Activation of the Water Molecule. 5. Rhodium(I) Hydride Catalyzed Water Gas Shift Reaction. Identification of the Elemental Reactions Comprising the Catalytic Cycles

Article abstract of DOI:10.1021/ja00402a027

Rhodium(I)-hydrido compounds, e. g., RhHL3 (1, L = PEt3; 2, L = P(i-Pr)3, Rh2H2(μ-N2)4 (3), trans-RhH(N2) (4), and RhH (5), serve as catalyst precursors for the water gas shift (wgs) reaction active under relatively mild conditions (>50 deg C).Types of Rh compounds isolated from the wgs reaction were BPh4 (6b, L = P(i-Pr)3; 7b, L = P(c-C₁₁); 8b, L = PEt3; py = pyridine) trans-RhH(CO)11)3>2 (9), trans-Rh(OH)(CO)2 (11), and Rh2(CO)42 (22).The key intermediates appear to be trans-RhH(CO)L(9; 10, L = P(i-Pr)3 and trans-Rh(OH)(CO)L2 (11;12, L = P(c-C6H11)3) which in pyridine is in equilibrium with the solvated form )6a, L = P(i-Pr)3; 7a, L = P(c-C6H11)3).The formation of 6a and 7a from 2 and 3, respectively, was shown to proceed through oxidative addition of H2O to give (OH) (14a, L = P(i-Pr)3; 15a, L = P(c-C6H11)3) (step 1) and subsequent reaction of the adduct with CO releasing H2 (step 2).H2 evolution from the water adduct OH (16a) of 1 with CO requires more drastic conditions (80 deg C) and takes place through the intermediate ⁺ (18).The water adducts 14a and 16a are bases stronger than NaOH in aqueous pyridine.Alternatively,the catalyst precursors may react first with CO to give a Rh(0) compound such as Rh2(CO)33 (21) and Rh2(CO)4L2 (22; 23, L = PPh(t-Bu)2; 24, L = P(c-C6H11)3) via trans-RhH(CO)L2 (9,10) (step 3).Then these carbonyl compounds 21, 9, and 10 react with H2O to give 6a or 7a with concomitant evolution of H2 (step 4).Under a CO2 atmosphere 11 and 12 yield trans-RhH(CO)2L2 (9, 10) through a nucleophilic attack of OH^- to the coordinated CO to give Rh(CO2H)(CO)L2 as a transient species (step 5), followed by decarboxylation (step 6).The intermediacy of Rh(CO2H)(CO)L2 was suggested by isolating the closely related trans-Rh(CO2CH3)(CO)2 (25) from the reaction of trans-Rh(OCH3)(CO)2 (13) with CO.Accordingly, we infer that one cycle starts from 11 or 12 and involves a sequence of steps 5 -> 6 -> 4 to regenerate 11 or 12.As the wgs reaction proceeds, another cycle becomes important.This is shown for the P(i-Pr)3 compound 10.The cycle is comprised to the reaction with CO2-H2O to give Rh(OCO2H)(CO)2 (27) and H2 (step 7), transformation of 27 into Rh2(μ-OCO2)(CO)4 (26) (step 8), and hydrolysis of 26 regenerating 11 or its solvated ion-pair compound 6a (step 9).In support of the proposed reaction scheme, all the isolated Rh compounds, i.e., trans-Rh(OH)(CO)L2, trans-RhH(CO)L2, Rh(μ-OCO2)(CO)2L4, and RhH2(OCO2H)L2 (precursor of 27) (L = P(i-Pr)3 and/or P(c-C6H11)3), as well as 21 were found to exhibit catalytic activity comparable to that of the catalyst precursor carrying the corresponding phosphine ligand.