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 PCcarbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PCNHCP functionalized iron complex [(PCNHCP)FeCl2] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PCNHCP)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.
Highlights
Driven by their pre-eminent two-electron chemistry, the predictable reactivity and selectivity of precious metals have made them the premier choice as catalysts in many synthetic processes.[1]
Realizing the lack of current synthetic methodologies for preparing earth-abundant metal PCNHCP pincer complexes, we became interested in a ligand platform known as dipyrido[1,2-c;2′,1′e]imidazolin-6-ylidenes,[17] whose rigid framework might allow for strong binding of earth-abundant metals
We commenced our studies by synthesizing azolium salt A1 via a modification of a known literature procedure.17a Subsequent deprotonation of A1 with potassium tert-butoxide (KOtBu) in THF resulted in the formation of free carbene A2 (Scheme 1)
Summary
Driven by their pre-eminent two-electron chemistry, the predictable reactivity and selectivity of precious metals have made them the premier choice as catalysts in many synthetic processes.[1]. The presented mechanism is similar to that proposed by Leitner and Milstein, whose ruthenium PNP complex catalyzes H/D exchange between aromatic hydrocarbons and benzene-d6.24a Besides the mechanism outlined, a modified version is energetically accessible (Figure S76; path B) In this mechanism, instead from the trans-dideuteride, σ-CAM is initiated from the iron(II) cis-dideuteride with a very similar transition state that is only 5.2 kcal mol−1 higher in energy. Π-donation of the coplanar NMe2 substituent effectively destabilizes this negative charge, resulting in an overall higher activation energy for C−H bond activation (Table 2) Overall, these computational studies show that for the majority of substrates the observed regioselectivity can be explained by a combination of steric and electronic factors (Table 2) and that a σ-CAM based mechanism is able to explain the observed HIE (Figure 3)
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