23 research outputs found
Intramolecular C–H Oxidative Addition to Iridium(I) in Complexes Containing a <i>N</i>,<i>N</i>′‑Diphosphanosilanediamine Ligand
The
iridium(I) complexes of formula Ir(cod)(SiNP)<sup>+</sup> (<b>1</b><sup><b>+</b></sup>) and IrCl(cod)(SiNP) (<b>2</b>) are
easily obtained from the reaction of SiMe<sub>2</sub>{N(4-C<sub>6</sub>H<sub>4</sub>CH<sub>3</sub>)PPh<sub>2</sub>}<sub>2</sub> (SiNP)
with [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]<sup>+</sup> or [IrCl(cod)]<sub>2</sub>, respectively. The carbonylation of [<b>1</b>][PF<sub>6</sub>] affords the cationic pentacoordinated complex [Ir(CO)(cod)(SiNP)]<sup>+</sup> (<b>3</b><sup>+</sup>), while the treatment <b>2</b> with CO gives the cation <b>3</b><sup>+</sup> as an intermediate,
finally affording an equilibrium mixture of IrCl(CO)(SiNP) (<b>4</b>) and the hydride derivative of formula IrHCl(CO)(SiNP–H)
(<b>5</b>) resulting from the intramolecular oxidative addition
of the C–H bond of the SiCH<sub>3</sub> moiety to the iridium(I)
center. Furthermore, the prolonged exposure of [<b>3</b>]Cl
or <b>2</b> to CO resulted in the formation of the iridium(I)
pentacoordinated complex Ir(SiNP–H)(CO)<sub>2</sub> (<b>6</b>). The unprecedented κ<sup>3</sup><i>C</i>,<i>P</i>,<i>P</i>′ coordination mode
of the [SiNP–H] ligand observed in <b>5</b> and <b>6</b> has been fully characterized in solution by NMR spectroscopy.
In addition, the single-crystal X-ray structure of <b>6</b> is
reported
Experimental and Theoretical Mechanistic Investigation on the Catalytic CO<sub>2</sub> Hydrogenation to Formate by a Carboxylate-Functionalized Bis(<i>N</i>‑heterocyclic carbene) Zwitterionic Iridium(I) Compound
The bis-imidazolium
salt, 1,1-bis(<i>N</i>-methylimidazolium)
acetate bromide, is a convenient precursor for the synthesis of zwitterionic
iridium(I) [Ir(cod){(MeIm)<sub>2</sub>CHCOO}] and cationic iridium(III)
[IrH(cod){(MeIm)<sub>2</sub>CHCOO}]<sup>+</sup> compounds (MeIm =
3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized
bis(NHC) ligand. The [Ir(cod){(MeIm)<sub>2</sub>CHCOO}] compound catalyzes
the hydrogenation of CO<sub>2</sub> to formate in water using NEt<sub>3</sub> as base, reaching turnover numbers of approximately 1500.
Reactivity studies have shown that activation of the catalyst precursor
involves the reaction with H<sub>2</sub> in a multistep process that
under catalytic conditions results in the formation of a dihydrido
iridium(III) octahedral [IrH<sub>2</sub>(H<sub>2</sub>O){(MeIm)<sub>2</sub>CHCOO}] species stabilized by the κ<sup>3</sup>-<i>C</i>,<i>C′</i>,<i>O</i> coordination
of the ligand. DFT studies on the mechanism were carried out to elucidate
two possible roles of the base. In the first one, NEt<sub>3</sub> neutralizes
only the produced formic acid, whereas in the second it assists the
proton transfer in heterolytic cleavage of the H<sub>2</sub> molecule.
Although this base-involved mechanism is more favorable in that it
exhibits a lower energy span for the overall reaction, the energy
barrier obtained from kinetic experiments suggests that both mechanisms
could be operative under the experimental reaction conditions
Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity
A series
of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized
bis-N-heterocyclic carbene ligand of formula [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOO}] and [M<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh,
Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene
(cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)<sub>2</sub>CHCOO]Br and suitable metal precursor. The solid-state
structure of both types of complexes shows a boat-shaped six-membered
metallacycle derived of the κ<sup>2</sup>C,C′ coordination
mode of the bis-NHC ligand. The uncoordinated carboxylate fragment
is found at the <i>bowsprit</i> position in the Cp*M<sup>III</sup> complexes, whereas in the M<sup>I</sup>(diene) complexes
it is at the <i>flagpole</i> position of the metallacycle.
The complexes [Rh<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOO}]
(diene = cod, nbd) exist as two conformational isomers in dichloromethane, <i>bowsprit</i> and <i>flagpole</i>, that interconvert
through the boat-to-boat inversion of the metallacycle. An inversion
barrier of ∼17 kcal·mol<sup>–1</sup> was determined
by two-dimensional exchange spectroscopy NMR measurements for [Rh<sup>I</sup>(cod){(MeIm)<sub>2</sub>CHCOO}]. Reaction of zwitterionic
Cp*M<sup>III</sup> complexes with methyl triflate or tetrafluoroboric
acid affords the cationic complexes [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOOMe}]<sup>+</sup> or [Cp*M<sup>III</sup>Cl{(MeIm)<sub>2</sub>CHCOOH}]<sup>+</sup> (M = Rh, Ir) featuring carboxy
and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands,
respectively. Similarly, complexes [M<sup>I</sup>(diene){(MeIm)<sub>2</sub>CHCOOMe}]<sup>+</sup> (M = Rh, Ir) were prepared by
alkylation of the corresponding zwitterionic M<sup>I</sup>(diene)
complexes with methyl triflate. In contrast, reaction of [Ir<sup>I</sup>(cod){(MeIm)<sub>2</sub>CHCOO}] with HBF<sub>4</sub>·Et<sub>2</sub>O (Et = ethyl), CH<sub>3</sub>OTf, CH<sub>3</sub>I, or I<sub>2</sub> gives cationic iridium(III) octahedral complexes [Ir<sup>III</sup>X(cod){(MeIm)<sub>2</sub>CHCOO}]<sup>+</sup> (X = H, Me, or I) featuring a tripodal coordination mode of the
carboxylate bridge-functionalized bis-NHC ligand. The switch from
κ<sup>2</sup>C,C′ to κ<sup>3</sup>C,C′,O
coordination of the bis-NHC ligand accompanying the oxidative addition
prevents the coordination of the anions eventually formed in the process
that remain as counterions
Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)<sub>2</sub>]<sup>+</sup> Complexes with Functionalized N‑Heterocyclic Carbene Ligands
The synthesis of unbridged biscarbene
iridium(I) [Ir(cod)(MeIm∩Z)<sub>2</sub>]<sup>+</sup> complexes
having N- or O-functionalized NHC
ligands (∩Z = 2-methoxybenzyl, pyridin-2-ylmethyl, quinolin-8-ylmethyl)
is described. The molecular structures of the complexes show an antiparallel
disposition of the carbene ligands that minimize the steric repulsions
between the bulky substituents. However, the complexes were found
to be dynamic in solution, due to the restricted rotation about the
C(carbene)–Ir bond that results in two interconverting diasteromers
having different dispositions of the functionalized NHC ligands. A
rotational barrier of around 80 kJ mol<sup>–1</sup> (298 K)
has been determined by 2D EXSY NMR spectroscopy. The iridium(III)
dihydride complex [IrH<sub>2</sub>(MeIm∩Z)<sub>2</sub>]<sup>+</sup> (∩Z = pyridin-2-ylmethyl) has been prepared by reaction
of the corresponding iridium(I) complex with molecular hydrogen. These
complexes efficiently catalyzed the transfer hydrogenation of cyclohexanone
using 2-propanol as a hydrogen source and KOH as base at 80 °C
with average TOF values of 117–155 h<sup>–1</sup> at
0.1 mol % iridium catalyst loading. All of the catalyst precursors
showed comparable activity independent of both the wingtip type at
the NHC ligands and the counterion. Mechanistic studies support the
involvement of diene free bis-NHC iridium(I) intermediates in these
catalytic systems. DFT calculations have shown that a MPV-like concerted
mechanism (Meerwein–Ponndorf–Verley mechanism), involving
the direct hydrogen transfer at the coordination sphere of the iridium
center, might compete with the well-established hydrido mechanism.
Indirect evidence of a MPV-like mechanism has been found for the catalyst
precursor having NHC ligands having with a pyridin-2-ylmethyl wingtip
Labile Rhodium(I)–N-Heterocyclic Carbene Complexes
The neutral square-planar complexes
Rh(acac)(IPr)(η<sup>2</sup><i>-</i>olefin) have been
prepared from [Rh(μ-Cl)(IPr)(η<sup>2</sup><i>-</i>olefin)]<sub>2</sub> (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-carbene;
olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac).
Protonation of the acetylacetonato complexes with triflic acid opens
the way to the formation of the putative bare [Rh-IPr]<sup>+</sup> fragment that has been stabilized at low temperature by labile ligands
such as triflate, cyclooctene, and acetonitrile to generate Rh(OTf)(IPr)(η<sup>2</sup><i>-</i>coe), [Rh(IPr)(η<sup>2</sup><i>-</i>coe)(NCCH<sub>3</sub>)<sub>2</sub>]OTf, and [Rh(IPr)(NCCH<sub>3</sub>)<sub>3</sub>]OTf complexes. The derivative [Rh(IPr)(η<sup>2</sup><i>-</i>coe)(NCCH<sub>3</sub>)<sub>2</sub>]OTf was
further characterized by an X-ray diffraction analysis
Rhodium(I)-N-Heterocyclic Carbene Catalyst for Selective Coupling of <i>N</i>‑Vinylpyrazoles with Alkynes via C–H Activation
The complex [Rh(μ-Cl)(<i>I</i>Pr)(η<sup>2</sup><i>-</i>coe)]<sub>2</sub> {<i>I</i>Pr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene,
coe = <i>cis</i>-cyclooctene} efficiently catalyzes the
coupling of alkynes and <i>N</i>-vinylpyrazole via C–H
activation, leading to Markovnikov-selective butadienylpyrazole derivatives
under mild conditions. A straightforward approach to cross-conjugated
acyclic trienes is also operative through a one-pot alkyne dimerization-hydrovinylation
tandem reaction. The proposed mechanism involves C–H activation
of vinylpyrazole directed by nitrogen coordination to the metallic
center. Subsequent alkyne coordination, insertion, and reductive elimination
steps lead to the coupling products. Several key intermediates participating
in the catalytic cycle have been detected and characterized, including
a κ-N, η<sup>2</sup>-CC coordinated vinylpyrazole
complex and a Rh<sup>III</sup>-hydride-alkenyl species resulting from
the C–H activation of the vinylpyrazol
Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation
The new Rh–hydroxo dinuclear
complexes stabilized by an
N-heterocyclic carbene (NHC) ligand of type [Rh(μ-OH)(NHC)(η<sup>2</sup>-olefin)]<sub>2</sub> (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst
precursors for alkyne hydrothiolation under mild conditions, presenting
high selectivity toward α-vinyl sulfides for a varied set of
substrates, which is enhanced by pyridine addition. The structure
of complex <b>3</b> has been determined by X-ray diffraction
analysis. Several intermediates relevant for the catalytic process
have been identified, including Rh<sup>I</sup>-thiolato species Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-coe)(py) (<b>6</b>) and
Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-HCCCH<sub>2</sub>Ph)(py) (<b>7</b>), and the Rh<sup>III</sup>-hydride-dithiolato
derivative RhH(SCH<sub>2</sub>Ph)<sub>2</sub>(IPr)(py) (<b>8</b>) as the catalytically active species. Computational DFT studies
reveal an operational mechanism consisting of sequential thiol deprotonation
by the hydroxo ligand, subsequent S–H oxidative addition, alkyne
insertion, and reductive elimination. The insertion step is rate-limiting
with a 1,2 thiometalation of the alkyne as the more favorable pathway
in accordance with the observed Markovnikov-type selectivity
Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation
The new Rh–hydroxo dinuclear
complexes stabilized by an
N-heterocyclic carbene (NHC) ligand of type [Rh(μ-OH)(NHC)(η<sup>2</sup>-olefin)]<sub>2</sub> (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst
precursors for alkyne hydrothiolation under mild conditions, presenting
high selectivity toward α-vinyl sulfides for a varied set of
substrates, which is enhanced by pyridine addition. The structure
of complex <b>3</b> has been determined by X-ray diffraction
analysis. Several intermediates relevant for the catalytic process
have been identified, including Rh<sup>I</sup>-thiolato species Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-coe)(py) (<b>6</b>) and
Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-HCCCH<sub>2</sub>Ph)(py) (<b>7</b>), and the Rh<sup>III</sup>-hydride-dithiolato
derivative RhH(SCH<sub>2</sub>Ph)<sub>2</sub>(IPr)(py) (<b>8</b>) as the catalytically active species. Computational DFT studies
reveal an operational mechanism consisting of sequential thiol deprotonation
by the hydroxo ligand, subsequent S–H oxidative addition, alkyne
insertion, and reductive elimination. The insertion step is rate-limiting
with a 1,2 thiometalation of the alkyne as the more favorable pathway
in accordance with the observed Markovnikov-type selectivity
Labile Rhodium(I)–N-Heterocyclic Carbene Complexes
The neutral square-planar complexes
Rh(acac)(IPr)(η<sup>2</sup><i>-</i>olefin) have been
prepared from [Rh(μ-Cl)(IPr)(η<sup>2</sup><i>-</i>olefin)]<sub>2</sub> (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-carbene;
olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac).
Protonation of the acetylacetonato complexes with triflic acid opens
the way to the formation of the putative bare [Rh-IPr]<sup>+</sup> fragment that has been stabilized at low temperature by labile ligands
such as triflate, cyclooctene, and acetonitrile to generate Rh(OTf)(IPr)(η<sup>2</sup><i>-</i>coe), [Rh(IPr)(η<sup>2</sup><i>-</i>coe)(NCCH<sub>3</sub>)<sub>2</sub>]OTf, and [Rh(IPr)(NCCH<sub>3</sub>)<sub>3</sub>]OTf complexes. The derivative [Rh(IPr)(η<sup>2</sup><i>-</i>coe)(NCCH<sub>3</sub>)<sub>2</sub>]OTf was
further characterized by an X-ray diffraction analysis
ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry
The
σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ<sup>3</sup>-pydc-X)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)]<sub>2</sub> and the corresponding 4-substituted pyridine-2,6-dicarboxylic
acids (H<sub>2</sub>pydc-X) or, alternatively, from their lithium
salts (X = H) and [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]PF<sub>6</sub>. Deuterium labeling studies in combination with theoretical calculations
have shown that formation of <b>1</b> involves a metal-mediated
proton transfer in the reactive intermediate [Ir(κ<sup>2</sup>-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ<sup>3</sup>-pydc)(cod)(CH<sub>3</sub>OH)], followed by olefin insertion.
The formation of this hydrido intermediate results from solvent-assisted
proton transfer through a hydrogen-bonding network, forming an eight-membered
metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)]<sub>2</sub> with iminodiacetic acid derivatives, RN(CH<sub>2</sub>COOH)<sub>2</sub>, gave the stable iridium(I) mononuclear [Ir{κ<sup>2</sup>-MeN(CH<sub>2</sub>COOH)(CH<sub>2</sub>COO)}(cod)] (R = Me) complex
having a free carboxymethyl group and the tetranuclear complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(cod)<sub>4</sub>] (R = Ph) with doubly deprotonated ligands.
The molecular structure of the related cyclooctene complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(coe)<sub>8</sub>] has been determined by X-ray analysis.
Reaction of <b>1</b> with monodentate N- and P-donor ligands
gave the compounds [Ir(κ<sup>3</sup>-pydc)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)(L)] (L = py, BnNH<sub>2</sub>, PPh<sub>3</sub>, PMe<sub>3</sub>). Reaction of <b>1</b> with the short-bite
bis(diphenylphosphino)methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear
complex <b>1</b><sub><b>2</b></sub><b>-dppm</b> as
a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b><sub><b>2</b></sub><b>-dppe</b> and <b>1</b><sub><b>2</b></sub><b>-dppp</b> have been prepared using
1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane
(dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies
have been obtained as two diastereoisomers in a 1:1 ratio due to the
chirality of the mononuclear building block. The single-crystal X-ray
structures of <b>1-py</b> and <b>1-dppm</b> are reported