RATE AND EQUILIBRIUM STUDY OF THE REVERSIBLE OXIDATIVE ADDITION OF SILANES TO THE IRIDIUM CENTER IN CP2TA(MU-CH2)2IR(CO)2 AND OF ALKENE HYDROSILATION ISOMERIZATION CATALYZED BY THIS SYSTEM

The mechanism of the catalytic hydrosilation of ethylene by the early-late transition metal heterodinuclear complex Cp2Ta(mu-CH2)2Ir(CO)2 (1) has been studied. The second-order rate constant for catalytic ethylene hydrosilation at 45-degrees-C in toluene-d8 is (2.65 +/- 0.43) X 10(-3) M-1 s-1; the rate is dependent upon [ 1 ] and [C2H4] but is independent of [Et3SiH]. We have found that the first step in the catalytic reaction is the oxidative addition of R3SiH to form CP2Ta(mu-CH2)2Ir9SiR3)(H)(CO)2 [R = Me (2a); Et (2b); Ph (2c)], which is in equilibrium with 1 and free silane. In all three cases, only the cis Si/H isomer, in which the hydride is trans to a CO and the silane trans to a mu-CH2, is detectable over a range of temperatures. The thermodynamic parameters for all three reactions have been measured in THF-d8: 2a, DELTAH-degrees (-9.60 +/- 0.45 kcal/ mol), DELTAS-degrees (-19.1 +/- 1.5 eu); 2b, DELTAH-degrees (-10.8 +/- 0.4 kcal/mol), DELTAS-degrees (-24.8 +/- 1.4 eu); 2c, DELTAH-degrees (-12.4 +/- 0.4 kcal/mol), DELTAS-degrees (-24.0 +/- 1.4 eu). The parameters were also measured for 2b in toluene-d8: DELTAH-degrees (-11.2 +/- 0.7 kcal/mol), DELTAS-degrees (-25.0 +/- 2.5 eu). The rate of oxidative addition of Et3SiH to 1 was found to be first-order in [Et3SiH] and [1]; at 0-degrees-C in toluene the second-order rate constant is 3.57 +/- 0.07 M-1 s-1 and in THF, 2.16 +/- 0.03 M-1 s-1. The primary kinetic isotope effect for this reaction was measured to be 1.13. The reductive elimination of Bt3SiH from 2b at 0-degrees-C in toluene has a rate constant of (1.20 +/- 0.02) x 10(-3) s-1 in toluene and (1.32 +/- 0.02) X 10(-3) s-1 in THF. The primary kinetic isotope effect for this step was calculated to be 1.45. The activation parameters for the addition of Et3SiH have also been measured: DELTAH* = 9.4 +/- 0.2 kcal/mol and DELTAS(double dagger) = -21.1 eu, and for the reductive elimination of Et3SiH from 2b: DELTAH(double dagger) = 20.6 +/- 0.2 kcal/mol and DELTAS(double dagger) = 3.9 eu. The reaction of Et3SiD with 1 results in the incorporation of deuterium into the methylene bridges. We suggest that this reaction occurs by oxidative addition of Et3SiD, reductive elimination of au-methylene deuteride to form a TaCH2D group, C-H oxidative addition of this group across the iridium center, and reductive elimination of Et3SiH. We have studied the kinetics of this reaction and found that the rate is dependent upon [1] and independent of [Et3SiD] (since addition of more than 2 equiv of Et3SiD drives the equilibrium completely toward 2b) and that the rate constant at 10-degrees-C is (6.46 +/- 0.82) X 10(-4) s-1. The activation parameters for this process have been measured: DELTAH(double dagger) = 15.8 +/- 1.2 kcal/mol and DELTAS(double dagger) = -17 +/- 5 eu. However, this reaction pathway for deuterium exchange is not thought to be along the route for catalytic hydrosilation because addition of CO to the reaction mixture severely inhibits the reaction. Instead, after oxidative addition of R3SiH, a CO ligand dissociates to create an open coordination site. Ethylene then binds to the iridium center, inserts into the Ir-H bond (at this point beta-elimination can occur, and in the case of substituted alkenes results in isomerization without hydrosilation), and reductive elimination of an ethyl and a silyl group completes the catalytic cycle.