Page - 95 - in Advanced Chemical Kinetics

([RuCl2(p-cymene)]2/0.5 TMEDA) (155 vs. 161 h−1, respectively). The authors did not carry out activity studies using similar Ru loadings, and it is therefore not possible to make a direct comparison between the two systems. The trend of the catalyst loading effect in the 2007 paper does suggest, nevertheless, that this system would in fact be less active than the 2005 system when using similar Ru loadings. The fact that lowering the catalyst loading of [RuCl2(p-cymene)]2/TMEDA leads to a more active catalytic system might indicate that an associative/dissociative process is involved (vide infra) [10]. However, this was not discussed in the paper [7]. Moreover, the 2.0 ppm ([RuCl2(p-cymene)]2/five TMEDA) system was allowed to run for 11 days, at which point it was still active and it had reached a turnover number (TON) of 17,215, corresponding to an overall TOF of 64 h−1 [7]. No mechanism was proposed. In 2011, Beller reported on a bis-isopropyl phosphorous substituted phosphorous-nitrogen- phosphorous (PNPiPr, see Figure 3) pincer ruthenium catalyst [RuH2(PNPiPr)CO] [8, 9] that, when formed in situ from mixing 1:1 [RuH2(PPh3)3CO] and PNPiPr ligand, dehydrogenates isopropanol via a proposed outer-sphere β-hydride elimination contrary to the until then sug- gested inner-sphere approaches [9]. The PNP ligand holds an amine unit which deprotonates under somewhat mild conditions leading to an amide-ruthenium bond. Hence, the ligand plays a cooperative role during the catalytic cycle. This setup allowed for conducting the AAD under neutral conditions without using any additives. Moreover, this led to a drastic increase in TOF with an observed TOFmax of 14,145 h −1 when employing a 4.0 ppm loading of the catalyst in refluxing isopropanol. This corresponds to a more than 25-fold increase in catalyst turnover frequency compared to the previous state-of- the-art [7]. Interestingly, adding merely 1.3 equivalent of NaiOPr in fact led to an approxi- mately 10% decrease in TOF. Moreover, screening results suggested that the CO was vital for obtaining any AAD activity and that exchanging one of the hydrides with a chloride rendered the addition of 1.3 equivalent of NaiOPr necessary. The latter suggests that the role of the base in this case is to eliminate off the chloride, thus generating complex A in Figure 3. Figure 3. Proposed mechanism for the Beller isopropanol AAD system. Best results: TOF = 14,145 h−1. TON > 40,000. Stable for more than 12 h. Catalyst Kinetics and Stability in Homogeneous Alcohol Acceptorless Dehydrogenation http://dx.doi.org/10.5772/intechopen.70654 95