(CO) 7 in the presence of substantial concentrations of CO to yield HCo(CO) 4 and
Rh 4 (CO) 12 [91]. Importantly, the hydrogen activation kinetics are zero order in CO
– no dissociation step is needed prior to H 2 oxidative addition [92]. The typically
observed half-life of the reaction in the range of 250–293 K is 1–2 min which
corresponds to the mixing time of the reagents used.
The above reaction suggested that it would be instructive to mix HRe(CO) 5 and
Rh 4 (CO) 12 in n-hexane solvent both in the presence and absence of dissolved
CO. In both cases, reaction occurred on the timescale of mixing, and the product
was all-terminal RhRe(CO) 9 [61–63]. By varying the hydrogen partial pressure, it
was possible to show that the reaction is reversible and very rapid, again on the
mixing timescale, pushing the organometallics back towards HRe(CO) 5 and
Rh 4 (CO) 12 . As of the present writing, direct hydrogen activation experiments on
pure RhRe(CO) 9 in n-hexane have not been performed, since no attempts to isolate
RhRe(CO) 9 have ever been made.
3.5.2 Hydride Pools and Minimal Cluster Concentrations
From an in situ spectroscopic and chemometric view point, the lack of measurable
quantities of Rh 2 (CO) 8 and Rh 6 (CO) 16 in heterobimetallic CBERs is one of the
most obvious differences with homometallic rhodium hydroformylations. This is
clearly related to the very aggressive nature of the hydrides HMn(CO) 5 , HRe(CO) 5 ,
HWCp(CO) 3 and HMoCp(CO) 3 towards higher nuclearity rhodium carbonyls. It
also suggests that in the heterobimetallic systems, there is slightly better utilization
of rhodium due to the deduced concentrations of these di- and multinuclear
reservoirs.
3.5.3 Hydrogen Bonding and Deactivation
There are many other details to explain in the CBER systems ½M ¼ M
f g,
M À M
f
g CBER+UNI and [ M ¼ M
f g, M
0
È É
, M À M
0
È
É
] CBER+UNI , but we will
end with just one of these. In the [M ¼ Mo
f g, Rh
f g, Mo À Rh
f
g ] CBER+UNI and
[M ¼ W
f g, Rh
f g, W À Rh
f
g] CBER+UNI systems, additional dinuclear organometallics without M–M
0 bonds were observed. These new complexes RCORh
(CO) 4 –HWCp(CO) 3 and RCORh(CO) 4 –HMoCp(CO) 3 existed due to hydrogen
bonding between the Cp moieties and the carbonyls on rhodium. This example
is noted just as a further justification for in situ studies and signal processing.
The chemistry of catalytic systems is usually so very complex and there are
often new exciting results if one is able to investigate in an in situ manner.
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
225
Rh 4 (CO) 12 [91]. Importantly, the hydrogen activation kinetics are zero order in CO
– no dissociation step is needed prior to H 2 oxidative addition [92]. The typically
observed half-life of the reaction in the range of 250–293 K is 1–2 min which
corresponds to the mixing time of the reagents used.
The above reaction suggested that it would be instructive to mix HRe(CO) 5 and
Rh 4 (CO) 12 in n-hexane solvent both in the presence and absence of dissolved
CO. In both cases, reaction occurred on the timescale of mixing, and the product
was all-terminal RhRe(CO) 9 [61–63]. By varying the hydrogen partial pressure, it
was possible to show that the reaction is reversible and very rapid, again on the
mixing timescale, pushing the organometallics back towards HRe(CO) 5 and
Rh 4 (CO) 12 . As of the present writing, direct hydrogen activation experiments on
pure RhRe(CO) 9 in n-hexane have not been performed, since no attempts to isolate
RhRe(CO) 9 have ever been made.
3.5.2 Hydride Pools and Minimal Cluster Concentrations
From an in situ spectroscopic and chemometric view point, the lack of measurable
quantities of Rh 2 (CO) 8 and Rh 6 (CO) 16 in heterobimetallic CBERs is one of the
most obvious differences with homometallic rhodium hydroformylations. This is
clearly related to the very aggressive nature of the hydrides HMn(CO) 5 , HRe(CO) 5 ,
HWCp(CO) 3 and HMoCp(CO) 3 towards higher nuclearity rhodium carbonyls. It
also suggests that in the heterobimetallic systems, there is slightly better utilization
of rhodium due to the deduced concentrations of these di- and multinuclear
reservoirs.
3.5.3 Hydrogen Bonding and Deactivation
There are many other details to explain in the CBER systems ½M ¼ M
f g,
M À M
f
g CBER+UNI and [ M ¼ M
f g, M
0
È É
, M À M
0
È
É
] CBER+UNI , but we will
end with just one of these. In the [M ¼ Mo
f g, Rh
f g, Mo À Rh
f
g ] CBER+UNI and
[M ¼ W
f g, Rh
f g, W À Rh
f
g] CBER+UNI systems, additional dinuclear organometallics without M–M
0 bonds were observed. These new complexes RCORh
(CO) 4 –HWCp(CO) 3 and RCORh(CO) 4 –HMoCp(CO) 3 existed due to hydrogen
bonding between the Cp moieties and the carbonyls on rhodium. This example
is noted just as a further justification for in situ studies and signal processing.
The chemistry of catalytic systems is usually so very complex and there are
often new exciting results if one is able to investigate in an in situ manner.
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
225