Solvent dependent mechanistic pathways for η-O2CCH 3 substitution from the [Mo3(μ3-O) 2(μ-O2CCH3)6(η-O 2CCH3)3]- anion

摘要

Rate coefficients for terminal acetate (η-O2CCH3) substitution from the [Mo3(μ3-O)2(μ- O2CCH3)6(η-O2CCH 3)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-d 7). 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).