Solvent dependent mechanistic pathways for η-O2CCH3 substitution from the [Mo3(Î1⁄43-O)2(Î1⁄4-O2CCH3)6(η-O2CCH3)3]− anion

Rate coefficients for terminal acetate (g-O2CCH3) substitution from the [Mo3(l3-O)2(l-O2CCH3)6 (g-O2CCH3)3] anion were examined in polar protic (D2O and methanol-d4), polar aprotic (DMSO-d6 and DMF-d7) and acidic solvents (acetic acid-d4). Reaction rates were determined using variable-temperature H NMR and found to span orders of magnitude depending on the solvent (k K (s!1) = 7.2 " 10!6 for D2O, 2.6 " 10!5 for methanol-d4, 3.7 " 10!4 for acetic acid-d4; no reaction for DMSO-d6 and DMF-d7). Unlike D2O and methanol-d4, little to no reaction occurs in DMSO-d6 and DMF-d7 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: DH = 126 (±6) kJ mol!1 and DS = 80 (±18) J mol!1 K!1; methanol-d4: DH = 115 (±3) kJ mol!1 and DS = 52 (±9) J mol!1 K!1) while parameters for acetate exchange in acetic acid-d4 are markedly different and suggest a mechanism more complex than a simple ligand exchange reaction (DHobs = 63 (±12) kJ mol!1 and DSobs = !97 (±40) J mol!1 K!1). ! 2013 Elsevier B.V. All rights reserved.