Multiedge X-ray Absorption Spectroscopy Part II: XANES Analysis of Bridging and Terminal Chlorides in Hexachlorodipalladate(II) Complex

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory

The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character in these orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO₂²⁺) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (O═P). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO₂²⁺ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO₂²⁺ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σ- and π-type interactions with U 5f and 6d orbitals in the UO₂Cl₂(TRPO)₂ complexes. Although both the axial (O_yl) and equatorial (O_eq) oxygen atoms in the UO₂Cl₂(TRPO)₂ complexes contribute to the X-ray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O_eq and is assigned to the transition from O_eq 1s orbitals to the unoccupied molecular orbitals of 1b_1u + 1b_2u + 1b_3u symmetries resulting from the σ- and π-type mixing between U 5f and O_eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O_eq 2p character in these orbitals for the four UO₂Cl₂(TRPO)₂ complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U-TRPO bonding interactions in the UO₂Cl₂(TRPO)₂ complexes. The covalent bonding interactions between TRPO and UO₂²⁺, especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

Quantifying the Interdependence of Metal-Ligand Covalency and Bond Distance Using Ligand K-edge XAS

Bond distance is a common structural metric used to assess changes in metal-ligand bonds, but it is not clear how sensitive changes in bond distances are with respect to changes in metal-ligand covalency. Here we report ligand K-edge XAS studies on Ni and Pd complexes containing different phosphorus(III) ligands. Despite the large number of electronic and structural permutations, P K-edge pre-edge peak intensities reveal a remarkable correlation that spectroscopically quantifies the linear interdependence of covalent M-P σ bonding and bond distance. Cl K-edge studies conducted on many of the same Ni and Pd compounds revealed a poor correlation between M-Cl bond distance and covalency, but a strong correlation was established by analyzing Cl K-edge data for Ti complexes with a wider range of Ti-Cl bond distances. Together these results establish a quantitative framework to begin making more accurate assessments of metal-ligand covalency using bond distances from readily-available crystallographic data.

Mono- and binuclear tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate bismuth(III) dichloride complexes: a soft scorpionate ligand can coordinate to p-block elements

Tris(pyrazolyl)hydroborate ligands have been utilized in the fields of inorganic and coordination chemistry due to the ease of introduction of steric and electronic substitutions at the pyrazole rings. The development and use of the tris(pyrazolyl)hydroborate ligand, called a `scorpionate', were pioneered by the late Professor Swiatoslaw Trofimenko. He developed a second generation for his ligand system by the introduction of 3-tert-butyl and 3-phenyl substituents and this new ligand system accounted for many remarkable developments in inorganic and coordination chemistry in stabilizing monomeric species while maintaining an open coordination site. Bismuth is remarkably harmless among the toxic heavy metal p-block elements and is now becoming popular as a replacement for highly toxic metal elements, such as lead. Two bismuth(III) complexes of the anionic sulfur-containing tripod tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate ligand were prepared. By recrystallization from MeOH/CH2Cl2, orange crystals of dichlorido(methanol-κO)[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III), [Bi(C21H34BN6S3)Cl2(CH4O)], (I), were obtained, manifesting a mononuclear structure. By using a noncoordinating solvent, red crystals of the binuclear structure with bridging Cl atoms were obtained, namely di-μ-chlorido-bis{chlorido[tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl-κS)hydroborato]bismuth(III)}, [Bi2(C21H34BN6S3)2Cl4], (II). These complexes show {BiIIIS3Cl2O} and {BiIIIS3Cl3} coordination geometries with average BiIII-S bond lengths of 2.73 and 2.78 Å in (I) and (II), respectively. The overall BiIII coordination geometry is distorted octahedral due to stereochemically active lone pairs. The three BiIII-S bond lengths are almost equal in (I) but show considerable differences in (II), with one long and two shorter distances that also correlate with changes in the UV-Vis and 1H NMR spectra. For direct measurements of the Bi-S/Cl coordination, ligand K-edge X-ray absorption measurements were carried out in combination with ground and excited-state electronic structure analyses. For p-block elements, these sulfur-containing ligands are useful for preparing the appropriate complexes due to their flexible coordination geometry.