Catalyst properties and mechanism of reaction

The identity of active catalytic species for the TH of ketones with our iron carbonyl [6.5.6]-P-N-N-P complexes was still unclear. Did the imine or imines on the ligand get reduced in situ, allowing catalysis to occur through a bifunctional outer sphere mechanism, as seen with the analogous ruthenium systems This question drove us to further investigate the mechanism of transfer hydrogenation with our first generation [6.5.6]-P-N-N-P systems. 

Both complexes 39 and 40 are poor catalysts for the TH of acetophenone in basic isopropanol, suggesting that the protonated or deprotonated form is not within the catalytic cycle. When a catalytic mixture with 32 is evaluated by P NMR, the major species detected in solution is 40 along with free ligand and oxidized free ligand. Mass balance experiments, as evaluated by P NMR, showed that a maximum of 75% of the iron in solution is NMR inactive, suggesting that the active species is either NMR inactive or NMR active and found in extremely low concentrations. Furthermore, an IR spectmm of a cmde solution of 40 revealed two distinct carbonyl stretching frequencies at 1862 and 1870 cmwhich are indicative of electron-rich iron carbonyl complexes. 

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