Chelate and trans effect of P,O donor phosphine ligands on rhodium catalyzed carbonylation of methanol (original) (raw)
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Ligand effects in the rhodium-catalyzed carbonylation of methanol
Coordination Chemistry Reviews, 2003
The carbonylation of methanol to give acetic acid is one of the most important homogeneously catalyzed industrial processes. The original [Rh(CO)2I2]− catalyst, developed at the Monsanto laboratories and studied in detail by Forster and co-workers, is largely used for the industrial production of acetic acid and anhydride. The conditions used (30–60 bar pressure and 150–200°C) have spurred the search for
Dalton Transactions, 2003
The complexes [Rh(CO)Cl(2-Ph 2 PC 6 H 4 COOMe)], 1, and trans-[Rh(CO)Cl(2-Ph 2 PC 6 H 4 COOMe) 2 ], 2, have been synthesized by the reaction of the dimer [Rh(CO) 2 Cl] 2 with 2 and 4 molar equivalents of 2-(diphenylphosphino)methyl benzoate. The complexes 1 and 2 show terminal ν(CO) bands at 1979 and 1949 cm Ϫ1 respectively indicating high electron density at the metal centre. The molecular structure of the complex 2 has been determined by single crystal X-ray diffraction. The rhodium atom is in a square planar coordination environment with the two phosphorus atoms trans to each other; the ester carbonyl oxygen atom of the two phosphine ligands points towards the rhodium centre above and below the vacant axial sites of the planar complex. The rhodium-oxygen distances (Rh ؒ ؒ ؒ O(49) 3.18 Å; Rh ؒ ؒ ؒ O(19) 3.08 Å) and the angle O(19) ؒ ؒ ؒ Rh ؒ ؒ ؒ O(49) 179Њ indicate long range intramolecular secondary Rh ؒ ؒ ؒ O interactions leading to a pseudo-hexacoordinated complex. The complexes 1 and 2 undergo oxidative addition (OA) reactions with CH 3 I to produce acyl complexes [Rh(COCH 3 )ClI(2-Ph 2 PC 6 H 4 COOMe)], 4, and trans-[Rh(COCH 3 )ClI(2-Ph 2 PC 6 H 4 COO-Me)(2-Ph 2 PC 6 H 4 COOMe)]
Chemistry - A European Journal, 2002
The new diphosphine ligands Ph 2 PC 6 H 4 C(O)X(CH 2 ) 2 OC(O)C 6 H 4 PPh 2 (1: X NH; 2: X NPh; 3: X O) and Ph 2 PC 6 H 4 C(O)O(CH 2 ) 2 O(CH 2 ) 2 OC(O)-C 6 H 4 PPh 2 (5) as well as the monophosphine ligand Ph 2 PC 6 H 4 C(O)X(CH 2 ) 2 OH (4) have been prepared from 2-diphenylphosphinobenzoic acid and the corresponding amino alcohols or diols. Coordination of the diphosphine ligands to rhodium, iridium, and platinum resulted in the formation of the square-planar complexes [(P À P)Rh(CO)Cl] (6: P À P 1; 7: P À P 2; 8:
Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation
Organometallics, 2011
Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H 2. Residues from catalytic reactions contain a Rh(III) acetyl complex, [Rh(xantphos)(COMe)I 2 ] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a "pincer" κ 3-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh(CO) 2 I] 2. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh(xantphos)(COMe)I(L)] + , whereas in CH 2 Cl 2 migratory CO deinsertion gives [Rh(xantphos)(Me)I(CO)] + (4), which reacts with H 2 to liberate methane, as observed in catalytic reactions using syngas. A number of Rh(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh(xantphos)(CO)] + is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh(xantphos)(CO)I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh−O distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh-O interaction is supported by DFT calculations for the S N 2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P−M−P bite angle and M−O distance and division into two broad categories with bite angle <120°(cis) or >143°(trans).
2013
refluxing/stirring under N2 affords neutral, mononuclear complexes, viz., trans-[RuCl2{η 1 -(P)-PPh2(p-C6H4NMe2)}2(PPh3)2], [Rh(coe)2Cl{η 1 -(P)PPh2(p-C6H4NMe2)}] and [Ir(cod)Cl{η 1 -(P)PPh2(p-C6H4NMe2)}] respectively. The complexes are characterized by elemental analysis, ESI(+)MS, UV-vis, conductivity measurements, cyclic voltammetry, thermal analysis and 1 H and 31 P{ 1 H}NMR studies. The catalytic efficiency of the rhodium complex for the carbonylation of methanol to acetic acid and its ester is evaluated at varying CO pressure (15, 20, 33 bar) at 130 °C. Under similar experimental conditions, these catalysts have a higher turnover number (1266-1896) than the well-known Monsanto’s species [Rh(CO)2I2] – (TON = 463-1000)
Tetrahedron: Asymmetry, 2003
The enantiomerically pure hydroxylated phosphine (1S,2S,5R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol 3 was prepared in a two-step stereoselective process from the commercially available and low cost (−)-menthone. Reaction of this new ligand with the appropriate rhodium precursor gave the respective Rh(I) and Rh(III) complexes. For rhodium(I), the binuclear complex trans-(OC-6-32)-{m-chloro-m-hydroxo-tetrachlorobis[(1S,2S,5R)-1-diphenylphosphinomethyl-2-isopropyl-5-methyl-cyclohexanol-O,P]dirhodium(III)}8 were characterized in solid state by single-crystal X-ray diffraction analysis. The O-axial-P-equatorial trans-O,O arrangements observed in compounds 7a,b and 8 are preferred over the more common O-equatorial-P-equatorial structure, presumably due to the stereo-electronic coordination preferences of the chiral hydroxylated phosphine. To the best of our knowledge, the ligand arrangement observed in 7b and 8 is unique among ESBO complexes. The rhodium(I) complex 6 is active as a catalyst precursor in the hydroformylation of styrene, yielding initial turnover frequencies up to 440 h −1 and selectivities up to 74% in the branched aldehyde. Unfortunately, no enantiomeric excesses have been observed within experimental error.
Journal of Organometallic Chemistry, 2011
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Dalton Transactions, 2003
To provide further insight into electronic and steric factors and to quantify their relative importance, we studied in detail the migratory CO insertion step for RhMe(CO)I 2 (L-L) systems (L-L = dppms (PPh 2 CH 2 P(S)Ph 2 ) or dppe (PPh 2 CH 2 CH 2 PPh 2 )), Monsanto catalysts and some electronically unsymmetrical diphosphine model systems. The difference in the reaction rates of dppms and dppe has a clear electronic origin that reflects the different properties of sulfide phosphine (π-donor) and phosphine (π-acid) ligands. Molecular orbital calculations clearly show that dppms strongly increases back-bonding to CO and favors the overlap between CO and methyl. Steric effects modulate the barrier, which decreases more for dppe than it does for dppms. For dppms, the electronic contribution that phenyl phosphine substituents make to lower the barrier is greater than that made by purely steric effects. The sulfide phosphine ligand dppms accelerates the carbonyl insertion because of its π-donor capability. For the diphosphine ligands we studied, the energy barrier varied gradually as basicity varied, and the slowest kinetics is shown by the most electron-donating ligand. The basicity dependence is stronger when the phosphine ligand occupies a trans position to CO. On the other hand and in unsymmetrical diphosphine complexes, phosphine basicity affects stability and reactivity in opposite ways. D a l t o n T r a n s . , 2 0 0 3 , 9 2 -9 8 T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3