Homoleptic Uranium-Bis(acyl)phosphide Complexes.

The first uranium bis(acyl)phosphide (BAP) complexes were synthesized from the reaction between sodium bis(mesitoyl)phosphide (Na(mesBAP)) or sodium bis(2,4,6-triisopropylbenzoyl)phosphide (Na(trippBAP)) and UI3(1,4-dioxane)1.5. Thermally stable, homoleptic BAP complexes were characterized by single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy, when appropriate, for the elucidation of the electronic structure and bonding of these complexes. EPR spectroscopy revealed that the BAP ligands on the uranium center retain a significant amount of electron density. The EPR spectrum of the trivalent U(trippBAP)3 has a rhombic signal near g = 2 (g1 = 2.03; g2 = 2.01; and g3 = 1.98) that is consistent with the EPR-observed unpaired electron being located in a molecular orbital that appears ligand-derived. However, upon warming the complex to room temperature, no resonance was observed, indicating the presence of uranium character.

Experimental and computational studies of the mechanism of iron-catalysed C-H activation/functionalisation with allyl electrophiles.

Synthetic methods that utilise iron to facilitate C-H bond activation to yield new C-C and C-heteroatom bonds continue to attract significant interest. However, the development of these systems is still hampered by a limited molecular-level understanding of the key iron intermediates and reaction pathways that enable selective product formation. While recent studies have established the mechanism for iron-catalysed C-H arylation from aryl-nucleophiles, the underlying mechanistic pathway of iron-catalysed C-H activation/functionalisation systems which utilise electrophiles to establish C-C and C-heteroatom bonds has not been determined. The present study focuses on an iron-catalysed C-H allylation system, which utilises allyl chlorides as electrophiles to establish a C-allyl bond. Freeze-trapped inorganic spectroscopic methods (57Fe Mössbauer, EPR, and MCD) are combined with correlated reaction studies and kinetic analyses to reveal a unique and rapid reaction pathway by which the allyl electrophile reacts with a C-H activated iron intermediate. Supporting computational analysis defines this novel reaction coordinate as an inner-sphere radical process which features a partial iron-bisphosphine dissociation. Highlighting the role of the bisphosphine in this reaction pathway, a complementary study performed on the reaction of allyl electrophile with an analogous C-H activated intermediate bearing a more rigid bisphosphine ligand exhibits stifled yield and selectivity towards allylated product. An additional spectroscopic analysis of an iron-catalysed C-H amination system, which incorporates N-chloromorpholine as the C-N bond-forming electrophile, reveals a rapid reaction of electrophile with an analogous C-H activated iron intermediate consistent with the inner-sphere radical process defined for the C-H allylation system, demonstrating the prevalence of this novel reaction coordinate in this sub-class of iron-catalysed C-H functionalisation systems. Overall, these results provide a critical mechanistic foundation for the rational design and development of improved systems that are efficient, selective, and useful across a broad range of C-H functionalisations.