Zeolite Framework Stabilized Copper Complex
A R T I C L E S
Figure 1. Research strategy to create an artificial Cu(II)-2-His-1-carboxylate facial triad: (a) structure of the facial triad (X, Y, Z ) weakly coordinated
ligands, such as water molecules); (b) newly synthesized NNO-ligand (MIm2Pr); (c) coordination around the metal in the homogeneous 1:2 complex; and
(d) a schematic drawing of the complex inside the zeolite Y pore (X ) zeolite framework oxygen).
To structurally mimic the 2-His-1-carboxylate facial triad
more closely, we recently reported the synthesis of a new ligand
MIm2Pr), which provides an N,N,O donor set incorporating a
separation of the catalyst from the products or the selectivity
of a catalytic reaction. A zeolite supercage can also be seen as
a protective structure around the active center of the catalyst,
similar to the protein mantle surrounding the active site of
enzymes. Our group has studied in the past decade the
incorporation of copper histidine (His) type complexes inside
the supercages of zeolite Y in great detail with the aim to mimic
(
carboxylate group (Figure 1b). Using Cu(II) as a probe, we
studied the coordination chemistry of this new ligand and found
that the complexation of this ligand with half an equivalent of
a Cu(II) salt resulted in a centrosymmetrical 1:2 complex with
1
7
the ligand facially capping the copper atom (Figure 1c). The
formation of mononuclear complexes with one ligand coordi-
nated, however, proved troublesome. The reaction with 1 equiv
of copper(II) precursor yields structures of higher nuclearity;
for example, Cu(OTf)2 (OTf ) trifluoromethanesulfonate) forms
dinuclear complexes with the MIm2Pr ligand.18 However, to
mimic the coordination environment around the metal in 2-His-
19-23,33,34
the active site of the copper enzyme galactose oxidase.
Copper histidines are intriguing biomimetic complexes, which
have been widely studied in homogeneous and heterogeneous
systems. However, upon immobilization, these compounds face
a problem in isolating only one type of species inside the zeolite
framework. Instead, there are two species present in variable
amounts depending on the pH, Si/Al ratio of the zeolite support,
1-carboxylate-type enzymes, a mononuclear complex should be
19,20,23,33,34
and complex concentration in the impregnation solution.
formed with only one MIm2Pr ligand coordinated to the metal
and the other sites occupied by weakly coordinating ligands.
So far, this has been difficult to achieve in a homogeneous
system.
It would be convenient to have a ligand system in which the
likelihood of different coordination modes would be diminished.
The novel MIm2Pr ligand is not as pH sensitive as histidine as
there are not so many sites which could protonate/deprotonate.
MIm2Pr has two substituted imidazole complexes in one ligand
connected via a carbon bridge, which limits the possibilities in
which the ligand could coordinate to a metal ion. It is known
that MIm2Pr forms bis-ligand and dinuclear complexes in
solution, and therefore, the direct impregnation or ion exchange
of these complexes, as was done with copper histidines, cannot
be applied. Instead, the complexes were prepared by a “ship-
Therefore, we decided to investigate the possibility of
stabilizing the mononuclear complex inside a microporous
zeolitic support, such as zeolite Y (Figure 1d). Specifically, our
rationale for using this zeolite was to make use of the 13 Å
pores of zeolite Y to prevent the dimerization of the complex
and to stabilize it using the oxygens from the zeolite framework
to occupy the newly created free coordination sites around
copper, thus providing an anchoring site in the zeolite. In
addition, there also exists the consideration that the zeolite
support could influence the properties of the immobilized
complex. By changing the properties of the zeolite material,
also the properties of the immobilized complex can be influ-
enced.
3
1,36-39
in-a-bottle” synthesis method,
which allows for the
complex formation inside the zeolite framework, and the
(
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