77645-97-7Relevant academic research and scientific papers
Synthesis and structure of three (tricyclohexylphosphine)rhodium complexes and their interconversion with carbon monoxide and hydrogen
Freeman, Michael A.,Young, David A.
, p. 1556 - 1560 (2008/10/08)
The reaction of Rh(CO)2acac with CO and H2 in the presence of excess tricyclohexylphosphine (PCy3) gave an easily separated mixture of HRh(CO)(PCy3)2 (3) and (PCy3)2Rh(μ-CO)2Rh(CO)PCy3 (7). The structures of 3 and 7 were determined by X-ray crystallography and related to appropriate structures previously reported. Crystal data for 3: monoclinic, P21/c (C2h5, No. 14), a = 10.066 (2) A?, b = 15.910 (4) A?, c = 23.438 (6) A?, a = 90.00°, β = 94.13 (2)°, γ = 90.00°, Z = 4. Crystal data for 7: triclinic, P1 (Ci1), a = 13.681 (3) A?, b = 14.261 (4) A?, c = 19.235 (5) A?, a = 104.15 (2)°, β= 99.70 (2)°, γ = 84.98 (2)°, Z = 2. 3 has a trans square-planar configuration. 7 contains a square-planar Rh(CO)PCy3 and a distorted-tetrahedral Rh(PCy3)2. The 31P NMR spectra of 7 indicated an intramolecular geminal phosphine site exchange process on Rh(PCy3)2. 3 and 7 both react with CO, forming PCy3(CO)2Rh(μ-CO)2Rh(CO)2PCy 3 (8), which was stable only in the presence of CO, reverting back to 7 in the absence of CO. 7 reacted with H2, re-forming 3.
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.
