21 research outputs found
Reversible Intramolecular P–S Bond Formation Coupled with a Ni(0)/Ni(II) Redox Process
P–S
bond formation/cleavage mediated by a nickel ion supported
by a PPP ligand (PPP = PÂ[2-P<sup><i>i</i></sup>Pr<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>]<sub>2</sub><sup>–</sup>) has
been investigated herein. To gain an entry into this chemistry, a
mononuclear thiolato nickel complex, (PPP)ÂNiÂ(SAr) (<b>1a</b>,<b>b</b>) was prepared by treating the chloride starting material
with NaSPh. Upon carbonylation, this complex produces a nickel(0)
monocarbonyl species, (PP<sup>SAr</sup>P)ÂNiÂ(CO) (<b>2a</b>,<b>b</b>), in which the thiolate migrates onto the central P of the
ligand to give a P–S bond and two-electron reduction of a nickelÂ(II)
center. The reaction undergoes via a pseudo-first-order decay with
respect to consumption of a nickelÂ(II) thiolato species, suggesting
an intramolecular reaction under the excess COÂ(g) conditions. The
reverse reaction involving P–S bond cleavage with concomitant
decarbonylation occurs to regenerate <b>1a</b>,<b>b</b> in benzene. Reaction of <b>2a</b> with trityl chloride results
in Ph<sub>3</sub>CSPh formation, whereas the reaction with MeI gives
methylation at a phosphide moiety or a thiolate group
Reversible Intramolecular P–S Bond Formation Coupled with a Ni(0)/Ni(II) Redox Process
P–S
bond formation/cleavage mediated by a nickel ion supported
by a PPP ligand (PPP = PÂ[2-P<sup><i>i</i></sup>Pr<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>]<sub>2</sub><sup>–</sup>) has
been investigated herein. To gain an entry into this chemistry, a
mononuclear thiolato nickel complex, (PPP)ÂNiÂ(SAr) (<b>1a</b>,<b>b</b>) was prepared by treating the chloride starting material
with NaSPh. Upon carbonylation, this complex produces a nickel(0)
monocarbonyl species, (PP<sup>SAr</sup>P)ÂNiÂ(CO) (<b>2a</b>,<b>b</b>), in which the thiolate migrates onto the central P of the
ligand to give a P–S bond and two-electron reduction of a nickelÂ(II)
center. The reaction undergoes via a pseudo-first-order decay with
respect to consumption of a nickelÂ(II) thiolato species, suggesting
an intramolecular reaction under the excess COÂ(g) conditions. The
reverse reaction involving P–S bond cleavage with concomitant
decarbonylation occurs to regenerate <b>1a</b>,<b>b</b> in benzene. Reaction of <b>2a</b> with trityl chloride results
in Ph<sub>3</sub>CSPh formation, whereas the reaction with MeI gives
methylation at a phosphide moiety or a thiolate group
Synthesis and Reactivity of Nickel(II) Hydroxycarbonyl Species, NiCOOH‑κ<i>C</i>
Reactions
of nickel complexes supported by an anionic PNP pincer
ligand (PNP<sup>–</sup> = NÂ[2-P<sup><i>i</i></sup>Pr<sub>2</sub>-4-Me-C<sub>6</sub>H<sub>3</sub>]<sub>2</sub>) toward
CO<sub>2</sub> and CO are investigated, particularly for interrogating
their C–O bond formation/cleavage chemistry. The formation
of a nickel formate species (<b>2</b>) was accomplished by the
reaction of (PNP)ÂNiH with CO<sub>2</sub>, while the structural isomer
complex (PNP)ÂNiCOOH-κ<i>C</i> (<b>4</b>) was
successfully produced from the corresponding nickel hydroxyl compound
by exposing it to COÂ(g). Its structurally unique character was gleaned
by obtaining two solid-state structures for (PNP)ÂNiCOOH-κ<i>C</i> (<b>4</b>) and {(PNP)ÂNi}<sub>2</sub>-μ-CO<sub>2</sub>-κ<sup>2</sup><i>C</i>,<i>O</i> (<b>6</b>); the latter was obtained from the reaction of <b>4</b> with a nickel hydroxyl complex. Both species possess a NiCOO-κ<i>C</i> binding mode, which is reminiscent of the binding mode
found at the carbon monoxide dehydrogenase (CODH) active site with
its Ni–COO–Fe fragment. The cationic species {(PNP)ÂNiCO}<sup>+</sup> (<b>7</b>) was also prepared via the protonation of <b>4</b>, which then led to the investigation of the C–O bond
formation in <b>7</b> by adding a nucleophile such as OH<sup>–</sup>
Direct CO<sub>2</sub> Addition to a Ni(0)–CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO
Addition
of CO<sub>2</sub> to a low-valent nickel species has been
explored with a newly designed <sup>acri</sup>PNP pincer ligand (<sup>acri</sup>PNP<sup>–</sup> = 4,5-bisÂ(diisopropylphosphino)-2,7,9,9-tetramethyl-9<i>H</i>-acridin-10-ide). This is a crucial step in understanding
biological CO<sub>2</sub> conversion to CO found in carbon monoxide
dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably
accessed in the presence of a CO ligand, which can be prepared from
a stepwise reduction of a cationic {(<sup>acri</sup>PNP)ÂNiÂ(II)–CO}<sup>+</sup> species. All three NiÂ(II), NiÂ(I), and Ni(0) monocarbonyl
species were cleanly isolated and spectroscopically characterized.
Addition of electrons to the nickelÂ(II) species significantly alters
its geometry from square planar toward tetrahedral because of the
filling of the d<sub><i>x</i><sup>2</sup>–<i>y</i><sup>2</sup></sub> orbital. Accordingly, the CO ligand
position changes from <i>equatorial</i> to <i>axial</i>, ∠N–Ni–C of 176.2(2)° to 129.1(4)°,
allowing opening of a CO<sub>2</sub> binding site. Upon addition of
CO<sub>2</sub> to a nickel(0)–CO species, a nickelÂ(II) carboxylate
species with a NiÂ(η<sup>1</sup>-CO<sub>2</sub>-κ<i>C</i>) moiety was formed and isolated (75%). This reaction occurs
with the concomitant expulsion of COÂ(g). This is a unique result markedly
different from our previous report involving the flexible analogous
PNP ligand, which revealed the formation of multiple products including
a tetrameric cluster from the reaction with CO<sub>2</sub>. Finally,
the carbon dioxide conversion to CO at a single nickel center is modeled
by the successful isolation of all relevant intermediates, such as
Ni–CO<sub>2</sub>, Ni–COOH, and Ni–CO
Reaction of Ferrate(VI) with ABTS and Self-Decay of Ferrate(VI): Kinetics and Mechanisms
Reactions of ferrateÂ(VI)
during water treatment generate perferrylÂ(V)
or ferrylÂ(IV) as primary intermediates. To better understand the fate
of perferrylÂ(V) or ferrylÂ(IV) during ferrateÂ(VI) oxidation, this study
investigates the kinetics, products, and mechanisms for the reaction
of ferrateÂ(VI) with 2,2′-azino-bisÂ(3-ethylbenzothiazoline-6-sulfonate)
(ABTS) and self-decay of ferrateÂ(VI) in phosphate-buffered solutions.
The oxidation of ABTS by ferrateÂ(VI) via a one-electron transfer process
produces ABTS<sup>•+</sup> and perferrylÂ(V) (<i>k</i> = 1.2 × 10<sup>6</sup> M<sup>–1</sup> s<sup>–1</sup> at pH 7). The perferrylÂ(V) mainly self-decays into H<sub>2</sub>O<sub>2</sub> and FeÂ(III) in acidic solution while with increasing
pH the reaction of perferrylÂ(V) with H<sub>2</sub>O<sub>2</sub> can
compete with the perferrylÂ(V) self-decay and produces FeÂ(III) and
O<sub>2</sub> as final products. The ferrateÂ(VI) self-decay generates
ferrylÂ(IV) and H<sub>2</sub>O<sub>2</sub> via a two-electron transfer
with the initial step being rate-limiting (<i>k</i> = 26
M<sup>–1</sup> s<sup>–1</sup> at pH 7). FerrylÂ(IV) reacts
with H<sub>2</sub>O<sub>2</sub> generating FeÂ(II) and O<sub>2</sub> and FeÂ(II) is oxidized by ferrateÂ(VI) producing FeÂ(III) and perferrylÂ(V)
(<i>k</i> = ∼10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup>). Due to these facile transformations of reactive
ferrateÂ(VI), perferrylÂ(V), and ferrylÂ(IV) to the much less reactive
FeÂ(III), H<sub>2</sub>O<sub>2</sub>, or O<sub>2</sub>, the observed
oxidation capacity of ferrateÂ(VI) is typically much lower than expected
from theoretical considerations (i.e., three or four electron equivalents
per ferrateÂ(VI)). This should be considered for optimizing water treatment
processes using ferrateÂ(VI)
Alkoxide Migration at a Nickel(II) Center Induced by a π‑Acidic Ligand: Migratory Insertion versus Metal–Ligand Cooperation
Two pathways of alkoxide
migration occurring at a nickelÂ(II) center
supported by a PPP ligand (PPP<sup>−</sup> = PÂ[2-P<sup><i>i</i></sup>Pr<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>]<sub>2</sub><sup>–</sup>) are presented in this Article. In the first
route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide
species reveals the formation of a P–O bond. This reaction
occurs via metal–ligand cooperation (MLC) involving a 2-electron
reduction at nickel. To demonstrate a P–O bond formation, a
nickelÂ(II) isopropoxide species (PPP)ÂNiÂ(O<sup><i>i</i></sup>Pr) (<b>4</b>) was prepared. Upon addition of a Ï€-acidic
isocyanide ligand CN<sup><i>t</i></sup>Bu, a nickel(0) isocyanide
species (PP<sup>O<i>i</i>Pr</sup>P)ÂNiÂ(CN<sup><i>t</i></sup>Bu) (<b>6b</b>) was generated; P–O bond formation
occurred via reductive elimination (RE). When CO is present, migratory
insertion (MI) occurs instead. The reaction of <b>4</b> with
COÂ(g) results in the formation of (PPP)ÂNiÂ(COO<sup><i>i</i></sup>Pr) (<b>5</b>), representing an alternative pathway.
The corresponding RE product (PP<sup>O<i>i</i>Pr</sup>P)ÂNiÂ(CO)
(<b>6a</b>) can be independently produced from the substitution
reaction of {(PP<sup>O<i>i</i>Pr</sup>P)ÂNi}<sub>2</sub>(μ-N<sub>2</sub>) (<b>3</b>) with COÂ(g). While two different carbonylation
pathways in <b>4</b> seem feasible, C–O bond forming
migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety
via a P–O bond cleavage was demonstrated by treating <b>3</b> with CO<sub>2</sub>(g). The formation of (PPP)ÂNiÂ(OCOO<sup><i>i</i></sup>Pr) (<b>7</b>) clearly shows that an
isopropoxide group migrates onto the bound CO<sub>2</sub> ligand,
and a P–Ni moiety is regenerated
Mechanistic Study on C–C Bond Formation of a Nickel(I) Monocarbonyl Species with Alkyl Iodides: Experimental and Computational Investigations
An open-shell reaction of the nickelÂ(I)
carbonyl species (PNP)ÂNi-CO
(<b>1</b>) with iodoalkanes has been explored experimentally
and theoretically. The initial iodine radical abstraction by a nickelÂ(I)
carbonyl species was suggested to produce (PNP)ÂNi-I (<b>4</b>) and the concomitant alkyl radical, according to a series of experimental
indications involving stoichiometric controls employing iodoalkanes.
Corresponding alkyl radical generation was also confirmed by radical
trapping experiments using Gomberg’s dimer. Molecular modeling
supports that the nickel acyl species (PNP)ÂNi-COCH<sub>3</sub> (<b>2</b>) can be formed by a direct C–C bond formation between
a carbonyl ligand of <b>1</b> and a methyl radical. As an alternative
pathway, the five-coordinate intermediate species (PNP)ÂNiÂ(CO)Â(CH<sub>3</sub>) (<b>5</b>) that involves both CO and CH<sub>3</sub> binding at a nickelÂ(II) center is also suggested with a comparable
activation barrier, although this pathway energetically favors the
formation of (PNP)ÂNi-CH<sub>3</sub> (<b>3</b>) via a barrierless
elimination of CO over a CO migratory insertion. Thus, our present
work supports that the direct C–C bond coupling occurs between
an alkyl radical and the carbonyl ligand at a monovalent nickel center
in the generation of an acyl product
Alkoxide Migration at a Nickel(II) Center Induced by a π‑Acidic Ligand: Migratory Insertion versus Metal–Ligand Cooperation
Two pathways of alkoxide
migration occurring at a nickelÂ(II) center
supported by a PPP ligand (PPP<sup>−</sup> = PÂ[2-P<sup><i>i</i></sup>Pr<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>]<sub>2</sub><sup>–</sup>) are presented in this Article. In the first
route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide
species reveals the formation of a P–O bond. This reaction
occurs via metal–ligand cooperation (MLC) involving a 2-electron
reduction at nickel. To demonstrate a P–O bond formation, a
nickelÂ(II) isopropoxide species (PPP)ÂNiÂ(O<sup><i>i</i></sup>Pr) (<b>4</b>) was prepared. Upon addition of a Ï€-acidic
isocyanide ligand CN<sup><i>t</i></sup>Bu, a nickel(0) isocyanide
species (PP<sup>O<i>i</i>Pr</sup>P)ÂNiÂ(CN<sup><i>t</i></sup>Bu) (<b>6b</b>) was generated; P–O bond formation
occurred via reductive elimination (RE). When CO is present, migratory
insertion (MI) occurs instead. The reaction of <b>4</b> with
COÂ(g) results in the formation of (PPP)ÂNiÂ(COO<sup><i>i</i></sup>Pr) (<b>5</b>), representing an alternative pathway.
The corresponding RE product (PP<sup>O<i>i</i>Pr</sup>P)ÂNiÂ(CO)
(<b>6a</b>) can be independently produced from the substitution
reaction of {(PP<sup>O<i>i</i>Pr</sup>P)ÂNi}<sub>2</sub>(μ-N<sub>2</sub>) (<b>3</b>) with COÂ(g). While two different carbonylation
pathways in <b>4</b> seem feasible, C–O bond forming
migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety
via a P–O bond cleavage was demonstrated by treating <b>3</b> with CO<sub>2</sub>(g). The formation of (PPP)ÂNiÂ(OCOO<sup><i>i</i></sup>Pr) (<b>7</b>) clearly shows that an
isopropoxide group migrates onto the bound CO<sub>2</sub> ligand,
and a P–Ni moiety is regenerated
Heterolytic H<sub>2</sub> Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane
Reversible,
heterolytic addition of H<sub>2</sub> across an iron–boron
bond in a ferraboratrane with formal hydride transfer to the boron
gives iron-borohydrido-hydride complexes. These compounds catalyze
the hydrogenation of alkenes and alkynes to the respective alkanes.
Notably, the boron is capable of acting as a shuttle for hydride transfer
to substrates. The results are interesting in the context of heterolytic
substrate addition across metal–boron bonds in metallaboratranes
and related systems, as well as metal–ligand bifunctional catalysis
Heterolytic H<sub>2</sub> Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane
Reversible,
heterolytic addition of H<sub>2</sub> across an iron–boron
bond in a ferraboratrane with formal hydride transfer to the boron
gives iron-borohydrido-hydride complexes. These compounds catalyze
the hydrogenation of alkenes and alkynes to the respective alkanes.
Notably, the boron is capable of acting as a shuttle for hydride transfer
to substrates. The results are interesting in the context of heterolytic
substrate addition across metal–boron bonds in metallaboratranes
and related systems, as well as metal–ligand bifunctional catalysis