Phosphorus-containing hybrid calixphyrins: Promising mixed-donor ligands for visible and efficient palladium catalysts

Article abstract of DOI:10.1021/ja0640039

Phosphorus-sulfur-containing hybrid calixphyrins were prepared by the BF₃-promoted dehydrative condensation between σ⁴-2,5-bis[(pyrrol-2-yl)methyl]phosphole and 2,5-bis[hydroxy(phenyl)methyl]thiophene. X-ray crystallographic analysis of the Pd-P,N₂,S-hybrid calixphyrin complex revealed that the Pd center was coordinated by the four heteroatoms to adopt a distorted square planar geometry. The Pd complex, displaying a characteristic reddish purple color in solution, catalyzed the Heck reaction of bromoarenes with n-butyl acrylate with high efficiency at elevated temperatures. Copyright

Full text of DOI:10.1021/ja0640039

Published on Web 08/19/2006
Phosphorus-Containing Hybrid Calixphyrins: Promising Mixed-Donor
Ligands for Visible and Efficient Palladium Catalysts
Yoshihiro Matano,* Tooru Miyajima, Takashi Nakabuchi, Hiroshi Imahori, Noriaki Ochi, and
Shigeyoshi Sakaki
Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku,
Kyoto 615-8510, Japan
Received June 7, 2006; E-mail: matano@scl.kyoto-u.ac.jp
There has been a great deal of interest in the coordination
chemistry of phosphorus-containing macrocyclic ligands,1 because
(1) the metal center is supported and stabilized by multidentate
platforms, and (2) various coordination environments are available
by changing the components of the macrocycles. However, the
potential utility of this class of ligands in transition metal catalysis
has not been explored much, partly because of a lack of information
on the relationship between their structural properties and their
catalytic activities. Calixphyrins are a class of compounds containing
multidentate platforms derived from porphyrins and calixpyrroles.2
Owing to the involvement of both sp2- and sp3-meso-carbons,
calixphyrins possess reasonable rigidity and flexibility that are
beneficial for designing efficient catalysts. We expected that
replacement of the pyrrole nitrogen atom of calixphyrin with a
phosphorus atom, namely replacing the pyrrole unit by phosphole,
would provide a new class of macrocyclic P,N-mixed-donor ligands
Figure 1. ORTEP diagram of 5 (30% probability ellipsoids). Hydrogen
atoms and the solvent molecules are omitted for clarity.
Scheme 1. Synthesis of Hybrid Calixphyrins.
capable of generating a coordinatively unsaturated metal center by
flipping motion under appropriate conditions.3 Here we report the
first examples of P,S-containing hybrid calixphyrins of the 5,10-
porphodimethene type. The palladium-P,N2,S-calixphyrin com-
plex, displaying a characteristic reddish purple color, has been found
to catalyze the Heck reaction with high efficiency at elevated
temperatures.
The synthesis of phosphole-containing hybrid calixphyrins is
summarized in Scheme 1. Treatment of a CH2Cl2 solution contain-
ing σ4-2,5-bis[(pyrrol-2-yl)methyl]phosphole 14 and 2,5-bis[hy-
droxy(phenyl)methyl]thiophene 25 with BF3‚OEt2 at room temper-
ature gave a mixture of condensation products, which was then
reacted with 2.2 equiv of 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ) to afford σ4-phosphorus-containing P,N2,S-hybrid calix-
phyrins 3 and 4 in 24% and 5% yield, respectively. When treated
with excess P(NMe2)3, the σ4-hybrid 3 was converted to the σ3-
P,N2,S-hybrid 5 in 66% yield with a small amount of the side-
product 6.
Scheme 2. Synthesis of Hybrid Calixphyrin-Pd Complex.
The calixphyrins 3-6 were fully characterized by standard
spectroscopic techniques. The 31P chemical shifts of 3 and 5 are
close to those of 4 and 6, suggesting that the pyrrole-thiophene-
pyrrole (N-S-N) ring reduction does not significantly affect the
electronic character of the phosphorus atom. The crystal structure
of 5 was further elucidated by X-ray diffraction analysis (Figure
1).6 The σ3-P,N2,S-hybrid 5 is composed of a flat N-S-N plane
palladium complex, 5 was treated with Pd(dba)2 in CH2Cl2 at room
temperature (Scheme 2). After 3 h, the palladium complex 7 was
obtained as an air and thermally stable, purple solid in 92% yield.
The metathesis of 6 with Pd(OAc)2 afforded the same complex, 7,
in quantitative yield. These results revealed that the σ3-P,N2,S-hybrid
calixphyrins would behave as redox-active macrocyclic ligands.
The crystal structure of 7 was elucidated by X-ray diffraction
analysis (Figure 2).7 The palladium center is coordinated by the
and the phosphole ring that stands almost perpendicular to the
N-S-N plane. Probably because of steric reasons, the P-phenyl
group is located outside the macrocycle. The observed structural
features are quite different from those of the 5,10-porphodimethene
type of calix[4]phyrin, which exhibits a nonplanar, twisted
conformation.2c
The σ3-P,N2,S-hybrid 5 is a new type of macrocyclic P,N,S-
mixed-donor ligand for transition metal complexes. To prepare a
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10.1021/ja0640039 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
In summary, we prepared P,S-containing hybrid calixphyrins for
the first time. The σ3-P,N2,S-hybrids were successfully converted
to a visible, macrocyclic palladium complex, which was found to
catalyze the Heck reaction with high efficiency. It should be noted
here that the coordination number and the oxidation state at the
metal center are controllable by changing the size and components
of the macrocyclic platform. We anticipate that these characteristics
of P-containing hybrid calixphyrins will be of great benefit for
designing new classes of transition metal catalysts.
Acknowledgment. This work was partially supported by Grants-
in-Aid (No. 17350018 and 21st Century COE on Kyoto University
Alliance for Chemistry) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan. We thank Dr. Yoshihide
Nakao for his assistance with DFT calculations.
Figure 2. ORTEP diagram of 7 (30% probability ellipsoids). Hydrogen
atoms and the solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Pd-P, 2.2135(8); Pd-N1, 2.079(2); Pd-
N2, 2.066(2); Pd-S, 2.2667(8); P-Pd-N1, 95.21(7); P-Pd-N2, 85.75-
(7); S-Pd-N1, 92.93(6); S-Pd-N2, 85.77(7).
Supporting Information Available: Experimental details, CIF files
for 5 and 7, and DFT computational results. This material is available
free of charge via the Internet at http://pubs.acs.org.
four heteroatoms to adopt a distorted square planar geometry. The
N-S-N unit in 7 is not on the same plane, and the edge-to-edge
distances between two meso carbons differ from those in 5 (Figure
S1). The phosphole ring leans toward the inside for binding the
palladium, and the sulfur atom is deviated from the thiophene ring
with a dihedral angle of 20°. The Pd-N bond lengths of 2.066-
(2)-2.079(2) Å are longer than typical values (2.012 ( 0.018 Å)
observed for Pd-coordinated porphyrin-type macrocycles.8
The observed carbon-carbon bond lengths of the N-S-N unit
in 7 are indicative of a noticeable contribution by the canonical
structure 7′, in which the formal oxidation state of Pd is considered
to be +2 (Figure S2). This was confirmed by density functional
theory calculations on model compounds (for details, see Supporting
Information). If the palladium in 7 is deviated from the N-S-N
plane by flipping motion, however, a coordinatively unsaturated,
highly reactive palladium center would be generated. This hypoth-
esis was verified by the variable-temperature NMR measurements
of 7. When heated at 125-135 °C in DMSO-d6 and DMA-d9, the
thiophene- and pyrrole-derived 1H peaks and the phosphole-31P peak
were considerably broadened as compared to those observed at 25
°C (Figure S3). This result indicates that the macrocyclic framework
in 7 becomes much more flexible at elevated temperatures, which
allows coordinative interaction between the solvents and the face-
up palladium center.
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As expected, the reactions proceeded at 135 °C to afford the Heck
products with turnover numbers of 9700-12300.9 Although the
literature contains more active Heck catalysts,10 the present results
demonstrate that the P-containing hybrid calixphyrins constitute a
useful addition to the existing efficient phosphine ligands. It is
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of the catalyst without isolating it.11
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(11) Complex 7: UV/vis (CH2Cl2) λmax 524 nm (ꢀ ) 14000).
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