Tuning the electronic properties of tetradentate iron-NHC complexes: Towards stable and selective epoxidation catalysts

The Effect oftransAxial Isocyanide Ligands on Iron(II) Tetra‐NHC Complexes and their Reactivity in Olefin Epoxidation

“…Moreover, the better catalyst performance of 3 compared to 2 a and 2 b [25] is a good indication of the exact location for the selective modification at the classical iron NHC complex with two axial acetonitrile ligands. Direct modifications on the base frame or the iron center obviously evoke widely divergent features with regard to the catalytic activity.…”

“…Conceivable would be, that these features may orient the catalyst selectivity to more challenging substrates as for example in the C−H oxidation. Analogous to derivative 3 (Figure 1), bearing a characteristic tetradentate macrocyclic benzimidazolylidene ligand, catalysts 2 a and 2 b show high stability combined with a high temperature tolerance (up to 60 °C), but an overall lower activity in contrast to the most active non‐heme iron epoxidation catalyst to date ( 2 ) due to the electronic‐finetuning at the iron center or the modified scaffold in case of 3 [25] . Interestingly, the initial oxidation step of catalyst 3 from Fe(II) to Fe(III) is slowed down by the withdrawing properties of the modified scaffold, coinciding with the significantly higher half‐cell potential (E 1/2 =0.44 V) [25] .…”

“…Analogous to derivative 3 (Figure 1), bearing a characteristic tetradentate macrocyclic benzimidazolylidene ligand, catalysts 2 a and 2 b show high stability combined with a high temperature tolerance (up to 60 °C), but an overall lower activity in contrast to the most active non‐heme iron epoxidation catalyst to date ( 2 ) due to the electronic‐finetuning at the iron center or the modified scaffold in case of 3 [25] . Interestingly, the initial oxidation step of catalyst 3 from Fe(II) to Fe(III) is slowed down by the withdrawing properties of the modified scaffold, coinciding with the significantly higher half‐cell potential (E 1/2 =0.44 V) [25] . This fact can be transferred to the more active mono‐( tert ‐butylisocyanide) substituted compound 2 a having a nearly related half‐cell potential (E 1/2 =0.35 V), resulting in a long‐lasting induction period even in the presence of Sc(OTf) 3 [25] …”

“…Another derivative 3 with a tetradentate macrocyclic NHC ligand cCCCC (Figure 2) showing similar electrochemical properties (E 1/2 =0.44 V) is also examined and serves as an interpretation basis for the obtained catalytic results of 2 a and 2 b . Compared to 2 a and 2 b , complex 3 is bearing a backbone‐modified imidazole structure without any modification at the axial positions [25] …”

“… Comparison of the mono‐ and bis( tert ‐butylisocyanide) substituted derivatives 2 a and 2 b with an already existing and in epoxidation catalysis tested system 3 [25] …”