A stable zerovalent palladium chain enveloped by a π-electron sheath of conjugated polyene ligands

Article abstract of DOI:10.1039/b714530f

A surprisingly stable homoleptic Pd⁰₄ chain complex of non-activated olefins was isolated and structurally characterized by X-ray crystallographic analysis, and the unique structure and bonding are compared to those of the corresponding dicationic [Pd₄]²⁺ chain sandwich complex. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714530f

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A stable zerovalent palladium chain enveloped by a p-electron sheath of
conjugated polyene ligands{
Yasuki Tatsumi, Tetsuro Murahashi,* Mitsue Okada, Sensuke Ogoshi and Hideo Kurosawa
Received (in Cambridge, UK) 20th September 2007, Accepted 31st October 2007
First published as an Advance Article on the web 13th November 2007
DOI: 10.1039/b714530f
A surprisingly stable homoleptic Pd0 chain complex of non-
delocalization over the Pd chain as well as over the polyene
ligands takes place.8g
4
activated olefins was isolated and structurally characterized by
Interestingly, treatment of the [Pd4]2+ complex [Pd4(m4-
X-ray crystallographic analysis, and the unique structure and
g3:g2:g2:g3-DPOT)2][B(Arf)4]2 (DPOT
=
1,8-diphenyl-1,3,5,7-
bonding are compared to those of the corresponding dicationic
[Pd4]2+ chain sandwich complex.
octatetraene and B(Arf)4
=
B{3,5-(CF3)2C6H3}4) (1)8a or
[Pd4(m4-g2:g2:g2:g2-DPOT)2(py)2][B(Arf)4]2 (1-py)8a with Cp2Co
(2 equiv.) in CH3CN gave a brown precipitate 2, which is insoluble
in most organic solvents. The composition of 2 was confirmed as
‘‘Pd4(DPOT)2’’ by the elemental analysis. Oxidation of 2 with
[Cp2Fe][B(Arf)4] (2 equiv.)10 in the presence of pyridine gave 1-py
almost quantitatively. The reduction of [Pd4(m4-g2:g2:g2:g2-tBu-
Small Pd clusters have been proposed to act as the active catalytic
species by themselves or the source of mononuclear Pd0 species in
heterogeneous Pd catalyzed reactions.1 However, it has not been
understood how small aggregates of Pd0 atoms are stabilized in the
absence of strong p-acceptor ligands such as CO, isocyanide, or
electron-deficient olefin ligands.2 Actually, without such strong
p-acceptor ligands, only a limited number of polynuclear Pd0
complexes with intermetallic distances close to that of bulk Pd
metal (2.76 s) have been isolated; e.g. Pd2(Cy2P(CH2)2PCy2)2 (Pd–
Pd = 2.761 s).3,4 In light of the fact that graphenes or other sp2-
carbon frameworks are widely used as the catalyst support for
palladium, ‘‘non-activated’’ olefins and arenes5 may be capable of
stabilizing small Pd0 clusters. However, this has not been proven at
the molecular level. Indeed, the corresponding homoleptic mono-
nuclear Pd0 complexes such as Pd(1,6-cyclooctadiene)2 and
Pd(norbornene)3 are thermally unstable, and usually decompose
in solution at around ambient temperature.6,7 Herein, we report
the isolation and structural characterization of a surprisingly stable
homoleptic olefin Pd0 cluster in which ‘‘non-activated’’ conjugated
polyene ligands flank a linear array of Pd0 atoms with Pd–Pd
separations shorter than 2.7 s. The bonding nature of the unique
Pd0 cluster was studied using quantum calculations.
DPOT)2(py)2][B(Arf)4]2 (3-py, tBu-DPOT
=
1,8-bis(3,5-di-
t-Bu)phenyl-1,3,5,7-octatetraene) gave complex 4 which is soluble
in benzene or THF (Scheme 1). It was confirmed that oxidation of
4 with [Cp2Fe][B(Arf)4] (2 equiv.) in the presence of pyridine gave
3-py almost quantitatively.
The molecular structure of 4 was determined from X-ray
crystallographic analysis (Fig. 1).{ The Pd4 chain is retained in 4.
The Pd–Pd distances (Pd1–Pd2 = 2.6272(8) s, Pd2–Pd3 =
2.6048(7) s, Pd3–Pd4 = 2.6662(8) s) are shorter than the Pd–Pd
distance in bulk Pd metal (2.76 s) or those in the [Pd4]2+
complexes 3-py(BF4) (2.7580(2) s, 2.7448(2) s)11 and
1
(2.7322(8) s, 2.654(1) s).8a The linear Pd0 (n . 2) chain cluster
n
with Pd–Pd separation shorter than 2.7 s is unprecedented.12
t
Interestingly, two Bu-DPOT ligands in 4 coordinate to the Pd4
chain in different coordination modes: i.e. m4-g3:g2:g2:g3 and
m4-g2:g2:g2:g2 modes. On the other hand, in the dicationic
complexes, the two 1,8-diaryltetraene ligands bind a [Pd4]2+ chain
in an identical mode: m4-g3:g2:g2:g3-mode to the [Pd4]2+ chain
in the absence of capping ligands at the Pd chain ends, or
Our group recently reported that a dicationic palladium chain or
sheet, which is generally formulated as [Pdn]2+, can be formed and
stabilized between parallel p-conjugated polyenes or aromatic
hydrocarbons in a discrete complex.8,9 Typically, a stable sandwich
chain complex [Pdn(a,v-diphenylpolyene)2]2+ (n = 3–5) is obtained
from a dicationic precursor [PdL4]2+ or [Pd2L6]2+ (L = CH3CN)
and a Pd0 complex in the presence of a,v-diphenylpolyene
ligands.8 On the other hand, the polyene-polypalladium(0)
complexes were not formed from a mixture of the Pd0 complex
Pd2(dba)3 and a,v-diphenylpolyenes. A recent theoretical study
showed that the dicationic charge is important in maintaining the
high stability of sandwich chain complexes where charge
Department of Applied Chemistry, Osaka University, & PRESTO,
Japan Science and Technology Agency (JST), Suita, Osaka, 565-0871,
Japan. E-mail: tetsu@chem.eng.osaka-u.ac.jp; Fax: (+81)6 6879 7394;
Tel: (+81)6 6879 7374
{ Electronic supplementary information (ESI) available: Experimental
details of the preparation and characterization of 2, 3-py, 4 and 49, details
of the X-ray single crystal structural analyses for 4 and 3-py(BF4), and
computational details. See DOI: 10.1039/b714530f
Scheme 1
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Scheme 2 A: Back-donating interactions between the tetraene LUMO
and bonding dp orbital; B: Back-donating interactions between pentaene
LUMO and antibonding ds orbital.
located on the Pd4 moiety (+1.57). For another dicationic model
[Pd4(m4-g2:g2:g2:g2-C8H10)2(py)2]2+, the polyene ligands are nearly
neutral (20.08 for each), while positive charge is mainly on the Pd4
moiety (+1.83). Thus, the wide range of charge capacities for the
p-conjugated polyene from negative to positive values is important
in stabilizing the Pd chain in different oxidation states. The MO
analysis showed that back-donating interactions from the bonding
dp orbital to the tetraene LUMO and from the antibonding ds
orbital to the pentaene LUMO are responsible for the bonding
between the polyene ligand and the polypalladium moiety in
Pd4(C8H10)(C10H12) (Scheme 2). However, these interactions do
not cause a direct Pd–Pd interaction;4,17 Wiberg bond indices
(WBIs)18 (0.095 for the inner Pd–Pd and zero for the outer Pd–Pd)
are much smaller than those for the corresponding dicationic
model compounds.19
In summary, the homoleptic Pd0 cluster of a ‘‘non-activated’’
olefin was isolated for the first time. The surprisingly stable
structure is mainly due to the variable hapticity as well as the
charge capacity of the conjugated polyene ligands. The present
results provide molecular evidence of the concept that sp2-carbon
frameworks could serve as excellent supports for small zerovalent
Pd clusters. Further study on the p-complexes of Pd0 clusters is
now underway in our laboratory.
Fig. 1 ORTEP drawing of 4 (50% probability thermal ellipsoids;
hydrogen atoms and solvent molecules are omitted for clarity). Selected
bond lengths (s) and angles (u): Pd1–Pd2 2.6272(8), Pd2–Pd3 2.6048(7),
Pd3–Pd4 2.6662(8), Pd1–C1 2.413(9), Pd1–C2 2.311(11), Pd1–C3 2.175(8),
Pd1–C18 2.206(8), Pd1–C17 2.223(7), Pd2–C4 2.227(10), Pd2–C5 2.192(8),
Pd2–C16 2.152(9), Pd2–C15 2.205(8), Pd3–C6 2.232(10), Pd3–C7 2.187(8),
Pd3–C14 2.173(9), Pd3–C13 2.207(8), Pd4–C8 2.358(10), Pd4–C9 2.122(8),
Pd4–C10 2.250(7), Pd4–C12 2.155(7), Pd4–C11 2.169(8), Pd1–Pd2–Pd3
176.33(2), Pd2–Pd3–Pd4 176.00(3).
m4-g2:g2:g2:g2-mode in the presence of capping ligands. Thus,
during the reduction, the pyridine ligands dissociate and one of the
tBu-DPOT ligands slips on the Pd4 chain by changing the
hapticity, stabilizing the formally zerovalent Pd4 chain. It is
noticeable that the total electron counting for 4 (58 e) is the same
as that for the dicationic complexes 1, 1-py, and 3-py.
Complex 4 isomerized slowly to 49 at 25 uC under N2
atmosphere within 2 days. The isomerization was promoted by
the presence of free tBu-DPOT (1 h at 20 uC). The structure of the
isomer 49 is assumed to be the eclipsed sandwich with respect to
the zigzag polyene part of tBu-DPOT ligands.13,14 No decomposi-
tion of 49 was observed even upon heating to 95 uC in toluene-d8
under N2 atmosphere, while complex 4 or 49 is air-sensitive.15 Such
a high thermal stability of 4 is surprising because known
homoleptic ‘‘non-activated’’ olefin Pd0 complexes decompose in
solution upon heating.
This work was supported by PRESTO, Japan Science and
Technology Agency (JST) and Grants-in-aid for Scientific
Research, Ministry of Education, Culture, Sports, Science and
Technology of Japan. Y. T. thanks the research fellowships for
young scientists from the Japan Society for the Promotion of
Science (JSPS).
Notes and references
Obvious differences in the reactivity toward electrophilic
substrates was observed between lower valent 4 and higher valent
3-py, 1-py, or 1. Complex 4 reacted with dibenzylideneacetone
(dba) or allyl chloride immediately to form Pd2(dba)3 or [Pd(g3-
allyl)Cl]2. On the other hand, 3-py, 1-py, or 1 did not react with
these substrates under the same conditions. These results in
reactivity emphasize the zerovalent character of the palladium
chain in 4.
{ Crystal data for 4: C78H111NOPd4, Mr = 1504.34, triclinic, space group
¯
P1 (no. 2), a = 13.8260(5), b = 15.3256(5), c = 18.6303(5) s, a =
71.3570(8)u, b = 83.1230(9)u, c = 71.5490(9)u, V = 3547.44(19) s3, Z = 2,
F(000) = 1556, Dc = 1.408 g cm23, m(Mo Ka) = 10.414 cm21, T = 123 K,
44 531 reflections collected, 21 391 unique (Rint = 0.072), 797 variables
refined with 11 296 reflections with I . 2s(I) to R1 = 0.099. CCDC
661329. For crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b714530f
1 For selected recent reviews: (a) D. Astruc, Inorg. Chem., 2007, 46, 1884;
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2 Most of the previously isolated molecular Pd0 clusters are stabilized by
the bridging coordination of strong p-acceptor ligands such as CO and
isocyanides. For a review: A. D. Burrows and D. M. P. Mingos,
Transition Met. Chem. (Dordrecht, Neth.), 1993, 18, 129 and references
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To gain an insight into the bonding of 4, we carried out
quantum calculations at the MP2 level on the model compound
Pd4(m4-g2:g2:g2:g2-C8H10)(m4-g3:g2:g2:g3-C10H12).16 The opti-
mized geometry agrees well with the structure of 4, where
calculated Pd–Pd distances are 2.65 s for outer bonds and
2.59 s for inner. Natural charge analysis shows that the Pd4
moiety possesses a considerable positive charge (+1.68), and the
negative charge is shared by the C8H10 ligand (20.83) and C10H12
ligand (20.85). The polyene ligands in the corresponding
dicationic model [Pd4(m4-g3:g2:g2:g3-C10H12)2]2+ are charged
positively (+0.21 for each), while the dominant positive charge is
478 | Chem. Commun., 2008, 477–479
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14 The 1H or 13C{1H} NMR signal pattern of 49 is similar to that of 4. 1H
NMR spectra of 4 or 49 were temperature dependent due to the
dynamic sliding behavior of the tBu-DPOT ligands on the Pd4 chain. In
THF-d8 (25 uC) or toluene-d8 at 25–95 uC, complex 4 exhibited one set
of 8 olefin proton resonances for the two tBu-DPOT ligands. 1H NMR
(C6D6, 25 uC) of the –(CHLCH)4– part of 4: d = 5.11 (1H, d, J =
12.7 Hz), 4.34 (1H, t, J = 11.8 Hz), 3.74 (1H, d, J = 12.5 Hz), 3.57 (1H, t,
J = 11.6 Hz), 2.33 (1H, t, J = 12.2 Hz), 2.28 (1H, t, J = 12.3 Hz), 1.55
(2H, overlapped). Cooling the THF-d8 solution sample down to
2105 uC resulted in decoalescence of these resonances. Although we are
unable to obtain well-characterized resonances at temperatures lower
than 2105 uC, the spectral aspects suggest occurrence of synchronous
sliding of the tBu-DPOT ligands (g3:g2:g2:g3 A g2:g2:g2:g2 and
g2:g2:g2:g2 A g3:g2:g2:g3) where the relative direction of these two
slidings is either parallel or antiparallel without intercrossing of the two
different directional movements. For details of the proposed dynamic
behavior, see ESI{.
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5 The term ‘‘non-activated’’ olefin is used for an olefin containing no
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10 h to unidentified products. In contrast, the dicationic complex 3-py is
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661330. See ESI{.
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13 The presence of two isomers for the dicationic polyene-polypalladium
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ref. 8b.
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Chem. Commun., 2008, 477–479 | 479