Dinitrosyl Iron Complex [K‐18‐crown‐6‐ether][(NO)₂Fe(^MePyrCO₂)]: Intermediate for Capture and Reduction of Carbon Dioxide

Abstract

Continued efforts are made for the utilization of CO2 as a C1 feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro‐/photo‐)catalysts disclosed that initial step for CO2 activation involves either nucleophilic insertion or direct reduction of CO2. In this study, nucleophilic activation of CO2 by complex [(NO)2Fe(μ‐MePyr)2Fe(NO)2]2− (2, MePyr=3‐methylpyrazolate) results in the formation of CO2‐captured complex [(NO)2Fe(MePyrCO2)]− (2‐CO2, MePyrCO2=3‐methyl‐pyrazole‐1‐carboxylate). Single‐crystal structure, spectroscopic, reactivity, and computational study unravels 2‐CO2 as a unique intermediate for reductive transformation of CO2 promoted by Ca2+. Moreover, sequential reaction of 2 with CO2, Ca(OTf)2, and KC8 established a synthetic cycle, 2 → 2‐CO2 → [(NO)2Fe(μ‐MePyr)2Fe(NO)2] (1) → 2, for selective conversion of CO2 into oxalate. Presumably, characterization of the unprecedented intermediate 2‐CO2 may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO2.

Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers

“…Given these results with CS 2 activation and homocoupling to C 2 S 4 2− , we probed whether the reduction of CO 2 to oxalate proceeded by a similar ligand-based pathway. Indeed, a nucleophilic attack by supporting ligands was proposed in other systems as the first step toward CO 2 reduction to oxalate, 23,56 and CO 2 adducts at BDI are known (Figure 2). However, these adducts have not exhibited further reactivity beyond reversible binding; in those cases, a lack of available reducing equivalents likely precludes reductive coupling.…”

“…20−22 Recently, Liaw et al reported that CO 2 reduction to oxalate by an iron nitrosyl complex traverses a C−N bond between a ligand N atom and CO 2 (Figure 1d). 23 Examples of reversible bond formation involving only ligands have been reported but are not in the context of catalytic transformations. For instance, various groups have reported reversible C−C bonds between the para-C of aromatic ligands (e.g., pyridine bound to iron(I) βdiketiminate or a perfluorophenylimidocobalt diketimide; Figure S1).…”

“…Of the modes of cooperativity, secondary coordination sphere effects and redox non-innocence have been widely explored synthetically and have strong precedent in enzyme systems. − Classic examples of metal–ligand redox cooperativity rely on a redox-active ligand serving as a reservoir of electrons with the metal center serving as the site of the bond-making and breaking events. , The opposite scenario in which the ligand serves as the reaction site for bonding changes in the substrate with the metal ion(s) as the electron reservoir has a precedent in catalysis only for proton reduction. − In this extreme of ligand cooperativity, the redox state of metal species can switch its reactivity on or off by altering the electron density on the reactive ligand site. ,, Intermediate cases (i.e., both the metal and ligand cooperate in redox and substrate activation) are well represented in the literature. For instance, seminal work by Milstein and co-workers followed by reports from de Vries and co-workers highlighted the generality of a de- and rearomatization sequence to effect the functionalization of unsaturated functional groups, particularly nitriles, at tridentate pincer complexes (Figure a–c). − Recently, Liaw et al reported that CO 2 reduction to oxalate by an iron nitrosyl complex traverses a C–N bond between a ligand N atom and CO 2 (Figure d) …”

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes

“…Chemical catalysis by metal complexes often relies on the cooperation of the ligand, either to aid in substrate bond activation or provide redox equivalents necessary for the process ( chemical vs redox noninnocence ). In contrast to the widespread applications of redox-active ligands, reports of productive catalysis by chemically noninnocent ligands are mostly limited to the support of protons or hydrogen atoms. − In particular, examples in which non-H groups are involved are rare for any ligand. − …”