104 research outputs found
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 bisÂ(trimethyl)Âsilylamide
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 actinideânitrogen 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 CâH bonds. Complexes with up to three uraniumânitrogen
double bonds are now being widely studied, yet those with one thoriumânitrogen
double bond are rare, and those with two are unknown. A new, simple
monoÂ(imido) thorium complex and the first bisÂ(imido) thorium complex,
KÂ[ThÂ(î»NAr)ÂNâł<sub>3</sub>] and K<sub>2</sub>[ThÂ(î»NAr)<sub>2</sub>Nâł<sub>2</sub>], are readily made from insertion reactions
(Ar = aryl, Nâł = NÂ(SiMe<sub>3</sub>)<sub>2</sub>) into the
ThâC bond of the cyclometalated thorium amides [ThNâł<sub>2</sub>(NÂ(SiMe<sub>3</sub>)Â(SiMe<sub>2</sub>CH<sub>2</sub>))] and
KÂ[ThNâł(NÂ(SiMe<sub>3</sub>)Â(SiMe<sub>2</sub>CH<sub>2</sub>))<sub>2</sub>]. X-ray and computational structural analyses show a âtransition-metal-likeâ <i>cis</i>-bisÂ(imido) geometry and polarized Thî»N bonds
with twice the Wiberg bond order of the formally single ThâN
bond in the same molecule
Thorium Mono- and Bis(imido) Complexes Made by Reprotonation of <i>cyclo</i>-Metalated Amides
Molecules
containing actinideânitrogen 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 CâH bonds. Complexes with up to three uraniumânitrogen
double bonds are now being widely studied, yet those with one thoriumânitrogen
double bond are rare, and those with two are unknown. A new, simple
monoÂ(imido) thorium complex and the first bisÂ(imido) thorium complex,
KÂ[ThÂ(î»NAr)ÂNâł<sub>3</sub>] and K<sub>2</sub>[ThÂ(î»NAr)<sub>2</sub>Nâł<sub>2</sub>], are readily made from insertion reactions
(Ar = aryl, Nâł = NÂ(SiMe<sub>3</sub>)<sub>2</sub>) into the
ThâC bond of the cyclometalated thorium amides [ThNâł<sub>2</sub>(NÂ(SiMe<sub>3</sub>)Â(SiMe<sub>2</sub>CH<sub>2</sub>))] and
KÂ[ThNâł(NÂ(SiMe<sub>3</sub>)Â(SiMe<sub>2</sub>CH<sub>2</sub>))<sub>2</sub>]. X-ray and computational structural analyses show a âtransition-metal-likeâ <i>cis</i>-bisÂ(imido) geometry and polarized Thî»N bonds
with twice the Wiberg bond order of the formally single ThâN
bond in the same molecule
Recommended from our members
Activation of White Phosphorus by Low-Valent Group 5 Complexes: Formation and Reactivity of <i>cyclo</i>-P<sub>4</sub> 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 (P<sub>4</sub>) reacts with niobiumÂ(III) and tantalumÂ(III) ÎČ-diketiminate
(BDI) <i>tert</i>-butylimido complexes to produce the bridging <i>cyclo</i>-P<sub>4</sub> phosphide species {[(BDI)Â(N<sup>t</sup>Bu)ÂM]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)} (<b>1</b>, M = Nb; <b>2</b>, M
= Ta) in fair yields. <b>1</b> is alternatively synthesized
upon hydrogenolysis of (BDI)ÂNbÂ(N<sup>t</sup>Bu)ÂMe<sub>2</sub> in the
presence of P<sub>4</sub>. The trinuclear side product {[(BDI)ÂNbN<sup>t</sup>Bu]<sub>3</sub>Â(ÎŒâP<sub>12</sub>)} (<b>3</b>) is also identified. Protonation of <b>1</b> with
[HOEt<sub>2</sub>]Â[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] does not occur at the phosphide ring but rather involves the BDI
ligand to yield {[(BDI<sup>#</sup>)ÂNbÂ(N<sup>t</sup>Bu)]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)}Â[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sub>2</sub> (<b>4</b>). The monocation and dication analogues {[(BDI)Â(N<sup>t</sup>Bu)ÂNb]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)}Â{BÂ(Ar<sup>F</sup>)<sub>4</sub>}<sub><i>n</i></sub> (<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<sup>F</sup>. DFT calculations were used in combination
with EPR and UVâvisible spectroscopies to probe the nature
of the metalâphosphorus bonding
A New Supporting Ligand in Actinide Chemistry Leads to Reactive Bis(NHC)borate-Supported Thorium Complexes
A versatile,
monoanionic, chelating (bis)Âcarbene 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 ThÂ(III)-bpy(â1)
(f<sup>1</sup>Ï*<sup>1</sup>) 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<sub>H</sub>O)ÂInCl]<sub>2</sub>(ÎŒ-Cl)Â(ÎŒ-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><sub>rel</sub> value of âŒ14,
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>-P<sub>4</sub> 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 (P<sub>4</sub>) reacts with niobiumÂ(III) and tantalumÂ(III) ÎČ-diketiminate
(BDI) <i>tert</i>-butylimido complexes to produce the bridging <i>cyclo</i>-P<sub>4</sub> phosphide species {[(BDI)Â(N<sup>t</sup>Bu)ÂM]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)} (<b>1</b>, M = Nb; <b>2</b>, M
= Ta) in fair yields. <b>1</b> is alternatively synthesized
upon hydrogenolysis of (BDI)ÂNbÂ(N<sup>t</sup>Bu)ÂMe<sub>2</sub> in the
presence of P<sub>4</sub>. The trinuclear side product {[(BDI)ÂNbN<sup>t</sup>Bu]<sub>3</sub>Â(ÎŒâP<sub>12</sub>)} (<b>3</b>) is also identified. Protonation of <b>1</b> with
[HOEt<sub>2</sub>]Â[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] does not occur at the phosphide ring but rather involves the BDI
ligand to yield {[(BDI<sup>#</sup>)ÂNbÂ(N<sup>t</sup>Bu)]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)}Â[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sub>2</sub> (<b>4</b>). The monocation and dication analogues {[(BDI)Â(N<sup>t</sup>Bu)ÂNb]<sub>2</sub>Â(ÎŒâη<sup>3</sup>:η<sup>3</sup>P<sub>4</sub>)}Â{BÂ(Ar<sup>F</sup>)<sub>4</sub>}<sub><i>n</i></sub> (<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<sup>F</sup>. DFT calculations were used in combination
with EPR and UVâvisible spectroscopies to probe the nature
of the metalâphosphorus bonding
Cationic Zirconium Hydrides Supported by an NNNN-Type Macrocyclic Ligand: Synthesis, Structure, and Reactivity
An air- and light-sensitive, but thermally stable trisÂ[(trimethylsilyl)Âmethyl]Âzirconium
complex containing an NNNN-type macrocyclic ligand [ZrÂ(Me<sub>3</sub>TACD)Â(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>] (<b>1</b>; Me<sub>3</sub>TACD = Me<sub>3</sub>[12]ÂaneN<sub>4</sub>: 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane)
was prepared by reacting [ZrÂ(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>4</sub>] with (Me<sub>3</sub>TACD)ÂH. Reaction of the zirconium trisÂ(alkyl) <b>1</b> with a Lewis or BrĂžnsted acid gave a dialkyl cation
with a weakly coordinating anion [ZrÂ(Me<sub>3</sub>TACD)Â(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>]Â[A] [A = AlÂ{OCÂ(CF<sub>3</sub>)<sub>3</sub>}<sub>4</sub> (<b>2a</b>), BÂ{3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>}<sub>4</sub> (<b>2b</b>), BÂ(3,5-C<sub>6</sub>H<sub>3</sub>Cl<sub>2</sub>)<sub>4</sub> (<b>2c</b>), and BPh<sub>4</sub>) (<b>2d</b>)]. Hydrogenolysis
of <b>2a</b>â<b>2c</b> resulted in the formation
of the dinuclear tetrahydride dication [{ZrÂ(Me<sub>3</sub>TACD)Â(ÎŒ-H)<sub>2</sub>}<sub>2</sub>]Â[A]<sub>2</sub> (<b>3a</b>â<b>3c</b>). Compounds <b>1</b>â<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 {Zr<sub>2</sub>(Ό-H)<sub>4</sub>} core in solution and in the solid state with a relatively
short Zr···Zr distance of 2.8752(11) Ă
. Density
functional theory computations at the B3PW91 level reproduced this
structure (Zr···Zr distance of 2.900 Ă
). 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 <sup>13</sup>C NMR spectra
of the reaction mixture using <sup>13</sup>CO, 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)<sub>2</sub>Â(ÎŒ<sub>2</sub>-CH<sub>2</sub>)Â(ÎŒ<sub>2</sub>-PDIPP)] (L = [MeCÂ(NDIPP)ÂCHCÂ(NDIPP)ÂMe]<sup>â</sup>, DIPP = 2,6-(<sup><i>i</i></sup>Pr)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) (<b>2</b>), has been synthesized,
and its reactivity has been investigated. Reaction of scandium methyl
phosphide [LScÂ(Me)Â{PÂ(H)ÂDIPP}] with 1 equiv of scandium dimethyl
complex [LScMe<sub>2</sub>] 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 CO<sub>2</sub>, CS<sub>2</sub>, benzonitrile, and <i>tert</i>-butyl
isocyanide. In the above reactions, the unsaturated substrates insert
into the ScâCÂ(methylidene) bond to give some interesting dianionic
ligands while the ScâPÂ(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 methylÂidene ligand
New Mechanism for the Ring-Opening Polymerization of Lactones? Uranyl Aryloxide-Induced Intermolecular Catalysis
The uranyl aryloxide [UO<sub>2</sub>(OAr)<sub>2</sub>(THF)<sub>2</sub>] (Ar = 2,6-<sup><i>t</i></sup>Bu<sub>2</sub>-C<sub>6</sub>H<sub>2</sub>) is an active catalyst
for the ring-opening <i>cyclo</i>-oligomerization of Δ-caprolactone
and ÎŽ-valerolactone but not for ÎČ-butyrolactone, Îł-butyrolactone,
and <i>rac</i>-lactide. <sup>1</sup>H 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 CâH···Ï
interactions stabilize the coordination of the less rigid monomers
- âŠ