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 carbene P pincer complexes has so far remained elusive. Here we report the synthesis of the first PC NHC P functionalized iron complex [(PC NHC P)FeCl 2 ] ( 1 ) and the reactivity of the corresponding trans -dihydride iron(II) dinitrogen complex [(PC NHC P)Fe(H) 2 N 2 )] ( 2 ). Complex 2 is stable under an atmosphere of N 2 and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N 2 ). With benzene- d 6 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(sp 2 )–H bond activation, which is further discussed in this study.

Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

“…SCS 2– can fold its core resulting in facial binding, which contrasts with the mer binding of the bis­(thioether) NHC analogue S Me CS Me . These examples illustrate the conformational flexibility of this scaffold, which contrasts with the more rigid scaffold in a set of iron complexes with an NHC-containing PCP pincer ligand recently reported by de Ruiter . Though these iron complexes were not precatalysts for N 2 reduction due to their decomposition in the presence of acid, we were able to generate complexes of thiolate, hydrazido, and amido groups that take advantage of the biomimetic scaffold.…”

“…These examples illustrate the conformational flexibility of this scaffold, which contrasts with the more rigid scaffold in a set of iron complexes with an NHC-containing PCP pincer ligand recently reported by de Ruiter. 73 Though these iron complexes were not precatalysts for N 2 reduction due to their decomposition in the presence of acid, we were able to generate complexes of thiolate, hydrazido, and amido groups that take advantage of the biomimetic scaffold. Most notable is the unique κ 1 -phenylhydrazido complex, which models a potential N 2 reduction intermediate and undergoes N−N cleavage.…”

Access to and Reactivity of Fe0, Fe−I, FeI, and FeII PCcarbeneP Pincer Complexes

“…Unfortunately, Milstein could not obtain structural or spectroscopic metrics for the iron‐carbene motif, preventing comparisons with data from previously reported Ni and Co systems, or with the Fe system in this report. It must also be noted that recently de Ruiter has reported a PC NHC P iron(II) pincer complex that was found to be an exceptional catalyst for H/D exchange reactions (Figure 1, IV ) [17] . However, PC NHC P ligands are electronically and structurally different from PC carbene P systems in that they have very little metal‐carbene double bond character, and rarely exhibit metal‐ligand cooperativity.…”

“…It must also be noted that recently de Ruiter has reported aP C NHC P iron(II) pincer complex that was found to be an exceptional catalyst for H/D exchange reactions (Figure 1, IV). [17] However,P C NHC Pl igands are electronically and structurally different from PC carbene Ps ystems in that they have very little metal-carbene double bond character,a nd rarely exhibit metal-ligand cooperativity.…”

Palladium(II) Complexes of a Neutral CCC-Tris(N-heterocyclic carbene) Pincer Ligand: Synthesis and Catalytic Applications

“…In particular, pincer architectures featuring terminal NHC donors are becoming a prominent design motif, combining the strong σ-donor characteristics of NHCs with the favorable thermal stability and reaction control possible with a mer-tridentate geometry . To date, a vast array of pincer ligands bearing NHCs have been prepared, including those based on neutral CNC, CNN, PCP, SCS, and CSC, anionic CNC, and dianionic OCO and SCS architectures.…”