Chromium catalysts that switch the selectivity

Abstract only

Chromium catalysts that switch the selectivity

Abstract only

Isolation of a chromium hydride single-component ethylene polymerization catalyst

Reaction of the divalent complex [(t-Bu)NPN(t-Bu)]2Cr (1) with different stoichiometric ratios of Al(i-Bu)3 afforded {µ-[(t-Bu)NP(i-Bu)N(t-Bu)]Al(i-Bu)2}2Cr (2) and [{(i-Bu)P{µ-N(t-Bu)}2Al(i-Bu)2}Cr(µ-H)]2 (3) as part of the same reaction sequence. Complex 2 arises from association of alane and the two ligands via alkylation of the P atom and retention of the AlR2 unit. Complex 3 appears to be generated from subsequent dissociation of one alkylated ligand and possible transfer of an i-Bu group to Cr followed by ß-H elimination or transfer of a hydride originating from isobutyl elimination of the aluminum-containing residue. Both species are potent ethylene polymerization catalysts with no need for further activation. It is assumed that 2 is transformed in situ into 3 under the influence of ethylene. Reactions with ethylene carried out in the presence of excess of Al(i-Bu)3 switch the selectivity completely toward selective trimerization

Isolation of a self-activating ethylene trimerization catalyst of a Cr-SNS system

Two pyridine-SNS ligands (2,6-CH2SR)2C5H5N (R = t-Bu (a), n-decyl (b)) were prepared and tested for the possibility of forming active and selective chromium trimerization catalyst precursors. The two trivalent derivatives [(2,6-CH2SR)2C5H5N]CrCl3 (1a,b), and the divalent [(2,6-CH2S(t-Bu)2C5H5N]CrCl2·THF (2a) were isolated and characterized. Upon treatment of 1a with Me3Al, the self-activating, single-component, highly selective chromium trimerization catalyst [2,6-CH2S(t-Bu)2C5H5N]CrMe[(µ-Cl)AlMe3)]2 (3a) was obtained and fully characterized

Highly active ethylene oligomerization catalysts

Reaction of RN(H)PBr(Ph)2N(H)R [R = t-Bu, i-Pr, Ph] with two equivalents of n-BuLi followed by reaction with CrCl2(THF)2 afforded the divalent chromium complexes [(t-Bu)NP(Ph)2N(t-Bu)]Cr(µ2-Cl)2Li(THF)2 (1), [(i-Pr)NP(Ph)2N(i-Pr)]Cr(µ2-Cl)2Li(THF)2 (2), and [{[(Ph)NP(Ph)2N(Ph)]Cr}2(µ2-Cl)3][Li(DME)3] (3). The trivalent analogue of 1, {[(t-Bu)NP(Ph)2N(t-Bu)]Cr(µ2-Cl)3(µ3-Cl)2}{Li (THF)2} (4), was obtained in a similar manner via treatment of the double deprotonated ligand with CrCl3(THF)3. Both reactions of the divalent1 or trivalent4 with AlMe3 yielded the trivalent and cationic complex {[(t-Bu)NP(Ph)2N(t-Bu)]2Cr}{(Me3Al)2Cl}·toluene (5). Upon activation with MAO, 1–3 produced unprecedented and potentially useful catalytic systems for nonselective ethylene oligomerization devoid of polymer. Divalent chromium is clearly the species responsible for the catalytic behavior, ruling out that nonselective oligomerization proceeds via a redox metallacycle mechanism. The absence of polymer in combination with the record activity makes 1 competitive with the best performing industrially used systems

Preparation and characterization of a switchable single-component chromium trimerization catalyst

Reaction of [(Ar)NPN(t-Bu)]2Cr (2) with Me3Al afforded {[(Ar)NP(Me)N(t-Bu)]AlMe2}Cr{[(Ar)NP(Me)(AlMe3)N(t-Bu)]AlMe(µ-Me)} (3), in which the ligands P atom has been alkylated and one AlR2 residue retained by the chelating framework. One of the two ligands also retained an additional Me3Al unit via coordination to the alkylated P atom. Complex 3 provides the first case of a selective ethylene trimerization catalyst with high activity and excellent selectivity, producing 1-hexene upon exposure to ethylene at 80 °C. Furthermore, upon treatment with MAO, complex 3 acts as a nonselective catalyst, producing a statistical mixture of oligomers with the highest ever observed activity. In addition, upon treatment with [(i-Bu)2Al]2O, the complex acts as a highly active polymerization catalyst

Isolation of a chromium hydride single-component ethylene polymerization catalyst

Reaction of the divalent complex [(t-Bu)NPN(t-Bu)]2Cr (1) with different stoichiometric ratios of Al(i-Bu)3 afforded {µ-[(t-Bu)NP(i-Bu)N(t-Bu)]Al(i-Bu)2}2Cr (2) and [{(i-Bu)P{µ-N(t-Bu)}2Al(i-Bu)2}Cr(µ-H)]2 (3) as part of the same reaction sequence. Complex 2 arises from association of alane and the two ligands via alkylation of the P atom and retention of the AlR2 unit. Complex 3 appears to be generated from subsequent dissociation of one alkylated ligand and possible transfer of an i-Bu group to Cr followed by ß-H elimination or transfer of a hydride originating from isobutyl elimination of the aluminum-containing residue. Both species are potent ethylene polymerization catalysts with no need for further activation. It is assumed that 2 is transformed in situ into 3 under the influence of ethylene. Reactions with ethylene carried out in the presence of excess of Al(i-Bu)3 switch the selectivity completely toward selective trimerization

A highly selective ethylene tetramerization catalyst

Small change, big difference: The introduction of an additional CH2 group into the bridge of ligands with two P/N units leads to a different selectivity of the corresponding chromium-based catalysts. Whereas 1 produces an ethylene trimerization system, 2 provides an unprecedented ethylene tetramerization system that produces 1-octene with high purity and little or no polymer side products (see scheme; DMAO=Me3Al-depleted methylaluminoxane)

Selective Ethylene Oligomerization with Chromium Complexes Bearing Pyridine-Phosphine Ligands:Influence of Ligand Structure on Catalytic Behavior

Chromium complexes bearing a series of pyridine–phosphine ligands have been synthesized and examined for their catalytic behavior in ethylene oligomerization. The choice of solvent, toluene versus methylcyclohexane, shows a pronounced influence on the catalytic activity for all these complexes. Variations of the ligand system have been introduced by modifying the phosphine substituents affecting ligand bite angles and flexibility. It has been demonstrated that minor differences in the ligand structure can result in remarkable changes not only in catalytic activity but also in selectivity toward a-olefins versus polyethylene and distribution of oligomeric products. Ligand PyCH2N(Me)PiPr2, in combination with CrCl3(THF)3 afforded selective ethylene tri- and tetramerization, giving 1-hexene and 1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE