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 = P­[2-P^<i>i</i>Pr₂-C₆H₄]₂^–) has been investigated herein. To gain an entry into this chemistry, a mononuclear thiolato nickel complex, (PPP)­Ni­(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^SArP)­Ni­(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 nickel­(II) center. The reaction undergoes via a pseudo-first-order decay with respect to consumption of a nickel­(II) thiolato species, suggesting an intramolecular reaction under the excess CO­(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 Ph₃CSPh 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 = P­[2-P^<i>i</i>Pr₂-C₆H₄]₂^–) has been investigated herein. To gain an entry into this chemistry, a mononuclear thiolato nickel complex, (PPP)­Ni­(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^SArP)­Ni­(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 nickel­(II) center. The reaction undergoes via a pseudo-first-order decay with respect to consumption of a nickel­(II) thiolato species, suggesting an intramolecular reaction under the excess CO­(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 Ph₃CSPh 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^– = N­[2-P^<i>i</i>Pr₂-4-Me-C₆H₃]₂) toward CO₂ 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)­NiH with CO₂, while the structural isomer complex (PNP)­NiCOOH-κ<i>C</i> (<b>4</b>) was successfully produced from the corresponding nickel hydroxyl compound by exposing it to CO­(g). Its structurally unique character was gleaned by obtaining two solid-state structures for (PNP)­NiCOOH-κ<i>C</i> (<b>4</b>) and {(PNP)­Ni}₂-μ-CO₂-κ²<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)­NiCO}⁺ (<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^–

Direct CO₂ Addition to a Ni(0)–CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO₂ to a low-valent nickel species has been explored with a newly designed ^acriPNP pincer ligand (^acriPNP^– = 4,5-bis­(diisopropylphosphino)-2,7,9,9-tetramethyl-9<i>H</i>-acridin-10-ide). This is a crucial step in understanding biological CO₂ 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 {(^acriPNP)­Ni­(II)–CO}⁺ species. All three Ni­(II), Ni­(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel­(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d_{<i>x</i>²–<i>y</i>²} 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 CO₂ binding site. Upon addition of CO₂ to a nickel(0)–CO species, a nickel­(II) carboxylate species with a Ni­(η¹-CO₂-κ<i>C</i>) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO­(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 CO₂. 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–CO₂, Ni–COOH, and Ni–CO

Reaction of Ferrate(VI) with ABTS and Self-Decay of Ferrate(VI): Kinetics and Mechanisms

Reactions of ferrate­(VI) during water treatment generate perferryl­(V) or ferryl­(IV) as primary intermediates. To better understand the fate of perferryl­(V) or ferryl­(IV) during ferrate­(VI) oxidation, this study investigates the kinetics, products, and mechanisms for the reaction of ferrate­(VI) with 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and self-decay of ferrate­(VI) in phosphate-buffered solutions. The oxidation of ABTS by ferrate­(VI) via a one-electron transfer process produces ABTS^•+ and perferryl­(V) (<i>k</i> = 1.2 × 10⁶ M^–1 s^–1 at pH 7). The perferryl­(V) mainly self-decays into H₂O₂ and Fe­(III) in acidic solution while with increasing pH the reaction of perferryl­(V) with H₂O₂ can compete with the perferryl­(V) self-decay and produces Fe­(III) and O₂ as final products. The ferrate­(VI) self-decay generates ferryl­(IV) and H₂O₂ via a two-electron transfer with the initial step being rate-limiting (<i>k</i> = 26 M^–1 s^–1 at pH 7). Ferryl­(IV) reacts with H₂O₂ generating Fe­(II) and O₂ and Fe­(II) is oxidized by ferrate­(VI) producing Fe­(III) and perferryl­(V) (<i>k</i> = ∼10⁷ M^–1 s^–1). Due to these facile transformations of reactive ferrate­(VI), perferryl­(V), and ferryl­(IV) to the much less reactive Fe­(III), H₂O₂, or O₂, the observed oxidation capacity of ferrate­(VI) is typically much lower than expected from theoretical considerations (i.e., three or four electron equivalents per ferrate­(VI)). This should be considered for optimizing water treatment processes using ferrate­(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 nickel­(II) center supported by a PPP ligand (PPP⁻ = P­[2-P^<i>i</i>Pr₂-C₆H₄]₂^–) 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 nickel­(II) isopropoxide species (PPP)­Ni­(O^<i>i</i>Pr) (<b>4</b>) was prepared. Upon addition of a π-acidic isocyanide ligand CN^<i>t</i>Bu, a nickel(0) isocyanide species (PP^O<i>i</i>PrP)­Ni­(CN^<i>t</i>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 CO­(g) results in the formation of (PPP)­Ni­(COO^<i>i</i>Pr) (<b>5</b>), representing an alternative pathway. The corresponding RE product (PP^O<i>i</i>PrP)­Ni­(CO) (<b>6a</b>) can be independently produced from the substitution reaction of {(PP^O<i>i</i>PrP)­Ni}₂(μ-N₂) (<b>3</b>) with CO­(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 CO₂(g). The formation of (PPP)­Ni­(OCOO^<i>i</i>Pr) (<b>7</b>) clearly shows that an isopropoxide group migrates onto the bound CO₂ 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 nickel­(I) carbonyl species (PNP)­Ni-CO (<b>1</b>) with iodoalkanes has been explored experimentally and theoretically. The initial iodine radical abstraction by a nickel­(I) carbonyl species was suggested to produce (PNP)­Ni-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)­Ni-COCH₃ (<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)­Ni­(CO)­(CH₃) (<b>5</b>) that involves both CO and CH₃ binding at a nickel­(II) center is also suggested with a comparable activation barrier, although this pathway energetically favors the formation of (PNP)­Ni-CH₃ (<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 nickel­(II) center supported by a PPP ligand (PPP⁻ = P­[2-P^<i>i</i>Pr₂-C₆H₄]₂^–) 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 nickel­(II) isopropoxide species (PPP)­Ni­(O^<i>i</i>Pr) (<b>4</b>) was prepared. Upon addition of a π-acidic isocyanide ligand CN^<i>t</i>Bu, a nickel(0) isocyanide species (PP^O<i>i</i>PrP)­Ni­(CN^<i>t</i>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 CO­(g) results in the formation of (PPP)­Ni­(COO^<i>i</i>Pr) (<b>5</b>), representing an alternative pathway. The corresponding RE product (PP^O<i>i</i>PrP)­Ni­(CO) (<b>6a</b>) can be independently produced from the substitution reaction of {(PP^O<i>i</i>PrP)­Ni}₂(μ-N₂) (<b>3</b>) with CO­(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 CO₂(g). The formation of (PPP)­Ni­(OCOO^<i>i</i>Pr) (<b>7</b>) clearly shows that an isopropoxide group migrates onto the bound CO₂ ligand, and a P–Ni moiety is regenerated

Heterolytic H₂ Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane

Reversible, heterolytic addition of H₂ 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 H₂ Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane

Reversible, heterolytic addition of H₂ 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