New vistas in the molecular chemistry of thorium: low oxidation state complexes

Abstract

Although the molecular chemistry of thorium is dominated by the +4 oxidation state accounts of Th(iii) complexes have continued to increase in frequency since the first structurally characterised example was reported thirty years ago. The isolation of the first Th(ii) complexes in 2015 and exciting recent Th(iii) and Th(ii) reactivity studies both indicate that this long-neglected area is set to undergo a rapid expansion in research activity over the next decade, as previously seen since the turn of the millennium for analogous U(iii) small molecule activation chemistry. In this perspective article, we review synthetic routes to Th(iii) and Th(ii) complexes and summarise their distinctive physical properties. We provide a near-chronological discussion of these systems, focusing on structurally characterised examples, and cover complementary theoretical studies that rationalise electronic structures. All reactivity studies of Th(iii) and Th(ii) complexes that have been reported to date are described in detail.

Concomitant Carboxylate and Oxalate Formation From the Activation of CO2 by a Thorium(III) Complex

“…The mapping of U III ‐mediated CO 2 activation by DFT calculations has provided key insights into possible mechanistic pathways . In contrast, Cloke reported the only example of CO 2 activation by a putative Th III intermediate as Th III small molecule activation is in its infancy . Herein we report the first reaction of an isolated Th III complex with CO 2 , and CS 2 for comparative studies.…”

Chemical structure and bonding in a thorium(iii)–aluminum heterobimetallic complex

“…In contrast, our understanding of Th chemistry has increased greatly upon discoveries of organometallics with thorium formally in the +3 and +2 oxidation state. 32 Th( iii ) and Th( ii ) complexes are potentially good candidates for providing insight into Th reactivity and performing new redox transformations in analogy to the recent progress associated with the study of low-valent U complexes. 33 – 35 However, the large energetic requirements for the reduction from Th( iv ) materials (calculated for ThCl 6 2– to be +3.7 V vs. NHE relative to +0.6 V for U( iv )/( iii ) reduction in UCl 6 2– ) inhibits the study of Th( iii ) and Th( ii ) complexes.…”

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

“…In contrast, the Th(III) complex remains rare due to the greatly negative reduction potential −2.35 V (vs. Fc + /Fc), where 0.65 V was used to correct the original value (vs. NHE) . Although recent advance in the synthetic technique leads to more accessibility of Th III complexes, the study of redox properties of Th VI /Th III complexes remains underexplored, to our best of knowledge. For example, above E 0 of Th VI/III ions in water is the only available value, and moreover was obtained with the assistance of electron‐transfer and f ‐ d absorption bands .…”

“…The molecular chemistry of thorium is dominated by the IV oxidation state in contrast to III and II ones. Although extremely low reduction potentials of Th IV/III (‐3.0 V versus NHE) and Th III/II (‐4.9 V) were presumed, several Th III complexes and even one Th II complex have been synthesized and structurally characterized via massive efforts . In addition to commonly used cyclopentadienyl (Cp), cyclooctatetraenyl (COT) and their substituted derivatives, a small‐cavity macrocyclic trans ‐calix[2]pyrrole[2]benzene (H 2 L, Figure ), synthesized by the Sessler's group, has been found suitable for chelating Th IV .…”

“…Although extremely low reduction potentials of Th IV/III (‐3.0 V versus NHE) and Th III/II (‐4.9 V) were presumed, several Th III complexes and even one Th II complex have been synthesized and structurally characterized via massive efforts . In addition to commonly used cyclopentadienyl (Cp), cyclooctatetraenyl (COT) and their substituted derivatives, a small‐cavity macrocyclic trans ‐calix[2]pyrrole[2]benzene (H 2 L, Figure ), synthesized by the Sessler's group, has been found suitable for chelating Th IV . Interestingly, the flexibility of H 2 L allows its two binding pockets (BP), bis (arene) (marked as Ar L) and bis (pyrrolide) ( Pl L), to hold the actinide ion.…”