Page - 94 - in Advanced Chemical Kinetics

The steps B to C and D to A both involve trifluoroacetic acid as a crucial player. In the former, the acid is extruded, whereas in the latter, it is used as hydride protonation agent and for the re-coordination of trifluoroacetate. Hence, the two steps prefer a low and high acid concen- tration, respectively, explaining an optimal situation of the 12 equivalents to the Ru complex with respect to obtaining the highest TOF. Moreover, at high acid concentration, the alcohol coordination step (A to B) appeared to be the rate-determining step. Contrary, the Cole-Hamilton approach utilises an alkaline-based mechanism, as outlined in Figure 2. Commencing with complex A, that is, the result of N2 dissociation from RuH2(N2) (PPh3)3, isopropylate coordinates leading to anionic intermediate B. A β-hydride elimination and acetone dissociation then form the trihydride complex C, which is sufficiently basic to deproton- ate isopropanol to yield the dihydride dihydrogen species D and regenerate isopropylate. Finally, loss of H2 closes the catalytic cycle. This step was found to be the rate-determining, which was cor- roborated by an incremental effect on the TOF by applying a 500 W tungsten halogen light source. In 2005 [6] and 2007 [7], Beller developed isopropanol AAD systems based on mixtures of a ligand and either a Ru(II) or Ru(III) precursor in refluxing isopropanol containing 0.8 M NaOiPr. Thus, using two equivalents of 2-di-tbutyl-phosphinyl-1-phenyl-1H-pyrrole to 315 ppm of RuCl3 resulted in a TOF of 155 h −1 after 2 h (TOF2h) [6], and five equivalents of tetra- methylethylenediamine (TMEDA) to 2.0 ppm of [RuCl2(p-cymene)]2 gave a TOF2h of 519 h−1 [7]. The authors note a clear catalyst activity dependence on the loading of [RuCl2(p-cymene)]2 [7]. Hence, with 8 and 40 ppm of [RuCl2(p-cymene)]2 with 0.5 equivalents TMEDA, TOF2h’s of 309 and 161 h−1, respectively, are observed. Therefore, similar activities are obtained when employing 315 ppm (RuCl3/two 2-di-tbutyl-phosphinyl-1-phenyl-1H-pyrrole) and 40 ppm Figure 2. Proposed mechanism for the Cole-Hamilton isopropanol AAD system. Best result: TOF = 330 h−1. Advanced Chemical Kinetics94