Two‐Electron Redox Reactivity of Thorium Supported by Redox‐Active Tripodal Frameworks

Abstract

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene‐tethered tris(siloxide) tripodal ligands [(KOSiR2Ar)3‐arene)]. The two‐electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(OtBu)2Ar)3‐arene)(THF)2] (2OtBu) and [K(2.2.2‐cryptand)][Th((OSiPh2Ar)3‐arene)(THF)2](2Ph‐crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two‐electron reduction of a broad range of substrates (N2O, COT, CHT, Ph2N2, Ph3PS and O2) while retaining the ligand framework. This work shows that arene‐tethered tris(siloxide) tripodal ligands allow implementation of two‐electron redox chemistry at the thorium center while retaining the ligand framework unchanged.

Reduction of Thorium Tris(amido)arene Complexes: Reversible Double and Single C–C Couplings

“…19,20,33,34 In addition, very recently, Mazzanti and co-workers showed that thorium(IV) complexes supported by redox-active tripodal frameworks could act as a thorium(II) synthon to enable twoelectron redox reactivity. 40 These all involve reduced arene anions as electron reservoirs. However, while transition-metal arene complexes could undergo reversible C−C couplings under redox conditions, 78,79 no such reactivity has been shown for f-block metal arene complexes.…”

“…27,35−40 Notably, all attempts to reduce thorium(IV) arene complexes led to ligand-based reduction with the formation of thorium(IV) complexes of reduced arenes (Figure 1a). 27,36,40 Such reduction reactivity is in contrast to that of uranium(IV) analogues, which usually result in uranium-centered reduction. 27,29,41 The different reactivity between thorium and uranium may be attributed to the facts that thorium(IV) is more difficult to reduce than uranium(IV) 42−44 and genuine thorium(III) and thorium(II) complexes are much rarer compared to their uranium(III) and uranium(II) counterparts.…”

“…Compared to uranium, thorium–arene interactions are much less studied and thorium arene complexes are scarce. In 2003, Gambarotta and co-workers reported the synthesis of thorium complexes of naphthalene dianion and their reactivity as thorium­(II) synthons. , Later on, a handful of thorium­(IV) complexes with either inter- or intramolecular arene coordination were reported. ,− Notably, all attempts to reduce thorium­(IV) arene complexes led to ligand-based reduction with the formation of thorium­(IV) complexes of reduced arenes (Figure a). ,, Such reduction reactivity is in contrast to that of uranium­(IV) analogues, which usually result in uranium-centered reduction. ,, The different reactivity between thorium and uranium may be attributed to the facts that thorium­(IV) is more difficult to reduce than uranium­(IV) − and genuine thorium­(III) and thorium­(II) complexes are much rarer compared to their uranium­(III) and uranium­(II) counterparts. − Furthermore, a comparison between thorium and uranium inverse-sandwich arene complexes shows that thorium has less tendency to form δ-bonding interactions with arenes than uranium …”

Investigating Steric and Electronic Effects in the Synthesis of Square Planar 6d1 Th(III) Complexes

“…Progress in the field of thorium redox chemistry has lagged behind that of uranium, even though thorium is over three times more abundant than uranium . In fact, most of the literature on thorium chemistry involves its most stable +4 oxidation state. − Although there are many studies showcasing the redox chemistry of thorium complexes, many of these require the use of redox-active ligands or reduced arenes. − To date, only 12 structurally characterized Th­(III) complexes have been reported. − There are also only six published Th­(II) crystal structures, five of which contain the {[C 5 H 3 (SiMe 3 ) 2 ] 3 Th II } 1– anion and one containing the [(C 5 H 4 Si i Pr 3 ) 3 Th II ] 1– anion. − As a result, the development of synthetic routes to new examples of Th­(III) and Th­(II) complexes is an important research goal.…”

Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum μ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer

“…These virtues derive from the synergy between the ancillary silanolates and the high-valent molybdenum alkylidyne unit, which has been studied in detail in the past. ,,− As a next step, investigating whether these tripodal silanolates also match molybdenum complexes beyond the alkylidyne estate seemed interesting and relevant. Nitrido complexes are obvious candidates, as they are isolobal with the alkylidynes. , As such, they are of interest in their own right and, at the same time, constitute a potential entry point into the realm of low-valent molybdenum silanolate coordination chemistry. , The results of our first foray are summarized below.…”