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

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

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

Aminophosphine-Based Chromium Catalysts for Selective Ethylene Tetramerization

Chromium complexes of three new ligands, Ph₂PN­(Me)­(CH₂)₂-X [X = NMe₂ <b>(PNN</b>); PPh₂ (<b>PNP</b>); Py <b>(PNPy</b>)], have been examined vis-à-vis their ability to promote ethylene tetramerization, (PNN)­CrCl₃(L) [L = THF (<b>1</b>); CH₃CN (<b>2</b>)], (PNPy)­CrCl₃(L) [L = THF (<b>3</b>); CH₃CN (<b>4</b>)], and (PNP)­CrCl₃(THF) (<b>5</b>). In the case of <b>2</b> and <b>4</b>, it was possible to grow crystals suitable for X-ray diffraction. The reaction of <b>3</b> with Et₃Al afforded the dinuclear [(HN­(Me)­(CH₂)₂Py)­CrCl₂Et]₂ (<b>6</b>) containing a trivalent chromium connected to an Et group. During the alkylation though, the ligand has been fragmented with removal of the side arm and protonation of the N atom of the remaining NP residue. All the complexes have been tested for ethylene oligomerization activity. Complex <b>1</b> displayed the highest selectivity for 1-octene, upon activation with DMAO in MeCy. Contrary to expectations, complex <b>6</b> is not a self-activating catalyst

Isolation and Characterization of a Class II Mixed-Valence Chromium(I)/(II) Self-Activating Ethylene Trimerization Catalyst

Reduction of the tetranuclear {[(<i>t</i>-Bu)­NPPh₂]­Cr­[μ-(<i>t</i>-Bu)­NPPh₂]₂Cr}₂(μ-Cl)₂ (<b>1</b>) with either KC₈ or vinyl Grignard afforded the dinuclear, mixed-valence (Me₃P)­Cr­[μ-(<i>t</i>-Bu)­NPPh₂]₃Cr (<b>2</b>) with the two metals possessing distinctively different coordination environments. According to the formulation of <b>2</b> as Cr­(I)/Cr­(II) mixed-valence species, <b>2</b> acts as a self-activating catalyst, producing under pressure of ethylene a mixture of 1-butene and 1-hexene. Activation with three different activators selectively produced three different products, namely, 1-butene with TEAL, 1-hexene with DMAO/TEAL, and LAO-free highly linear HMWPE with DMAO. Mixtures of 1-hexene and 1-butene were also obtained upon activation with vinyl Grignard. In this case it was possible to isolate, albeit in very low yield, an intriguing butadiene/butadiene-diyl cluster, {[(η⁴-butadiene)Cr (μ,η⁴-butadienediyl)­(μ-NP)­Mg]₂(μ-Cl)₄Mg­(THF)₂}­{[(THF)₃Mg]₂(μ-Cl)₃}₂ (<b>3</b>), which is also a highly selective self-activating trimerization catalyst

Heterometallic Aluminum–Chromium Phenazine and Thiophenazine Complexes. Formation of a Tetranuclear Chromium(I) Sandwich Complex

Treatment of a suspension of phenazine and CrCl₂(THF)₂ in toluene with 4 equiv of triethylaluminum (Scheme ) affords the pentanuclear, heterobimetallic, and divalent [AlEt₂(THF)­(μ₃-η¹:η¹-C₁₂H₈N₂)­(μ-κ¹-AlEt₂Cl)]₂(μ-Cr) (<b>1</b>). In this species the two nitrogen atoms of each phenazine ligand are connected to three metals. A similar reaction with thiophenazine afforded trimetallic [(η¹:η¹-C₁₂H₈NS)­(μ-κ¹-AlEt₂Cl)]₂(μ-Cr) (<b>2</b>), where the sulfur atoms remained uncoordinated. When the preparation of <b>1</b> was carried out in the presence of excess Et₃Al, the reaction took a different pathway, affording [AlEt₂]₂(μ-η¹:η¹-η⁶:η⁶-C₁₂H₈N₂Cr)₂ (<b>3</b>). The aluminate residues hold two parallel and eclipsed phenazine molecules, forming a rigid cage where the two chromium atoms are encapsulated. The ambiguity about the metal oxidation state has been solved by DFT calculations, revealing the formal oxidation state of chromium as being better described as monovalent

Isolation and Characterization of a Class II Mixed-Valence Chromium(I)/(II) Self-Activating Ethylene Trimerization Catalyst

Reduction of the tetranuclear {[(<i>t</i>-Bu)­NPPh₂]­Cr­[μ-(<i>t</i>-Bu)­NPPh₂]₂Cr}₂(μ-Cl)₂ (<b>1</b>) with either KC₈ or vinyl Grignard afforded the dinuclear, mixed-valence (Me₃P)­Cr­[μ-(<i>t</i>-Bu)­NPPh₂]₃Cr (<b>2</b>) with the two metals possessing distinctively different coordination environments. According to the formulation of <b>2</b> as Cr­(I)/Cr­(II) mixed-valence species, <b>2</b> acts as a self-activating catalyst, producing under pressure of ethylene a mixture of 1-butene and 1-hexene. Activation with three different activators selectively produced three different products, namely, 1-butene with TEAL, 1-hexene with DMAO/TEAL, and LAO-free highly linear HMWPE with DMAO. Mixtures of 1-hexene and 1-butene were also obtained upon activation with vinyl Grignard. In this case it was possible to isolate, albeit in very low yield, an intriguing butadiene/butadiene-diyl cluster, {[(η⁴-butadiene)Cr (μ,η⁴-butadienediyl)­(μ-NP)­Mg]₂(μ-Cl)₄Mg­(THF)₂}­{[(THF)₃Mg]₂(μ-Cl)₃}₂ (<b>3</b>), which is also a highly selective self-activating trimerization catalyst

Aminophosphine-Based Chromium Catalysts for Selective Ethylene Tetramerization

Chromium complexes of three new ligands, Ph₂PN­(Me)­(CH₂)₂-X [X = NMe₂ <b>(PNN</b>); PPh₂ (<b>PNP</b>); Py <b>(PNPy</b>)], have been examined vis-à-vis their ability to promote ethylene tetramerization, (PNN)­CrCl₃(L) [L = THF (<b>1</b>); CH₃CN (<b>2</b>)], (PNPy)­CrCl₃(L) [L = THF (<b>3</b>); CH₃CN (<b>4</b>)], and (PNP)­CrCl₃(THF) (<b>5</b>). In the case of <b>2</b> and <b>4</b>, it was possible to grow crystals suitable for X-ray diffraction. The reaction of <b>3</b> with Et₃Al afforded the dinuclear [(HN­(Me)­(CH₂)₂Py)­CrCl₂Et]₂ (<b>6</b>) containing a trivalent chromium connected to an Et group. During the alkylation though, the ligand has been fragmented with removal of the side arm and protonation of the N atom of the remaining NP residue. All the complexes have been tested for ethylene oligomerization activity. Complex <b>1</b> displayed the highest selectivity for 1-octene, upon activation with DMAO in MeCy. Contrary to expectations, complex <b>6</b> is not a self-activating catalyst