Effects of the Grafting of Lanthanum Complexes on a Silica Surface on the Reactivity: Influence on Ethylene, Propylene, and 1,3-Butadiene Homopolymerization

In this contribution, we report full details of the ethylene, 1,3-butadiene, and propylene homopolymerization processes mediated by alkylated bis­(trimethyl)­silylamide lanthanide-grafted complexes using a density functional theory (DFT) study of the initiation and first propagation steps. These systems allows us (i) to examine the role of the grafting mode on the kinetics and thermodynamics of the three processes considered, (ii) to confirm the catalytic behavior of these grafted complexes in ethylene polymerization, (iii) to rationalize the experimental preference for 1,4-cis polymerization of 1,3-butadiene, and (iv) to provide unprecedented information on the catalytic activity of the lanthanide-grafted complex as a propylene hompolymerization catalyst

Thorium Mono- and Bis(imido) Complexes Made by Reprotonation of <i>cyclo</i>-Metalated Amides

Molecules containing actinide–nitrogen multiple bonds are of current interest as simple models for new actinide nitride nuclear fuels, and for their potential for the catalytic activation of inert hydrocarbon C–H bonds. Complexes with up to three uranium–nitrogen double bonds are now being widely studied, yet those with one thorium–nitrogen double bond are rare, and those with two are unknown. A new, simple mono­(imido) thorium complex and the first bis­(imido) thorium complex, K­[Th­(NAr)­N″₃] and K₂[Th­(NAr)₂N″₂], are readily made from insertion reactions (Ar = aryl, N″ = N­(SiMe₃)₂) into the Th–C bond of the cyclometalated thorium amides [ThN″₂(N­(SiMe₃)­(SiMe₂CH₂))] and K­[ThN″(N­(SiMe₃)­(SiMe₂CH₂))₂]. X-ray and computational structural analyses show a “transition-metal-like” <i>cis</i>-bis­(imido) geometry and polarized ThN bonds with twice the Wiberg bond order of the formally single Th–N bond in the same molecule

Thorium Mono- and Bis(imido) Complexes Made by Reprotonation of <i>cyclo</i>-Metalated Amides

Molecules containing actinide–nitrogen multiple bonds are of current interest as simple models for new actinide nitride nuclear fuels, and for their potential for the catalytic activation of inert hydrocarbon C–H bonds. Complexes with up to three uranium–nitrogen double bonds are now being widely studied, yet those with one thorium–nitrogen double bond are rare, and those with two are unknown. A new, simple mono­(imido) thorium complex and the first bis­(imido) thorium complex, K­[Th­(NAr)­N″₃] and K₂[Th­(NAr)₂N″₂], are readily made from insertion reactions (Ar = aryl, N″ = N­(SiMe₃)₂) into the Th–C bond of the cyclometalated thorium amides [ThN″₂(N­(SiMe₃)­(SiMe₂CH₂))] and K­[ThN″(N­(SiMe₃)­(SiMe₂CH₂))₂]. X-ray and computational structural analyses show a “transition-metal-like” <i>cis</i>-bis­(imido) geometry and polarized ThN bonds with twice the Wiberg bond order of the formally single Th–N bond in the same molecule

Activation of White Phosphorus by Low-Valent Group 5 Complexes: Formation and Reactivity of <i>cyclo</i>-P₄ Inverted Sandwich Compounds

We report the synthesis and comprehensive study of the electronic structure of a unique series of dinuclear group 5 <i>cyclo</i>-tetraphosphide inverted sandwich complexes. White phosphorus (P₄) reacts with niobium­(III) and tantalum­(III) β-diketiminate (BDI) <i>tert</i>-butylimido complexes to produce the bridging <i>cyclo</i>-P₄ phosphide species {[(BDI)­(N^tBu)­M]₂­(μ‑η³:η³P₄)} (<b>1</b>, M = Nb; <b>2</b>, M = Ta) in fair yields. <b>1</b> is alternatively synthesized upon hydrogenolysis of (BDI)­Nb­(N^tBu)­Me₂ in the presence of P₄. The trinuclear side product {[(BDI)­NbN^tBu]₃­(μ‑P₁₂)} (<b>3</b>) is also identified. Protonation of <b>1</b> with [HOEt₂]­[B­(C₆F₅)₄] does not occur at the phosphide ring but rather involves the BDI ligand to yield {[(BDI^#)­Nb­(N^tBu)]₂­(μ‑η³:η³P₄)}­[B­(C₆F₅)₄]₂ (<b>4</b>). The monocation and dication analogues {[(BDI)­(N^tBu)­Nb]₂­(μ‑η³:η³P₄)}­{B­(Ar^F)₄}_<i>n</i> (<b>5</b>, <i>n</i> = 1; <b>6</b>, <i>n</i> = 2) are both synthesized by oxidation of <b>1</b> with AgBAr^F. DFT calculations were used in combination with EPR and UV–visible spectroscopies to probe the nature of the metal–phosphorus bonding

A New Supporting Ligand in Actinide Chemistry Leads to Reactive Bis(NHC)borate-Supported Thorium Complexes

A versatile, monoanionic, chelating (bis)­carbene ligand (<b>2</b>) was used to prepare a thorium dihalide complex (<b>3</b>) and a direduced-bpy derivative (<b>4</b>). CASSCF calculations suggest the involvement of a multiconfigurational open-shell singlet, with the main configuration corresponding to a Th­(III)-bpy(−1) (f¹π*¹) electronic structure. The reactivity of <b>4</b> was explored in various transformations, including reactions with carbonyls and organic azides; the latter gave rise to an unusual terminal Th-imido bpy complex (<b>6</b>)

Theoretical Investigation of Lactide Ring-Opening Polymerization Induced by a Dinuclear Indium Catalyst

A DFT study of the ring-opening polymerization of lactide (LA) induced by a dinuclear indium catalyst supported by a chiral diamino phenoxy ligand, [(NN_HO)­InCl]₂(μ-Cl)­(μ-OEt) (<b>1</b>), is reported. The nature of the active catalyst, mononuclear vs dinuclear, was investigated and was shown to be dinuclear because of the high energetic cost of its dissociation. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (<i>R,R</i>/<i>R,R</i>)-<b>1</b> catalyst. In complete agreement with experimental results we observed that (1) selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization, (2) a slight kinetic preference for the polymerization of l-LA over d-LA is found that translates to a <i>k</i>_rel value of ∼14, which is identical with the experimental value, and (3) when <i>rac</i>-LA is used, no clear preference for d- vs l-LA insertion is found, leading to isotactic PLA

Activation of White Phosphorus by Low-Valent Group 5 Complexes: Formation and Reactivity of <i>cyclo</i>-P₄ Inverted Sandwich Compounds

We report the synthesis and comprehensive study of the electronic structure of a unique series of dinuclear group 5 <i>cyclo</i>-tetraphosphide inverted sandwich complexes. White phosphorus (P₄) reacts with niobium­(III) and tantalum­(III) β-diketiminate (BDI) <i>tert</i>-butylimido complexes to produce the bridging <i>cyclo</i>-P₄ phosphide species {[(BDI)­(N^tBu)­M]₂­(μ‑η³:η³P₄)} (<b>1</b>, M = Nb; <b>2</b>, M = Ta) in fair yields. <b>1</b> is alternatively synthesized upon hydrogenolysis of (BDI)­Nb­(N^tBu)­Me₂ in the presence of P₄. The trinuclear side product {[(BDI)­NbN^tBu]₃­(μ‑P₁₂)} (<b>3</b>) is also identified. Protonation of <b>1</b> with [HOEt₂]­[B­(C₆F₅)₄] does not occur at the phosphide ring but rather involves the BDI ligand to yield {[(BDI^#)­Nb­(N^tBu)]₂­(μ‑η³:η³P₄)}­[B­(C₆F₅)₄]₂ (<b>4</b>). The monocation and dication analogues {[(BDI)­(N^tBu)­Nb]₂­(μ‑η³:η³P₄)}­{B­(Ar^F)₄}_<i>n</i> (<b>5</b>, <i>n</i> = 1; <b>6</b>, <i>n</i> = 2) are both synthesized by oxidation of <b>1</b> with AgBAr^F. DFT calculations were used in combination with EPR and UV–visible spectroscopies to probe the nature of the metal–phosphorus bonding

Cationic Zirconium Hydrides Supported by an NNNN-Type Macrocyclic Ligand: Synthesis, Structure, and Reactivity

An air- and light-sensitive, but thermally stable tris­[(trimethylsilyl)­methyl]­zirconium complex containing an NNNN-type macrocyclic ligand [Zr­(Me₃TACD)­(CH₂SiMe₃)₃] (<b>1</b>; Me₃TACD = Me₃[12]­aneN₄: 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) was prepared by reacting [Zr­(CH₂SiMe₃)₄] with (Me₃TACD)­H. Reaction of the zirconium tris­(alkyl) <b>1</b> with a Lewis or Brønsted acid gave a dialkyl cation with a weakly coordinating anion [Zr­(Me₃TACD)­(CH₂SiMe₃)₂]­[A] [A = Al­{OC­(CF₃)₃}₄ (<b>2a</b>), B­{3,5-C₆H₃(CF₃)₂}₄ (<b>2b</b>), B­(3,5-C₆H₃Cl₂)₄ (<b>2c</b>), and BPh₄) (<b>2d</b>)]. Hydrogenolysis of <b>2a</b>–<b>2c</b> resulted in the formation of the dinuclear tetrahydride dication [{Zr­(Me₃TACD)­(μ-H)₂}₂]­[A]₂ (<b>3a</b>–<b>3c</b>). Compounds <b>1</b>–<b>3</b> were characterized by multinuclear NMR spectroscopy, and the solid-state structures of <b>1</b>, <b>2c</b>, and <b>3b</b> were established by single-crystal X-ray diffraction studies. The dinuclear hydride complex <b>3b</b> exhibits a quadruply bridged {Zr₂(μ-H)₄} core in solution and in the solid state with a relatively short Zr···Zr distance of 2.8752(11) Å. Density functional theory computations at the B3PW91 level reproduced this structure (Zr···Zr distance of 2.900 Å). The cationic hydride complex <b>3b</b> reacted with excess carbon monoxide in tetrahydrofuran at room temperature to give ethylene in 25% yield based on <b>3b</b>. Upon analysis of ¹³C NMR spectra of the reaction mixture using ¹³CO, oxymethylene and enolate complexes were detected as intermediates among other complexes

A Scandium Complex Bearing Both Methylidene and Phosphinidene Ligands: Synthesis, Structure, and Reactivity

The scandium complex bearing both methylidene and phosphinidene ligands, [(LSc)₂­(μ₂-CH₂)­(μ₂-PDIPP)] (L = [MeC­(NDIPP)­CHC­(NDIPP)­Me]⁻, DIPP = 2,6-(^<i>i</i>Pr)₂C₆H₃) (<b>2</b>), has been synthesized, and its reactivity has been investigated. Reaction of scandium methyl phosphide [LSc­(Me)­{P­(H)­DIPP}] with 1 equiv of scandium dimethyl complex [LScMe₂] in toluene at 60 °C provided complex <b>2</b> in good yield, and the structure of complex <b>2</b> was determined by single-crystal X-ray diffraction. Complex <b>2</b> easily undergoes nucleophilic addition reactions with CO₂, CS₂, benzonitrile, and <i>tert</i>-butyl isocyanide. In the above reactions, the unsaturated substrates insert into the Sc–C­(methylidene) bond to give some interesting dianionic ligands while the Sc–P­(phosphinidene) bond remains untouched. The bonding situation of complex <b>2</b> was analyzed using DFT methods, indicating a more covalent bond between the scandium ion and the phosphinidene ligand than between the scandium ion and the methyl­idene ligand

New Mechanism for the Ring-Opening Polymerization of Lactones? Uranyl Aryloxide-Induced Intermolecular Catalysis

The uranyl aryloxide [UO₂(OAr)₂(THF)₂] (Ar = 2,6-^<i>t</i>Bu₂-C₆H₂) is an active catalyst for the ring-opening <i>cyclo</i>-oligomerization of ε-caprolactone and δ-valerolactone but not for β-butyrolactone, γ-butyrolactone, and <i>rac</i>-lactide. ¹H EXSY measurements give the thermodynamic parameters for exchange of monomer and coordinated THF, and rates of polymerization have been determined. A comprehensive theoretical examination of the mechanism is discussed. From both experiment and theory, the initiation step is intramolecular and in keeping with the accepted mechanism, while computational studies indicate that propagation can go via an intermolecular pathway, which is the first time this has been observed. The lack of polymerization for the inactive monomers has been investigated theoretically and C–H···π interactions stabilize the coordination of the less rigid monomers