An official website of the United States government
Abstract Hydrophosphination using calcium compounds as catalysts under irradiation is described as a foray into s‐block photocatalysis. Transition‐metal compounds have been highly successful hydrophosphination catalysts under photochemical conditions, utilizing substrates previously considered inaccessible. A calcium hydrophosphination precatalyst, Ca(nacnac) (THF) (N(SiMe3)2) (1, nacnac = HC[(C(Me)N‐2,6‐iPr2C6H3)]2), reported by Barrett and Hill, as well as the presumed intermediate, Ca(nacnac) (THF) (PPh2) (2), and the Schlenk equilibrium product, Ca[N(SiMe3)2]2(THF)2(3) were screened under photochemical conditions with a range of unsaturated substrates including styrenic alkenes, Michael acceptors, and dienes with modest to excellent conversions, though unactivated alkenes were inaccessible. All compounds exhibit enhanced catalysis under irradiation by light emitting diode (LED)‐generated blue light. Nacnac‐supported compounds generate radicals as evidenced by Electron Paramagnetic Resonance (EPR) spectroscopy and radical trapping reactions, whereas unsupported calcium compounds are EPR silent and appear to undergo hydrophosphination akin to thermal reactions with these compounds. These results buttress the notion that photoactivation of π‐basic ligands is a broad phenomenon, extending beyond the d‐block, but like d‐block metals, consideration of ancillary ligands is essential to avoid radical reactivity.
more »
« less
Effect of Photolysis on Zirconium Amino Phenoxides for the Hydrophosphination of Alkenes: Improving Catalysis
A comparative study of amino phenoxide zirconium catalysts in the hydrophosphination of alkenes with diphenylphosphine reveals enhanced activity upon irradiation during catalysis, with conversions up to 10-fold greater than reactions in ambient light. The origin of improved reactivity is hypothesized to result from substrate insertion upon an n→d charge transfer of a Zr–P bond in the excited state of putative phosphido (Zr–PR2) intermediates. TD-DFT analysis reveals the lowest lying excited state in the proposed active catalysts are dominated by a P 3p→Zr 4d MLCT, presumably leading to enhanced catalysis. This hypothesis follows from triamidoamine-supported zirconium catalysts but demonstrates the generality of photocatalytic hydrophosphination with d0 metals.
more »
« less
- Award ID(s):
- 2101766
- PAR ID:
- 10330926
- Date Published:
- Journal Name:
- Photochem
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2673-7256
- Page Range / eLocation ID:
- 77 to 87
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Carbamate formation and exchange catalysts enable efficient polyurethane (PU) manufacturing, as well as emerging recycling and reprocessing methods for PU thermosets. Zirconium β-diketonate complexes, such as Zr acetylacetonate [Zr(acac)4], are effective alternatives to toxic organotin catalysts that have been used for PU reprocessing. Here, we report that Zr(acac)4 undergoes a thermally activated process in the PU network during reprocessing that transforms it into a more active carbamate exchange catalyst. This process is associated with the irreversible loss of acetylacetonate ligands and is not observed for the more sterically hindered Zr 2,2,6,6-tetramethyl-3,5-heptanedione [Zr(tmhd)4] complex. Crossover experiments between PU thermoplastics indicated enhanced carbamate exchange after the thermal activation of Zr(acac)4 in the presence of one of the PUs, whereas a sample of Zr(acac)4 activated in the absence of the PU had no catalytic activity. Thermal gravimetric analysis suggested that this process is associated with the loss of one protonated acac ligand. Stress relaxation analysis of PU thermosets indicated a distinct change in the characteristic relaxation time associated with the thermal activation of Zr(acac)4 at temperatures above 140 °C; no such change was observed for samples reprocessed using Zr(tmhd)4. Density functional theory and molecular experiments suggest that irreversible ligand exchange of acac with alkoxide or carbamate reduces the activation energy for urethane formation and reversion. Furthermore, the Zr(acac)4 catalyst activated in the presence of a PU’s polyol precursor provided more porous and less dense PU foams compared to those made using the unactivated Zr(acac)4 catalyst. These findings are important for developing improved PU synthesis and recycling processes. Thermally activating a catalyst during reprocessing may provide more nuanced control of the in-use and reprocessing characteristics of PU thermosets.more » « less
-
An examination of several catalytic reactions among the group 15 elements is presented. The connections between the chemistry of the pnictogens can sometimes be challenging, but aspects of metal–pnictogen reactivity are the key. The connecting reactivity comes from metal-catalyzed transformations such as dehydrocoupling and hydrofunctionalization. Pivotal mechanistic insights from E–N heterodehydrocoupling have informed the development of highly active catalysts for these reactions. Metal–amido nucleophilicity is often at the core of this reactivity, which diverges from phosphine and arsine dehydrocoupling. Nucleophilicity connects to the earliest understanding of hydrophosphination catalysis, but more recent catalysts are leveraging enhanced insertion activity through photolysis. This photocatalysis extends to hydroarsination, which may also have more metal–arsenido nucleophilicity than anticipated. However, metal-catalyzed arsinidene chemistry foreshadowed related phosphinidene chemistry by years. This examination shows the potential for greater influence of individual discoveries and understanding to leverage new advances between these elements, and it also suggests that the chemistry of heavier elements may have more influence on what is possible with lighter elements.more » « less
-
Abstract Unique classes of active‐site motifs are needed for improved electrocatalysis. Herein, the activity of a new catalyst motif is engineered and isolated for the oxygen evolution reaction (OER) created by nickel–iron transition metal electrocatalysts confined within a layered zirconium phosphate matrix. It is found that with optimal intercalation, confined NiFe catalysts have an order of magnitude improved mass activity compared to more conventional surface‐adsorbed systems in 0.1mKOH. Interestingly, the confined environments within the layered structure also stabilize Fe‐rich compositions (90%) with exceptional mass activity compared to known Fe‐rich OER catalysts. Through controls and by grafting inert molecules to the outer surface, it is evidenced that the intercalated Ni/Fe species stay within the interlayer during catalysis and serve as the active site. After determining a possible structure (wycherproofite), density functional theory is shown to correlate with the observed experimental compositional trends. It is further demonstrated that the improved activity of this motif is correlated to the Fe and water content/composition within the confined space. This work highlights the catalytic enhancement possibilities available through zirconium phosphate and isolates the activity from the intercalated species versus surface/edge ones, thus opening new avenues to develop and understand catalysts within unique nanoscale chemical environments.more » « less
-
Abstract Organophosphines have garnered attention from many avenues ranging from agriculture to fine chemicals. One‐time use of phosphate resources has made sustainable use of phosphorus overall imperative. Hydrophosphination serves as an efficient method to selectively prepare P−C bonds, furnishing a range of phosphorus‐containing molecules while maximizing the efficient use of phosphorus. Since the first report in 1958, a wide array of catalysts have appeared for hydrophosphination, a reaction that is spontaneous in some instances. This review presents a representative view of the literature based on known catalysts through mid‐2022, highlighting extensions to unique substrates and advances in selectivity. While several excellent reviews have appeared for aspects of this transformation, this review is meant as a comprehensive guide to reported catalysts.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/photochem2010007
