Reactivity of Chromium Complexes of a Bis(imino)pyridine Ligand: Highly Active Ethylene Polymerization Catalysts Carrying the Metal in a Formally Low Oxidation State
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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| 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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".