Reactivity with Alkylaluminum of a Chromium Complex of a Pyridine-Containing PNP Ligand: Redox N–P Bond Cleavage

The ligand 2,6-[(Ph₂)₂PN]₂C₅H₃N based on the popular PNP motif has been used to generate the corresponding chromium adduct {2,6-[(Ph₂)₂PN]₂C₅H₃N}­CrCl₃·2.5THF (<b>1</b>). Its reaction with Et₂AlCl and Cl₂AlEt afforded the two nearly isostructural complexes {2,6-(Ph₂PNH)­[(Et₂ClAl)­NPPh₂]­C₅H₃N}­CrCl­(PEtPh₂)·0.5­(toluene) (<b>2</b>) and {2,6-(Ph₂PNH)­[(EtCl₂Al)­NPPh₂]­C₅H₃N}­CrCl­(PEtPh₂)·0.5­(toluene) (<b>3</b>). The formation of these two species is the result of a multiple attack of the activator at both the ligand system and the metal center. During the reaction, the two nitrogen atoms lost one phosphine residue each, the metal was reduced, one of the two nitrogens was protonated, and one EtPPh₂ molecule was formed and retained by the metal center. The three complexes characterized in this work display activity for catalytic and nonselective ethylene oligomerization

Synthesis, Structures, and Ethylene Oligomerization Activity of Bis(phosphanylamine)pyridine Chromium/Aluminate Complexes

A trivalent chromium complex of a PN­(pyridine) ligand system, {[(2,6-Ph₂P-NH)₂C₅H₃N]­CrCl₃}­(THF)₂ (<b>1</b>), was prepared and tested as a catalyst for ethylene oligomerization and polymerization, with the purpose of probing the ability of a pyridine ring substituent as a stabilizing factor on catalytically active intermediates. Its nonselective catalytic behavior indicated that ready reduction of the metal center to the divalent state occurred during the activation process. To substantiate this point, we have reacted <b>1</b> with a few common aluminate activators and isolated both the divalent complexes {[2,6-Ph₂PNHC₅H₃NAlClEt₂NPPh₂]­Cr­(μ-Cl)₂AlEt₂}­(toluene) (<b>3</b>) and {[2,6-Ph₂PNHC₅H₃NAlCl-<i>i</i>-Bu₂NPPh₂]­Cr­(μ-Cl)₂Al-<i>i</i>-Bu₂}₂ (toluene) (<b>4</b>) and the trivalent complexes {[2,6-Ph₂PNHC₅H₃NAlClMe₂NPPh₂]­CrMe­(μ-Cl)₂AlMe₂}­(toluene)_1.5 (<b>2</b>) and {[2,6-Ph₂PNHC₅H₃ NHNPPh₂]­CrEt­(μ-Cl)₂AlEt₂}­AlEtCl₃(hexane)_0.5 (<b>5</b>). The reaction of the ligand with the divalent chromium precursor CrCl₂(THF)₂ in the presence of alkylaluminum afforded {[2,6-Ph₂PNHC₅H₃NCl₂EtAlNPPh₂]­Cr­(μ-Cl)₂AlEt₂}₂(toluene) (<b>6</b>) containing aluminate residues, where the metal has preserved the initial divalent state. All of these species showed moderate to high activities toward ethylene oligomerization

Reactivity with Alkylaluminum of a Chromium Complex of a Pyridine-Containing PNP Ligand: Redox N–P Bond Cleavage

The ligand 2,6-[(Ph₂)₂PN]₂C₅H₃N based on the popular PNP motif has been used to generate the corresponding chromium adduct {2,6-[(Ph₂)₂PN]₂C₅H₃N}­CrCl₃·2.5THF (<b>1</b>). Its reaction with Et₂AlCl and Cl₂AlEt afforded the two nearly isostructural complexes {2,6-(Ph₂PNH)­[(Et₂ClAl)­NPPh₂]­C₅H₃N}­CrCl­(PEtPh₂)·0.5­(toluene) (<b>2</b>) and {2,6-(Ph₂PNH)­[(EtCl₂Al)­NPPh₂]­C₅H₃N}­CrCl­(PEtPh₂)·0.5­(toluene) (<b>3</b>). The formation of these two species is the result of a multiple attack of the activator at both the ligand system and the metal center. During the reaction, the two nitrogen atoms lost one phosphine residue each, the metal was reduced, one of the two nitrogens was protonated, and one EtPPh₂ molecule was formed and retained by the metal center. The three complexes characterized in this work display activity for catalytic and nonselective ethylene oligomerization

Polymer-Free Ethylene Oligomerization Using a Pyridine-Based Pincer PNP-Type of Ligand

Di- and trivalent chromium complexes of the pyridine-based ligand [2,6-(Ph₂PCH₂)₂ C₅H₃N]­CrCl₃ (<b>1</b>) and {[2,6-(Ph₂CH₂)₂C₅H₃N]­CrCl₂}.(THF) (<b>2</b>) and their aluminate aggregates [2,6-(Ph₂CH₂)₂C₅H₃NCrCl­(μ-Cl)­AlClMe₂] (<b>3</b>), {[(2,6-(Ph₂CH₂)₂C₅H₃NCrCl­(μ-Cl)­AlClEt₂]}. (toluene)_0.5 (<b>4</b>), {2,6-(Ph₂CH₂)₂C₅H₃NCrEt­(μ-Cl)₂AlEt₂}­{AlCl₃Et} (<b>5</b>), {2,6-(PPh₂CH₂) C₅H₃N (PPh₂CH)­Al­(<i>i</i>-Bu)₂(μ-Cl)­Cr­(μ-Cl)₂Al­(<i>i</i>-Bu)₂}.(toluene)_1.5 (<b>6</b>), and {[2,6-(PPh₂CH₂)₂C₅H₃ N]₂Cr} {(μ-Cl)­[Al­(<i>i</i>-Bu)₃]₂} (<b>7</b>) were prepared, isolated, and their activities toward ethylene oligomerization tested. While complexes <b>3</b>, <b>5</b>, and <b>6</b> were directly accessible by reacting catalyst precursor <b>1</b> with Me₃Al, DEAC, and TIBA, respectively, complexes <b>4</b> and <b>7</b> were prepared using catalyst precursor <b>2</b> with DEAC and TIBA, respectively. All these complexes, with the exception of <b>7</b>, showed good activities for a polymer-free ethylene oligomerization. Complex <b>7</b> contains cationic chromium in its monovalent state and its encapsulation in an octahedral ligand field as defined by two ligands is probably responsible for its failure as a catalyst

Unexpected Role of Zinc Hydride in Catalytic Hydrosilylation of Ketones and Nitriles

The hydride compound DippNacNacZnH (<b>1</b>) catalyzes chemoselective hydrosilylation of ketones and aldehydes under mild conditions and chemoselective reduction of nitriles to imines. Mechanistic studies showed that the product of nitrile insertion into the Zn–H bond of <b>1</b>, DippNacNacZn-NC­(H)­(Ph) (<b>2</b>), is not a potent catalyst. Kinetic studies under catalytic conditions suggest a reversible coordination of silane to <b>1</b> to form an intermediate which then reacts with the substrate (nitrile or ketone) via a cyclic transition state to give the silylated product

Unexpected Role of Zinc Hydride in Catalytic Hydrosilylation of Ketones and Nitriles

The hydride compound DippNacNacZnH (<b>1</b>) catalyzes chemoselective hydrosilylation of ketones and aldehydes under mild conditions and chemoselective reduction of nitriles to imines. Mechanistic studies showed that the product of nitrile insertion into the Zn–H bond of <b>1</b>, DippNacNacZn-NC­(H)­(Ph) (<b>2</b>), is not a potent catalyst. Kinetic studies under catalytic conditions suggest a reversible coordination of silane to <b>1</b> to form an intermediate which then reacts with the substrate (nitrile or ketone) via a cyclic transition state to give the silylated product

Oxidative Addition of σ Bonds to an Al(I) Center

The Al­(I) compound NacNacAl (<b>1</b>, NacNac = [ArNC­(Me)­CHC­(Me)­NAr]⁻ and Ar = 2,6-Prⁱ₂C₆H₃) reacts with H–X (X = H, Si, B, Al, C, N, P, O) σ bonds of H₂, silanes, borane (HBpin, pin = pinacolate), allane (NacNacAlH₂), phosphine (HPPh₂), amines, alcohol (PrⁱOH), and Cp*H (Cp* = pentamethylcyclopentadiene) to give a series of hydride derivatives of the four-coordinate aluminum NacNacAlH­(X), which are characterized herein by spectroscopic methods (NMR and IR) and X-ray diffraction. This method allows for the syntheses of the first boryl hydride of aluminum and novel silyl hydride and phosphido hydride derivatives. In the case of the addition of NacNacAlH₂, the reaction is reversible, proving the possibility of reductive elimination from the species NacNacAlH­(X)

Radical Mechanisms in the Reaction of Organic Halides with Diiminepyridine Cobalt Complexes

The formally Co(0) complex LCo­(N₂) (L = 2,6-bis­(2,6-dimethylphenyliminoethyl)­pyridine) can be prepared via either Na/Hg reduction of LCoCl₂ or hydrogenolysis of LCoCH₂SiMe₃. In the latter reaction, LCoH could be trapped by reaction with NCC₆H₄-4-Cl to give LCoNCHC₆H₄-4-Cl. LCo­(N₂) reacts with many alkyl and aryl halides RX, including aryl chlorides, to give a mixture of LCoR and LCoX in a halogen atom abstraction mechanism. Intermediacy of free alkyl and aryl radicals is confirmed by the ring-opening of cyclopropylmethyl to crotyl, and the rearrangement of 2,4,6-^<i>t</i>Bu₃C₆H₂ to 3,5-^<i>t</i>Bu₂C₆H₃CMe₂CH₂, before binding to Co. The organocobalt species generated in this way react further with activated halides R′X (alkyl iodides; allyl and benzyl halides) to give cross-coupling products RR′ in what is most likely again a halogen abstraction mechanism. DFT studies support the proposed radical pathways for both steps. MeI couples smoothly with LCoCH₂SiMe₃ to give LCoI and CH₃CH₂SiMe₃, but the analogous reaction of ^<i>t</i>BuI leads in part to radical attack at the 3 and 4 positions of the pyridine ring to form (^<i>t</i>Bu₂-L)­CoI and (^<i>t</i>Bu₂-L)­CoI₂

Radical Mechanisms in the Reaction of Organic Halides with Diiminepyridine Cobalt Complexes

The formally Co(0) complex LCo­(N₂) (L = 2,6-bis­(2,6-dimethylphenyliminoethyl)­pyridine) can be prepared via either Na/Hg reduction of LCoCl₂ or hydrogenolysis of LCoCH₂SiMe₃. In the latter reaction, LCoH could be trapped by reaction with NCC₆H₄-4-Cl to give LCoNCHC₆H₄-4-Cl. LCo­(N₂) reacts with many alkyl and aryl halides RX, including aryl chlorides, to give a mixture of LCoR and LCoX in a halogen atom abstraction mechanism. Intermediacy of free alkyl and aryl radicals is confirmed by the ring-opening of cyclopropylmethyl to crotyl, and the rearrangement of 2,4,6-^<i>t</i>Bu₃C₆H₂ to 3,5-^<i>t</i>Bu₂C₆H₃CMe₂CH₂, before binding to Co. The organocobalt species generated in this way react further with activated halides R′X (alkyl iodides; allyl and benzyl halides) to give cross-coupling products RR′ in what is most likely again a halogen abstraction mechanism. DFT studies support the proposed radical pathways for both steps. MeI couples smoothly with LCoCH₂SiMe₃ to give LCoI and CH₃CH₂SiMe₃, but the analogous reaction of ^<i>t</i>BuI leads in part to radical attack at the 3 and 4 positions of the pyridine ring to form (^<i>t</i>Bu₂-L)­CoI and (^<i>t</i>Bu₂-L)­CoI₂

Half-Sandwich Silane σ‑Complexes of Ruthenium Supported by NHC Carbene

Reactions of carbene complex [Cp­(IPr)­Ru­(pyr)₂]­[BF₄] (<b>6</b>, IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) with excess acetonitrile and LiCl in THF afford complexes [Cp­(IPr)­Ru­(NCCH₃)₂]­[PF₆] (<b>7</b>) and Cp­(IPr)­RuCl (<b>8</b>), respectively. Complex <b>8</b> was characterized by NMR and X-ray diffraction analysis. Addition of hydrosilanes to <b>8</b> results in silane σ-complexes Cp­(IPr)­Ru­(η²-HSiR₃)Cl (<b>4</b>), which were characterized by NMR and X-ray studies of Cp­(IPr)­Ru­(η²-HSiMeCl₂)Cl (<b>4b</b>) and Cp­(IPr)­Ru­(η²-H₃SiPh)Cl (<b>4d</b>). The hydrogen–silicon coupling constants of complexes <b>4</b> show an unusual trend in that the <i>J</i>(H–Si) values increase from the less-chlorinated complex Cp­(IPr)­Ru­(η²-HSiMe₂Cl)Cl (<b>4c</b>) to the trichloro derivative Cp­(IPr)­Ru­(η²-HSiCl₃)Cl (<b>4a</b>). Reaction of Cp­(IPr)­RuCl (<b>8</b>) with two equivalents of HSiCl₃ gave the ruthenate complex [IPrH]⁺­[CpRuCl­(H) (SiCl₃)₂]⁻, characterized by NMR and X-ray study. Addition of hydrosilanes to the cationic complex [Cp­(IPr)­Ru­(NCCH₃)₂]­[BAr^F₄] (<b>9</b>) furnished very unstable cationic silane σ-complexes [Cp­(IPr)­Ru­(η²-HSiR₃)­(NCCH₃)]⁺ (<b>5</b>), characterized by low-temperature NMR. Reaction of complex <b>9</b> with two equivalents of HSiCl₃ gives the neutral bis­(silyl) complex CpRu­(NCCH₃)­(H)­(SiCl₃)₂ and [IPrH]­[BAr^F₄]. Catalytic studies showed that <b>9</b> is a poorer catalyst for hydrosilylation of benzaldehyde, benzonitrile, and pyridine than its phosphine analogue [Cp­(^<i>i</i>Pr₃P)­Ru­(NCCH₃)₂]­[BAr^F₄]. The reason for this reduced activity was assigned to the easy dissociation of carbene from the former catalyst