Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 49

structural non-uniformity of supported species. As a tiny fraction of sup-
ported metals sometimes plays a dominant role in catalysis, it remains
challenging to understand catalytic chemistry of these catalysts. When
metals are atomically dispersed, supports act as ligands to control the
catalytic properties of metals. Consequently, non-uniformity of supports
leads to the formation of multiple supported species, leading to diverse
catalytic properties. Zeolites provide the opportunity to synthesize struc-
turally uniform supported species because they are crystalline materials
and have nearly uniform anchoring sites. Site-isolated cationic metal
complexes can be anchored on these sites via the surface organometallic
approach if zeolites have large apertures and low density of anchoring
sites. One of the advantages of using the surface organometallic approach
over other synthesis methods is that organometallic precursors can be
designed to incorporate ligands that mimic zeolite surface and those that
function as intermediates in a catalytic cycle. Among various organo-
metallic precursors, metal acac complexes are preferred because ligand
exchange of bidentate acac ligands with oxygen ligands of zeolites readily
occurs via protonation of acac ligands by zeolites. Acac ligands mimic the
surface of zeolites that also act as bidentate ligands so that the ligand
exchange causes minimal structural changes. A series of zeolite-supported
metal complexes have been synthesized using metal acac precursors such
as Rh(CO) 2 (acac), Rh(Z 2 -C 2 H 4 ) 2 (acac), Ir(Z 2 -C 2 H 4 ) 2 (acac), and Ru(Z 2 -
C 2 H 4 ) 2 (acac) 2 . Resultant supported metal complexes have a high degree of
uniformity as proven by IR spectroscopy using CO as a probe molecule
and/or variable-temperature 13 C MAS NMR spectroscopy. Uniformity of
supported metal complexes in these catalysts allows incisive character-
ization by multiple spectroscopic techniques such as EXAFS and IR
spectroscopies. Furthermore, these experimental results are corroborated
by rigorous structural modeling by DFT calculations. Structural uniformity
of supported species also allows detailed investigations on reactivity of
ligands, genesis of catalytically active species, and a whole catalytic cycle
by a combination of multiple characterization techniques with DFT cal-
culations at an unprecedented level. These investigations show that sup-
ported metal

d n 9 r 4 n g | 7

.

complexes in these catalysts function essentially as

molecules.
Remaining challenges in the synthesis of zeolite-supported molecular
metal complex catalysts are (i) how to anchor metal complexes at crystal-
lographically equivalent positions in a zeolite and (ii) how to anchor larger
metal complexes. Although zeolites are crystalline and have nearly uniform
anchoring sites, distribution of anchoring sites are often random and in
some cases zeolites have multiple anchoring sites that are not equivalent
crystallographically. If zeolites are synthesized with precise control of the
distribution of anchoring sites (acid sites), that will allow one to synthesize
zeolite-supported molecular metal complex catalysts more precisely, which
ultimately enables structural analysis of supported metal complexes by
single-crystal X-ray diffraction. Anchoring bulky metal complexes in zeolites