Influence of 1,2,4-Tri-tert-butylcyclopentadienyl Ligand on the Reactivity of the Thorium Bipyridyl Metallocene [η5-1,2,4-(Me3C)3C5H2]2Th(bipy)]

The thorium bipyridyl metallocene (Cp3tBu)2Th(bipy) (1; Cp3tBu = η5-1,2,4-(Me3C)3C5H2) shows a rich reactivity toward a series of small molecules. For example, complex 1 may act as a synthon for the (Cp3tBu)2Th(II) fragment as illustrated by its reactivity toward to CuI, hydrazine derivative (PhNH)2, Ph2E2 (E = S, Se), elemental sulfur (S8) and selenium (Se), organic azides, CS2, and isothiocyanates. Moreover, in the presence of polar multiple bonds, such as those in ketones Ph2CO and (CH2)5CO, aldehydes p-MePhCHO and p-ClPhCHO, seleno-ketone (p-MeOPh)2CSe, nitriles PhCN, Ph2CHCN, C6H11CN, and p-(NC)2Ph, and benzoyl cyanide PhCOCN, C-C coupling occurs to furnish (Cp3tBu)2Th[(bipy)(Ph2CO)] (10), (Cp3tBu)2Th[(bipy)((CH2)5CO)] (11), (Cp3tBu)2Th[(bipy)(p-MePhCHO)] (12), (Cp3tBu)2Th[(bipy)(p-ClPhCHO)] (13), (Cp3tBu)2Th[(bipy){(p-MeOPh)2CSe}] (14), (Cp3tBu)2Th[(bipy)(PhCN)] (16), (Cp3tBu)2Th[(bipy)(Ph2CHCN)] (17), (Cp3tBu)2Th[(bipy)(C6H11CN)] (18), [(Cp3tBu)2Th]2{μ-(bipy)[p-Ph(CN)2](bipy)} (20), and (Cp3tBu)2Th{(bipy)[PhC(CN)O]} (21), respectively. Nevertheless, ketazine (PhCH═N)2 or benzyl nitrile PhCH2CN forms the dimeric complexes [(Cp3tBu)Th]2[μ-NC(Ph)(bipy)]2 (15) and (Cp3tBu)2Th[(bipy){C(═CHPh)NH}] (19), respectively. In contrast, C-N bond cleavage and C-C coupling processes occur upon addition of isonitriles Me3CNC and C6H11NC to 1 to yield the thorium isocyanido amido complexes (Cp3tBu)2Th[4-(Me3C)bipy](NC) (22) and (Cp3tBu)2Th[4-(C6H11)bipy](NC) (23), respectively. Furthermore, a single-electron transfer (SET) process ensues when 1 equiv of CuI is added to 1 to yield the Th(VI) bipyridyl iodide complex (Cp3tBu)2Th(I)(bipy) (3).

Elucidating the Impact of Rare Earth or Transition Metal Identity on the Physical and Electronic Structural Properties of a Series of Redox-Active Tris(amido) Complexes

The use of redox-active ligands with the f-block elements has been employed to promote unique chemical transformations and explore their unique emergent electronic properties for a myriad of applications. In this study, we report eight new tris(amido) metal complexes: 1-Ln (Ln = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+), 1-La, and 1-Ti (an early transition metal analogue). The one-electron oxidation of the tris(amido) ligand was conducted to generate semi-iminato complexes 2-Ln, 2-La, and 2-Ti, and these complexes were studied using EPR. Tris(amido) complexes 1-Ln, 1-La, and 1-Ti were fully characterized using a range of spectroscopic (NMR and UV-vis/NIR) and physical techniques (X-ray diffraction and cyclic voltammetry, with the exception of 1-La). Computational methods were employed to further elucidate the electronic structures of these complexes. Lastly, complexes 1-Ln, 1-La, and 1-Ti were probed as catalysts for alkyl-alkyl cross-coupling, and the initial rate of the reaction was measured to explore the influence of the metal ion.

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.

N-Aryloxide-Amidinate Thorium Complexes

An N-aryloxide-amidine ligand (1), [ONNO] ligand, integrating phenoxide (PhO-) and amidine ligands through methylene linkers, was employed in actinide chemistry. Upon reaction of the deprotonated ligand with ThCl4(DME)2 in ether, the corresponding dimer complex 2 was obtained. Upon treatment of 2 with KCp* (Cp* = Cp(Me)5) in tetrahydrofuran, the corresponding {[ONNO]ThIVCp*(LiCl)}2 (4) was obtained. In complex 2, the two ArO- arms bonded from the same ligand to different ThIV centers. In contrast, both ArO- arms coordinated to the same metal center in 4. Notably, when a mixture of 2 and bipyridine was treated with one or two equiv of KC8, the [ONNO]ThIV-bipyridyl•̅ radical dimer complex (5) and [ONNO]ThIV-bipyridyl2- dianionic dimer species (6) were obtained, respectively.

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.

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as ThII Synthons

Thorium redox chemistry is extremely scarce due to the high stability of Th^IV . Here we report two unique examples of thorium arenide complexes prepared by reduction of a Th^IV -siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(O^t Bu)₃ )₃ Th(η⁶ -C₁₀ H₈ )] (1) and the inverse-sandwich complex [K(OSi(O^t Bu)₃ )₃ Th]₂ (μ-η⁶ ,η⁶ -C₁₀ H₈ )] (2). The electrons stored in these complexes allow the reduction of a broad range of substrates (N₂ O, AdN₃ , CO₂ , HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN₂ to yield the unexpected Th^IV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.

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).

Pentanuclear Thorium(IV) Coordination Cluster from the Use of Di(2-pyridyl) Ketone

The Th(NO₃)₄·5H₂O/di(2-pyridyl) ketone [(py)₂CO] reaction system gives a pentanuclear cluster containing the doubly deprotonated form of the gem-diol derivative of the ligand. The cluster consists of a tetrahedral arrangement of four Th^IV ions centered on the fifth ion, which is the first characterized Th^IV₅ complex. The analysis of its structure reveals that this is a Kuratowski-type coordination compound.

Metrical Oxidation States of 1,4-Diazadiene-Derived Ligands

The conventional method of assigning formal oxidation states (FOSs) to metals and ligands is an important tool for understanding and predicting the chemical reactivity, in particular, in catalysis research. For complexes containing redox-noninnocent ligands, the oxidation state of the ligand can be ambiguous (i.e., their spectroscopic oxidation state can differ from the FOS) and thus frustrates the assignment of the oxidation state of the metal. A quantitative correlation between the empirical metric data of redox-active ligands and their oxidation states using a metrical oxidation state (MOS) model has been developed for catecholate- and amidophenoxide-derived ligands by Brown. In the present work, we present a MOS model for 1,4-diazabutadiene (DADⁿ) ligands. This model is based on a similar approach as reported by Brown, correlating the intra-ligand bond lengths of the DADⁿ moiety in a quantitative manner with the MOS using geometrical information from X-ray structures in the Cambridge Crystallographic Data Center (CCDC) database. However, an accurate determination of the MOS of these ligands turned out to be dependent on the coordination mode of the DAD^2– moiety, which can adopt both a planar κ²-N₂-geometry and a η⁴-N₂C₂ π-coordination mode in (transition) metal complexes in its doubly reduced, dianionic enediamide oxidation state. A reliable MOS model was developed taking the intrinsic differences in intra-ligand bond distances between these coordination modes of the DAD^2– ligand into account. Three different models were defined and tested using different geometric parameters (C=C → M distance, M–N–C angle, and M–N–C–C torsion angle) to describe the C=C backbone coordination with the metal in the η⁴-N₂-C₂ π-coordination mode of the DAD^2– ligand. Statistical analysis revealed that the C=C → M distance best describes the η⁴-N₂-C₂ coordination mode using a cutoff value of 2.46 Å for π-coordination. The developed MOS model was used to validate the oxidation state assignment of elements not contained within the training set (Sr, Yb, and Ho), thus demonstrating the applicability of the MOS model to a wide range of complexes. Chromium complexes with complex electronic structures were also shown to be accurately described by MOS analysis. Furthermore, it is shown that a combination of MOS analysis and FOD calculations provides an inexpensive method to gain insight into the electronic structure of singlet spin state (S = 0) [M(trop₂dad)] transition-metal complexes showing (potential) singlet biradical character.

Assigning oxidation states to metals and ligands is an important tool for understanding and predicting the chemical reactivity. For complexes containing redox-noninnocent ligands, the oxidation state of the ligand can be ambiguous. We present a metrical oxidation state model for 1,4-diazabutadiene ligands, correlating the intra-ligand bond lengths with the oxidation state using information from X-ray structures. This model accounts for the difference in bond length distances between the different coordination modes of the fully reduced ligand.

White phosphorus activation by a Th(iii) complex

Lappert's original Th(iii) complex, [Th{C₅H₃(SiMe₃)₂-1,3}₃], reduces white phosphorus to give a cyclo-P₄ dianion, which exhibits an unprecedented μ–η¹:η¹-binding mode in the dinuclear Th(iv) product.

The ligand redox behavior and role in 1,2-bis[(2,6-diisopropylphenyl)imino]-acenaphthene nickel–TMA(MAO) systems for ethylene polymerization

The BIAN ligand in the BIAN nickel–TMA(MAO) system is redox-active and plays a more important role than just acting as a neutral ligand.

Synthesis, Spectroscopy, and Electrochemistry of (α-Diimine)M(CO)3Br, M = Mn, Re, Complexes: Ligands Isoelectronic to Bipyridyl Show Differences in CO2 Reduction

The synthesis and characterization of new Mn(I)- and Re(I)-centered organometallic complexes fashioned with 1,4-diazabutadiene (DAB) ligands is reported. Ten compounds of the type fac-(α-diimine)M(CO)3Br (M = Mn, Re) were obtained in moderate to excellent yield (35-80%) and high purity from the coordination of the five ligands with M(CO)5Br in refluxing ethanol. Despite the electronic similarity of DAB to 2,2'-bipyridyl, the complexes described herein were poor mediators of electrochemical CO2 conversion to CO, but provide insight into the role of redox-active ligands in catalysis. Additional characterization of the one-electron reduced rhenium compounds, relevant intermediates in CO2 reduction, by EPR and single-crystal X-ray analysis is described.