22 research outputs found
Rhodium and Iridium Complexes of Bulky Tertiary Phosphine Ligands. Searching for Isolable Cationic M<sup>III</sup> Alkylidenes
Cyclometalated chloride complexes
of rhodium and iridium based on (Ρ<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ÂM<sup>III</sup> fragments that result from the metalation
of the xylyl substituent of a coordinated PR<sub>2</sub>(Xyl) phosphine
(Xyl = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) have been prepared
by reaction of the appropriate metal precursor with the corresponding
phosphine. For iridium, the four complexes <b>1a</b>â<b>d</b>, derived from the phosphines P<sup>i</sup>Pr<sub>2</sub>(Xyl), PCy<sub>2</sub>(Xyl), PMe<sub>2</sub>(Xyl), and PPh<sub>2</sub>(Xyl), respectively, have been prepared, whereas for rhodium only
the complexes <b>2a</b>,<b>d</b>, derived from P<sup>i</sup>Pr<sub>2</sub>(Xyl) and PMe<sub>2</sub>(Xyl), respectively, have
been studied. Chloride abstraction from compounds <b>1</b> and <b>2</b> by NaBAr<sub>F</sub> (BAr<sub>F</sub> = BÂ(3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>) leads to
either cationic dichloromethane adducts or to cationic hydride alkylidene
structures resulting from Îą-H elimination. The rhodium complexes
investigated yield only dichloromethane adducts. However, in the iridium
system the less sterically demanding phosphines PMe<sub>2</sub>(Xyl)
and PPh<sub>2</sub>(Xyl) also provide dichloromethane adducts as the
only observable products, whereas for the bulkier P<sup>i</sup>Pr<sub>2</sub>(Xyl) and PCy<sub>2</sub>(Xyl) ligands the hydride alkylidene
formulation prevails. Nonetheless, variable-temperature NMR studies
reveal that in solution each of these two structures exists in equilibrium
with undetectable concentrations of the other by means of facile reversible
Îą-H elimination and migratory insertion reactions. Reactivity
studies on the cationic hydride alkylidene complexes of iridium are
reported as well
EfektivnĂ aritmetika eliptickĂ˝ch kĹivek nad koneÄnĂ˝mi tÄlesy
The thesis deals with arithmetics of elliptic curves over finite fields and methods to improve those calculations. In the first part, algebraic geometry helps to define elliptic curves and derive their basic properties including the group law. The second chapter seeks ways to speed up these calculations by means of time-memory tradeoff, i.e. adding redundancy. At last, the third part introduces a wholly new curve form, which is particularly effective for such purposes
Cationic Ir(III) Alkylidenes Are Key Intermediates in CâH Bond Activation and CâC Bond-Forming Reactions
This work describes the chemical reactivity of a cationic
(Ρ<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ÂIrÂ(III) complex that
contains
a bisÂ(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a Îş<sup>4</sup>-<i>P</i>,<i>C</i>,<i>Câ˛</i>,<i>Câł</i> fashion. The
complex (<b>1</b><sup><b>+</b></sup> in this paper) undergoes
very facile intramolecular CâH bond activation of all benzylic
sites, in all likelihood through an IrÂ(V) hydride intermediate. But
most importantly, it transforms into
a hydride phosphepine species <b>4</b><sup><b>+</b></sup> by means of an also facile, base-catalyzed, intramolecular dehydrogenative
CâC coupling reaction. Mechanistic studies demonstrate the
participation as a key intermediate of an electrophilic cationic IrÂ(III)
alkylidene, which has been characterized by low-temperature NMR spectroscopy
and by isolation of its trimethylphosphonium ylide. DFT calculations
provide theoretical support for these results
Cationic Ir(III) Alkylidenes Are Key Intermediates in CâH Bond Activation and CâC Bond-Forming Reactions
This work describes the chemical reactivity of a cationic
(Ρ<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ÂIrÂ(III) complex that
contains
a bisÂ(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a Îş<sup>4</sup>-<i>P</i>,<i>C</i>,<i>Câ˛</i>,<i>Câł</i> fashion. The
complex (<b>1</b><sup><b>+</b></sup> in this paper) undergoes
very facile intramolecular CâH bond activation of all benzylic
sites, in all likelihood through an IrÂ(V) hydride intermediate. But
most importantly, it transforms into
a hydride phosphepine species <b>4</b><sup><b>+</b></sup> by means of an also facile, base-catalyzed, intramolecular dehydrogenative
CâC coupling reaction. Mechanistic studies demonstrate the
participation as a key intermediate of an electrophilic cationic IrÂ(III)
alkylidene, which has been characterized by low-temperature NMR spectroscopy
and by isolation of its trimethylphosphonium ylide. DFT calculations
provide theoretical support for these results
Living Polymerization of Ethylene and Copolymerization of Ethylene/Methyl Acrylate Using âSandwichâ Diimine Palladium Catalysts
Cationic
PdÂ(II) catalysts incorporating bulky 8-<i>p</i>-tolylnaphthyl
substituted diimine ligands have been synthesized
and investigated for ethylene polymerization and ethylene/methyl acrylate
copolymerization. Homopolymerization of ethylene at room temperature
resulted in branched polyethylene with narrow <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> values (ca. 1.1), indicative
of a living polymerization. A mechanistic study revealed that the
catalyst resting state was an alkyl olefin complex and that the turnover-limiting
step was migratory insertion, thus the turnover frequency is independent
of ethylene concentration. Copolymerization of ethylene and methyl
acrylate (MA) was also achieved. MA incorporation was found to increase
linearly with MA concentration, and copolymers with up to 14 mol %
MA were prepared. Mechanistic studies revealed that acrylate insertion
into a PdâCH<sub>3</sub> bond occurs at â70 °C
to yield a four-membered chelate, which isomerizes first to a five-membered
chelate and then to a six-membered chelate. Barriers to migratory
insertion of both the (diimine)ÂPdCH<sub>3</sub>(C<sub>2</sub>H<sub>4</sub>)<sup>+</sup> (19.2 kcal/mol) and (diimine)ÂPdCH<sub>3</sub>(Ρ<sup>2</sup>-C<sub>2</sub>H<sub>3</sub>CO<sub>2</sub>Me)<sup>+</sup> (15.2 kcal/mol) were measured by low-temperature NMR kinetics
Cyclometalated Iridium Complexes of Bis(Aryl) Phosphine Ligands: Catalytic CâH/CâD Exchanges and CâC Coupling Reactions
This
work details the synthesis and structural identification of
a series of complexes of the (Ρ<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ÂIrÂ(III) unit coordinated to cyclometalated bisÂ(aryl)Âphosphine
ligands, PRâ˛(Ar)<sub>2</sub>, for RⲠ= Me and Ar = 2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>, <b>1b</b>; 2,6-Me<sub>2</sub>-4-OMe-C<sub>6</sub>H<sub>2</sub>, <b>1c</b>; 2,6-Me<sub>2</sub>-4-F-C<sub>6</sub>H<sub>2</sub>, <b>1d</b>; RⲠ= Et,
Ar = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>, <b>1e</b>. Both chloride- and hydride-containing compounds, <b>2b</b>â<b>2e</b> and <b>3b</b>â<b>3e</b>, respectively, are described. Reactions of chlorides <b>2</b> with NaBAr<sub>F</sub> (BAr<sub>F</sub> = BÂ(3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>) in the presence
of CO form cationic carbonyl complexes, <b>4</b><sup><b>+</b></sup>, with νÂ(CO) values in the narrow interval 2030â2040
cm<sup>â1</sup>, indicating similar Ď-basicity of the
IrÂ(III) center of these complexes. In the absence of CO, NaBAr<sub>F</sub> forces Îş<sup>4</sup>-<i>P</i>,<i>C</i>,<i>C</i>â˛,<i>C</i>âł coordination
of the metalated arm (studied for the selected complexes <b>5b</b>, <b>5d</b>, and <b>5e</b>), a binding mode so far encountered
only when the phosphine contains two benzylic groups. A base-catalyzed
intramolecular, dehydrogenative, CâC coupling reaction converts
the Îş<sup>4</sup> species <b>5d</b> and <b>5e</b> into the corresponding hydrido phosphepine complexes <b>6d</b> and <b>6e</b>. Using CD<sub>3</sub>OD as the source of deuterium,
the chlorides <b>2</b> undergo deuteration of their 11 benzylic
positions whereas hydrides <b>3</b> experience only D incorporation
into the IrâH and IrâCH<sub>2</sub> sites. Mechanistic
schemes that explain this diversity have come to light thanks to experimental
and theoretical DFT studies that are also reported
Cyclometalated Iridium Complexes of Bis(Aryl) Phosphine Ligands: Catalytic CâH/CâD Exchanges and CâC Coupling Reactions
This
work details the synthesis and structural identification of
a series of complexes of the (Ρ<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ÂIrÂ(III) unit coordinated to cyclometalated bisÂ(aryl)Âphosphine
ligands, PRâ˛(Ar)<sub>2</sub>, for RⲠ= Me and Ar = 2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>, <b>1b</b>; 2,6-Me<sub>2</sub>-4-OMe-C<sub>6</sub>H<sub>2</sub>, <b>1c</b>; 2,6-Me<sub>2</sub>-4-F-C<sub>6</sub>H<sub>2</sub>, <b>1d</b>; RⲠ= Et,
Ar = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>, <b>1e</b>. Both chloride- and hydride-containing compounds, <b>2b</b>â<b>2e</b> and <b>3b</b>â<b>3e</b>, respectively, are described. Reactions of chlorides <b>2</b> with NaBAr<sub>F</sub> (BAr<sub>F</sub> = BÂ(3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub>) in the presence
of CO form cationic carbonyl complexes, <b>4</b><sup><b>+</b></sup>, with νÂ(CO) values in the narrow interval 2030â2040
cm<sup>â1</sup>, indicating similar Ď-basicity of the
IrÂ(III) center of these complexes. In the absence of CO, NaBAr<sub>F</sub> forces Îş<sup>4</sup>-<i>P</i>,<i>C</i>,<i>C</i>â˛,<i>C</i>âł coordination
of the metalated arm (studied for the selected complexes <b>5b</b>, <b>5d</b>, and <b>5e</b>), a binding mode so far encountered
only when the phosphine contains two benzylic groups. A base-catalyzed
intramolecular, dehydrogenative, CâC coupling reaction converts
the Îş<sup>4</sup> species <b>5d</b> and <b>5e</b> into the corresponding hydrido phosphepine complexes <b>6d</b> and <b>6e</b>. Using CD<sub>3</sub>OD as the source of deuterium,
the chlorides <b>2</b> undergo deuteration of their 11 benzylic
positions whereas hydrides <b>3</b> experience only D incorporation
into the IrâH and IrâCH<sub>2</sub> sites. Mechanistic
schemes that explain this diversity have come to light thanks to experimental
and theoretical DFT studies that are also reported
Distortional Effects of Noncovalent Interactions in the Crystal Lattice of a Cp*Ir(III) Acylhydroxamic Acid Complex: A Joint ExperimentalâComputational Study
[Cp*IrÂ(Îź-OH)<sub>3</sub>IrCp*]ÂOH
reacts with PhCONHOH to
give [Cp*IrÂ(Ρ<sup>2</sup>-ONCOPh)], in which the doubly deprotonated
âNHOH unit binds side-on via N and O, an otherwise unrecorded
binding mode. The X-ray structure shows pyramidalization at Ir together
with secondary bonding between the carbonyl oxygen and Ir (<i>d</i><sub>Ir¡¡¡O</sub> = 2.873(8) Ă
). The
related <i>o</i>-hydroxyphenylÂhydroxamic acid gives
a conventional chelate structure in which both sp<sup>3</sup> O atoms
are bound in deprotonated form. In contrast, PhSO<sub>2</sub>NHOH
reacts with SâN cleavage to give the nitrosyl, [Cp*IrÂ(NO)Â(SO<sub>2</sub>Ph)]. A detailed computational analysis identifies noncovalent
interactions in the crystal lattice (crystal-packing effects) as responsible
for the distortion in [Cp*IrÂ(Ρ<sup>2</sup>-ONCOPh)]
Distortional Effects of Noncovalent Interactions in the Crystal Lattice of a Cp*Ir(III) Acylhydroxamic Acid Complex: A Joint ExperimentalâComputational Study
[Cp*IrÂ(Îź-OH)<sub>3</sub>IrCp*]ÂOH
reacts with PhCONHOH to
give [Cp*IrÂ(Ρ<sup>2</sup>-ONCOPh)], in which the doubly deprotonated
âNHOH unit binds side-on via N and O, an otherwise unrecorded
binding mode. The X-ray structure shows pyramidalization at Ir together
with secondary bonding between the carbonyl oxygen and Ir (<i>d</i><sub>Ir¡¡¡O</sub> = 2.873(8) Ă
). The
related <i>o</i>-hydroxyphenylÂhydroxamic acid gives
a conventional chelate structure in which both sp<sup>3</sup> O atoms
are bound in deprotonated form. In contrast, PhSO<sub>2</sub>NHOH
reacts with SâN cleavage to give the nitrosyl, [Cp*IrÂ(NO)Â(SO<sub>2</sub>Ph)]. A detailed computational analysis identifies noncovalent
interactions in the crystal lattice (crystal-packing effects) as responsible
for the distortion in [Cp*IrÂ(Ρ<sup>2</sup>-ONCOPh)]
Catalyst Activation by Loss of Cyclopentadienyl Ligands in Hydrogen Transfer Catalysis with Cp*Ir<sup>III</sup> Complexes
The activity of the two related complexes
[Cp*IrÂ(IMe)<sub>2</sub>X]ÂBF<sub>4</sub> (X = Cl (<b>1</b>),
H (<b>2</b>)) in
transfer hydrogenation from isopropyl alcohol to acetophenone was
investigated. The results suggest that the commonly accepted monohydride
mechanism for transfer hydrogenation mediated by cyclopentadienyl
iridium species does not apply to chloride <b>1</b>. We have
found evidence that, although the two monodentate NHC ligands are
retained in the coordination sphere, the Cp* ligand is completely
released under mild conditions in a precatalytic activation step.
Synthesis of modified versions of the initial precatalyst <b>1</b> with different cyclopentadienyl and NHC ligands demonstrated that
increasing the steric pressure around the iridium center facilitates
precatalyst activation and thus enhances the catalytic performance.
Study of five new iridiumÂ(III) complexes bearing mono- or diphosphines
helped us monitor Cp* ligand loss under mild conditions. An unusual
PâC bond cleavage was also noted in a 1,2-bisÂ(dimethylphosphino)Âmethane
(dmpm) ligand. On the basis of these findings, a novel catalyst activation
mechanism is proposed for [(Ρ<sup>5</sup>-C<sub>5</sub>R<sub>5</sub>)ÂIr] transfer hydrogenation based on the lability of the cyclopentadienyl
ligand