“Anion-Switchable” Pincer-Mn(I) Catalyst for the Reductive N-Methylation of Amines with Formic Acid and CO2

We report a manganese(I) complex of formula Mn(NDI-CNC)(CO)3(BArF4), in which NDI-CNC refers to a pincer pyridine-bis-imidazolylidene ligand functionalized with a naphthalene-diimide (NDI) moiety. Due to the presence of the NDI fragment, the electron-donating strength of the pincer ligand can be increased by producing an electrochemical reduction of the NDI moiety or by the addition of tetrabutylammonium chloride (TBACl). The extent of the changes produced in the electron-donating power of the pincer ligand can be quantified by studying the variation of the C–O stretching frequencies by infrared spectroscopy. It is observed that the catalytic activity of the manganese complex in the reductive methylation of a series of secondary amines with formic acid (or CO2) in the presence of PhSiH3 is almost negligible, but the catalyst can be turned very active in the presence of TBACl. This study constitutes a rare example of an anion-sensitive catalyst. Furthermore, the activity of the catalyst can be switched on and off for several cycles by subsequent addition of TBACl or NOBF4, respectively.