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

Abstract

The performance of trans axially substituted mono‐(2 a) and bis(tert‐butylisocyanide) (2 b) complexes derived from the highly active bio‐inspired iron(II) (pre‐ )catalyst 2 containing an equatorial macrocyclic tetra N‐heterocyclic carbene in homogenous olefin epoxidation catalysis is reported. H2O2 is used as oxidant in combination with the Lewis acid Sc(OTf)3 as additive resulting in a considerable improvement of catalytic activity. In contrast to other iron epoxidation catalysts, the introduction of π‐accepting isocyanide ligands does not improve the catalytic performance of 2 a and 2 b posed by cyclic voltammetry. However, besides their lower activity, a high temperature tolerance of both compounds is found as a unique feature for iron NHC epoxidation catalysts.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

“…Substitution of the axial MeCN ligands of 1 with one ( 25 ) or two t BuNC molecules ( 26 , Scheme 5) lowers the electron density at the iron center through their π-accepting character. Catalytic tests at room temperature show significantly lower TOFs ( 25 : 1400 h −1 ; 26 : 240 h −1 ), 265 which can be explained by lower electron density at iron and by stronger coordinating axial ligands, corroborated by thermogravimetric analysis. 59,265 Higher temperatures (60 °C) might facilitate the dissociation of the t BuNC ligands, as higher TOFs can be achieved ( 25 : 25 900 h −1 ; 26 : 4300 h −1 ), 265 underlining the importance of accessible axial coordination sites for catalysis.…”

“…Catalytic tests at room temperature show significantly lower TOFs ( 25 : 1400 h −1 ; 26 : 240 h −1 ), 265 which can be explained by lower electron density at iron and by stronger coordinating axial ligands, corroborated by thermogravimetric analysis. 59,265 Higher temperatures (60 °C) might facilitate the dissociation of the t BuNC ligands, as higher TOFs can be achieved ( 25 : 25 900 h −1 ; 26 : 4300 h −1 ), 265 underlining the importance of accessible axial coordination sites for catalysis. Iron( ii ) 13 and iron( iii ) 14 complexes contain a tetracarbene ligand with benzimidazole units, which is providing less σ-donation compared to 1 and 2 and thus leading to a less electron rich iron atom (see Section 3).…”

Organometallic 3d transition metal NHC complexes in oxidation catalysis

“…95 In this context, the catalytic performance of iron(II) complexes 11c and 11d substituted by trans axially mono-and bis-CN t Bu ligands has been investigated (Scheme 6). 96 The catalytic activity was expected to be between that of Fe(II) NCCN complex 10a and Fe(II) macrocyclic NHC complex 11a with labile MeCN ligands. 97,98 However, both complexes exhibit comparatively low epoxidation activities due to the π-accepting character of t -BuCN.…”

“…4000 h −1 , while TONs stay low, but largely temperature independent (330 for 11c and 150 for 11d ), proving high temperature tolerance. 96 Additional modifications of the NHC scaffold may be required to achieve higher activities with comparable stability.…”