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Reductive Elimination • Migratory Insertion •  - Hydrogen Elimination 1

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Transition metal complexes in organic synthesis Organometallic catalysts in industrial synthesis : Three Nobel Prizes 2000, 2005 and 2010 Hydrogenation RHC CH2 + H2 RCH2CH3 Methanol to acetic acid process CH3OH + CO CH3COOH Olefin polymerization and oligomerization * * * *n *n *n Isotactic polypropylene Syndiotactic polypropylene Atactic polypropylene C4-C8 40% n C10- C18 40 % C20 & > 20 % Organometallic complexes Organometaliic complexes allow impossible reactions to occur. Have direct metal to carbon bond. The bond may be metal-carbon sigma bond to give metal alkyls such as R4Ti or by the interaction of metals with pi-system of unsaturated organic systems. Metals whose energetically accessible d-orbitals allow efficient overlap with pi-orbitals of the carbon-carbon and carbon-heteroatom multiple bonds to give pi-complexes. + The moiety containing pi-bond may be neutral (R-CH=CH2), cation (R-CH=CH2 ) or anion (R-CH=CH2-). Organometallic Chemistry M-C bond can be a type or type bond Organo transition metal compounds can be divided into two broad classes: 1. Sigma bonded organometallics: In which a single carbon atom of the ligand is attached directly to the metal through a sigma bond (two electron two centre bond). Ligands of this type increases electron density on the central metal atom. Examples are: R-Cu, R-TiCl3, R2TiCl2 etc. 2. Pi-bonded organometallics In which a metal atom (ion or complex species) is bonded to ligands such as CO, RNC, C=C and other unsaturated molecules. These ligands in addition to the presence of lone pair of electrons have in common the presence of low lying vacant orbitals of correct symmetry to form pi bonds by accepting electrons from transition metal d-orbitals (back bonding). This decreases the electron density on metal atom. These can be further subdivided as; a) in which the ligand is bonded through entire unsaturated hydrocarbon molecule and b) in which only one carbon of the ligand is within the bonding distance of the metal, examples CO, carbene, benzyne etc. M-C bond can be a type or type bond. The metal therefore acts as both Lewis acid (electron acceptor) and a Lewis base (electron donor). Back donation into pi-antibonding orbitals leads to weakening of carbon-carbon double bond. C C M M * C C donation from back donation to * 18 electron rule The rule states that thermodynamically stable transition metal organometallic compounds are formed when the sum of the metal d electrons and the electrons conventionally considered as being supplied by the surrounding ligands equals 18. When this happens the metal acquires the electronic configuaration of next higher noble gas. In general, the conditions favouring adherence to the 18 electron rule are, an electron rich metal (one that is in a low oxidation state) and ligands that are good -acceptors. The requirement for 18 electrons comes from the need to fill one s, three p and five d orbitals There are exceptions to 18 electron rule including complexes of Ni, Pt, Pd, Zr and Ti which all form stable 16- electron complexes. The 16 electron complexes are very important in catalytic processes, which have one high energy vacant d- orbital, which act as a site for other ligands in chemical reactions. Examples: 1 1 2 1 3 4 5 6 8 2 5 -R, -Ar -C2R4 -allyl, -allyl, - Cb, -Cp, -C6H6 -C8H8 -C60, -R5C60 The symbol indicates bridging normally we have 2 and rarely 3 bridging Examples: 2-CO, 3-CO, 2-CH3, 2-H, 2-Cl, , 3-Cl, 2-OR, 2-PR2, 2-NR2 Hapto ligands and Sandwich compounds The hapto symbol, (eta) with a numerical superscript, provides a topological description by indicating the number of carbon atoms at a bonding distance to the metal. For example, if all the five carbon atoms of a cyclopentadienyl moiety are equidistant from a metal atom, we term it as 5-cyclopentadienyl. Sandwich 6 5 ( -C6H6)2Cr ( -C5H5)2Fe Bent Sandwich Half Sandwich Triple decker & polycyclic Unique reactions in organometallic chemistry • Ligand exchange • Oxidative Addition • Reductive Elimination • Migratory Insertion • - Hydrogen Elimination 1. Ligand exchange: A transition metal complex can lose a ligand by dissociation and combine with another ligand by association process. For example, the rhodium complex can react with an alkene as follows: Kr 4d8 5s1 It takes place in two steps; in the first step, one of the ligands dissociates. This leads to a complex in which rhodium has only 16 valence electrons and is therefore coordinatively unsaturated. In the second step, rhodium associates with other ligand to become coordinatively saturated again. 2. Oxidative addition When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction L n M + X Y L n (X ) (Y )M d n d n -2 H OX state of metal increases by 2 units H2 oxidative Ph3P PPh3 addition H PPh3 Coordination number increases by 2 units Rh Rh Ph P Cl 2 new anionic ligands are added to the 3 Ph3P Cl metal PPh3 Requirements for oxidative addition Rh+1 Rh+3 • availability of nonbonded electron density on the metal, • two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated, • a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. • ligands can be H-H, H-X, R-X, RCO-H, R-CO-X etc. Examples of Oxidative addition : Cis or trans ? Cl Cl PPh3 Ir 18E Cl2 Ph3P CO Cl O Cl PPh3 O2 O PPh3 Ir Ir Ph3P CO Ph3P CO 16E Cl Me Me MeI Cl PPh 3 I PPh3 Ir Ir Ph P CO 3 Ph3P CO I Cl Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans 3. Reductive elimination Almost the exact reverse of Oxidative Addition CH3 Ph2 Ph2 P CH P CH3 reductive elimination 3 Pt Pt + H3C CH3 165 °C, days P CH P CH3 3 Ph2 Ph2 CH3 Pt2+ Pt4+ Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable bond and leave the metal Factors which facilitate reductive elimination • a high formal positive charge on the metal, • the presence of bulky groups on the metal, and • an electronically stable organic product. Cis orientation of the groups taking part in reductive elimination is a MUST Final step in many catalytic cycles Hydroformylation ( conversion of an alkene to an aldehyde) Sonogashira Coupling (coupling of a terminal alkyne to an aryl group Cativa Process (Methanol to Acetic acid) 4. Migratory Insertion X L + L M Y [M-Y-X] M Y X d n d n No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another CH 3 Ph3P O OC OC CO C CH Mn + PPh3 Mn 3 OC CO OC CO OC OC These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates Types of Migratory Insertion X X M A B M A 1, 1 - migratory insertion B X X A B 1, 2 - migratory insertion M M A B CO O 1, 1-migratory CH2CH2R CCH2CH2R Ph3P insertion Ph3P Rh Rh OC PPh3 OC PPh3 H R 1, 2-migratory CH CH R Ph3P 2 2 Ph3P insertion Rh Rh OC PPh Ph3P 3 CO 5. -Hydride elimination Beta-hydride elimination is a reaction in which an alkyl group having a hydrogen, bonded to a metal centre is converted into the corresponding metal-bonded hydride and a bonded alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. No change in the formal oxidation state of the metal mechanism H H H H H H H H C H C C M M C M C C H H H H H H Can either be a vital step in a reaction or an unwanted side reaction -hydrogen elimination does not happen when • the alkyl has no -hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2) • (ii) the -hydrogen on the alkyl is unable to approach the metal (as in C≡CH) • the M–C–C–H unit cannot become coplanar Select the most unstable platinum complex from the given list. Justify your answer H Ph P C Ph P Ph P 3 SiMe Et3P C 3 3 Pt 3 Pt Pt Pt SiMe3 Ph3P Et P C Ph3P Ph3P 3 C H A B C D No -H -H unable to MCCH unit will not be approach M coplanar.
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