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2. Catalysis Involving CO 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 "-donation of CO to the metal. 3 is contributing the most when the # back donation from the metal to CO is weak. The carbon is more electrophilic here. 2a is the extreme structure that evidences the # back donation of the metal to the #* of CO. A possible alternative is 2b, where the electrophilicity of the carbon is shown. With strong # 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.
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