Redox Activity of Iron Diazine-Diimine Carbonyl and Dinitrogen Complexes: A Comparative Study of the Influence of the Heterocyclic Ring

Abstract

A detailed investigation of the electronic structure of diazinediimine iron complexes and their comparison with the pyridine analogues reveals subtle but important differences, imparted by the supporting heterocycle. In the case of LFe­(CO)2 complexes (L = pyrazine- and pyrimidine­diimine), the characterization of three available redox states confirmed that whereas the nature of the electron-transfer processes is similar, the differences in π-acidity of the supporting heterocycle significantly affect the redox potentials. The reduction of LFe­(CO)2 can yield either a ligand-centered radical (for L = pyrimidine) or a C–C-bonded dimer (for L = pyrazine), supported by a dearomatized core. In the latter case, the C–C bond can be reversibly cleaved oxidatively. Compared to the carbonyl analogues, employing weak-field N2 ligands triggers changes in electronic structure for the neutral and reduced LFe­(N2) complexes (L = pyrimidine­diimine). En route to the synthesis of the nitrogen complexes, the square-planar LFeCl (L = pyrimidine­diimine) was isolated. The monoradical character of the supporting chelate triggers the asymmetric distribution of electron density around the heterocycle.

Metal–Ligand Cooperativity in Iron Dinitrogen Complexes: Proton-Coupled Electron Transfer Disproportionation and an Anionic Fe(0)N2 Hydride

“…As a result of the increased electron density, the imine N1C15 bond is also significantly elongated while the adjacent C15–C1 bond is contracted, compared to those of parent compound 2 . The trend is consistent with the metric data previously collected for reduced pyridine- and diazinediimine-based iron dinitrogen complexes . Interestingly, depending on the chelating ligand employed for the potassium cation, two coordination modes of dinitrogen can be observed by single-crystal X-ray diffraction.…”

“…In the latter case, the overlap integral UCOs is close to unity, suggesting that the PNN ligand does not display redox non-innocence in this case. This feature differentiates it from the redox-active pyridine and diazinediimine ligands, where the additional imine functionality accepts one electron . Taking the EPR, crystallographic, and computational data into account, we suggest two resonance forms for 4 (Figure ).…”

Iron-Catalyzed Synthesis of Conformationally Restricted Bicyclic N-Heterocycles via [2+2]-Cycloaddition: Exploring Ring Expansion─Mechanistic Insights and Challenges

“…This increased electron density is directly linked to increased catalytic activity in the cycloaddition reaction (Figure B). The ground-state electronic structures of 2 and 6 , however, show strong similarities with the analogous pyridinediimine complexes. , Using the broken symmetry (BS) density-functional theory (DFT) approach, the ground state solution with the lowest energy is based on a BS­(2,2) description, corresponding to a spin pairing scheme (L ↑↑ –Fe ↓↓ ) consistent with a direduced pyrimidinediimine chelate, antiferromagnetically coupled with an intermediate spin ( S = 1) iron center. Using this solution as an input for the calculation of the Mössbauer spectrum yielded parameters which are in good agreement with the experimental data (δ calc = 0.35 mm·s –1 , Δ E Q(calc) = 2.31 mm·s –1 ).…”

“…Alternatively, the same products were accessible starting from the Boc-protected analogues (11 and 20) under standard acidic conditions. With the exception of amine 26, the other building blocks are either not known (28 and 29) or only accessible through multi-step reaction sequences (27). We expect that the very facile and high yielding access to these building blocks would enable their exploration as conformationally rigid bioisosteres of traditional motifs currently employed in drug design.…”

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis

“…Pyridine­(diimine) ligands are relatively straightforward to prepare, are sterically and electronically modular, − and offer the potential to support a host of electronically and sterically distinct structures. Despite the availability of ligand variants, catalytically active iron complexes, more specifically iron dinitrogen complexes, for unique cycloaddition reactions remain limited (Scheme ).…”