Page - 96 - in Advanced Chemical Kinetics

When increasing the catalyst loading eight times from 4.0 to 32 ppm, a fourfold decrease in TOF2h was observed (8342–2048 h −1). This was not addressed further, but it might be specu- lated whether a detrimental association of two catalyst molecules is more likely at a higher concentration (vide infra for more discussions on this topic) [10]. A TON of more than 40,000 was achieved after merely 12 h, compared to the 17,215 after 11 days in the previous AAD report by Beller [7]. Notably, the system was noted to be still highly active after the 12 h. In this regard, it might be speculated whether the chelating pincer ligand renders the catalytic complex particularly robust and thus enables a prolonged lifetime of the system. Figure 3 shows the proposed mechanism. Because no additives or protonation/deprotonation steps are involved during the catalytic cycle, the entire cycle is composed of two elemental steps, β-hydride elimination of the isopropanol and dehydrogenation of the catalyst. This fact might contribute to the markedly enhanced catalytic activity. As mentioned, the two steps involve the outer-sphere β-hydride elimination of isopropanol by complex A leading to acetone and complex B followed by H2 formation and extrusion, thus regenerating A. According to the suggested mechanism, the ligand nitrogen atom plays a fun- damental role in both steps. Hence, during the first step, the amide functionality coordinates to the alcohol proton thereby enhancing the carbon-based hydride abstraction. Similarly, the amine serves as proton transferring source to the alkaline hydride leading to the H2 production. Overall, the kinetics and longevity of the catalytic systems for isopropanol AAD seem to be highly influenced by the catalytic mechanism and of the necessity of an additive involved in it. Moreover, it is clearly feasible to effectively both dehydrogenate isopropanol and subse- quently extrude H2 without any additive-mediated catalyst activation. Thus, devising a sys- tem that employs as simple a mechanism as possible and that are in the absence of catalytic sinks might be important facets to strive for designing new AAD catalysts in the future. It remains to be disclosed whether an inner- or outer-sphere β-hydride elimination is the key to reaching the superior catalyst activity demonstrated by Beller. Thus, more investigations on this topic would be interesting. 3. Primary alcohols 3.1. Ethanol In 1987, Cole-Hamilton demonstrated that EtOH can be dehydrogenated with a TOF of 96 h−1 by 1 × 10−3 M (61 ppm) [Rh(bipy)2]Cl in EtOH containing 5% v/v and 1.0 M NaOH at 120°C [11]. The same group improved on this in 1988 [5] and 1989 [12] with a TOF of 210 h−1 by use of 3.48 × 10−4 M (20 ppm) RuH2(N2)(PPh3)3, 1 M NaOH, and an intense light source at 150°C. Mechanistic considerations similar to those described for isopropanol (Figure 2) were discussed. In 2012, Beller demonstrated that the same catalytic system that showed superiority with respect to isopropanol AAD (Figure 3) also provide interesting results with ethanol [9] Hence, a TOF2h of 1483 h −1 could be achieved when using 3.1 ppm 1:1 [RuH2(PPh3)3CO] and PNPiPr ligand in refluxing neutral ethanol. Both acetaldehyde and ethyl acetate were observed as Advanced Chemical Kinetics96