Multiple Bonds between Main-Group Elements and Transition Metals. 152.1 Hydrolysis and Polymerization−Precipitation of Methyltrioxorhenium in Aqueous Solution

Abstract

Methyltrioxorhenium, CH3ReO3 (1; MTO), hydrolyzes rapidly in basic aqueous solutions and much more slowly in acidic media. At low concentrations (cMTO < 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 (C0.92H3.3ReO3.0)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.