77661-49-5Relevant articles and documents
Activation of the Water Molecule. 5. Rhodium(I) Hydride Catalyzed Water Gas Shift Reaction. Identification of the Elemental Reactions Comprising the Catalytic Cycles
Yoshida, T.,Okano, T.,Ueda, Y.,Otsuka, S.
, p. 3411 - 3422 (2007/10/02)
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.
