Synthesis and catalytic activities of porphyrin-based PCP pincer complexes

Article abstract of DOI:10.1002/anie.201308551

2,18-Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition-metal salts to afford the corresponding porphyrin-based PCP pincer complexes. The porphyrinic plane and the PCP-pincer unit are apparently coplanar, with sma

Full text of DOI:10.1002/anie.201308551

Angewandte
Chemie
DOI: 10.1002/anie.201308551
Ligand Design
Synthesis and Catalytic Activities of Porphyrin-Based PCP Pincer
Complexes**
Keisuke Fujimoto, Tomoki Yoneda, Hideki Yorimitsu,* and Atsuhiro Osuka*
Abstract: 2,18-Bis(diphenylphosphino)porphyrins undergo
peripheral cyclometalation with group 10 transition-metal
salts to afford the corresponding porphyrin-based PCP
pincer complexes. The porphyrinic plane and the PCP-pincer
unit are apparently coplanar, with small strain. The catalytic
activities of the porphyrin-based pincer complexes at the
periphery were investigated in the allylation of benzaldehyde
with allylstannane and in the 1,4-reduction of chalcone to
discover the electronic interplay between the inner metal and
the outer metal in catalysis.
W
hereas a porphyrin scaffold usually accommodates
a metal in its inner cavity to alter its electronic and structural
properties, porphyrins bearing a porphyrinic carbon–transi-
tion metal s bond on the periphery have been emerging as
a new class of porphyrin complexes.[1–3] They are not only
structurally novel but also intriguing to investigate because of
the electronic interplay between the inner metal and the outer
metal.[2b,e,h,3a,b]
to a porphyrinic carbon atom.[3a] Both groups found that the
catalytic activities of the pincer complexes in the Heck
reaction depend on the inner metal, and thus implies
electronic interplay between the inner metalloporphyrin
core and the outer palladium pincer units.[7]
Despite the observations that imply inner and outer
electronic interplay, the mechanism of the Heck reaction
catalyzed by a pincer palladium complex has been under
intense debate. Many researchers suggested that pincer
palladium complexes are most likely to act simply as
precursors of highly active palladium(0) colloids at higher
temperatures in the Heck reaction.[8] In other words, 1M1Pd
would be structurally labile under the reaction conditions and
Many conventional porphyrin/transition metal complexes
show important catalytic activities such as oxygenation.[4] In
contrast, the catalytic activities of peripherally metalated
porphyrins remain unexplored.[1] Among peripherally meta-
lated porphyrins, porphyrin-based pincer complexes have
been attracting attention because of the chemical stabilities,
physicochemical properties, and catalytic activities of pincer
structures.[5]
In pioneering work, Klein Gebbink et al. synthesized
porphyrins which have meso aryl substituents bearing a pincer
unit (E = coordinating heteroatom, Ar= 3,5-tBu2C6H3
throughout the manuscript).[6] After their interesting reports
about the indirectly peripherally metalated porphyrin pincer
complexes, we reported the synthesis of the NCN pincer
palladium complexes 1M1Pd as the first porphyrin-based
pincer complexes wherein the palladium is directly attached
À
result in irreversible dissociation of the porphyrinic C Pd
s bond. Thus, there are no distinct reports on the catalytic
activity of porphyrin-based pincer complexes with regard to
electronic interplay between the inner metal and the outer
metal.[9,10]
The structures of the NCN pincer complexes 1M1Pd are
significantly distorted because of the fused six-membered
À
À
rings with long C Pd and N Pd bonds, as well as the rigid
square-planar geometry of the palladium center.[3a] In pursuit
of peripherally metalated porphyrins that are catalytically
active without considerable decomposition, we have now
developed the porphyrin-based PCP pincer complexes
2M1M2. The PCP pincer unit of 2M1M2 consists of two fused
five-membered rings and is hence expected to take a less
biased conformation. As phosphines represent the most
popular ligands for transition-metal catalysts, we envisioned
more possibilities for 2M1M2 to serve as catalysts, but not as
merely a metal source.
[*] K. Fujimoto, T. Yoneda, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science, Kyoto
University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
Prof. Dr. H. Yorimitsu
ACT-C, Japan Science and Technology Agency (Japan)
[**] This work was supported by Grants-in-Aid from the MEXT (Nos.:
24106721 “Reaction Integration” and 25107002 “Science of Atomic
Layers”) and from the JSPS [Nos.: 25220802 (Scientific Research
(S)), 24685007 (Young Scientists (A)), 23655037 (Exploratory
Research)]. T.Y. acknowledges a JSPS Fellowship for Young
Scientists.
The synthesis of the pincer ligand 4M1 (M1 = Ni and Zn)
was achieved through palladium-catalyzed phosphination[11]
of the porphyrinyl ditriflate 3M1 (Scheme 1). We were
anxious about possible oxidation of the trivalent phosphines
to phosphine oxides in air. Indeed, 4Zn is sensitive to air and
was gradually oxidized in solution, thus leading to a decreased
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308551.
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Electronic perturbations induced by the outer metals were
investigated through UV/Vis absorption spectroscopy (Fig-
ure S33). Compared to the parent bidentate ligand 4Ni,
palladium (2NiPd) and platinum (2NiPt) pincer complexes
exhibit similar yet enhanced and red-shifted Soret bands,
which could be ascribed to their more rigid conformations. In
contrast, the homobimetallic nickel complex 2NiNi displays
a broad and largely split Soret band at l = 397 and 458 nm and
the most red-shifted Q-band. Since 2NiM2 are structurally
similar (see Figures S37–S39 in the Supporting Information),
the unique absorption of 2NiNi is attributable to a rather
strong interaction between the outer nickel d orbitals and the
porphyrinic p orbitals.[13]
Pincer complexes are known to have a wide spectrum of
catalytic activity.[5] In exploring the catalytic activity of
porphyrin-based pincer complexes which sustain their
pincer structures during catalytic cycles, we firstly selected
catalytic allylation of benzaldehyde with allyltributyltin[14] as
a model reaction. Unlike the Heck reaction, the allylation is
known to proceed by maintaining the valence of the transition
metal during the catalytic cycle and without significant
decomposition of the catalyst.[14b,15] Therefore, we considered
that the allylation an ideal reaction to investigate the genuine
catalytic activity of porphyrin-based pincer complexes.
The allylation indeed occurred in the presence of 2M1M2
(Table 1). Additions of AgPF6 are essential for generating
catalytically active cationic pincer complexes by removal of
the chloride on M2. The allylation that was catalyzed by 2NiNi
Scheme 1. Synthesis of porphyrin-based PCP pincer complexes.
a) 5 equiv HPPh2, 20 mol% Pd(OAc)2, 20 mol% dppb, 10 equiv NEt3,
DMF, 908C, 12 h; b) 1.1 equiv [NiCl2(PPh3)2], 1.1 equiv NaOAc, tolu-
ene, 1108C, 24 h; c) 1.1 equiv [PdCl2(MeCN)2], 1.1 equiv NaOAc,
toluene, 808C, 3 h; d) 1.1 equiv K2PtCl4, 1.1 equiv NaOAc, toluene/
DMF, 1008C, 10 h; e) TFA/CH2Cl2, 208C, 10 min.
yield of 4Zn. However, 4Ni is stable in air and can be handled
without special care, and its structure has been confirmed by
X-ray diffraction analysis (see Figure S36). The porphyrin 4Ni
exhibits a Soret band at l = 427 nm, which is red-shifted by
18 nm from the parent b-unsubstituted NiII porphyrin because
of the influence of b,b-diphosphination (see Figure S33). The
following cyclometalation with soluble group 10 metal salts
proceeded smoothly in the presence of sodium acetate as
a base. The bimetallic pincer complexes 2M1M2 were isolated
as stable solids in good yields by recrystallization. Freebase
porphyrin pincer complexes (2H2M2) were obtained by
selective removal of the inner zinc of 2ZnM2 under acidic
conditions, which underscores the robustness of the
PCP pincer structure.
Table 1: Allylation of benzaldehyde with allyltin catalyzed by 2M1M2.
The structures of 2NiM2 were unambiguously determined
by X-ray crystallographic analysis (Figure 1 for 2NiPd,
Figures S37 and S39 for 2NiNi and 2NiPt, respectively). The
porphyrinic plane and the PCP pincer unit are flat and,
especially for 2NiPd, constitute an almost perfect plane,
which is in sharp contrast to the previous NCN pincer
complexes having highly distorted structures.[3a] The porphyr-
2M1M2 Yield [%][a]
2M1M2 Yield [%][a]
2M1M2 Yield [%][a]
2NiNi
2NiPd
2NiPt
68
95
95
2ZnNi
2ZnPd
2ZnPt
90
97
88
2H2Ni
2H2Pd
2H2Pt
70
94
93
[a] Yields are those of isolated products.
À
inic C P bonds are directed inward to the outer metal
because of strong coordination, with P1-C1-C2 and P2-C5-C4
or 2H2Ni did not go to completion because of decomposition
of the catalysts during the reaction. With these two excep-
tions, the other 2M1M2 catalysts were robust enough to
complete the allylation and exhibited very similar catalytic
activities regardless of the inner and outer metals.
À
angles of 114.24(14)8 for both in 2NiPd. The length of the C3
Pd bond is 2.014(3) ꢀ, which is apparently longer than that of
our previous NCN pincer palladium complex[3a] [1.977(7) ꢀ]
and is similar to that of the closely related anthracene-based
PCP pincer palladium complex[12] [2.010(5) ꢀ].
According to Szabꢁꢂs report,[14b] the allylation should
begin with the formation of the cationic pincer complex by
means of AgPF6 (see Scheme S1). The catalytic cycle of the
allylation consists of a) transmetalation between allyltribu-
tyltin and the cationic pincer complex to form the corre-
sponding h1-allyl M2 complex and Bu3SnPF6, b) allylation of
the aldehyde with the h1-allyl M2 complex through a six-
membered cyclic transition state to form the corresponding
group 10 metal homoallyloxide, and c) alkoxide exchange
with Bu3SnPF6 to form tributyltin homoallyloxide and to
regenerate the initial cationic pincer complex. The similar
behavior of 2M1M2 in the catalytic activities regardless of the
inner and outer metals implies that the inner and outer metals
Figure 1. ORTEP drawings of 2NiPd. Thermal ellipsoids represent 50%
probability. The meso aryl groups are omitted for clarity.
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Scheme 2. Plausible mechanism of 1,4-reduction according to the
report of Szabꢀ et al.
Figure 2. Effect of inner metals on 1,4-reduction catalyzed by 2M1Pd.
Each reaction was performed three times under the same reaction
conditions. The top and bottom of each error bar indicate the highest
and lowest yields, respectively. The line plots were made with the
average yields of the three runs.
As shown in Scheme 2, the reaction mechanism of the 1,4-
reduction would comprise a) formation of the palladium
butoxide 5 from 2M1Pd, b) generation of the palladium
hydride 6 with concomitant elimination of butanal, c) 1,4-
reduction to form the palladium enolate 7, and d) protonation
of 7 with 1-butanol to complete the catalytic cycle. It is not
easy to identify the rate-limiting step in our system because
the rate of each step depends on reaction conditions.
Alternatively, to discuss the electronic interplay between
the inner metal and the outer palladium, the most closely
related and reliable information may come from the thought-
ful mechanistic analysis, by Goldberg and co-workers, on the
behaviors of PCP pincer palladium alkoxide complexes.[17b]
According to Goldbergꢂs report, the generation of a palladium
hydride pincer complex from the corresponding palladium
alkoxide complex should be slow and proceed through an
alcohol-promoted dissociative b-hydride abstraction mecha-
nism[18] (Scheme 3). The dissociative mechanism involves the
have only little influence in the rate-limiting step[16] of the
three steps.
We have also found that the palladium complexes 2M1Pd
catalyze 1,4-reduction of chalcone in 1-butanol through
transfer hydrogenation.[17] Interestingly, the catalytic activities
depend on the inner metals. Notably, 2ZnPd showed the
highest catalytic activity among the three 2M1Pd complexes
(Figure 2) as well as all of the pincer complexes reported so
far.[17] Notably, the presence of elemental mercury had no
influence on the 1,4-reduction. In addition, the reduction did
not take place when either [PdCl2(PPh3)2], [Pd2(dba)3], or
Pd(OAc)2 was used instead of 2M1Pd. These experimental
results indicate that the pincer complexes, and not palladium
colloids, have catalytic activity.
To reveal the origin of the interesting electronic interplay
between the inner metal and the outer palladium in the 1,4-
reduction, we looked to the electronic nature of the palladium
pincer complexes for clues. We measured the oxidation
potentials of the complexes by cyclic voltammetry (see
Table 2 and Figure S41). It is clear that 2ZnPd is much
more readily oxidized than 2NiPd and 2H2Pd and that the
oxidation potentials of 2NiPd and of 2H2Pd are similar. This
trend (2ZnPd > 2NiPd ꢀ 2H2Pd) parallels the catalytic activ-
ities of the three outer palladium complexes (Figure 2). The
electron-rich nature of 2ZnPd or the ligand 4Zn would be
mainly responsible for its highest catalytic activity.
Scheme 3. Formation of palladium hydride through alcohol-promoted
dissociative b-hydride abstraction.
formation of the cationic pincer complex. We assume that the
electron-rich nature of 2ZnPd or 4Zn would facilitate the
solvation-assisted formation of the cationic complex through
stabilizing the cation, which leads to the highest activity of
2ZnPd.
In conclusion, we have synthesized a family of new
peripherally metalated porphyrins, that is, porphyrin-based
PCP pincer complexes bearing a meso carbon–metal s bond.
The complexes take rather unbiased and flat geometry. The
legitimate catalytic activities of 2M1M2 were investigated in
the allylation of benzaldehyde with allylstannane and in the
1,4-reduction of chalcone. In the latter catalytic reaction, the
apparent electronic interplay within 2M1Pd in the catalysis
Table 2: Oxidation potentials of 2M1Pd.[a]
2M1Pd
E1/2ox1 [V]
E1/2ox2 [V]
2ZnPd
2NiPd
2H2Pd
0.26
0.51
0.41
0.73
0.91
1.01
[a] Potentials vs ferrocene/ferrocenium cation. In CH2Cl2. Scan rate,
0.2 VsÀ1. Working electrode: Pt. Counter electrode: Pt wire. Supporting
electrolyte: 0.1m Bu4NPF6.
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Received: October 1, 2013
Revised: November 14, 2013
Published online: December 11, 2013
Keywords: ligand design · metalation · porphyrinoid ·
.
reaction mechanisms · transition metals
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