Structural and mechanistic studies of ore-transfer reactions of bis(arylimido)oxorhenium (VII)

Kinetic, spectroscopic, and crystallographic studies have been carried out on products derived from the compound MeRe(NAr)(2)(O) (1, Ar = 2,6-diisopro pylphenyl) in reactions with PRn(OR)(3-n), abbreviated PY3. This reaction y ields Y3P=O and a mononuclear Re(V) compound, MeRe(NAr)(2)(PY3)(2). The mol ecular 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 cryst allography. In benzene solution at room temperature 3a exchanges with free PMe2Ph, based on variable-temperature H-1 and P-31 NMR data. A kinetic stud y of the reactions between 1 and PRn(OR)(3-n) revealed that 3 forms at a ra te proportional to the concentrations of PY3 and 1. Alkoxy-containing compo unds are highly reactive compared with others; for example, for P(OMe)(2)Ph and PMe2Ph, the respective values of k/L mol(-1) s(-1) at 298 K in benzene solution are 7.2 x 10(1) and 2.1 x 10(-4). The rate-enhancing effect of al koxy groups can be found in all comparable reagents used in this work. The kinetic data were analyzed by considering sigma -bonding effects (as repres ented by pK(a) values), steric effects (in terms of cone angles), and compo site quantities (such as the stereoelectronic parameter chi) The pi -activi ty of PY3 appears to play a dominant role, suggesting an intermediate, [Re] -O-PY3, the stability of which depends strongly upon the pi -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 comp etitive Lewis pi -acids appears to be predominant in these reactions. This mechanism is supported by the activation parameters: DeltaS(double dagger) is nearly invariant, whereas the variation of DeltaH(double dagger) determi nes the trend in reactivity. When the reaction between 1 and an insufficien t quantity of P(OMe)(n)Ph3-n (n = 1-3) was carried out, the product was a R e-VII-Re-V adduct, MeRe(NAr)(2)O.MeRe(NAr)(2) (2'), suggesting a step in wh ich the nonligated Re(V) intermediate MeRe(NAr)(2) reacts with 1, when the concentration of P(OMe)(n)Phs(3-n) does not allow its reaction with MeRe(NA r)(2). Compound 2' slowly rearranged to an isolable compound, {MeRe(NAr)(2) }(2)(mu -O) (2), which has been structurally characterized.