Solvent dependent mechanistic pathways for η-O2CCH3 substitution from the [Mo3(μ3-O)2(μ-O2CCH3)6(η-O2CCH3)3]- anion

Rate coefficients for terminal acetate (eta-O2CCH3) substitution from the [Mo-3(mu(3)-O)(2)(mu-O2CCH3)(6) (eta-O2CCH3)(3)]-anion were examined in polar protic (D2O and methanol-d(4)), polar aprotic (DMSO-d(6) and DMF-d(7)) and acidic solvents (acetic acid-d(4)). Reaction rates were determined using variable-temperature H-1 NMR and found to span orders of magnitude depending on the solvent (k(298 K) (s(-1)) = 7.2 x 10(-6) for D2O, 2.6 x 10(-5) for methanol-d(4), 3.7 x 10(-4) for acetic acid-d(4); no reaction for DMSO-d(6) and DMF-d(7)). Unlike D2O and methanol-d(4), little to no reaction occurs in DMSO-d(6) and DMF-d(7) most likely because the aprotic solvent is not able to solvate the leaving acetate ligand through hydrogen bonding. Activation parameters show that acetate substitution in polar protic solvents follows a dissociative pathway (D2O: Delta H-double dagger = 126 (+/- 6) kJ mol(-1) and Delta S-double dagger = 80 (+/- 18) J mol(-1) K-1; methanol-d(4): Delta H-double dagger = 115 (+/- 3) kJ mol(-1) and Delta S-double dagger = 52 (+/- 9) J mol(-1) K-1) while parameters for acetate exchange in acetic acid-d(4) are markedly different and suggest a mechanism more complex than a simple ligand exchange reaction (Delta H-obs(double dagger) = 63 (+/- 12) kJ mol(-1) and Delta S-obs(double dagger) = -97 (+/- 40) J mol(-1) K-1). (C) 2013 Elsevier B.V. All rights reserved.