Hydrogen Isotope Exchange in Pyridine Catalyzed by an Iron(II) Imido Complex: Counterion‐Directed Regioselectivity

High‐spin ( S = 2) iron(II) imido complexes [Ph 2 B( t BuIm) 2 Fe═NDipp]M (M = Li + , K + , K(18‐c‐6) + ) are catalysts for the hydrogen isotope exchange (HIE) reaction with pyridine as the substrate. As dictated by the counter‐cation, these complexes catalyze site‐selective α ‐, α , β , γ ‐, and β , γ ‐deuteration of pyridine. Experimental and computational mechanistic investigations reveal the critical role of the counter‐cation in catalysis, which activates the substrate, facilitates deuteration, and dictates HIE regioselectivity. The stoichiometric reaction of pyridine with [Ph 2 B( t BuIm) 2 Fe═NDipp]Li affords the catalytically active bis(2‐pyridyl) complex [Ph 2 B( t BuIm) 2 Fe(2‐Py) 2 Li(THF) 2 ]. By maintaining coordination to the substrate during the catalytic cycle, Li + preorganizes pyridine for regioselective α ‐deuteration by this catalyst. On the other hand, [Ph 2 B( t BuIm) 2 Fe═NDipp]K reacts with pyridine to afford the 2‐pyridyl amido complex [Ph 2 B( t BuIm) 2 Fe(2‐Py)NHDipp] − , which has been structurally characterized with [K(18‐c‐6)(THF) 2 ] + and [K(dibenzo‐18‐c‐6)(THF) 2 ] + counterions. As dictated by the size of the counter‐cation, [Ph 2 B( t BuIm) 2 Fe═NDipp] − catalyzes regioselective α , β , γ ‐ and β , γ ‐deuteration of pyridine. Here, the counter‐cation stabilizes the appropriate pyridyl regioisomer for the selectivity‐determining deuteration step.