Structural Evidence for Aromatic Heterocycle N–O Bond Activation via Oxidative Addition

Many methods report the scission of N–O bonds of aromatic heterocycles and their subsequent functionalization. Oxidative addition is one of the presumed pathways through which aromatic N–O bond activation with transition metals is achieved. We report the first well-defined pathway of (benz)isoxazole’s aromatic N–O bond activation through oxidative addition. We also provide control experiments, which show that aromatic N–O bonds may be broken by strong inorganic reductants. These results highlight that N–O bonds are susceptible to both reduction and oxidative addition, which has important implications for catalysis. Exploring the reactivity of one of these complexes toward a series of electrophiles leads to the discovery of a Staudinger-type β-lactam synthesis upon the reaction with a ketene. Finally, we demonstrate that the choice of different metal/ligand combinations allows for selective oxidative addition into either C–I bonds or N–O bonds in the presence of the other

Multistate-Mediated Rearrangements and FeCl₂ Elimination in Dinuclear FePd Complexes

Mass spectrometric, spectroscopic, and computational characterization of a novel bifunctional iron–palladium complex proves a change of coordination upon solvation. Collisional excitation reveals FeCl₂ and HCl elimination in a solvent-modulated competition. Hereby, <i>syn</i> and <i>anti</i> isomers, identified by theoretical calculations, favor and disfavor FeCl₂ elimination, respectively. The FeCl₂ elimination likely proceeds by chlorido and Cp ligand exchange among the metallic centers in a concerted, ballet-like manner. A multitude of stationary points were identified along the computed multistep reaction coordinates of the three conceivable spin states. The quintet state shows a static Jahn–Teller type relaxation by a tilt away of the Cp ligand at the iron center. The direct singlet–quintet spin crossover is an unprecedented assumption, leaving behind the triplet state as a spectator without involvement. The FeCl₂ elimination would decrease catalytic activity. It is kinetically hindered within a range of applicable temperatures in conceivable technical applications

The Connection between NHC Ligand Count and Photophysical Properties in Fe(II) Photosensitizers: An Experimental Study

Four homo- and heteroleptic complexes bearing both polypyridyl units and N-heterocyclic carbene (NHC) donor functions are studied as potential noble metal-free photosensitizers. The complexes [Fe^II(L1)­(terpy)]­[PF₆]₂, [Fe^II(L2)₂]­[PF₆]₂, [Fe^II(L1)­(L3)]­[PF₆]₂, and [Fe^II(L3)₂]­[PF₆]₂ (terpy = 2,2′:6′,2″ terpyridine, L1 = 2,6-bis­[3-(2,6-diisopropylphenyl)­imidazol-2-ylidene]­pyridine, L2 = 2,6-bis­[3-isopropylimidazol-2-ylidene]­pyridine, L3 = 1-(2,2′-bipyridyl)-3-methylimidazol-2-ylidene) contain tridentate ligands of the C^N^C and N^N^C type, respectively, resulting in a Fe-NHC number between two and four. Thorough ground state characterization by single crystal diffraction, electrochemistry, valence-to-core X-ray emission spectroscopy (VtC-XES), and high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) in combination with ab initio calculations show a correlation between the geometric and electronic structure of these new compounds and the number of the NHC donor functions. These results serve as a basis for the investigation of the excited states by ultrafast transient absorption spectroscopy, where the lifetime of the ³MLCT states is found to increase with the NHC donor count. The results demonstrate for the first time the close interplay between the number of NHC functionalities in Fe­(II) complexes and their photochemical properties, as revealed in a comparison of the activity as photosensitizers in photocatalytic proton reduction