Complexes with Nitrogen‐Centered Radical Ligands: Classification, Spectroscopic Features, Reactivity, and Catalytic Applications

Abstract

The electronic structure, spectroscopic features, and (catalytic) reactivity of complexes with nitrogen-centered radical ligands are described. Complexes with aminyl ([M(˙NR2)]), nitrene/imidyl ([M(˙NR)]), and nitridyl radical ligands ([M(˙N)]) are detectable and sometimes even isolable species, and despite their radical nature frequently reveal selective reactivity patterns towards a variety of organic substrates. A classification system for complexes with nitrogen-centered radical ligands based on their electronic structure leads to their description as one-electron-reduced Fischer-type systems, one-electron-oxidized Schrock-type systems, or systems with a (nearly) covalent M-N π bond. Experimental data relevant for the assignment of the radical locus (i.e. metal or ligand) are discussed, and the application of complexes with nitrogen-centered radical ligands in the (catalytic) syntheses of nitrogen-containing organic molecules such as aziridines and amines is demonstrated with recent examples. This Review should contribute to a better understanding of the (catalytic) reactivity of nitrogen-centered radical ligands and the role they play in tuning the reactivity of coordination compounds.

Bimetallic Oxidative Addition in Nickel-Catalyzed Alkyl–Aryl Kumada Coupling Reactions

“…PBE0-dDsC computations used the TZ2P slater-type orbital basis set in ADF. 42,43 Solvation corrections, also computed at the PBE0-dDsC/TZ2P//M06/def2-SVP level, were determined in THF solvent using Klamt's Continuum Solvation The alternative escape-rebound pathway commences with structure E1, which, based analysis of the computed spin density is a doublet Ni(III) halide (Br) complex and not a Ni(II) ligand cation complex 45,46 owing to majority of the spin density being located on the nickel center (see SI Figure S10 Based on these findings, the bimetallic oxidative addition pathway is strongly preferred over the escape-rebound pathway based on its superior energetic profile (notably the barrierless association of the alkyl radical and small TS associated with reductive elimination of the alkyl-aryl product). In principle, these two pathways are interconnected by a reaction in which B1 abstracts a bromine atom from .…”

Radical ligand-containing single-molecule magnets

“…17). Structurally related to the double-decker lanthanide phthalocyanine complexes are the neutral heteroleptic complexes Dy(Pc)(TClPP) (20) and Dy(Pc(˛-OC 5 H 11 ) 4 )(TClPP) (21), where Pc(␣-OC 5 H 11 ) 4 is a pentyloxy-substituted phthalocyaninate and TClPP is a chlorophenyl-substituted porphyrinate (Fig. 18) [233].…”

“…While not intended to provide a comprehensive review on other metal-ligand radical complexes that do not exhibit slow magnetic relaxation, this article will nevertheless briefly survey classes of molecules that could potentially be exploited to realize new, metal-radical ligand singlemolecule magnets. For a more comprehensive account of other metal-radical ligand complexes, the reader is referred to broader and more inclusive review articles [9][10][11][12][13][14][15][16][17][18][19][20][21]. For the latest advances on single-molecule magnets, the reader is referred to some recent review articles [22][23][24][25][26][27][28][29][30].…”

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

“…However, they are interesting synthetic targets for catalytic applications involving functionalization of hydrocarbons, olefin aziridination, nitrene transfer as well as dinitrogen fixation [45]. Furthermore, their electronic structure is often complex and usually requires a combination of EPR and DFT studies to gain insight information about their spin density distribution.…”

“…X-band EPR spectroscopy Characterized imidyl complexes are even scarcer than their aminyl counterparts, in part due to the lower steric protection. A number of imidyl complexes have been reported [45] and are employed in stoichiometric reactions. However, reports wherein imidyl complexes are unambiguously detected intermediates in catalytic reactions are extremely scarce.…”