Glenn J. Sunley

Methanol carbonylation revisited: thirty years on

Monsanto initiated development of its rhodium- and iodide-catalysed process for the carbonylation of methanol to acetic acid in 1966. Ownership of the technology was acquired in 1986 by BP Chemicals who have further extended it. The work of the Sheffield group, in developing a deeper understanding of the mechanism of the process, is reviewed. The rate-determining step in the rhodium–iodide catalysed reaction is the oxidative addition of methyl iodide to [Rh(CO)2I2]–1a: the product from this reaction, the reactive intermediate [MeRh(CO)2I3]–2a has been detected and fully characterised spectroscopically. The rates of the reversible reactions linking 1a, 2a and the acetyl complex [(MeCO)Rh(CO)I3]–3a, as well as activation parameters for several of the processes involved, have been measured. The efficiency of methanol carbonylation arises primarily from rapid conversion of 2a into 3a, leading to a low standing concentration of 2a, and minimising side reactions such as methane formation. By contrast, in the iridium-catalysed carbonylation, for which similar cycles can be written, the reaction of [MeIr(CO)2I3]–2b with CO to give [(MeCO)Ir(CO)2I3]–4b is rate determining. Model studies show that while kRh/kIr is ca. 1 : 150 for the oxidative addition, it is ca. 105–106 : 1 for migratory CO insertion. The migratory insertion for iridium can be substantially accelerated by adding either methanol or a Lewis acid (SnI2); both appear to facilitate substitution of an iodide ligand by CO, resulting in easier methyl migration. The carbonylation of higher alcohols (ROH) has also been successfully modelled; the corresponding alkyl iodides (RI) react much more slowly than MeI with [M(CO)2I2]–, but again give the acyls [(RCO)M(CO)I3]– for M = Rh and the alkyls [RM(CO)2I3]– for M = Ir. The greater stability of [MeIr(CO)2I3]– compared with [MeRh(CO)2I3]– accounts for the very different characters of the reactions catalysed by the two metals. It is suggested that the broad features of the Rh/Ir reactivities can be rationalised since the M–C bond to a 5d metal is generally stronger than that to the corresponding 4d metal; thus if metal–ligand bond making plays a key role in a step, then the 5d metal is more likely to react faster (e.g. in the oxidative addition), but if a metal–ligand bond-weakening or -breaking step plays a key role in a process (e.g. in the migration), it is likely that the 4d metal will be faster.

Reaction of the rhodium and iridium complexes [C5Me5MMe2(Me2SO)] with aldehydes to give [C5Me5MMe(R)(CO)], and related reactions

Abstract The complexes [C 5 Me 5 MMe 2 (Me 2 SO)] ( 1a , M = Rh; 1b , M = Ir) react with aldehydes (RCHO) (cyclohexane, 80°C) to give [C 5 Me 5 M(Me)R(CO)], methane, and Me 2 SO; the reaction is strongly inhibited by dimethyl sulphoxide but is unaffected by addition of di-t-butyl peroxide. Benzaldehyde reacts with the rhodium complex ca. 30 times as fast as with the iridium complex; the relative rates of reaction of tolualdehydes with both 1a and 1b are in the ratio, 1/2/2 for the o -, m - and p -isomers. The complexes [C 5 Me 5 M(Me)R(CO)] are also formed by carbonylation of [C 5 Me 5 M(Me)R(Me 2 SO)]. Reaction of [C 5 Me 5 IrMe(phenyl)(Me 2 SO)] with ArCHO gave a mixture of [C 5 Me 5 Ir-Me(Ar)(CO)] and [C 5 Me 5 Ir(Ar)(phenyl)(CO)] (ca. 4/1); replacement of aryl being favoured over replacement of methyl. [C 5 Me 5 Ir(phenyl) 2 (Me 2 SO)] gave [C 5 Me 5 Ir(phenyl)(tolyl)(CO)] on reaction with p -MeC 6 H 4 CHO. Possible mechanisms are discussed; the evidence favours one involving metal(V) intermediates.

New acetyls techonologies from BP chemicals

Abstract The major features of three catalyst systems for ‘acetyls’ processes developed by BP Chemicals are described: 1. the homogeneous, promoted iridium methanol carbonylation system for acetic acid manufacture recently commercialised in the USA and Korea (CATIVA), 2. a non-commercialised ruthenium promoted rhodium system, also for methanol carbonylation, 3. a vapour phase reaction of ethylene with acetic acid over silicotungstic acid supported on silica giving commercially viable activity and catalyst lifetime for the manufacture of ethyl acetate. All three examples illustrate the importance of exploring process conditions to reveal the advantages of new catalyst systems, or transform known catalysts into commercial viability.

Syntheses, structures, and reactions of alkenyl-dirhodium complexes: coupling μ-methylene and σ-vinyl to allyl; an entry to μ-ethylidene-μ-methylene dirhodium complexes. Crystal structures of [{(C5Me5Rh)(μ-CH2)(CHCH2)}2] and [(C5Me5Rh)2(μ-CH2)(μ-CHMe)Cl2]

Abstract Reaction of [(C 5 Me 5 Rh-μ-CH 2 ) 2 Cl 2 ] with R′CHCHMgBr gave the dialkenyl-di-μ-methylenedirhodium complexes [(C 5 Me 5 Rh-μ-CH 2 ) 2 (CHCHR′) 2 ], ( 4 , R′ = H; 5 , R′ = Me). Coupling of the μ-methylene and the σ-alkenyl occurs very easily: on heating (giving ca. 90% R′C 3 H 5 olefins), and on reaction with AgBF 4 in MeCN (giving η 3 -allylic complexes, [C 5 Me 5 Rh(R′CHCHCH 2 )(MeCN)] + ; the anti -methylallyl isomer is the first product observed from reaction of 5 with AgBF 4 , indicating that the coupling is stereospecific). The divinyl complex 4 also reacted with HCl in polar solvents to give the allyl, [C 5 Me 5 Rh(η 3 -C 3 H 5 )Cl] ( 10 ), and then [(C 5 Me 5 RhCl 2 ) 2 ] and propene, and in non-polar solvents to yield the μ-methylene-μ-ethylidene complex [(C 5 Me 5 Rh) 2 (μ-CH 2 )(μ-CHMe)(Cl) 2 ] ( 13a ) and propene. The reversible reaction of 4 with HCl/pentane gave an unstable complex, identified by 1 H NMR as cis -[(C 5 Me 5 Rh) 2 (μ-CH 2 ) 2 (CHClMe) 2 ] ( 14 ). The complexes have been identified spectroscopically and by X-ray crystal structure determinations on di-σ-vinyldi-μ-methylenebis(pentamethylcyclopentadienylrhodium) ( 4 ), and dichloro-μ-methylene-μ-ethylidenebis(pentamethylcyclopentadienylrhodium) ( 13a ).

Selective Methylative Homologation: An Alternate Route to Alkane Upgrading

InI3 catalyzes the reaction of branched alkanes with methanol to produce heavier and more highly branched alkanes, which are more valuable fuels. The reaction of 2,3-dimethylbutane with methanol in the presence of InI3 at 180-200 degrees C affords the maximally branched C7 alkane, 2,2,3-trimethylbutane (triptane). With the addition of catalytic amounts of adamantane the selectivity of this transformation can be increased up to 60%. The lighter branched alkanes isobutane and isopentane also react with methanol to generate triptane, while 2-methylpentane is converted into 2,3-dimethylpentane and other more highly branched species. Observations implicate a chain mechanism in which InI3 activates branched alkanes to produce tertiary carbocations which are in equilibrium with olefins. The latter react with a methylating species generated from methanol and InI3 to give the next-higher carbocation, which accepts a hydride from the starting alkane to form the homologated alkane and regenerate the original carbocation. Adamantane functions as a hydride transfer agent and thus helps to minimize competing side reactions, such as isomerization and cracking, that are detrimental to selectivity.

Hydrogen bonding directs the H2O2 oxidation of platinum(II) to a cis-dihydroxo platinum(IV) complex.

The use of ligands with proximate hydrogen bonding substituents in the oxidation of platinum(II) dimethyl complexes with H2O2 leads to the exclusive formation of an unusual cis-dihydroxo platinum(IV) complex, which can dehydrate to form a trinuclear metalla-azacrown complex.

Evidence for vinylic intermediates in the Fischer–Tropsch reaction to give alkenes and alkanes

Labelling experiments showing that 13C2[from Si(13C2H3)4] can be incorporated into the C3 and C4 products from CO–H2 over Rh–CeO2–SiO2 are consistent with the intermediary of surface vinyls, as had been suggested by the analysis of decomposition reactions of model dirhodium complexes.

Cyclometallation of unsaturated carboxylic acids by pentamethylcyclopentadienyl-rhodium and -iridium complexes. Crystal structures of [(C5Me5)Ir(CPhCHCO–O)(Me2SO)] and [(C5Me5)Ir(CPhCHCO–OMe)I]+

Reaction of [(C5Me5)MMe2(Me2SO)](C5Me5=η-pentamethylcyclopentadienyl) with cinnamic acid gave [(C5Me5)[graphic omitted])(Me2SO)](2; M = Rh) and (3a; M = Ir), while the complexes [(C5Me5)[graphic omitted](Me2SO)](3b; R1= Me, R2= H), (3c; R1= R2= Me), and (3d; R1= H, R2= Me) were made from the appropriate unsaturated acids. Complex (3b) was also made by reaction of [(C5Me5)IrCl2(Me2SO)] with silver crotonate. Reaction of (2), (3a), (3b), and (3d) with methyl iodide gave the complexes [(C5Me5)[graphic omitted]–OMe)I](6; M = Rh, R1= Ph, R2= H), (7a; M = Ir, R1= Ph, R2= H), (7b; M = Ir, R1= Me, R2= H), and (7d- M = Ir R1= H, R2= Me). The complexes were characterised by their n.m.r. spectra and by single crystl X-ray determinations for (3a) and (7a). In each case these showed the presence of a cyclometallated five-membered ring. The MC bonds in (6) and (7), but not in (2) or (3), are associated with quite substantial carbenoid character, MC[graphic omitted] Reaction of (3a) or (3b) with CO merely caused displacement of the dimethyl sulphoxide to give [(C5Me5) [graphic omitted](CO)](5); however, (7b) ring-opened with CO to give the σ-alkenyl complex [(C5Me5)Ir(CMeCHCO2Me)(CO)I](8).

Methane formation during the iridium/iodide catalysed carbonylation of methanol

Abstract [Ir(CO) 2 I 3 Me] − reacts with carboxylic acids or hydrogen (but not with mineral acids) at elevated temperature to cleave the iridium(III)–methyl bond liberating methane; a cyclic transition state is proposed for the reactions with RCOOH.

The migratory insertion of carbon monoxide in pentamethylcyclopentadienyliridium (III) complexes. Structural effects on reactivity and mechanism for rhodium and iridium systems

The reactions of complex (C5Me5)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh2, PMePh2, PMe2Ph) give the products of alkyl migratory insertion (C5Me5Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C5Me5)Ir(Cl)(CO)(Ph) or the iodide complexes (C5Me5)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C5Me5)Ir(Cl)(CO)(Me) with PMePh2 and PMe2Ph in acetonitrile yields the chloride substitution product [(C5Me5)Ir(CO)(L)(Me)]+Cl−. Kinetic measurements for the reactions of (C5Me5)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C5Me5)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step (K = 1.5 (CyNC), 7.3 (PHPh2), 7.1 (PMePh2) dm3 mol−1 at 323 K) and k2 for the slow carboncarbon bond formation (k2 (105) = 6.9 (CyNC), 1.2 (PHPh2), 1.0 (PMePh2) s−1 at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh2 obtained between 308 and 338 K, are ΔH≠ = 106±16 kJ mol−1 and ΔS≠ = − 14±5 J K−1 mol−1. The reaction of 1a with PMePh2 proceeds at similar rates in tetrahydrofuran (K = 3.7 dm3 mol−1, k2 (105) = 1.2 s−1, 323 K). The crystal structure of (C5Me5)Ir(Cl)(COMe) (PMe2Ph) has been determined by X-ray diffraction. C20H29ClOPIr: Mr = 544.1, monoclinic, P21/n, a = 8.084 (2), b = 9.030(2), c = 28.715 (3) A, β = 91.41 (3)°, Z = 4, Dc = 1.71 g cm−3, V = 2095.5 A3, room temperatyre, Mo Kα, γ = 0.71069, μ = 65.55 cm−1, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η5-C5Me5 molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the IrP and the IrCOMe bonds are eclipsed with respect to C(ring)-methyl bonds.