Fixation of both O2 and CO2 from air by a crystalline palladium complex bearing N-heterocyclic carbene ligands

Article abstract of DOI:10.1021/ja051054h

Crystals of the two-coordinate palladium(0) complex 1 bearing the new N-heterocyclic carbene ligand, ITmt, directly and rapidly fixed both O₂ and CO₂ from air to produce the corresponding palladium(II) peroxocarbonate complex 2. The present reaction consists of dioxygenation of the palladium(0) complex 1 to the palladium(II) peroxo complex 3 and the subsequent CO₂ insertion to produce the peroxocarbonate complex 2. Reaction of the crystals of 1 with air was monitored by microscopic IR spectroscopy to confirm the sequence of the two-step solid-state reaction. The unique reactivity of solid 1 toward air was explained in terms of the structural features of the carbene ligand, ITmt. Copyright

Full text of DOI:10.1021/ja051054h

Published on Web 04/27/2005
Fixation of Both O2 and CO2 from Air by a Crystalline Palladium Complex
Bearing N-Heterocyclic Carbene Ligands
Makoto Yamashita, Kei Goto,* and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
113-0033 Tokyo, Japan
Received February 18, 2005; E-mail: goto@chem.s.u-tokyo.ac.jp; takayuki@chem.s.u-tokyo.ac.jp
Scheme 1
Fixation of CO2 is generally achieved in biological systems
through the dark reaction (Calvin-Benson cycle)1 of photosynthe-
2
sis. This process can fix a low concentration (0.035%) of CO2
from air at room temperature. Many elemental reactions of CO2
with transition metal complexes3,4 and catalytic reactions of CO2
with organic substrates5-9 as well as CO2 sensing by use of lithium-
ion conductor10,11 have been developed to date. Most of these
positions in the Pd(0) complex 1. All four m-terphenyl units are in
reactions, however, usually require a high pressure of CO2 or a
gear with one another, forming a cavity to accommodate the
CO2 atmosphere in a reaction vessel, in contrast to the case of
peroxocarbonate chelate ring. There have been some reports of the
biological CO2 fixation. Otherwise, a high temperature of the device
formation of group 10 peroxocarbonate complexes by the reaction
with O2 and CO2 gases,22,23 and one report about a reversible CO2
absorption reaction from air to a solution of transition metal alkoxide
complex.24 However, the present reaction is the first example of
the solid-state fixation of O2 and CO2 from air to a transition metal
complex.
is necessary. Here we report a rapid fixation reaction of both O2
and CO2 from air to the palladium(0) complex 1 bearing the novel
N-heterocyclic carbene (NHC) ligands, ITmt [1,3-bis(2,2′′,6,6′′-
tetramethyl-m-terphenyl-5′-yl)imidazol-2-ylidene]. This reaction
rapidly proceeds even in the solid state to produce the corresponding
palladium(II) peroxocarbonate complex 2.
Several control experiments for the reaction of the palladium(0)
complex 1 with O2 or CO2 revealed that the intermediate of the
reaction is the palladium(II) peroxo complex 3 (Scheme 2). Direct
treatment of a THF solution of 1 with dry ice under an argon
1
atmosphere caused no change in its H NMR spectrum. On the
other hand, the subsequent reaction of the resulting solution
containing 1 and CO2 with O2 gas produced the peroxocarbonate
complex 2 within seconds. In the reaction of crystalline 1 with O2
In the course of our study on the stabilization of highly reactive
species, we previously developed a novel steric protection group,
a Bpq group, with a denderimer-type framework.12-14 The ligand
ITmt was designed as its NHC analogue, which is expected to
facilitate the formation of low-coordinate species. As a whole
molecule, ITmt has a much higher degree of bulkiness than other
diaryl NHC ligands such as 1,3-bis(2,4,6-trimethylphenyl)imid-
azoline-2-ylidene (IMes).15 On the other hand, the steric congestion
in the vicinity of the carbene center is less severe in ITmt because
of the absence of any substituent in the ortho positions of nitrogen.
The ligand ITmt can be readily prepared by the usual procedures.16
The Pd(0)(ITmt)2 complex 1 was synthesized by the reaction of
gas, the peroxo complex 3 formed, with a color change from red
to brownish-yellow. A recent report about the formation of
bis(NHC)-ligated palladium peroxo complex (IMes)2PdO2 (7) by
the solid-state reaction of (IMes)2Pd (4) with air also supports the
formation of peroxo complex 3.18,25,26 The peroxo complex 3 was
independently synthesized by reaction of 1 with O2 in C6D6, and
1
was fully characterized by H NMR, 13C NMR, IR spectroscopy,
and X-ray crystallography (Figure 1b).
The solid-state reactions of other two-coordinate palladium(0)
complexes bearing bulky phosphines or carbenes with O2 and air
were carried out as control experiments, but none of these reac-
tions produced a palladium(II) peroxocarbonate complex. Solid
Pd(PCy3)2, Pd(PtBu3)2, and Pd(IPr)2 (IPr ) 1,3-bis(2,6-diisopropyl-
phenyl)imidazoline-2-ylidene)27 did not react with O2 in air at all,
although Pd(PCy3)2 was converted to the corresponding peroxo-
carbonate complex by treatment with O2 and CO2 gases in toluene.22
Whereas (IMes)2Pd (4) was reported to react with O2 in air to form
the corresponding peroxo complex (IMes)2PdO2 (6) in the solid
state,17 no carbonyl band was observed in the IR spectrum when
solid 4 was exposed to air for 12 h at room temperature, indicating
that 6 does not undergo further reaction with CO2 (Scheme 2).
To confirm the solid-state fixation of O2 and CO2 by 1,
microscopic IR spectra were recorded during the course of the
reaction of 1 with air.28 Exposure of 1 to air at room temperature
for 1 h led to a disappearance of its red color from the edge of the
crystals in the early stage of the reaction. Subsequently, new bands
17
Pd[P(o-tol)3]2 with ITmt (Scheme 1). The neutral complex 1
showed a similar crystal structure (Figure 1a) to those of other
reported Pd(NHC)2 complexes.18-21
Exposure of the crystalline palladium(0) complex 1 to air at room
temperature for 3 h caused a dramatic color change from deep red
to pale yellow, and the formation of the palladium(II) peroxocar-
bonate complex 2 was confirmed by NMR, IR spectroscopy, and
elemental analysis. Complete structural characterization of 2 as a
peroxocarbonate complex22,23 was achieved by X-ray crystal-
lography (Figure 1c). In the solid state, 2 has a palladium atom on
the C2 axis and two disordered peroxocarbonate groups, forming a
palladacycle with an occupancy of 0.5 for each. The two NHC
ligands are placed at cis positions to each other, although these
ligands have considerable steric bulkiness and occupy the trans
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10.1021/ja051054h CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Solid-state structures of (a) palladium bis(NHC) complex 1, (b) peroxo complex 3, and (c) peroxocarbonate complex 2. Hydrogens, disordered
aromatic rings, and solvents are omitted for clarity. All carbene rings, phenyl rings on each nitrogen, and palladacycles are illustrated as colored planes.
Scheme 2
Supporting Information Available: All experimental procedures
and spectroscopic data of ITmt and 1-3; IR monitoring of the reaction
of 1 with air; space filling model of 3 and 6; CIF files for 1-3. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Acknowledgment. We thank Prof. Shigehiro Yamaguchi and
Dr. Atsushi Wakamiya at Nagoya University for the measurement
of X-ray crystallography of 3. This work was partly supported by
Grants-in-Aid for The 21st Century COE Program (T.K.) and for
Scientific Research (K.G. and T.K.) from the Ministry of Education,
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