For over 40 years following the polyolefin catalyst discoveries of Hogan and Banks (Phillips) and Ziegler (Max Planck Institute), chemists traversed the periodic table searching for new transition ...metal and lanthanide-based olefin polymerization systems. Remarkably, none of these “hits” employed iron, that is, until three groups independently reported iron catalysts for olefin polymerization in the late 1990’s. The history surrounding the discovery of these catalysts was only the beginning of their uniqueness, as the ensuing years have proven these systems remarkable in several regards. Of primary importance are the pyridine-bis(imine) ligands (herein referred to as PDI), which produced iron catalysts that are among the world’s most active for ethylene polymerization, demonstrated “staying power” despite over 15 years of ligand improvement efforts, and generated highly active polymerization systems with cobalt, chromium, and vanadium. Although many ligands have been employed in iron-catalyzed polymerization, the PDI family has thus far provided the most information about iron’s capabilities and tendencies. For example, iron systems tend to be highly selective for ethylene over higher olefins, making them strong candidates for producing highly crystalline polyethylene, or highly linear α-olefins. Iron PDI polymerizes propylene with 2,1-regiochemistry via a predominantly isotactic, chain end control mechanism. Because the first insertion proceeds via 1,2-regiochemistry, iron (and cobalt) PDI systems can be tailored to make highly linear dimers of α-olefins by “head-to-head” coupling, resulting from a switch in regiochemistry after the first insertion. Finally, PDI ligands, while not being surpassed in activity, have inspired the development of related ligand families and complexes, such as pendant donor diimines (PDD), which are also highly efficient at producing linear α-olefins. This Account will detail a variety of oligomerization and polymerization results achieved with PDI and PDD catalysts. Our studies on ligand modification are discussed, but numerous ligands have been synthesized by others. Computational approaches, identification of catalyst active sites, noninnocent ligand studies, commercialization efforts, and other outstanding research are only briefly mentioned, at most. The reader is directed to review articles where appropriate, in order to address the cursory treatment of these areas.
Cobalt(II) dichloride complexes supported by a variety of neutral, tridentate pincer ligands have been prepared and, following in situ activation with NaBEt3H, evaluated for the catalytic borylation ...of 2-methylfuran, 2,6-lutidine, and benzene using both HBPin and B2Pin2 (Pin = pinacolate) as boron sources. Preparation of well-defined organometallic compounds in combination with stoichiometric experiments with HBPin and B2Pin2 provided insight into the nature and kinetic stability of the catalytically relevant species. In cases where sufficiently electron donating pincers are present, such as with bis(phosphino)pyridine chelates, Co(III) resting states are preferred and catalytic C–H borylation is efficient. Introduction of a redox-active subunit into the pincer reduces its donating ability and, as a consequence, the accessibility of a Co(III) resting state. In these cases, unusual mixed-valent μ-hydride cobalt complexes have been crystallographically and spectroscopically characterized. These studies have also shed light on the active species formed during in situ activated cobalt alkene hydroboration catalysis and provide important design criteria in base metal catalyzed C–B bond forming reactions.
Herein we establish the utility of a three‐coordinate (N‐phosphinoamidinate)cobalt(amido) pre‐catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room ...temperature, leading to the selective terminal addition of the boron group.
Chain‐walking hydroborations: A three‐coordinate (N‐phosphinoamidinate)cobalt(amido) pre‐catalyst is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.
Warming ocean temperatures have been linked to kelp forest declines worldwide, and elevated temperatures can act synergistically with other local stressors to exacerbate kelp loss. The bull kelp ...Nereocystis luetkeana is the primary canopy-forming kelp species in the Salish Sea, where it is declining in areas with elevated summer water temperatures and low nutrient concentrations. To determine the interactive effects of these two stressors on microscopic stages of N. luetkeana, we cultured gametophytes and microscopic sporophytes from seven different Salish Sea populations across seven different temperatures (10-22°C) and two nitrogen concentrations. The thermal tolerance of microscopic gametophytes and sporophytes was similar across populations, and high temperatures were more stressful than low nitrogen levels. Additional nitrogen did not improve gametophyte or sporophyte survival at high temperatures. Gametophyte densities were highest between 10 and 16°C and declined sharply at 18°C, and temperatures of 20 and 22°C were lethal. The window for successful sporophyte production was narrower, peaking at 10-14°C. Across all populations, the warmest temperature at which sporophytes were produced was 16 or 18°C, but sporophyte densities were 78% lower at 16°C and 95% lower at 18°C compared to cooler temperatures. In the field, bottom temperatures revealed that the thermal limits of gametophyte growth (18°C) and sporophyte production (16-18°C) were reached during the summer at multiple sites. Prolonged exposure of bull kelp gametophytes to temperatures of 16°C and above could limit reproduction, and therefore recruitment, of adult kelp sporophytes.
The synthesis and structural characterization of three-coordinate iron(II) and cobalt(II) complexes supported by new N-phosphinoamidinate ligands is reported, along with the successful application of ...these complexes as precatalysts for the challenging room-temperature hydrosilylation of carbonyl compounds to afford alcohols upon workup. Under the rigorous screening conditions employed (0.015 mol % Fe) for the reduction of acetophenone, the well-defined iron(II) amido precatalyst 2b, featuring bulky N-2,6-diisopropylphenyl and di-tert-butylphosphino moieties within the N-phosphinoamidinate ligand, exhibited exceptional catalytic performance. Further experimentation revealed that the yield achieved in the hydrosilylation of acetophenone employing 2b was unaltered when conducting reactions in the absence of light, in the presence of excess mercury, or under solvent-free conditions. Notably, precatalyst 2b was found to exhibit the broadest substrate scope reported to date for such room-temperature iron-catalyzed carbonyl hydrosilylations en route to alcohols, enabling the chemoselective reduction of structurally diverse aldehydes and ketones, as well as for the first time esters, at remarkably low loadings (0.01–1.0 mol % Fe) and using only 1 equiv of phenylsilane reductant.
Molecular, Fe-catalyzed ethylene oligomerization provides access to a range of linear α-olefins (LAOs) that are used to produce polyethylene, lubricants, surfactants, and other commercial products. ...This work provides an experimental example of an Fe pendant donor diimine ((PDD)Fe) catalyzed ethylene oligomerization that showcases very high olefin oligomer purity without branching and provides K (propagation/(propagation + termination)) values of LAOs fractions, which show larger K values as a function of carbon chain length. This experimental example provided an anchor point to try to identify a practical density functional theory (DFT) protocol to model ethylene oligomerization branching, propagation/termination, and K values. Using M06-L DFT calculations, we successfully modeled the very high oligomerization purity for the (PDD)Fe catalyst, compared to the lower purity for the Fe tridentate pyridine bisimine (PBI)Fe catalyst, which showed enhanced regioselectivity for migratory insertion between Fe–H intermediates and LAOs. Modeling propagation/termination and K values were significantly more challenging with most oxidation and spin states incorrectly predicting a significant preference for oligomerization termination. Therefore, caution should be used when trying to model these types of quantitative catalysis values.
A new route to single-component iron ethylene oligomerization and polymerization catalysts is described. Treatment of readily synthesized iron butadiene complexes with B(C6F5)3 generated the ...corresponding betaine compounds, active catalysts for the oligomerization and polymerization of ethylene. The electronic structures of a family of iron compounds bearing tridentate, α-diimine phosphine ligands have been determined, including cases where the neutral donor has dissociated from the metal. In iron-catalyzed ethylene oligomerization with these compounds, the hemilability of the chelate has been identified as a catalyst deactiviation pathway.
A new family of highly active ethylene tri-/tetramerization catalysts based on N-phosphinoamidinechromium complexes has been investigated. The 1-hexene to 1-octene molar ratio can be tuned from 140 ...to 1.5 by varying the steric environment around the chromium center, and product purities are very good to excellent. Precatalyst tridentate coordination effectively shuts down catalytic activity, suggesting that THF abstraction from the chromium center by the Lewis acidic aluminum activator is necessary to achieve an active catalyst system.
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