Wanjian Ding

Influence of the 5f Orbitals on the Bonding and Reactivity in Organoactinides: Experimental and Computational Studies on a Uranium Metallacyclopropene

The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η(5)-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-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-(η(2)-C═C) moiety, leading to more covalent bonds between the (η(5)-C5Me5)2U(2+) and [η(2)-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 (η(5)-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.

Experimental and Computational Studies on the Formation of Thorium-Copper Heterobimetallics.

The formation of actinide-transition metal heterobimetallics mediated by a terminal actinide imido complex was comprehensively studied. The reaction of the thorium imido complex [(η5 -C5 Me5 )2 Th=N(mesityl)(DMAP)] (3), prepared from [(η5 -C5 Me5 )2 ThMe2 ] (1) and mesitylNH2 or [(η5 -C5 Me5 )2 Th(NHmesityl)2 ] (2) in the presence of 4-(dimethylamino)pyridine (DMAP), with copper(I) halides gave the first thorium-copper heterobimetallic compounds [(η5 -C5 Me5 )2 Th(X){N(mesityl)Cu(DMAP)}] (X=Cl (4), Br (5), I (6)). Complexes 4-6 feature an unusual geometry with a short Th-Cu distance, which DFT studies attribute to a weak donor-acceptor bond from the Cu+ atom to the electropositive Th4+ atom. They are reactive species, as was shown by their reaction with the dimethyl complex [(η5 -C5 Me5 )2 ThMe2 ] (1). Furthermore, a comparison between Th and early transition metals confirmed that Th4+ exhibits distinctively different reactivity from d-transition metals.

Intrinsic reactivity of a uranium metallacyclopropene toward unsaturated organic molecules

The uranium metallacyclopropene (η5-C5Me5)2U[η2-C2(SiMe3)2] (1) reacts with various small unsaturated organic molecules. For example, replacement of bis(trimethylsilyl)acetylene occurs when complex 1 is exposed to alkynes, conjugated alkenes, nitriles and quinones. Reaction of 1 with internal phenyl(alkyl)acetylene PhC[triple bond, length as m-dash]CMe selectively yields the Cs symmetric uranium metallacyclopentadiene (η5-C5Me5)2U[η2-C(Ph)[double bond, length as m-dash]C(Me)-C(Ph)[double bond, length as m-dash]C(Me)] (6) after the loss of bis(trimethylsilyl)acetylene, while treatment of 1 with phenyl(silyl)acetylenes (PhC[triple bond, length as m-dash]CR, R = SiHMe2, SiMe3) gives the corresponding C2v symmetric isomers (η5-C5Me5)2U[η2-C(R)[double bond, length as m-dash]C(Ph)-C(Ph)[double bond, length as m-dash]C(R)] (R = SiHMe2 (7), SiMe3 (8)). Furthermore, while no deprotonation occurs between complex 1 and pyridine derivatives, cyclohexanone can be inserted into the uranium metallacyclopropene moiety of 1 to yield the five-membered, heterocyclic complex (η5-C5Me5)2U[OC(CH2)5(C2(SiMe3)2)] (14) in quantitative conversion. Density functional theory (DFT) studies have been performed to complement the experimental studies.

A Base-Free Terminal Actinide Phosphinidene Metallocene: Synthesis, Structure, Reactivity, and Computational Studies

The synthesis, structure, and reactivity of a base-free terminal actinide phosphinidene metallocene have been comprehensively studied. The salt metathesis reaction of the thorium methyl iodide complex Cp‴2Th(I)Me (2; Cp‴ = η5-1,2,4-(Me3C)3C5H2) with Mes*PHK (Mes* = 2,4,6-(Me3C)3C6H2) in THF furnishes the first stable base-free terminal phosphinidene actinide metallocene, Cp‴2Th═PMes* (3). Density functional theory (DFT) shows that the bonds between the Cp‴2Th2+ and [PMes*]2- fragments are more covalent than those in the related thorium imido complex. While the phosphinidene complex 3 shows no reactivity toward alkynes, it reacts with a variety of heterounsaturated molecules such as CS2, isothiocyanate, nitriles, isonitriles, and organic azides, forming carbodithioates, imido complexes, metallaaziridines, and azido compounds. These experimental observations are complemented by DFT computations.

Computational comparative mechanistic study of C-E (E=C,N,O,S) couplings via CO2 activation mediated by uranium(III) complexes

A DFT mechanistic study is undertaken on the functionalization of CO2 to form C-C, C-N, C-S, and C-O bonds promoted by trivalent uranium complexes (Tp*)2 UR [Tp*=hydrotris(3,5-dimethylpyrazolyl)-borate ligand, R= -C≡CPh (Cpda-CC), -C≡CSiMe3 (Cpda-CSi), -NHPh (Cpda-N), -SPh (Cpda-S), and -OPh (Cpda-O)]. These model systems are similar in view of their two-step reaction mechanisms, that is, the insertion of CO2 into the U-E (E=C, N, O, S) bond to form a [U-κ1 -O2 C] intermediate, followed by the reorientation of the carboxylate group to coordinate with the U atom in the κ2 manner (Cpdb-X, X=CC, CSi, N, S, O). However, the free energy barriers to the rate-determining steps are substantially different, increasing in the order Cpda-S<Cpda-CC<Cpda-Csi<Cpda-N<Cpda-O, which suggests that the bond property of the U-E bonds and the nucleophilicity of the R groups govern the reactivity of the trivalent U complexes. The insertion product may then be silylated in the presence of trimethylsilyl iodide. Two potential mechanisms have been investigated with the -O2 CR group attacking the Si atom from the side cis (frontside) or trans (backside) to the I atom. The backside pathway was found to be more feasible in view of the free energy barriers and thermicity.