Coordination and Redox Isomerization in the Reduction of a Uranium(III) Monoarene Complex

Abstract

Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand ((Ad,Me) ArO)3 mes(3-) . Cyclic voltammetry of [{((Ad,Me) ArO)3 mes}U(III) ] (1) revealed a nearly reversible and chemically accessible reduction at -2.495 V vs. Fc/Fc(+) -the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

δ and φ back-donation in AnIV metallacycles

“…Optical properties. UV-visible and near-IR spectroscopy are among the most used experimental tools to probe chemical bonding interactions 3,4,9,17,18,22,[24][25][26]34,39,[51][52][53] . The UV-visible-NIR spectra for the cumulene complex (C 5 Me 5 ) 2 U[η 4 -1,2,3,4-PhC 4 Ph] has been recently reported 32 , providing an opportunity for experimental validation of the orbital analysis discussed above.…”

“…In contrast to the M-M δ bonds, metal-ligand (M-L) interactions involving a formation of δ bonds are less common. These bonds qualitatively differ from the M-M δ bonds for several reasons: first, they are formed due to the covalent overlap between occupied d or f metal orbitals and unoccupied ligand orbital(s), and, hence, are called δ backbonds; [16][17][18][19][20][21][22][23][24][25][26] second, such M-L interactions occur with "head-tohead" orbital overlap (Fig. 1c).…”

“…Although the M-L δ or φ back-bonding is commonly considered a weak interaction, it may be as necessary as σ and π bonding interactions for the full description of electronic structure. The M-L δ back-bonding has been found to be instrumental in explaining certain features of several compounds [16][17][18][19][20][21][22][23][24][25][26] . For example, considering classical sandwich complexes of actinides, it was previously noted for cycloheptatrienyl sandwich compounds An(η 7 -C 7 H 7 ) 2 (An=Th-Am) that the f δ orbitals not only participate in bonding via the e 2 ″ p π orbitals of the C 7 H 7 rings, but are as essential as the d δ orbitals in stabilizing the frontier p π orbitals of these ligands 19 .…”

“…In other inverted sandwich complexes M 2 -2 2 -μ-C 10 H 8 (M=Na, K) and 2 2 -μ-C 8 H 8 , δ bonding was invoked to explain the appreciably longer U−C arene bond lengths in the latter complex (2.822 Å) in comparison to the former (2.634 Å), wherein a better covalent overlap was found 23 . Several other studies have also pointed to the need of using δ backbonding for a comprehensive description of the M-L interaction between the divalent or trivalent U atoms and aromatic hydrocarbons 17,[24][25][26] . In these structures, the U atom is bound equatorially to three O atoms of the scaffolding ligand, and axially to the arene allowing for a formation of two singly occupied δ bonds.…”

A Relativistic DFT Probe of Thorium and Protactinium Complexes Supported by Heterocalix[4]arene and Redox Properties of Early‐Middle Actinides

“…Therefore, a reduction mechanism including both metal and bis (arene)‐part ligand is proposed. This differs from the previous hypothesis that the tris (alkyloxide) arene uranium complexes feature a metal‐centered reduction process; moreover, the metal center plays more significant role in the reduction process as it becomes heavier. Interestingly, this assumption is held while considering even heavier Np and Pu metal complexes …”