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.
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.
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.
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.
Spectroscopy and theory enable broader insight into electronic structure and bonding in U(
iii
) coordination complexes, focusing on systems with Tp* ligands.
Homoleptic Aryl Complexes of Uranium (IV) Wolford, Nikki J.; Sergentu, Dumitru‐Claudiu; Brennessel, William W. ...
Angewandte Chemie (International ed.),
July 22, 2019, Letnik:
58, Številka:
30
Journal Article
Recenzirano
Odprti dostop
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)62− (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.
Uranium unencumbered: The first structurally characterized and sterically unencumbered uranium (IV) homoleptic aryl‐ate species are reported. MCD spectroscopy and computational studies provide insight into electronic structure and bonding interactions across this series of organouranium complexes with exclusively U−C σ‐bonds.
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.
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 β‐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.
Upon reaction of Fe(OAc)2, NHC, and EtMgBr, it was found that the NHC utilized determined the rate of β‐hydride elimination. Fe(0)‐ethylene and Fe(II)‐ethyl species were isolated and characterized by X‐ray crystallography, 57Fe Mössbauer, and MCD spectroscopy. The Fe(0)‐ethylene complexes were found to have a rare S=1 ground state. The Fe(II)‐ethyl species were found to form cross‐coupled product under stoichiometric reaction conditions.
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.
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.
Homoleptic Aryl Complexes of Uranium (IV) Wolford, Nikki J.; Sergentu, Dumitru‐Claudiu; Brennessel, William W. ...
Angewandte Chemie,
July 22, 2019, Letnik:
131, Številka:
30
Journal Article
Recenzirano
Odprti dostop
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)62− (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.
Ungehindertes Uran: Die ersten strukturell charakterisierten und sterisch ungehinderten homoleptischen at‐Arylkomplexe von Uran(IV) werden vorgestellt. MCD‐Spektroskopie und Computerstudien geben Einblick in die Elektronenstruktur und Bindungswechselwirkungen dieser Komplexe, die ausschließlich U‐C‐σ‐Bindungen enthalten.
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