Bifunctional Mechanism with Unconventional Intermediates for the Hydrogenation of Ketones Catalyzed by an Iridium(III) Complex Containing an N-Heterocyclic Carbene with a Primary Amine Donor
Bibliographic record
Abstract
An alcohol-assisted outer-sphere bifunctional mechanism for the H 2 hydrogenation of ketones catalyzed by an iridium system is presented on the basis of experimental and theoretical evidence. An iridium(III) complex containing an N-heterocyclic carbene (NHC) with a tethered primary amine donor (C–NH 2 ), [IrCp*(C–NH 2 )Cl]PF 6 ( 3; Cp* = pentamethylcyclopentadienyl ligand), when activated by an alkoxide base, catalyzed the H 2 hydrogenation of acetophenone and benzophenone under 25 bar of H 2 pressure at 50 °C, achieving a maximum turnover frequency (TOF) of 416 h –1 . The presence of 2-propanol accelerates catalysis only when the alkoxide base is used in large excess with respect to iridium. This system has activity for ketone hydrogenation that is inferior to that of the structurally similar ruthenium(II) complex [RuCp*(C–NH 2 )py]PF 6 ( 2; py = pyridine, TOF of up to 17 600 h –1 for the hydrogenation of acetophenone). On the other hand, an iridium(III) complex that contains a cyclopentadienyl ligand (Cp), [IrCp(C–NH 2 )Cl]PF 6 ( 5 ), has activity that is superior to that of its Cp* analogue ( 3 ) in the catalytic H 2 hydrogenation of acetophenone when activated by an alkoxide base, reaching a TOF of up to 687 h –1 . Consistent with our previous computational studies, an iridium(III) hydride–amine complex, [IrCp*(C–NH 2 )H]PF 6 ( 6 ), was isolated and was found to be inactive as a catalyst for ketone hydrogenation. The cationic charge is thought to contribute to a diminished hydricity and reactivity of the iridium(III) hydride in comparison to the neutral ruthenium(II) analogue. The important role of the N–H group is illustrated as well by the poor catalytic activity of the structurally similar iridium(III) complex [IrCp*(C–NMe 2 )Cl]PF 6 ( 8 ), which does not contain an N–H group. Nevertheless, we present evidence that complex 3, in the presence of an excess alkoxide base, does efficiently hydrogenate ketones via an outer-sphere bifunctional mechanism involving the novel, neutral hydride intermediate Ir(η 4 -Cp*H)(C–NH 2 )H. The formation of this intermediate relies on the uncommon migration of a hydride ligand to the η 5 -Cp* ligand which appears to be promoted by the unique NHC ligand of the present system.
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How this classification was reachedexpand
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 | 0.000 | 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 | 0.001 |
| Open science | 0.000 | 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 teacher head, 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".