Terminal Titanyl Complexes of Tri- and Tetrametaphosphate: Synthesis, Structures, and Reactivity with Hydrogen Peroxide

The synthesis and characterization of tri- and tetrametaphosphate titanium­(IV) oxo and peroxo complexes is described. Addition of 0.5 equiv of [OTi­(acac)₂]₂ to dihydrogen tetrametaphosphate ([P₄O₁₂H₂]^2–) and monohydrogen trimetaphosphate ([P₃O₉H]^2–) provided a bis­(μ₂,κ²,κ²) tetrametaphosphate titanyl dimer, [OTiP₄O₁₂]₂^4– (<b>1</b>; 70% yield), and a trimetaphosphate titanyl acetylacetonate complex, [OTiP₃O₉(acac)]^2– (<b>2</b>; 59% yield). Both <b>1</b> and <b>2</b> have been structurally characterized, crystallizing in the triclinic <i>P</i>1̅ and monoclinic <i>P</i>2₁ space groups, respectively. These complexes contain TiO units with distances of 1.624(7) and 1.644(2) Å, 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 peroxotitanium­(IV) metaphosphate complexes [O₂TiP₄O₁₂]₂^4–(<b>3</b>; 61% yield) and [O₂TiP₃O₉(acac)]^2– (<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 phosphorus­(III) compounds (P­(OMe)₃, PPh₃) and SMe₂ 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]⁺ salts ([PPN]⁺ = [N­(PPh₃)₂]⁺) of tri-, tetra-, and hexametaphosphates results in five new metaphosphate acids, [PPN]₂[P₃O₉H] (<b>2</b>), [PPN]₄[(P₄O₁₂)₃H₈] (<b>3</b>), [PPN]₄[P₆O₁₈H₂]·2H₂O (<b>4</b>), [PPN]₃[P₆O₁₈H₃] (<b>5</b>), and [PPN]₂[P₆O₁₈H₂(H₃O)₂] (<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]­[P₂O₇H₃] (<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]₂[P₄O₁₂H₂] (<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 chemo­selective cysteine arylation of unprotected peptides and proteins using Au­(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, poly­(ethylene glycol) tags, and a stapled peptide. A library of Au­(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 chemo­selective cysteine arylation of unprotected peptides and proteins using Au­(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, poly­(ethylene glycol) tags, and a stapled peptide. A library of Au­(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 Pd⁰ 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 Pd­(CNAr^Dipp2)₂ 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 Pd­(CNAr^Dipp2) as the catalytically active intermediate. Attempts to access this fleeting species via activation of the Pd^II monoisocyanide PdCl­(η³-C₃H₅)­(CNAr^Dipp2) with alkoxide base yielded the dinuclear Pd^I species (μ-C₃H₅)­(μ-O^<i>i</i>Pr)­[Pd­(CNAr^Dipp2)]₂. Although dinuclear Pd^I complexes are often produced as off-cycle species when using complexes of the type PdCl­(η³-allyl)­L as precatalysts, this represents the first time that the comproportionation product (μ-allyl)­(μ-Cl)­[PdL]₂ 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 Pd­(CNAr^Dipp2), which can self-aggregate to yield the isolable tripalladium cluster Pd₃(η²-Dipp-μ-CNAr^Dipp2)₃. This cluster is catalytically competent for the Suzuki–Miyaura reaction and functions as a formal source of monoligated Pd­(CNAr^Dipp2) in solution

The Stannylphosphide Anion Reagent Sodium Bis(triphenylstannyl) Phosphide: Synthesis, Structural Characterization, and Reactions with Indium, Tin, and Gold Electrophiles

Treatment of P₄with in situ generated [Na]­[SnPh₃] leads to the formation of the sodium monophosphide [Na]­[P­(SnPh₃)₂] and the Zintl salt [Na]₃[P₇]. The former was isolated in 46% yield as the crystalline salt [Na­(benzo-15-crown-5)]­[P­(SnPh₃)₂] and used to prepare the homoleptic phosphine P­(SnPh₃)₃, isolated in 67% yield, as well as the indium derivative (XL)₂InP­(SnPh₃)₂ (XL = S­(CH₂)₂NMe₂), isolated in 84% yield, and the gold complex (Ph₃P)­AuP­(SnPh₃)₂. The compounds [Na­(benzo-15-crown-5)]­[P­(SnPh₃)₂], P­(SnPh₃)₃, (XL)₂InP­(SnPh₃)₂, and (Ph₃P)­AuP­(SnPh₃)₂ were characterized using multinuclear NMR spectroscopy and X-ray crystallography. The bonding in (Ph₃P)­AuP­(SnPh₃)₂ 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)₂InP­(SnPh₃)₂ was also interrogated using ³¹P and ¹³C solid-state NMR and computational methods. Co-product [Na]₃[P₇] was isolated in 57% yield as the stannyl heptaphosphide P₇(SnPh₃)₃, following salt metathesis with ClSnPh₃. Additionally, we report that treatment of P₄ with sodium naphthalenide in dimethoxyethane at 22 °C is a convenient and selective method for the independent synthesis of Zintl ion [Na]₃[P₇]. The latter was isolated as the silylated heptaphosphide P₇(SiMe₃)₃, in 67% yield, or as the stannyl heptaphosphide P₇(SnPh₃)₃ in 65% yield by salt metathesis with ClSiMe₃ or ClSnPh₃, 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 V­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>, forming OV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–O, and the enthalpies of sulfur atom transfer (SAT) to <b>1</b> and V­(N­[<i>t</i>-Bu]­Ar)₃, <b>2</b> (Ar = 3,5-C₆H₃Me₂), forming the corresponding sulfides SV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–S, and SV­(N­[<i>t</i>-Bu]­Ar)₃, <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]­hepta-2,5-diene) and Ph₃SbS as chalcogen atom transfer reagents. The V–O BDE in <b>1</b>–O is 6.3 ± 3.2 kcal·mol^–1 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^–1 lower than that in <b>2</b>–S. These differences are attributed primarily to a weakening of the V–N_axial 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 V­(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^–1, 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_axial 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