Characterization of a rhodium-sparteine complex, [((−)-sparteine)Rh(η4-COD)]+: Crystal structure and DNMR/DFT studies on ligand-rotation dynamics
Bibliographic record
Abstract
A cationic rhodium-sparteine complex, [((-)-sparteine)Rh(eta(4)-COD)](+) (1(+); COD = 1,5-cyclooctadiene) was obtained, isolated as its tetrafluoroborate salt (1BF(4)), and characterized using X-ray crystallography and multinuclear ((1)H, (13)C) NMR spectroscopy. This is the first structurally characterized sparteine complex of rhodium. The Rh-N bonds are unusually long (2.214(3) and 2.242(3) A), apparently due to steric repulsion between COD and sparteine. (1)H NMR exchange experiments (EXSY) demonstrate a dynamic process that results in an overall 180 degrees rotation of the COD methine protons in solution (CD(2)Cl(2)) with a first-order rate constant of 460 s(-1) at the coalescence temperature (314 K) and interpolated rate constant of 150 s(-1) at 298 K. Temperature-dependent NMR studies yield DeltaH(++) = 13.0 +/- 0.3 kcal mol(-1), DeltaS(++) = -5 +/- 1 cal mol(-1) K(-1), such that DeltaG(298)(++) = 14.3 +/- 0.3 kcal mol(-1). DFT studies (B3LYP) indicate that the loosely bound (-)-sparteine ligand rotates through a pseudo-tetrahedral transition state where both ligands are rotated approximately 90 degrees relative to each other. While both ligands remain bound (eta(4)-COD, kappa(2)-sparteine), bonding to sparteine is weakened much more than bonding to COD in the transition state. DFT computed DeltaG(298)(++) and DeltaS(++) values (15.55 kcal mol(-1) and -2.67 cal mol(-1) K(-1), respectively) agree very well with the experimental values. Attempts to find alternative mechanisms involving partial dechelation of COD and (-)-sparteine yielded slightly higher barriers along with positive DeltaS values for intermediate formation.
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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".