Exploring Differences in Lanthanide Excited State Reactivity Using a Simple Example: The Photophysics of La and Ce Thenoyltrifluoroacetone Complexes.

A series of four lanthanide thenoyltrifluoroacetone (TTA) complexes consisting of two f0 (La3+ and Ce4+) and two f1 (Ce3+) complexes was examined using steady-state and time-resolved spectroscopic techniques. The wide range of spectroscopic techniques presented herein have enabled us to discern the nature of the excited states (charge transfer, CT vs ligand localized, LL) as well as construct a Jablonski diagram for detailing the excited state reactivity within the series of molecules. The wavelength and excitation power dependence for these series of complexes are the first direct verification for the presence of simultaneous competing, noninteracting CT and LL excited states. Additionally, a computational framework is described that can be used to support spectroscopic assignments as a guide for future studies. Finally, the relationship between the obtained photophysics and possible photochemical separation mechanisms is described.

Mechanistic Studies of Iron-PyBOX-Catalyzed Olefin Amino-Oxygenation with Functionalized Hydroxylamines.

Iron-catalyzed amino-oxygenation of olefins often uses discrete ligands to increase reactivity and broaden substrate scope. This work is focused on examining ligand effects on reactivity and in situ iron speciation in a system which utilizes a bisoxazoline ligand. Freeze-trapped 57Fe Mössbauer and EPR spectroscopies as well as SC-XRD experiments were utilized to isolate and identify the species formed during the catalytic reaction of amino-oxygenation of olefins with functionalized hydroxylamines, as well as in the precatalytic mixture of iron salt and ligand. Experiments revealed significant influence of ligand and solvent on the speciation in the precatalytic mixture which led to the formation of different species which had significant influence on the reactivity. In situ experiments showed no evidence for the formation of an Fe(IV)-nitrene intermediate, and the isolation of a reactive intermediate was unsuccessful, suggesting that the use of the PyBOX ligand led to the formation of more reactive intermediates than observed in the previously studied system, preventing direct detection of intermediate species. However, isolation of the seven coordinate Fe(III) species with three carboxylate units of the hydroxylamine and spin-trap EPR experiments suggest formation of a species with unpaired electron density on the hydroxylamine nitrogen, which is in accordance with formation of a potential iron iminyl radical species, as recently proposed in literature. An observed increase in yield when substrates devoid of C-H bonds as well as isolation of a ring-closed dead-end species with substrates containing these bonds suggests the identity of the functionalized hydroxylamine can dictate the reactivity observed in these reactions.

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.

Creation of an unexpected plane of enhanced covalency in cerium(III) and berkelium(III) terpyridyl 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.

NHC Effects on Reduction Dynamics in Iron-Catalyzed Organic Transformations*.

The high abundance, low toxicity and rich redox chemistry of iron has resulted in a surge of iron-catalyzed organic transformations over the last two decades. Within this area, N-heterocyclic carbene (NHC) ligands have been widely utilized to achieve high yields across reactions including cross-coupling and C-H alkylation, amongst others. Central to the development of iron-NHC catalytic methods is the understanding of iron speciation and the propensity of these species to undergo reduction events, as low-valent iron species can be advantageous or undesirable from one system to the next. This study highlights the importance of the identity of the NHC on iron speciation upon reaction with EtMgBr, where reactions with SIMes and IMes NHCs were shown to undergo ß-hydride elimination more readily than those with SIPr and IPr NHCs. This insight is vital to developing new iron-NHC catalyzed transformations as understanding how to control this reduction by simply changing the NHC is central to improving the reactivity in iron-NHC catalysis.

[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction.

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

Near-infrared C-term MCD spectroscopy of octahedral uranium(V) complexes.

C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d-d and f-f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(v) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f-f transitions of [UCl6]- and [UF6]- are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]-. The experimental NIR MCD studies were further extended to [U(OC6F5)6]-, [U(CH2SiMe3)6]-, and [U(NC(tBu)(Ph))6]- to evaluate the effects of ligand-type on the f-f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f-f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(v) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(v) complexes and identify areas for continued theoretical development.

C-Term magnetic circular dichroism (MCD) spectroscopy in paramagnetic transition metal and f-element organometallic chemistry.

Magnetic circular dichroism (MCD) spectroscopy is a powerful experiment used to probe the electronic structure and bonding in paramagnetic metal-based complexes. While C-term MCD spectroscopy has been utilized in many areas of chemistry, it has been underutilized in studying paramagnetic organometallic transition metal and f-element complexes. From the analysis of isolated organometallic complexes to the study of in situ generated species, MCD can provide information regarding ligand interactions, oxidation and spin state, and geometry and coordination environment of paramagnetic species. The pratical aspects of this technique, such as air-free sample preparation and cryogenic experimental temperatures, allow for the study of highly unstable species, something that is often difficult with other spectroscopic techniques. This perspective highlights MCD studies of both transition metal and f-element organometallic complexes, including in situ generated reactive intermediates, to demonstrate the utility of this technique in probing electronic structure, bonding and mechanism in paramagnetic organometallic chemistry.

Ligand effects on electronic structure and bonding in U(III) coordination complexes: a combined MCD, EPR and computational study.

The trivalent oxidation state of uranium has been shown to undergo unique reactivity, from its ability to activate a variety of small molecules to its role in the catalytic reduction of ethene to ethane amongst others. Central to this unique reactivity and ability to rationally design ligands for isotope separation is the underlying uranium electronic structure. While electronic structure studies of U(iv), U(v), and U(vi) have been extensive, by comparison, analogous studies of more reduced oxidation states such as U(iii) remains underdeveloped. Herein we report a combined MCD and EPR spectroscopic approach along with density functional theory and multireference wavefunction calculations to elucidate the effects of ligand perturbation in three uranium(iii) Tp* complexes. Overall, the experimental and computational insight suggests that the change in ligand environment across this series of U(iii) complexes resulted in only minor perturbations in the uranium electronic structure. This combined approach was also used to redefine the electronic ground state of a U(iii) complex with a redox non-innocent Bipy- ligand. Overall, these studies demonstrate the efficacy of the combined experimental and theoretical approach towards evaluating electronic structure and bonding in U(iii) complexes and provide important insight into the challenges in altering ligand environments to modify bonding and reactivity in uranium coordination chemistry.

Homoleptic Aryl Complexes of Uranium (IV).

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

Development and Evolution of Mechanistic Understanding in Iron-Catalyzed Cross-Coupling.

Since the pioneering work of Kochi in the 1970s, iron has attracted great interest for cross-coupling 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, 57Fe 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 carbon-carbon 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 ß-hydrogens. In iron-bisphosphine 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 coordinating functional groups. As with the bisphosphine case, high-spin iron(II) complexes were shown to be reactive and selective in cross-coupling. Overall, these studies have demonstrated key aspects of iron cross-coupling and the utility of detailed speciation and mechanistic studies for the rational improvement and development of iron cross-coupling methods.