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

Abstract

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^EtN^MeP^Et)(CO)₃ (P^EtN^MeP^Et = (Et₂PCH₂)₂NMe) at Fe or N. Protonation of Fe(P^EtN^MeP^Et)(CO)₃ by [(Et₂O)₂H]⁺[B(C₆F₅)₄]^- occurs at the metal to give the iron hydride [Fe(P^EtN^MeP^Et)(CO)₃H]⁺[B(C₆F₅)₄]^-. In contrast, treatment with HBF₄·OEt₂ 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 BF₄ ^- 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 BF₄ ^- and to OTf^-. Treatment of Fe(P^EtN^MeP^Et)(CO)₃ 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.

Fig. 1. Solid state structure of Fe(P^EtN^MeP^Et)(CO)₃ (Fe⁰). Thermal ellipsoids shown at the 50% probability level. Hydrogen atoms are omitted.

Scheme 1

Fig. 2. Solid state structure of [Fe(P^EtN^MeP^Et)(CO)₃H]⁺[B(C₆F₅)₄]^–, [FeH]⁺. Thermal ellipsoids shown at the 50% probability level. All other hydrogen atoms except the Fe–H are omitted.

Fig. 3. IR spectra (ν̃_CO region) of Fe⁰ treated with varying amounts of HBF₄·OEt₂ in CH₂Cl₂. Blue = Fe⁰ in CH₂Cl₂ solution before addition of acid. Red = spectrum after 1 equiv. of HBF₄·OEt₂.

Fig. 4. Solid state structure of [FeNH]⁺BF₄^–. Thermal ellipsoids shown at the 50% probability level. Hydrogen atoms other than the NH are omitted.

Fig. 5. Solid state structure of [FeNH]⁺OTf^–. Thermal ellipsoids shown at the 50% probability level. All hydrogen atoms have been omitted except for the ammonium hydrogen.

Fig. 6. IR spectra of a solution (≈10 mM) of [FeH]⁺[B(C₆F₅)₄]^– in CH₂Cl₂ treated with increasing amounts of [Et₄N]⁺BF₄^–. Blue = [FeH]⁺[B(C₆F₅)₄]^–. Green = [FeH]⁺[B(C₆F₅)₄]^– with ∼1 equiv. of [Et₄N]⁺BF₄^–. Red = [FeH]⁺[B(C₆F₅)₄]^– with ∼5 equiv. of [Et₄N]⁺BF₄^–.

Fig. 7. The tautomer with a protonated amine ligand is favored with a OTf^– or BF₄^– counterion, as shown by DFT calculations. The computed structures of [FeNH]⁺OTf^– and [FeNH]⁺BF₄^– are shown in the lower part of the figure. Hydrogen atoms have been omitted, except for the protonated ammonium.

Scheme 2

Fig. 8. Solid state structure of [FeHNH]²⁺[OTf^–]₂. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted, except the N–H and Fe–H.

Scheme 3

Fig. 9. Cyclic voltammogram of Fe⁰ in PhF under Ar at various scan rates, showing a reversible oxidation at E_1/2 = –0.37 V vs. [Cp₂Fe]^0/+. Conditions: [Bu₄N][B(C₆F₅)₄] (100 mM) as the supporting electrolyte, glassy carbon as the working electrode, a silver wire as a pseudo reference, and a Pt wire as the counter electrode.

Fig. 10. Solid state structure of [Fe^I]⁺[BAr^F₄]^–. Thermal ellipsoids are shown at the 15% probability level. X-ray diffraction data were collected at 230 K because of fracture of the crystal at lower temperatures. Two molecules were present in the asymmetric unit; both are similar, and one molecule was chosen arbitrarily to be represented here.