Received: October 10, 2014 | Accepted: October 25, 2014 | Web Released: November 1, 2014
CL-140935
Oxygen-atom-transfer Reactions of a Palladium(II) Peroxocarbonate Complex
Shohei Sase, Maki Hashimoto, and Kei Goto*
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
(
E-mail: goto@chem.titech.ac.jp)
Reactivity of a palladium(II) peroxocarbonate complex was
investigated. It was found that a bis(N-heterocyclic carbene)
palladium(II) peroxocarbonate acts as an oxygen-atom-transfer
we report the reactivity of the palladium(II) peroxocarbonate
as an oxygen-atom-transfer reagent toward a phosphine and a
sulfoxide.
(
OAT) reagent toward a phosphine and a sulfoxide. In these
In the literature, there has been no example of an oxygen-
atom-transfer reaction from a transition-metal peroxocarbonate
complex to a sulfur compound. During the course of our studies
OAT reactions, the peroxocarbonate was converted to the
corresponding palladium(II) carbonate. While the OAT reaction
toward triphenylphosphine proceeded only sluggishly, addition
of lithium chloride remarkably facilitated the reaction. It was
proposed that the chloride ion induces the liberation of the
peroxocarbonate moiety from the coordination sphere of
palladium.
7
on the properties of 2, it was found that 2 was quantitatively
II
transformed to the [Pd (CO )(NHC) ] complex 3 in DMSO-d
3
2
6
at room temperature. In a mixed solvent of benzene/dimethyl
sulfoxide (1:1 v/v), the transformation also proceeded to afford
3, which was isolated in 82% yield (Scheme 2). GC-MS
analysis of the reaction mixture confirmed the formation of
dimethyl sulfone (m/z = 94), proving the oxygen-atom transfer
from 2 to dimethyl sulfoxide. The amount of dimethyl sulfoxide
was able to be reduced to 100 equivalents without decreasing the
yield of 3 (Table 1, Entry 1). The reaction of 2 with diphenyl
sulfoxide (30 equiv) resulted in no reaction, while that with
dimethyl sulfoxide under the same conditions produced 3 in
39% yield (Entries 2 and 3). These results indicate that the
oxygen-atom transfer from 2 to a sulfoxide is sensitive to the
steric bulkiness of the substrate.
Transition-metal peroxocarbonate complexes represent an
interesting class of compounds in view of their ability as
oxygen-atom-transfer reagents as well as their potential for
1
application to oxidative CO2 fixation to organic molecules.
They can be synthesized by the reaction of low-valent transition-
metal complexes with O2 and CO2. To date, the syntheses of
peroxocarbonates containing various transition metals such as
2
3
4
5
iron(III), rhodium(III), palladium(II), and platinum(II) have
been reported. However, information on the reactivity of
transition-metal peroxocarbonates has been limited only to
rhodium(III) and platinum(II) complexes, which were found to
behave as oxygen-atom-transfer reagents toward organic mole-
The structure of the palladium(II) carbonate 3 was un-
ambiguously determined by X-ray crystallographic analysis
8
(Figure 1). The coordination geometry around the Pd atom is
distorted square planar with two carbene ligands in the cis
cules such as phosphines3
a3c,5a,5b
and olefins.
3d3f
configuration and one η -coordinated carbonate ligand. The
2
As for the palladium(II) counterpart, while there is a report
on the synthesis of a bis(phosphine) peroxocarbonate complex,
the reactivity has not been investigated due to its instability in
structural parameters for the palladium carbonate moiety are in
the range typical for palladium(II) carbonate complexes. The
CPdC bond angle (98.1(2)°) in 3 is slightly larger than that of
the related palladium(II) carbonate 4 (Chart 1) bearing two NHC
9
4
solution. It is desirable to elucidate the reactivity of a well-
defined palladium(II) peroxocarbonate complex. Recently, we
have reported that the palladium(0) complex 1 bearing N-
heterocyclic carbene (NHC) ligands (denoted as ITmt) bearing
m-terphenyl groups on the nitrogen atom captures both O and
2
CO2 from air in the solid state to produce the bis(N-heterocyclic
carbene) palladium(II) peroxocarbonate complex 2 (Scheme 1),
which represents the first example of the solid-state fixation of
6
O2 and CO2 from air to a transition metal complex. In contrast
Scheme 2. Oxygen-atom-transfer reaction from 2 to dimethyl
sulfoxide to form the palladium(II) carbonate complex 3.
to the aforementioned bis(phosphine) palladium(II) peroxo-
carbonate, the NHC-ligated complex 2 is stable enough to
manipulate under laboratory conditions. In this communication,
Table 1. Oxygen-atom transfer from 2 to sulfoxides
a
Entry
Sulfoxide
equiv
Time/h
Yield of 3 /%
1
2
3
R = Me
R = Me
R = Ph
1
100
30
30
13
39
39
quant.
39
N.R.
Scheme 1. Solid-state fixation of O2 and CO2 from air by
Pd(0) complex 1 to produce the palladium(II) peroxocarbonate
complex 2.
b
a
b
Estimated by H NMR. N.R.: No reaction.
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