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DOI: 10.1039/A900388F (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 1205-1208

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

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Malcolm H. Chisholm and Ann M. Macintosh

Abstract

The reaction between Mo2(O2CBut)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(O2CBut)3(O2CCF3) 1, cis- and trans-Mo2(O2CtBu)2(O2CCF3)2 (cis-2, trans-2) and Mo2(O2CBut)(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-But2-py) and [Mo2(O2CBut)2(CH3CN)4]2+[BF4–]2 all failed. In the presence of [Mo2(PhNCHNPh)2(CH3CN)4]2+[BF4–]2 and 2,6-But2-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(O2CBut)4 and Mo2(O2CCF3)4 to give Mo2O2(CBut)n(O2CCF3)4 – n, where n = 0–4. Similarly, the reaction between Mo2(O2CBut)4 and [Mo2(O2CBut)2(CH3CN)4]2+[BF4–]2 to give [Mo2(O2CBut)3(CH3CN)2]+[BF4–] is suppressed in the presence of 2,6-di-tert-butylpyridine.

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