Free cyclooctadiene is observed when the reaction is performed in CD3CN. When 10 is generated in CH3CN, isolated, and then redissolved in CD3CN, the two peaks for iridium- coordinated CH3CN appear in the 1H NMR spectrum at 2.53 and 2.68 ppm with an approximate ratio of 2:1. These peaks are not present when 10 is generated in CD3CN.
Furthermore, these peaks disappear over time in CD3CN, while the peak for uncoordinated CH3CN at 1.97 increases by approximately the same amount. Mass spectrometry confirmed the assignment of 10 as tris(acetonitrile){1,3-di(2-hydroxy-5- tert-butylphenyl)imidazolyl}iridium(III) hexafluorophosphate (scheme 4.9). The fact that cyclooctadiene can simply be displaced from 9 by acetonitrile indicates weak binding to iridium(III) by cyclooctadiene.
shown by the graph in figure 4.8a, diffusion of dihydrogen from the gas phase into solution was slow, and the concentration of dihydrogen in solution was lower than the concentration of olefin. Surprisingly, the disappearance of cyclohexene and the generation of cyclohexane still followed a first-order dependence on cyclohexene concentration (figure 4.8b). This seems to indicate a zero-order dependence on dihydrogen concentration, meaning that reaction of the catalyst with dihydrogen is much faster than reaction of the catalyst with cyclohexene; however, further studies need to be performed in order to confirm this preliminary result.
a)
b)
Figure 4.8. Catalysis of cyclohexene hydrogenation using catalyst precursor 9 in THF-d8
under 900 psi of dihydrogen. a) Plot of cyclohexene, cyclohexane, and dihydrogen concentrations vs. time. b) The first-order dependence of the reaction on cyclohexene
0 0.05 0.1 0.15 0.2
0 5000 10000 15000 20000
Concentration (M)
time (s)
cyclohexene dihydrogen cyclohexane
y = 1E-04x + 1.6 R² = 0.9976 0
0.5 1 1.5 2 2.5 3 3.5
0 5000 10000 15000 20000
-ln[A]
time (s)
"[A] = [cyclohexene]"
"[A] = [cyclohexene]0 - [cyclohexane]"
Linear("[A] = [cyclohexene]")
concentration as determined both from cyclohexene disappearance ([A] = [cyclohexene]) and from cyclohexane appearance ([A] = [cyclohexene]0 – [cyclohexane]). It should be noted that [cyclohexene]0 represents the initial concentration of cyclohexene.
While further studies need to be performed in order to determine the exact rate of catalysis, a minimum first-order rate constant for hydrogenation of cyclohexene can be approximated at 1 x 10-4 s-1 (figure 4.8b). This rate is fairly low in comparison to many other iridium-catalyzed olefin hydrogenations, but most other iridium-catalyzed hydrogenations involve either an iridium(I) catalyst precursor or a dihydridoiridium(III) catalyst precursor.42-45 While generation of an iridium(I) complex or a dihydridoiridium(III) complex from 9 is unlikely to occur at least initially, 9 may be converted to a monohydrido iridium(III) complex by displacement of acetonitrile with dihydrogen, followed by deprotonation with a coordinated phenoxide. Similar monohydrido iridium(III) complexes have been generated by coordination of dihydrogen and subsequent deprotonation; furthermore, some of these monohydrido iridium(III) complexes have proved to be competent precursors to hydrogenation catalysts.45-49
In an attempt to determine the fate of iridium in hydrogenations with 9, 9 was dissolved in THF-d8 and sealed in a J-Young NMR tube at room temperature under 1 atm of dihydrogen (the ratio of H2 to 9 in the tube was approximately 10:1). By 1H NMR spectroscopy, only trace amounts of cyclooctene were ever observed; no iridium-hydride signals were observed; and, only broad peaks were observed for the diphenolate carbene ligand. When the solvent and other volatiles were then removed under reduced pressure and the resulting solid was headed in CD3CN at 90 ˚C, 10 was generated. The generation
of 10 seems to indicate that the diphenolate imidazolyl-carbene ligand remains bound to iridium throughout the catalysis; however, it is still unclear what other ligands are bound to iridium during the catalysis. While iridium hydrides were not observed during hydrogenation, it is still possible that they are generated in small amounts or that they react rapidly. Furthermore, it would be difficult to observe an iridium hydride due the broadening which occurs for the peaks of the other ligands bound to iridium.
In order to further probe the coordination environment of the diphenolate carbene iridium(III) complexes, 9 was reacted, in separate experiments, with trimethylphosphine and tricyclohexylphosphine as shown in scheme 4.10. In each case, greater than three equivalents of phosphine were used. Based on 1H NMR spectroscopy, the reactions occurred slowly at room temperature, but they went to completion when heated for greater than 12 hours at 90 ˚C. During this time, cyclooctadiene was cleanly displaced from the metal center. In the reaction with trimethylphosphine, three equivalents of phosphine reacted to generate tris(trimethylphosphine){1,3-di(2-hydroxy-5-tert- butylphenyl)imidazolyl}iridium(III) hexafluorophosphate (11) as shown in scheme 4.10;
however when greater than three equivalents of tricyclohexylphosphine were reacted with 9, only two equivalents of phosphine bound to the metal center, resulting in the formation of (acetonitrile)bis(tricyclohexylphosphine){1,3-di(2-hydroxy-5-tert- butylphenyl)imidazolyl}iridium(III) hexafluorophosphate (12) (scheme 4.10).
With respect to 12, only one singlet is observed for the coordinated tricyclohexylphosphines in the 31P NMR spectrum, and only one set of peaks is observed in the 1H NMR spectrum for both phenolates. Taken together, this data indicates that the tricyclohexylphosphine ligands bind trans to each other, while the diphenolate carbene is
bound meridionally as shown in scheme 4.10. The 1H NMR spectrum of 11 also contains only one set of peaks for both phenolates, but the 31P NMR spectrum of 11 contains a doublet and a triplet with a 2:1 ratio of intensities. The identity of each complex was confirmed by high resolution mass spectrometry (FAB+), and an elemental analysis was obtained for 12.