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Inorganic Chemistry Year 3 Transition Metal Catalysis Eighteen Electron Rule Get the number of the group that the metal is in (this will be the number of d electrons) 2. Add to this the charge 3. 1. Negative charge More electrons Add to the count 2. Positive charge Less electrons Remove from the count Add the number of donating ligands to the count A stable complex has a total count of 18 Revision 1. Eighteen Electron Rule - Test 16 HCo(CO)4 18 [RhI2(CO)2]- 16 Ni(η3-C3H5)2 16 Fe(CO)4(PPh3) 18 Revision Rh(PPh3)3Cl 16 Electron Square Planar Complexes Fe Ru Os Have a strong preference for square planar 16 electron arrangements Why? - Ligands provide 8 electrons, and the metal provides 8 electrons - dx2-y2 raised in energy above dz2 - Back – bonding becomes less efficient Revision Complexes with d8 configuration Oxidation State Start at zero 2. Ignore all formally neutral ligands (ie CO, PPh3) 3. Add 1 for anionic ligand 4. Positive change, electron removed, add 1 5. Negative charge, electron added, subtract 1 6. Give your result in roman numerals Revision 1. Oxidation State - Test (I) HCo(CO)4 (I) [RhI2(CO)2]- (I) Ni(η3-C3H5)2 (II) Fe(CO)4(PPh3) (0) Revision Rh(PPh3)3Cl LnL’M - L’ LnM(S) Ln M A + L’ Product B Reaction Steps Catalytic Cycles LnM(S) Substrate LnL’M - L’ Ln M A + L’ Product B Reaction Steps Catalytic Cycles Reasonable 18 electron counts and coordination numbers Types of reactions Ligand association Ligand dissociation Migration Elimination Oxidative Addition Reductive Elimination Metallacycle formation Transmetallation Reaction Steps Constructing Catalytic Cycles 18 electron complexes electronically saturated Most are coordination saturated Therefore exchange of ligands require initial dissociation The rate of this reaction can be slow Reaction Steps Ligand Association / Dissociation Complementary process Requires two accessible oxidation states of 2 units apart Change of co-ordination number is also 2 Reaction Steps Oxidative Addition / Reductive Elimination Types of A – B Favoured by: Low oxidation number H-H Electron donating ligands X-X Anionic complexes C-C Vacant sites H-X C-X Square planar complexes are ideal for oxidative addition M-X Reaction Steps Oxidative Addition Low valent systems can attack the C-H bonds of an already bound ligand Reaction Steps Oxidative Addition Ortho Metallation The reverse of oxidative addition Takes place in a cis sense Favoured by the opposite factors: High oxidation state Crowded co-ordination sphere Electron withdrawing ligands Reaction Steps Reductive Elimination Insertion Reaction Ligand Y migrates which enlarges ligand X Can often be seen as a nucleophilic attack of Y on X Migrating Ligand Y can be alkyl or halide X Ligands can be a wide range, CO, CNR, CR2, C=C, O2, SO2 Reaction Steps Migration Alkyl / Hydride Reaction Steps Hydride Migration Regiochemistry at Alkene β-elimination Reverse of hydride migration to an alkene Coordination number increases during intermediate Needs a vacant site M-C-C-H group must be syn coplanar Must have a beta-hydrogen for beta-elimination to take place Reaction Steps β - elimination α-elimination Alpha-elimination is the reverse of hydride migration to an alkylidene (Carbene) Usually not seen if beta-elimination is possible due to the less favourable transition state Reaction Steps α - elimination What reaction steps are required for the conversion… Mn(CO)5Me + CO Mn(CO)5(COMe) Reaction Steps Reaction Steps - Test Draw a balanced scheme showing beta-hydride elimation from [Cp2Ti(n-Pr)]+ Reaction Steps Reaction Steps - Test Reaction Steps - Test Reaction Steps Complex of the type Pt(PR3)4 can form PtCl2 (PR3)2 when reacted with HCl: suggest an explanation for this result, including a reaction scheme naming the elementary reactions involved and showing the byproducts. Catalysed by: Wilkinson’s Catalysis RhCl(PPh3)3 Addition to Alkene Hydrogenation Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes (a) Draw a catalytic cycle for the conversion of ethene and hydrogen to ethane with this system Addition to Alkene Hydrogenation - Test Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes (a) Draw a catalytic cycle for the conversion of ethene and hydrogen to ethane with this system Addition to Alkene Hydrogenation - Test Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes (b) Give electrons counts for each of the intermediates. 1 NA 2 12 3 14 4 16 5 14 Addition to Alkene Hydrogenation - Test Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes (c) State the oxidation state of the metal centre in each of the intermediates 1 NA NA 2 12 (I) 3 14 (III) 4 16 (III) 5 14 (III) Addition to Alkene Hydrogenation - Test Treatment of the ionic rhodium complex [Rh(COD)2][BF4] (COD = cyclo-octa-1,4-diene) with bis(diphenylphosphino)ethane (‘dppe’) in n-butanol leads to the formation of a highlyeffective system for the hydrogenation of alkenes (d) Name each of the reaction steps in the catalytic cycle. A Dis/Associaton Ligand Exchange B Oxidative Addition C Alkene Binding D Hydride Migration E Reductive Elimination Addition to Alkene Hydrogenation - Test The phosphine ligand is one of the best ligands for hydrogenation Good two electron donor with steric tuning of coordination sphere and metal accessibility by changing the phosphine substituents (pi-acidity and tolman cone angle) Fast turnover needs Bulky group favours dissociation Good donors favour oxidative addition Addition to Alkene Phosphine Ligands The hydrogenation of an unsymmetrical alkene like this will result in stereoisomers due to a chiral compound If you need a specific isomer you need to use a chiral catalyst, and to do this you need chiral ligands. Chiral phosphines are the best due to the above reasons BINAP-(R) Addition to Alkene Chiral Hydrogenation You do not need to use pure hydrogen and an alkene. A secondary alcohol and an imine works just as good. Remember nitrogen has lone pair to coordionate Arene Hydrogenation Has cis stereospecificity. Only one face of an aromatic is hydrogenated Addition to Alkene Transfer Hydrogenation Catalysed by: A range of well-defined complex: Fe, Mn, Co, Ru, Rh, Ir, Pd and Pt examples. Related to hydrogenation but with SiR3 as a ‘super-proton’ First developed by H2PtCl6 Strong tendency to form linear products Addition to Alkene Hydrosilation Catalysed by: NiL4 where L = P[O(o-tolyl)]3 Addition of H-C≡N to double bonds catalysed by Ni, Cu and Pd complexes Strong donor phosphines do not give turnover Reaction tends to isomerise internal alkenes to give terminal products Most important hydrocyanation is hydrocyanation of butadiene to give adiponitrile Addition to Alkene Hydrocyanation oxo-reaction Catalysed by: HCo(CO)4 / HRh(PPh3)2(CO) Cobalt: Harsh Conditions. Moderate selectivity, competitive hydrogenation Rhodium: Mild conditions. Excess phosphine needed for high selectivity Addition of a hydrogen atom and a formyl (CHO) group to an alkene Addition to Alkene Hydroformylation Suggest a plausible mechanism for the reaction and catalysed by Wilkinsons catalyst Addition to Alkene Addition to Alkene - Test Suggest a plausible mechanism for the reaction and catalysed by Wilkinsons catalyst Addition to Alkene Addition to Alkene - Test Hydroformylation Carbonylation Adding CO + H2 to a substrate Adding just a CO to a substrate The most important carbonylation reaction is the conversion of methanol to acetic acid R-OH + CO R-COOH Acetate Derivatives Acetic Acid Acetaldehyde Methyl Acetate Vinyl Acetate H3CCOOH H3CCHO H3CCOOMe H3CCOOCH=H2 Acetic acid is manufactured on large scale! Carbonylation Carbonylation Cobalt-based Reppe/BASF Rhodium-based Monsanto [RhI2CO2]Iridium-based BP Cativa • Better than rhodium at forming alkyl bond • Rate-limiting step is methyl migration • Kinetics more complicated than Monsanto process • Reaction ‘promoted’ by species such as [Ru(CO)3I2]2 • Mechanism likely to involve neutral and anionic species and competing processes Carbonylation Carbonylation Catalysts Carbonylation The Monsanto Process Rhodium-based Polyketone formation Carbonylation Catalyst: [PdL2(OR)]+ Grignard reagents couple with aryl halides Catalyst: Ni(acac)2 / (dppe)NiCl2 / FeCl3 / CoCl2 / CrCl2 Coupling Reactions Coupling Grignard Reagents Negishi coupling Catalyst: (Ph3P)4Ni / (Ph3P)2PdCl2 | Coupling an aryl halide with an organozinc (RZnX) Stille coupling Catalyst: Pd(dba)3 | Coupling an aryl halide with an organotin reagent (RSnR’3) Make sure you have no beta-hydrogens Alkyl < benzyl ~ allyl < aryl < alkenyl < alkynyl (reactivity from transfer from tin) Me3Sn best choice for control but tBu3Sn much safer Bulky phosphine ligands and highly basic additives (CsF) facilitates cross coupling of aryl chlorides Coupling Reactions Organometallic Coupling Sonogashira coupling Catalyst: (MeCN)2PdCl2 Copper alkynyls (Cu-C≡C-R) couple to organic electrophiles under harsh conditions Palladium salts allow same reaction under mild conditions A base is required to remove the HX liberated Coupling Reactions Organometallic Coupling Catalyst: Can be Pd(0) or Pd(II) pre-catalyst Coupling Reactions Organometallic Coupling Mechanism Catalyst: Can be Pd(0) or Pd(II) pre-catalyst | Note number of ligands can change PdLn Coupling Reactions Mizoroki-Heck reaction Mechanism A palladium-catalysed coupling may be carried out using either a Suzuki or a Stille protocol. What are the advantages of each approach? Stille • Organotin complexes • Reaction is facile (low temp, fast), but starting materials are still stable in air so practically east Suzuki • Organoboron complexes • Low toxicity of starting material and byproducts • Some boronic acids commercially availiable Coupling Reactions Coupling Reactions - Test Carbenes Carbene is defined as a neutral divalent carbon. Free carbenes can be formed. Looking at metal-carbine complexes Metal in low oxidation state Heteroatom substituents on carbene cation Carbenes Fischer Carbenes ` Schrock Carbenes Metal in high oxidation state Substituents on carbene carbon usually carbon or hydrogen Carbene may be called an alkylidene Nucleophilic Carbenes Ideally carbine ligands introduced like phosphines using free carbine Wanzlick proposed stabilisation using heteroatom substiuents ` ` ` Metal-Carbene Bonding • As with phosphines, this allows bonding from carbene to metal (lone pair in sp2 orbital) and back-bonding (to empty p orbital) • Back bonding is significant in fisher and schrock carbenes but not with nucleophilic carbenes • For electron counting all carbenes donate two electrons • For oxidation state: • Fisher / Shrock carbenes (alylidenes) are dianionic (-2) • Nucleophillic carbenes are neutral (phosphines) Carbenes • Free carbenes may be either in triplet (sp) or singlet (sp2) state • Bonding to metals is best described by invoking singlet carbenes Carbenes / Coupling - Test [Ar = 2,6-(i-Pr)2C6H3]? N-Heterocyclic carbene ligand will bind tightly to metal preventing formation of Pd metal, pyridine ligand is labile so rapidly forms free site Carbenes Why might the complex below be a better catalyst for both of these coupling routes than the more ‘traditional’ palladium-phosphine systems Metathesis Alkene Methesis – Very small free energy differences Schrock’s Catalysts Highly active React with hindered alkenes Control reactivity by altering alkoxide groups Poor functional group tolerance Grubb’s Catalysts 3 different types Undergo the following mechanism Metathesis Equilibria must be driven (for example loss of gas) Metathesis Metathesis Mechanism The Chauvin Mechanism Metathesis in synthesis Metathesis Metathesis / Carbene - Test Metathesis Treating EtOCH=CH2 with RuCl2(PCy3)2(CHPh) (Grubbs’ I) does not lead to catalytic turnover but gives a stable metal complex and one equivalent of a styrene (PhCH=CH2) as a byproduct. Predict the structure of this product and explain why the reaction is stoichiometric. Ziegler Catalyst TiCl4 Activated by AlEt3 Mild Conditions Controlled process: Yields HIGH DENSITY linear polymer TACTICITY ISOTATIC SYNDIOTATIC ATATIC Alkene Polymerisation Alkene Polymerisation Alkene Polymerisatioin Cossee-Arlman Mechanism Alkene Polymerisatioin Metallocene Mechanism Hydrolysis of AlMe3 gives MAO [-Al(Me)-O-]n Alkene Polymerisation MAO Methylaluminoxane Triphenylcarbenum (trityl) salts [Ph3C]+ powerful alkyl abstractor Alkene Polymerisation Co-Catalysts and Activators Alkene Polymerisation Ligand Symmetry and Tacticity Essential Features: • Electron Deficiency • Vacant coordination site adjacent to the growing polymer chain • Steric protection at the metal centre to prevent beta-hydride elimination Cations and Anions Alkene Polymerisation Alkene Polymerisation Catalysts Show how MAO activates metal halide complexes containing bidentate ligands to act as polymerisation catalysts: use the general formula (L-L)MX2 to represent the pre-catalyst What other role does MAO play in the reaction? What is the mechanism for polymerisation of alkenes by the activated catalyst? Alkene Polymerisation Alkene Polymerisation - Test Alkene Polymerisation PAST EXAM PAPER MODEL ANSWERS Hydroformylation Alkene Metathesis Alkene Polymerisation Alkene Polymerisation Transfer Hydrogenation Alkene Polymerisation Alkene Polymerisation