Electronic Finetuning of a Bio‐inspired Iron(II) tetra‐NHC Complex by trans Axial Isocyanide Substitution

Abstract

The synthesis of trans axially substituted mono-(1 a) and bis(tert-butylisocyanide) (1 b) derivatives of the highly active homogeneous bio-inspired iron(II) olefin epoxidation (pre-)catalyst 1 bearing an equatorial macrocyclic tetra N-heterocyclic carbene and two trans axial labile acetonitrile ligands is reported. NMR spectroscopy and SC-XRD indicate a considerable π-backdonation from the iron(II) centres to the isocyanide ligand(s). The impact of isocyanide substitution on the electronic features of the complexes is studied by cyclic voltammetry revealing a significant increase in half-cell potential assignable to the reversible Fe(II)/Fe(III) redox couple with an increasing number of isocyanides as a result of their π-accepting properties: E 1/2 = 0.15 V (1), E 1/2 = 0.35 V (1 a), E 1/2 = 0.44 V (1 b).

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

“…First, accessible coordination sites must be present for the attack of the oxidant in order to form the active species [11b] . Accordingly to the examined reversibility of the redox process in cyclic voltammetry for compound 2 b , no isocyanide ligand dissociation after the oxidation step is expected, so that ligand exchange reactions at the beginning are hampered [21b] …”

“…Even the reduced π‐backbonding at the iron center because of two competing axial isocyanide ligands with π‐acceptor character leads only to weak dissociation of the ligands in acetonitrile, determined by 1 H‐NMR spectroscopy [21b] . Contrarily, the more active (pre‐)catalyst 2 a exhibits one accessible coordination site with a weakly coordinating acetonitrile ligand which can be easily exchanged.…”

“…Preceding cyclic voltammetry studies of both new compounds reveal the same redox potential trend, rising from mono‐ to bis( tert ‐butylisocyanide) ligand substitution. In comparison to 2 (E 1/2 =0.15 V) and 2 a (E 1/2 =0.35 V), 2 b displays the highest half‐cell potential (E 1/2 =0.44 V) corresponding to the reversible Fe(II)/Fe(III) redox process [21b] . 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 .…”

“…Structures of complex 1 as an example for the application of one or two isoyanide ligands at complex 2 leading to mono‐ and bis( tert ‐butylisocyanide) substituted derivatives 2 a and 2 b . Their isolobal carbonyl analogs 2 c and 2 d containing tetradentate macrocyclic NHC ligand cCCCC are also depicted (right) [10a,21b] …”

“…This concept of isocyanide ligand substitution was transferred to the notably more active Fe‐cCCCC complex 2 to create novel complexes with mixed catalytic properties released by axial π‐acceptor ligands [4,21b] . Accordingly, the mono‐ ( 2 a ) and bis( tert ‐butylisocyanide) derivatives ( 2 b ) were already synthesized and characterized by NMR‐spectroscopy, elemental analysis, ESI‐MS, and single crystal X‐ray diffraction (SC‐XRD) before [21b] . Preceding cyclic voltammetry studies of both new compounds reveal the same redox potential trend, rising from mono‐ to bis( tert ‐butylisocyanide) ligand substitution.…”