Bond Covalency and Oxidation State of Actinide Ions Complexed with Therapeutic Chelating Agent 3,4,3-LI(1,2-HOPO)

Abstract

for heavier actinides, reflecting increased energy degeneracy driven covalency and concomitant orbital mixing between the 5f 24 orbitals of the An ions and the π orbitals of the ligand. Notably, the ability of this ligand to either accept or donate electron 25 density as needed from its pyridine rings is found to be key to its extraordinary stability across the actinide series. ■ INTRODUCTION27 Radiological contamination incidents can result in widespread 28 radiation exposure to both local and remote regions. 29 Representing an extreme recent example, the 2011 Fukushima 30 Daiichi Nuclear Power Plant accident resulted in the dispersal 31 of several radionuclides across a wide area, including portions of 32 the continental U.S.1 Actinide (An) and lanthanide (Ln) fission 33 product species are likely to be major components of such 34 contamination events, and it is therefore necessary to 35 thoroughly understand and study the behavior of these ions 36 in environmental and biological systems. Internal contamination of human populations in the event of 38 a radiological incident, whether accidental or intentional, is of 39 critical concern. Once internalized, An ions transit rapidly 40 throughout the bloodstream and are primarily deposited in the 41 liver and bones (uranium is an exception and preferentially 42 deposits in the kidneys rather than in the liver), from which 43 elimination occurs very slowly. 3,4 Uptake and deposition of 44 these ions present severe health risks due to both their 45 The present work uses density functional theory (DFT) 120 combined with extended X-ray absorption fine structure 121 (EXAFS) measurements to advance our current understanding 122 of An-3,4,3-LI(1,2-HOPO) complexes. Their structure, ther-123 modynamics, electronic structures, and redox properties are 124 investigated across the An series up to Es, with both formally 125 An(III) and An(IV) ions. The similarity of 3,4,3-LI(1,2-126 HOPO) to other biological complexants and the wealth of 127 excellent experimental information on this system ensures that 128 a fundamental understanding of An-3,4,3-LI(1,2-HOPO) 129 complexation will have applications beyond this single ligand, 130 and presents an opportunity to study trends in An-ligand 131 Other than the exceptions described above, the fit model 243 obtained from calculated structures describes the data well 244 (Figure 2). Generally speaking, the EXAFS data are consistent 245 with the HOPO complex structure calculations for the nearest 246 neighbor M(III)−O pair distances with the largest deviation 247 occurring with [Am(HOPO)] − . In addition, the Cf−C/N 248 average bond length differs from calculation, but the calculation 249 also shows a broad distribution width for the 8 bonds in this t1 250 shell. Table 1 compares the EXAFS bond length results to 251 those derived from the DFT calculations for the first two shells. 252 Further details of the methods and results are available in 253 Supporting Information (pp S11−S15). Figure S6 for thermodynamic calculations...

Energy-Degeneracy-Driven Covalency in Actinide Bonding

“…For example, there is a body of evidence that demonstrates -in certain systemssubstantial 5f-and 6d-covalency exists. (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) These data are juxtaposed with numerous studies suggesting that An-ligand bonding is primarily ionic. (21) The later conclusions are fueled by several observables that include (but are not limited to) An-ligand bond distances being routinely predicted using actinide ionic radii and an appreciable body of spectroscopic data that shows the 5f  5f optical transitions are marginally impacted by the ligand field.…”

Periodic trends and complexation chemistry of tetravalent actinide ions with a potential actinide decorporation agent 5‐LIO(Me‐3,2‐HOPO): A relativistic density functional theory exploration

“…Abergel and co‐workers have carried out extensive studies using various spectroscopic methods to understand the coordination chemistry and solution thermodynamics of [AnL1] complexes. In conjugation with these experiments, relativistic density functional theory (DFT) studies also have been highly informative to understand the bonding nature of actinides with L1 at a molecular level . These studies demonstrated that the stabilities of [AnL1] complexes depend on many inter‐dependent parameters such as charge and ionic radius of the metal, ligand to metal charge transfer (LMCT) effect, degree of covalency of the metal–ligand bond and also on the chelatoaromatic effect.…”

“…These studies demonstrated that the stabilities of [AnL1] complexes depend on many inter‐dependent parameters such as charge and ionic radius of the metal, ligand to metal charge transfer (LMCT) effect, degree of covalency of the metal–ligand bond and also on the chelatoaromatic effect. Undoubtedly, the relative participation of valence f and d orbitals in actinide–ligand bonding plays a pivotal role in the alteration of the values of the metal binding stability constants …”

The duality of electron localization and covalency in lanthanide and actinide metallocenes

“…As described above, Ce(C 8 H 8 ) 2 and U(C 8 H 8 ) 2 have qualitatively similar amounts of C 2p mixing with the f-orbitals of e 2u symmetry (28% vs. 24% C 2p character, respectively), but different amounts of f-orbital overlap. Consequently, similar amounts of f-orbital mixing does not equate to similar strengths of f-orbital bonding, 83,[127][128][129][130][131][132][133][134][135][136][137] which highlights the counterintuitive relationship between covalency and stability for f-element molecules. 138 Specically, the decrease in overlap for Ce(C 8 H 8 ) 2 results in a smaller covalent contribution to the bond energy and more localized electrons.…”

“…Without a similar degree of overlap, 4f-orbital mixing in Ce(C 8 H 8 ) 2 is accompanied by an overall weakening of the complex due to a comparatively large drop in the strength of ionic bonding. 62,83,84,[127][128][129]…”