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

The reaction between Mo-2(O2CBut)(4) and Mo-2(O2CCF3)(4) has been studied by K-1 and F-19 NMR spectroscopy in the solvents C6D6, CD2Cl2, CD3CN, pyridine-d(5) and THF-d(8). In each solvent ligand exchange is observed with the formation of Mo-2(O2CBut)(3)(O2CCF3) 1, cis- and trans-Mo-2((O2CBu)-Bu-t)(2)(O2CCF3)(2) (cis-2, trans-2) and Mo-2(O2CBut)(O2CCF3)(3) 3. The approach to equilibrium is solvent dependent with the rate being C6D6 similar to CD2Cl2 > CD3CN and THF-d(8). 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-2(t)-py) and [Mo-2(O2CBut)(2)(CH3CN)(4)](2+)[BF4-](2) all failed. In the presence of [Mo-2(PhNCHNPh)(2)(CH3CN)(4)](2+)[BF4-], and 2,6-Bu-2(t)-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 Mo-2(O2CBut)(4) and Mo-2(O2CCF3)(4) to give Mo2O2(CBut)(n)(O2CCF3)(4) where n = 0-4. Similarly the reaction between Mo-2(O2CBut)(4) and [Mo-2(O2CBut)(2)(CH3CN)(4)](2+)[BF4-](2) to give [Mo-2(O2CBut)(3)(CH3CN)(2)](+)[BF4-] is suppressed in the presence of 2,6-di-tert-butylpyridine.