14841-12-4

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Time-resolved infrared spectroscopic study of reactive acyl intermediates relevant to cobalt-catalyzed carbonylations

Time-resolved infrared spectroscopic studies have been used to characterize the reactive intermediate CH3C(O)Co(CO)2PPh3 (I(Co)), which is relevant to the mechanism of the catalysis of alkene hydroformylation by the phosphine-modified cobalt carbonyls. Step-scan FTIR and (variable) single-frequency time-resolved infrared detection on the microsecond time scale were used to record the spectrum of I(Co) and to demonstrate that the principal photoproduct of the subsequent reaction of this species at P(CO) = 1 atm is the methyl cobalt complex CH3Co(CO)3PPh3 (M(Co)). At higher P(CO) the trapping of I(Co) with CO to re-form CH3C(O)Co(CO)3PPh3 (A(Co)) (rate = k(CO)[CO][I(Co)]) was shown to become competitive with the rate of acetyl-to-cobalt methyl migration to give M(Co) (rate = k(M)[I(Co)]). Activation parameters for the competing pathways in benzene were determined to be ΔH((+))(CO) = 5.7 ± 0.4 kJ mol-1, ΔS((+))(CO) = -91 ± 12 J mol-1 K-1 and ΔH((+))(M) = 40 ± 2 kJ mol-1, ΔS((+))(M)= -19 ± 5 J mol-1 K-1. The effects of varying the solvent on the competitive reactions of I(Co) were also explored, and the mechanistic implications of these results are discussed.

Activation parameters for the reactive intermediates relevant to carbonylation catalysts based on cobalt carbonyls

Time-resolved spectroscopic techniques have been used to prepare and to interrogate transient species that are models for reactive intermediates in cobalt-catalyzed hydroformylation. Flash photolysis of acetylcobalt carbonyl complexes of the type RC(O)Co(CO)3(PR′3) (A; R = CH3, CD3, or C2H5; R′ = Ph or nBu) leads to CO photodissociation to give the unsaturated intermediate [RC(O)Co(CO)2(PR′3)] (I), which decays by two competitive pathways, alkyl migration to the cobalt to give RCo(CO)3PR′3 (M) and reaction with CO to re-form A. With the perdeuterioacetyl complex (R = CD3, R′ = Ph), rate constants both of CO trapping (kCO) and of methyl migration (kM) were just slightly smaller than those of the perprotio analogue (kh/kd = 1.04 ± 0.01 and 1.07 ± 0.09, respectively). Thus, any stabilization of the vacant coordination site of I by agostic interactions with the acetyl methyl group appears to be kinetically insignificant, consistent with the previous conclusion (Inorg. Chem. 2000, 39, 3098-3106) that this site is stabilized by an η2-coordinated carbonyl. Changing the phosphine ligand has a greater influence on the kinetics of I. The species generated by the flash photolysis of the trialkyl phosphine complex CH3C(O)Co(CO)3(P(nBu3)) exhibited a much larger kM than was the case for the PPh3 analogue, although there was little difference in the kCO values. Similarly, kM proved to be sensitive to the nature of R as demonstrated by the slower alkyl migration (at 298 K) for the intermediate formed by CO photodissociation from the propionyl complex C2H5C(O)Co(CO)3PPh3 relative to the acetyl analogue. Nonetheless, all these intermediates displayed analogous time-resolved infrared spectra and general kinetics behavior in benzene solution (implying common mechanisms for decay), so it is concluded that all are present as the η2-chelated acyl structure under these conditions.