Iron(II) Active Species in Iron–Bisphosphine Catalyzed Kumada and Suzuki–Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides

While previous studies have identified FeMes₂(SciOPP) as the active catalyst species in iron–SciOPP catalyzed Kumada cross-coupling of mesitylmagnesium bromide and primary alkyl halides, the active catalyst species in cross-couplings with phenyl nucleophiles, where low valent iron species might be prevalent due to accessible reductive elimination pathways, remains undefined. In the present study, in situ Mössbauer and magnetic circular dichroism spectroscopic studies combined with inorganic syntheses and reaction studies are employed to evaluate the in situ formed iron species and identify the active catalytic species in iron–SciOPP catalyzed Suzuki–Miyaura and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl halides. While reductive elimination to form Fe­(η⁶-biphenyl)­(SciOPP) occurs upon reaction of FeCl₂(SciOPP) with phenyl nucleophiles, this iron(0) species is not found to be kinetically competent for catalysis. Importantly, mono- and bis-phenylated iron­(II)–SciOPP species that form prior to reductive elimination are identified, where both species are found to be reactive toward electrophile at catalytically relevant rates. The higher selectivity toward the formation of cross-coupled product observed for the monophenylated species combined with the undertransmetalated nature of the in situ iron species in both Kumada and Suzuki–Miyaura reactions indicates that Fe­(Ph)­X­(SciOPP) (X = Br, Cl) is the predominant reactive species in cross-coupling. Overall, these studies demonstrate that low-valent iron is not required for the generation of highly reactive species for effective aryl-alkyl cross-couplings

A Combined Mössbauer, Magnetic Circular Dichroism, and Density Functional Theory Approach for Iron Cross-Coupling Catalysis: Electronic Structure, In Situ Formation, and Reactivity of Iron-Mesityl-Bisphosphines

While iron-bisphosphines have emerged as effective catalysts for C–C cross-coupling, the nature of the in situ formed iron species, elucidation of the active catalysts and the mechanisms of catalysis have remained elusive. A combination of ⁵⁷Fe Mössbauer and magnetic circular dichroism (MCD) spectroscopies of well-defined and in situ formed mesityl-iron­(II)-SciOPP species combined with density functional theory (DFT) investigations provides the first direct insight into electronic structure, bonding and in situ speciation of mesityl-iron­(II)-bisphosphines in the Kumada cross-coupling of MesMgBr and primary alkyl halides using FeCl₂(SciOPP). Combined with freeze-trapped solution Mössbauer studies of reactions with primary alkyl halides, these studies demonstrate that distorted square-planar FeMes₂(SciOPP) is the active catalyst for cross-coupling and provide insight into the molecular-level mechanism of catalysis. These studies also define the effects of key reaction protocol details, including the role of the slow Grignard addition method and the addition of excess SciOPP ligand, in leading to high product yields and selectivities

New Insights into Structure and Luminescence of Eu^III and Sm^III Complexes of the 3,4,3-LI(1,2-HOPO) Ligand

We report the preparation and new insight into photophysical properties of luminescent hydroxypyridonate complexes [M^III<b>L</b>]⁻ (M = Eu or Sm) of the versatile 3,4,3-LI­(1,2-HOPO) ligand (<b>L</b>). We report the crystal structure of this ligand with Eu^III as well as insights into the coordination behavior and geometry in solution by using magnetic circular dichroism. In addition TD-DFT calculations were used to examine the excited states of the two different chromophores present in the 3,4,3-LI­(1,2-HOPO) ligand. We find that the Eu^III and Sm^III complexes of this ligand undergo a transformation after in situ preparation to yield complexes with higher quantum yield (QY) over time. It is proposed that the lower QY in the in situ complexes is not only due to water quenching but could also be due to a lower degree of f-orbital overlap (in a kinetic isomer) as indicated by magnetic circular dichroism measurements

Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation

Two distinct [Cu–O–Cu]²⁺ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]²⁺ active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu–O–Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu–O–Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites

Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe­(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis­(diphosphine)­iron­(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mössbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an <i>S</i> = ¹/₂ species consistent with the formation of a mono­(diphosphine)­iron­(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe­(I) structure, as this is predicted to be high spin