Anion control of tautomeric equilibria: Fe-H vs. N-H influenced by NH···F hydrogen bonding

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(P N P )(CO) (P N P = (Et PCH ) NMe) at Fe or N. Protonation of Fe(P N P )(CO) by [(Et O) H] [B(C F ) ] occurs at the metal to give the iron hydride [Fe(P N P )(CO) H] [B(C F ) ] . In contrast, treatment with HBF ·OEt gives protonation at the iron and at the pendant amine. Both the FeH and NH tautomers were characterized by single crystal X-ray diffraction. Addition of excess BF to the equilibrium mixture leads to the NH tautomer being exclusively observed, due to NH···F hydrogen bonding. A quantum chemical analysis of the bonding properties of these systems provided a quantification of hydrogen bonding of the NH to BF and to OTf . Treatment of Fe(P N P )(CO) with excess HOTf gives a dicationic complex where both the iron and nitrogen are protonated. Isomerization of the dicationic complex was studied by NOESY NMR spectroscopy.