Chromium(II) Complexes

Reinholdt, Anders; Staples, Orion; Mindiola, Daniel J.

doi:10.1016/b978-0-08-102688-5.00012-x

Cited by 2 publications

(1 citation statement)

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“…Two lithium atoms are situated around the two Cr–O motifs, illustrating the impact of lithiation that leads to the reduction of chromium. In Cr/LTO , the local environment of chromium has a trigonal bipyramidal structure with a coordination number of five. − This includes a Cr–C bond (2.034 Å), two Cr–O bonds (1.940 and 1.944 Å) where the oxygen atoms originate from terminal hydroxyl groups, and two Cr–O bonds (1.944 and 2.271 Å) resulting from interactions with lattice oxygen atoms from the LTO support. Similar five-coordinate chromium(II) complexes exhibiting trigonal bipyramidal geometry have been previously reported but are generally uncommon. − …”

Section: Resultsmentioning

confidence: 99%

Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation

Kanbur,

Hall,

Kim

et al. 2024

ACS Catal.

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In this work, supported organochromium ethylene polymerization catalysts have been tuned to mediate ethylene oligomerization via surface lithiation, which provides a generalizable protocol to control stereoelectronics and redox states of surface organometallic active sites. The homoleptic chromium(IV) alkyl complex Cr(CH 2 SiMe 3 ) 4 was grafted on high-surface-area anatase titania (TiO 2 ) nanoparticles as well as on silica to produce Cr/TiO 2 and Cr/SiO 2 , respectively. Treatment of these materials with excess n-butyllithium led to the reduced chromium complexes Cr/Li x TiO 2 and Cr/Li/SiO 2 , each of which still retains one hydrocarbyl ligand on chromium. A set of heterogeneous complexes were studied by electron paramagnetic resonance and X-ray absorption spectroscopy, which indicate a reduction in the oxidation state of the major chromium species to Cr II upon lithiation. Cr/ Li x TiO 2 converts ethylene to hexenes with a high selectivity (>80%), which was persistent over 10 days at 80 °C, achieving >950 turnovers. The exclusive formation of C 4 and C 6 olefins, preferring the trimerization product, without a statistical (Flory−Schulz) distribution is characteristic of the oxidative cyclization oligomerization mechanism rather than the traditional Cossee−Arlman mechanism, whereas Cr/Li/SiO 2 produced a mixture of trimerization and polymerization products, suggesting site heterogeneity in the silica-based material. On the other hand, the unreduced chromium(IV) materials as well as low lithium-containing Cr/Li x TiO 2 (x < 0.16) exclusively produced ultrahigh molecular weight polyethylene, determined by differential scanning calorimetry and gel permeation chromatography analysis, likely formed via a linear-insertion mechanism, with a crossover from the polymerization to oligomerization regime observed at ∼16% Li intercalation.

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