OA & RE.pptx

This includes the material related to oxidative addition and reduction elimination.

1. OXIDATIVE ADDITION & REDUCTIVE ELIMINATION (Organometallic Compounds) M. Umer Farooq 208-PHD-2022

2. Oxidative Addition: Addition of a pair of ligands followed by the oxidation of central metal is known as oxidative addition e.g. Addition of A-B type molecule such as H 2 or CH 3 -I etc In oxidative addition A-B molecule breaks and form a M-A and M-B bonds A and B are X type ligands (L, X & Z) During this reaction, oxidation state, electron count, and coordination number all are increased by 2 Due to this increase in OS of metal, the reaction is given the name oxidative addition EC = +2 CN = +2 OS = +2

3. A vacancy for 2e - s must be present for reaction to occur, means, complex should be 16e In other way, metal should be coordinatively unsaturated, means CN should be less than 6 If the complex is of 18e, or CN is 6, then the dissociation of a ligand must take place 1 st A metal complex of a given OS must also be stable on the OS if it is 2 units higher i.e. Here metal in both oxidation states should be stable Central metal should be electron rich, i.e. d 8 and d 10 give fast reactions Metals with d do not participate in oxidative addition General requirements for Oxidative addition reactions:

4. Types of Oxidative addition mechanisms Concerted Addition S N 2 Reaction Radical mechanism Ionic mechanism

5. Concerted addition Concerted means three center A transition state is formed in this mechanism Non-polar or slightly polar ligands (H-H, C-H alkanes, Si-H Silanes etc ) react via transition state 1 st step (slow) is σ complex formation and 2 nd step (fast) is oxidation, in which metal transfers 2 e - s to σ * orbital 2 nd order reaction

6. Addition of H 2 in sq. planer d 8 metal with 16e i.e. Vaska’s Complex Ligand (H-H) 1 st bind with the metal through σ bond and form σ complex In the 2 nd step, H-H σ bond breaks As a result, strong back donation of a pair of e - s from metal to σ * of H 2 Here L = PPh 3 (Phosphine) Consider the example:

7. Trans ligands fold back to give cis dihydrides In the above reaction, trans ligands set ( Cl,CO ) foldback to become cis Phosphines are rarely seen to be foldback Being a strong pi acceptor ligand, CO wants to acquire equatorial plan of TBP transition complex Tendency of a pair of trans pi acceptor ligands to foldback can be so strong that the starting material distorts even in the absence of incoming ligands (as shown in diagram) Points to be noted:

8. S N 2 reaction: A pair of e - s from metal is used to break A –B bond Polar substrates (halides of the following) undergo this mechanism i.e. (Methyl R, Allyl CH 2 =CH – CH 2 – , Acyl RCO – , Benzyl C 6 H 5 – CH 2 – ) Polar solvent is used to accelerate the reaction Just as Concerted addition, it is also a 2 nd order reaction e - pair of metal attacks the σ * orbital of A –B by an inline attack at the least electronegative atom, to give LnM 2+ , A - and B - ions

9. In SN2 mechanism, R and X may end up cis or trans to one another in the final product, whereas in Concerted mechanism, both incoming ligands are at cis position

10. Of the two steps, the first involves oxidation by two units but no change in the electron count The second step involves an increase by 2e in the electron count (I − is a 2e reagent) but no change in the oxidation state.

11. Two steps together constitute the full oxidative addition When an 18e complex is involved, the first step can therefore proceed without the necessity of losing a ligand first Only the second step requires a vacant 2e site Reactivity of the substrate is directly proportional to the nucleophilicity of the metal (d 6 , d 8 , M ⁰) If the ligands attached with metal are electron donating, then the reaction is favorable, otherwise less favorable Steric hindrance of the carbon of incoming ligand slows down the reaction. So, the reactivity order may be Methyl halide> primary halide > secondary halide > tertiary halide Better leaving group X with carbon accelerates the reaction. So, the order may be R–I > R–Br > R–Cl Points to be noted:

12. Radical mechanism: Two types of radical mechanism are being observed non-chain chain mechanism Normally photoinitiated Reaction is carried out in solvent

13. Non-Chain mechanism: The non‐chain operates in the addition of certain alkyl halides, R‐X, to Pt(PPh 3 ) 3 (R = Me, Et; X = I OR R = PhCH 2 ; X = Br) One electron from metal is transfer to σ* of RX As a result, M • + and RX • − are formed From RX • − , X − transfers to M • + , and R • radical is liberated. This produces the pair of radicals which rapidly recombine to give the product before either of the radical can escape from the solvent cage Radical mechanism is fast for stable R radical t° > s° > p° > Me The order of the rate of reaction depending up on the halogen is as follows RI > RBr > RCl

14. Radical Chain mechanism: Radical chain mechanism observed for EtBr and PhCH 2 Br Vaska’s complex as substrate and PMe 3 as Ligands Reactions mentioned in non-chain mechanism may lead to chain mechanism, if any of the radicals formed, escapes from the solvent cage without recombination Otherwise to start the chain mechanism, radical initiator R • is required In both non-chain and chain mechanism, metal radical extracts X - from RX, to form R • Radical chain mechanism may slow or stop in the presence of radical inhibitor e.g. hindered phenol 2,6-di- t -butylphenol Such inhibitors quench R radical to form stable compound RH As a result, unactive aryloxy radical ( ArO • ) is form

15. Ionic Mechanism: This mechanism is common for the addition of hydrogen halide in its dissociated form H + and X - Hydrogen halide largely dissociate in solution form Proton and halide ion tend to add to the metal complex in two steps Based on above facts, Ionic mechanism usually follow two different pathways of the HX addition; Protonation prior to halogenation Halogenation prior to protonation In the 1 st one, proton addition takes place prior to the addition of halide In the 2 nd , halide is added prior to the addition of proton Presence of polar solvent is the common favored condition for both pathways

16. 1 st pathway-Protonation prior to halogenation: To promote this pathway, complex should be basic enough to protonate 1 st i.e. Ligands attached to the metal should be basic (having σ donation capacity) Central metal should have low oxidation state (electron rich) The reaction is followed by the leaving of two ligands and the change of geometry from tetrahedral to square planar

17. Isolation of reactive cation as stable salt: “Non-coordinating anion is used to isolate reactive cations as stable salts” Rate of the 1 st step suggests that protonation is a slow step Only the protonation step can also be carried out by using an acid with non-coordinating anion HBF 4 and HPF 6 as examples of acids with non-coordination anions The anion has insufficient nucleophilicity to carry out 2 nd step As a result the intermediate can be isolated This intermediate is basically the reactive cation but does not have any specie to react

18. In this pathway anion attacks 1 st followed by the protonation of intermediate Positive charge on the complex is favored for this reaction Ligands attached with central metal must be π acceptor (having electron accepting capacity) The geometry of the complex changes from square planar to trigonal bipyramid (TBP) on halogenation and finally changes from TBP to octahedral 2 nd pathway-Protonation prior to halogenation:

19. In this pathway, halogenation is a slow step, following the above rate equation Halogenation step can be carried out independently by using only LiCl For above mentioned Iridium complex, no reaction is observed if HBF 4 is used alone. There two reasons for this; Iridium complex is not basic enough to protonate BF 4 - is a non-coordination anion Other acids which are ionized to some extent in solution, such as RCO 2 H and HgCl 2 may well react by the same mechanism, but this has not yet been studied in detail Points to be noted:

20. Reductive Elimination: Reverse of Oxidative Addition is Reductive Elimination Two cis ligands bonded with metal are removed Oxidation state (OS), electron count (EC) and coordination number (CN) are reduced by two units Higher oxidation state metal complexes are favorable Reaction is suitable for; d 8 metals e.g., Ni(II), Pd(II), Au(III) d 6 metals e.g., Pt(IV), Ir (III), Rh(III) This reaction can be stimulated by oxidation or photolysis The best-known example of elimination through photolysis is elimination of H 2 from dihydrides

21. General Mechanism of Reductive Elimination:

22. Common Examples: Reactions that involve H, are fast, may be due to the fact that transition energy is lowered by the formation of stable sigma-bond complex L n M (H-X) Such complexes are stable only if at least one H has to be eliminated All above reactions are analogous to the concerted oxidative addition Intermediate is considered to be non-polar, non-radical, three centered transition state

23. References: THE ORGANOMETALLIC CHEMISTRY OF THE TRANSITION METALS Fourth Edition ROBERT H. CRABTREE Yale University, New Haven, Connecticut