Page - 104 - in Advanced Chemical Kinetics

4 M afforded TOF’s of 37 and 29 h−1, respectively. This was partially explained by the decrease in reaction temperature as a result of the lower base concentration (79°C with the 6 M and 76°C with the 4 M). It was also revealed that [Ru(H)2 (PiPr3)2(CO)2] catalyses the reaction with practically the same efficiency (TOF = 50 h−1), leaving some speculations as to whether this is the real catalyst. Moreover, mechanistic investigations suggest that the CO moiety of [Ru(salbinapht)(CO) (PiPr3)] plays an active role during the catalytic cycle by reacting with hydroxide and thus forming formic acid and H2. This might open for the possibility that a similar mechanism might (partially) take place with the Beller and Milstein systems. Other metal complexes have also been shown to conduct MeOH reforming by AAD, spe- cifically iron [16, 24], manganese [25], and iridium [26–28]. Several of them are comprised of PNPiPr pincer ligand complexes, such as the iron-based compounds shown in Figure 11. Beller initially showed that borohydride coordinated species afforded a TOF2h of 644 h−1 when using a 4.5 ppm catalyst loading in a 9:1 (v/v) MeOH/H2O mixture containing 8.0 M KOH and stir- ring at 91°C [24]. A long-term experiment allowed for a TON46h of 9834. When comparing this result using the iron-based catalyst with the TOF values obtained when using the ruthenium-based congener, the latter is superior with respect to catalyst activity and longevity. Nevertheless, showcasing the feasibility of conducting MeOH reforming using a non-noble metal catalyst is an important step towards applicability, which these findings therefore represent. Bernskoetter, Hazari, and Holthausen improved the iron-based MeOH reforming by exchang- ing the borohydride with a formate, dissolving minute amounts of MeOH and H2O in EtOAc, Figure 10. Reek catalyst for MeOH reforming. Best result: TOF = 55 h−1. Figure 9. Milstein PNN ruthenium catalyst for MeOH reforming. Best results: TOF = 45 h−1. TON = 29,000. Yield = 82%. Reusable system. Advanced Chemical Kinetics104