11.4.1 Properties and reactivity of carbon monoxide

Carbon monoxide (CO) is an important building blockin industrial organic chemistry. Organic substratescontaining a carbon-carbon unsaturation may undergoan addition of CO, normally resulting in the formationof a new carbon-carbon sequence, where one carbonatom is provided by carbon monoxide, the other onesoriginating from the unsaturated substrate, e.g. anolefin or an acetylenic derivative. Carbonylationreactions perhaps most appropriately exemplify howthe nature of the metal catalyst can influence theformation of the products. Coordinative addition of thereagents to the metal catalyst is believed to be anessential step on the way to products. Selectivity canoriginate from either the intrinsic thermodynamicstability of the product or from the preferentialcoordination of the substrates within the activatedmetal centre. For example, the formation ofconsecutive carbon-carbon bonds from CO forming apolyketone such as �[ C(O)�C(O)�] is believed tobe thermodynamically less favoured than the sequence�[ CH2�CH2�C(O)�] , the latter beingtherefore the preferred one. On the other hand,selectivity induced by the metal is evidenced by theabsence of olefin polymerization when a CO/olefinmixture is used, e.g. in the hydroformylation reactionwhich consists of converting an olefin into the higheraldehyde in the presence of molecular hydrogen. As itwill be discussed in the following sections, only adetailed study of the coordination properties oftransition elements will help to explain the specificityof catalytic processes.

Carbon monoxide, which is required for preparingbinary metal carbonyls, is the product of partialcombustion of carbon (Df H°=�110.4 kJ/mol) and canbe prepared by the syn-proportionation reaction

(DG°=�120.1 kJ/mol) between carbon dioxide andcarbon, the equilibrium to CO being favoured at hightemperature:

CO2(g)�C(s)����2CO(g)

In valence bond terms, CO can be represented by aformula containing a triple bond between the twoelements. In agreement with this representation, thebond dissociation enthalpy is 1,073 kJ/mol, i.e. thehighest for a diatomic molecule, and the IR stretchingvibration is about n�CO=2,140 cm�1 (where n�CO is thewave number) in an organic solvent. The concentrationof CO in solution is around 7·10�3 M at atmosphericpressure, the nature of the solvent having a minoreffect. The bond description of CO confines a lonepair of electrons on each of the atoms constituting themolecule, thus suggesting that carbon monoxide can,in principle, behave as a Lewis base through both theoxygen and the carbon atoms. However, normally COis bonded to metals through its carbon atom; if onlyone metal is involved, terminal carbonyl groups willbe present. When two or more metal atoms areinvolved, double or triple bridges will develop. Fewcases are known where CO acts as a bridging ligandbetween two different metal atoms, engaging both thecarbon and the oxygen atoms in the bond.

Carbon monoxide can undergo base-catalysedreactions with, for example, alcohols or secondaryamines, yielding formates HCO2R or formamidesHCONR2, respectively. Also, acid-catalysed reactionsof CO are known, such as the conversion of propyleneto isobutyric acid, catalysed by proton-activesubstances (Orchin and Wender, 1957), as discussedwithin this article (see Section 11.4.4). Normally,branched carboxylic acids are obtained (Koch, 1955)from alkenes and CO in the presence of concentratedsulphuric acid (Weissermel and Arpe, 1978). Theformyl cation HCO�, characterized by a carbonyl

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11.4

Homogeneous catalysis with carbonmonoxide in carbon-carbon bond

forming processes

stretching vibration at 2110 cm�1, is generated in thepresence of HF/SbF5 with CO under pressure (De Regeet al., 1997); it may be involved in catalytic processesoccurring in strongly acidic media. Concentratedsulphuric acid, or the HCl/CuCl system, activatecarbon monoxide towards the electrophilic attack onaromatic hydrocarbons to form aromatic aldehydes(Gatterman-Koch reaction).

11.4.2 Metal carbonylsand their derivatives:synthesis and structure

As catalytic processes involving carbon monoxidefrequently require the presence of metal carbonylderivatives, a short summary of the properties of thesecompounds will be outlined here. Typical metal-COcombinations are known for the three d series (3d, 4d,5d), the central metal atom frequently possessing anincomplete d shell, i.e. an electronic configurationdn (0�n�10). Carbonyl derivatives of transitionmetals are characterized by IR-active carbonylstretching vibrations around 2,000 cm�1. Somecompounds of this class, characterized by a completelyfilled (d10) configuration, are also known (Calderazzoand Belli Dell’Amico, 1986), for example Au(I) inAuCl(CO), or by an empty (d0) shell, for exampleCa(II) in Ca(C5Me5)2 where Me�CH3, which forms acarbon monoxide adduct with n�CO at 2,158 cm�1 (Selget al., 2002), or Ti(IV) (Calderazzo et al., 1997) in[Ti(C5H5)2(CO)2]

2+, with n�CO�2,119 and 2,099 cm�1.A few carbonyl adducts are also known forcyclopentadienyl derivatives of 4f and 5f elements,such as Yb(C5Me5)2 (Schultz et al., 2001), and thecrystallographically established (Del Mar Conejo etal., 1999) U(C5Me4H)3(CO) whose n�CO was observedat 1,880 cm�1. Carbonyl derivatives have also beenreported for elements of the s-p series, examples beingthose of boron(III), such as the structurallycharacterized B(CF3)3(CO), n�CO�2,251 cm�1

(Finze et al., 2002). This is a recent addition to thelong-standing borine-carbonyl BH3(CO), preparedfrom CO and diborane B2H6 (Burg, 1952).

Vibrational spectra give important informationabout several properties of metal carbonyls: theelectronic distribution within the M�CO bond interminal carbonyl groups; the molecular structureassociated with the number of observed carbonylstretching vibrations; and the type of bonding,terminal or bridging, in polynuclear compounds. TheM�CO bond is regarded as resulting from a s-bondbetween the lone pair on carbon and an empty orbitalon the metal, reinforced by p back-bonding from ametal orbital of appropriate symmetry to an

anti-bonding orbital of CO. The s and p contributionsare viewed as synergic, the result being a mutualreinforcement of the bond. As the p back-bondingcontribution increases, the carbonyl stretchingvibration is expected to shift to lower frequencies. Thecarbonyl stretching vibrations also depend on the totalcharge on the complex. This is shown by the data forthe isoelectronic series of the hexacarbonyls, wherethe same 3d6 electronic configuration is present fromiron(II) to titanium(�II) (the in cm�1 of the uniqueIR-active carbonyl stretching vibration is reported inparenthesis): [Fe(CO)6]

2+ (2,204), [Mn(CO)6]+

(2,090), Cr(CO)6 (2,000), [V(CO)6]� (1,859), and

[Ti(CO)6]2 (1,748). A wide range of oxidation states

are known for metal carbonyls: from �III in[Ir(CO)6]

3+ (Willner and Aubke, 2003) to �IV in[M(CO)4]

4�, M�Cr, Mo, W (Ellis, 2003).Well-established examples of this class of

compounds are the neutral metal carbonyls of the 3dseries: V(CO)6, Cr(CO)6, Mn2(CO)10, Fe(CO)5,Co2(CO)8, Ni(CO)4. In these compounds the centralmetal atom is in the zero oxidation state, thecorresponding electronic configurations going from3d5 for vanadium(0) to 3d10 for nickel(0). Taking intoaccount the presence of a metal-metal bond for theelements with an odd atomic number, it is easilyverified that these systems attain the effective atomicnumber of the next inert gas, the sole exception beingrepresented by vanadium.

When carbon monoxide is bonded to a transitionelement, in a cationic, or even a neutral compound, theelectrophilic character of carbon increases, ascompared with the isolated CO molecule. Thus, forexample, the reaction of methyl lithium with W(CO)6(Fischer and Massböl, 1964) gives the anionic acylcomplex having the formula [W(CO)5(COMe)]−,which can be further alkylated by [Me3O]+ to thecarbene derivative W(CO)5�C(OMe)Me.

As far as the synthesis of metal carbonyls isconcerned, nickel is the only metal which, when finelydivided and activated, readily reacts with Co to formthe corresponding carbonyl Ni(CO)4 under mildconditions of temperature and pressure (Mond et al.,1890). Tetracarbonylnickel(0) is a liquid (boiling point40-41°C at atmospheric pressure). An electron-diffraction experiment in the gas phaseand an X-ray diffraction study at low temperature onsolid Ni(CO)4 have shown the compound to have aregular tetrahedral geometry; an accurate refinementof the structural data has established the carbonylligands to be C-bonded to nickel.

In most other cases, reduction of a readilyavailable inorganic precursor is required to preparethe corresponding metal carbonyl. If CO itself isused as the reducing agent, the oxidation products

724 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

are CO2 (Df G°��394 kJ/mol) or COCl2(Df G°��206 kJ/mol), when the starting material isa metal oxide or a metal chloride, respectively.Grignard reagents, lithium alkyls or aryls, andaluminum alkyls have often been used for thepreparation of metal carbonyl derivatives, with thealleged intermediacy of metal-alkyl derivativesundergoing reduction by homolytic cleavage,followed by the coordinative addition of CO.Electropositive metals characterized by alow-standard reduction potential (alkali metals, Mg,Zn) have frequently been used for the reduction oftransition metal halides in the presence of carbonmonoxide. The reducing metal is finely divided, andpreviously activated by one of the conventionalmethods. As a dispersing and/or solvent medium, adry organic solvent is frequently employed, normallya hydrocarbon or an ether. The preparativeprocedures reported in the literature usually requirehigh temperature (50-200°C) and pressure (50-300bar). Methods of preparation which use mildreaction conditions have been developed in recentyears. Preparative and structural aspects of metalcarbonyl derivatives have been reviewed by severalauthors (Brauer, 1975-1981; Calderazzo, et al.,1968; Cotton, 1976).

While vanadium(III) salts are carbonylated to[V(CO)6]

− at high temperature and pressure in thepresence of the Mg/Zn/pyridine reducing system,niobium derivatives undergo carbonylation to[Nb(CO)6]

− at atmospheric pressure and ambienttemperature, with the same reducing system(Calderazzo et al., 1983). The neutral mononuclearcarbonyls of group 6, with the formula M(CO)6(M�Cr, Mo, W) are normally prepared starting from areadily available material in a positive oxidation state;consequently, the use of a reducing agent incombination with carbon monoxide is required.Hexacarbonylchromium(0) Cr(CO)6 can be preparedby treating anhydrous CrCl3 with the phenyl Grignardreagent PhMgBr in tetrahydrofuran (THF) with COunder pressure at temperatures between �4 and�10°C. Another preparation of Cr(CO)6 involves thecarbonylation, in the presence of Mg/Zn, of severalchromium(III) salts under CO pressure (100-300 bar)at 130-180°C in a pyridine medium. Thehexacarbonyls of chromium, molybdenum, andtungsten are mononuclear, the central metal atombeing octahedrally coordinated to six carbonyl groups.Accordingly, the idealized molecular symmetry is Oh.

The best available method for preparingMn2(CO)10 is the alkylation of anhydrousmanganese(II) acetate with AlR3 and CO underpressure in isopropyl ether (i-Pr)2O at 60-140°C. Themethylcyclopentadienyl derivative of manganese(I),

Mn(MeC5H4)(CO)3, is reduced to Mn2(CO)10 atatmospheric pressure of CO by sodium in diglyme(16-20% yield).

Decacarbonyldirhenium(0), Re2(CO)10, is preparedstarting from KReO4 by reductive carbonylation underpressure (300 bar) at 300°C, in the presence of copper.With NH4ReO4 under CO pressure (100 bar) at about190°C, a 69% yield of Re2(CO)10 was secured.Application of this method to NH4TcO4 gave a 90%yield of Tc2(CO)10 under less drastic conditions thanthose required for NH4ReO4 (Calderazzo et al., 1989).The use of Al(i-Bu)2H as a reducing agent allows thecarbonylation of NH4ReO4 to be carried out, atatmospheric pressure of CO, at 70-80°C, the yield ofRe2(CO)10 being about 60% (Top et al., 1996). Thedecacarbonyls of manganese, technetium, andrhenium, having the formula M2(CO)10, have terminalcarbonyl groups and a metal-metal bond. Themolecular symmetry is D4d with the two M(CO)5fragments in a staggered conformation.

Substantially quantitative yields of Fe(CO)5 areobtained by carbonylation of anhydrous FeI2 underdrastic conditions (200°C, 200 bar of CO pressure) inthe presence of copper as halogen acceptor(Calderazzo et al., 1968). Starting from solutions of preformed Fe(CO)5 in glacial acetic acid at 10-15°C,the slightly soluble enneacarbenyldiiron(0), Fe2(CO)9is formed by visible−light irradiation and recovered byfiltration. The molecular structure of Fe(CO)5, asdetermined on the compound below its melting point(�20.5°C) by X-ray diffraction, is trigonalbipyramidal (D3h symmetry). The enneacarbonylFe2(CO)9 contains three bridging carbonyl groupsconnecting the two Fe(CO)3 units, i.e.(OC)3Fe(m2�CO)3Fe(CO)3.

Molecular hydrogen H2 can be used as a reducingagent in the presence of CO, as in the case of cobalt(II)carboxylates, the carboxylato group being released asthe corresponding carboxylic acid. Good yields of Co2(CO)8 are obtained from bis(2-ethylhexanoato)cobalt(II) at 30°C, H2 and COpressures being, respectively, 180 and 80 bar:

2Co(O2CR)2�8CO�2 H2�� Co2(CO)8�4RCOOH

The best solvents for the synthesis are: diisopropylketone (i-Pr)2CO, methyl isopropyl ketone MeCO(i-Bu), diethylene glycol diethyl etherEtOCH2CH2OCH2CH2OEt, followed by THF. Thispoint will be further examined in connection with theexperimental findings concerning the cobalt-catalysedhydroformylation (Chini, 1960a). It has been furtherdemonstrated (Chini, 1960b) that Co2(CO)8 issynthesized in an autocatalytic reaction and that therate of CO absorption decreases by increasing the COpressure (pCO) above a certain limiting value.

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Dodecacarbonyltetracobalt(0), Co4(CO)12 is theproduct of partial thermal decarbonylation of Co2(CO)8, the formation of Co4(CO)12 beingendothermic:

2Co2(CO)8����Co4(CO)12�4CO

The tetranuclear cobalt derivative can also bedirectly prepared by reducing cobalt(II) salts (2-ethylhexanoate or acetylacetonate) with H2 in thepresence of the stoichiometric amount of Co2(CO)8, intoluene as a medium. The equation below describesthe conversion of a cobalt(II) carboxylato complex:

2Co(O2CR)2 �3Co2(CO)8�2H2�� 2Co4(CO)12�

�4RCOOH

Yields are 90% or better (Ercoli et al., 1959). Thisreaction is completely inhibited by carbonmonoxide.The dinuclear Co2(CO)8 has a solid-statestructure with two doubly bridging carbonyl groups;each cobalt atom is hexacoordinated to three terminalcarbonyl groups, to the other cobalt atom, and to twobridging carbon atoms, i.e. (OC)3Co(m2-CO)2Co(CO)3,the cobalt-cobalt distance being 0.2530 nm. Indodecacarbonyltetracobalt(0) Co4(CO)12, which has atetrahedral arrangement of the four cobalt atoms, anapical Co(CO)3 group is connected by Co�Co bondsto a basal Co3(CO)9 fragment with three bridging andsix terminal carbonyl groups and cobalt-cobalt bonds,i.e. [(CO)2Co(m 2-CO)]3�Co(CO)3.

Dodecacarbonyltetrarhodium(0), Rh4(CO)12, isprepared from RhCl3 and CO at 200 bar in thepresence of a halogen acceptor, such as copper, silver,cadmium, or zinc: at 50-80°C the tetranuclearcompound is mainly formed, whereas at 80-230°C theproduct is Rh6(CO)16, in agreement with theendothermic nature of the entropy-driven nucleationprocess in these systems.

Octacarbonyldirhodium(0) Rh2(CO)8 (n�CO: 2,084;2,060; 1,862; 1,847 cm�1) can only be observed atrelatively low temperatures (between 19.5 and�15.2°C) and at an elevated CO pressure of about 200bar (Oldani and Bor, 1983, 1985).Dodecacarbonyltetrarhodium(0) has essentially thesame structure as the cobalt analogue, i.e.[(CO)2Rh(m2-CO)]3�Rh(CO)3.

Hexadecacarbonylhexacobalt(0), Co6(CO)16 isisomorphous with the corresponding rhodiumcompound Rh6(CO)16 (Leung and Coppens, 1983),with the six metal atoms placed at the vertices of anoctahedron: each metal atom binds two terminalcarbonyl groups, the remaining four carbonyl groupsbeing triply bridging on two couples of oppositetriangular faces, i.e. M6(CO)12(m3-CO)4, M�Co, Rh.

At variance with the easy accessibility of Ni(CO)4,the corresponding tetracarbonyls of palladium(0) and

platinum(0), Pd(CO)4 and Pt(CO)4, have beenidentified only spectroscopically (IR) at lowtemperature, both being synthesized by vapourizationof the metal followed by reaction with CO in a solidmatrix at about 20 K (Kündig et al., 1973). Thesecompounds decompose just above 60 K.

Especially with 4d and 5d elements, polynuclearcompounds with metal-metal bonds (metal carbonylclusters) are formed under reduced carbon monoxidepressure and/or high temperature (Chini et al., 1976;Roth et al., 1992; Ceriotti et al., 1994; Hughes andWade, 2000).

11.4.3 Reactivity of metalcarbonyls

Metal carbonyls can undergo several types ofreactions: substitution of the carbonyl groups;oxidation; or reduction. No change of the oxidationstate of the metal occurs for the first case, while anincrease or a decrease of the oxidation state occurs forthe second and third cases, respectively.

Substitution reactions on metal carbonyls may becarried out thermally, photochemically, andchemically. In a thermal substitution, heat is used toboth increase the kinetics of the process and shift theequilibrium; when gaseous carbon monoxide isproduced, the DS° of the reaction is normally positive,as for the formation of the tricarbonylarenemetal(0)complexes of group 6, M�Cr, Mo, W:

M(CO)6�arene����M(CO)3(arene)�3CO

The tricarbonyl-h6-arene metal complexes ofchromium, molybdenum, and tungsten have a pseudo-octahedral geometry with the arene occupyinga triangular face of the octahedron, the oppositetriangular face being defined by the three carbonylgroups.

Tetracarbonylnickel(0) Ni(CO4) is quite reactiveand several substitution products are known. Forexample, with tertiary phosphines and similarpnicogen-containing derivatives, many complexes withthe formula Ni(CO)4�n(ER3)n have been prepared, Ebeing an element of group 15. The tertiary phosphinecomplexes of nickel(0) are important catalyticprecursors, as in the trimerization of substitutedacetylenes or in the dimerization of butadiene tocyclo-octa-1,5-diene.

Compounds containing metals belonging to thesame group within the periodic table, i.e. with thesame number of valence electrons for the central metalatom, normally are isostructural: examples are thehexacarbonyls of group 6. For this type of compoundsand for low oxidation states, the order of substitutional

726 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

reactivity is frequently 3d�4d�5d (Basolo, 1990;Freeman and Basolo, 1991), with the exception of thecyclopentadienyl (Cp) derivatives of group 5,MCp(CO)4 (M�V, Nb, Ta), for which the order ofsubstitutional reactivity is V�Nb�Ta. Relevant tothese findings are the calculated data (Li et al., 1995)suggesting that the first CO dissociation energy for thehexacarbonyls of group 6 and the tetracarbonyls ofgroup 10 is the lowest for the 4d term (molibdenumand nickel respectively). Thus, at least for dissociativemechanisms, the order of reactivity usually observedhas a theoretical support. These observations arerelevant for catalysis.

A special case of substitution reaction is themigratory insertion reaction of alkyl or aryl metalcarbonyls, by which an acyl- or aroyl-metal carbonylis obtained by the action of a Lewis base. Thisreaction, first reported in 1957 (Coffield et al.,1957), converts a carbyl-pentacarbonyl derivative ofmanganese(I) MnR(CO)5 into Mn(COR)(CO)5 underthe action of CO. The mechanism has been studiedextensively and in the case of Mn(Me)(CO)5 thereaction was found to proceed through acoordinatively unsaturated tetracarbonyl speciesresulting from methyl migration onto one of theterminal CO groups in a cis position (Calderazzo andCotton, 1962; Calderazzo, 1977). In the migratoryinsertion reaction, the central metal atom does notalter its oxidation state and a new carbon-carbonbond is formed. As such, this reaction is believed toconstitute a fundamental step in several carbon-carbon forming catalytic processes(hydroformylation, homologation, CO-olefincopolymerization, alkene polymerization).

In oxidation processes, the order of reactivity isgenerally 5d �4d�3d. This point, which is in keepingwith the increasing stability of the higher oxidationstates for compounds of d transition elements, isimportant in connection with the catalytic processesdiscussed below.

Relevant to the forthcoming discussion on themechanism of metal-catalysed processes is theobservation that the migratory insertion of CO inbetween metal-hydrido bonds has been observed onlyin a few specific systems:

M�H�CO�� MC(O)H

No case of 3d hydrido metal complexes beingconverted to the corresponding formyl derivative isknown; on the contrary, formyl derivatives of 3dmetals, indirectly prepared, normally decarbonylate tothe corresponding hydrides.

An important redox reaction, producing thetetracarbonylcobaltate(�I) anion, [Co(CO)4]

−, andsolvated cobalt(II), is encountered with metal

carbonyls in the presence of nitrogen- or oxygen-containing Lewis bases (L), as exemplified forCo2(CO)8,

3Co2(CO)8�12L����2 [CoL6][Co(CO)4]2�8CO

The reaction has been shown to be reversed in thepresence of CO under pressure (Chini, 1960a). Asimilar disproportionation reaction occurs withhexacarbonylvanadium(0) (Calderazzo et al., 1968):

3V(CO)6�6L��[VL6][V(CO)6]2�6CO

The solid-state structure of the ionic productarising from the disproportionation in the presence ofTHF, i.e. [V(THF)4][V(CO)6]2, has been reported(Schneider and Weiss, 1976).

Noteworthy is the observation (Tucci and Gwynn,1964) that Co2(CO)8 and alcohols produce a redoxprocess to [Co(CO)4(ROH)]�[Co(CO)4]

�, stable attemperatures below 0°C. Also Rh4(CO)12 andRh6(CO)16 undergo disproportionation with pyridine(py) and the pyridinium derivative[(py)2H][Rh5(CO)13(py)2] has been structurallycharacterized (Fachinetti et al., 1993).

The dinuclear carbonyls of group 7 are reduced byalkali metals M� in THF and the dinuclear derivativeof group 9, Co2(CO)8, behaves similarly:

M2(CO)10�2M��� 2M�[M(CO)5]Co2(CO)8�2M��� 2 M�[Co(CO)4]

The pentacarbonylmetalate(�I) anions of group 7and [Co(CO)4]

− are appropriate entries into the carbyl-carbonyl derivatives, R�M(CO)5 and R�Co(CO)4,respectively, by reaction with an alkyl halide:

[M(CO)5]−�RX�� R�M(CO)5�X−

[Co(CO)4]−�RX�� R�Co(CO)4�X−

The resulting products have pseudo-octahedral ortrigonal bipyramidal structures of C4v or C3vsymmetry with the hydrido or carbyl ligandoccupying the apical position of the coordinationpolyhedron (Calderazzo et al., 1981; Brammer et al.,1992; Brammer, 2003).

11.4.4 Catalytic reactions of carbon monoxide

In this section a number of catalyzed reactions areillustrated by which carbon monoxide is directedtowards the formation of new carbon-carbon bonds: a) the synthesis of carboxylic acids catalyzed bystrong acids; b) the transition-metal-catalyzedformation of various organic substances (alcohols,glycols, aldehydes, etc.) from carbon monoxide underreducing conditions (alcohol homologation and CO

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hydrogenative coupling), or from CO and an olefin(hydroformylation and related reactions); c) theCO/olefin copolymerization; d ) the carbonylationof methanol to acetic acid, which can be formallyregarded as an insertion of CO in between theC�O bond of methanol. A detailed knowledge ofthe fundamental properties of organometallicderivatives is essential for a close understanding ofthe elementary steps involved in catalytic processesand, consequently, for quantifying the effect ofreaction parameters on both yields and nature of the products (Collman, 1968; Halpern, 1970; Stilleand Lau, 1977).

Synthesis of branched carboxylic acidsCarboxylic acids can be produced by olefin

carbonylation in the presence of water (Reppecarbonylation, see below). An alternative route to theC8 carboxylic acid (2-ethylhexanoic) is byhydroformylation of propylene, followed bydimerization of the C4-aldehyde and by oxidation.Carboxylic acids are now mostly synthesized by thereaction originally reported by Koch (Koch, 1955).Branched carboxylic acids having the formulaR1R2R3C�COOH are formed in the presence ofacids such as H2SO4, H3PO4, BF3 or SbF5, where R1,R2 and R3 are alkyl residues CnH2n�1, with n�1, the lowest member of the series being 2,2-dimethylpropanoic acid (pivalic acid),(CH3)3C�COOH, melting point �37°C, boilingpoint 176.5°C (Keenan and Krevalis, 1993). Thescheme below represents the formation of this acidfrom isobutene, whereby the intermediate carbocationis carbonylated forming the acylium ion which isfurther hydrolysed to yield the product, the netreaction being represented by the sum of the followingsingle steps:

(CH3)2C�CH2�H��� (CH3)3C�

(CH3)3C��CO�� (CH3)3C�C(O)�

(CH3)3 C�C(O)��H2O�� (CH3)3C�COOH�H�

(CH3)2C�CH2�CO�H2O�� (CH3)3C�COOH

All butene isomers are converted to pivalic aciddue to isomerization under the reaction conditions. Inthe manufacture of C10 trialkylacetic acids, a C9branched olefin stream is used leading to a mixture ofcarboxylic acids, which is liquid at room temperatureboiling in the 250-257°C range. Versatic Acid 911 is amixture of C9-C11 carboxylic acids from a C8-C10olefin stream. The commercial process is carried outin two stages: the olefin is first reacted with CO underpressure (50-100 bar) in the presence of the acidcatalyst, and the resulting product is treated withwater. The tertiary carboxylic acids thus obtained arepurified by distillation.

The products marketed by Shell (Versatic Acid)and by Exxon (Neo Acid) are carboxylic acids with ahighly branched structure and a tertiary carbon atomnext to the carboxylic group; these two features allowthe acids to be used in a wide variety of applications,as agrochemicals, pharmaceuticals, peroxides andcatalysts, with a range of uses quite larger thanprimary and secondary carboxylic acids. The highlybranched Versatic Acid 10 allows metals to bedissolved in organic solvents. Such salts findapplication as paint dryers, tyre adhesives, polymeradditives and catalysts. Esters of Versatic Acid 10 arestable to hydrolysis because of steric protection ofthe ester bond. Peroxyesters are used aspolymerization initiators.

C2 products from CO: methanol homologationand CO hydrogenative coupling

These reactions have the common feature offorming new carbon-carbon bonds from CO or otherC1 products under reducing conditions, leading to C2products. The reactions are normally catalysed bytransition d metals, in most cases cobalt, ruthenium orrhodium.

A process forming carbon-carbon bonds fromcarbon monoxide under reducing conditions is theDuPont production of ethylene glycol(HOCH2CH2OH) from CO, H2, and formaldehyde.However, the main source of this chemical still isethylene oxidation to ethylene oxide, followed byhydrolysis.

The first case of homologation reaction wasreported in 1949 (Wender et al., 1949), whereby ethylalcohol was produced from methanol with 1:1 CO/H2at about 200 bar and 160°C (Wender et al., 1951):

CH3�OH�CO�2H2�� CH3CH2OH�H2O

The reaction is apparently of general applicability,and the best yield (63%) at that time was obtainedwith ter-butyl alcohol being converted to isoamylalcohol. These results were later confirmed (Berty etal., 1956), while selectivities around 80% in thehomologation of methanol to ethyl alcohol have beenannounced in a review article (Pruett, 1981). Alsoiron-carbonyl derivatives in the presence of a tertiaryamine were found to catalyse the reaction; under ratherharsh conditions similar to those specified above, noproducts were obtained from methanol other than ethylalcohol. Moreover, the product is substantiallyanhydrous (Roth et al., 1984).

Paraformaldehyde was used as a C1 startingmaterial to produce glycolaldehyde HOCH2�CHO(Marchionna et al., 1989). Selectivities as high as 90%were obtained in acetone at 110°C at a total pressureof 125 bar of a 1:1 CO/H2 mixture,

728 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Rh4(CO)12/[N(PPh3)2]Cl or[Rh(CO)2Cl2]

�/[Rh5(CO)15�x(PPh3)x]� being used as

catalyst precursor. Formaldehyde had been suggested(Fahey, 1981) to be an intermediate in several metal-catalysed reactions involving the 1:1 CO/H2mixture; however, the synthesis gas, pressurized at about 1,970 bar in the presence of Rh(C5H7O2)(CO)2in a tetraglyme medium, only yielded ethylene glycol,methanol and ethanol as main products. In view of theresults with rhodium carbonyl clusters, the importanceof the metal precursor in catalytic processes is onceagain demonstrated, and the intermediacy offormaldehyde in C2 forming processes is furthersubstantiated. Also, preformed CoH(CO)4 has beenfound to react with formaldehyde at 0°C and atatmospheric pressure of CO giving a high yield ofglycolaldehyde.

An interesting C�C coupling reaction involvingcarbon monoxide is the synthesis of ethylene glycolfrom CO in the presence of H2, formally representedas shown below:

2CO�3H2�� HOCH2CH2OH

Metal carbonyls were used as promoters for thisreaction. Particularly studied were systems containingrhodium carbonyl clusters up to Rh13 or higher (Pruett,1981). As usual, the catalytic species responsible forthe process has not been established conclusively, theproposal being that the catalytic cycle goes through arhodium species containing a bond to a hydroxymethylgroup of the type (L)nRh�CH2OH, of unknownmolecular complexity, on which coupling of thecarbon-containing residue would occur.

Olefin hydroformylation and related reactionsThe hydroformylation of olefins, i.e. the addition

of a formyl group (�CHO) across a carbon-carbondouble bond in the presence of H2, was discovered as asideline of the studies, first announced in 1926, on theFischer-Tropsch reaction. In 1938 olefins RCH�CH2were reported by Ruhrchemie (Weissermel and Arpe,1978) to be converted to the corresponding aldehydescontaining one additional carbon atom by thecombined action of carbon monoxide and hydrogen(Roelen, 1943, 1944, 1948, 1952). Some years earlier,workers at the US Bureau of Mines of Pittsburgh,Experiment Station (Smith et al., 1930), had reactedethylene with CO/H2, obtaining oxygen-containingproducts which apparently were not furtherinvestigated; only later did Roelen establish that theproducts are essentially propionaldehyde anddiethylketone. In the USA, the first cobalt-based plantwent into stream in 1948 in Baton Rouge, Louisiana.In Italy, a hydroformylation plant of ethylene topropionaldehyde had been projected since 1941 by

Società Bomprini Parodi Delfino; however, war eventsdid not allow the plant to become operative (Natta andPino, 1949).

The worldwide installed capacity for thehydroformylation of olefins (oxo process) has beenreported to be about 7.0·106 tons of aldehydes andalcohols in the year 1990. Thus, it is the largesthomogenously catalysed process operated worldwide(Billig and Bryant, 1996). Several aspects of thehydroformylation of olefins have been reviewed(Beller et al., 1996; Bahrmann et al., 1996). Thefollowing equation describes the conversion ofethylene to propionaldehyde:

CH2�CH2�CO�H2�� CH3CH2CHO

Hydrogenation of carbon−carbon double bonds tothe corresponding alkanes is in generalthermodynamically more favoured thanhydroformylation. The same applies to thehydrogenation of CO to methane. As both CO andolefin coexist during the hydroformylation process,the substantially exclusive formation of aldehydes asprimary products sets a good example ofchemoselectivity, as directed by the nature of thecatalyst.

A few transition metals have been shown to catalysethe hydroformylation reaction, namely cobalt-,rhodium- and platinum-based precursors (Clark andJain, 1984), which are converted into the catalytically-active species under the reactionconditions. In more recent times, also rutheniumcomplexes were employed as catalyst precursors (Kalcket al., 1991) in the form of phosphine-substitutedderivatives, e.g. Ru(CO)3(PR3)2, as originally proposedby G. Wilkinson and co-workers (Evans et al., 1965).The activity, however, is considerably lower than that ofthe cobalt-based systems, with consequently lowerconversions. Mechanistic aspects of the olefinhydroformylation have been reviewed (Stille, 1991).

Cobalt carboxylates are still used as catalystprecursors for large-scale industrial operations; theyare relatively inexpensive, easily prepared and readilyconverted by CO/H2 to the isolatable carbonylderivatives Co2(CO)8, CoH(CO)4, or Co4(CO)12, theirrelative amount under reaction conditions being afunction of both pressure and temperature. For thecobalt-catalysed reaction, the nature of the solvent hasno large effect on the rate of hydroformylation ofcyclohexene at 110°C, while the structure of the olefingreatly affects rates, branched olefins being theslowest ones (Wender et al., 1956, 1957).

Important progress in the hydroformylation ofolefins was made in 1965 when rhodium-complexedtrisubstituted phosphines were reported to promote thehydroformylation of olefins under mild conditions of

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HOMOGENEOUS CATALYSIS WITH CARBON MONOXIDE IN CARBON-CARBON BOND FORMING PROCESSES

temperature and pressure (Osborn et al., 1965; Bairdet al., 1967; Evans et al., 1968), while a significantchange of regioselectivity induced by the phosphineligand was announced later (Slaugh and Mullineaux,1968). As a matter of fact, rhodium-carbonylderivatives show a much higher activity than thecobalt-based systems; rhodium, used as preformedRh4(CO)12, was found to perform an activity ca. 104

higher than that of cobalt (Heil and Markó, 1968).However, rhodium has a much higher price thancobalt. The rates of the rhodium-catalysed reactiondepend on the nature of the olefin, styrene being thefastest in the list (Heil and Markó, 1969).

Tertiary phosphines reduce the rate of the cobalt-catalysed hydroformylation, which iscompensated by a higher selectivity in terms of then/iso ratio of the produced aldehydes using alkyl-substituted olefins (Whyman et al., 2002).Assuming, as it has commonly been done, thatCoH(CO)4, or some other hydrido CoH(CO)x orformyl species CoC(O)H(CO)x (x4), is the actualcatalyst of the reaction, with alkyl-substitutedterminal olefins RCH�CH2, the addition to theolefin double bond may produce linear or branchedaldehydes (Bianchi et al., 1977a, 1977b):

��RCH2CH2�CHORCH�CH2�CO�H2––��RCH(CHO)CH3

Normally, the linear aldehyde predominates(represented at the top of the above scheme) arisingfrom the formal anti-Markownikoff addition of thefunctional group to the less alkyl-substituted carbonatom (Beller et al., 2004). Obviously, if double bondmigration occurs under the reaction conditions, thenumber of isomeric products increases considerably.

Styrene and preformed CoH(CO)4 react in thepresence of CO giving the branched acyl derivative,which slowly converts to the linear isomer (Ungváryand Markó, 1982). It is a relevant mechanisticobservation that high CO pressures, while decreasingthe reaction rate, increase the n/iso ratio of theproduced aldehydes from substituted olefins (Piacentiet al., 1966). For example, with propylene, thestraight-chain aldehyde varies from 62% at pCO�2.5bar to 81% at pCO�30 bar: higher CO pressures up to90 bar leave the n/iso ratio substantially unchanged.

Rhodium(I), initially introduced as a carbenecomplex and in benzene as solvent, catalyses theaddition of CO/H2 to p-substituted styrenes with thebranched isomer of the corresponding aldehyde beingaround 95% (Chen et al., 2000). On the other hand, theuse of a water-soluble rhodium precursor with asulphonated bidentate phosphine led to propylenehydroformylation with a n/iso ratio of 99 for the

corresponding aldehydes (Bahrmann et al., 1996).This sets an example of how the electronic and stericproperties of the catalytic system, as induced by thereaction medium, are important for the productselectivity.

Asymmetric hydroformylation (Agbossou et al.,1995; Breit, 2003; Nozaki et al., 2003) was firstreported in 1972 with a cobalt-based catalyst (Botteghiet al., 1972) or in the presence of a rhodium catalyst(Ogata and Ikeda, 1972; Tanaka et al., 1972) with aSchiff base or a chiral monodentate phosphine assupporting ligand.

A point of major interest from a mechanistic andan operational viewpoint is the effect of the COpressure on the rates of the cobalt-catalysedhydroformylation of olefins, discovered in the courseof kinetic studies with cyclohexene. It had in factbeen noted during kinetic studies with cyclohexene(Natta and Ercoli, 1952) that the yields of aldehydesare unaffected by pressure, in the range between 120and 380 bar, by using the conventional 1:1 CO/H2mixture. Subsequently (Natta et al., 1954), the so-called negative effect of carbon monoxide pressurewas discovered, a result which was almostsimultaneously confirmed by two other researchlaboratories (Martin, 1954; Greenfield et al., 1954);the yields of aldehyde at a given time increase up to amaximum value of CO pressure and then decrease athigher pressures, other relevant conditions beingmaintained constant. Also, the rates of the rhodium-catalysed hydroformylation are affected bythe CO partial pressure (Heil and Markó, 1968), withthe maximum of activity being observed for a COpressure of 40 bar, as the result of a series ofexperiments carried out at constant partial pressure ofH2 (40 bar). The effect of the olefin structure on therate of reaction has been studied, a-olefinsperforming the highest rates as expected (Heil andMarkó, 1969).

The equilibrium between Co2(CO)8 and CoH(CO)4mediated by H2 is well established. Moreover, theaddition of an olefin to CoH(CO)4 has been shown togive an alkyl derivative, the latter undergoing carbonylinsertion quite readily. A simplified, generallyaccepted, picture of the elementary steps involved inthe hydroformilation process is represented in thefollowing equations:

1/2 Co2(CO)8�1/2H2����CoH(CO)4

CoH(CO)4�RCH�CH2����Co(CH2CH2R)(CO)4

Co(CH2CH2R)(CO)4�CO����Co(COCH2CH2R)(CO)4

The equilibrium between Co2(CO)8 andCoH(CO)4 is endothermic, i.e. it is favoured byincreasing the temperature. The final step ishydrogenolysis of the acyl derivative yielding the

730 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

aldehyde and CoH(CO)4 (only the generallypredominant linear product is shown):

Co(COCH2CH2R)(CO)4�H2�� CoH(CO)4�

�RCH2CH2CHO

In situ spectroscopic studies under pressure haveshown that the hydroformylation of 1-octene inheptane (Whyman, 1974; Whyman et al., 2002; Dwyeret al., 2004) is characterized by higher concentrationsof aldehydes by increasing the CO/H2 ratio. Atintermediate times, the acyl derivative Co(COR)(CO)4was detected.

On the basis of the experimentally observed effectof CO pressure on yields, the hypothesis wasformulated that a cobalt-carbonyl complex with acontent of carbon monoxide lower than that inCoH(CO)4, possibly CoH(CO)3, could be operativeunder the reaction conditions.

The rates of the cobalt-catalysed hydroformylationreaction at 110°C slightly increase with the dielectricconstant of the medium. More recently, thehydroformylation has been carried in supercriticalcarbon dioxide (Koch et al., 1998). Moreover, theformation of ion pairs in both cobalt- (Fachinetti et al.,1987, 1988) and rhodium-carbonyl (Fachinetti et al.,1993) chemistry, as a consequence ofdisproportionation phenomena, has been firmlyestablished chemically and crystallographically. Theseare important, mechanistically relevant, phenomena,see Section 11.4.5.

Reactions similar to hydroformylation, involvingthe use of either water or alcohols as proton-activesubstances instead of H2, have been reported. In thepresence of water the carbonylation of the olefinsleads to carboxylic acids, whereas in the presence ofalcohols esters are produced (Reppe et al., 1953a,1953b; Fenton, 1973; Kiss, 2001):

��RCH2CH2�COARCH�CH2�CO�HA––��RCH(COA)CH3

(A= OH, OR)

Effects of CO monoxide pressure on rates, similar to those mentioned above for the olefinhydroformylation, have been found (Ercoli et al.,1955, 1960) also in these systems.

As several carbonylation reactions are negativelyaffected by the CO partial pressure above a certainlimiting value depending on the type of reaction andon temperature, uncharged carbonyl species with alower content of carbonyl groups with respect to thenumber required by the complete saturation of thecoordination shell is generally recognized. However,the possibility should also be taken into consideration,

in connection with the facile disproportionationexperienced by some metal carbonyl derivatives in thepresence of Lewis bases, that cationic metallic speciesmay actually take care of the coordinative elementarysteps leading from reagents to products. In thisconnection it is interesting to note that also thesynthesis of Co2(CO)8 is best carried out in ketones orethers as solvents.

CO/olefin co-polymerizationThe copolymerization of CO and ethylene was

first reported in 1941 by Farbenfabriken Bayer inGermany under rather harsh reaction conditions,temperatures up to 230°C and pressures as high as2,000 bar being used in the nickel-catalysed processstudied by Reppe (Sen, 1993; Drent and Budzelaar,1996; Sommazzi and Garbassi, 1997). CO/olefincopolymerization can also be promoted via a free-radical mechanism or by g-irradiation, thusgenerally obtaining random non-alternatingcopolymers. Later, considerable progress in thealternating copolymerization of CO with olefins to[C(O)CH(R)CH2]n was achieved by using palladiumprecursors in the presence of tertiary phosphines. TheCO/ethylene perfectly alternating copolymer, usuallyprepared in methanol solvent,

nCH2�CH2�nCO�CH3OH��

�� H(CH2CH2CO)nOCH3

is characterized by some interesting properties; theinsertion of the low-cost carbon monoxide allows themass of the ethylene-based polymer to be doubled, themechanical properties being good, with a high meltingpoint in a highly crystalline material.

A remarkable achievement in this area was thediscovery by the Shell Laboratories in Amsterdam thatbidentate phosphines, and 1,3-bis(phosphino)propanesin particular, increase the rate of copolymerization, thecatalyst lifetime and, consequently, the activity of thecatalyst in terms of the mass of copolymer per unitweight of catalyst precursor and per unit time. Theeffect of ligand bite on the metal-catalysedcopolymerization reaction has been reviewed (vanLeeuwen et al., 2000).

A typical catalyst composition may be produced insitu from a palladium derivative, normallybis(acetato)palladium(II) orbis(trifluoroacetato)palladium(II), a bidentate tertiaryphosphine or a chelating nitrogen donor, an acid suchas CF3COOH or p-MeC6H4SO3H, and an oxidizingorganic substance such as 1,4-benzoquinone, whosefunction is believed to be that of bringing backreduced palladium to the oxidation state II, underreaction conditions. It is interesting to note that thepresence of strong acids, such as those specified

731VOLUME II / REFINING AND PETROCHEMICALS

HOMOGENEOUS CATALYSIS WITH CARBON MONOXIDE IN CARBON-CARBON BOND FORMING PROCESSES

above, i.e. containing a weak conjugate base, isessential for high turn-over numbers. As an alternative,palladium can be introduced in the form of cationiccomplexes of the type [Pd(L�L)R(L)]�, whereL�L is a nitrogen- or a phosphorous-based bidentateligand, R is a carbyl group, and L is a monodentateligand completing the coordination sphere of square-planar palladium(II) in its resting state(Bianchini et al., 2002). The anion is generally aweakly coordinating one.

The CO/C2H4 copolymer thus produced, inagreement with its nature of a polyketone, shows asharp, highly diagnostic, IR band at about 1,700 cm�1.

Two experimental observations appear to beconnected with this process. The first one deals withthe hydroformylation of ethylene, which was shown,in the early days of the hydroformylation reaction, toyield diethylketone C2H5C(O)C2H5 predominantly,suggesting that the addition of a second molecule ofethylene to the metal-bonded acyl intermediateM�C(O)C2H5 is competing successfully with thehydrogenolysis to C2H5CHO. The second point ofinterest is that the alternate insertion of CO andethylene, taken as a reference olefin, to givepolyketone in the palladium-catalysed process is inagreement with simple thermodynamicconsiderations on bond dissociation enthalpies ofM�C and C�C bonds. Once the firstM�C(O)C2H4OMe sequence has been formed, thenext event must be carried out in the presence of COand ethylene. Although coordination of CO wouldnow be preferred (Calderazzo et al., 2004) due to thegenerally higher bond strength for M�CO than forM(C2H4), ethylene insertion should be regarded aspreferred. As a matter of fact, a second CO insertionwould generate the sequenceM�C(O)C(O)C2H4OMe, which should beconsidered to be less stable thanM�CH2CH2C(O)C2H4OMe. This is suggested bythe bond dissociation enthalpy inCH3C(O)�C(O)CH3 and PhC(O)�C(O)Ph, 282.0and 277.8 kJ/mol respectively, which are lower byabout 75 kJ/mol from that of Me�C(O)Ph,estimated to be 355.6 kJ/mol. It is noteworthy that inCH3C(O)�C(O)CH3 a high value of 0.1540(6) nmwas found crystallographically for the central carbon-carbon bond distance (Eriks et al., 1983).

This kind of reasoning, in terms of relativestability, which applies to ground-state molecules,should be regarded to be valid also for transition-statemetal-bonded species.

The CO/olefin copolymers and the terpolymershave been commercialized, and the patent portfolio onCarilon and Carilite polyketones has recently beendonated by Shell to Stanford Research Institute.

Carbonylation of methanol to acetic acidFormally, the synthesis of acetic acid from

methanol represents the insertion of CO in betweenthe carbon-oxygen bond of the alcohol:

CH3OH(l)�CO(g)�� CH3COOH(l)

This thermodynamically favoured process(DG°��87.8 kJ/mol) is characterized by a negativeentropy change.

However, mechanistic studies suggest that thefundamental step of the reaction is the carbonylationof a metal-bonded methyl group giving thecorresponding acetyl, followed by hydrolysis.

The first high-yielding homogeneous carbonylationof methanol to acetic acid was reported by Reppe atBadische Anilin und Soda Fabrik (BASF) in Germany(Reppe et al., 1953b) by a process first patented in1941. Several metals or metal complexes (iron, cobalt,nickel, tungsten) were used in the presence of iodidesas activators and yields as high as 90% were secured byoperating under drastic conditions of temperature andpressure (250°C, ca. 200 bar). Later, BASF (vonKutepow et al., 1965) announced a process forproducing acetic acid based on the carbonylation ofmethanol in the presence of cobalt-based catalysts andwith iodide-promoters, at pressures and temperaturesas high as 600 bar and 230°C, respectively, with aselectivity of about 90%. BASF has built a unit atLudwigshafen in 1960 with a capacity of 3,000 t/y,increased to 12,000 t/y in 1964. The process operates inthe presence of water at about 200°C with CO at 200-700 bar, with CoI2 as catalyst precursor.

In the case of the cobalt-catalysed reaction, whichis carried out in the presence of water, and with thepresent knowledge of cobalt-carbonyl chemistry, thefollowing reaction steps may be indicated to occur,after conversion of CoI2 to cobalt-carbonyl substances,exemplified by the tetracarbonylcobaltate(�I) in thefollowing scheme:

[Co(CO)4]− �MeI�� Co(Me)(CO)4�I−

Co(Me)(CO)4�CO�� Co(COMe)(CO)4Co(COMe)(CO)4�H2O�� H��MeCOOH�[Co(CO)4]

−

A few years later, Monsanto Company (Paulik andRoth, 1968) reported the carbonylation process to becatalysed by iodide-promoted rhodium or iridiumprecursors, under less harsh conditions of temperatureand pressure (150-200°C, 30-60 bar), and with ahigher selectivity (Forster, 1979). The rate of therhodium-catalysed carbonylation of methanol wasreported to be independent of CO partial pressure(Roth et al., 1971).

Commercial production with rhodium initiated in1970 (Robinson et al., 1972). In 1986, Monsantotechnology was transferred to BP Chemicals; a further

732 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

improvement of the process was then introduced byBP in 1996 with the use of iridium-based catalysts inthe presence of RuI2(CO)4 as promoter, the best one ina list of nine iodides (Haynes et al., 2004) in the so-called Cativa process (Sunley and Watson, 2000;Jones, 2000).

The transition-metal-catalysed carbonylation ofmethanol in 1984 accounted for about 60% of thetotal acetic acid produced in the USA, while about30% was the percentage in Europe (Chauvel andLefebvre, 1989).

Mechanistic studies of methanol carbonylation toacetic acid (Maitlis et al., 1996) evidenced thekinetically relevant steps in the rhodium system, ascompared with those occurring in the iridium-basedprocess. A simplified scheme which accounts for thecarbonylation to methanol in the presence of iodidepromoters is shown below:

[MI2(CO)2]−�MeI�� [M(Me)I3(CO)2]

−

[M(Me)I3(CO)2]−�CO�� [M(COMe)I3(CO)2]

−

[M(COMe)I3(CO)2]−�H2O�� HI�MeCOOH�

�[MI2(CO)2]−

(M�Rh, Ir)

where methyl iodide (MeI) is suggested to derivethrough iodide exchange between MeOH and HI. In[M(Me)I3(CO)2]

−, the methyl and carbonyl groupswithin the coordination sphere of the metal arebelieved to be in a relative cis position. In the rhodiumsystem the rate-determining step is the oxidativeaddition of MeI to the rhodium (I) system beingconverted to the hexacoordinated methyl derivative ofrhodium(III), detected spectroscopically, furtherundergoing carbonyl insertion to the acetyl product. Inthe iridium system, under the operative conditions ofwater content, the carbonylation of [Ir(Me)I3(CO)2]

�

to [Ir(COMe)I3(CO)2]� is relatively slow and rate-

determining. However, at low water concentrations,oxidative addition of MeI to iridium(I) becomes slowerand rate-determining (Sunley and Watson, 2000).Moreover, a direct comparison between the Ir/Ru (1:2molar ratio) system and the rhodium-catalysed processshows the former to be superior to the latter, while areverse situation was found for elevated waterconcentrations, similar to those used in the Monsantoprocess. Advantages of the Cativa process are lowerlevels of water (8% by weight), lower amounts ofby-products and improved carbon monoxideefficiency. It is to be pointed out that the iridium-basedsystem per se is catalytically less active than therhodium-based one. This problem can be overcome bythe presence of some additives; as mentioned earlier,the most significant effect has been found with acarbonyl-iodide of ruthenium(II), initially introducedas RuI2(CO)4, presumably undergoing decarbonylation

and nucleation under the rather drastic conditions ofthe process. Interestingly, the bimetallic Ru/Rh systemwas found to catalyse the hydrogenation of carbonmonoxide to ethylene glycol under drastic conditionsof temperature and pressure (Dombek, 1985).

Mention should also be made of a recent acquisitionin the production of acetic acid via the carbonylation ofmethanol, carried out with rhodium catalytic precursorsimmobilized on a resin derived from thecopolymerization of vinylpyridine and vinyl acetate. Thereaction was reported to have been carried out at 160-200°C under a pressure of 30-60 bar. The catalystdid not show deactivation over a period of about one yearof continuous operation (Thomas and Süss-Fink, 2003).

11.4.5 Conclusions

Some fundamental aspects of transition metalchemistry are relevant to the experimental findings oncatalytic carbonylation reactions. Concerning thecarbonylation of methanol, it is worth mentioning thatoxidative addition of alkyl halides to transition-metalcomplexes – first reported for square-planar complexesof platinum(II), e.g. Pt(Me)I(PEt3)2 being converted byMeI to the hexacoordinated derivative of 5d6

platinum(IV), PtMe2(PEt3)2I2 – generally occurs fasterin the sequence 3d�4d�5d (Collman, 1968; Halpern,1970; Stille and Lau, 1977). Thus, within group 9,iridium(I) is the most reactive towards the oxidativeaddition and the following trend was established:

IrCl(CO)(PPh3)2 > RhCl(CO)(PPh3)2IrCl(PPh3)3 > RhCl(PPh3)3

Within the domain of the cobalt-catalysedhydroformylation of olefins in the presence of tertiaryphosphines, it has been established that Co2(CO)8reacts with PR3 forming both ionic and substitutionneutral products, [Co(CO)3(PR3)2]

�[Co(CO)4]� and

[Co(CO)3PR3]2, respectively. Relevant to this point arethe catalytic studies carried out with a bimetallicprecursor (Broussard et al., 1993), the highest activitybeing found with a bicationic complex of rhodium. Itis also to be noted that the best yields of Co2(CO)8from bis(2-ethylhexanoato)cobalt(II) and CO/H2 areobtained in solvents such as (i-Pr)2CO, MeCO(i-Bu),EtOCH2CH2OCH2CH2OEt, THF, i.e. under conditionsfavourable to the possible formation of cobalt-containing ion pairs (Fachinetti et al., 1988). Sincecarbon monoxide is released as in the case of thepreviously noted equilibrium:

3Co2(CO)8�12L���� 2[CoL6][Co(CO)4]2�8CO

the experimentally observed negative influence of COpressure, above a certain value, on the yields of several

733VOLUME II / REFINING AND PETROCHEMICALS

HOMOGENEOUS CATALYSIS WITH CARBON MONOXIDE IN CARBON-CARBON BOND FORMING PROCESSES

carbonylation processes could find an explanation, ifthe elementary steps, e.g. substitution by the incomingligands (CO, olefin, H2), occur in cationic rather thanin neutral cobalt-containing species. As a matter of fact, the concentration of the neutralcarbonyl species would decrease at higher pCOs.Further clarification of these points should helpimprove the performance of new and well-establishedprocesses involving carbon monoxide.

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Fausto Calderazzo

Dipartimento di Chimica e Chimica IndustrialeUniversità degli Studi di Pisa

Pisa, Italy

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