Predicting the redox properties of uranyl complexes using electronic structure calculations

Abstract

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO2(V)‐UO2(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO2F4]1‐/2‐ and [UO2Cl4]1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO2]1+ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.

Redox Potentials of Uranyl Ions in Macrocyclic Complexes: Quantifying the Role of Counter-Ions

“…Among the five DFs, the M06 functional predicted a value of −1.56 V close to the experimental estimate of −1.54 V within 100 mV. We have earlier shown that the M06 functional predicts vertical and adiabatic detachment energies of uranyl complexes in close agreement with electron spray ionization photoelectron spectroscopic techniques . All other four functionals underestimate the redox potential by more than 500 mV.…”

Design and Optimization of Catalysts Based on Mechanistic Insights Derived from Quantum Chemical Reaction Modeling

“…Grimme showed that the B3LYP density functional can be systematically improved through geometrical counterpoise correction . Much effort has been spent to develop new functionals, and recently, several highly optimized density functionals have been developed and many have been conceived specifically with transition metals in mind. − Nonetheless, transition metals continue to challenge quantum chemical methods. For example, the performance of the new generation density functionals developed by Head-Gordon should be assessed in future studies, as they show very promising behavior and impressively small errors for a variety of properties such as thermochemistry, vibrational frequencies, and barrier heights.…”

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N–Ar)X Complexes

“…At the dawn of the 21st century, the redox chemistry of organoactinide complexes has experienced a remarkable revival and growth both experimentally and theoretically. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Indeed, in addition to the usual ligands such as chloride, carbocyclic ligands (C 5 R 5 , C 7 H 7 , C 8 H 8 ) and amides NR 2 , the use of a wider range of functionalized groups have led to high oxidation states actinide compounds (> +3) exploiting the stabilization induced by metal-ligand multiple bonds. 17,18 Furthermore, contrarily to the 4f lanthanide electrons which are essentially core electrons, 19 the 5f actinide electrons are involved in the bonding.…”

“…20,21 Moreover, the investigation of new ligands that could provide thermodynamic stabilization to high-valent uranium species, is interesting first on a fundamental point of view, but also to elaborate new separation techniques and storage methods and for the processing of nuclear wastes from the nuclear plants. 4,8,13,22,23 Indeed, uranium complexes are able to access several oxidation states ranging from U II to U VI . 24,25 Their redox properties and the availability of valence 5f/6d-orbitals to interact make uranium complexes remarkably suitable for exploring new catalytic reactions especially for small molecule activation chemistry.…”

“…At the dawn of the 21st century, the redox chemistry of organoactinide complexes has experienced a remarkable revival and growth both experimentally and theoretically. − Indeed, in addition to the usual ligands such as chloride, carbocyclic ligands (C 5 R 5 , C 7 H 7 , C 8 H 8 ), and amides, NR 2 , the use of a wider range of functionalized groups have led to high oxidation states actinide compounds (>+3) exploiting the stabilization induced by metal–ligand multiple bonds. , Furthermore, contrarily to the 4f lanthanide electrons, which are essentially core electrons, the 5f actinide electrons are involved in the bonding. The nature of the ligands influences the electrochemical, magnetic, and optical properties of actinide systems. , Moreover, the investigation of new ligands that could provide thermodynamic stabilization to high-valent uranium species, is interesting, first on a fundamental point of view but also to elaborate new separation techniques and storage methods and for the processing of nuclear wastes from the nuclear plants. ,,,, Indeed, uranium complexes are able to access several oxidation states ranging from U II to U VI . , Their redox properties and the availability of valence 5f/6d-orbitals to interact make uranium complexes remarkably suitable for exploring new catalytic reactions especially for small-molecule activation chemistry. − …”