The synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η5-C5Me5)2UCl2] (1) with potassium graphite (KC8) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me3SiC≡C–C≡CSiMe3) forms the uranium metallacyclocumulene [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) in good yield. Magnetic susceptibility data confirm that 2 behaves as a U(IV) complex, and density functional theory (DFT) studies indicate a substantial 5f orbital contribution to the bonding of the metallacyclopentatriene U(η4-C═C═C═C) moiety, leading to more covalent bonds between the [(η5-C5Me5)2U]2+ and [η4-C4(SiMe3)2]2– fragments than those found in the related Th(IV) compound. Consequently, very different reactivity patterns emerge; e.g., 2 can act as a synthetic equivalent for the (η5-C5Me5)2U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η4-Me3SiC═C═C═CSiMe3)]2– moiety in 2 may react as a nucleophile when exposed to a variety of simple heterounsaturated molecules such as aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, pyridines, and organic azides. DFT studies are included to complement the experimental observations.

We reported a density functional theory study on the complexation of six hydrated actinyl cations, AnO2(H2O)52+/+ (aq) (An = U/Np/Pu), with three expanded porphyrins, amethyrin (H4L1), oxasapphyrin (H2L2), and grandephyrin (H3L3). The geometries have been fully optimized and analyzed, and the electronic structures, the binding free energies, and the NMR properties were calculated. Natural population analysis and Quantum Theory of Atoms in Molecules (QTAIM) topology analysis techniques were applied to understand the interaction modes between two entities of each complex. The calculations show that for the same ligand, PuO22+ and NpO22+ display stronger binding affinity than UO22+, UO2+, NpO2+, and PuO2+, and among the three ligands tested, L22− fits better with the actinyl cations than L33− and H2L12−. The redox process was observed in the complexation of PuO22+ and NpO22+ with specific ligands, which agrees with the experimental results. In the characterization of the nature of the coordination bonding interactions, QTAIM gives a consistent description with the natural population analysis method and shows that the interaction between An and the electron donor atoms in the first coordination shell has a strong ionic feature, while the interaction between An and Oyl atoms of the actinyls in the complexes remains covalent. This work complements the earlier experimental studies by providing a molecular level of understanding on the interaction between actinyls and expanded porphyrins, and is expected to contribute to the communities of the chemistry of actinides and expanded porphyrins.

The uranium metallacyclocumulene, [η5-1,3-(Me3C)2C5H3]2U(η4-C4Ph2) (2) was isolated by the reduction of [η5-1,3-(Me3C)2C5H3]2UCl2 (1) with potassium graphite (KC8) in the presence of 1,4-diphenylbutadiyne (PhCC–CCPh) in good yield. Furthermore it was fully characterized including the determination of its molecular structure; and the reactivity of 2 towards various small unsaturated organic molecules was explored. For example, while complex 2 shows no reactivity with alkynes and 2,2′-bipyridine (bipy), it reacts as a nucleophile when exposed to carbodiimides, diazabutadienes, isothiocyanates, ketones, and pyridine derivatives, leading to five-, seven- or nine-membered heterometallacycles. In contrast, treatment of complex 2 with CS2 results in CS bond cleavage and forms the binuclear complex [η5-1,3-(Me3C)2C5H3]2U[μ-η4:η3-PhCCC(S)C(Ph)CS]U[η5-1,3-(Me3C)2C5H3]2 (10). Density functional theory (DFT) studies complement the experimental study.

Transferrins have been proposed to be responsible for the in vivo transportation of uranyl. In this work, the binding mechanism of uranyl to transferrin has been studied using density functional theory method. Three possible stepwise pathways have been investigated and compared, differing in the sequence of the three residues to bind with uranyl, i.e. Tyr* → Tyr* → Asp* (YYD) and Tyr* → Asp* → Tyr* (YDY) and Asp* → Tyr* → Tyr* (DYY). Compared with the activation energies and the reaction heat of these three possible mechanisms, it is concluded that the YYD pathway is a more plausible description for the binding of uranyl. According to the calculations, the binding process is described as a ligand exchange process assisted by the hydrolysis of uranyl tricarbonate complex, and the role of carbonate ligand which determines the optimal pathway is identified. The QTAIM analysis was used to compare the bond nature of uranyl complexes in its free form and its complex with the amino acid residues. The results are expected to benefit our understanding of the uptake of uranyl by serum transferrins, and have implications on protein engineering and the development of decorporation agents towards improved binding kinetics and thermodynamics of uranyl in a specific pH range.

The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η5-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η5-C5Me5)2U[η2-C2(SiMe3)2] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(IV) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(η2-C═C) moiety, leading to more covalent bonds between the (η5-C5Me5)2U2+ and [η2-C2(SiMe3)2]2– fragments than those in the related Th(IV) compound. Consequently, very different reactivity patterns emerge, e.g., 2 can act as a source for the (η5-C5Me5)2U(II) fragment when reacted with alkynes and a variety of heterounsaturated molecules such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.

The formation of thorium metallacyclopentadiene and metallacyclopropene complexes is significantly influenced by the steric and electronic properties of the internal alkyne employed during their syntheses. The reduction of (η5-C5Me5)2ThCl2 (2) with potassium graphite (KC8) or lithium in the presence of internal phenyl(alkyl)acetylenes (PhC≡CR) selectively yields the corresponding Cs-symmetric thorium metallacyclopentadienes (η5-C5Me5)2Th[η2-C(Ph)═C(R)C(Ph)═C(R)] (R = Me (4), iPr (5), C6H11 (6)), while the phenyl(silyl)acetylene PhC≡CSiHMe2 gives the C2v-symmetric metallacyclopentadiene (η5-C5Me5)2Th[η2-C(SiHMe2)═C(Ph)C(Ph)═C(SiHMe2)] (7). However, the sterically more encumbered acetylene derivative PhC≡CSiMe3 affords the chloro metallacyclopropene complex [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)][Li(EDM)2] (8), whereas Me3SiC≡CSiMe3 forms the chloro alkenyl complex [(η5-C5Me5)2Th[C(SiMe3)═CHSiMe3](Cl) (9), in which the chloro metallacyclopropene intermolecularly activates a C–H bond of the solvent (C7H8). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C–C bond formation.

The mechanisms of CO2 insertion into the U–N bond of a silylamido NHC (N-heterocyclic carbene) U(III) complex were investigated theoretically. An earlier reported mechanism involving a reversible NHC carboxylation was found to be energetically too demanding, and a novel four-step mechanism featuring a direct CO2 insertion into the U–N bond is proposed.

We report our recent DFT mechanistic study on the functionalization of CO2 and CS2 promoted by a trivalent uranium complex (Tp*)2UCH2Ph. In the calculations, the uranium atom is described by a quasi-relativistic 5f-in-core ECP basis set (LPP) developed for the trivalent uranium cation, which was qualified by the calculations with a quasi-relativistic small core ECP basis set (SPP) for uranium. According to our calculations, the functionalization proceeds in a stepwise manner, and the CO2 or CS2 does not interact with the central uranium atom to form a stable complex prior to the reaction due to the steric hindrance from the bulky ligands but directly cleaves the U–C (benzyl) bond by forming a C–C covalent bond. The released coordination site of uranium is concomitantly occupied by one chalcogen atom of the incoming molecule and gives an intermediate with the uranium atom interacting with the functionalized CO2 or CS2 in an η1 fasion. This step is followed by a reorientation of the (dithio)carboxylate side chain of the newly formed PhCH2CE2– (E = O, S) ligand to give the corresponding product. Energetically, the first step is characterized as the rate-determining step with a barrier of 9.5 (CO2) or 25.0 (CS2) kcal/mol, and during the reaction, the chalcogen atoms are reduced, while the methylene of the benzyl group is oxidized. Comparison of the results from SPP and LPP calculations indicates that our calculations qualify the use of an LPP treatment of the uranium atom toward a reasonable description of the model systems in the present study.

The uranium metallacyclocumulene, [η5-1,3-(Me3C)2C5H3]2U(η4-C4Ph2) (2) was isolated by the reduction of [η5-1,3-(Me3C)2C5H3]2UCl2 (1) with potassium graphite (KC8) in the presence of 1,4-diphenylbutadiyne (PhCC–CCPh) in good yield. Furthermore it was fully characterized including the determination of its molecular structure; and the reactivity of 2 towards various small unsaturated organic molecules was explored. For example, while complex 2 shows no reactivity with alkynes and 2,2′-bipyridine (bipy), it reacts as a nucleophile when exposed to carbodiimides, diazabutadienes, isothiocyanates, ketones, and pyridine derivatives, leading to five-, seven- or nine-membered heterometallacycles. In contrast, treatment of complex 2 with CS2 results in CS bond cleavage and forms the binuclear complex [η5-1,3-(Me3C)2C5H3]2U[μ-η4:η3-PhCCC(S)C(Ph)CS]U[η5-1,3-(Me3C)2C5H3]2 (10). Density functional theory (DFT) studies complement the experimental study.