The First Thorium Arene Complex: A Divalent Synthon

Abstract

Very strong reducing power [1] and a marked inclination toward performing cooperative attack on the same substrate [2] are the basis of the distinctive reactivity of divalent f-block metals. [3] In spite of the great interest in this field, divalent actinides remain basically unknown, possibly as a result of an intrinsic instability of this oxidation state. Moreover, the significance of the low oxidation states remains questionable, since the f electrons are expected to be more likely engaged in backbonding with the ligand rather than being metal-centered, nonbonding electrons. As a consequence, even simple salts such as UI 2 [4] and ThI 2[5] are regarded as divalent only from the point of view of formula, since the f electrons are believed to be located in a type of conduction band. Nevertheless, the electron-rich nature of these species raises expectations for high reactivity and strong reducing power. This idea is substantiated by the chemical behavior of the two known U II synthons, [6] and by some recent findings that a trivalent calix-tetrapyrrolide uranium compound resulted, upon reduction, in a complex series of transformations including dinitrogen cleavage, solvent deoxygenation, and depolymerization of silicone grease. [7] For thorium such chemistry is limited to a handful of trivalent compounds or synthons. [8] Given this scenario, we became interested in exploring the chemistry of divalent Th complexes, most likely synthons, with the dual purpose of: 1) probing the stability of an oxidation state unknown for thorium and 2) preparing potent two-electron reductants for further reactivity studies. For this purpose, we focused on pyrrole-based polyanions as ligands. These species have proved their ability both to stabilize low oxidation states in large cluster structures [9] and to increase the high reactivity of the low-valent compounds even further. [3f-n] Here we describe the reduction of a tetravalent polypyrrolide thorium complex, which resulted in both solvent fragmentation and formation of the first divalent synthon thorium arene complex.The reactions

Thermally stable potassium N-heterocyclic carbene complexes with alkoxide ligands, and a polymeric crystal structure with distorted, bridging carbenes

“…For comparison, the average K-C distance in potassium cyclopentadienide complexes is 3.10 A ˚.17 Close contacts with arene, phenol, and pyrrole ligands are increasingly reported in the organometallic literature, since often the additional reducing electrons provided by incorporated potassium affords unusual reduced dinitrogen and carbon dioxide adducts of the most electropositive metals. [18][19][20] In these complexes, K-C distances of about 3.3 A ˚are reported.…”

Expanding Thorium Hydride Chemistry Through Th2+, Including the Synthesis of a Mixed-Valent Th4+/Th3+ Hydride Complex

“…An additional aspect of the chemistry of 1 is that it is the first thorium complex capable of a two-electron metal-based reduction from a single metal center . Redox-active ligands have been used with Th 4+ complexes to enable reduction chemistry, − but prior examples of metal-based reductions were limited to one-electron processes because thorium chemistry was previously limited to only the +3 and +4 oxidation states. The formal two-electron metal-based reduction chemistry of 1 was first identified in its reaction with cyclooctatetraene to form (C 8 H 8 ) 2– (eq ).…”

π-Metal complexes of tetrapyrrolic systems. A novel coordination mode in “porphyrin-like” chemistry

“…18). 54 In complex 48, as common for other organometallic complexes of calix [4]pyrrole, two of the pyrrolic rings are s-bonded to the metal and two are p-bonded. However, only one of the latter subunits is symmetrically p-bonded, with the other displaying considerable ''slippage''; presumably, this is due to the concurrent Z 4 -coordination of the thorium center by one ring of a co-bound naphthalene molecule.…”

“…To the extent this interpretation is correct, it highlights the possible reactivity of complex 48. In fact, intriguing reactions between compound 48 and Me 3 SiN 3 54 and CO 2 were observed; 55 this led the authors to suggest that this particular complex could be considered as a Th(II) synthon. During the course of their studies of lanthanide and uranium complexes of calix [4]pyrrole, Gambarotta et al observed that the calix [4]pyrrole displayed a tendency to rearrange to the corresponding N-confused-calix [4]pyrrole.…”