DOI: 10.1002/cctc.201501178
Communications
Co-Immobilization of a Palladium–Bisphosphine Complex
and Strong Organic Base on a Silica Surface for
Heterogeneous Synergistic Catalysis
Ken Motokura,[a] Koki Saitoh,[a] Hiroto Noda,[a] Yohei Uemura,[b] Wang-Jae Chun,[c]
Akimitsu Miyaji,[a] Sho Yamaguchi,[a] and Toshihide Baba*[a]
Co-immobilization of a palladium–bisphosphine complex and
a strong organic base, 1,4-diazabicyclo[2.2.2]octane (DABCO),
on a silica support was successfully achieved. The new catalyst
structure was characterized by X-ray photoelectron spectrosco-
py, solid-state NMR spectroscopy, X-ray absorption fine struc-
ture spectroscopy, and elemental analysis. Although the local
structure of the Pd–bisphosphine complex was unaffected by
the presence of DABCO on the same silica surface, its catalytic
activity in allylation reactions of nucleophiles was significantly
enhanced, achieving turnover numbers (TON) up to >10000,
owing to dual-activation of substrate molecules by the Pd
complex and DABCO.
idine functionalities in proximity to Pt nanoparticles to increase
the catalytic activity for sugar oxidation.[5] For metal com-
plexes, we have already reported palladium and rhodium com-
plexes with diamine ligands coexisting with tertiary amine
groups on silica surfaces.[6,7] These bifunctional surfaces signifi-
cantly accelerated the Tsuji–Trost allylations and the 1,4-addi-
tion of organoboronic acids through the synergistic effect of
metal complexes and tertiary amines. To expand the variation
of metal complexes and organic functional groups on the
same surface, we aimed to prepare and characterize a SiO2 sur-
face with two specifically controlled molecules: a palladium–
bisphosphine complex[8] and a strong organic base, 1,4-diaza-
bicyclo[2.2.2]octane (DABCO).[9] Oxidation of phosphine ligands
on the silica surface[10] was effectively avoided by pre-complex-
ation of the Pd species before immobilization. Spectroscopic
techniques, namely, X-ray photoelectron spectroscopy (XPS), X-
ray absorption fine structure spectroscopy (XAFS), and solid-
state NMR spectroscopy enabled atomic-level characterization
of the immobilized Pd complexes and DABCO functionality.
The prepared catalysts were applied in the Tsuji–Trost allyla-
tions of a ketoester and nitroalkane.
Synergies between immobilization methods for well-defined
molecules, such as metal complexes and organocatalysts, on
solid surfaces and atomic-level characterization techniques for
these functionalized surfaces provide new opportunities for
the creation of highly efficient heterogeneous catalysts.[1] Het-
erogeneous synergistic catalysis by immobilized molecules is
a new concept in catalysis. Since Kubota and co-workers re-
ported acid–base catalysis by silica-supported amines,[2] various
types of heterogeneous synergistic catalysis by acidic inorganic
material-supported basic amines have been developed.[3,4]
The next targets for heterogeneous synergistic catalysis are
combinations of metal complexes or nanoparticles with organ-
ic functional molecules, which would enable a wide range of
catalytic transformations. Tada and co-workers have demon-
strated synergistic catalysis between metal nanoparticles and
surface-immobilized functionalities by immobilizing ethynylpyr-
Scheme 1 shows the synthetic procedure for preparation of
the SiO2-supported Pd–bisphosphine complex and DABCO
skeleton (SiO2/DABCO/PP-Pd). A silane-coupling reaction suc-
cessfully attached a chloropropyl group to the SiO2 surface
(SiO2/Cl), which was subsequently used to quaternarize
DABCO, affording SiO2-supported DABCO (SiO2/DABCO). Before
immobilizing the palladium complex on SiO2, a palladium–bis-
phosphine complex containing a silane-coupling moiety (PP-
Pd) was prepared from the appropriate bisphosphine ligand
and [PdCl(h3-allyl)]2.[11] The reaction between PP-Pd and SiO2/
DABCO gave the dual SiO2-supported DABCO and Pd–bisphos-
phine complex (SiO2/DABCO/PP-Pd). A SiO2-supported palladi-
um complex (SiO2/PP-Pd) was prepared by using a similar pro-
cedure.
[a] Dr. K. Motokura, K. Saitoh, H. Noda, Dr. A. Miyaji, Dr. S. Yamaguchi,
Prof. Dr. T. Baba
Department of Environmental Chemistry and Engineering
Tokyo Institute of Technology
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502 (Japan)
The elemental analysis results for the prepared samples are
summarized in Table S1 (in the Supporting Information). Chlor-
ine (0.8 mmolgÀ1) was observed in SiO2/Cl. After the reaction
of SiO2/Cl with DABCO, the ratio of Cl (0.6 mmolgÀ1) to N
(1.4 mmolgÀ1) was approximately 1:2, suggesting that quater-
narized DABCO had formed on the SiO2 surface (SiO2/DABCO).
SiO2/DABCO/PP-Pd contained 0.17 mmolgÀ1 of the Pd species.
In the case of SiO2/PP-Pd, the amounts of Pd, N, and Cl were
almost identical (ca. 0.3 mmolgÀ1). P/Pd ratio in SiO2/PP-Pd
was determined by XPS analysis: the atomic ratio was approxi-
[b] Dr. Y. Uemura
Division of Electronic Structure
Department of Materials Molecular Science
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
[c] Prof. Dr. W.-J. Chun
Graduate School of Arts and Sciences
International Christian University
Mitaka, Tokyo 181-8585 (Japan)
Supporting Information for this article is available on the WWW under
ChemCatChem 2016, 8, 331 – 335
331
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