Since the pioneering work of Kochi in the 1970s, iron has attracted great interest for crosscoupling catalysis due to its low cost and toxicity as well as its potential for novel reactivity compared to analogous reactions with precious metals like palladium. Today there are numerous iron-based cross-coupling methodologies available, including challenging alkyl-alkyl and enantioselective methods. Furthermore, cross-couplings with simple ferric salts and additives like NMP and TMEDA (N-methylpyrrolidone and tetramethylethylenediamine) continue to attract interest in pharmaceutical applications. Despite the tremendous advances in iron cross-coupling methodologies, in situ formed and reactive iron species and the underlying mechanisms of catalysis remain poorly understood in many cases, inhibiting mechanism-driven methodology development in this field. This lack of mechanism-driven development has been due, in part, to the challenges of applying traditional characterization methods such as nuclear magnetic resonance (NMR) spectroscopy to iron chemistry due to the multitude of paramagnetic species that can form in situ. The application of a broad array of inorganic spectroscopic methods (e.g., electron paramagnetic resonance, 57 Fe Mössbauer, and magnetic circular dichroism) removes this barrier and has revolutionized our ability to evaluate iron speciation. In conjunction with inorganic syntheses of unstable organoiron intermediates and combined inorganic spectroscopy/gas chromatography studies to evaluate in situ iron reactivity, this approach has dramatically evolved our understanding of in situ iron speciation, reactivity, and mechanisms in iron-catalyzed cross-coupling over the past 5 years. This Account focuses on the key advances made in obtaining mechanistic insight in iron-catalyzed carboncarbon cross-couplings using simple ferric salts, iron-bisphosphines, and iron-N-heterocyclic carbenes (NHCs). Our studies of ferric salt catalysis have resulted in the isolation of an unprecedented iron-methyl cluster, allowing us to identify a novel reaction pathway and solve a decades-old mystery in iron chemistry. NMP has also been identified as a key to accessing more stable intermediates in reactions containing nucleophiles with and without β-hydrogens. In ironbisphosphine chemistry, we have identified several series of transmetalated iron(II)-bisphosphine complexes containing mesityl, phenyl, and alkynyl nucleophile-derived ligands, where mesityl systems were found to be unreliable analogues to phenyls. Finally, in iron-NHC cross-coupling, unique chelation effects were observed in cases where nucleophile-derived ligands contained *

The synthesis and characterization of sterically unencumbered homoleptic organouranium aryl complexes containing U À C s-bonds has been of interest to the chemical community for over 70 years.R eported herein are the first structurally characterized,s terically unencumbered homoleptic uranium (IV) aryl-ate species of the form [U(Ar) 6 ] 2À (Ar = Ph, p-tolyl, p-Cl-Ph). Magnetic circular dichroism (MCD) spectroscopya nd computational studies provide insight into electronic structure and bonding interactions in the U À C sbond across this series of complexes.O verall, these studies solve adecades-long challenge in synthetic uranium chemistry, enabling new insight into electronic structure and bonding in organouranium complexes.

The first uranium bis­(acyl)­phosphide (BAP) complexes were synthesized from the reaction between sodium bis­(mesitoyl)­phosphide (Na­( mes BAP)) or sodium bis­(2,4,6-triisopropylbenzoyl)­phosphide (Na­( tripp BAP)) 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( tripp BAP) 3 has a rhombic signal near g = 2 (g 1 = 2.03; g 2 = 2.01; and g 3 = 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.

Magnetic circular dichroism (MCD) spectroscopy has been utilized to evaluate the electronic structure of the tris­(cyclopentadienyl) rare-earth complexes [K­(2.2.2-cryptand)]­[LnCp′3] (Ln = Y, La, Pr, Eu, Gd; Cp′ = C5H4SiMe3), which contain ions in the formal +2 oxidation state. These complexes were chosen to evaluate the 4f n 5d1 electron configuration assignments of the recently discovered La­(II), Pr­(II), and Gd­(II) ions versus the traditional 4f n+1 configuration of the long-known Eu­(II) ion. The 4d1 Y­(II) complex provided another benchmark in the MCD study. Transitions with f-orbital character were observed in the NIR MCD spectra of the 4f25d1 complex [PrCp′3]−. This study provides the first direct observation of f–f transitions in such Ln­(II) species. The broadening of these transition for Pr­(II) provides further confirmation of the 4f n 5d1 versus 4f n+1 electronic configurations previously proposed and supported by restricted active-space (RAS) calculations. For further insight into the electronic structure of these [LnCp′3]− complexes, experimental UV–vis MCD spectroscopy was coupled with spectral calculations, which allowed for the assignment of transitions. The sensitivity of UV–vis MCD to spin–orbit coupling (SOC) and the increased spectral resolution in comparison to electronic absorption spectroscopy enabled identification of low-energy nd to (n + 1)p transitions in this class of complexes. Combined, these studies provide further insight into the electronic transitions and overall electronic structure of low-valent lanthanide­(II) organometallic complexes.

Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M‒L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating BkIII with 4’-(4-nitrophenyl)-2,2’:6’,2”-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO3)3(H2O)·THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO3)3(H2O)·THF. Here, we show that enhanced Bk‒N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M‒L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the CeIII complex are reported.