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Syntheses, kinetic and homogeneous hydrogenation studies of ditertiary phosphine rhodium(I) complexes Fung, Dawning Chui Mun


The original purpose of this work was to investigate the catalytic properties of a series of Rh₂(CO)₄(P-P)₂ complexes (where P-P = ditertiary phosphines of the type PR₂(CH₂)nPR₂, R = alkyl or aryl) for hydroformylation. The preparation of Rh₂(CO)₄(P-P)₂ involves the synthesis of Rh(P-P)₂Cl, followed by reaction with NaBH₄ to give RhH(P-P)₂, which when treated with CO in benzene yields Rh₂(CO)₄(P-P)₂, as reported in the literature. The dimer, Rh₂(CO)₄(dpp)₂, where dpp = PPh₂(CH₂)₃PPh₂, was prepared and examined for its interaction with H₂, and H₂/CO, in order to test its capabilities for catalytic homogeneous hydroformylation. The interaction of Rh₂(CO)₄(dpp)₂ (49) with H₂, and the reaction of CO with RhH(dpp)₂ (52) to yield 49, are summarized as follows: [Formula Omitted] All the species shown, except 59, have been detected by ¹H and ³¹P{¹H} NMR spectroscopy. Formation of the monomeric hydride, 50, from 49, occurs at high [dpp]. The reaction of Rh₂(CO)₄(dpp)₂ and 6 equivalents dpp with synthesis gas (H₂ : CO = 1 : 1) gives initially 50 and R₂(CO)₄(dpp)₂ reforms after 30 minutes of interaction. This is consistent with the previous finding of low turnover rate for hydroformylation of 1-hexene using as catalyst the co-ordinatively saturated Rh₂(CO)₄(dpp)₂. Treatment of 52 in toluene with ~1 atm CO, followed by treatment with ~1 atm H₂, sets up the following equilibria (where dpp* = monodentate dpp): [Formula Omitted] The homogeneous hydrogenation of 1-hexene at 31° C, - 1 atm H₂, catalyzed by "the RhH(dpp)₂/CO/H₂ system" in toluene is ascribed to the formation of an unidentified "RhH" from 50 and/or 51. The H₂-uptake curve displayed an initial ("inductive") period required for the generation of an active species "RhH", a second period of maximum rate, and a final slowing down period. The mechanism suggested for homogeneous hydrogenation of 1-hexene catalyzed by the "RhH(dpp)₂/CO/H₂ system" is presented as follows: [Formula Omitted] The corresponding rate law for the maximum rate, consistent with the kinetic data, is given by: [Formula Omitted] where ["Rh"]t is total concentration of the active "RhH" catalyst. At high [1-hexene], where k₃[1-hexene] >> k₋₃ + k₄[H₂], the rate law is simplified to: Rate = k₄[H₂],["RhH"]t where ["RhH"]t ~ total rhodium concentration in solution. The values of k₃ and k₄ at 31° C were found to be 0.42 M⁻¹ s⁻¹ and 20 M⁻¹ s⁻¹ respectively. The Rh(dcpe)₂ ⁺X⁻ complexes (X = CI, BF₄, PF₆; dcpe PCy₂(CH₂)₂PCy₂) were prepared and found to have no reactions with NaBH₄ or LiAlH₄. Consequently, the dcpe carbonyl dimer could not be prepared. The Rh(p = p)₂ ⁺Cl⁻ complex, where p = p = PPh₂C₂H₂PPh₂, was isolated and characterized; its reaction with NaBH₄ was incomplete, partially generating RhH(p=p)₂. Treatment of the mixture with CO gave partially Rh(CO)(p=p)₂ ⁺Cl⁻ and another uncharacterized carbonyl complex. A single crystal X-ray structure determination of Rh(dcpe)₂ ⁺Cl⁻ showed that the geometry around Rh is distorted square planar. Also, the extremely air-sensitive species [RhCl(dcpe)• solv]n (solv = THF or 0.1 C₆H₆) and RhCl(dcpe)(CH₂Cl₂)•C₆H₆ were isolated. The interaction of Rh(dcpe)₂ ⁺Cl⁻ with small gas molecules was studied in order to test its potential as a catalyst. There is interaction between Rh(dcpe)₂ ⁺Cl⁻ and HCI, Cl₂, and CO, in CH₂C1₂. The reaction with HCI to give cis-RhHCl(dcpe)₂ ⁺Cl⁻ is extremely rapid. The use of stopped-flow kinetics and UV-VIS spectrophotometric techniques at 25° C gave an equilibrium constant of 4.2 x 10⁷ M⁻¹ for the reaction. The forward reaction was first-order in both [Rh(dcpe)₂ ⁺Cl⁻] and [HCI], indicating a concerted oxidative addition reaction. The RhHCl(dcpe)₂⁺ species reacts further with HCI to give RhHCl₂(dcpe) and the diphosphonium salt, dcpe(HCl)₂. The Rh(dcpe)₂ ⁺Cl⁻ complex reacts with Cl₂ to give RhCl₂(dcpe)₂ ⁺Cl⁻, which was also obtained by prolonged treatment of RhHCl(dcpe)₂ ⁺Cl⁻ with CDCl₃ The reaction of Rh(dcpe)₂ ⁺Cl⁻ with CO to give Rh(CO)(dcpe)₂ ⁺Cl⁻ yielded k on and k off values of 2.2 x 10⁻² M⁻¹ s⁻¹ and 5.02 x 10⁻⁴ s⁻¹ respectively at 25° C. The Rh(dcpe)₂ ⁺Cl⁻, complex was inactive as a catalyst for decarbonylation of benzaldehyde, or hydrogenation of 1-hexene.

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