Bismuthanes as Hemilabile Donors in an O2-Activating Palladium(0) Complex

Article abstract of DOI:10.1002/chem.201703489

A xanthene-based bismuthane/phosphane chelating ligand has been accessed that has enabled the synthesis of a palladium(0) bismuthane complex. The bismuthane donor proved to be hemilabile as it switched to a dangling position upon addition of O₂ that gave a palladium(II) peroxide complex. Unlike the corresponding 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) palladium peroxide, the bismuth analogue could be employed for catalytic phosphane oxidation and oxidative phenol coupling.

Full text of DOI:10.1002/chem.201703489

DOI: 10.1002/chem.201703489
Communication
&
Bismuth
Bismuthanes as Hemilabile Donors in an O2-Activating
Palladium(0) Complex
Karolin Materne, Beatrice Braun-Cula, Christian Herwig, Nicolas Frank, and
Christian Limberg*[a]
Dedicated to Professor Dieter Fenske on occasion of his 75th birthday
character of the bismuthane changed from acceptor to pre-
dominantly donor.[7]
Abstract: A xanthene-based bismuthane/phosphane che-
lating ligand has been accessed that has enabled the syn-
thesis of a palladium(0) bismuthane complex. The bis-
muthane donor proved to be hemilabile as it switched to
a dangling position upon addition of O2 that gave a palla-
dium(II) peroxide complex. Unlike the corresponding 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos)
palladium peroxide, the bismuth analogue could be em-
ployed for catalytic phosphane oxidation and oxidative
phenol coupling.
The insights gained tethering bismuthane functions with
phosphane units as in the PBiP ligand mentioned above for
the complexation of late transition metals, led us to pursue fur-
ther promising bismuthane/phosphane combinations. The xan-
thene backbone has been used for the construction of a varie-
ty of potent bidentate ligands and complexes that exhibit in-
teresting reactivity.[8] Mostly, these ligands contain two aryl
phosphane units (rarely also mixed P/N donors), the most
prominent representative being 4,5-bis(diphenylphosphino)-
9,9-dimethylxanthene (Xantphos; Figure 1, left).
A lively area of current research is concerned with the directed
construction of compounds, in which two different metals in-
teract. Such investigations are motivated to uncover new prop-
erties, emerging through the M···M’ contact or to develop new
modes of cooperative reactivity.[1] Focusing on bismuth(III) as
part of such heterometallic entities, it is noted that only few
complexes exist, in which a transition metal neighbors Bi di-
rectly, and most of them feature a covalent bonding situation,
that is, a metal–metal bond.[2] Some compounds have been
isolated, which showed that bismuthanes principally can act as
donor ligands;[3] however, their number is rather limited, and
because of their reduced donor abilities, bismuthanes were
called the Cinderellas among Group 15/16 ligands.[3b] Those
bismuthane complexes, which are known, mainly contain
Group 6–8 central atoms,[2] and until recently, closed-shell
M···Bi interactions, for M representing a late-transition metal,
remained virtually unexplored.[4] In 2012, we and concomitant-
ly Gabba¨ı and co-workers established bismuth–late-metal
bonds through a different kind of metallophilic interaction.
The Lewis acidity of BiIII in an ambiphilic PBiP ligand system
(Ph2P-C6H4-Bi(Cl)-C6H4-PPh2), employed as such[5] or generated
in situ,[6] was exploited to bind AuI, PtII and PdII centers, which
acted as s donors. It was further shown that upon replacement
of the Cl residue at Bi against less electronegative ligands, the
Figure 1. Selected complexes with xanthene-based phosphane ligands
(P=PPh2).[8a–c]
Hence, we were interested to replace (formally) one of the
phosphane functions in Xantphos by a bismuthane unit (BiPh2)
and to then investigate the coordination properties of the new
potential ligand Xan(PPh2)(BiPh2) towards a Group 10 metal in
the oxidation state 0. Herein, we present the synthesis of
Xan(PPh2)(BiPh2), its reactivity towards a Pd0 precursor, and the
activation of O2 by the resulting complex.
The synthesis of Xan(PPh2)(BiPh2) started with the introduc-
tion of the phosphane fragment to the xanthene backbone ac-
cording to a modified literature procedure.[9] For this, XanBr2
was lithiated with one equivalent of phenyllithium and then
treated with one equivalent of Ph2PCl in tetrahydrofuran. The
bismuthane unit was then introduced through a second lithia-
tion and a subsequent reaction with Ph2BiCl (Scheme 1). After
work-up, a white solid was isolated, which was identified as
Xan(PPh2)(BiPh2) by NMR spectroscopy, elemental analysis, and
ESI MS data. Single-crystals of Xan(PPh2)(BiPh2) that were suit-
able for an X-ray diffraction analysis were obtained by diffusion
of hexane into a saturated solution of Xan(PPh2)(BiPh2) in tolu-
ene (Figure 2).
[a] K. Materne, Dr. B. Braun-Cula, Dr. C. Herwig, N. Frank, Prof. Dr. C. Limberg
Institut fꢀr Chemie, Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: christian.limberg@chemie.hu-berlin.de
Homepage: http://www.chemie.hu-berlin.de/aglimberg
The bismuth atom exhibits the typical structural features of
triply arylated bismuthanes, so that the sum of the angles
The ORCID identification number(s) for the author(s) of this article can be
found under https://doi.org/10.1002/chem.201703489.
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In the 31P{1H} NMR spectrum of a solution in C6D6, the phos-
phane donor gave rise to a signal at d=20.8 ppm, shifted by
37 ppm in comparison to unbound Xan(PPh2)(BiPh2). The
proton NMR spectrum displayed two new singlet signals for
the methyl groups (d=1.56 and 1.71 ppm) of the xanthene
backbone. Suitable crystals for an X-ray diffraction analysis
could be grown by diffusion of diethylether into a saturated
solution of the product in acetonitrile at room temperature
and the corresponding molecular structure determined is
shown in Figure 3.
Scheme 1. Synthesis of the potential ligand Xan(PPh2)(BiPh2).
Figure 2. Molecular structure of Xan(PPh2)(BiPh2): (left) front view, (right) top
view. All hydrogen atoms are omitted for clarity. Selected bond lengths [ꢁ]
and angles [8]: C2ꢀBi1 2.273(2), C29ꢀBi1 2.223(3), Bi1ꢀC30A 2.256(4), C36ꢀP1
1.865(3), C42ꢀP1 1.886(3), C14ꢀP1 1.832(3), P1···Bi1 4.2096(15); C29-Bi1-C2
91.53(10), C30A-Bi1-C2 93.38(12), C14-P1-C36 103.59(14), C14-P1-C42
102.11(13), C29-Bi1-C30A 94.90(13).
Figure 3. Molecular structure of 1·MeCN. The co-crystallized MeCN solvent
molecule and hydrogen atoms are omitted for clarity. Selected bond lengths
[ꢁ] and angles [8]: P1AꢀPd1A 2.290(3), P2AꢀPd1A 2.297(3), Bi1AꢀPd1A
2.7342(11), Bi2AꢀPd1A 2.7845 (11); P1A-Pd1A-P2A 129.07(12), P1A-Pd1A-
Bi1A 105.67(9), P2A-Pd1A-Bi1A 106.86(8), P1A-Pd1A-Bi2A 105.97(8), P2A-
Pd1A-Bi2A 107.51(8), Bi1A-Pd1A-Bi2A 97.27(3).
around the BiIII atom amounts to only 279.818.[10] The phenyl
residues at the P and Bi atoms are pointing away from each
other, and the BiIII···PIII distance is 4.2096(15) ꢁ.
Bearing in mind the scarcity of complexes with Pd···Bi inter-
actions (apart from a cluster[11] and a complex[10a] containing
PdꢀBi metal bonds, only a PdII!Bi complex of the above-men-
tioned ambiphilic pincer ligand is known[6]) the potential of
Xan(PPh2)(BiPh2) to bind Pd0 was investigated. Therefore, a
mixture of Xan(PPh2)(BiPh2) and 0.25 equivalents of [Pd2(dba)3]
(dba=bis(dibenzylideneacetone)) was dissolved in tetrahydro-
furan, and a color change from dark red to yellow was ob-
served within a few minutes (Scheme 2).
The complex was identified as [(Xan(PPh2)(BiPh2))2Pd] (1),
that is, the Pd0 center is surrounded by two ligand molecules.
Both phosphorus and bismuth donors are bound, so that a tet-
rahedral coordination sphere results. The distances of
Bi1A···Pd1A 2.7342(11) ꢁ and Bi2A···Pd1A 2.7845(11) ꢁ indicate
a significant interaction between these atoms, because the
values are significantly smaller than the sum of the corre-
sponding van der Waals radii (3.70 ꢁ).[12] To clarify the nature of
the metallophilic Bi···Pd interactions present, DFT calculations
(B3LYP) were carried out for 1, and a natural bond orbital
(NBO) analysis revealed one dominating donor–acceptor inter-
action between the filled atomic orbital of bismuth (6s) and
the empty atomic orbital of palladium (5s), and thus, an unpre-
cedented to date Bi!Pd ligation. The corresponding average
NBO deletion energy amounts to 61.9 kcalmolꢀ1 (Bi1A!Pd1A
67.1 and Bi2A!Pd1A 56.8 kcalmolꢀ1).
Because a series of different P-ligand-based Pd0 complexes
are known, which are able to react with dioxygen forming pal-
ladium(II) peroxido complexes,[13] the reaction behavior of com-
plex 1 towards the activation of O2 was tested.
Scheme 2. Synthesis of a Pd0 complex 1 (P=PPh2, Bi=BiPh2).
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Scheme 3. Synthesis of a PdII peroxido complex 2 (P=PPh2, Bi=BiPh2).
After treatment of complex 1 with dioxygen in n-hexane, a
light yellow precipitate was formed within a few minutes,
which was analyzed by infrared spectroscopy (Scheme 3). The
solid-state IR spectrum (ATR) showed one characteristic absorp-
tion band at u˜ =920 cmꢀ1, which can be assigned to a OꢀO
stretching vibration of a h2-peroxido ligand.[13] For the 18O iso-
topically labeled complex a shift of Du˜ =52 cmꢀ1 to higher
wavelengths could be observed (Figure 4).
Figure 5. Molecular structure of 2. All hydrogen atoms are omitted for clari-
ty. Selected bond lengths [ꢁ] and angles [8]: Pd1ꢀO2 2.015(3), Pd1ꢀP1
2.3175(9), O2ꢀO2’ 1.433(5), Bi1···P1 4.4507(12), Bi1···Pd1 4.2233(16); O2-Pd1-
O2’ 41.65(15), O2-Pd1-P1 104.14(8), O2’-Pd1-P1 144.76(8), P1-Pd1-P1’
110.84(5).
solvent leads to the loss of the peroxido unit with regeneration
of the Pd0 complex 1. The hemilabile behavior of the bismuth-
anes triggered investigations on the potential of 2 as an oxi-
dant, because some of the known PdꢀO2 complexes have
been reported to perform oxidations.[15]
Due to the low stability of 2 in solution, dioxygen was
added to a mixture of complex 1 and five equivalents of PPh3
in tetrahydrofuran.[17] A discoloration of the solution was ob-
served immediately. By using 31P{1H} NMR spectroscopy, the
formation of OPPh3 (80%, TON=4.0; based on PPh3) was de-
tected (Scheme 4). Although we could not obtain evidence for
any intermediate of the catalytic cycle, a conceivable mecha-
nism could include a bis(m-oxido) species generated after the
first O atom transfer or the insertion of phosphane into the
PdꢀO2 bonds.[16]
Figure 4. ATR-FTIR spectra of 16O-2 (black) and 18O-2 (red) between 1150 and
400 cmꢀ1
.
Upon cooling of a saturated solution of the product in tolu-
ene/n-hexane (1:2) to ꢀ308C for 16 hours, crystals were grown,
and an X-ray diffraction analysis revealed the molecular struc-
ture of [(Xan(PPh2)(BiPh2))2PdO2] (2; Figure 5).
The PdII center is coordinated in a distorted square-planar
coordination sphere by the phosphane units and the peroxide
ligand, whereas the bismuthane units remain dangling. Ac-
cordingly, these bismuthane donors in 1 can be considered as
hemilabile[14]: they readily give way to an external substrate,
such as O2, but in 1, they serve to fill the otherwise vacant co-
ordination sites. The crystallographically determined O2ꢀO2’
bond length of 1.433(5) ꢁ is within the typical range of known
PdII(h2-O2) complexes,[13] and also the Pd1ꢀO2/O2’ distances
are with 2.015(3) ꢁ characteristic.[13]
Scheme 4. Reactions of complex 1 with PPh3 and 2,4-di-tert-butylphenol in
the presence of dioxygen.
Employing 2,4-di-tert-butylphenol as a substrate (5 equiv in
THF) led to the formation of the coupling product 3,3’,5,5’-
tetra-tert-butyl-2,4-bisphenol (37%; based on complex 1). For
the analogous reaction with 9,10-dihydroanthracene, no reac-
tivity was observed.
Compound 2 decomposes rapidly in solution, but as a solid
2 is stable for months at room temperature. Dissolving 2 in a
To investigate the influence of the bismuthane units in 1 for
O2 activation and subsequent substrate oxidation, the analo-
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dation or coupling product was observed. Also attempts to
perform CꢀH bond activations failed.
In conclusion, first reactivity studies exhibit a higher reactivi-
ty for the bismuthane based complex 2 in comparison to 4
concerning the tested substrates PPh3 and 2,4-di-tert-butylphe-
nol. Apparently, the hemilabile character of the bismuth
groups exerts a positive effect on the reactivity of the palladi-
um peroxide complex 2 and restricts decomposition under
turnover conditions. Further studies, especially with regard to
the complexation behavior of the new bismuth-based ligand
Xan(PPh2)(BiPh2) towards other late transition metals and their
reactivity towards small molecules are ongoing.
Scheme 5. Synthesis of a PdII peroxido complex 4 (P=PPh2).
gous bis-ligated Pd0 complex [(Xantphos)2Pd] (3) based on the
Xantphos ligand[8e] was prepared in situ and also reacted with
molecular dioxygen in THF to generate a PdII peroxido com-
plex (Scheme 5).
Acknowledgements
We thank the DFG (LI 714/8-1) for funding as well as the Hum-
boldt-Universitꢂt zu Berlin.
Formation of a crystalline, yellow green solid was observed,
which was analyzed by infrared spectroscopy. A characteristic
absorption band at u˜ =907 cmꢀ1, shifting by 67 cmꢀ1 upon
usage of 18O2 was observed again, suggestive of a OꢀO
stretching vibration and formation of a peroxide complex.[13]
Upon cooling a saturated solution of the product in tetrahy-
drofuran to ꢀ308C for 16 hours, crystals could be grown, and
an X-ray diffraction analysis identified the product as [(Xant-
phos)2PdO2] (4; Figure 6). In comparison to 2, complex 4 con-
tains only one ligand molecule, coordinating to the metal
center by both phosphane donors. The palladium(II) center is
found in a square planar coordination sphere, and the O2ꢀO2’
distance of 1.422(5) ꢁ, as well as the Pd1ꢀO2 distance of
2.015(3) ꢁ (Pd1ꢀO2’ is the same) are similar to those found for
compound 2.
Conflict of interest
The authors declare no conflict of interest.
Keywords: bismuth · hemilabile · metallophilic interactions ·
palladium · peroxido ligands
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made proceeding this way, all reactions with PPh3 and 2,4-di-tert-butyl-
phenol were also carried out in a different order (first the palladium
peroxo complex 2 or 4 were generated, and afterwards the substrate
was added). In these cases, the formation of OPPh3 and the bisphenol,
respectively, was observed, too, proving that the PdII peroxo com-
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described procedure was preferred.
Manuscript received: July 27, 2017
Accepted manuscript online: && &&, 0000
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COMMUNICATION
&
Bismuth
K. Materne, B. Braun-Cula, C. Herwig,
N. Frank, C. Limberg*
&& – &&
Bismuthanes as Hemilabile Donors in
an O2-Activating Palladium(0)
Complex
New character trait of bismuthanes:
Pd0 chelated by a phosphino/bismuthi-
no ligand reacted with dioxygen to
form a palladium(II) peroxido complex
(see scheme). In this reaction, the bis-
muthane donor acts as a hemilabile
ligand that leaves when O2 enters and
comes back, upon O2 elimination. This
feature proved to be favorable for the
properties of the peroxide as an oxi-
dant.
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