16 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 (Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀM<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>ā<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> = BĀ(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 Ī±-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
Ī±-H elimination and migratory insertion reactions. Reactivity
studies on the cationic hydride alkylidene complexes of iridium are
reported as well
Electronic and Structural Effects of Low-Hapticity Coordination of Arene Rings to Transition Metals
A DFT computational
study and a structural analysis of the coordination
of arenes to transition metals in low -hapticity (Ī·<sup>1</sup> and Ī·<sup>2</sup>) modes have been developed, including a
pseudosymmetry analysis of the molecular orbitals and the introduction
of a hapticity map that makes evident the different degrees of intermediate
hapticities. Calculations on [Pt<sup>II</sup>L<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)] model complexes reveal a preference for the Ī·<sup>2</sup> mode, while the Ī·<sup>1</sup> coordination is found
to be a low-energy transition state for a haptotropic shift. The attachment
of the arene to a side group that is coordinated to the metal introduces
geometrical constraints, which result in hapticities intermediate
between one and two. Comparison of the Ī·<sup>1</sup> arene complexes
with benzonium cations shows that in the former case the bonding to
the metal involves essentially the Ļ system of the arene, affecting
only slightly the delocalized nature of the carbonācarbon bonds.
This behavior is in sharp contrast with the frequently found Ī·<sup>1</sup> coordination of Cp that involves Ļ bonding and full
dearomatization of the ring
Reactivity of a Cationic (C<sub>5</sub>Me<sub>5</sub>)Ir<sup>III</sup>-Cyclometalated Phosphine Complex with Alkynes
The
electrophilic cationic complex [(Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(C<sup>ā§</sup>P)]<sup>+</sup>, which contains a metalated
phosphine derived from PMeĀ(Xyl)<sub>2</sub> (Xyl = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>), reacts at ā60 Ā°C with the
alkynes HCī¼CH, PhCī¼CH, and PhCī¼CMe, with formation
of the corresponding Ļ adducts [(Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(C<sup>ā§</sup>P)Ā(Ī·<sup>2</sup>-alkyne)]<sup>+</sup>. Thermal activation of these complexes leads to products
that result from the CāC coupling of the alkyne and the Ļ
IrāC bond of the metalated phosphine, whose nature depends
markedly upon the alkyne involved. Thus, for HCī¼CH the carbonācarbon
bond -forming reaction leads to an iridium-bound alkene moiety as
the thermodynamic product, whereas the analogous complexes derived
from the bulkier PhCī¼CH and PhCī¼CMe alkynes undergo
further transformation into allylic structures. Mechanistic studies
supported by the use of PhCī¼CD demonstrate the implication
of an undetected vinylidene structure, Irī»Cī»CĀ(H)ĀPh,
in the key carbonācarbon bond-forming step of the PhCī¼CH
reaction system, whereas for the internal alkyne PhCī¼CMe a
migratory insertion mechanism is operative. However, no clear distinction
between these two routes can be made for the CāC bond-forming
reaction for which HCī¼CH is responsible
A Cationic Terminal Methylene Complex of Ir(I) Supported by a Pincer Ligand
A rare
example of a cationic methylene complex of IrĀ(I), [(PONOP)ĀIrĀ(CH<sub>2</sub>)]<sup>+</sup>, <b>2</b> (PONOP = 2,6-bisĀ(di-<i>tert</i>-butylphosphinito)Āpyridine), has been prepared by Ī±-hydride
abstraction from the previously described neutral methyl complex [(PONOP)ĀIrĀ(CH<sub>3</sub>)]. The intrinsic high reactivity of the compound prevented
both the isolation of a pure solid and its full characterization in
the solid state. Nevertheless, the proposed molecular structure finds
unequivocal support in multinuclear NMR spectroscopy and in reactivity
studies that include reactions with Lewis bases, dihydrogen, and ethyl
diazoacetate. An ylide compound, <b>3</b>, resulted from the
reaction between [(PONOP)ĀIrĀ(CH<sub>2</sub>)]<sup>+</sup> and PMe<sub>3</sub>, while the Ī·<sup>2</sup>-alkene complex <b>5</b> formed in a Cī»C coupling reaction involving the methylene
ligand of <b>2</b> and ethyl diazoacetate. Hydrogenolysis of
the Irī»CH<sub>2</sub> linkage of <b>2</b> led to several,
previously known, hydride and dihydrogen iridium complexes
EfektivnĆ aritmetika eliptickĆ½ch kÅivek nad koneÄnĆ½mi 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 CāH Bond Activation and CāC Bond-Forming Reactions
This work describes the chemical reactivity of a cationic
(Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(III) complex that
contains
a bisĀ(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a Īŗ<sup>4</sup>-<i>P</i>,<i>C</i>,<i>Cā²</i>,<i>Cā³</i> fashion. The
complex (<b>1</b><sup><b>+</b></sup> in this paper) undergoes
very facile intramolecular CāH bond activation of all benzylic
sites, in all likelihood through an IrĀ(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
CāC coupling reaction. Mechanistic studies demonstrate the
participation as a key intermediate of an electrophilic cationic IrĀ(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 CāH Bond Activation and CāC Bond-Forming Reactions
This work describes the chemical reactivity of a cationic
(Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(III) complex that
contains
a bisĀ(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a Īŗ<sup>4</sup>-<i>P</i>,<i>C</i>,<i>Cā²</i>,<i>Cā³</i> fashion. The
complex (<b>1</b><sup><b>+</b></sup> in this paper) undergoes
very facile intramolecular CāH bond activation of all benzylic
sites, in all likelihood through an IrĀ(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
CāC coupling reaction. Mechanistic studies demonstrate the
participation as a key intermediate of an electrophilic cationic IrĀ(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
Cyclometalated Iridium Complexes of Bis(Aryl) Phosphine Ligands: Catalytic CāH/CāD Exchanges and CāC Coupling Reactions
This
work details the synthesis and structural identification of
a series of complexes of the (Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(III) unit coordinated to cyclometalated bisĀ(aryl)Āphosphine
ligands, PRā²(Ar)<sub>2</sub>, for Rā² = 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>; Rā² = 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>ā<b>2e</b> and <b>3b</b>ā<b>3e</b>, respectively, are described. Reactions of chlorides <b>2</b> with NaBAr<sub>F</sub> (BAr<sub>F</sub> = BĀ(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 Ī½Ā(CO) values in the narrow interval 2030ā2040
cm<sup>ā1</sup>, indicating similar Ļ-basicity of the
IrĀ(III) center of these complexes. In the absence of CO, NaBAr<sub>F</sub> forces Īŗ<sup>4</sup>-<i>P</i>,<i>C</i>,<i>C</i>ā²,<i>C</i>ā³ 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, CāC coupling reaction converts
the Īŗ<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 IrāH and IrāCH<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 CāH/CāD Exchanges and CāC Coupling Reactions
This
work details the synthesis and structural identification of
a series of complexes of the (Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀIrĀ(III) unit coordinated to cyclometalated bisĀ(aryl)Āphosphine
ligands, PRā²(Ar)<sub>2</sub>, for Rā² = 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>; Rā² = 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>ā<b>2e</b> and <b>3b</b>ā<b>3e</b>, respectively, are described. Reactions of chlorides <b>2</b> with NaBAr<sub>F</sub> (BAr<sub>F</sub> = BĀ(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 Ī½Ā(CO) values in the narrow interval 2030ā2040
cm<sup>ā1</sup>, indicating similar Ļ-basicity of the
IrĀ(III) center of these complexes. In the absence of CO, NaBAr<sub>F</sub> forces Īŗ<sup>4</sup>-<i>P</i>,<i>C</i>,<i>C</i>ā²,<i>C</i>ā³ 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, CāC coupling reaction converts
the Īŗ<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 IrāH and IrāCH<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
Activation of Small Molecules by the MetalāAmido Bond of Rhodium(III) and Iridium(III) (Ī·<sup>5</sup>āC<sub>5</sub>Me<sub>5</sub>)M-Aminopyridinate Complexes
We
report the synthesis and structural characterization of five-coordinate
complexes of rhodium and iridium of the type [(Ī·<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)ĀMĀ(N^N)]<sup>+</sup> (<b>3-M</b><sup><b>+</b></sup>), where N^N represents the aminopyridinate
ligand derived from 2-NHĀ(Ph)-6-(Xyl)ĀC<sub>5</sub>H<sub>3</sub>N (Xyl
= 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>). The two complexes
were isolated as salts of the BAr<sub>F</sub> anion (BAr<sub>F</sub> = BĀ[3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>]<sub>4</sub>). The MāN<sub>amido</sub> bond of complexes <b>3-M</b><sup><b>+</b></sup> readily activated CO, C<sub>2</sub>H<sub>4</sub>, and H<sub>2</sub>. Thus, compounds <b>3-M</b><sup><b>+</b></sup> reacted with CO under ambient conditions,
but whereas for <b>3-Rh</b><sup><b>+</b></sup>, CO migratory
insertion was fast, yielding a carbamoyl carbonyl species, <b>4-Rh</b><sup><b>+</b></sup>, the stronger IrāN<sub>amido</sub> bond of complex <b>3-Ir</b><sup><b>+</b></sup> caused
the reaction to stop at the CO coordination stage. In contrast, <b>3-Ir</b><sup><b>+</b></sup> reacted reversibly with C<sub>2</sub>H<sub>4</sub>, forming adduct <b>5-Ir</b><sup><b>+</b></sup>, which subsequently rearranged irreversibly to [Ir]Ā(H)Ā(ī»CĀ(Me)ĀNĀ(Ph)ā)
complex <b>6-Ir</b><sup><b>+</b></sup>, which contains
an N-stabilized carbene ligand. Computational studies supported a
migratory insertion mechanism, giving first a Ī²-stabilized linear
alkyl unit, [Ir]ĀCH<sub>2</sub>CH<sub>2</sub>NĀ(Ph)ā, followed
by a multistep rearrangement that led to the final product <b>6-Ir</b><sup><b>+</b></sup>. Both Ī²- and Ī±-H eliminations,
as well as their microscopic reverse migratory insertion reactions,
were implicated in the alkyl-to-hydrideācarbene reorganization.
The analogous reaction of <b>3-Rh</b><sup><b>+</b></sup> with C<sub>2</sub>H<sub>4</sub> originated a complex mixture of
products from which only a branched alkyl [Rh]ĀCĀ(H)Ā(Me)ĀNĀ(Ph)ā
(<b>5-Rh</b><sup><b>+</b></sup>) could be isolated, featuring
a Ī²-agostic methyl interaction. Reactions of <b>3-M</b><sup><b>+</b></sup> with H<sub>2</sub> promoted a catalytic
isomerization of the Ap ligand from classical Īŗ<sup>2</sup>-N,Nā²
binding to Īŗ-N plus Ī·<sup>3</sup>-pseudoallyl coordination
mode