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.
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.
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.
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.
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) Wolford, Nikki J.; Sergentu, Dumitru‐Claudiu; Brennessel, William W. ...
Angewandte Chemie (International ed.),
06/2019, 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.
Abstract The catalytic relevance of Fe(IV) species in non‐heme iron catalysis has motivated synthetic advances in well‐defined five‐ and six‐coordinate Fe(IV) complexes for a better understanding of ...their fundamental electronic structures and reactivities. Herein, we report the syntheses of FeDipp 2 and FeMes 2 , a pair of unusual four‐coordinate non‐heme formally Fe(IV) complexes with S =1 ground states supported by strongly donating bisamide ligands. By combining spectroscopic characterization and computational modeling, we found that small variations in ligand aryl substituents resulted in substantial changes in both structures and bonding. This work highlights the strong donor capabilities and modularity of the bisamide ligand set. More broadly, it is a critical contribution to the utilization of ligand design to modulate molecular geometries and electronic structures of low‐coordinate, high‐valent iron 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.
A fundamental understanding of electronic structure and bonding in uranium and iron coordination complexes is essential, in the former for the study of separations chemistry and fundamental bonding ...theory, and in the latter for understanding reactivity, as iron has become a favorable catalyst in organic transformations. This work applies the use of the physical inorganic spectroscopic techniques of magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), and 57Fe Mössbauer in combination with computational insight to probe electronic structure and bonding in both uranium and iron coordination complexes.In uranium chemistry, low temperature synthetic techniques allowed for the synthesis and characterization of a series of uranium (IV) complexes of the form U(Ar)62− (Ar = Ph, tolyl, p-Cl-Ph). In this study it was found that the donor ability of the aryl ligand had little to no effect on the uranium center and instead it was the coordination environment deviations that resulted in changes in the experimental MCD spectra. Similarly, a series of U (III) complexes with systematic changes to the ligand environment (Tp*UI2, Tp*2UI, Tp*2UBn) were studied. Much like what was observed in the U (IV) aryl study, the changes in the ligand environment had little effect on the spectroscopic features from both MCD and EPR spectroscopies, an observation that was supported by density functional theory (DFT) calculations. This combined experimental and computational method was also applied to a species with the redox non-innocent Bipy− ligand (Tp*2UBipy) and it was determined that at cryogenic temperatures (5 K), antiferromagnetic coupling was observed between the ligand radical and the uranium (III) center. Future work is focused on expanding the oxidation states and ligand environments of uranium utilized in these electronic structure studies.A similar combined spectroscopic and computational approach was utilized to study the reduction dynamics in pairs of iron (II) and iron (0) complexes with N-heterocyclic carbene (NHC) ligands. Under identical reaction conditions it was observed that the identity of the NHC ligand resulted in either an Fe (II) species of the form NHC(Fe)(ethyl)2 or an Fe (0) species of the form NHC(Fe)(ethylene)2. This observation has important implications in the field of iron catalysis where NHC ligands are often utilized. The Fe (II)-ethyl complexes were characterized using MCD spectroscopy. The Fe (0)-ethylene complexes were found to have a rare high-spin (S = 1) ground state and were characterized by MCD and DFT. Future work is focused on further understanding the reactivity related to these species and using what has been learned to target reactive species in catalysis.
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