Investigations of a MnIV-oxo adduct supported by an N5 ligand with mixed pyridyl and benzimidazolyl ligation uncovers distinct reactivity trends for MnIV-oxo and FeIV-oxo adducts at parity of coordination sphere.

Treatment of [Rh(COD)(μ-Cl)] 2 with excess t BuOK and subsequent addition of 2 equiv of PIN•HBr in THF afforded [Rh(COD)(κC 2 -PIN)Br] (1) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of 1 confirms ligand coordination to "Rh(COD)Br" through the carbene carbon featuring an unbound naphthyridine. Compound 1 is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h −1 was achieved for the acrylonitrile. A similar Rh(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst−water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle.

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe-oxo adducts has been extensively investigated, studies of analogous Mn-oxo systems are less common. There are several recent reports of Mn-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C-H bonds at rates approaching those of analogous Fe-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn-oxo complexes, [Mn(O)(2pyN2Q)], which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C-H bonds of cyclohexane with rates exceeding those of Fe-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn(O)(2pyN2Q)] is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn(O)(2pyN2Q)] was faster than that predicted by a previously published Bells-Evans-Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C-H bond, albeit with a shallow slope. The implications of this result with regard to Mn-oxo and Fe-oxo reactivity are discussed.

Chemical modification of the hair dye para-phenylenediamine results in colors that avoid in situ oxidation and toxicity.