On the mechanism of carboxylate ligand scrambling at Mo24+ centers: evidence for a catalyzed mechanism

Abstract

The reaction between Mo₂(O₂CBu^t)₄ and Mo₂(O₂CCF₃)₄ has been studied by ¹H and ¹⁹F NMR spectroscopy in the solvents C₆D₆, CD₂Cl₂, CD₃CN, pyridine-d₅ and THF-d₈. In each solvent ligand exchange is observed with the formation of Mo₂(O₂CBu^t)₃(O₂CCF₃) 1, cis- and trans-Mo₂(O₂C^tBu)₂(O₂CCF₃)₂ (cis-2, trans-2) and Mo₂(O₂CBu^t)(O₂CCF₃)₃ 3. The approach to equilibrium is solvent dependent with the rate being C₆D₆ ≈ CD₂Cl₂ > CD₃CN and THF-d₈. Attempts to quench the ligand exchange by the addition of proton and carboxylate anion traps such as BaCO₃, Cs₂CO₃, proton sponge, 2,6-di-tert-butylpyridine (2,6-Bu^t₂-py) and [Mo₂(O₂CBu^t)₂(CH₃CN)₄]²⁺[BF₄^–]₂ all failed. In the presence of [Mo₂(PhNCHNPh)₂(CH₃CN)₄]²⁺[BF₄^–]₂ and 2,6-Bu^t₂-py the ligand exchange reaction is halted. These data are used to argue for a catalyzed ligand exchange reaction involving free carboxylate anion or carboxylic acid in the reaction between Mo₂(O₂CBu^t)₄ and Mo₂(O₂CCF₃)₄ to give Mo₂O₂(CBu^t)_n(O₂CCF₃)_4 – n, where n = 0–4. Similarly, the reaction between Mo₂(O₂CBu^t)₄ and [Mo₂(O₂CBu^t)₂(CH₃CN)₄]²⁺[BF₄^–]₂ to give [Mo₂(O₂CBu^t)₃(CH₃CN)₂]⁺[BF₄^–] is suppressed in the presence of 2,6-di-tert-butylpyridine.