CHAPTER 1 CHAPTER 1 HOMOGENEOUS CATALYSIS WITH TRANSITION METAL CATALYSTS OBJECTIVES INTRODUCTION

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1 APTER 1 MGENEUS ATALYSIS WIT TRANSITIN METAL ATALYSTS APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 2 INTRDUTIN Industrial homogeneous catalysis is based on the development of organometallic catalysts. Transition metals has been driven by their potential applications as industrial catalysts. The chemistry of transition metal catalysis is explained in terms of the reactivity of organic ligands bound to the metal center. Professor Bassam El Ali 3 1

2 INTRDUTIN The d orbitals of the transition metals allow ligands such as (hydride),, and alkenes to be bounded in such a way that they are activated towards further reactions. The molecular transformations generally require a loose coordination of the reactants to the central atom and facile release of the products from the coordination sphere. Both processes must proceed with an activation energy that is as low as possible. Extremely labile metal complexes are required. Such complexes have a vacant coordination site or at least one weakly bound ligand. Professor Bassam El Ali 4 INTRDUTIN Transition metals can exist in various oxidation states and that they can exhibit a range of coordination numbers. The coordination complexes can be classified by dividing the ligands into two groups: ionic and neutral ligands. Ionic ligands include: -, l -, -, Alkyl -, Aryl -, 3 - Neutral ligands are:, alkenes, phosphines, phosphites, arsine, 2, amines, Professor Bassam El Ali 5 INTRDUTIN This distinction is useful for assigning oxidation states and in describing the course of reactions. Although, hydrogen ligand mostly react as - and alkyl groups as R -, it is also possible that, for example, methyl groups react as 3- or 3+. Professor Bassam El Ali 6 2

3 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 7 oordination and Exchange of Ligands Ligands can be released from the complex or undergo exchange, or free coordination sites can be occupied by solvent molecules. Most complexes do not react in their coordinatively saturated form, but via an intermediate of lower coordination number with which they are in equilibrium. For example, triphenylphosphine platinum complexes are involved in the following equilibrium reactions: Professor Bassam El Ali 8 oordination and Exchange of Ligands K 1 [Pt(PPh 3 ) 4 ] [Pt(PPh 3 ) 3 ] + PPh 3 K 300k 2 = 1 mol/l K [Pt(PPh 3 ) 3 ] 2 [Pt(PPh 3 ) 2 ] + PPh 3 K 300k 2 = 10-6 mol/l In aromatic solvents, the first equilibrium constant K 1 indicates rapid dissociation, but the second equilibrium constant K 2 is very small. The extremely high reactivity of [(Pt(PPh 3 ) 2 ] compensates for this concentration effect, and complete reaction occurs with π-acidic molecules such as and N: Professor Bassam El Ali 9 3

4 oordination and Exchange of Ligands [P t(p P h 3 ) 2 ] 2 2 N [P t( ) 2 (P P h 3 ) 2 ] [P t(n ) 2 (P P h 3 ) 2 ] The rapid dissociation of many complexes is explained in terms of steric hindrance of the ligands. With increasing space requirements of the phosphine or phosphite ligands, the rate of dissociation increases. A semi-quantitative measure for steric demand is the cone angle of the ligand, as introduced by Tolman. Professor Bassam El Ali 10 oordination and Exchange of Ligands The sterically most demanding ligands should exhibit the fastest dissociation. NiL 4 K N il 3 + L For the reaction the following sequence, which correlates with the cone angles was found. L = P(Et) 3 < PMe 3 < P(- i Pr) 3 < PEt 3 < PMe 2 Ph < PPh 3 K [mol] completely dissociated Professor Bassam El Ali 11 oordination and Exchange of Ligands The cone angles refer to is a constant metal-phosphorus bond length and do not reflect the true space filling in the coordinated state. Even complexes containing voluminous ligands can undergo addition of one or two small molecules: [R h l{p ( t Bu) 3 } 2 ] 2 [R h 2 l{p( t Bu) 3 } 2 ] [R h l( ){P( t Bu) 3 }] Professor Bassam El Ali 12 4

5 oordination and Exchange of Ligands Typical cone angles for trivalent phosphorus ligands Ligand one angle [º] P 3 87 P(Me) P(Et) PMe P(Ph) P(- i Pr) PEt PMe 2Ph 136 PPh P( i Pr) P(yclohexyl) P( i Bu) Professor Bassam El Ali 13 oordination and Exchange of Ligands For ligand dissociation/association processes, Tolman introduced the 16/18-electron rule. For each covalently bonded ligand, two electrons are added to the number of d electrons of the central transition metal atom (corresponding to its formal oxidation state) to give a total valence electron count. A thermal dissociative equilibrium is the well known Wilkinson's catalyst: -PPh 3 -PPh 3 Rh()(PPh 3 ) 3 Rh()(PPh 3 ) 2 Rh()(PPh 3 ) Professor Bassam El Ali 14 oordination and Exchange of Ligands The active form of the catalyst is generated by loss of PPh 3 ligands in solution. The complexation of the substrate at the transition metal center to give a so-called π complex. K M( 2 4 )(P P h 3 ) 2 M (P P h 3 ) M = Pd > Pt > Ni K Professor Bassam El Ali 15 5

6 oordination and Exchange of Ligands The tendency of ethylene complexes to dissociate can be explained in terms of the strength of the backbonding from the metal (M) to the alkene ( 2 4 ) (M = Ni > Pt > Pd; o > Ir > Rh; Fe > s > Ru). The coordination of certain ligands to a transition metal center can be facilitated by exploiting the trans effect. For example, the reaction is slow, but can be accelerated by adding Snl 2. This leads to formation of a Snl 3- group, whose strong trans effect labilizes the chloro ligand in the trans position. Professor Bassam El Ali 16 oordination and Exchange of Ligands Slow [Ptl 4 ] [Ptl 3 ( 2 4 )] - + l - l l 3 Sn l Pt l 2- Fast [Pt(Snl 3 )l 2 ( 2 4 ] - + l - Professor Bassam El Ali 17 oordination and Exchange of Ligands The trichlorotin(ii) ) ion Snl 3- can replace the ligands l -,, and PF 3 in nucleophilic ligand-exchange reactions. Ptl Snl 3 - [Ptl 2 (Snl 3 ) 2 ] Snl 3 [Pt(Snl 3 ) 5 ] 3- Ligand-substitution reactions (such as square-planar Pd II or Pt II complexes) are often used as model reactions for ranking ligands in order of their nucleophilicity. [PtX 4 ] 2- + Y - [PtX 3 Y] 2- + X - Professor Bassam El Ali 18 6

7 oordination and Exchange of Ligands The reactions, which proceed by an SN 2 mechanism, give the following series for the nucleophilicity of the incoming ligand Y: F - ~ 2 ~ - < l - < Br - ~ N 3 ~ 2 4 < py <N 2- < N 3- < I - ~ SN - ~ R 3 P The Pt II complex [PtX 4 ] 2- has a soft electrophilic center. Therefore, according to the SAB (hard and soft acids and bases) concept, fast substitution reactions should occur with soft reagents such as phosphines, iodide, and olefins. Professor Bassam El Ali 19 oordination and Exchange of Ligands Ligand-exchange processes can often be explained in terms of the higher stability of the product complex: [o(n 3 ) 5 I] [o(n 3 ) 5 ( 2 )] 3+ + I - A B Each soft or hard fragment strives for stabilization on a corresponding center (symbiotic effect). omplex A exhibits a hard/soft dissymmetry (N 3 is hard, I - soft), whereas in complex B the hard o 3+ center is stabilized exclusively by hard ligands. Professor Bassam El Ali 20 oordination and Exchange of Ligands The heterolytic addition of reagents: a substrate XY undergoes addition to the metal center without changing the formal oxidation state or coordination number of the metal center. M x L y + XY M x L y-1 + X + Y + + L - ne anionic ligand is replaced by another, as in the addition of hydrogen to ruthenium (II) complexes: [Ru II l 2 (PPh 3 ) 3 ] + 2 [Ru II l(pph 3 ) 3 ] l - Professor Bassam El Ali 21 7

8 oordination and Exchange of Ligands The activation of molecular hydrogen by Pt II, Ru III, and Pd/Sn catalyst systems can be explained analogously. 2 [Pd(Snl 3 ) 2 (PPh 3 ) 2 ] [Pd(Snl 3 )(PPh 3 ) 2 ] Snl 3- Professor Bassam El Ali 22 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 23 omplex Formation In alkene complexes, the transition metal can have oxidation state 0 or higher. The olefin ligands are bound to the transition metal through one or more double bonds, the exact number depending on the number of free sites in the electron shell of the metal atom. Generally sufficient olefins or other Lewis bases are added to give the transition metal the electron configuration of the next higher noble gas: Professor Bassam El Ali 24 8

9 omplex Formation Fe- Ni Fe- yclooctatetraene 1,5,9-yclododecatriene yclobutadiene irontricarbonyl. nickel. irontricarbonyl. Form al Fe charge: 0. Form al Ni charge: 0. Form al Fe charge: 0. Num ber of π electrons Num ber of π electrons Num ber of π electrons involved: 4 involved: 6 involved: 4 Professor Bassam El Ali 25 omplex Formation In olefin-metal bonding, a distinction is made between σ and π bonding contributions. The π bonding contribution for several metals increases as follows: Al 3+ «Ti 4+ < Pt 2+ < Ni 0 d 0 d 8 d 10 Pt II < Rh I < Fe 0 < Ni 0 Ag I < Pd II «Rh II ~ Pt II < Rh I π-bonding contribution (softness), Stability of the metal olefin complex Professor Bassam El Ali 26 omplex Formation Metal-olefin backbonding is particularly strong for soft metals that are rich in d electrons, but negligible at low d electron densities. For silver and palladium complexes, with their dominant σ contributions, the metal-olefin bond strength can be increased by donor substituents on the olefin, while in the case of the soft platinum complexes, it is increased by electron-withdrawing groups on the olefin. For a given metal ion, the σ-acceptor property becomes stronger with increasing positive charge. Rh II (d 7 ) is a stronger σ acceptor than Rh I (d 8 ). Professor Bassam El Ali 27 9

10 omplex Formation The coordination ability of olefins can also be compared. The following series, obtained for a nickel(0) complex, illustrates the importance of electronic effects in the olefin: Ethene > Propene > 1-Butene 1-exene N-=-N > 2 =-N > 2 =- 3 > 2 =-- 3 > 2 =- 6 5 > 1-exene > 2 =--( 2 ) 3-3 Professor Bassam El Ali 28 omplex Formation The strength of the nickel-olefin bond is increased by electron-withdrawing substituents such as cyano and carboxyl groups, and decreased by electron-donating groups. Donor ability increase in the series: Methyl < Ethyl < Alkoxyl With their delocalized π-electron system, allyl ligands can bond to metals in a manner similar to olefins. Professor Bassam El Ali 29 omplex Formation Allyl complexes have been detected as intermediates in catalytic processes involving propene or higher olefins and dienes (the cyclooligomerization of butadiene and the codimerization of butadiene with ethylene). The abstraction of a hydrogen atom from an alkyl group next to a double bond (1,3 hydride shift) leads to formation of hydrido metal π-allyl complexes via intermediate σ-alkyl compounds: R L n M + 2 L n M R 2 R Acceptor L n M Donor 2 σ-alkyl complex π-allyl complex Professor Bassam El Ali 30 10

11 omplex Formation This reaction occurs mainly in metal complexes of low oxidation state. Typical examples of this class of compounds are [Mn(n )() 4 ] and the dimmer [{Pd(n )l} 2 ] with the structure: 2 Pd 2 l l Pd 2 2 Professor Bassam El Ali 31 omplex Formation The equilibrium between σ- and π-allyl complexes can be influenced by the ligands. Strongly basic alkyl phosphine ligands favor the σ structure, as has been shown for allyl metal halide complexes of Pt II and Ni II. Soft π-acceptor ligands such as favor the formation of π-allyl complexes. Professor Bassam El Ali 32 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 33 11

12 Acid-Base Reactions According to the general acid-base concepts of Brønstëd and Lewis, metal cations are generally regarded as acids. Transition metal cations or coordinatively unsaturated compounds can undergo addition of neutral or anionic nucleophiles to give cationic, anionic, and π-acceptor complexes. u N 3 [u(n 3 ) 4 ] 2+ [Pdl 4 ] 2- + l - [Pdl 5 ] 3- A g( 2 ) n Ag( 2 ) n Professor Bassam El Ali 34 Acid-Base Reactions Another example of Lewis acid behavior: an iridium complex takes up a ligand to form a dicarbonyl complex. trans-[irl()(pph 3 ) 2 ] + [Irl() 2 (PPh 3 ) 2 ] Professor Bassam El Ali 35 Acid-Base Reactions 16-electron species can add a ligand to give 18-electron complexes: [(acac)rh( 2 4 ) 2 ] [(acac)rh( 2 4 ) 3 ] 16e 18e Professor Bassam El Ali 36 12

13 Acid-Base Reactions The Brønstëd theory: the acid/base character of a compound depends on its reaction partner and is therefore not an absolute. Protonation reactions of transition metal complexes, generally of low oxidation state. An example is cobalt carbonyl hydride, the true catalyst in many carbonylation reactions: [o() 4 ] [o() 4 ] Professor Bassam El Ali 37 Acid-Base Reactions Metal basicity is also exhibited by phosphine and phosphite complexes of nickel (0), which can be protonated by acids of various strengths: [Ni{P(Et) 3 } 4 ] + + [Ni{P(Et) 3 } 4 ] + The hydride formation constant K for the general reaction can be strongly influenced by the donor character of the phosphine ligand L: Professor Bassam El Ali 38 Acid-Base Reactions L = Ph 2 P PPh 2 PPh(Et) 2 P(Et) 3 K P(Me) 3 P( 2-2 I) 3 P( 2 -l 3 ) 3 With very good σ donors like diphosphines,, nickel (0) becomes a strong metal base, and the corresponding hydride is highly stable. Professor Bassam El Ali 39 13

14 Acid-Base Reactions Phosphine ligands that remove electron density from the metal center lower the complex-formation constant. Trialkylphosphine ligands, which primarily act as σ donors, increase the electron density at nickel atom and give rise to strong metal bases. For example, [Ni(PEt 3 ) 4 ] can be protonated with weak acids such as ethanol. An intermediate basicity is obtained with triarylphosphines and phosphites, and protonation of the corresponding nickel complexes requires strong mineral acids. PF 3 complexes exhibit negligible basicity because PF 3 is a strong electron acceptor, like. Professor Bassam El Ali 40 Acid-Base Reactions Some transition metal hydrides are also strong bases; [π -p 2 Re] has a basicity comparable to that of ammonia. [(π p) 2 Re] + + [π-p) 2 Re 2 ] + Many neutral carbonyl complexes can also be protonated. [s 3 () 12 ] + + [s 3 () 12 ] + [Ru() 3 (PPh 3 ) 2 ] + + [Ru() 3 (PPh 3 ) 2 ] + Professor Bassam El Ali 41 Acid-Base Reactions Shriver has described, on the basis of IR spectroscopic data, trends according to the position of the metal in the periodic table: 1. Low oxidation states, especially negative ones or metal (0) complexes, increase the metal basicity. With increasing oxidation state, metals become more acidic. 2. Transition metal basicity increases from right to left in a period, and from top to bottom in a group: [Mn() 5 ] - > [Fe() 4 ] - > [o() 4 ] - [Re() 5 ] -»[Mn() 5 ] - 3. Electron-donor ligands such as phosphines increase the metal basicity: (Fe() 4 PPh 3 ] > [Fe() 5 ] Professor Bassam El Ali 42 14

15 Acid-Base Reactions Numerous stable adducts can be regarded as the result of acid-base reactions of transition metal complexes. [(π-p) 2 W 2 ] + BF 3 [(π-p) 2 W 2 -BF 3 ] [(π-p)o() 2 ] + gl 2 [(π-p)o() 2 -gl 2 ] Professor Bassam El Ali 43 Acid-Base Reactions In the oxidative addition of hydrogen to a square-planar d 8 iridium complex, the transition metal complex acts as an electron-providing metal base, and the substance undergoing addition can be regarded as an acid: [Ir I l()(pph 3 ) 2 ] + 2 [Ir III 2 l()(pph 3 ) 2 ] Reducing agent xidizing agent (metal base) (acid) Professor Bassam El Ali 44 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 45 15

16 xidative Addition and Reductive Elimination xid. Add. L x M n + X -Y L x M n +2 XY Red. Elim. The bonds of small covalent molecules XY (-X, -X, -, -, -, etc.) add to a low oxidation state transition metal, whose oxidation state then increases by two units. The reaction is mainly observed with complexes of d 8 and d 10 transition metals (e.g., Fe 0, Ru 0, s 0, Rh I, Ir I, Ni 0, Pd 0, Pt 0, Pd II, Pt II ). Professor Bassam El Ali 46 xidative Addition and Reductive Elimination The reaction can take two possible courses: 1.The molecules being added split into two η 1 ligands, which are both formally anionically bound to the metal center (a square-planar iridium complex whose central atom gives up two electrons and is oxidized to Ir III ). trans-[ir I l()(pph 3 ) 2 ] + l [Ir III l 2 ()(PPh 3 ) 2 ] d 8 d 6 Professor Bassam El Ali 47 xidative Addition and Reductive Elimination 2. The molecules being added contain multiple bonds and are bound as η 2 ligands, without bond cleavage. The resulting complexes contain threemembered rings. l Ph 3 P PPh 3 Ir + 2 PPh 3 l Ir PPh3 F3 Pt(PPh3)4 + (F3)2= Ph 3 P Ph 3 P Pt F3 + 2 PPh3 Professor Bassam El Ali 48 16

17 xidative Addition and Reductive Elimination The addition and simultaneous activation of molecular hydrogen, an important step in homogeneous hydrogenation reactions: [Pt II l(snl 3 )(PPh 3 ) 2 ] + 2 [Pt IV l 2 (Snl 3 )(PPh 3 ) 2 ] A model reaction for the first step of hydrosilylation. [Irl()(PPh 3 ) 2 ] + R 3 Si [Irl(SiR 3 )()(PPh 3 ) 2 ] Anionic RhI complexes readily undergo addition of alkyl halides. [RhI() 2 I 2 ] I [ 3 Rh III () 2 I 3 ] - Professor Bassam El Ali 49 xidative Addition and Reductive Elimination The formation of η 3 -allyl complexes can also be regarded as an oxidative addition reaction. Proton abstraction from an olefin leads to the formally anionic allyl group. 3 2 Ni 0 PF 3 2 II Ni 2 PF 3 Professor Bassam El Ali 50 xidative Addition and Reductive Elimination xidative addition reactions on transition metal complexes; lassification of the adding compounds Bond cleavage No bond cleavage (Addends dissociate) (Addends stay associated) 2 X2 X (X = al, N, R, l4) 2S 65S RX RX RS2X R3SnX R3SiX gx2 3gX SiI4 2 S2 S2 F2=F2 (N)2=(N)2 R-?-R (F3)2 RN R2== 66 R = alkyl, aryl, F3 etc. Professor Bassam El Ali 51 X=al 17

18 xidative Addition and Reductive Elimination Nickel, palladium, and platinum d 10 complexes preferentially add polar reagents (acids, alkyl, acyl, and metal halides), whereby a ligand must dissociate to give a free coordination site. [Ni() 4 ] + l [Ni II l() 2 ] + 2 Addition of alkyl halide occurs first and followed by dissociation of a phosphine ligand. PEt 3 Rl + [Pt(PEt 3 ) 3 ] [RPtl(PEt 3 ) 3 ] R Pt l + PEt 3 PEt 3 Professor Bassam El Ali 52 xidative Addition and Reductive Elimination With Brønstëd acids the reaction can proceed via an ionic intermediate. [Pt 0 X - PPh 3 (PPh 3 ) 3 ] [Pt II (PPh 3 ) + 3 X - [Pt II X(PPh 3 ) 2 ] ther oxidative addition reactions that involve simultaneous ligand dissociation can be explained by applying the 18-electron rule. Professor Bassam El Ali 53 xidative Addition and Reductive Elimination oordinatively saturated 18-electron complexes (d electrons + electron lone pairs of the ligands) must first lose a ligand to provide a vacant coordination site for oxidative addition. [Ru 0 ()3(PPh3)2] + I2 d 8 6e 4e [Mo 0 ()4bipy] + gl2 d6 8e 4e [Ru II I2()2(PPh3)2] + [Mo II l(gl)()3bipy] + For the metals of group VIII, the trend was found for oxidative addition reactions of the type d 8 d 6, given the same ligands. Professor Bassam El Ali 54 18

19 xidative Addition and Reductive Elimination Fe 0 > o I > Ni II ^ ^ ^ Ru 0 > Rh I > Pd II xidative ^ ^ ^ addition s 0 > Ir I > Pt II xidative addition Tendency to undergo oxidative addition for the metals of groups 8-10 Professor Bassam El Ali 55 xidative Addition and Reductive Elimination The tendency to undergo oxidative addition increases from top to bottom in a group and from right to left in a period, as does the metal basicity. [lr I (PPh 2 Me) 2 ()l] > [Ir I PPh 3 )()l] > [Rh I PPh 3 ) 2 ()I] [lr I (PPh 3 ) 2 ()l] > [Pt II (PPh 3 ) 2 ()l] + Ir I > Pt» Au III For the metals of group VIII, the trend was found for oxidative addition reactions of the type d8 d6, given the same ligands. Professor Bassam El Ali 56 xidative Addition and Reductive Elimination Ligand effects are of major importance in oxidative addition reactions. Increasing donor character of a ligand increases the electron density at the metal center and favors oxidative addition. Electron-releasing (basic) ligands make the metal base stronger, while electron-withdrawing ligands weaken it. Professor Bassam El Ali 57 19

20 xidative Addition and Reductive Elimination Alkylphosphines, which are good σ donors, facilitate oxidative addition, while π-acceptor ligands make it more difficult. A low reaction rate is observed for the strongly basic, bulky ligand tri-tert-butylphosphine (steric effects). Professor Bassam El Ali 58 xidative Addition and Reductive Elimination The ligand effects for the square-planar iridium complex [Ir()(PPh 3 ) 2 X]: [Ir()(PPh 3 ) 2 X] [Ir()() 2 (PPh 3 ) 2 X] [Ir()( 2 )(PPh 3 ) 2 X] X = I > Br > l; F > Br > l X = I > Br, l Professor Bassam El Ali 59 xidative Addition and Reductive Elimination Although the fluoro ligand lowers the σ basicity, it is also a good π donor that increases the π basicity of the metal, and it is the latter effect that predominates in the oxidative addition of hydrogen. The complex [Ir()(PPh 3 ) 2 l] reacts with hydrogen at room temperature to give a dihydride complex, but the analogous rhodium complex [Rh()(PPh 3 ) 2 l] does not; only the chloro complex [Rh(PPh 3 ) 3 l] forms a hydrogen adduct. Professor Bassam El Ali 60 20

21 xidative Addition and Reductive Elimination The dinitrogen complex [Irl(PPh 3 ) 2 N 2 ] does not undergo addition of hydrogen. If σ-donor and π-acceptor ligands are approximately in balance, as in the complex [Ni() 2 (PPh 3 ) 2 ], then the compound is relatively stable and unreactive towards oxidative addition. Dissociation of ligands is also more difficult. Reductive elimination, the reverse of oxidative addition, is favored by ligands that lower the electron density at the metal center. The last step of a catalytic cycle is often an irreversible reductive elimination in which the product is released. [Rh III l( 2 5 )(PPh 3 ) 3 ] [Rh I l(pph 3 ) 3 ] Professor Bassam El Ali 61 xidative Addition and Reductive Elimination In the rhodium-catalyzed carbonylation of methanol via methyl iodide, acetyl iodide is formed by reductive elimination from an anionic Rh III acyl complex: I I Rh III I 3 - [Rh I I2(2] I Similarly, aldehydes are formed as the final products of cobaltcatalyzed hydroformylation: o III () 2 (-R)() 2 L o I () 2 L + R- Professor Bassam El Ali 62 xidative Addition and Reductive Elimination As with oxidative addition, electron-donating ligands such as trialkylphosphines increase the rate of reaction. For hydrogen addition: o 2 () 6 (PBu 3 ) 2 > o 2 () 8 When hydrogen is passed into an aqueous cobalt cyanide solution, hydridopentacyanocobalt ions are formed and can be used for the reduction of organic and inorganic substrates: 2 [o II (N) 5 ] [o III (N) 5 ] 3- Professor Bassam El Ali 63 21

22 xidative Addition and Reductive Elimination The addition of hydrogen halides to metal-metal bonds [Mo II 2 X 8 ] 4- (X = l, Br) [Mo II 2 X 8 ] 4- + X [Mo III 2 X 8 ] 3- + X - xidative coupling is a reaction in which the oxidation state of the metal increases by two units and the coordination number remains unchanged. [( 2 4 )N i 0 (PPh 3 ) 2 ] + 2 F 2 = F d 10 Ph3P F2 N i II Ph3P F 2 X F F ( 2 =-X) 2 Fe() 3 d 8 Fe()3 X = - 3 d 6 X Professor Bassam El Ali 64 xidative Addition and Reductive Elimination yclooligomerization reactions of unsaturated hydrocarbons: Ni 0 (DT)PR3 R3P-Ni 0 -DT R3P Ni II A B + First, a ligand displacement reaction with butadiene gives a nickel(0) π complex A, which undergoes oxidative coupling to give the metalcontaining ring B, a π -allyl σ-alkyl complex. Finally, reductive elimination gives the main products 1,5-cyclooctadiene and 4- vinylcyclohexene. Professor Bassam El Ali 65 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 66 22

23 Insertion and Elimination Reactions Insertion reactions: LxMn-X + YZ LxMn-(YZ)-X X =,, N,, l, metal YZ =, olefin, diene, alkyne, nitrile, etc. A molecule YZ is inserted into an M-X bond without changing the formal oxidation state of the metal M. A simple example is the insertion of an olefin into a Pt- bond to give an alkyl complex. This reaction is a nucleophilic attack of a base (hydride ion) on a positively polarized olefin. 2 = 2 PEt 3 PEt l Pt 2 3 l Pt PEt 3 Et 3 P π complex l Pt cis insertion PEt 3 formation Et 3 P Professor Bassam El Ali 67 Insertion and Elimination Reactions lefin insertion is particularly facile in the case of the complexes [Pt(Snl 3 )(PR 3 ) 2 ]. The soft π-acceptor ligand [Snl 3 ] - stabilizes the metal-hydride bond. Pt/Sn systems are known to be good hydrogenation catalysts. R [Ptl(PEt 3 ) 2 ] + R R Ptl(PEt 3 ) 2 R Professor Bassam El Ali 68 Insertion and Elimination Reactions The insertion of into metal-carbon bonds. [R-Mn() 5 ] + [R--Mn() 5 ] An alkyl group migration to a group coordinated in the cis position occurs. This migration probably occurs via a three-center transition state. R M M R R M Professor Bassam El Ali 69 23

24 Insertion and Elimination Reactions Influence of the substituents in the carbonylation of manganese complexes of the type (RMn() 5 ]: R = n- 3 7 > Et > Ph > Me» 2 Ph, F 3, l insertion in (R-Mn() 5 ], reactivity Professor Bassam El Ali 70 Insertion and Elimination Reactions The electron-releasing alkyl groups cause a stronger polarization of the metal-carbon bond, but more electronegative electron-withdrawing ligands lower the reaction rate. If the stability and reactivity of the metal complexes are compared, two trends become apparent: 4d Stability of M- bond, 5d polarizability, Reactivity in 6d softness of metal insertion reactions Professor Bassam El Ali 71 Insertion and Elimination Reactions The harder metals at the top of the groups are more reactive towards carbonyl insertion. Thus iridium carbonyl complexes are less reactive than the rhodium and cobalt homologues: Pd II > Pt II ; Mn > Re; r > Mo > W The influence of nucleophilic ligands on the insertion reaction has been investigated for molybdenum complexes. [ 3 -Mo(π-p)() 3 ] + L ( 3 --Mo(π-p)() 2 L] In the nonpolar solvent toluene, the reaction rate decreases in the sequence: L = P(nBu) 3 > P(-nBu) 3 > PPh 3 > P(Ph) 3 Professor Bassam El Ali 72 24

25 Insertion and Elimination Reactions The alkylphosphines of higher σ basicity activate the insertion reaction, as do polar solvents such as ether, which can increase the reaction rate by a factor of 10 3 to Examples of very fast insertions are the reactions of carbenes with M-, M-, and M-l bonds. [Mn() 5 ] + : 2 [ 3 Mn() 5 ] (from 2 N 2 ) [Irl()(PPh 3 ) 2 ] + : 2 [Ir( 2 l)()(pph 3 ) 2 ] (from 2 N 2 ) Professor Bassam El Ali 73 Insertion and Elimination Reactions arboxylation reactions with the hard Lewis acid 2 are of potential interest to future industrial syntheses. δ + δ δ δ + M -R + = = M R The benzyl complex of titanium is a very hard starting material, as is the tungsten dialkylamide. Ti( ) Ti [W(NMe 2 ) 6 ] [W(NMe 2 ) 3 ( 2 NMe 2 ) 3 ] Professor Bassam El Ali 74 Insertion and Elimination Reactions Elimination reactions: the elimination of from acyl complexes and of 2 from carboxylates can result in the formation of metal-aryl bonds. Such eliminations occur under the influence of heat and light. Me Mn() 5 Me Mn() 5 + [Ni(bipy)(Ph) 2 ] [Ni(bipy)Ph 2 ] Professor Bassam El Ali 75 25

26 Insertion and Elimination Reactions Decomposition reactions by another mechanism: β elimination. β- hydride elimination is an important mechanism for the decomposition of σ-organyl complexes. L x M X 2 R L x M X α β R L x M + R =X The SAB concept predicts that β-hydride elimination is favored when the metal center is made softer and the unsaturated product harder. For example, acetaldehyde is more readily eliminated than ethylene. Ph 3P 2 3 Pt Ph3P l Ph3P l Pt 3 Ph3P Ph 3P Pt + 3 l PPh3 Professor Bassam El Ali 76 Insertion and Elimination Reactions Alkoxy complexes of transition metals are generally less stable because of the presence of a hard (R) / soft (M) dissymmetry. The elimination of alkene from the more stable alkyl metal complexes generally requires drastic conditions. 180 o trans-[pet 3 ) 2 Ptl( 2 5 ) trans-[pet 3 ) 2 Ptl] + 2 = 2 95 o, 40 bar Metal complexes containing alkyl groups that have no hydrogen atoms in the β position, such as methyl, benzyl, and neopentyl, is more stable than other alkyl derivatives. Professor Bassam El Ali 77 Insertion and Elimination Reactions The α elimination reaction should also be mentioned here. It is mainly of importance in W and Mo complexes. Extraction of an a hydrogen atom from methyl compounds leads to intermediate alkylidene species: W 3 W 2 Professor Bassam El Ali 78 26

27 Insertion and Elimination Reactions The decomposition of methyltungsten compounds with formation of methane is believed to involve such hydrido carbene intermediates: l 4 W 3 l 4 W= 2 l 4 W= In ethyltungsten complexes, for which β elimination of alkene would be expected, the α elimination is favored. l 5 W 3 l 5 W= 3 Professor Bassam El Ali 79 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 80 Reactions at oordinated Ligands Nucleophilic attack on coordinated ligands is a widely encountered type of reaction. arbonyl complexes are readily attacked by various nucleophiles, including -, R -, NR 3, NR 2-, -, and 3-. δ+ δ+ δ - 2 () 4 Fe + - () 4 Fe () 4 Fe The carbonyl carbon atom of carbonyl complexes is an electrophilic center that according to the SAB concept can be regarded as a hard acid (similar to + ). Professor Bassam El Ali 81 27

28 Reactions at oordinated Ligands The attack of the hard base - initially gives a hydroxycarbonyl species, which, however, is unstable and loses 2, forming a carbonyl metallate anion. The effectiveness of nucleophiles with respect to the carbonyl carbon atom decreases in the following sequence: Et - > Ph - > - > Ac - > N 3- > F - > 2 > Br - ~ I - Alkoxide ions attack coordinated carbon monoxide to form alkoxy carbonyl complexes (Mn, Re, Fe, Ru, s, o, Rh, Ir, Pd, Pt, and g). [Ir() 3 (PPh 3 ) 2 ] Ir () 2 (PPh 3 ) 2 3 Professor Bassam El Ali 82 Reactions at oordinated Ligands Anionic rhodium complexes such as [Rh() 2 I 2 ] - undergo nucleophilic attack by water with formation of 2. δ + R h III + 2 R h I The resulting rhodium (I) carbonyl complex can be oxidized back to rhodium (III) by protons; the final products are 2 and 2. Rh I () Rh III () + 2 Professor Bassam El Ali 83 Reactions at oordinated Ligands When π backbonding from the metal to the olefin predominates, electrons flow from the metal to the olefin, which then exhibits carbanion behavior. (electrophilic attack readily occurs). Low metal oxidation states (0, +1) and anionic complexes favor electrophilic attack on coordinated ligands. Substituent effects also play a role: electron-withdrawing groups can inhibit electrophilic attack. Professor Bassam El Ali 84 28

29 Reactions at oordinated Ligands The reactivity of ligands towards nucleophiles increases for higher oxidation states of the metal (+2, +3) and for cationic complexes. A proton attacks a diene ligand to give an π-allyl complex. 2 + Fe 0 ( ) 3 + BF 4 F e II ( ) 3 BF 4-3 ydride ions are removed from a diene complex to give an arene complex. o(π-p) + 2 Ph 3 + o(π-p) Ph 3 Professor Bassam El Ali 85 Reactions at oordinated Ligands arbon monoxide is classified according to the SAB concept as a very soft Lewis base. Thus activation occurs by coordination of the atom to soft transition metals. The ligand can also react as a hard Lewis base via the oxygen atom. Sufficiently hard Lewis acids A can coordinate to the oxygen atom and further weaken the - bond. ard Lewis acids (A=All 3, AlR 3, Bl 3 ) preferentially attack bridging ligands: M--A A le t 3 p p F e F e Professor Bassam El Ali E t 3 A l 86 Reactions at oordinated Ligands The coordination of Lewis acids to terminal ligands. N N PPh 3 AlR 3 Mo AlR PPh 3 3 Bifunctional activation of leads to carbene-like resonance structures. L n M + AlX 3 L n M AlX 3 Professor Bassam El Ali 87 29

30 Reactions at oordinated Ligands The coordination of Lewis acids to terminal ligands. N N PPh 3 AlR 3 Mo AlR PPh 3 3 Bifunctional activation of leads to carbene-like resonance structures. L n M + AlX 3 L n M AlX 3 The presence of Lewis acids or protons can accelerate carbonyl insertion reactions. M + + A M A Professor Bassam El Ali 88 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. haracterization of omogeneous atalysts. Professor Bassam El Ali 89 atalyst oncepts in omogeneous atalysis The 16/18-Electron Rule All ligands bound covalently to the metal center contribute two electrons to the valence shell, and the metal atom provides all the d electrons, corresponding to its formal oxidation state. Examples: [Rhl(PPh 3 ) 3 ] has 8 + (4 x 2) = 16 valence electrons 8e [ 3 Mn() 5 ] has 6 + (6 x 2) = 18 valence electrons 6e Professor Bassam El Ali 90 30

31 atalyst oncepts in omogeneous atalysis The 16/18-Electron Rule The second rule can be depicted schematically for the key reactions of homogeneous catalysis. Saturated 18 e complex Dissociation Product Unsaturated 16 e complex Substrate Elimination Addition Saturated 18 e complex Saturated π complex 18 e Addition Insertion Unsaturated Professor 16 e complex Bassam El Ali 91 atalyst oncepts in omogeneous atalysis atalytic ycles ydroformylation of a terminal alkene in terms of a cyclic process. Professor Bassam El Ali 92 atalyst oncepts in omogeneous atalysis ard and Soft atalysis atalytic processes generally consist of complicated series of reactions, whereby the activation of individual steps can place different demands on the catalyst. The homogeneous catalysis of organic reactions can be has classified on the basis of the SAB concept. The first step of a reaction cycle: an acid-base reaction between the catalyst and the organic substrate, then a distinction can be made between "hard" and "soft" catalysis. Professor Bassam El Ali 93 31

32 atalyst oncepts in omogeneous atalysis atalytic ycles omogeneous catalysis ard catalysis Soft catalysis - With + or transition metal ions in - With transition metal complexes in high oxidation states, low oxidation states, e.g. Mo 6+, V 2+, Fel3, Til4, Zn 2+ e.g. o -, Rh +, Ni 0, Fe 0, u +, Ir + - Good electron exchange between metal and substrate (covalent interaction) - Acid-base catalysis: generation of - Soft substrates (olefins, dienes, electrophilic and nucleophilic centers aromatics) - Soft ligands and reagents (2,, N -, PR3, Snl3 - etc.) - Soft solvents (benzene, acetone, Me2S) Examples: Examples: Friedel-rafts reactions, oxidation arbonylation, hydrogenation, olefin Professor Bassam El Ali 94 Processes, epoxidation, ester hydrolysis oligomerization atalyst oncepts in omogeneous atalysis ard and Soft atalysis Petrochemical catalytic reactions are predominantly soft; hard catalysis with transition metal ions is less important. A second possibility for classifying homogeneous catalysis with transition metal complexes is the redox mechanism of such reactions. The transition metal formally changes its oxidation state by two units. Professor Bassam El Ali 95 atalyst oncepts in omogeneous atalysis ard atalysis with Transition Metal ompounds An example of hard catalysis is the oxidation of aldehydes with o III or Mn III salts Mn 3+, o 3+ 3 Acetaldehyde is oxidized to peracetic acid via hard acetyl radical intermediates. The peracetic acid then oxidizes acetaldehyde to acetic acid M 3+ 3 h h - +, -M 2+ 3 h Professor Bassam El Ali 96 32

33 atalyst oncepts in omogeneous atalysis ard atalysis with Transition Metal ompounds xidation catalysts often have a large proportion of ionic bonding, mostly with simple σ bonding of hard ligands ( 2, R, RN 2, -, - ) to the metal ion. The selective epoxidation of olefins with organic hydroperoxides. The key step of this process is the non-dissociative coordination of the hydroperoxide molecule by a hard-hard interaction of the type. M h h R Professor Bassam El Ali 97 atalyst oncepts in omogeneous atalysis ard atalysis with Transition Metal ompounds The metal center lowers the electron density on the peroxide oxygen atom, activating it towards nucleophilic attack of the olefin. Typical catalysts are Mo VI, W VI, and Ti IV compounds. M n+ + R [M n+ R] = + R + M n+ If the metal complex contains M= groups (e. g., oxo complexes of molybdenum or vanadium), oxygen transfer from the metal hydroperoxide complex to the alkene proceeds via a cyclic transition state. M R M R M + R Professor Bassam + El Ali 98 atalyst oncepts in omogeneous atalysis ard atalysis with Transition Metal ompounds The catalytic effectivity increases with increasing Lewis acidity of the complex: Mo 3 > W 3 ; electron-withdrawing ligands also increase the activity: (Mo 2 (acac) 2 ] > (Mo 2 (diol) 2 ]. The oxirane process for the epoxidation of propene is of industrial importance. In this process, isobutane is oxidized with air to tert-butyl hydroperoxide, preferably with hard Mo V and Mo VI salts as catalysts. The hydroperoxide then oxidizes the propene.. Professor Bassam El Ali 99 33

34 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Typical catalysts for the isomerization, hydrogenation, oligomerization, and carbonylation of olefins are characterized by a low oxidation state of the central atom, which is stabilized by σ-π interactions with soft ligands such as -,, PR 3, and X -. Metal hydrides, such as [o() 4 ] and [Rh()(PPh 3 ) 3 ], or combinations of a metal complex and a hydride source (e.g., [o 2 () 8 ]/ 2, [Ni{P(Et 3 )} 4 ]/ 2 S 4 catalyze the isomerization of 1-alkenes to 2-alkenes. In the industrial carbonylation of α-olefins, this double-bond isomerization is undesirable since the linear end products are of greater industrial importance. Professor Bassam El Ali 100 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Two mechanisms are discussed for the double-bond isomerization of olefins: The metal alkyl mechanism The metal allyl mechanism The addition of M- to the double bond can proceed by a Markovnikov (a) or anti-markovnikov route (b). nly after Markovnikov addition is the 2-olefin formed by β-elimination. Professor Bassam El Ali 101 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The isomerization depends crucially on the hydride character of the hydrogen atom. The hydride ligands of soft complexes such as [Rh()(PPh 3 ) 3 ] preferably undergo anti-markovnikov addition. δ - δ + 2 R δ + M 2 s δ - With harder compounds such as [o() 4 ], in which the hydrogen atom has more protic than hydridic character, Markovnikov addition is followed by isomerization. δ+ δ- R 2 2 R 2 3 R 3 ()xo δ+ o + ()x o() x Professor Bassam El Ali

35 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Thus the harder cobalt carbonyl compounds are more strongly isomerizing than the softer rhodium species. Soft ligands like PPh 3 also favor anti-markovnikov addition for steric reasons. Isomerization of α-olefins by the metal alkyl mechanism M 2 R Mark. M + R 2 2 Anti-Mark. 3 M 2 R R 3 M (a) (b) M 2 2 R R 2 2 M 2 2 R M Professor Bassam El Ali 103 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The active Rh I catalyst A undergoes oxidative addition of l and insertion of ethylene into the Rh- bond to give the Rh III alkyl complex B. The following ethylene insertion reaction is the rate-determining step and is favored by the medium hard Rh III center. The resulting Rh III butyl complex has a hard-soft dissymmetry, and the system is stabilized by reductive elimination of l to give the Rh I butene complex D, from which the desired product 1-butene is released in a displacement reaction with ethylene. Professor Bassam El Ali 104 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Rhl L, 2 4 L 2 Rh I ( 2 4 ) 2 + l A L 2 2 L 3 Rh III 2 5 l B L 3 Rh I ( 2 = 2 5 ) L 3 Rh III l s l D m Dimerization of ethylene to 1-butene 1 with a rhodium catalyst (m = medium hard; s = soft) Professor Bassam El Ali

36 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The homogeneously catalyzed hydrogenation of olefins and dienes: The advantage of the homogeneous reactions are the high selectivities. For example, with the weak catalysts [Rhl(PPh 3 ) 3 ], [Rul 2 (PPh 3 ) 3 ], and [Rh()(PPh 3 ) 3 ], only alkene and alkyne groups are attacked, while other, harder unsaturated groups such as,, N, and N 2 remain unchanged. Wilkinson's catalyst [RhI(PPh 3 ) 3 ] allows the hydrogenation of alkenes and alkynes to be carried out at 25 and 1 bar hydrogen pressure. The rate-determining step in catalytic hydrogenation: the olefin-hydride migration (insertion reaction) to form a metal alkyl complex. Professor Bassam El Ali 106 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds This insertion reaction can be regarded as the nucleophilic attack of a hydride ligand on an activated double bond. This explains why groups that increase the electron density on the hydrido group or lower the electron density in the olefinic double bond generally increase the reaction rate. In the hydrogenation of cyclohexene with [RhlL 3 ], the following ligand influence has been found: L = I > Br > l Rate of hydrogenation Softness of σ donors Professor Bassam El Ali 107 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Soft catalyst systems such as [o(n) 5 ] - /N - are effective in hydrogenating soft substrates like conjugated dienes. ther soft homogeneous catalysts such as [Fe() 5 ], [η5 - pm() 3 ] (M = r, Mo, W), [Ru()l(PPh 3 ) 3 ], and trans- [Pt(Snl 3 )(PPh 3 ) 2 ] also reduce conjugated dienes selectively to monoenes. The selectivity for the hydrogenation of dienes in the presence of mono-olefins depends on the stability of the π-allyl intermediates formed. Professor Bassam El Ali

37 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Pt Pt σ -Alkyl com plex (a) Pt Pt Pt π-allyl com plex (b) ydrogenation of dienes and monoolefins with Pt/Sn catalysts Professor Bassam El Ali 109 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds Reaction route (b), in which the softer π-allyl complex is formed, is preferentially followed by soft catalyst systems. This is the case when excess ligand R 3 P,, or [Snl 3 ] - is present. ydroformylation of olefins with soft rhodium catalysts gives exclusively aldehydes as oxo products, with the harder cobalt catalysts alcohols can also be obtained. The initially formed aldehydes, which can be regarded as relatively hard, are better able to form complexes with the hard cobalt center. R + o() nl m R R 2 o() nl m o() nl m + 2 R 2 + o() nl m Professor Bassam El Ali 110 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The SAB concept can also be applied to the related hydrocarboxylation reaction, in which carboxylic acids are produced from olefins,, water, and small amounts of hydrogen. With hard cobalt/tert-amine catalysts, the products are the hard carboxylic acids, whereas rhodium catalysts give mainly aldehydes. The reaction of methanol with to give acetic acid, catalyzed by carbonyls of Fe, o, and especially Rh in the presence of halides. rhodium catalysts allow the process to be carried out at low temperatures and pressures (ca. 180, 35 bar, Monsanto process) Professor Bassam El Ali

38 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds At the beginning of the reaction, iodide promoters convert the hard substrate methanol to the soft methyl iodide. 3 + I 3 I + 2 Rhodium(III) halide is used as catalyst precursor. Under the reaction conditions, it is reductively carbonylated to the active catalyst species, the anionic rhodium (I) complex [Rh() 2 I 2 ] -. [Rh I () 2I 2] - + 3I [ 3Rh III () 2I 3] - + [ 3 Rh III () 2I 3] - s s s h s s A B + 3 3I 3 + 3I -A h s h h Professor Bassam El Ali 112 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The soft RhI complex anion A readily undergoes oxidative addition of methyl iodide. Insertion of into the Rh- bond of the resulting complex B then gives the acetyl rhodium complex. wing to a hard-soft dissymmetry, rapid elimination of acetyl iodide occurs. This initial product of the reaction is immediately solvolyzed by methanol to give acetic acid. The rate-determining step is believed to be the oxidative addition of methyl iodide to the RhI complex.. Professor Bassam El Ali 113 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds The selective oxidation of ethylene to acetaldehyde with Pd II /u II chloride solutions has attained major industrial importance (Wacker process). This reaction can be regarded as an oxidative olefin substitution (oxypalladation). - -l - + 2, -l - [Pdl 4 ] [Pd( 2 4 )l 3 ] - Pd( 2 4 )( 2 )l 2 A B δ- 2 s ( 2 )l 2 Pd + 2 h δ+ h [( 2 )l 2 Pd II 22)] h m s h D Pd 0 + l Professor Bassam El Ali

39 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds After coordination of the ethylene to the tetrachloropalladate A, the strong trans effect of the ethylene ligand in complex B facilitates ligand substitution to give the aquo complex. The function of this neutral aquo complex is possibly that it exhibits less π backbonding from the metal to the olefin than the anionic complex. The soft palladium (II) center in the hydroxyalkyl complex D is coordinated by several hard ligands. Professor Bassam El Ali 115 atalyst oncepts in omogeneous atalysis Soft atalysis with Transition Metal ompounds With the hard base water, oxidative olefin substitution leads to acetaldehyde; with acetic acid, vinyl acetate is formed. The metallic palladium (0) is oxidized by atmospheric oxygen in the presence of u 2+, re-forming the starting complex. Another example of a soft catalysis is the hydrocyanation of butadiene to adiponitrile. Since both the substrate and the reagent N are very soft, soft Ni 0 complexes such as [Ni{P(Ar) 3 } 4 ] are preferred as catalysts. s Ni(0) + 2 N N ( 2 ) 4 N s Professor Bassam El Ali 116 APTER 1 BJETIVES Introduction to omogeneous atalysis. : oordination and Exchange of Ligands. omplex Formation. Acid-Base Reactions. xidative Addition and Reductive Elimination. Insertion and Elimination Reactions. Reactions at oordinated Ligands. atalyst oncepts in omogeneous atalysis: The 16/18-Electron Rule. atalytic ycles. ard and Soft atalysis. Professor Bassam El Ali