temperature unmodified catalytic hydroformylation of 3,3-dimethybut-1-ene
[85]. The dinuclear heterobimetallic cobalt–rhodium carbonyl complex CoRh(CO) 7
disappeared completely from solution in circa 5 min. The yield of rhodium intermediates, specifically the observable RCORh(CO) 4 , was basically 100%. The system
exhibited a very high rate of reaction after the 5 min induction period. The observable
RCOCo(CO) 4 was found to contribute no statistically significant amount of product
formation at the conditions used. The other monometallic rhodium complexes
exhibited half-lives to RCORh(CO) 4 on the order of 1 h to circa 8 h. Moreover, the
selectivity to RCORh(CO) 4 was in some cases significantly less than unity (particularly when using RhCl 3 ), and hence the yield of RCORh(CO) 4 was seriously diminished. As expected, these latter systems exhibited considerably lower rates than the
system generated from CoRh(CO) 7 . To make sure that no inadvertent error was made
with the mass balances, the turnover frequencies TOF based on instantaneous RCORh
(CO) 4 in each system were evaluated and were found to be the same.
System 2 The polynuclear precursor Ru 3 (CO) 12 can be used to carbonylate piperidine under CO with 100% selectivity. The system is particularly sensitive to the
sequence of additions and holding times of the reagents, or in other words, the start
up [86]. Thus, if Ru 3 (CO) 12 is added to piperidine under 1 bar CO, a very gradual
disappearance of Ru 3 (CO) 12 occurs, almost no detectable new mononuclear species
can be observed, and the rate of product formation is negligible. If, however,
Ru 3 (CO) 12 is added to piperidine under 50 bar CO, a very rapid disappearance of
Ru 3 (CO) 12 occurs, with nearly quantitative formation of Ru(CO) 5 . Under these
conditions the rate of product is negligible. However, then by lowering the system
pressure to say 5 bar, a very active system is created where the concentration of
mononuclear intermediates appears entirely stable in time. Thus System 2 illustrates
the stunning difference between circa 0% and 100% yield of intermediates using
exactly the same set of reagents.
2.8.2 Other Non-linear Systems and Another Possible Non-linear
Mechanism
As mentioned in Sect. 1, the field of cooperative or synergistic behaviour is very
broad and this chapter is certainly not the appropriate place to survey the area.
Having said that, mention was made of Jacobsen’s quadratic systems in Sect. 2.2.1
although there are some dissimilarities with monometallic CBER. Other groups of
reactions which have attracted this author’s attention are the Pt–Sn hydroformylation systems and the Ir–Ru Cativa process for acetic acid [87]. A common
theme in the Pt–Sn and Ir–Ru systems appears to be the need of the second metal
(Sn or Ru) in order to abstract a halogen from the first metal, thereby freeing a
coordination site. Catalytic bimetallic systems where a second metal is needed to
abstract a halogen should, at some level, exhibit a bilinear term to reflect the
abstraction.
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
217
[85]. The dinuclear heterobimetallic cobalt–rhodium carbonyl complex CoRh(CO) 7
disappeared completely from solution in circa 5 min. The yield of rhodium intermediates, specifically the observable RCORh(CO) 4 , was basically 100%. The system
exhibited a very high rate of reaction after the 5 min induction period. The observable
RCOCo(CO) 4 was found to contribute no statistically significant amount of product
formation at the conditions used. The other monometallic rhodium complexes
exhibited half-lives to RCORh(CO) 4 on the order of 1 h to circa 8 h. Moreover, the
selectivity to RCORh(CO) 4 was in some cases significantly less than unity (particularly when using RhCl 3 ), and hence the yield of RCORh(CO) 4 was seriously diminished. As expected, these latter systems exhibited considerably lower rates than the
system generated from CoRh(CO) 7 . To make sure that no inadvertent error was made
with the mass balances, the turnover frequencies TOF based on instantaneous RCORh
(CO) 4 in each system were evaluated and were found to be the same.
System 2 The polynuclear precursor Ru 3 (CO) 12 can be used to carbonylate piperidine under CO with 100% selectivity. The system is particularly sensitive to the
sequence of additions and holding times of the reagents, or in other words, the start
up [86]. Thus, if Ru 3 (CO) 12 is added to piperidine under 1 bar CO, a very gradual
disappearance of Ru 3 (CO) 12 occurs, almost no detectable new mononuclear species
can be observed, and the rate of product formation is negligible. If, however,
Ru 3 (CO) 12 is added to piperidine under 50 bar CO, a very rapid disappearance of
Ru 3 (CO) 12 occurs, with nearly quantitative formation of Ru(CO) 5 . Under these
conditions the rate of product is negligible. However, then by lowering the system
pressure to say 5 bar, a very active system is created where the concentration of
mononuclear intermediates appears entirely stable in time. Thus System 2 illustrates
the stunning difference between circa 0% and 100% yield of intermediates using
exactly the same set of reagents.
2.8.2 Other Non-linear Systems and Another Possible Non-linear
Mechanism
As mentioned in Sect. 1, the field of cooperative or synergistic behaviour is very
broad and this chapter is certainly not the appropriate place to survey the area.
Having said that, mention was made of Jacobsen’s quadratic systems in Sect. 2.2.1
although there are some dissimilarities with monometallic CBER. Other groups of
reactions which have attracted this author’s attention are the Pt–Sn hydroformylation systems and the Ir–Ru Cativa process for acetic acid [87]. A common
theme in the Pt–Sn and Ir–Ru systems appears to be the need of the second metal
(Sn or Ru) in order to abstract a halogen from the first metal, thereby freeing a
coordination site. Catalytic bimetallic systems where a second metal is needed to
abstract a halogen should, at some level, exhibit a bilinear term to reflect the
abstraction.
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
217