Palladium(I) carbonyl carboxylate clusters cyclo-[Pd2(μ-CO)2(μ-OCOR)2]n (n = 2 or 3): Structure and reactivity

Article abstract of DOI:10.1016/j.jorganchem.2006.03.033

The interaction of palladium(+1) cluster Pd₄(μ-CO)₄(μ-OAc)₄ with saturated and unsaturated carboxylic acids was studied. It was found, that the substitution of acetates groups on others carboxylates leads to the clusters w

Full text of DOI:10.1016/j.jorganchem.2006.03.033

Journal of Organometallic Chemistry 691 (2006) 3730–3736
www.elsevier.com/locate/jorganchem
Palladium(I) carbonyl carboxylate clusters
cyclo-[Pd2(l-CO)2(l-OCOR)2]n (n = 2 or 3): Structure and reactivity
a,
a
c
a
Tatiana A. Stromnova *, Oleg N. Shishilov , Mikhail V. Dayneko , Kirill Yu. Monakhov ,
a
a
b
Andrei V. Churakov , Ludmila G. Kuz’mina , Judith A.K. Howard
a
Institute of General and Inorganic Chemistry, Russian Academy of Science (RAS), Leninsky prospekt, 31, Moscow 119991, Russia
b
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
Moscow State Academy of Fine Chemical Technology, Vernadskogo pr., 86, Moscow 119571, Russia
c
Received 7 December 2005; received in revised form 26 February 2006; accepted 21 March 2006
Available online 28 March 2006
Abstract
The interaction of palladium(+1) cluster Pd4(l-CO)4(l-OAc)4 with saturated and unsaturated carboxylic acids was studied. It was
found, that the substitution of acetates groups on others carboxylates leads to the clusters with different nuclearity. Palladium(+1) car-
bonyl carboxylate complexes of composition [Pd(l-CO)(l-OCOR)]n, where R = CF3, CCl3, CH2Cl, MeCH = CMe, Me, Pri, Bu, Bui,
Butert, n-C5H11 and n = 4 or 6 were synthesized. According to X-ray data all clusters possess cyclic planar metal cores with alternate
pairs of l-carbonyl and l-carboxylate ligands. The presence of bulky alkyl fragments in the carboxylate ligand increases the nuclearity
of the cluster compared to that of the starting palladium(+1) carbonyl acetate of composition Pd4(l-CO)4(l-OAc)4 due, apparently, to
steric hindrance.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Palladium; Carbonyl carboxylate; Cluster; Crystal structure
1. Introduction
For example, according to preliminary data, the substitu-
tion of chlorine on different carboxylates in Pt and Pd
leads to the less-toxic complexes with good anti-cancer
activity [3].
Thus, this paper deals with Pd(+1) complexes that con-
tain neutral ligands (CO) and carboxylate ligands and can
serve as the convenient starting compounds for synthesis of
various new classes of complexes as, for example, palla-
dium nitrosyl complexes [4].
Palladium carbonyl complexes have attached the partic-
ular attention as potential intermediates of catalytical pro-
cesses associated with CO transformation. The catalytic
oxidation of CO to CO2, syntheses of alkyl oxalates and
alkyl carbonates, the carbonylation of alcohols, unsatu-
rated hydrocarbons and nitroarenes, etc., are among these
reactions. The processes, where formal oxidation state of
Pd atoms plays key role, keep the special place. This is
especially preferable for redox processes where easiness of
Pd(+1) transformation into Pd(0) or Pd(+2) is very impor-
tant feature [1,2].
2. Results and discussion
The first Pd(+1) carbonyl acetate cluster Pd4(l-CO)4(l-
OOCMe)4 (1) was obtained earlier by reduction of Pd(+2)
acetate with carbon monoxide in glacial acetic acid media
[5].
On other side, Pd carboxylates are very attractive in
systems where the absence of halogen should be realized.
*
According to X-ray diffraction data, cluster 1 has an
almost rectangular metal core. The opposite sides of the
Corresponding author. Tel.: +7 095 9554805; fax: +7 095 9541279.
E-mail address: strom@igic.ras.ru (T.A. Stromnova).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.03.033

T.A. Stromnova et al. / Journal of Organometallic Chemistry 691 (2006) 3730–3736
3731
core are linked by the same pairs of ligands – two pairs of
acetate groups and two pairs of carbonyls (see Fig. 1).
The coordinated acetate groups in cluster 1 can be
replaced by other carboxylates in the reactions with the
corresponding carboxylic acids (both saturated and unsat-
urated ones):
All complexes obtained contain one carbonyl and one
carboxylate groups per one Pd atom and have a composi-
tion [Pd(CO)(OCOR)]n. The total composition of com-
pounds allows assuming that the formal oxidation state
of Pd atoms remains like in initial cluster (that is +1).
According to IR spectroscopic data, all the complexes
contain bridging carbonyl groups (see Table 1).
One can see that the increasing of electron-acceptor
capacity of R group in carboxylate ligands is accompanied
with increasing of mCO value in IR spectra. Perhaps, the
presence of electron-withdrawing groups such as CF3 or
CCl3 favours to decrease the electron density on Pd atom
and to reduce the back-donation from Pd atom to CO
Pd4ðl-COÞ4ðl-OOCMeÞ4 þ RCOOH
! ½PdðCOÞðOCORÞꢀn þ MeCOOH
R ¼ CF3; CCl3; CH2Cl;
MeCH ¼ CMe; Me; Pri; Bu; Butert; Bui; C5H11
group, and, as result, increasing of mCO
.
Thus, all the complexes contain bridging carbonyl
groups and this fact allows to consider that the [Pd2(l-
CO)2(OOCR)2] structural moiety is common for each clus-
ter obtained.
Nevertheless, the structures of clusters are quite
different.
The structure of cluster [Pd2(l-CO)2(l-OOCR)2]n (2:
R = CH2Cl, n = 2) is very similar to of 1 [5,6]. The molecule
2 is centrosymmetric with approximately rectangular Pd4
core. The neighboring palladium atoms are linked by pairs
of carbonyl and carboxyl ligands forming two Pd2(l-CO)2
and two Pd2(l-OOCCH2Cl)2 fragments (see Fig. 2). The
˚
Pd–Pd distances are 2.6594(3) and 2.9653(3) A, while Pd–
Pd–Pd angles are equal 91.670(9)° and 88.330(9)°. The coor-
dination environment of Pd atoms (two carbon and two
oxygen atoms) is almost square-planar. The displacements
Fig. 1. Structure of cluster 1.
Table 1
[Pd(CO)(OCOR)]n R=
CF3
CCl3
CH2Cl
MeCH@CMe
Me
Pri
Bu
Butert
C5H11
m
CO, cmꢁ1
2000
1968a
1999
1967a
1979
1934a
1975
1954a
1976
1934a
1974
1944a
1972
1936a
1976
1932a
1968
1936a
a
Wide band.
Fig. 2. Molecular structure of 2. Displacement ellipsoids are shown at 50% probability level. Hydrogen atoms are omitted for clarity.

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T.A. Stromnova et al. / Journal of Organometallic Chemistry 691 (2006) 3730–3736
of Pd(1) and Pd(2) atoms from least-squares planes of these
coordinated atoms are 0.009(1) and 0.017(1) A, respec-
alternating pairs of carbonyl and carboxyl ligands forming
three Pd2(l-CO)2 and three Pd2(l-OOCPri)2 fragments.
The Pd–Pd distances lie within the ranges 2.6484(10)–
˚
tively. Since the O–O distances in carboxyl ligands
˚
˚
(2.247(3) and 2.242(3) A) are significantly shorter than
2.6658(9) and 2.8767(10)–2.9493(10) A, for carbonyl and
the distance between carboxyl bridged Pd atoms
(2.9653(3) A), all carbonyl ligands are noticeably shifted
carboxyl bridged fragments, respectively. The displace-
ments of 12 Pd atoms from least-squares planes of coordi-
nated atoms (two carbons and two oxygen atoms) range
˚
outwards the molecular center to maintain the square-pla-
nar coordination of Pd atoms. The bridging carbonyl
ligands are symmetrical with Pd–C distances ranging from
˚
within 0.022(4)–0.084(4) A. In contrast to 1 and 2, all car-
bonyl ligands in 6 are slightly shifted towards the center of
molecule to maintain the planarity of Pd coordination
environment. The latter difference with the structure of 2
is a consequence of steric requirements of larger Pd6 cycle.
As for 2, bridging carbonyl ligands in 5 are rather symmet-
rical with Pd–C distances ranging from 1.948(9) to
˚
1.972(3) to 1.981(3) A and Pd–C–O angles varying from
136.4(3)° to 138.9(3)°. One of the independent CH2Cl
groups was found rotationally disordered with chlorine
atoms occupying two positions in the ratio 0.54/0.46. In
the crystal, molecules of 2 form columns spread along c-axis
(see Fig. 3). These columns are bridged by short intermolec-
˚
2.004(8) A and Pd–C–O angles varying from 136.2(8) to
˚
ular Pdꢂ ꢂ ꢂCl contacts (3.1245(8) A). The space between col-
139.8(8)°. In the structure of 6, five of 12 isopropyl groups
were found to be rotationally disordered over two posi-
tions. The crystal of 6 contains solvent benzene molecules.
The center of benzene molecule (CR1) is positioned
approximately on the line connecting the centers of adja-
cent Pd2(l-CO)2 moieties (CR2 and CR3) (see Fig. 5).
The distances CR1–CR2 and CR1–CR3 (3.250 and
umns is filled with disordered solvent toluene molecules
lying on crystallographic inversion centers.
Thus, the clusters 1 and 2, as well as the clusters [Pd2(l-
CO)2(l-OOCR)2]n, where R = CF3 (3) or CCl3(4) (the
structures of last two complexes were determine by EXAFS
[7]) possess cyclic tetranuclear metal core with pairs of car-
bonyl and carboxylate bridging ligands.
˚
3.254 A) are significantly shorter than the doubled van
˚
Three next representatives of carbonyl carboxylate clus-
ters – namely clusters [Pd2(l-CO)2(l-OOCR)2]n – (5,
R = CMe3; 6, R = Pri; 7, R = C5H11) possess the hexanu-
clear metal core. The structure of 5 was published early
[8]. The structure of 6 contains two independent hexanu-
clear molecules with similar geometrical parameters (bond
lengths and angles). In both molecules, cyclic Pd6 cores are
der Waals radius of carbon atom (3.4 A) [9]. and indicate
the presence of p–p interaction. The angle CR2–CR1–
CR3 is equal 165.4°. The same arrangement of solvent ben-
zene molecule was observed previously in the closely
related structure of 5. Moreover, the neighboring molecules
of 6 0are linked by short intermolecular Pd(6)–Pd(40) and
˚
Pd(1 )–O(12) contacts – 3.1772(8) and 2.899(7) A, respec-
˚
planar within 0.1387(5) and 0.1442(5) A (see Fig. 4). In the
tively. These three intermolecular interactions form dis-
torted hexagonal layers in the plane [101] (see Fig. 6).
Pd6 cycles, the neighboring palladium atoms are linked by
Fig. 3. Packing motif in the crystal structure of 2. Intermolecular Pdꢂ ꢂ ꢂCl contacts are shown by dashed lines.

T.A. Stromnova et al. / Journal of Organometallic Chemistry 691 (2006) 3730–3736
3733
Fig. 4. Molecular structure of 6. Only one independent molecule is shown. Displacement ellipsoids are shown at 50% probability level. Hydrogen atoms
are omitted for clarity.
Fig. 5. Arrangement of solvent benzene molecule in structure of 6. i-Pr groups are omitted for clarity.
Preliminary X-ray analyses of complex [Pd(CO)-
(OOCC5H11)]6 (7) showed that the structure of 7 is similar
to that of 6. The metal core in 7 represents approximately
planar Pd6 fragment with alternating pairs of carbonyl and
carboxyl ligands. The details concerning the crystal struc-
ture 7 will be published elsewhere later.
The presence of two bands mCO in IR spectra (see Table
1) can be explained by the fact that strong polar groups
(such as CO, NO, etc.) coordinated on the similar metal
center have more then one bands.
an arrangement of CO-groups towards metal core in clus-
ters is slightly non-symmetrical. For example, in
Pd4(CO)4(OOCCH2Cl)4 the length of both Pd–C(carbonyl)
bonds is the same, but dihedral angles between two planes
– Pd4 (metal core plane) and Pd2C(carbonyl) – are different
and equal 76.8° and 77.5°. More difference were found in
structure of Pd6(CO)6(OOCPri)6. In this molecule the dihe-
dral angles between two planes – Pd6 (metal core plane)
and Pd2C(carbonyl) – change from 81.4° to 77.2° and
diversity between two dihedral angles for CO-groups coor-
dinated on one side of metal core varies from 0.9° up to
3.9°. Such difference can lead to different overlapping of
The second possible reason of the presence of two bands
mCO in IR spectra can be sequent of their structures. Thus,

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T.A. Stromnova et al. / Journal of Organometallic Chemistry 691 (2006) 3730–3736
Fig. 6. Distorted hexagonal layers [101] in the crystal structure of 6. Short intermolecular contacts are shown by dashed lines.
corresponding orbitals responsible for binding Pd–CO and
to change of back-donation from Pd to CO, which displays
of water is a plausible reason of change of the solution’s
color from yellow to greenish in the course of syntheses.
The transformation of tetranuclear Pd4(l-CO)4(l-
OOCMe)4 into tetra- or hexanuclear clusters II can include
the substitution of acetate groups by different carboxylates:
in changing of mCO
.
One can assume that the wide band in every IR spec-
trum is a result of overlapping of some near-placed bonds
in solid sample of corresponding complex. The absence of
effect of crystal-packing in solution of complex should lead
to appearance only one mCO band – as a result of overage of
position of CO-groups. IR spectra of complexes in CH2Cl2
solution verify this suggestion. Thus, in IR spectrum of
Pd4ðl-COÞ4ðl-OOCMeÞ4 þ RCOOH
! ½PdðCOÞðO2CRÞꢀn þ MeCOOH
Perhaps, the substitution is accompanied by dissociation
of tetranuclear clusters into binuclear fragments
(RCOO)Pd(l-CO)2Pd(OOCR). It can be the other reason
of color’s change during the syntheses. The subsequent
enlargement of these fragments gives rise to clusters in
which the number of metal atoms is a multiple of two –
Pd4 or Pd6. The origin of different nuclearity of clusters for-
matted can be both electron and steric factors. Based on
analyses of the structures of all cluster structures, namely
the positions of atoms in quite different radical R in carbox-
ylate groups, gives no evidence of the influence of steric fac-
tors. Hence, we can suggest the electron factors as the main
origin of observed difference in the cluster structured. For
example, the presence of electron acceptors in carboxylate
group leads to decreasing of electron density on Pd atom.
And as consequence such atom will climb to have planar
square environment. On the contrary, if there are electron
donors in carboxylate group, the degree of approaching
to such square becomes smaller.
complex Pd6(CO)6(OOCButert
) two bands at 1976 and
6
1932 cmꢁ1 in solid state transform to one band at
1952 cmꢁ1 in CH2Cl2 solution. The same picture – trans-
formation of two bands at 1975 and 1954 cmꢁ1 in solid
state into one band at 1932 cmꢁ1 in CH2Cl2 solution –
you can see in IR spectrum of complex [Pd2(CO)2(OOCC-
Me@CHMe)2] (8).
All clusters [Pd2(CO)2(OOCR)2]n are readily soluble in
benzene and toluene, and poorly soluble in diethyl ether
and hexane. Noteworthy that the solubility of [Pd2(CO)2(O-
COR)2]n increases in parallel to decreasing of electron
acceptor properties of radical R in their carboxylate groups
according to row CF3 < CCl3 < CH2Cl ꢃ MeCH@CMe <
Me ꢃ Pri ꢃ Bu ꢃ Butert <C5H11. In the presence of highly
polar organic solvents (aliphatic alcohols, acetone) and
water, the complexes decompose to form metallic palla-
dium. This capability increases according to the same
row. The partial decomposition in the presence of traces

T.A. Stromnova et al. / Journal of Organometallic Chemistry 691 (2006) 3730–3736
3735
3. Conclusion
4.4. Preparation of [Pd(CO)(OCOC5H11)]6 (7)
The presence of carboxylate groups promotes the for-
mation palladium clusters with planar cyclic metal core.
The nuclearity of such clusters depends on the steric factor
of substitute in carboxylate groups.
Cluster 1 (0.23 g, 1 mg-atom of Pd), benzene (30 ml) and
freshly distilled hexanoic acid (0.3 ml, an approximately two-
fold excess with respect to palladium) were placed in a round-
bottom flask. The synthesis and isolation of the product were
carried out as described for cluster 6. The pale yellow precip-
itate was dried under vacuum. The yield was 76% (0.24 g).
For [Pd(CO)(OOCC5H11)]6 Anal. Calc.: C, 33.68; H,
4.41. Found: C, 33.61; H 3.67%.
4. Experimental
4.1. General techniques
All organic solvents and liquid organic reagents were
purified and dried according to standard procedures.
Microanalyses were conducted on a Carlo Erba Analyzer
CHND-OEA 1108. The spectroscopic instrument used
was a Carl Zeiss SPECORD-M82 for IR spectra. The com-
pound cyclo-[Pd4(l-CO)4(l-OOCMe)4] (1) was prepared
according to a published procedure [5] by reductive car-
bonylation of palladium diacetate in glacial acetic acid.
The compound [Pd(CO)(OCOCMe3)]6 (5) was prepared
according to a published procedure [8]. Solid carboxylic
acids were commercially supplied.
4.5. Preparation of [Pd(CO)(OOCCMe@CHMe2)]n (8)
Cluster 1 (0.2 g, 1 mg-atom of Pd), THF (10 ml) and
1-methylcroton acid (0.2 g, 2 mmol, a twofold excess with
respect to the palladium) were placed in a round-bottom
flask. The reaction mixture was being stirred during
30 min. The resulting suspension was filtered and concen-
trated up to 1/3 part by volume under vacuum using an
oil pump. After that 5 ml of hexane were added and
dark-yellow precipitate formed. The precipitate was filtered
and washed by 3 ml of THF. The yellow powder was dried
under vacuum. The yield was 50% (0.117 g).
4.2. Preparation of [Pd(CO)(OCOR)]n, R = CH2Cl (2),
CF3 (3), CCl3 (4)
For [Pd(CO)(OCO(CH3)C@CHCH3)]n Anal. Calc.: C,
30.85; H, 3.00. Found: C, 30.81; H, 3.03%.
IR spectrum: m(CO) = 1972, 1946 cmꢁ1, m(C@C) =
1653 cmꢁ1, mas(COO) = 1557 cmꢁ1, ms(COO) = 1430 cmꢁ1
.
Complexes 2–4 were obtained according to [7] by inter-
action of cluster 1 with corresponding acids in toluene solu-
tions. The yield is usually about 75–80%. Crystals of
clusters 2 suitable for X-ray diffraction analyses were
grown from a CH2Cl2–benzene–hexane mixture at reduced
temperatures during 2–3 weeks.
4.6. X-ray crystallography
X-ray data for compounds 2 and 6 were collected on a
Bruker SMART CCD diffractometer (graphite-monochro-
˚
matized Mo Ka radiation, 0.71073 A) at 120 K using x scan
4.3. Preparation of [Pd(CO)(OCOCHMe2)]6 (6)
mode. Experimental intensities were corrected for Lorentz
and polarization effects. Semi-empirical absorption correc-
tion based on measurements of equivalent reflections was
applied. The structures were solved by direct methods [10]
and refined by full-matrix least-squares on F2 [11]. All non-
hydrogen atoms were refined with anisotropic thermal
parameters (except disordered methyl carbon atoms in 6).
In the structure of 2, one of –CH2Cl groups was found to
be rotationally disordered over two positions with occupancy
ratio 0.54/0.46. In the structure of 6, five of 12 isopropyl
groups were also found to be rotationally disordered over
two positions. Refinement of occupancy factors led to the
ratios 0.53/0.47, 0.52/0.48, 0.64/0.36, 0.58/0.42 and 0.64/
0.36. In both structures, all hydrogen atoms were placed in
calculated positions and refined using a riding model. The
crystal structure of 2 contains disordered toluene solvent mol-
ecule; the structure of 6 contains solvent benzene molecule.
Details of X-ray structural investigations are listed in Table 1.
Cluster 1 (0.15 g, 0.64 mg-atom of Pd), benzene (10 ml)
and freshly distilled i-butyric acid (0.2 ml, a twofold excess
with respect to palladium) were placed in a round-bottom
flask. The resulting suspension was twice evacuated, purged
with argon to remove the residual oxygen, and stirred under
an inert atmosphere. The suspension rapidly (with 5–
10 min) dissolved to give a bright yellow solution. The color
of the solution changed to greenish in the course of synthesis
(1 h). The optimum reaction timewas determinedina seriesof
experiments using the IR spectra of the final product (based
on the absence of stretching bands of the carbonyl groups
of starting complex). After completion of the synthesis, the
reaction mixture was concentrated to dryness under vacuum
using an oil pump and washed two or three times with hexane
(to removethe residual i-butyric acid). The pale yellow precip-
itate was dried under vacuum. The yield was 75% (0.129 g).
For [Pd(CO)(OCOCHMe2)]6 Anal. Calc.: C, 27.10; H,
3.16. Found: C, 27.29; H 3.68%.
Acknowledgements
Crystals of clusters 6, suitable for X-ray diffraction anal-
yses, were grown from a benzene–hexane mixture by low
reducing of temperatures from 20°C to ꢁ18°C during 3–6
days.
Authors are grateful to the Russian Foundation for Ba-
sic Research (Project 04-03-32436) and to the grant Rus-
sian Science Support Foundation (A.V.C).

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[2] T.A. Stromnova, Platinum Metals Rev. 4 (1) (2003) 20–27.
[3] T.A.K. Al-Allaf, L.J. Rashan, A.S. Abu-Surrah, R. Fawzi, M.
Steimann, Transit. Metal Chem. 2 (4) (1998) 403–406.
[4] T.A. Stromnova, D.V. Paschenko, L.I. Boganova, M.V. Daineko,
S.B. Katser, A.V. Churakov, L.G. Kuz’mina, J.A.K. Howard, Inorg.
Chim. Acta 350 (2003) 283–288.
[5] I.I. Moiseev, T.A. Stromnova, M.N. Vargaftik, G.Ja. Mazo, L.G.
Kuz’mina, Yu.T. Struchkov, J. Chem. Soc., Chem. Commun. (1978)
27–28.
[6] L.G. Kuz’mina, Yu.T. Struchkov, Koord. Khim. (Russ.) 5 (1979)
1558.
[7] T.A. Stromnova, N.Yu. Tihonova, L.K. Shubochkin, Koord. Khim.
(Russ.) 19 (1993) 460–466.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplemen-
tary publication no. CCDC-278722 and 278723. Copies of
the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax:
+44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk]. Supple-
mentary data associated with this article can be found, in the
online version, at doi:10.1016/j.jorganchem.2006.03.033.
[8] T.A. Stromnova, O.N. Shishilov, L.I. Boganova, N.A. Minaeva,
A.V. Churakov, L.G. Kuz’mina, J.A.K. Howard, Russ. J. Inorg.
Chem. 50 (2005) 179.
References
[9] A. Bondy, J. Phys. Chem. 68 (1964) 441.
[10] G.M. Sheldrick, Acta Crystallogr. Sect. A 46 (1990) 467.
[11] G.M. Sheldrick, SHELXL-97. Program for the Refinement of Crystal
Structures, University of Go¨ttingen, Germany, 1997.
[1] I.I. Moiseev, T.A. Stromnova, M.N. Vargaftik, Izv. Akad. Nauk,
Ser. Khim. 4 (5) (1998) 777, Russ. Chem. Bull. (Engl. Transl.).