Probing the Mechanism of Carbon−Hydrogen Bond Activation by Photochemically Generated Hydridotris(pyrazolyl)borato Carbonyl Rhodium Complexes: New Experimental and Theoretical Investigations
Bibliographic record
Abstract
Fast time-resolved infrared (TRIR) experiments and density functional (DFT) calculations have been used to elucidate the complete reaction mechanism between alkanes and photolytically activated hydridotris(pyrazoly-1-yl)boratodicarbonylrhodium. TRIR spectra were obtained after photolysis of Rh(Tp 4-tBu-3,5-Me )(CO) 2 in n -heptane, n -decane, and cyclohexane and of Rh(Tp 3,5-Me )(CO) 2 in n -heptane and cyclohexane. Initial photolysis produces a coordinatively unsaturated, 16-electron monocarbonyl species that vibrationally relaxes to an intermediate with ν CO of 1971 cm −1 in n -heptane solution (species A ). DFT calculations on Rh(Tp 3,5-Me )(CO)−RH (RH = C 2 H 6, C 6 H 12 ) suggest that A is the triplet state of a five-coordinate, square-pyramidal Rh(κ 3 -Tp 3,5-Me )(CO)−RH, in which the alkane is weakly bound. Within the first 2 ns, a new transient grew in at 1993 cm −1 (species B ). The calculations show that the observed species B is the singlet state of a four-coordinate Rh(κ 2 -Tp 3,5-Me )(CO)(RH), in which the alkane is strongly bound and one pyrazolyl ring is rotated, decoordinating one N. The transient due to B grew at the same rate as A partially decayed. However, A did not decay completely, but persisted in equilibrium with B throughout the time up to 2500 ps. The ν(CO) bands due to A and B decayed at the same rate as a band at 2026 cm −1 grew in (τ ca. 29 ns, n -heptane). The latter band can be readily assigned to the final alkyl hydride products, Rh(κ 3 -Tp 4-tBu-3,5-Me )(CO)R(H) and Rh(κ 3 -Tp 3,5-Me )(CO)R(H) (species D ). The experimental data do not allow the elucidation of which of the two alkane complexes, A or B, is C−H activating, or whether both of the complexes react to form the final product. The calculations suggest that a third intermediate (species C ) is the C−H activating species, that is, the final product D is formed from C and not directly from either A or B . Species C is nominally a five-coordinate, square-pyramidal Rh(κ 2 1 / 2 -Tp 3,5-Me )(CO)(RH) complex with a strongly bound alkane and one pyrazolyl partially decoordinated, but occupying the apical position of the square pyramid. Intermediate C is unobserved, as the calculations predict it possesses the same CO stretching frequency as the parent dicarbonyl. The unobserved species is predicted to lie on the reaction path between A and B and to be in rapid equilibrium with the four-coordinate species B .
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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 | 0.000 | 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 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".