HETEROMETALLIC CLUSTERS AS CATALYST PRECURSORS - SYNERGISM ARISING FROM THE FACILE GENERATION OF A REACTIVE FRAGMENT

The homogeneous rhodium-catalyzed hydroformylation of 3,3-dimethylbut-1-ene has been studied at 293 K, 2.0 MPa of hydrogen, and 2.0 MPa of carbon monoxide in n-hexane as solvent, using Rh4(CO)12, Rh6(CO)16, Rh2(CO)4Cl2, RhCl3.2H2O, CoRh(CO)7, and Co2Rh2(CO)12 as catalyst precursors. The time-dependent concentrations of RCORh(CO)4 (R = -CH2CH2C(CH3)3), the one and only observable rhodium intermediate, and of tl e organic product 4,4-dimethylpentanal (44DMP) were measured in situ using high-pressure infrared spectroscopy. Maximum yields for the transformation of the above precursors to RCORh(CO)4 were approximately 0.56, 0.62, 0.56, 0.35, 0.97, and 1.00 respectively. Definitions for precursor conversion, precursor selectivity to intermediates, and intermediate yields are introduced. The reactions were carried out to 17% conversion of 3,3-dimethylbut-1-ene, corresponding to a turnover number (TON) of 80, at a nominal rhodium concentration of 5 x 10(-5) mole fraction. The induction periods were less than 10 min starting with the heterometallic carbonyl clusters CoRh(CO)7 and Co2Rh2(CO)12 and 6-12 h starting with the homometallic carbonyl clusters Rh4(CO)12 and Rh6(CO)16. It is shown (i) that the activities of all the systems can be expressed as (d[44DMP]/dt)t = TOF-[RCORh(CO)4]t and (ii) that the turnover frequency has the value TOF = 0.138 +/- 0.028 min-1 for all systems. As a consequence, it is concluded that the ''synergism'' observed in this study, starting with CoRh(CO)7 and Co2Rh2(CO)12 as catalyst precursors, arises exclusively from their facile fragmentation under reaction conditions, and the rapid and selective formation of RCORh(CO)4. The synergism does not arise either from cluster catalysis or from a catalytic binuclear elimination reaction.