Porphyrin pincer complexes: Peripherally cyclometalated porphyrins and their catalytic activities controlled by central metals

Article abstract of DOI:10.1021/ja072218s

A series of heterobimetallic porphyrin pincer palladium complexes have been synthesized through direct cyclometalation of β,β′-dipyridylporphyrin. X-ray analyses elucidated their highly distorted structures of porphyrin cores induced by the outer metalation. The catalytic activity has been investigated in the typical Heck reaction of iodobenzene with butyl acrylate. These complexes exhibit markedly different catalytic activities in the initial phase of the reaction, depending on the central metals. Copyright

Full text of DOI:10.1021/ja072218s

Published on Web 05/01/2007
Porphyrin Pincer Complexes: Peripherally Cyclometalated Porphyrins and
Their Catalytic Activities Controlled by Central Metals
Shigeru Yamaguchi, Taisuke Katoh, Hiroshi Shinokubo,* and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, and PRESTO & CREST,
Japan Science and Technology Agency (JST), Sakyo-ku, Kyoto 606-8502, Japan
Received March 29, 2007; E-mail: hshino@kuchem.kyoto-u.ac.jp; osuka@kuchem.kyoto-u.ac.jp
Scheme 1 a
Porphyrin metal complexes generally employ the inner cavity
as the coordination site. The central metal often plays a critical
role to dominate electronic and structural properties of a metal-
loporphyrin. In contrast, transition metal porphyrin complexes that
have a carbon-metal σ-bond at the porphyrin peripheral are still
rare.1 Nevertheless, peripherally metalated porphyrins should offer
a nice scaffold to investigate electronic and steric effects of the
inner metal and porphyrin core onto the outer metal. Such effects
likely influence physical and chemical properties such as magne-
tism, photophysics, and reactivity in metal-catalyzed reactions of
the complexes.
Pincer type organometallic complexes have received much
attention as highly active and stable catalysts in a number of organic
transformations, as well as light emitting materials.2 The tridentate
ligand strongly supports a metal to prevent ligand dissociation,
achieving high thermal stability. Although there have been numer-
ous pincer complexes reported, heterobimetallic pincer complexes
are not well-precedented.3 More importantly, the effect of one metal
a Reagents and conditions: (a) 2-iodopyridine, Pd2(dba)3, PPh3, Cs2CO3,
upon the other catalytic center has not been investigated.
CsF,toluene/DMF/H2O,reflux;(b)(i)concdH2SO4/CH2Cl2,(ii)Zn(OAc)2‚2H2O
We undertook the synthesis of a heterobimetallic porphyrin pincer
or Cu(OAc)2‚H2O, CHCl3/MeOH; (c) K2PdCl4, toluene/DMF, 100 °C; (d)
K2PdCl4, NaOAc, DMF, 60 °C; (e) concd H2SO4/TFA, reflux.
complex, namely, a hybrid of a peripherally metalated porphyrin
and a pincer type complex (Scheme 1).4 Two 2-pyridyl groups were
introduced at the â-positions adjacent to the unsubstituted meso-
position of 5,10,15-triarylporphyrin to set up the outer coordination
site, starting from â,â′-diborylporphyrin 1Ni via Suzuki-Miyaura
cross-coupling with 2-iodopyridine. â,â′-Difunctionalized porphy-
rins have been difficult to prepare but have now become readily
available through iridium-catalyzed borylation of porphyrins.5
Metalation of nickel porphyrin 2Ni with K2PdCl4 in toluene-DMF
at 100 °C proceeded efficiently with vivid color change from purple
to green via smooth C-H bond cleavage to furnish heterobimetallic
complex 3NiPd as an air- and moisture-stable solid in 92% yield
after recrystallization. Its parent mass ion peak was observed at
m/z ) 1191.4577 (calcd for (C72H77N6NiPd)+ ) 1191.4600 [(M -
Cl)+]) in its high-resolution electrospray-ionization time-of-flight
(HR ESI-TOF) mass spectrum. Accordingly, disappearance of the
1
meso-proton in the H NMR spectrum of 3NiPd supported the
formation of a carbon-metal bond. Upon metalation, the proton
H1 on the pyridyl group exhibited substantial downfield shift (from
Figure 1. X-ray structures of porphyrin pincer complexes. Top view (a),
δ ) 9.01 to 9.84). Palladation of 2Cu also proceeded smoothly to
front view (b), side view (c) of 3NiPd, and top view (d) of 3HPd. The
provide 3CuPd in 72% yield, and 3ZnPd was obtained under
thermal ellipsoids are scaled to the 50% probability level. meso-Aryl
substituents are omitted for clarity.
milder conditions at 60 °C in 83% yield. Interestingly, treatment
of 3NiPd with sulfuric acid in refluxing trifluoroacetic acid (TFA)
1 and Supporting Information).6 Maximum displacements of the
â-carbon from the 4N plane are 1.067 Å for 3NiPd, 0.982 Å for
3CuPd, and 1.054 Å for 3HPd, and the mean plane deviations are
0.436 Å for 3NiPd, 0.384 Å for 3CuPd, and 0.380 Å for 3HPd.
The bond lengths of C-Pd are 1.977(7) Å for 3NiPd, 1.972(6) Å
for 3HPd, and 1.969(6) Å for 3CuPd. The pyridyl groups are rather
coplanar (ca. 6-9°) to the adjacent pyrrole units in the deformed
induced preferential demetalation of the inner nickel ion to provide
free base porphyrin 3HPd in 71% yield, thus highlighting the
strength of the C-Pd bond. X-ray crystallographic analyses of
3NiPd, 3CuPd, and 3HPd unambiguously elucidated their struc-
tures. Interestingly, the structure of the porphyrin macrocycle is
highly distorted to take a saddle conformation, while the outer
palladium exhibits almost ideal square planar coordination (Figure
9
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J. AM. CHEM. SOC. 2007, 129, 6392-6393
10.1021/ja072218s CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
In conclusion, we have achieved the synthesis of heterobimetallic
porphyrin pincer complexes. Highly distorted structures of them
have been revealed by X-ray analyses. These complexes exhibit
markedly different catalytic activity in the typical Heck reaction
depending on the central metals. Highly tunable nature of porphyrins
by central metals and peripheral substituents should allow further
creation of effective complexes for various transition-metal-
catalyzed reactions.
Acknowledgment. This paper is dedicated to the memory of
Professor Yoshihiko Ito. This work was supported by a Grant-in-
Aid for Scientific Research (No. 18685013) from MEXT. H.S.
acknowledges Asahi Glass Foundation and Mitsubishi Chemical
Corporation Fund for financial support.
Figure 2. UV/vis absorption spectra of 2Ni, 2H, 3NiPd, and 3HPd in
CH2Cl2.
Supporting Information Available: General procedures, spectral
data for compounds, crystal structures, and CIF files for 2Zn, 3NiPd,
3CuPd, and 3HPd. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 3. Reaction profile of the Heck reaction with 3MPd.
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dihedral angles (ca. 46-57°) between the aromatic substituents and
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Figure 2 shows UV/vis absorption spectra of 3 along with 2,
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The attractive feature of porphyrins is that their structural and
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catalytic activity in the typical Heck reaction.8 All of the pincer
complexes exhibited catalytic activity, but comparative experiments
revealed notable differences in the initial reaction profile (Figure
3). Although the mechanism of the present Heck system is unclear,
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(6) Crystal data for 3NiPd:
C72H77ClN6NiPd‚CCl4‚1.84CH3CN, Mw )
1917.58, monoclinic, space group P21/c (No. 14), a ) 13.689(3) Å, b )
19.250(3) Å, c ) 33.159(6) Å, â ) 91.498(4)°, V ) 8740(3) Å3, Z ) 4,
Dcalcd ) 1.457 g/cm3, T ) 90 K, 43 280 measured reflections, 15 262
unique reflections, R ) 0.0998 (I > 2.0σ(I)), Rw ) 0.2816 (all data),
GOF ) 1.151 (I > 2.0σ(I)). 3CuPd: C72H77ClCuN6Pd‚C2H4Cl2‚water,
Mw ) 1346.74, orthorhombic, space group Pbca (No. 61), a ) 19.161(3)
Å, b ) 13.9554(18) Å, c ) 49.017(6) Å, V ) 13107(3) Å3, Z ) 8, Dcalcd
) 1.365 g/cm3, T ) 90 K, 65 466 measured reflections, 11 547 unique
reflections, R ) 0.0883 (I > 2.0σ(I)), Rw ) 0.2080 (all data), GOF )
1.052 (I > 2.0σ(I)). 3HPd:
C72H79ClN6Pd‚C2H4Cl2‚CH3CN, Mw )
1310.27, orthorhombic, space group Pcab (No. 61), a ) 14.106(5) Å, b
) 19.030(5) Å, c ) 48.863(5) Å, V ) 13117(6) Å3, Z ) 8, Dcalcd
)
1.327 g/cm3, T ) 90 K, 65 200 measured reflections, 11 560 unique
reflections, R ) 0.0779 (I > 2.0σ(I)), Rw ) 0.2175 (all data), GOF )
1.043 (I > 2.0σ(I)).
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