Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Abstract

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PC_carbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PC_NHCP functionalized iron complex [(PC_NHCP)FeCl₂] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PC_NHCP)Fe(H)₂N₂)] (2). Complex 2 is stable under an atmosphere of N₂ and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N₂). With benzene-d₆ as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp²)-H bond activation, which is further discussed in this study.

Figure 1

Selected examples of first-row transition-metal PC_carbeneP pincer complexes and the herein reported reactivity of [(PC_NHCP)Fe(H)₂N₂)] (2).

Scheme 1. Synthesis of PC_NHCP Iron Complexes 1 and 2 and the Observed H/D Exchange in Benzene-d₆

Figure 2

Solid state structures of [(PC_NHCP)FeCl₂] (1, left) and [(PC_NHCP)Fe(H)₂N₂] (2, right). Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms (except H1 and H2) and cocrystallized solvent molecules are omitted for clarity.

Figure 3

Calculated free energy profiles (ΔH/ΔG) in kcal mol^–1 and plausible mechanistic pathways for hydrogen isotope exchange (HIE) of 3 with benzene. Hydrogen atoms (except H_b) are omitted for clarity. For computational details, see the Supporting Information.