Christopher Masters

The Fischer-Tropsch Reaction

Publisher Summary The chapter focuses on homogeneous systems of Fischer–Tropsch reaction that are either capable of catalyzing the reactions between carbon monoxide and hydrogen or that provide insight into the ways in which these two molecules can be induced to interact at a metal center(s). There are two main reasons for the economic importance of Fischer–Tropsch production in South Africa: (1) the existence of large coal deposits, which can be mined at low cost, and (2) that nations need to become independent of external oil supplies. The chapter discusses some aspects of the chemistry of metal complexes in solution that are related to the conversion of carbon monoxide and hydrogen to organic products. The chapter also describes the preparation and properties of such complexes. It also provides information about stoichiometric reductions of both metal carbonyl and metal acyl species. All the homogeneous catalyst systems discussed are at a preliminary stage of development, yet it is significant that in all cases more than one metal center is thought to be involved in the catalytic species. However, as a working hypothesis it suggests many possibilities. Carbon monoxide and hydrogen are two of the most widely studied molecules in homogeneous catalysis, cf. hydroformylation, carbonylation, and hydrogenation. The design of systems capable of persuading them to interact in a selective manner remains a worthwhile challenge for the organometallic chemist.

Boron hydride reduction of transition metal—acetyl complexes

Abstract Acetyl complexes of iron(II) and ruthenium(II) of the type (π-C 5 H 5 )(CO)LM(COCH 3 ), where L = PPh 3 , P(OPh) 3 , P(cyclohexyl) 3 , PMe 2 Ph or CO for M = Fe, and PPh 3 for M = Ru, are rapidly reduced to the corresponding ethyl complexes by BH 3 · THF or B 2 H 6 /C 6 H 6 . In some cases hydrido complexes of the type (π-C 5 H 5 )(CO)LMH are also formed. The reaction has been studied by use of 1 H NMR and the spectrum of (π-C 5 H 5 )(CO)(PPh 3 )FeC 2 H 5 , which shows several unusual features, is discussed in detail. It is suggested that the rate of reduction increases with increasing electron density at the metal centre. Acetyl complexes of other transition metals, i.e. Ir, Pt, Pd, Co and Mo, are also reduced to the corresponding ethyl compounds by B 2 H 6 /C 6 H 6 .

‘Capped’ tri-ruthenium carbonyl cluster: X-ray crystal structure of [Ru3(CO)9{MeSi(PBu2)3}]

The tridentate tertiary-phosphine ligand MeSi(PBu2)3 reacts with Ru3(CO)12 in refluxing benzene to give the symmetrical complex [Ru3(CO)9{MeSi(PBu2)3}], containing three bridging carbonyl ligands and one phosphorus centre co-ordinated to each of the ruthenium atoms as shown by X-ray crystallography.

Addition of acetic formic anhydride to trans-[ IrCl(CO)(PMe2Ph)2]; anion-, including hydrido-, transfer reactions

Addition of acetic formic anhydride to trans-[ IrCl(CO)(PMe2Ph)2] leads, via a series of intermolecular anion (hydrido, formato and chloro) ligand exchange reactions, to the formation of the cis-dihydrido, trans-tertiary-phosphino complex [ IrClH2(CO)(PMe2Ph)2].

Bi-heterometallic hydrido transfer between iridium and platinum

Abstract Hydrido transfer from IrH5L2 to Pt2Cl4L2 or Pd2Cl4L2 (where L = PPr3) occurs readily at room temperature, and in the case of the platinum dimer is shown to proceed via a hydrido-bridged platinumiridium complex.

Homogeneous platinum(II)-catalysed hydrogen–deuterium exchange at a saturated carbon atom

Alkenes of the type RC(CH3)2CHCH2(R = Et, Pr, or Bu) undergo hydrogen–deuterium exchange with a deuterium oxide–acetic [2H1]acid solvent containing perchloric acid in the presence of a homogeneous platinum(II) catalyst. Incorporation of deuterium into the alkyl part of the alkene occurs exclusively at C-5. It is suggested that exchange occurs via a dimeric platinum(II) species. Exchange of the olefinic protons is also found; evidence suggests that this latter exchange is essentially an acid-catalysed reaction. Dimeric complexes of the type [Pt2Cl4{RC-(CH3)2CHCH2}2] have been isolated from the reaction medium.

Hydrogen–deuterium exchange at a saturated carbon atom in tertiary phosphine complexes of platinum(II)

Chloro-bridged diplatinum(II) complexes of the type [Pt2Cl4L2](L = PPr3, PBu3, PButPr2, PBut2Pr, PPrPh2, PPr2Ph, or PButPh2), undergo a regiospecific hydrogen-deuterium exchange in an aqueous (D2O) acetic acid (CH3CO2D) medium to give complexes containing deuterium in the alkyl groups of the tertiary phosphine moiety and, in the case of L = PButPh2, also in the aryl groups. The steric requirements of the tertiary phosphine have a marked influence on both the rate and the position of deuterium incorporation. The results presented show that, in internal-metallation reactions of platinum, the ease of ring formation decreases in the order five-membered > six-membered > four-membered rings. It is suggested that in such reactions steric rather than electronic factors are dominant.

Disproportionation of a halogen-bridged complex containing platinum and palladium

31 P N.m.r. spectroscopy shows that complexes of the type [PdPtCl4LaLb][Laand Lb are either the same or different tertiary-phosphide ligands (PPr3 or Pbu3)] are formed in chloroform solution when equimolar amounts of [Pd2Cl4(La)2] and [Pt2Cl4(Lb)2] are mixed at room temperature. All three complexes are in dynamic equilibrium, and a kinetic study indicates that the exchange occurs via a tetrameric intermediate involving four metal centres. Attempts to isolate the mixed metal complexes have proved unsuccessful. The first measurement of platinum–platinum coupling constants is reported for the complexes [Pt2Cl4(PBu3)2] and [Pt2I4(Pbu3)2].

Inorganic disproportionation. A 31P nuclear magnetic resonance study of a bi-hetero-metallic system containing platinum and palladium

31 P N.m.r. spectra show that on mixing chloroform solutions of [Pt2Cl4(PBu3)2] and [Pd2Cl4(PBu3)2] bi-hetero-metallic complexes of type [PdPtCl4(PBu3)2] are formed; the three complexes are in dynamic equilibrium and a kinetic study indicates that the exchange occurs via a tetrameric intermediate involving four metal centres.

Oxidative addition of carboxylic acids to trans-carbonylhalogenobis-(tertiary phosphine)iridium(I) complexes

Complexes of the type trans-[lrX(CO)L2](L = PEt3,PMe2Ph, or PPh3; X = Cl, Br, or l) undergo rapid oxidative addition with carboxylic acids RCO2H (R = H, Me, CF3, Ph, or 1-naphthyl) to give iridium (III) complexes [lrXH(O2CR)(CO)L2] corresponding to both (formal)cis and trans addition of the carboxylic acid to the iridium(I) species. In solution these complexes undergo rapid anion exchange such that, at equilibrium, two additional hydrido-species, [lrX2H(CO)L2] and [lrH(O2CR)2(CO)L2], are present. In all these octahedral complexes the tertiary phosphine groups are mutually trans, the hydride and carbonyl groups mutually cis, and the carboxylic unit is unidentate. The ease of formation of the different complexes depends on the nature of the carboxylic acid. The cis adduct containing chloride and having hydride and carbonyl mutually trans can be prepared by the action of carbon monoxide on complexes [lrCl(H)(O2CR)L2] which contain a bidentate carboxylate ligand. With weak acids, e.g. acetic, conversion of the iridium(I) into the iridium(III) species is incomplete; the exchange between free and co-ordinated acid is, however, slow on the n.m.r. time scale over the range –60 to 30 °C. The adducts formed between trans-[lrX(CO)L2](X = Cl, Br, or I) and formic acid are smoothly converted on heating in solution into dihydrido-complexes [lrXH2(CO)L2] with expulsion of carbon dioxide; with L = PPh3 or PEt3, trihydrido-complexes [lrH3(CO)L2] are also formed.