Uncovering f-element bonding differences and electronic structure in a series of 1 : 3 and 1 : 4 complexes with a diselenophosphinate ligand

Abstract

Understanding the bonding trends within, and the differences between, the 4f and 5f element series with soft donor atom ligands will aid elucidation of the fundamental origins of actinide (An) versus lanthanide (Ln) selectivity that is integral to many advanced nuclear fuel cycle separation concepts. One of the principal obstacles to acquiring such knowledge is the dearth of well characterized transuranic molecules that prevents the necessary comparison of 4f versus 5f coordination chemistry, electronic structure, and bonding. Reported herein is new chemistry of selenium analogues of dithiophosphinate actinide extractants. Ln III and An III/IV complexes with the diselenophosphinate [Se 2 PPh 2 ] À anion have been synthesized, structurally and spectroscopically characterized, and quantum chemical calculations performed on model compounds in which the phenyl rings have been replaced by methyl groups. The complexes [Ln III (Se 2 PPh 2) 3 (THF) 2 ] (Ln ¼ La (1), Ce (2), Nd (3)), [La III (Se 2 PPh 2) 3 (MeCN) 2 ] (4), [Pu III (Se 2 PPh 2) 3 (THF) 2 ] (5), [Et 4 N][M III (Se 2 PPh 2) 4 ] (M ¼ Ce (6), Pu (7)), and [An IV (Se 2 PPh 2) 4 ] (An ¼ U (8), Np (9)), represent the first f-element diselenophosphinates. In conjunction with the calculated models, complexes 1-9 were utilized to examine two important factors: firstly, bonding trends/differences between trivalent 4f and 5f cations of near identical ionic radii; secondly, bonding trend differences across the 5f series within the An IV oxidation state. Analysis of both experimental and computational data supports the conclusion of enhanced covalent bonding contributions in Pu III-Se versus Ce III-Se bonding, while differences between U IV-Se and Np IV-Se bonding is satisfactorily accounted for by changes in the strength of ionic interactions as a result of the increased positive charge density on Np IV compared to U IV ions. These findings improve understanding of soft donor ligand binding to the f-elements, and are of relevance to the design and manipulation of f-element extraction processes.

Should environmental effects be included when performing QTAIM calculations on actinide systems? A comparison of QTAIM metrics for Cs2UO2Cl4, U(Se2PPh2)4 and Np(Se2PPh2)4 in gas phase, COSMO and PEECM

“…As noted previously [11], the small values of ρ b and H b for the actinide-selenium bonds calculated in gas phase indicate that these are mostly ionic. ρ b , with up to 5% difference for the Se-P bond and 9% difference for the P-C bond, most notably for the COSMO calculations.…”

“…4 , Carbon atoms are shown in grey, hydrogen atoms in white, phosphorus atoms in pink, sulfur atoms in yellow and the uranium atom in blue. Atomic positions taken from reference [11].…”

“…Experimental crystal structures for Cs 2 UO 2 Cl 4 [23], U(Se 2 PPh 2 ) 4 [11] and Np(Se 2 PPh 2 ) 4 [11] were used to provide the atomic positions, as well as the positions of the point charges in the embedding regions for the PEECM.…”

“…We target Cs 2 UO 2 Cl 4 , and U(Se 2 PPh 2 ) 4 and Np(Se 2 PPh 2 ) 4 , which have also recently been studied by the QTAIM. [11] The electron densities are obtained from calculations in the gas-phase, in a polarizable continuum with the Conductor-Like Screening Model (COSMO) [31] and embedded in point charges with the PEECM. Should environmental effects reduce the differences between theory and experiment it is clear that including them will be important in future QTAIM studies.…”

“…This is due partly to the experimental difficulties associated with obtaining high quality experimental electron densities on radioactive systems featuring very heavy elements, and partly a function of the QTAIM only recently being extended to quantum chemical studies of the actinide elements. Indeed, we were the first group to extensively employ the QTAIM in this capacity, and have used it to study both covalency [6][7][8][9][10][11][12][13][14][15][16][17][18] and bond strength [19][20][21] in a range of molecular f element systems.…”