Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 44

chemisorbed and physisorbed species at steady state. Because a short resi-
dence time yields a higher 1-butene yield (longer residence times allows
1-butene to isomerize to other isomers), their data suggest that ethene
spends nearly the same time in a flow reactor with respect to the number of
'chemisorbed species' and thus the data show that chemisorbed species are
the catalytically active species for this dimerization reaction. These data
demonstrate the important role of the zeolite as ligands; 4,84 coordination of
oxygen atoms of the zeolite to the ruthenium atoms is essential.
Lu et al. investigated roles of ligands using Ir(Z 2 -C 2 H 4 ) 2 (acac) supported
on various zeolites (HSSZ-53, Hb, and HY), g-Al 2 O 3 , and MgO. Supported
iridium species in these samples are isostructural (Ir(Z 2 -C 2 H 4 ) 2 ). Ethene
ligands in all catalysts undergo facile ligand exchange with CO by the
treatment of 10% CO at 300 K at 1 bar, forming supported gem-dicarbonyl
iridium complexes, Ir(CO) 2 . However, the supported iridium complexes
exhibit markedly different catalytic activities in ethene hydrogenation. When
the electron density of iridium atoms is characterized by IR spectroscopy (CO
as a probe molecule and XANES spectroscopy (comparison of the white line
intensities), the results show a systematic variation of the electron density;
electron density of iridium atoms becomes higher in the order of
MgO o Al 2 O 3 «HSSZ-53 o zeolite Hb o zeolite HY (Figure 2.9).
The subtle differences of the frequencies of n CO among the three zeolites
are resolved by the sharpness of n CO bands that is enabled by the uniformity
of supported iridium species. For example, slight

d n 9 r 4 n g | 7

red-shifts in n CO

4cm 1 ) was also observed for zeolite Hb-supported rhodium complexes
as compared with zeolite HY. 25,91 The IR data are complemented by XANES
data (Figure 2.8). In addition to the variation of the electron density of
iridium atoms in these catalysts, Lu et al. show a correlation between the
electron density and the easiness of ligand exchange of carbonyl ligands of
the supported Ir(CO) 2 with C 2 H 4 . Their data show that CO ligands co-
ordinated to electron-deficient iridium atoms are more easily exchanged
with ethene ligands. This result may be explained by weaker bonds for Ir-CO
(caused by less back-donation from the iridium to CO) as well as easier co-
ordination of ethene to a vacant orbital for electron-deficient iridium atoms
as in the case of supported rhodium complex (vide infra). The results high-
light the analogy between roles of supports in catalysis by anchored metal
complexes and those of ligands in organometallic solution catalysis. The
mechanism for the ligand exchange of CO with C 2 H 4 (e.g., associative versus
dissociative) has not been fully investigated yet and requires further in-
vestigations. The classical examples in organometallic complexes with 16e,
d 8 square symmetry (e.g., rhodium( I ) complexes) is known to proceed via the
associative mechanism with an 18e intermediate having a trigonal bi-
pyramid with the incoming ligand (C 2 H 4 in the example shown above) in the
equatorial plane. 94 In this mechanism, good p-acid ligands are known to
facilitate leaving of a ligand in the trans position. Furthermore, because
organometallic 16e complexes including rhodium( I ) often undergo ligand
exchange via the associative mechanism (see Scheme 2.2), 94

( E

.

a similar