Living Polymerization of Ethylene and Copolymerization of Ethylene/Methyl Acrylate Using “Sandwich” Diimine Palladium Catalysts

Cationic Pd­(II) catalysts incorporating bulky 8-<i>p</i>-tolylnaphthyl substituted diimine ligands have been synthesized and investigated for ethylene polymerization and ethylene/methyl acrylate copolymerization. Homopolymerization of ethylene at room temperature resulted in branched polyethylene with narrow <i>M</i>_w/<i>M</i>_n values (ca. 1.1), indicative of a living polymerization. A mechanistic study revealed that the catalyst resting state was an alkyl olefin complex and that the turnover-limiting step was migratory insertion, thus the turnover frequency is independent of ethylene concentration. Copolymerization of ethylene and methyl acrylate (MA) was also achieved. MA incorporation was found to increase linearly with MA concentration, and copolymers with up to 14 mol % MA were prepared. Mechanistic studies revealed that acrylate insertion into a Pd–CH₃ bond occurs at −70 °C to yield a four-membered chelate, which isomerizes first to a five-membered chelate and then to a six-membered chelate. Barriers to migratory insertion of both the (diimine)­PdCH₃(C₂H₄)⁺ (19.2 kcal/mol) and (diimine)­PdCH₃(η²-C₂H₃CO₂Me)⁺ (15.2 kcal/mol) were measured by low-temperature NMR kinetics

Understanding the Effect of Ancillary Ligands on Concerted Metalation–Deprotonation by (^dmPhebox)Ir(OAc)₂(H₂O) Complexes: A DFT Study

A DFT study of (^dmPhebox)­Ir­(OAc)₂(H₂O) was undertaken to understand ligand effects that control the propensity of this complex to undergo intermolecular alkane C–H activation by a concerted metalation–deprotonation (CMD) mechanism. Substitution of electronically diverse substituents at the <i>para</i> position of the aryl ring and exchange of the pincer oxazolinyl arms with other donor groups were calculated to minimally impact the barriers to C–H activation. It is suggested that these modifications do not influence the orbitals directly involved in the six-membered CMD transition state. The base and spectator carboxylate ligands were calculated to play two distinct roles in the activation with different electronic preferences, namely, their intrinsic basicity and <i>trans</i> influence/effect, respectively. Heteroaryl linkers in the pincer backbone were identified as a promising lead, yielding noticeably lower computed C–H activation barriers, due to increased electrophilicity of the cationic metal center

Alkane Dehydrogenation by C–H Activation at Iridium(III)

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>), has been described. The reaction between <b>1a</b> and octane resulted in quantitative formation of (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>3a</b>) and octene. At early reaction times 1-octene is the major product, indicative of terminal C–H activation by <b>1a</b>. In contrast to prior reports of alkane dehydrogenation with Ir, C–H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems

Alkane Dehydrogenation by C–H Activation at Iridium(III)

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>), has been described. The reaction between <b>1a</b> and octane resulted in quantitative formation of (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>3a</b>) and octene. At early reaction times 1-octene is the major product, indicative of terminal C–H activation by <b>1a</b>. In contrast to prior reports of alkane dehydrogenation with Ir, C–H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems

Synthesis of Branched Polyethylene with “Half-Sandwich” Pyridine-Imine Nickel Complexes

Traditional cationic Pd­(II) and Ni­(II) ethylene polymerization catalysts are supported by ortho-disubstituted aryl diimine ligands. These catalysts are capable of producing high-molecular-weight polyethylene due to positioning of bulk in the two axial sites of the square coordination plane which retards chain transfer. Similar pyridine-imine complexes bearing a single ortho-disubstituted aryl imine moiety were reported to yield very low <i>M</i>_n polyethylene. In earlier studies, “sandwich” diimine nickel catalysts incorporating two 8-arylnaphthylimino groups which provide exceptional shielding of the two axial sites were shown to yield ultrahigh-molecular-weight polyethylene. Here we demonstrate that 8-arylnaphthyl groups incorporated into pyridine-imine nickel catalysts that block only a single axial site are highly effective in retarding chain transfer. These catalysts produce branched polyethylene (ca. 30–90 branches per 1000 carbons) with <i>M</i>_n values up to 2.6 × 10⁴ g/mol. Effects on the catalyst lifetimes and polymerization behavior as a function of substituent variations at the imine carbon and the aryl group are reported

Synthesis of Branched Polyethylene with “Half-Sandwich” Pyridine-Imine Nickel Complexes

Traditional cationic Pd­(II) and Ni­(II) ethylene polymerization catalysts are supported by ortho-disubstituted aryl diimine ligands. These catalysts are capable of producing high-molecular-weight polyethylene due to positioning of bulk in the two axial sites of the square coordination plane which retards chain transfer. Similar pyridine-imine complexes bearing a single ortho-disubstituted aryl imine moiety were reported to yield very low <i>M</i>_n polyethylene. In earlier studies, “sandwich” diimine nickel catalysts incorporating two 8-arylnaphthylimino groups which provide exceptional shielding of the two axial sites were shown to yield ultrahigh-molecular-weight polyethylene. Here we demonstrate that 8-arylnaphthyl groups incorporated into pyridine-imine nickel catalysts that block only a single axial site are highly effective in retarding chain transfer. These catalysts produce branched polyethylene (ca. 30–90 branches per 1000 carbons) with <i>M</i>_n values up to 2.6 × 10⁴ g/mol. Effects on the catalyst lifetimes and polymerization behavior as a function of substituent variations at the imine carbon and the aryl group are reported