J. Am. Chem. Soc. 1997, 119, 12673-12674
12673
provides a good starting point for electrocatalytic studies using
{Co(salen)}NaY, where CoII(salen) is synthetically encapsulated
within the 13-Å supercages of zeolite Y through the “ship-in-
the-bottle” method.14-16 In this method, once the adsorbed,
flexible ligand (salen) coordinates to the metal center (Co2+ is
pre-exchanged for extraframework Na+), the resultant complex
is trapped by its own rigid structure and cannot pass readily
through the 7.4-Å pore openings of the host.17 {M(L)}Z is
typically purified by multiple extractions to remove as much
excess ligand and partially encapsulated complex as possible.
{Co(salen)}NaY was previously characterized with results
consistent with complex formation within the supercages of the
zeolite.14-16
The electrochemical properties of {Co(salen)}NaY and related
zeolite-encapsulated complexes have been much debated.15,16,19-23
The most recent research19 indicates that electron transfer for
{M(L)}Z proceeds only for electroactive species at the external
surface (boundary) of the zeolite. A boundary-associated
process implies that M(L) is (1) adsorbed at or occluded in
zeolite defect sites (such as truncated or partially broken zeolite
supercages) or possibly (2) electroactive only in the outermost
layer of the supercages.19,24 Boundary siting is particularly
consistent with voltammetric data which indicate that only ca.
0.5% of {Co(salen)}NaY is redox active.16,19
To avoid unencapsulating the zeolite-associated Co(salen)
through the mechanical work often necessary to prepare a
zeolite-modified electrode,19,23,25-27 the electrocatalytic activity
of {Co(salen)}NaY was studied as a microheterogeneous
dispersion undergoing controlled potential electrolysis at a large
surface area reticulated vitreous carbon (RVC) electrode. We
previously showed that the physical collision of zeolite particles
into the RVC transfers electrons to boundary-associated Co-
(salen) and that no significant shifts in the Co(III/II) or Co(II/I)
couples are observed for {Co(salen)}NaY as compared to
homogeneous Co(salen).19
Electrocatalytic Reactivity of Zeolite-Encapsulated
Co(salen) with Benzyl Chloride
Carol A. Bessel† and Debra R. Rolison*
Surface Chemistry Branch
NaVal Research Laboratory
Code 6170, Washington, D.C. 20375
Department of Chemistry
VillanoVa UniVersity
VillanoVa, PennsylVania 19085
ReceiVed May 30, 1997
Zeolites are microporous aluminosilicate materials capable
of restricting the size and shape of the molecules that enter,
reside within, or exit the crystalline lattice.1 The syntheses of
zeolite-encapsulated transition metal complexes (designated as
{M(L)}Z) have evoked interest in regard to the actual coordina-
tion of a supercage-confined transition metal complex, the
effects of pore and supercage steric limitations on its reactivity,
and the effect of site isolation on its stability.2-9 We are
interested in these effects due to our recent work on electrified
microheterogeneous catalysis (EMC), a method in which
voltages (>10 Vdc) are passed through aqueous suspensions of
zeolite particles to decompose environmental toxins (e.g.,
polychlorinated biphenyls) or selectively produce industrially
relevant oxidation products (propylene oxide from propene) in
the absence of added oxidants.10 This study was undertaken to
explore the physicochemical effects of the zeolite on the
electrocatalytic activity of {Co(salen)}NaY (salen ) N,N′-bis-
(salicylidene)ethylenediamine) as compared to the same complex
in homogeneous solution.
The reaction of benzyl chloride with CO2, catalyzed by
electrogenerated [CoI(salen)]-, produces a mixture of pheny-
lacetic acid, 1,2-diphenylethane, and toluene in homogeneous
solution.11 The oxidative addition-reductive elimination mech-
anism of this reaction has been thoroughly tested11-13 and thus
A comparison of reaction yields and product distributions
between homogeneous [CoI(salen)]- and {CoI(salen)}NaY for
the carboxylation of benzyl chloride under zero-order conditions
(Table 1) demonstrates the importance of the zeolitic environ-
* To whom correspondence should be addressed at the Naval Research
† Villanova University.
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S0002-7863(97)01779-4 CCC: $14.00 © 1997 American Chemical Society