7 research outputs found
Terminal Titanyl Complexes of Tri- and Tetrametaphosphate: Synthesis, Structures, and Reactivity with Hydrogen Peroxide
The
synthesis and characterization of tri- and tetrametaphosphate titaniumÂ(IV)
oxo and peroxo complexes is described. Addition of 0.5 equiv of [OTiÂ(acac)<sub>2</sub>]<sub>2</sub> to dihydrogen tetrametaphosphate ([P<sub>4</sub>O<sub>12</sub>H<sub>2</sub>]<sup>2–</sup>) and monohydrogen
trimetaphosphate ([P<sub>3</sub>O<sub>9</sub>H]<sup>2–</sup>) provided a bisÂ(μ<sub>2</sub>,κ<sup>2</sup>,κ<sup>2</sup>) tetrametaphosphate titanyl dimer, [OTiP<sub>4</sub>O<sub>12</sub>]<sub>2</sub><sup>4–</sup> (<b>1</b>; 70% yield),
and a trimetaphosphate titanyl acetylacetonate complex, [OTiP<sub>3</sub>O<sub>9</sub>(acac)]<sup>2–</sup> (<b>2</b>;
59% yield). Both <b>1</b> and <b>2</b> have been structurally
characterized, crystallizing in the triclinic <i>P</i>1Ì…
and monoclinic <i>P</i>2<sub>1</sub> space groups, respectively.
These complexes contain Tiî—¼O units with distances of 1.624(7)
and 1.644(2) Ã…, respectively, and represent rare examples of
structurally characterized terminal titanyls within an all-oxygen
coordination environment. Complexes <b>1</b> and <b>2</b> react with hydrogen peroxide to produce the corresponding peroxotitaniumÂ(IV)
metaphosphate complexes [O<sub>2</sub>TiP<sub>4</sub>O<sub>12</sub>]<sub>2</sub><sup>4–</sup>(<b>3</b>; 61% yield) and
[O<sub>2</sub>TiP<sub>3</sub>O<sub>9</sub>(acac)]<sup>2–</sup> (<b>4</b>; 65% yield), respectively. Both <b>3</b> and <b>4</b> have been characterized by single-crystal X-ray diffraction
studies, and their solid-state structures are presented. Complex <b>3</b> functions as an oxygen atom transfer (OAT) reagent capable
of oxidizing phosphorusÂ(III) compounds (PÂ(OMe)<sub>3</sub>, PPh<sub>3</sub>) and SMe<sub>2</sub> at ambient temperature to result in
the corresponding organic oxide with regeneration of dimer <b>1</b>
Crystalline Metaphosphate Acid Salts: Synthesis in Organic Media, Structures, Hydrogen-Bonding Capability, and Implication of Superacidity
Metaphosphate acids cannot be thoroughly
studied in aqueous media
because their acidity is leveled by the solvent, and the resulting
metaphosphates are susceptible to acid-catalyzed hydrolysis. Exploration
of metaphosphate acid chemistry has now been made possible with the
development of a general synthetic method for organic media soluble
metaphosphate acids. Protonation of the [PPN]<sup>+</sup> salts ([PPN]<sup>+</sup> = [NÂ(PPh<sub>3</sub>)<sub>2</sub>]<sup>+</sup>) of tri-,
tetra-, and hexametaphosphates results in five new metaphosphate acids,
[PPN]<sub>2</sub>[P<sub>3</sub>O<sub>9</sub>H] (<b>2</b>), [PPN]<sub>4</sub>[(P<sub>4</sub>O<sub>12</sub>)<sub>3</sub>H<sub>8</sub>] (<b>3</b>), [PPN]<sub>4</sub>[P<sub>6</sub>O<sub>18</sub>H<sub>2</sub>]·2H<sub>2</sub>O (<b>4</b>), [PPN]<sub>3</sub>[P<sub>6</sub>O<sub>18</sub>H<sub>3</sub>] (<b>5</b>), and [PPN]<sub>2</sub>[P<sub>6</sub>O<sub>18</sub>H<sub>2</sub>(H<sub>3</sub>O)<sub>2</sub>] (<b>6</b>), obtained in yields of 80, 71, 66, 88,
and 76%, respectively. Additionally, our synthetic method can be extended
to pyrophosphate to produce [PPN]Â[P<sub>2</sub>O<sub>7</sub>H<sub>3</sub>] (<b>7</b>) in 77% yield. The structural configurations
of these oxoacids are dictated by strong hydrogen bonds and the anticooperative
effect. Intramolecular hydrogen bonds are observed in <b>2</b>, <b>4</b>, and <b>5</b> and the previously reported
[PPN]<sub>2</sub>[P<sub>4</sub>O<sub>12</sub>H<sub>2</sub>] (<b>1</b>), while intermolecular hydrogen bonds are observed in <b>3</b>, <b>6</b>, and <b>7</b>. The hydrogen bonds
in <b>3</b>–<b>7</b> possess short distances and
are classified as low-barrier hydrogen bonds. Gas-phase acidity computations
reveal that the parent tri- and tetrametaphosphoric acids are superacids.
Their remarkable acidity is attributable to the stabilization of their
corresponding conjugate bases via intramolecular hydrogen bonding
Organometallic Gold(III) Reagents for Cysteine Arylation
An
efficient method for chemoÂselective cysteine arylation
of unprotected peptides and proteins using AuÂ(III) organometallic
complexes is reported. The bioconjugation reactions proceed rapidly
(<5 min) at ambient temperature in various buffers and within a
wide pH range (0.5–14). This approach provides access to a
diverse array of <i>S</i>-aryl bioconjugates including fluorescent
dye, complex drug molecule, affinity label, polyÂ(ethylene glycol)
tags, and a stapled peptide. A library of AuÂ(III) arylation reagents
can be prepared as air-stable, crystalline solids in one step from
commercial reagents. The selective and efficient arylation procedures
presented in this work broaden the synthetic scope of cysteine bioconjugation
and serve as promising routes for the modification of complex biomolecules
Organometallic Gold(III) Reagents for Cysteine Arylation
An
efficient method for chemoÂselective cysteine arylation
of unprotected peptides and proteins using AuÂ(III) organometallic
complexes is reported. The bioconjugation reactions proceed rapidly
(<5 min) at ambient temperature in various buffers and within a
wide pH range (0.5–14). This approach provides access to a
diverse array of <i>S</i>-aryl bioconjugates including fluorescent
dye, complex drug molecule, affinity label, polyÂ(ethylene glycol)
tags, and a stapled peptide. A library of AuÂ(III) arylation reagents
can be prepared as air-stable, crystalline solids in one step from
commercial reagents. The selective and efficient arylation procedures
presented in this work broaden the synthetic scope of cysteine bioconjugation
and serve as promising routes for the modification of complex biomolecules
Synthetic and Mechanistic Interrogation of Pd/Isocyanide-Catalyzed Cross-Coupling: π‑Acidic Ligands Enable Self-Aggregating Monoligated Pd(0) Intermediates
Despite the large number of judiciously
designed ligands that have
been exploited in palladium-catalyzed cross-coupling protocols, the
incorporation of ligands bearing appreciable π-acidic properties
has remained significantly underexplored. Herein, we demonstrate that
well-defined and low-coordinate Pd<sup>0</sup> complexes supported
by <i>m</i>-terphenyl isocyanides function as competent
catalysts for the Suzuki–Miyaura cross-coupling of aryl bromides
and arylboronic acids. Two-coordinate PdÂ(CNAr<sup>Dipp2</sup>)<sub>2</sub> was active for the coupling of unhindered aryl bromides at
room temperature in 2-propanol, while increasing the temperature to
60 °C allowed for the use of mono- or di-<i>ortho</i>-substituted aryl bromides. Oxidative addition of the aryl bromide
was shown to proceed via a dissociative mechanism, implicating monoligated
PdÂ(CNAr<sup>Dipp2</sup>) as the catalytically active intermediate.
Attempts to access this fleeting species via activation of the Pd<sup>II</sup> monoisocyanide PdClÂ(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Â(CNAr<sup>Dipp2</sup>) with alkoxide base yielded the dinuclear
Pd<sup>I</sup> species (μ-C<sub>3</sub>H<sub>5</sub>)Â(μ-O<sup><i>i</i></sup>Pr)Â[PdÂ(CNAr<sup>Dipp2</sup>)]<sub>2</sub>. Although dinuclear Pd<sup>I</sup> complexes are often produced
as off-cycle species when using complexes of the type PdClÂ(η<sup>3</sup>-allyl)ÂL as precatalysts, this represents the first time that
the comproportionation product (μ-allyl)Â(μ-Cl)Â[PdL]<sub>2</sub> has been observed to undergo nucleophilic substitution with
alkoxide, despite the fact that activating conditions for these precatalysts
typically employ alkoxide bases. Remarkably, this alkoxide complex
can undergo β-hydride elimination with expulsion of acetone
and propene to produce two equivalents of catalytically active PdÂ(CNAr<sup>Dipp2</sup>), which can self-aggregate to yield the isolable tripalladium
cluster Pd<sub>3</sub>(η<sup>2</sup>-Dipp-μ-CNAr<sup>Dipp2</sup>)<sub>3</sub>. This cluster is catalytically competent for the Suzuki–Miyaura
reaction and functions as a formal source of monoligated PdÂ(CNAr<sup>Dipp2</sup>) in solution
The Stannylphosphide Anion Reagent Sodium Bis(triphenylstannyl) Phosphide: Synthesis, Structural Characterization, and Reactions with Indium, Tin, and Gold Electrophiles
Treatment of P<sub>4Â </sub>with
in situ generated [Na]Â[SnPh<sub>3</sub>] leads to the formation of
the sodium monophosphide [Na]Â[PÂ(SnPh<sub>3</sub>)<sub>2</sub>] and
the Zintl salt [Na]<sub>3</sub>[P<sub>7</sub>]. The former was isolated
in 46% yield as the crystalline salt [NaÂ(benzo-15-crown-5)]Â[PÂ(SnPh<sub>3</sub>)<sub>2</sub>] and used to prepare the homoleptic phosphine
PÂ(SnPh<sub>3</sub>)<sub>3</sub>, isolated in 67% yield, as well as
the indium derivative (XL)<sub>2</sub>InPÂ(SnPh<sub>3</sub>)<sub>2</sub> (XL = SÂ(CH<sub>2</sub>)<sub>2</sub>NMe<sub>2</sub>), isolated in
84% yield, and the gold complex (Ph<sub>3</sub>P)ÂAuPÂ(SnPh<sub>3</sub>)<sub>2</sub>. The compounds [NaÂ(benzo-15-crown-5)]Â[PÂ(SnPh<sub>3</sub>)<sub>2</sub>], PÂ(SnPh<sub>3</sub>)<sub>3</sub>, (XL)<sub>2</sub>InPÂ(SnPh<sub>3</sub>)<sub>2</sub>, and (Ph<sub>3</sub>P)ÂAuPÂ(SnPh<sub>3</sub>)<sub>2</sub> were characterized using multinuclear NMR spectroscopy
and X-ray crystallography. The bonding in (Ph<sub>3</sub>P)ÂAuPÂ(SnPh<sub>3</sub>)<sub>2</sub> was dissected using natural bond orbital (NBO)
methods, in response to the observation from the X-ray crystal structure
that the dative P:→Au bond is slightly <i>shorter</i> than the shared electron-pair P–Au bond. The bonding in (XL)<sub>2</sub>InPÂ(SnPh<sub>3</sub>)<sub>2</sub> was also interrogated using <sup>31</sup>P and <sup>13</sup>C solid-state NMR and computational methods.
Co-product [Na]<sub>3</sub>[P<sub>7</sub>] was isolated in 57% yield
as the stannyl heptaphosphide P<sub>7</sub>(SnPh<sub>3</sub>)<sub>3</sub>, following salt metathesis with ClSnPh<sub>3</sub>. Additionally,
we report that treatment of P<sub>4</sub> with sodium naphthalenide
in dimethoxyethane at 22 °C is a convenient and selective method
for the independent synthesis of Zintl ion [Na]<sub>3</sub>[P<sub>7</sub>]. The latter was isolated as the silylated heptaphosphide
P<sub>7</sub>(SiMe<sub>3</sub>)<sub>3</sub>, in 67% yield, or as the
stannyl heptaphosphide P<sub>7</sub>(SnPh<sub>3</sub>)<sub>3</sub> in 65% yield by salt metathesis with ClSiMe<sub>3</sub> or ClSnPh<sub>3</sub>, respectively
Role of Axial Base Coordination in Isonitrile Binding and Chalcogen Atom Transfer to Vanadium(III) Complexes
The enthalpy of oxygen atom transfer
(OAT) to VÂ[(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N], <b>1</b>, forming OVÂ[(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N], <b>1</b>–O, and the enthalpies
of sulfur atom transfer (SAT) to <b>1</b> and VÂ(NÂ[<i>t</i>-Bu]ÂAr)<sub>3</sub>, <b>2</b> (Ar = 3,5-C<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>), forming the corresponding sulfides SVÂ[(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N], <b>1</b>–S, and SVÂ(NÂ[<i>t</i>-Bu]ÂAr)<sub>3</sub>, <b>2</b>–S, have been measured by solution calorimetry in
toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]Âhepta-2,5-diene)
and Ph<sub>3</sub>SbS as chalcogen atom transfer reagents. The V–O
BDE in <b>1</b>–O is 6.3 ± 3.2 kcal·mol<sup>–1</sup> lower than the previously reported value for <b>2</b>–O and the V–S BDE in <b>1</b>–S
is 3.3 ± 3.1 kcal·mol<sup>–1</sup> lower than that
in <b>2</b>–S. These differences are attributed primarily
to a weakening of the V–N<sub>axial</sub> bond present in complexes
of <b>1</b> upon oxidation. The rate of reaction of <b>1</b> with dbabhNO has been studied by low temperature stopped-flow kinetics.
Rate constants for OAT are over 20 times greater than those reported
for <b>2</b>. Adamantyl isonitrile (AdNC) binds rapidly and
quantitatively to both <b>1</b> and <b>2</b> forming high
spin adducts of VÂ(III). The enthalpies of ligand addition to <b>1</b> and <b>2</b> in toluene solution are −19.9
± 0.6 and −17.1 ± 0.7 kcal·mol<sup>–1</sup>, respectively. The more exothermic ligand addition to <b>1</b> as compared to <b>2</b> is opposite to what was observed for
OAT and SAT. This is attributed to less weakening of the V–N<sub>axial</sub> bond in ligand
binding as opposed to chalcogen atom transfer and is in keeping with
structural data and computations. The structures of <b>1</b>, <b>1</b>–O, <b>1</b>–S, <b>1</b>–CNAd, and <b>2</b>–CNAd have been determined
by X-ray crystallography and are reported