Reactivity of Chromium Complexes of a Bis(imino)pyridine Ligand: Highly Active Ethylene Polymerization Catalysts Carrying the Metal in a Formally Low Oxidation State
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Bibliographic record
Abstract
A divalent chromium complex of bis(imino)pyridine, {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrCl 2 ( 1 ), was prepared with the aim of studying its reactivity with alkylating agents. Upon treatment with MeLi, the metal center was both reduced and alkylated, forming {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrMe(μ-Me)Li(THF) 3 ( 2 ). Complex 1 is also conveniently reduced with either NaH or metallic sodium to give the new species {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrCl ( 3 ). Despite the appearance of the metal center in a rare monovalent oxidation state, the square-planar geometry of the Cr atom suggests that the metal is most likely divalent, with the electron housed in the ligand π* orbital. When it is activated with MAO, complex 3 is a very and even more active catalyst for the polymerization of ethylene than either the −CrCl 2 or −CrCl 3 derivative of this ligand system and yet produces polymers with similar properties. Subsequent reactivity studies of complex 3 have allowed the isolation of several products. Reaction with either LiCH 2 Si(CH 3 ) 3 or MeLi resulted in deprotonation of one of the methyl groups on the ligand backbone, forming {2-[2,6-(i-Pr) 2 PhN C(CH 3 )]-6-[2,6-(i-Pr) 2 PhNC CH 2 ](C 5 H 3 N)}Cr(THF) ( 4 ) and {2-[2,6-(i-Pr) 2 PhN C(CH 3 )]-6-[2,6-(i-Pr) 2 PhNC CH 2 ](C 5 H 3 N)}Cr(μ-Me)Li(THF) 3 ( 5 ), respectively. On the other hand, alkylation with AlMe 3 allowed the successful preparation of another organochromium species, {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}CrCH 3 ( 7 ), along with small amounts of the byproduct {2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}Cr(μ-Cl) 2 Al(CH 3 ) 2 ( 6 ). Interestingly, complex 7, which also has the deceiving connectivity of a monovalent species, displays an even greater activity for ethylene polymerization than all of the other species reported herein, again producing a polymer with nearly identical characteristics. Activation with IBAO revealed a deactivation pathway similar to that observed with the FeCl 2 system. In this case, the stronger reducing power of IBAO resulted in the usual reduction not only of the ligand backbone but also of the metal center. As a result of the metal reduction, partial transmetalation of the ligand system occurred, with formation of [η 4 -{2,6-[2,6-(i-Pr) 2 PhN C(CH 3 )] 2 (C 5 H 3 N)}Al 2 (i-Bu) 3 (μ-Cl)]Cr-(η 6 -C 7 H 8 ) ( 8 ). By being catalytically inactive, the partly transmetalated 8 suggests that ligand demetalation is a possible catalyst deactivation pathway.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it