Customizing Polyolefin Morphology by Selective Pairing of Alkali Ions with Nickel Phenoxyimine-Polyethylene Glycol Catalysts

In the present work, we have prepared nickel phenoxyimine-polyethylene glycol (PEG) catalysts with sterically bulky <i>N</i>-aryl substituents and investigated their ethylene homo- and copolymerization behavior. We have found that different nickel catalyst and alkali ion (Na⁺ or K⁺) combinations yielded polyethylene with different branching microstructures and molecular weights. Our heterobimetallic catalysts can copolymerize ethylene and nonpolar α-olefins with high activity but are strongly inhibited in the presence of polar vinyl olefins. We demonstrate that our heterobimetallic catalysts are significantly more stable in ethylene homopolymerization in comparison to conventional nickel phenoxyimine systems on the basis of time-dependent activity studies. This work showcases the versatility of Lewis acid tunable catalyst constructs to prepare customized polyolefins and suggests that similar design strategies could be applied to other catalyst systems

Triazolecarboxamidate Donors as Supporting Ligands for Nickel Olefin Polymerization Catalysts

To increase the structural diversity of dinucleating platforms that are used in the construction of olefin polymerization catalysts, we are exploring new ligand designs that feature non-alkoxide/phenoxide bridging groups. In the current study, we demonstrate that 1,2,3-triazole-4-carboxamidate donors are excellent <i>N</i>,<i>N</i>-chelators for nickel and can readily bind secondary metal ions. We found that sterically bulky nickel triazolecarboxamidate complexes are active as ethylene homopolymerization catalysts and can afford low molecular weight polyethylene with about 80–130 branches per 1000 carbon atoms. The addition of zinc salts to our nickel complexes led to catalyst inhibition in some cases, which we have attributed to the formation of catalytically inactive mixed-metal species. To circumvent this problem, we anticipate that further elaboration of the triazolecarboxamidate ligand could provide discrete heterobimetallic complexes that will be useful as single-site catalysts with unique reactivity patterns

Catalytic Hydrogenation of Cytotoxic Aldehydes Using Nicotinamide Adenine Dinucleotide (NADH) in Cell Growth Media

We demonstrate, for the first time, that pentamethylcyclopentadienyl (Cp*) iridium pyridinecarboxamidate complexes (<b>5</b>) can promote <i>catalytic</i> hydride transfer from nicotinamide adenine dinucleotide to aldehydes in pH 7.4 buffered cell growth media at 37 °C and in the presence of various biomolecules and metal ions. Stoichiometric hydride transfer studies suggest that the unique reactivity of <b>5</b>, compared to other common Cp*Ir complexes, is at least in part due to the increased hydride transfer efficiency of its iridium hydride species <b>5-H</b>. Complex <b>5</b> exhibits excellent reductase enzyme-like activity in the hydrogenation of cytotoxic aldehydes that have been implicated in a variety of diseases

Fine-Tuning Nickel Phenoxyimine Olefin Polymerization Catalysts: Performance Boosting by Alkali Cations

To gain a better understanding of the influence of cationic additives on coordination–insertion polymerization and to leverage this knowledge in the construction of enhanced olefin polymerization catalysts, we have synthesized a new family of nickel phenoxyimine–polyethylene glycol complexes (<b>NiL0</b>, <b>NiL2</b>–<b>NiL4</b>) that form discrete molecular species with alkali metal ions (M⁺ = Li⁺, Na⁺, K⁺). Metal binding titration studies and structural characterization by X-ray crystallography provide evidence for the self-assembly of both 1:1 and 2:1 <b>NiL</b>:M⁺ species in solution, except for <b>NiL4</b>/Na⁺ which form only the 1:1 complex. It was found that upon treatment with a phosphine scavenger, these <b>NiL</b> complexes are active catalysts for ethylene polymerization. We demonstrate that the addition of M⁺ to <b>NiL</b> can result in up to a 20-fold increase in catalytic efficiency as well as enhancement in polymer molecular weight and branching frequency compared to the use of <b>NiL</b> without coadditives. To the best of our knowledge, this work provides the first systematic study of the effect of secondary metal ions on metal-catalyzed polymerization processes and offers a new general design strategy for developing the next generation of high performance olefin polymerization catalysts

Intracellular Transfer Hydrogenation Mediated by Unprotected Organoiridium Catalysts

In the present work, we show for the first time that the conversion of aldehydes to alcohols can be achieved using “unprotected” iridium transfer hydrogenation catalysts inside living cells. The reactions were observed in real time by confocal fluorescence microscopy using a Bodipy fluorogenic substrate. We propose that the reduced cofactor nicotinamide adenine dinucleotide (NADH) is a possible hydride source inside the cell based on studies using pyruvate as a cellular redox modulator. We expect that this biocompatible reductive chemistry will be broadly useful to practitioners working at the interface of chemistry and the life sciences

Selective Acceptorless Dehydrogenation and Hydrogenation by Iridium Catalysts Enabling Facile Interconversion of Glucocorticoids

An iridium­(III) pentamethylcyclopentadienyl catalyst supported by 6,6′-dihydroxy-2,2′-bipyridine displays exquisite selectivity in acceptorless alcohol dehydrogenation of cyclic α,β-unsaturated alcohols over benzylic and aliphatic alcohols under mild aqueous reaction conditions. Hydrogenation of aldehydes and ketones occurs indiscriminately using the same catalyst under hydrogen, although chemoselectivity could be achieved when other potentially reactive carbonyl groups present are sterically inaccessible. This chemistry was demonstrated in the reversible hydrogenation and dehydrogenation of the A ring of glucocorticoids, despite the presence of other alcohol/or carbonyl functionalities in rings C and D. NMR studies suggest that an iridium­(III) hydride species is a key intermediate in both hydrogenation and dehydrogenation processes

Mechanistic Studies of Ethylene and α-Olefin Co-Oligomerization Catalyzed by Chromium–PNP Complexes

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons from ethylene as a chemical feedstock, co-oligomerization of ethylene and linear α-olefins (LAOs) was investigated, using a previously reported chromium complex, [CrCl₃(PNP^OMe)] (<b>1</b>, where PNP^OMe = <i>N</i>,<i>N</i>-bis­(bis­(<i>o</i>-methoxyphenyl)­phosphino)­methylamine). Activation of <b>1</b> by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C₆ and C₁₀ alkene products. The identities of the C₁₀ isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprised of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO cotrimers and that cotrimers derived from one ethylene and two LAO molecules are also generated. Complex <b>1/</b>MMAO is also capable of converting 1-hexene to C₁₂ dimers and C₁₈ trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed and compared to previous reports of olefin cotrimerization

Spectral Studies of a Cr(PNP)–MAO System for Selective Ethylene Trimerization Catalysis: Searching for the Active Species

Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble Cr^IIICl₃(PNP) (PNP = bis­(diarylphosphino)­alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of Cr^IIICl₃(PNP) with MAO leads first to replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower–valent species. Reactivity studies demonstrate that the majority of the chromium species detected are not catalytically active

Evaluating the Identity and Diiron Core Transformations of a (μ-Oxo)diiron(III) Complex Supported by Electron-Rich Tris(pyridyl-2-methyl)amine Ligands

The composition of a (μ-oxo)­diiron­(III) complex coordinated by tris­[(3,5-dimethyl-4-methoxy)­pyridyl-2-methyl]­amine (R₃TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron­(III) perchlorate, sodium hydroxide, and R₃TPA affords [Fe₂(μ-O)­(μ-OH)­(R₃TPA)₂]­(ClO₄)₃ (<b>2</b>) rather than the previously reported species [Fe₂(μ-O)­(OH)­(H₂O)­(R₃TPA)₂]­(ClO₄)₃ (<b>1</b>). Facile conversion of the (μ-oxo)­(μ-hydroxo)­diiron­(III) core of <b>2</b> to the (μ-oxo)­(hydroxo)­(aqua)­diiron­(III) core of <b>1</b> occurs in the presence of water and at low temperature. When <b>2</b> is exposed to wet acetonitrile at room temperature, the CH₃CN adduct is hydrolyzed to CH₃COO^–, which forms the compound [Fe₂(μ-O)­(μ-CH₃COO)­(R₃TPA)₂]­(ClO₄)₃ (<b>10</b>). The identity of <b>10</b> was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not <b>1</b> and <b>2</b> are capable of generating the diiron­(IV) species [Fe₂(μ-O)­(OH)­(O)­(R₃TPA)₂]³⁺ (<b>4</b>), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because <b>2</b> reacts rapidly with hydrogen peroxide in CH₃CN but not in CH₃CN/H₂O, conditions that favor conversion to <b>1</b>, complex <b>1</b> is not a likely precursor to <b>4</b>. Compound <b>4</b> also forms in the reaction of <b>2</b> with H₂O₂ in solvents lacking a nitrile, suggesting that hydrolysis of CH₃CN is not involved in the H₂O₂ activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R₃TPA ligands and elaborate on conditions required to generate synthetic models of diiron­(IV) protein intermediates with this ligand framework

2‑Azaaryl-1-methylpyridinium Halides: Aqueous-Soluble Activating Reagents for Efficient Amide Coupling in Water

In this work, a class of 2-azaaryl-1-methylpyridinium (AMPx) reagents capable of promoting amidation processes in 100% water is reported. The process mass intensity of the AMPx-promoted reactions is similar to or lower than that of reactions using conventional coupling reagents, which suggests that the former has potential as a green amide synthesis method. It was found that the N-methylimidazole-based AMPim1 could be used to couple a wide range of carboxylic acids with amines, including natural amino acids to form peptide bonds. Tandem oxidation–amidation and reduction–amidation reactions in the presence of AMPim1 were achieved with up to moderate efficiency. It is proposed that the azaarene in AMPx plays multiple roles in the amide bond forming process, including as a leaving group, activator, and base