Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 46

mechanism may be applied to the supported Ir(CO) 2 system. Another issue
that remains to be elucidated is whether a support zeolite plays any
important role in the ligand exchange. 81 For metal complexes catalysts in
solution, reactivity of ligands is often influenced by polarity and nature of
solvents.
Lu et al. also reported a correlation between the electron density on sup-
ported diethene iridium complexes and the rate of ethene hydrogenation.
The rate of ethene hydrogenation increases as the Ir atom becomes more
electron-deficient. They reported that H-D exchange rate became signifi-
cantly higher when switching the support from the electron-donating MgO
to the electron-withdrawing HSSZ-53.
Liang et al. investigated details of ligand exchange process of zeolite-
supported Rh(Z 2 -C 2 H 4 ) 2 . They show that p-bonded C 2 H 4 ligands readily
undergo ligand exchange with C 2 H 4 molecules present in the gas phase over
the sample via the formation of an intermediate, ethyl ligands that is as-
sisted by reverse spillover. 25 Ethene ligands in the supported Rh(Z 2 -C 2 H 4 ) 2
are readily exchanged with CO ligands, forming supported Rh(CO) 2 , con-
sistent with calculated average bond energy of R-L (53 kJ mol 1 for L ¼ CO as
compared with 40 kJ mol 1 for L ¼ C 2 H 4 ). Upon exposure of the supported
Rh(CO) 2 to a C 2 H 4 flow, one of two CO ligands is exchanged by C 2 H 4 ,
forming supported Rh(CO)(Z 2 -C 2 H 4 ). This exchange occurs because the ex-
periment was conducted in flow conditions far from equilibrium. When
supported Rh(Z 2 -C 2 H 4 ) 2 complex is contacted with H 2 in helium, it forms
supported rhodium monohydride with C 2 H 4 . When the same supported
Rh(Z 2 -C 2 H 4 ) 2 complex is contacted with H 2 in a nitrogen flow at room
temperature, it forms supported rhodium dinitrogen Rh(N 2 ) 2 via a rhodium
nitrogen complex (Rh(C 2 H 5 )(N 2 ) or Rh(N 2 )). These results suggest new
catalysis for supported rhodium complexes to convert dinitrogen.

d n 9 r 4 n g | 7

.

2.5.2 Catalytic Cycle
Structural uniformity of zeolite-supported metal complexes offers the unique
opportunity to investigate catalytic cycles rigorously. Kletnieks et al.in-
vestigated the catalytic cycle of acetylene cyclotrimerization on zeolite-
supported rhodium metal complex. 26 The complete cycle is shown in
Figure 2.10. The initial structure of the supported rhodium complexes
(denoted as the precursor, PRE) determined by a combination of IR,
NMR and EXAFS spectroscopies aided by DFT calculations is site-isolated
Rh(Z 2 -C 2 H 4 ) 2 species bonded to the surface of the zeolite via two Rh-O
bonds (similar to that shown in Figure 2.7). PRE has highly uniform struc-
ture as described in the preceding sections. When acetylene is passed over
PRE, they readily replace the reactive ethene ligands and form a supported
bis-acetylene complex (SI 1 ). PRE enters into a catalytic cycle of acetylene
cyclotrimerization by being converted to the stable intermediate SI 1 . The
intermediate SI 1 was detected by solid state 13 C MAS NMR spectroscopy
(Figure 2.11) and assignments of chemical shifts were aided by DFT