SCOPE AND MECHANISM OF ALKENE HYDROGENATION ISOMERIZATION CATALYZED BY COMPLEXES OF THE TYPE R2E(CH2)2M(CO)(L) (R = CP, ME, PH - E = P, TA - M = RH, IR - L = CO, PPH3)

The mechanism of the catalytic hydrogenation of alkenes by the early-late transition metal heterobimetallic (ELHB) complex Cp2Ta(CH2)2Ir(CO)2 (1a) has been studied. The first step is the oxidative addition of H-2, the product of which cannot be spectroscopically detected for la but has been characterized by NMR spectroscopy for the related compound Cp2Ta(CH2)2Ir(CO)(PPh3) (2a). The reaction of D2 with 1a and 2a results in deuterium incorporation into the methylene bridges. We suggest that this reaction occurs by oxidative addition of D2, reductive elimination of a mu-methylene deuteride to form a Ta-CH2D group, C-H oxidative addition of this group across the iridium center, and reductive elimination of HD. We have studied the kinetics of this reaction and found that the rate is dependent upon [1a] and [D2] and that the rate constant for this second-order reaction (k2nd) at 35-degrees-C is 2.57 x 10(-2) M-1 s-1 is much larger than that for ethylene hydrogenation. In the hydrogenation, we further propose that the alkene binds to the species formed by reductive elimination of the A-methylene hydride. Alkene then inserts into the Ir-H bond to form an iridium alkyl. This complex is a 4-coordinate iridium species and can thus readily beta-eliminate, the result of which is that substituted alkenes are isomerized. Hydrogenation is completed by oxidative addition of the tantalum methyl C-H bond across the iridium center to form a 6-coordinate Ir(III) species, followed by reductive elimination of the alkyl hydride to form the free alkane. In the case of ethylene hydrogenation, the rate is dependent upon [1a], [H-2], and [C2H4], and the rate constant k3rd for this third-order process at 45-degrees-C is 9.21 X 10(-2) M-2 s-1. The rate of isomerization of 1-butene is approximately half that of hydrogenation; this process leads to a thermodynamic mixture of cis- and trans-2-butene. We have performed studies on the rate and scope (with ethylene, propene, 1-butene, and cis-2-butene) of hydrogenation/isomerization by seven other related catalyst systems: Cp2Ta(CH2)2Ir(CO)(PPh3); the Ta-Rh analogues of the Ta-Ir catalysts, Cp2Ta(CH2)2Rh(CO)2 and Cp2Ta(CH2)2Rh(CO)(PPh3); and the phosphorus ylide analogues of the Ta compounds bearing CO and PPh3 ligands, Ph2P(CH2)2Ir(CO)(PPh3), Me2P(CH2)2Ir(CO)(PPh3), Ph2P(CH2)2Rh(CO)(PPh3), and Me2P(CH2)2Rh(CO)(PPh3). The tantalum-iridium species hydrogenate alkenes up to 150 times faster than the ylide complexes, but the Ta-Rh compounds hydrogenate alkenes at about the same rate as their P-Rh analogues. Both Ta-Ir species undergo exchange of deuterium into the mu-CH2 groups faster than hydrogenation, both Ta-Rh complexes undergo exchange more slowly than hydrogenation, and none of the four ylide compounds exchange. Only Cp2Ta(CH2)2Rh(CO)(PPh3) isomerizes 1-butene in the absence of hydrogen. Addition of mercury normally has no effect on these hydrogenations, indicating that they are homogeneous processes. The sole exception is in reactions of Cp2Ta(CH2)2Rh(CO)2 where hydrogenation is spoiled upon addition of mercury to the system, indicating that a colloid may be the active species in this case. Hydrogenation by all of the ylide complexes is inhibited by PPh3, and thus in these cases the mechanism is proposed to involve PPh3 dissociation.