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 (η-O2CCH3) substitution from the [Mo3(μ3-O)2(μ-O2CCH3)6(η-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 1H NMR and found to span orders of magnitude depending on the solvent (k298 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: ΔH‡ = 126 (±6) kJ mol−1 and ΔS‡ = 80 (±18) J mol−1 K−1; methanol-d4: ΔH‡ = 115 (±3) kJ mol−1 and ΔS‡ = 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 (ΔH‡obs = 63 (±12) kJ mol−1 and ΔS‡obs = −97 (±40) J mol−1 K−1).