| Abstrakt: |
Kinetic, spectroscopic, and crystallographic studies have been carried out on products derived from the compound MeRe(NAr)2(O) (1, Ar = 2,6-diisopropylphenyl) in reactions with PRn(OR)3-n, abbreviated PY3. This reaction yields Y3P&dbd;O and a mononuclear Re(V) compound, MeRe(NAr)2(PY3)2. The molecular structures of MeRe(NAr)2(PMe2Ph)2 (3a) and MeRe(NAr)2(dmpe)2 (3j, dmpe = 1,2-bis(dimethylphosphino)ethane) were determined by X-ray crystallography. In benzene solution at room temperature 3a exchanges with free PMe2Ph, based on variable-temperature 1H and 31P NMR data. A kinetic study of the reactions between 1 and PRn(OR)3-n revealed that 3 forms at a rate proportional to the concentrations of PY3 and 1. Alkoxy-containing compounds are highly reactive compared with others; for example, for P(OMe)2Ph and PMe2Ph, the respective values of k/L mol-1 s-1 at 298 K in benzene solution are 7.2 × 101 and 2.1 × 10-4. The rate-enhancing effect of alkoxy groups can be found in all comparable reagents used in this work. The kinetic data were analyzed by considering σ-bonding effects (as represented by pKa values), steric effects (in terms of cone angles), and composite quantities (such as the stereoelectronic parameter χ). The π-acidity of PY3 appears to play a dominant role, suggesting an intermediate, [Re]−O−PY3, the stability of which depends strongly upon the π-acidity of PY3 and thus governs the reaction rate. This strong acceleration of reactivity by an OR group bound to phosphorus appears unprecedented. The role of competitive Lewis π-acids appears to be predominant in these reactions. This mechanism is supported by the activation parameters: ΔS&thermod; is nearly invariant, whereas the variation of ΔH&thermod; determines the trend in reactivity. When the reaction between 1 and an insufficient quantity of P(OMe)nPh3-n (n = 1−3) was carried out, the product was a ReVII−ReV adduct, MeRe(NAr)2O·MeRe(NAr)2 (2), suggesting a step in which the nonligated Re(V) intermediate MeRe(NAr)2 reacts with 1, when the concentration of P(OMe)nPh3-n does not allow its reaction with MeRe(NAr)2. Compound 2 slowly rearranged to an isolable compound, {MeRe(NAr)2}2(μ-O) (2), which has been structurally characterized. |
