Page - 102 - in Advanced Chemical Kinetics

In 2013, Grützmacher devised another catalytic system for methanol reforming by homoge- neously ruthenium catalysed AAD under neutral conditions [20]. Conducting the MeOH reform- ing using 500 ppm of [K(dme)2][Ru(H)(trop2dad)] (A in Figure 8) in refluxing THF containing a 1:1.3 mixture of MeOH/H2O (90°C applied temperature) afforded 90% conversion after 10 hours, corresponding to an overall TOF of 54 h−1 and TON of 540. Moreover, the yield was 84% yield. The proposed mechanism using [K(dme)2][Ru(H)(trop2dad)] is markedly different from that using [RuHCl(PNPiPr)CO] as catalyst. As shown in Figure 8, the ruthenium is redox active dur- ing the catalytic cycle. Hence, commencing with complex A ([Ru(H)(trop2dad)]−) containing a RuII center, a water assisted hydride protonation and subsequent dehydrogenation to species B with unspecified oxidation state is occurring. MeOH then adds to the Ru-N bond affording com- plex C, which has a RuII center. A β-hydride elimination then leads to the extrusion of formalde- hyde, which reacts with water to give methanediol. Furthermore, the metal center is reduced to Ru0 and the singly protonated 1,2-enediamide moiety in C becomes further protonated to yield the amino imine complex D. The methanediol is then dehydrogenated by D to yield formic acid and the imine moiety in D is reduced to afford diamine complex E, which upon de-coordination of the formic acid leads to species F. Hence, at this stage, complex B has taken up two equiva- lents of H2. Finally, a base-assisted dehydrogenation of the ligand framework and consequently oxidation of the ruthenium converts F back to A, thereby closing the catalytic cycle. Computational studies indicate that the conversion of methanediol to formic is faster than the conversion of MeOH to methanediol, explaining the absence of formaldehyde during the reaction. Moreover, it was demonstrated that complex A efficiently catalyses the dehydroge- nation of formic acid to H2 and CO2. Thus, employing a 100 ppm catalyst loading in a 1 M formic acid solution in dioxane at 90°C provided an initial TOF of 24,000 h−1. In 2014, Milstein also disclosed a catalytic system for MeOH reforming by AAD [21]. A cata- lyst loading of 250 ppm (with respect to MeOH) of the PNN ruthenium complex shown in Figure 9 in a 5.55:1 mixture of MeOH/H2O in toluene at 100–105°C (115°C applied tempera- ture) in the presence of two equivalents of KOH with respect to MeOH was employed. This led to a H2-based yield of 77% after 9 days. Interestingly, the organic layer of the reaction could be isolated and reused for another round of MeOH reforming. Doing so twice led to an overall TON of approximately 29,000 with a yield after the third round of 80%, again after 9 days. Hence, the system seems feasible for reusing, which is an important factor for devel- oping applicable MeOH reforming systems. The decomposition of formic acid to H2 and CO2 was also studied. When employing 900 ppm of the catalyst, 1.2 equivalents of KOtBu with respect to catalyst, and pure formic acid in a 1:1 (v/v) THF/H2O mixture a mere 25% conversion was observed after 24 h. This was improved to >99% upon exchanging the KOtBu with two equivalents of KOH. Interestingly, when no H2O was present, a reaction containing two equivalents of Et3N as base leads to 98% conversion after 24 h at room temperature. It was thus concluded that formic acid decomposition is markedly more facile than its formation from methanol. Hence, the first two steps of the MeOH reforming were suggested to be rate-determining. Moreover, mechanistic studies suggest that one of the ligand methylene hydrogens takes part of the catalytic cycle via dearomatization of the central pyridine unit, such as Milstein has previously demonstrated with other AAD systems [22]. Advanced Chemical Kinetics102