CHARACTERIZATION OF THE ENERGY SURFACE FOR THE OXIDATIVE ADDITION OF SILANES TO CPMN(CO)2(HEPTANE)

The energetic surface for the oxidative addition reaction of silanes to CpMn(CO)2(heptane) (2) has been studied by a combination of photoacoustic calorimetry and variable-temperature kinetic studies. For seven different silanes the enthalpy for the oxidative addition reaction, DELTA-H-1, ranged from -22.1 to -1 3.5 kcal/mol. DELTA-H-1 was found to depend both on the electron donating ability of the silane as well as on the cone angle of the silane. A threshold cone angle of 135-degrees must be reached before the steric effects tum on. The rate constants for the oxidative addition reactions at 25-degrees-C were all similar, ca. 2.5 x 10(6) L/(mol.s), indicative of an early transition state where steric and electronic effects have not yet manifested themselves. Kinetic studies of the reductive elimination of Et3SiH from CpMn(CO)2(H)(Et3Si) are consistent with a mechanism involving a pre-equilibrium step with the reactive intermediate 2 which is then trapped by PPh3. The activation parameters are the following: DELTA-H-1 double dagger = 27.4 +/- 0.8 kcal/mol and DELTA-S1 double dagger = 11.5 +/- 2.5 eu. The values for DELTA-H-1 double dagger determined by Wrighton and the results of DELTA-H-1 double dagger and DELTA-H-1 reported here are consistent with one another. Trapping of the CpMn(CO)2(heptane) intermediate is favored by PPh3 over Et3SiH with DELTA-DELTA-H(double dagger) = -2.1 +/- 0.5 kcal/mol and DELTA-DELTA-S(double dagger) = -2.7 +/- 1.0 eu. From the values of DELTA-H-1 double dagger and DELTA-H-1 double dagger, the strength of the Mn-heptane interaction is estimated to be 8 kcal/mol.