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

High-spin (S = 2) iron(II) imido complexes [Ph₂B(^tBuIm)₂Fe═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 [Ph₂B(^tBuIm)₂Fe═NDipp]Li affords the catalytically active bis(2-pyridyl) complex [Ph₂B(^tBuIm)₂Fe(2-Py)₂Li(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, [Ph₂B(^tBuIm)₂Fe═NDipp]K reacts with pyridine to afford the 2-pyridyl amido complex [Ph₂B(^tBuIm)₂Fe(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, [Ph₂B(^tBuIm)₂Fe═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.