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

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

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

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

Radical chemistry of alkyl aluminum with quinoxaline ligands

<p>The behavior of organo-aluminum species with 2,3-bis(2-pyridyl) quinoxaline (DPQ), a well-known polyazine capable of performing interesting radical transformations, was examined in the presence and absence of chromium. In spite of proving the presence of chromium as essential for reactivity, only organic radicals, coupled to aluminum-containing residues, have been isolated and characterized. The electronic structure of the organic radicals has been elucidated by a combination of crystallographic, DFT calculations and EPR studies. Experimental and computational work has highlighted the co-existence of both singlet and triplet forms in one di-radical complex.</p

Chromium–Chromium Interaction in a Binuclear Mixed-Valent Cr^I–Cr^II Complex

A mixed-valent Cr^I–Cr^II binuclear complex, {κ¹,κ²,κ³-<i>N</i>,<i>P</i>,<i>P</i>-cyclo­[(Ph)­PCH₂N­(CH₂Ph)­CH₂]}₂(CrCl₂)­[Cr­(μ-Cl)­(AlClMe₂)]·4toluene (<b>1</b>), of a P₂N₂ cyclic ligand was obtained upon treatment of the chromium precursor with alkylaluminum. Complex <b>1</b> was accessible from either its trivalent or divalent precursors, and density functional theory calculations revealed the presence of only σ- and π-orbital interactions in the Cr–Cr bond

Chromium–Chromium Interaction in a Binuclear Mixed-Valent Cr^I–Cr^II Complex

A mixed-valent Cr^I–Cr^II binuclear complex, {κ¹,κ²,κ³-<i>N</i>,<i>P</i>,<i>P</i>-cyclo­[(Ph)­PCH₂N­(CH₂Ph)­CH₂]}₂(CrCl₂)­[Cr­(μ-Cl)­(AlClMe₂)]·4toluene (<b>1</b>), of a P₂N₂ cyclic ligand was obtained upon treatment of the chromium precursor with alkylaluminum. Complex <b>1</b> was accessible from either its trivalent or divalent precursors, and density functional theory calculations revealed the presence of only σ- and π-orbital interactions in the Cr–Cr bond

Isolation of a Hexanuclear Chromium Cluster with a Tetrahedral Hydridic Core and Its Catalytic Behavior for Ethylene Oligomerization

A chromium complex [2-(NHCH₂PPh₂)­C₅H₄N]­CrCl₃·THF₂ (<b>1</b>) of the ligand PyNHCH₂PPh₂ has been synthesized, characterized, and examined for its catalytic behavior toward ethylene oligomerization. When complex <b>1</b> was treated with (<i>i</i>-Bu)₃Al, an unprecedented divalent polyhydride chromium cluster μ,κ¹,κ²,κ³-<i>N,N</i>,P-{[2-(NCH₂PPh₂)­C₅H₄N]­Cr­(μ-H)}₄­[(μ-Cl)­Cr­(μ-Cl)­Al­(<i>i</i>-Bu)₂Cl]₂ (<b>2</b>) was obtained. The complex contains a Cr₄H₄ core, which is expected to be diamagnetic, and which remains coordinated to two additional divalent high-spin Cr atoms via bridging interactions. Two aluminate residues remain bonded to the peripheral chromium atoms. The structure, magnetism, and electronic configuration are herein discussed