Elucidation of Thorium Redox-Active Ligand Complexes: Evidence for a Thorium-Tri(radical) Species

Metal-Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-phenyl) amine (H6Clamp) proligand was synthesized for comparison of its cerium complex with a potassium-cerium heterobimetallic complex of the 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where CeIII(CH3CN)1.5(H3Clamp) (1-Ce(H3Clamp)) exhibits shorter Ce-O distances and longer Ce-N bond distances compared to the analogous distances in K3(THF)3CeIII(dippap)3 (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H3Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand- and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H3Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H3Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium-ligand bonds upon oxidation.

Synthesis and Structure of [η5-1,2,4-(Me3Si)3C5H2]2Th(bipy) and Its Reactivity toward Small Molecules

Halide exchange of (Cp3tms)2ThCl2 (1; Cp3tms = η5-1,2,4-(Me3Si)3C5H2) with Me3SiI furnishes (Cp3tms)2ThI2 (2), which is then reduced with potassium graphite (KC8) in the presence of 2,2'-bipyridine to give the thorium bipyridyl metallocene (Cp3tms)2Th(bipy) (3) in good yield. Complex 3 was fully characterized and readily reacted with various small molecules. For example, 3 may serve as a synthetic equivalent for the (Cp3tms)2Th(II) fragment when exposed to CuI, Ph2S2, organic azides, and CS2. Moreover, upon the addition of thiobenzophenone Ph2CS, p-methylbenzaldehyde (p-MeC6H4)CHO, benzophenone Ph2CO, amidate PhCONH(p-tolyl), seleno-ketone (p,p'-dimethoxy), selenobenzophenone (p-MeOPh)2CSe, di(p-tolyl)methanimine (p-tolyl)2C═NH, 1,2-di(benzylidene)hydrazine (PhCH═N)2, and nitriles PhCN, PhCH2CN, and Ph2CHCN C-C coupling results to give (Cp3tms)2Th[(bipy)(Ph2CS)] (8), (Cp3tms)2Th[(bipy)(p-MePhCHO)] (9), (Cp3tms)2Th[(bipy)(Ph2CO)] (10), (Cp3tms)2Th[(bipy){(p-tolylNH)(Ph)CO}] (11), (Cp3tms)2Th[(bipy){(p-MeOPh)2CSe}] (12), (Cp3tms)2Th[(bipy){(p-tolyl)2CNH}] (13), (Cp3tms)2Th[(bipy)(PhCHNN═CHPh)] (14), (Cp3tms)2Th[(bipy)(PhCN)] (16), (Cp3tms)2Th[(bipy)(PhCH2CN)] (17), and (Cp3tms)2Th[(bipy)(Ph2CHCN)] (18), respectively. However, when thiazole is added to 3, the dimeric sulfido complex [(Cp3tms)2Th]2[μ-(bipy)CH2NCHCHS]2 (15) can be isolated. Moreover, the addition of isonitriles such as Me3CNC and PhCH2NC to 3 results in C-N bond cleavage and C-C coupling processes to form the thorium isocyanido amido complexes (Cp3tms)2Th[4-(Me3C)bipy](NC) (19) and (Cp3tms)2Th[4-(PhCH2)bipy](NC) (20), respectively. Nevertheless, upon exposure of 3 to (trimethylsilyl)diazomethane Me3SiCHN2, the bis-amido complex (Cp3tms)2Th[5,6-(Me3SiCH)bipy] (21), concomitant with N2 release, is isolated.

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

The factors affecting the formation and crystal structures of unusual 6d1 Th(III) square planar aryloxide complexes, as exemplified by [Th(OArMe)4]1- (OArMe = OC6H2tBu2-2,6-Me-4), were explored by synthetic and reduction studies of a series of related Th(IV) tetrakis(aryloxide) complexes, Th(OArR)4 (OArR = OC6H2tBu2-2,6-R-4). Specifically, electronic, steric, and countercation effects were explored by varying the aryloxide ligand, the alkali metal reducing agent, and the alkali metal chelating agent. Salt metathesis reactions between ThBr4(DME)2 (DME = 1,2-dimethoxyethane) and 4 equiv of the appropriate potassium aryloxide salt were used to prepare a series of Th(IV) aryloxide complexes in high yields: Th(OArH)4 (OArH = OC6H3tBu2-2,6), Th(OArtBu)4 (OArtBu = OC6H2tBu3-2,4,6), Th(OArOMe)4 (OArOMe = OC6H2tBu2-2,6-OMe-4), and Th(OArPh)4 (OArPh = OC6H2tBu2-2,6-Ph-4). Th(OArH)4 can be reduced by KC8, Na, or Li in the absence or presence of 2.2.2-cryptand (crypt) or 18-crown-6 (crown) to form dark purple solutions that have EPR and UV-visible spectra similar to those of the square planar Th(III) complex, [Th(OArMe)4]1-. Hence, the para position of the aryloxide ligand does not have to be alkylated to obtain the Th(III) complexes. Furthermore, reduction of Th(OArOMe)4, Th(OArtBu)4, and Th(OArPh)4 with KC8 in THF generated purple solutions with EPR and UV-visible spectra that are similar to those of the previously reported Th(III) anion, [Th(OArMe)4]1-. Although many of these reduction reactions did not produce single crystals suitable for study by X-ray diffraction, reduction of Th(OArH)4, Th(OArtBu)4, and Th(OArOMe)4 with Li provided X-ray quality crystals whose structures had square planar coordination geometries. Reduction of Th(OArPh)4 with Li also gave a product with EPR and UV-visible spectra that matched those of [Th(OArMe)4]1-, but X-ray quality crystals of the reduction product were too unstable to provide data. Neither Th(Odipp)4(THF)2 (Odipp = OC6H3iPr2-2,6) nor Th(Odmp)4(THF)2 (Odmp = OC6H3Me2-2,6) could be reduced to Th(III) products under similar conditions. Reduction of U(OArH)3(THF) with KC8 in the presence of 2.2.2-cryptand (crypt) was examined for comparison and formed [K(crypt)][U(OArH)4], which has a tetrahedral arrangement of the aryloxide ligands. Moreover, no further reduction was observed when either [K(crypt)][U(OArH)4] or [K(crown)(THF)2][U(OArH)4] were treated with KC8 or Li.

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

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 [(KOSiR2 Ar)3 -arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(Ot Bu)2 Ar)3 -arene)(THF)2 ] (2OtBu ) and [K(2.2.2-cryptand)][Th((OSiPh2 Ar)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 (N2 O, COT, CHT, Ph2 N2 , Ph3 PS 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.

Synthesis, Characterization, and Reduction of Thorium Pyridinediimine Complexes

A family of thorium complexes featuring the redox-noninnocent pyridinediimine ligand MesPDIMe was synthesized, including (MesPDIMe)ThCl4 (1-Th), (MesPDIMe)ThCl3(THF) (2-Th), (MesPDIMe)ThCl2(THF)2 (3-Th) and [(MesPDIMe)Th(THF)]2 (5-Th) Full characterization of these species shows that these complexes feature MesPDIMe in four different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, X-ray crystallography, and SQUID magnetometry where appropriate.

N-aryloxide-amidinate group 4 metal complexes

A N-aryloxide-amidine ligand (H3L), integrating phenoxide (PhO-) and amidine ligands through methylene linkers, has been synthesized from 2-(aminomethyl)-6-(tert-butyl)phenol in two steps. Upon reacting the deprotonated H3L ligand with group 4 metal chloride MIVCl4, a corresponding (LMIV-Cl)2 dimer could be obtained. The coordination modes exhibit variation depending on the radius of the metal ions. In the case of (LTiIV-Cl)2, the two ArO- arms from the same ligand bond to two different Ti(IV) centers, while in the case of (LZrIV/HfIV-Cl)2, both ArO- arms coordinate with the same metal center. Moreover, the two C-N bonds in the amidinate moiety are localized in (LTiIV-Cl)2, whereas they delocalize in (LZrIV-Cl)2. Notably, (LHfIV-Cl)2 could further react with one equivalent of HfCl4, yielding the binuclear metal azide in the presence of KN3 and LiCl, where the coordination mode of the amidinate moiety changed from the bidentate chelating type to the bimetallic bridging coordination.

Actinide-Oxygen Multiple Bonds from Air: Synthesis and Characterization of a Thorium Oxo Supported by Redox-Active Ligands

The first non-uranyl, f-element oxo complex synthesized from dioxygen in dry air is presented in this work. The synthesis was accomplished by treating the redox-active thorium amidophenolate complex, [Th(^dippap)₃][K(15-c-5)₂]₂ (1-ap crown), with dioxygen in dry air, forming a rare terminal thorium oxo, [O═Th(^dippisq)₂(^dippap)][K(15-c-5)₂]₂ (2-oxo). Compound 1-ap crown was regenerated by treating 2-oxo with potassium graphite. X-ray crystallography of 2-oxo revealed a comparatively longer bond length for the thorium-oxygen double bond when compared to other thorium oxos. As such, several thorium-oxygen single bonds were synthesized for comparison, including Th(^dippisq)₂(OSiMe₃)₂(THF) (4-OSiMe₃), Th(OSiMe₃)₄(bipy)₂ (5-OSiMe₃), and [Th(OH)₂ (^dippHap)₄][K(15-c-5)₂]₂ (6-OH). Full spectroscopic and structural characterization of the complexes was performed via ¹H NMR spectroscopy, X-ray crystallography, EPR spectroscopy, and electronic absorption spectroscopy as well as SQUID magnetometry, which all confirmed the electronic structure of these complexes.

Intrinsic reactivity of [η5-1,3-(Me3Si)2C5H3]2U(η4-C4Ph2) in small molecule activation

The uranium metallacyclocumulene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(η⁴-C₄Ph₂) (3) was isolated from the reaction mixture containing [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1), potassium graphite (KC₈) and 1,4-diphenylbutadiyne (PhCC-CCPh) in good yield. The reactivity of 3 towards various small organic molecules was evaluated. For example, while complex 3 shows no reactivity towards alkynes and 2,2'-bipyridine, it may deliver the [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se) and Ph₃CN₃, or react as a nucleophile in the presence of carbodiimides, isothiocyanates, aldehydes, ketones, and pyridine derivatives, forming five-, seven- or nine-membered heterometallacycles. On the contrary, addition of Ph₂CS to 3 induces CS bond cleavage yielding the dithiolate complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[S₂(C₁₂H₅Ph₅)] (14). In contrast, the closely related, but sterically more encumbered uranium metallacyclocumulene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(η⁴-C₄Ph₂) (4) features a more limited reactivity which is restricted to mono- and double insertions with small unsaturated organic molecules such as isothiocyanates, ketones and nitriles.

Small-Molecule Activation Mediated by [η5-1,3-(Me3Si)2C5H3]2U(bipy)

The uranium bipyridyl metallocene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(bipy) (2), is readily accessible in good yield by adding potassium graphite (KC₈) to a mixture of [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1) and 2,2'-bipyridine. Compound 2 was fully characterized and employed for small-molecule activation. It has been demonstrated that 2 may serve as a synthon for [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se), alkynes, and a variety of hetero-unsaturated molecules such as diazabutadienes, azine (Ph₂C═N)₂, o-benzoquinone, pyridine N-oxide, CS₂, isothiocyanates, and organic azides. However, upon exposure of 2 to thio-ketone Ph₂CS, aldehyde p-MePhCHO, ketone Ph₂CO, imine PhCH═NPh, azine (PhCH═N)₂, and nitrile PhCN, it may also promote C-C coupling reactions forming [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CS)] (16), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(p-MePhCHO)] (17), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CO)] (18), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNPh)] (19), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNN═CHPh)] (20), and [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(N₂C₁₀H₇C(Ph)NH)] (22), respectively, in quantitative conversion. Furthermore, in the presence of CuI, a single-electron transfer (SET) process is observed to yield the uranium(III) iodide complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(I)(bipy) (15).