2. Catalysis Involving CO

(Source: Collman / Hegedus + Chiusoli / Maitlis + original papers mentioned below)

1 General Reactivity of CO-Complexes

1 is the resonance representing the pure σ-donation of CO to the metal.

3 is contributing the most when the π back donation from the metal to CO is weak. The carbon is more electrophilic here.

2a is the extreme structure that evidences the π back donation of the metal to the π* of CO. A possible alternative is 2b, where the electrophilicity of the carbon is shown. With strong π back donation 2b is insigniﬁcant. 2 – Synthesis Gas and Water Gas Shift Reaction CO / H2 as feedstock.

– Hydrocarbonylation (or Hydroformylation) of Oleﬁns / Oxo Reaction Synthesis of aldehydes and alcohols from alkenes with cobalt and rhodium catalysts.

– Carbonylation of Alcohols: Monsanto’s Acetic Acid Process Preparation of acetic acid from methanol and CO.

3 Synthesis Gas (Syn Gas, CO / H2) as Feedstock Steam over coal:

C + H2O → CO + H2 0 0 (ΔH 298 K = 131 kJ/mol; ΔG1073 K= –12 kJ/mol)

Steam reforming of methane:

CH4 + H2O → CO + 3 H2 0 0 (ΔH 298 K = 206 kJ/mol; ΔG1073 K= –24 kJ/mol) Coupled with partial oxidation to give an endothermic overall reaction: H 0 2 C + H2O + O2 → CO + CO2 + H2 (Δ 298 K = –285 kJ/mol)

1 H 0 CH4 + 2 O2 → CO + 2 H2 (Δ 298 K = –36 kJ/mol)

4 Water-Gas-Shift Reaction (WGSR)

Allows adjust the CO : H2 ratio by converting CO to H2:

CO + H2O ↽!!!⇀! CO2 + H2

Drawback: CO2 as byproduct.

Catalysts: Heterogeneous Cr2O3 (T = 350°C) Cu-Zn-oxide (T = 200 – 300°C) Fe3O4

Homogeneous Carbonyl complexes: [FeH(CO)4]–, [RhI2(CO)2]–, [RuCl(bipy)2(CO)]+ 5 Homogeneously Catalyzed WGSR

Principle:

O CO OH– – CO2 M M CO M C M H – OH

– + H O – M H 2 M + OH + H2

6 Catalytic WGSR

7 Hydroformylation or Oxo Synthesis Synthesis of aldehydes and alcohols from alkenes.

– Most desired product: n-butanol. The most important commercial process is the reaction

of propene to n- / i-butanal (R = CH3) and other C4 oxygenates: O CH Co or Rh 3 C + O 2 + 2 CO + 2 H2 R H R R catalyst H – Several million tons per year of oxo products are produced worldwide!

– Catalysts are carbonyl complexes of cobalt or rhodium.

– Most important process that uses a transition metal carbonyl complex.

– Linear aldehydes are more desirable than branched ones.

8 Thermodynamics

Hydroformylation

G = –42 kJ/mol CH3CH=CH2 + H2 + CO CH3CH2CH2CHO Δ

Hydrogenation

ΔG = –88 kJ/mol CH3CH=CH2 + H2 CH3CH2CH3

The hydroformylation reaction is highly exothermic but less exergonic due to the decreasing entropy.

The thermodynamically favored product in hydroformylation is the hydrogenation product.

9 Most Important Hydroformylation Products Butanol and Derivatives: OH H O 2 H C – H2O 2 2-ethylhexanol H catalyst CHO catalyst CHO H2 catalyst [O] COOH OH

Higher α-Oleﬁns: CO, H2 H catalyst 1-hexene 1-heptanal O

Other reactions: 1-octene to nonaldehyde (pelargonic aldehyde) and decene to the C11 aldehyde (target: detergent alcohols). This development is driven by the increasing availability of the appropriate α-oleﬁns either from Fischer-Tropsch product mixtures or from the catalytic oligomerizaiton of ethylene (see later sections). Internal oleﬁns can also be hydroformylated. Such reactions have been less exploited commercially, as double bond isomerization competes strongly.

10 Cobalt Catalyst (1938, von Roelen)

As the most desired product is n-butanal, considerable attention has been devoted to increasing the linear:branched selectivity. This focused attention on the mechanism, especially the step where the propene inserts into the Co–H bond, as this can be either Markovnikov or anti-Markovnikov.

The observed rate law for the catalytic reaction (above a minimum threshold of CO pressure) is:

d[aldehyde] −1 = kobs[alkene][H2 ][Co][CO] dt

From the above rate laws, a simpliﬁed cycle based on [CoH(CO)4] as pre-catalyst is generally accepted (see below).

The inverse dependence on CO pressure suggests a step involving CO dissociation from the catalyst.

11 The dependence on alkene concentration and on H2 pressure suggests that alkene coordination and hydrogen activation occur either before or during the rate determining step.

Formation of the Catalyst from the Precatalyst

H2 [Co2(CO)8] !!!→ 2 [CoH(CO)4]

– CO ↽!!!!!!!!!⇀! [CoH(CO)4] + CO [CoH(CO)3] active species (CO dissociation: negative order in CO)

12 Oleﬁn Coordination and Migratory Insertion

linear faster R H (CO)3Co – CO OC Co R both equilibria are reversible [CoH(CO)4] + R OC + CO R CO fast (CO)3Co CH branched 3 Primary insertion → linear alkyl (anti-Markovnikov) Secondary insertion → branched alkyl (Markovnikov)

As the olefin insertion into the Co–H bond is fast and reversible, [CoH(CO)4] also catalyzes – besides hydroformylation – both olefin isomerization and H / D isotopic exchange in the olefin. However, because a vacant coordination site is needed for elimination, these side reactions are inhibited at higher partial pressures of CO. The linear-to-branched ratio is determined by the kind of insertion (1ary vs. 2ary, see preceding slide) and by the rate of CO-insertion into the Co-alkyl bond (see next slide).

13 CO-Insertion into the Cobalt-Alkyl Bond

R + CO R + CO O (CO)3Co (CO) Co (CO) Co R – CO 4 – CO 4 faster H B C D OC Co R OC CO fast R + CO R + CO O A (CO) Co (CO) Co (CO) Co 3 – CO 4 – CO 4 CH3 CH3 CH3 B' C' D' R Observation: High CO pressure increases the n : i aldehyde ratio. Explanation: CO scavenges the 16-electron alkyl complex B to give C. Thus, the inverse reaction (β-elimination to A) is inhibited ® the insertion is under kinetic (not thermodynamic) control. At lower CO pressure, the coordinatively unsaturated, 16-electron complex [Co(R)(CO)3] (B) will have a long enough lifetime to undergo β-hydrogen elimination and alkene reinsertion to give the branched alkyls, which are slightly favored thermodynamically (why?). The formyl complexes D are the only detectable species (resting species) when the steady-state reaction is examined by IR spectroscopy. Under standard catalytic conditions (linear oleﬁns such as 1-octene, [Co2(CO)8], 130-175°C, 250 atm)

14 C–H-Bond Formation (Rate Determining Step)

Two Possibilities:

O [CoH(CO)4] R + [Co2(CO)7] O O – CO H (CO) Co (CO) Co 4 3 H O R' R' 2 R + [CoH(CO)3] H

15 Catalytic Cycle

16 Comments

d[aldehyde] = k [alkene][H ][Co][CO]−1 dt obs 2

If a 1:1 H2/CO ratio is maintained, the rate will be independent of total pressure, since the rate is proportional to pH2 and inversely proportional to pCO. However, a certain minimum CO partial pressure is required to maintain the stability of [CoH(CO)4], which decomposes to cobalt metal at low pCO.

Thus, reasonable reaction rates in the temperature range 110–180 °C require rather high CO partial pressures (pCO), and total H2/CO pressures of 200–300 bar.

Disadvantage: high CO partial pressure decreases the hydroformylation reaction rate (why?) Advantage: high pCO increases linear-:branched-ratio (why?). Advantage: high pCO decreases alkene isomerization (why?).

→ Compromise between rate and regioselectivity!

17 Higher temperatures increase the rate but decrease the selectivity for the linear product and increase side reactions.

Typical side reactions are isomerization, alkene hydrogenation (typically ca. 1 %), and aldehyde hydrogenation to alcohol (typically 5–12 %). The latter is not unwelcome, as aldehydes are usually later hydrogenated to alcohols.

Drawbacks of Co-Catalysts: – High temperatures (140 – 175°C) and pressures (200 bar). – Branched aldehydes are the major product, but linear ones are the desired ones.

18 Rhodium Catalysts Binary rhodium carbonyls are not useful because of cluster aggregation:

[RhH(CO)4] is a very active hydroformylation catalyst, but gives oleﬁn hydrogenation and isomerization, and lower linear : branched ratio than cobalt carbonyl catalysts. But … Phosphine Ligands as Additives – Stabilize mononuclear complexes by inhibiting cluster formation → higher activity – Suppress oleﬁn hydrogenation and isomerization – Increases the linear : branched ratio (up to 30 : 1) – Are active at ambient temperature and pressure

Therefore, rhodium catalysts have been used in commercial production since 1976. Union Carbide Process: Propene Hydroformylation with [RhH(CO)(PPh3)3] as catalyst

Molten PPh3 (m. p. 79°C) as solvent, 100°C, 50 atm pressure → 92 % linear aldeyhde, negligible hydrogenation / isomerization. Problems: Cost of rhodium, degradation of PPh3.

19 Mechanism

Under standard conditions, CO intercepts the coordinatively unsaturated alkyl complex and the insertion is irreversible → kinetic control. The primary insertion (antimarkovnikov) is favored because the corresponding transition state is less crowded. Under process conditions, the linear:branched ratio is typically of 8–9 : 1 (the stereochemistry of insertion (1ary/2ary) is not shown).

20 Support for the Proposed Mechanism

Effect of [PPh3]: – reduces reaction rate (because the precatalyst must dissociate a PPh3 ligand): !!⇀ [RhH(CO)(PPh3)3] ↽!! [RhH(CO)(PPh3)2] + PPh3

– increases linear : branched product ratio

– suppresses oleﬁn hydrogenation and isomerization

Effect of P(CO): – higher CO partial pressures cause high linear:branched ratio up to a limit, too high CO partial pressures lower linear:branched ratio.

Effect of P(H2): – the rate law is first-order in P(H2) → step (d) is rate- determining. But: Effect of [olefin]: – the rate law is zero-order in olefin at high olefin concentration

– at lower [olefin], the rate law becomes first order in [olefin], a step before step (d) becomes rate-determining.

21 Stereochemistry of Oleﬁn and CO Insertion

1) [RhD(CO)(L) ] Me H 3 Me H CO / D2, 80°C Et Me

Et Me 2) [O], MeOH D CO2Me L = PPh3 cis-addition only!

The reaction was run with a 1:1 CO:D2 ratio and was stopped at 50 % conversion.

There was no incorporation of deuterium in the unreacted oleﬁn, which implies that oleﬁn insertion is irreversible.

The formation of the threo diastereoisomer indicates that the CO insertion occurs with cis stereochemistry and with retention of conﬁguration.

A. Stefani, C. Consiglio, P. Pino, J. Am. Chem. Soc. 1973, 95, 6504. 22 Hydrogen Activation

Possible Pathways:

[RhIIIH (COR)(CO)(L) ] H2 2 2 fast I I (a) [Rh (COR)(CO)(L)2] or [Rh H(CO)(L)2] + RCHO I 2 [Rh (η -H2)(COR)(CO)(L)2]

I (b) [Rh (COR)(CO)(L)2] + [RhH(CO)2)(L2)] [Rh2(CO)2(L)4] + RCHO

I 2 [Rh H(CO)(L)2]

All processes require one vacant coordination site on [RhI(COR)(CO)(L)2] and would thus be inhibited by CO.

23 Two-Phase (Water-Soluble) Rhodium Hydroformylation Catalysts

Introduced by Kuntz at Rhône-Poulenc in 1981. Celanese-Ruhrchemie currently operates several hydroformylation plants based on this technology (C. W. Kohlpaintner, R. W. Fischer, B. Cornils, Applied Catalysis A: General, 2001, 221, 219.) Water-soluble sulfonated triphenylphosphine ligand P(C6H4-m-SO3Na)3 (TPPTS) Water-soluble catalyst [RhH(CO)(TPPTS)3], very high (9–) formal charge, totally insoluble in all but the most polar solvents. A two-phase catalytic system results, in which the butanal product is essentially in the organic phase and can be easily separated. Similarly, the recovery of the catalyst is straightforward as it stays in the aqueous phase. An excess of the phosphine is required for good linear/branched selectivitities, as with conventional Rh/PPh3 catalysts, but lower concentration is required because the dissociation equilibrium of TPPTS in water is shifted towards the rhodium complex.

The solubility in water of shorter chain alkenes (C2–C4) is high enough to allow hydroformylation.

→ Rather high linear-to-branched regioselectivities (16–18:1) can be obtained for propylene

→ Rates are slower than with conventional Rh/PPh3 catalysts due to lower alkene concentration in the water phase.

→ Alkenes higher than 1-pentene are not soluble in water and cannot be used.

24 Carbonylation of Alcohols Most important industrial processes based on carbonylation (only CO, not CO + H2!):

CH3OH + CO → CH3COOH ΔG = –86 kJ/mol

O CH3 → H C O CH + CO 3 3 H3C C O O O

Acetic acid is one of the major commoditiy chemicals, with a current (2008) word production of ca. 9 Mt/a.

History of CH3COOH production:

Vinegar (5–10 % CH3COOH in H2O) as food additive since time immemorial. Not suitable to pruduce glacial CH3COOH, because water removal is extremely energy demanding. Destructive distillation of wood gives methanol, acetone, and some acetic acid, not used anymore. Oxidation of acetaldehyde (from ethanol oxidation now superseded by Wacker oxidation of ethene in water, see later)

25 Cost Advantages of Methanol Carbonylation – Highly selective process – Syngas is the only feedstock, as methanol is made from syngas by heterogeneous catalysis:

CO + 2 H2 → CH3OH Thus, both C atoms in CH3COOH arise from CO. If natural gas is used as the feedstock for syngas production, the conversion to acetic acid can be represented in three steps based only upon methane and water as reagents:

CH4 + H2O → CO + 3 H2

CO + 2 H2 → CH3OH

CH3OH + CO → CH3COOH The overall stoichiometry is:

2 CH4 + 2 H2O → CH3COOH + 4 H2

The excess H2 produced can be used in other processes: An integrated chemical plant might produce acetic acid and use the hydrogen to make ammonia: N2 + 3 H2 → 2 NH3 26 Monsanto’s Acetic Acid Process First large-scale industrial application of homogeneous catalysis The global annual manufacturing capacity for acetic acid is ca. 9 million tons, about 80 % of which is based on methanol carbonylation technology

Catalyst: Rhodium + iodide as promoter

Rh catalyst, I – !!!!!!!!!→ CH3OH + CO 180°C, 30 – 40 atm CH3COOH (99 %)

Any source of rhodium or of iodide may be introduced as precatalysts. These are converted into [RhI2(CO)2]– and CH3I under reaction conditions.

Review Article: C. M. Thomas, G. Süss-Fink, Coord. Chem. Rev. 2003, 243, 125.

27 Role of Iodide

For all catalysts – cobalt, rhodium, iridium (see below) – the actual substrate of carbonylation is methyl iodide.

CH3OH must be converted to CH3I, otherwise methanol would insert CO into the O–H bond (to give methyl formate) and not into the C–O bond (to give acetic acid).

The rhodium-catalyzed reaction depends on iodide, which is simultaneously a good nucleophile, a very weak base, a good leaving group, and an effective ligand for rhodium. No alternative to iodide has been found yet.

Drawbacks:

– Hydrogen iodide is corrosive → major engineering problem.

– A substantial amount of water (up to 15 wt.%) is required to achieve high catalyst activity and stability. H2O assists the reductive elimination of CH3C(O)I, which becomes rate-determining at lower [H2O]. As Rh also catalyzes the WGSR, the presence of water is a serious problem.

28 Mechanistic Features Rate determining is the oxidative addition of MeI to [RhI2(CO)2]– (see rate law below: the active form of iodide is methyl iodide).

Remember: (a) Iodide is a good leaving group, speeds up oxidative addition. (b) Iodide is the best σ-donor among the halides → increases the electron density at rhodium. After the r.d.s., migratory insertion and reductive elimination occur. They are fast and therefore not rate-limiting here. The reductive elimination has been observed in a model reaction. Finally, methyl is a special alkyl group, as it undergoes the fastest oxidative addition and the resulting alkyl complex is incapable of β-hydride elimination .

rate = k [Rh]1[I–]1[CH3OH]0[CO]0 29 Hydrolysis / Methanolysis The reaction conditions can be tuned to give other products by direct hydrolysis of the rhodium acyl:

O CH + CH OH 3 3 – [RhI2(CO)2] + HI + H3C C – CH3 O (CO)2I3Rh + CH COOH H C O CH O 3 – 3 3 [RhI2(CO)2] + HI + O O

30 Commercial Plant

31 – Continuous CO / MeOH feed into the reactor

– "Flash tank" where the pressure is released to separate most of the volatiles (including CH3COOH). The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel.

– The distillation train is fed with the product stream from the ﬂash tank. MeI is removed and recycled in the ﬁrst column, water is the second, and heavier by-products (such as propionic acid) are removed in the third one.

Besides being the product, CH3COOH also acts as the major solvent component. Therefore, the methanol feedstock is largely esteriﬁed into methyl acetate under process conditions:

MeOH + MeCO2H → MeCO2Me + H2O

As well as the water produced by esteriﬁcation, quite a high concentration of water (ca. 10 M) is required to maintain high rates and prevent deactivation by precipitation of the rhodium catalyst as insoluble Rh(III) iodides. Indeed, water promotes reduction of Rh(III) to Rh(I) via the water-gas shift reaction (WGSR), then Rh(I) reacts with CO and forms soluble carbonyl complexes.

Low water content is desirable: 1) Separation of water by distillation is expensive (energy!).

2) H2O causes signiﬁcant loss of CO as CO2 by the WGSR. → Solution: Cativa (see below)

32 The Cativa™ Process

Iridium catalyst + iodide + Lewis-acidic promoter → higher activity and selectivity.

Commercialized in 1996 by BP-Amoco. Operated in four plants worldwide (stand 2004).

Advantages: Higher catalyst stability and activity (allows lower water content), higher solubility.

Mechanistically more complicated than rhodium, as neutral intermediates are involved, too.

33 A. Haynes, P. M. Maitlis, et al., J. Am. Chem. Soc. I CO – Ir 2004, 126, 2847. I CO 1

The oxidative addition of CH3I to iridium is CH3COI CH3I ca. 100 times faster than to rhodium (why?) → H3C O – is not rate-determining! C HI CH3 – I CO I CO Resting state of the catalyst: Ir Ir H2O I CO I CO fac,cis-[IrI3(CH3)(CO)2]– (2) I I (with water content ≥ ca. 5% w/w. Observed by in 6 CH COOH CH OH 2 + 3 3 situ IR spect.) CO mer,trans – I R.d.s.: Substitution of an iodo ligand by CO isomer CH 3 (2 → 4) I CO Ir The promoter accelerates this step by I CO I– abstracting an iodo ligand from 2. (why is this step slow?) CO 4 Why is step 2 → 4 needed? Migratory CO insertion is ca. 700 times faster for 4 than for 2 because of decreased π–back-donation to the CO ligands in 4 as compared to 2. (Why?)

34 Role of the Promoter

Promoters are (a) carbonyl complexes of W, Re, Ru, and Os, or (b) simple iodides of Zn, Cd, Hg, Ga, and In:

[RuI2(CO)3]2, [RuI2(CO)2]n, InI3, GaI3, and ZnI2.

None of the promoters show any detectable activity in the absence of the iridium catalyst. Typical promoter : iridium ratios are between 1 and 10. Soluble iodide salts (LiI and (Bu4N)I) are catalyst poisons.

35 Explanation The stoichiometric carbonylation of 2 to 6 is a stepwise – H C O – CH3 3 C reaction that involves dissociation of one ligand. I CO CO I CO Therefore, it is accelerated by protic solvents (CH3OH), Ir I CO Ir I CO Ru which solvate the iodide ion. I CO I CO I I 2 6 The neutral complexes [RuI2(CO)3]2, [RuI2(CO)4], and [RuI2(CO)2]n (Ru/Ir molar ratio < 0.2) give rate H3C O C CO – CH3 enhancements of 15 – 20 times (at 93°C in PhCl). I CO I CO I CO Anionic ruthenium iodo complexes, such as Ru Ir Ir I CO – 2– I CO I I CO [RuI3(CO)3] and [RuI4(CO)2] , do not promote the 5 3 carbonylation of 2.

CH3 I CO CO The ability to accept an iodo ligand is a key property of Ir I CO the promoter! CO 4

A. Haynes, P. M. Maitlis, et al., J. Am. Chem. Soc. 2004, 126, 2847.