studies are correctly carried out in
Dependence of Y on temperature: (
1
) palladium acetate;
3
η
(
2) bis[(
ꢀallyl)palladium acetate]. Reaction time 3 h.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 37
No. 6
2011

462
BERENBLYUM et al.
2. Berenblyum, A.S., Aseeva, A.P., Lakhman, L.I., and
where R = Me, Ph, CH2OMe, CHO.
Moiseev, I.I., J. Organomet. Chem., 1982, vol. 234,
no. 2, p. 237.
The results are presented in Table 2. The Table preꢀ
sents as well the published Mulliken charges on the
allylic carbon atom C(1) for such complexes deterꢀ
mined by quantum chemical calculations [16]. It can
be seen from these data that Y is well correlated with
the charge on the C(1) atom. Moreover, as was to be
expected for the heterolytic mechanism, an increase in
the negative charge on the carbon atom results in an
increase in the reactivity of complexes with respect to
H2. No correlation between the charge on other carꢀ
bon atoms and the Y value was observed for the studied
allyl complexes [16]. Hence, presumably, the activaꢀ
tion of the H–H bond involves the carbon atom linked
to palladium and the substituent R.
Thus, the results indicate that the reaction of pallaꢀ
dium(II) complexes with molecular hydrogen in soluꢀ
tion and on the support surface has certain analogy
and occurs by a heterolytic mechanism. This can be
used to predict the reactivity of the complexes dependꢀ
ing on their composition and to select the starting
compounds for the targeted synthesis of heterogeꢀ
neous palladium catalysts at different conditions of
their formation.
3. Berenblyum, A.S., Lakhman, L.I., Radchenko, E.D.,
and Moiseev, I.I., Izv. Akad. Nauk SSSR, Ser. Khim.
,
1976, no. 11, p. 2417.
4. A.S. Berenblyum, Zh. Vsesoyuzn. Khim. Obꢀva, 1987,
vol. 32, no. 1, p. 82.
5. Zhu, Y., Kang, Y., Zou, Z., et al., Electrochem. Comꢀ
mun., 2008, vol. 10, no. 5, p. 802.
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,
2008, vol. 34, no. 10, p. 788 [Russ. J. Coord. Chem.
(Engl. Transl.), vol. 34, no. 10, p. 780].
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Koord. Khim., 1989, vol. 15, no. 5, p. 688.
8. Dent, W.T., Long, R., and Wilkinson, A.J., J. Chem.
Soc., 1964, p. 1585.
9. Atkinson, L.K. and Smith, D.C., J. Chem. Soc., Sect. A
1971, p. 3592.
,
10. Robinson, S.D. and Show, B.L., J. Chem. Soc., 1963,
p. 4806.
11. Ogoshi, S., Hiraako, K., Nakanishi, J., et al., J. Orgaꢀ
nomet. Chem., 1993, vol. 445, no. 1ꢀ2, p. 13.
12. Evstigneeva, E.M. and Flid, V.R., Izv. Akad. Nauk, Ser.
Khim., 2008, no. 6, p. 1171.
ACKNOWLEDGMENTS
The authors are grateful to R. E. Podobedov for the
synthesis of palladium(II) trifluoroacetate and pivꢀ
alate.
13. Berenblyum, A.S. and AlꢀWadhaf, H.A., Vestnik MITKhT
,
2010, vol. 5, no. 5, p. 43.
14. Berenblyum, A.S., AlꢀWadhaf, H.A., Katsman, E.A.,
and Flid, V.P., Kinet. Katal., 2011, vol. 52, no. 2, p. 305.
15. Albert, A. and Serjeant, E., Ionization Constants of Acids
and Bases, New York: Wiley, 1962.
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 37
No. 6
2011