Multiple bonds between main-group elements and transition metals. 152. Hydrolysis and polymerization-precipitation of methyltrioxorhenium in aqueous solution

Methyltrioxorhenium, CH₃ReO₃ (1; MTO), hydrolyzes rapidly in basic aqueous solutions and much more slowly in acidic media. At low concentrations (c_MTO < 8.0 × 10^-3 M) the formation of CH₄ gas and perrhenate was detected. The hydrolysis was followed by spectrophotometry, and the rate constants were determined as a function of temperature and pressure according to the rate law -d[MTO]/dt = k₁[OH^-][MTO]. The rate constant, activation enthalpy, entropy, and volume at 298.2 K are as follows: k₁/M^-1 s^-1 = (8.6 ± 0.3) × 10², ΔH₁^‡/kJ mol^-1 = 15.9 ± 1, ΔS₁^‡/J mol^-1 K^-1 = -135 ± 3, and ΔV₁^‡/cm³ mol^-1 = -2.4 ± 0.4. Attempts to perform the reverse reaction, the synthesis of MTO from [ReO₄]^- and CH₄ (200 MPa), failed: the decomposition of MTO seems to be irreversible. At higher MTO concentrations a second reaction, a faster reversible polymerization-precipitation, takes place to yield a gold solid of net formula (C_0.92H_3.3ReO_3.0)_n. The rate of polymerization-precipitation was studied as a function of temperature in D₂O by ¹H NMR spectroscopy; it follows first-order reversible kinetics. The rate constant, activation enthalpy, and entropy of polymerization-precipitation at 298.2 K are as follows: k₂/s^-1 = (2.1 ± 0.4) × 10^-6, ΔH₂^‡/ kJ mol^-1 = 111.9 ± 5, ΔS₂^‡/J mol^-1 K^-1 = 25 ± 16 (the parameters for the dissolution of the polymer are estimated data). The rate of the polymerization-precipitation is independent of the concentration of H⁺, and the reaction does not occur in the presence of [ClO₄]^-, [NO₃]^-, and other oxidants.