43 research outputs found
Intramolecular Arene C–H to C–P Functionalization Mediated by Nickel(II) and Palladium(II)
A trisÂ(phosphine)
ligand with a triarylbenzene backbone was employed to support mono-nickelÂ(II)
and
-palladiumÂ(II) complexes. Two phosphine arms coordinated to the metal
center, while the third phosphine was found to form a C–P bond
with dearomatization of the central arene. Deprotonation effected
the rearomatization of the central ring and metal reduction from MÂ(II)
to M(0). The overall conversion corresponds to a functionalization
of an unactivated arene C–H bond to a C–P bond. This
transformation represents a rare type of mechanism of C–H functionalization,
facilitated by the interactions of the group 10 metal with the arene
Ï€ system. This conversion is reminiscent of and expands the
scope of recently reported intramolecular rearrangements of biaryl
phosphine ligands common in group 10 catalysis
Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation
A new
series of tetranuclear Fe clusters displaying an interstitial μ<sub>4</sub>-F ligand was prepared for a comparison to previously reported
μ<sub>4</sub>-O analogues. With a single nitric oxide (NO) coordinated
as a reporter of small-molecule activation, the μ<sub>4</sub>-F clusters were characterized
in <i>five</i> redox states, from Fe<sup>II</sup><sub>3</sub>{FeNO}<sup>8</sup> to Fe<sup>III</sup><sub>3</sub>{FeNO}<sup>7</sup>, with NO stretching frequencies ranging from 1680 to 1855 cm<sup>–1</sup>, respectively. Despite accessing more reduced states
with an F<sup>–</sup> bridge, a two-electron reduction of the
distal Fe centers is necessary for the μ<sub>4</sub>-F clusters
to activate NO to the same degree as the μ<sub>4</sub>-O system;
consequently, NO reactivity is observed at more positive potentials
with μ<sub>4</sub>-O than μ<sub>4</sub>-F. Moreover, the
μ<sub>4</sub>-O ligand better translates redox changes of remote
metal centers to diatomic ligand activation. The implication for biological
active sites is that the higher-charge bridging ligand is more effective
in tuning cluster properties, including the involvement of remote
metal centers, for small-molecule activation
Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes
A new, stoichiometric activation
mode is presented for Cr-PNP (PNP
= diphosphinoamine) complexes for ethylene tetramerization catalysis.
To access suitable precatalysts, two robust CrÂ(III) multiaryl compounds
were synthesized as THF adducts. These complexes are supported by
a facially coordinated bisÂ(aryl) ligand with an additional ether donor.
From these precursors, Cr-PNP trisÂ(hydrocarbyl) complexes were synthesized.
Using 1 equiv of Brønsted acid as an activator, an active species
for the catalytic tetramerization of ethylene was produced, without
the need for excess alkylaluminum reagents
A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex
The reduction of nitric oxide (NO)
to nitrous oxide (N<sub>2</sub>O) is a process relevant to biological
chemistry as well as to the
abatement of certain environmental pollutants. One of the proposed
key intermediates in NO reduction is hyponitrite (N<sub>2</sub>O<sub>2</sub><sup>2–</sup>), the product of reductive coupling of
two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper
complex generating a <i>trans</i>-hyponitrite moiety supported
by two μ-O-bimetallic (Y,Cu) cores, a previously unreported
coordination mode. Reaction of the hyponitrite species with Brønsted
acids leads to the generation of N<sub>2</sub>O, demonstrating the
viability of the hyponitrite complex as an intermediate in NO reduction
to N<sub>2</sub>O. The additional reducing equivalents stored in each
tricopper unit are employed in a subsequent step for N<sub>2</sub>O reduction to N<sub>2</sub>, for an overall (partial) conversion
of NO to N<sub>2</sub>. The combination of Lewis acid and multiple
redox active metals facilitates this four electron conversion via
an isolable hyponitrite intermediate
Combination of Redox-Active Ligand and Lewis Acid for Dioxygen Reduction with π‑Bound Molybdenum−Quinonoid Complexes
A series
of π-bound Mo−quinonoid complexes supported
by pendant phosphines have been synthesized. Structural characterization
revealed strong metal–arene interactions between Mo and the
π system of the quinonoid fragment. The Mo–catechol complex
(<b>2a</b>) was found to react within minutes with 0.5 equiv
of O<sub>2</sub> to yield a Mo–quinone complex (<b>3</b>), H<sub>2</sub>O, and CO. Si- and B-protected Mo–catecholate
complexes also react with O<sub>2</sub> to yield <b>3</b> along
with (R<sub>2</sub>SiO)<sub><i>n</i></sub> and (ArBO)<sub>3</sub> byproducts, respectively. Formally, the Mo–catecholate
fragment provides two electrons, while the elements bound to the catecholate
moiety act as acceptors for the O<sub>2</sub> oxygens. Unreactive
by itself, the Mo–dimethyl catecholate analogue reduces O<sub>2</sub> in the presence of added Lewis acid, BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, to generate a Mo<sup>I</sup> species and a
bisÂ(borane)-supported peroxide dianion, [[(F<sub>5</sub>C<sub>6</sub>)<sub>3</sub>B]<sub>2</sub>O<sub>2</sub><sup>2–</sup>], demonstrating
single-electron-transfer chemistry from Mo to the O<sub>2</sub> moiety.
The intramolecular combination of a molybdenum center, redox-active
ligand, and Lewis acid reduces O<sub>2</sub> with pendant acids weaker
than BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. Overall, the Ï€-bound
catecholate moiety acts as a two-electron donor. A mechanism is proposed
in which O<sub>2</sub> is reduced through an initial one-electron
transfer, coupled with transfer of the Lewis acidic moiety bound to
the quinonoid oxygen atoms to the reduced O<sub>2</sub> species
A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex
The reduction of nitric oxide (NO)
to nitrous oxide (N<sub>2</sub>O) is a process relevant to biological
chemistry as well as to the
abatement of certain environmental pollutants. One of the proposed
key intermediates in NO reduction is hyponitrite (N<sub>2</sub>O<sub>2</sub><sup>2–</sup>), the product of reductive coupling of
two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper
complex generating a <i>trans</i>-hyponitrite moiety supported
by two μ-O-bimetallic (Y,Cu) cores, a previously unreported
coordination mode. Reaction of the hyponitrite species with Brønsted
acids leads to the generation of N<sub>2</sub>O, demonstrating the
viability of the hyponitrite complex as an intermediate in NO reduction
to N<sub>2</sub>O. The additional reducing equivalents stored in each
tricopper unit are employed in a subsequent step for N<sub>2</sub>O reduction to N<sub>2</sub>, for an overall (partial) conversion
of NO to N<sub>2</sub>. The combination of Lewis acid and multiple
redox active metals facilitates this four electron conversion via
an isolable hyponitrite intermediate
Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins
Dinuclear nickel phenoxyiminato olefin polymerization
catalysts
based on rigid <i>p</i>-terphenyl frameworks are reported.
Permethylation of the central arene of the terphenyl unit and oxygen
substitution of the peripheral rings ortho to the aryl–aryl
linkages blocks rotation around these linkages, allowing atropisomers
of the ligand to be isolated. The corresponding syn and anti dinickel
complexes (<b>25-s</b> and <b>25-a</b>) were synthesized
and characterized by single-crystal X-ray diffraction. These frameworks
limit the relative movement of the metal centers, restricting the
metal–metal distance. Kinetics studies of isomerization of
a ligand precursor (<b>7-a</b>) allowed the calculation of the
activation parameters for the isomerization process (Δ<i>H</i><sup></sup><sup>⧧</sup> = 28.0 ± 0.4 kcal ×
mol<sup>–1</sup> and Δ<i>S</i><sup></sup><sup>⧧</sup> = −12.3 ± 0.4 cal mol<sup>–1</sup> K<sup>–1</sup>). The reported nickel complexes are active
for ethylene polymerization (TOF up to 3700 (mol C<sub>2</sub>H<sub>4</sub>) (mol Ni)<sup>−1</sup> h<sup>–1</sup>) and
ethylene/α-olefin copolymerization. Only methyl branches are
observed in the polymerization of ethylene, while α-olefins
are incorporated without apparent chain walking. These catalysts are
active in the presence of polar additives and in neat tetrahydrofuran.
The syn and anti isomers differ in polymerization activity, polymer
branching, and polymer molecular weight. For comparison, a series
of mononuclear nickel complexes (<b>26</b>, <b>27-s</b>, <b>27-a</b>, <b>28</b>, <b>30</b>) was prepared
and studied. The effects of structure and catalyst nuclearity on reactivity
are discussed
Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins
Dinuclear nickel phenoxyiminato olefin polymerization
catalysts
based on rigid <i>p</i>-terphenyl frameworks are reported.
Permethylation of the central arene of the terphenyl unit and oxygen
substitution of the peripheral rings ortho to the aryl–aryl
linkages blocks rotation around these linkages, allowing atropisomers
of the ligand to be isolated. The corresponding syn and anti dinickel
complexes (<b>25-s</b> and <b>25-a</b>) were synthesized
and characterized by single-crystal X-ray diffraction. These frameworks
limit the relative movement of the metal centers, restricting the
metal–metal distance. Kinetics studies of isomerization of
a ligand precursor (<b>7-a</b>) allowed the calculation of the
activation parameters for the isomerization process (Δ<i>H</i><sup></sup><sup>⧧</sup> = 28.0 ± 0.4 kcal ×
mol<sup>–1</sup> and Δ<i>S</i><sup></sup><sup>⧧</sup> = −12.3 ± 0.4 cal mol<sup>–1</sup> K<sup>–1</sup>). The reported nickel complexes are active
for ethylene polymerization (TOF up to 3700 (mol C<sub>2</sub>H<sub>4</sub>) (mol Ni)<sup>−1</sup> h<sup>–1</sup>) and
ethylene/α-olefin copolymerization. Only methyl branches are
observed in the polymerization of ethylene, while α-olefins
are incorporated without apparent chain walking. These catalysts are
active in the presence of polar additives and in neat tetrahydrofuran.
The syn and anti isomers differ in polymerization activity, polymer
branching, and polymer molecular weight. For comparison, a series
of mononuclear nickel complexes (<b>26</b>, <b>27-s</b>, <b>27-a</b>, <b>28</b>, <b>30</b>) was prepared
and studied. The effects of structure and catalyst nuclearity on reactivity
are discussed
Bimetallic Effects on Ethylene Polymerization in the Presence of Amines: Inhibition of the Deactivation by Lewis Bases
Dinickel complexes supported by terphenyl ligands appended
with
phenoxy and imine donors were synthesized. Full substitution of the
central arene blocks rotation around the aryl–aryl bond and
allows for the isolation of atropisomers. The reported complexes perform
ethylene polymerization in the presence of amines. The inhibiting
effect of polar additives is up to 250 times lower for the syn isomer
than the anti isomer. Comparisons with mononuclear systems indicate
that the proximity of the metal centers leads to the observed inhibitory
effect on the deactivation of the catalysts
A <i>trans</i>-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex
The reduction of nitric oxide (NO)
to nitrous oxide (N<sub>2</sub>O) is a process relevant to biological
chemistry as well as to the
abatement of certain environmental pollutants. One of the proposed
key intermediates in NO reduction is hyponitrite (N<sub>2</sub>O<sub>2</sub><sup>2–</sup>), the product of reductive coupling of
two NO molecules. We report the reductive coupling of NO by an yttrium–tricopper
complex generating a <i>trans</i>-hyponitrite moiety supported
by two μ-O-bimetallic (Y,Cu) cores, a previously unreported
coordination mode. Reaction of the hyponitrite species with Brønsted
acids leads to the generation of N<sub>2</sub>O, demonstrating the
viability of the hyponitrite complex as an intermediate in NO reduction
to N<sub>2</sub>O. The additional reducing equivalents stored in each
tricopper unit are employed in a subsequent step for N<sub>2</sub>O reduction to N<sub>2</sub>, for an overall (partial) conversion
of NO to N<sub>2</sub>. The combination of Lewis acid and multiple
redox active metals facilitates this four electron conversion via
an isolable hyponitrite intermediate