Organometallic Chemistry: Oxidative Addition and Reductive Elimination PDF

Organometallic Chemistry CHE Oxidative Addition and Reductive Elimination H H2 Cl PPh3 Cl PPh3 Ir...

Organometallic Chemistry CHE Oxidative Addition and Reductive Elimination H H2 Cl PPh3 Cl PPh3 Ir Ir Ph3P CO Ph3P H CO ©2022 Halawisi,KFU 1 Organometallic Chemistry CHE Oxidative addition Insertion of metals into σ-bonds Transition metals are capable of inserting into σ-bonds of (two group X-Y) to generate new metal-ligand bonds. In doing so, the metal is formally oxidized by two electrons. Hence the name oxidative addition. Both polar and non-polar σ-bonds are capable of undergoing this reaction. The reaction is reversible, and the reverse is termed reductive elimination. oxidative addition X dn LnM X-Y LnM dn−2 reductive elimination Y ©2022 Halawisi,KFU 2 Organometallic Chemistry CHE Oxidative addition and reductive elimination increased the following two units higher oxidation state coordination number electron count Mn+2 C.N +2 dn−2 oxidative addition X dn LnM X-Y LnM dn−2 reductive elimination Y decreased the following two units higher oxidation state coordination number electron count Mn-2 C.N - 2 dn+2 ©2022 Halawisi,KFU 3 Organometallic Chemistry CHE Example: We have already seen some examples of formal oxidative addition when discussing the the formation of Grignard reagent S = Solvent = THF or Ether S Mg0 + Ph—Br Ph—Mg—Br OX=0 σ-bonds OX=+2 Also, we have already seen some examples of formal oxidative addition when discussing the reactivity and synthesis of metals complexes occur on transition metal complexes with electron counts of 16 or fewer. Addition is possible to 18-electron complexes, but loss of a ligand (dissociation) must occur first. ©2022 Halawisi,KFU 4 Organometallic Chemistry CHE note: count the OX, C.N, and dn Example: A binuclear carbonyl complex with X2 + Br2 2 Br Mn(CO)5 (CO)5Mn−Mn(CO)5 OA may occur intramolecularly : cyclometallations ©2022 Halawisi,KFU 5 Organometallic Chemistry CHE Example: Alkylation/protonation of metals We have already seen some examples of formal oxidative addition when discussing the reactivity of metals with certain electrophiles. Fe + H 3C I Fe CO + I Fe(0) CO OC CO H 3C Fe(II) CO CO 2− + CO Fe H H Fe CO CO OC CO Fe(-II) CO Fe(0) Both alkylation and protonation reactions constitute formal oxidative additions. In these examples, however, the conjugate base (e.g. iodide) does not bind to the metal. As a result, there is no change to the total electron count. ©2022 Halawisi,KFU 6 Organometallic Chemistry CHE Vaska’s compound: One of the first detailed studies of OA was on 16-electron square planar iridium complexes. ©2022 Halawisi,KFU 7 Organometallic Chemistry CHE Mechanisms for oxidative addition There are three general pathways for oxidative addition: 1. Concerted oxidative addition 2. SN2 mechanism 3. Single electron processes (radical mechanisms) It is important to note that all three of these pathways can give rise to products that may be identical. Important features of the reaction (i.e. stereo- and regiochemistry) will allow one to distinguish which mechanism is operative. General trends among transition metal complexes can also be instructive. ©2022 Halawisi,KFU 8 Organometallic Chemistry CHE 1. Concerted oxidative additions Concerted oxidative additions, exemplified by the oxidative addition of H2 to Vaska’s complex, may be viewed as “classical oxidative additions”. In general, non-polar σ-bonds will undergo concerted oxidative addition in favor of the other two mechanisms. These reactions proceed through a three-membered ring transition state giving rise to a product containing a cis disposition of the atoms that underwent oxidative addition. As with other concerted processes, both atoms undergoing the oxidative addition retain their configuration. X X X LnM LnM LnM Y Y Y Concerted oxidative additions typically proceed with prior formation of a sigma complex between the X-Y bond and the metal. ©2022 Halawisi,KFU 9 Organometallic Chemistry CHE Orbital considerations Let us consider the orbital interactions in oxidative addition by using the dihydrogen complex as an example. H2 complexes are examples of relatively stable σ-complexes. [W(CO)3(PiPr3)2(H2)] H H H H CO CO e− density R 3P W PR 3 R 3P W PR 3 e− density OC OC CO CO (HOMO)of H2 (LUMO)dz2 (HOMO) dxz (LUMO)of H2 Dihydrogen complexes are stabilized by a combination of donation from the σ-bonding orbital of H2 to an empty metal d orbital, and by donation from a filled metal d orbital to the σ* orbital of H2. Both of these interactions weaken the σ-bond in H2. ©2022 Halawisi,KFU 10 Organometallic Chemistry CHE H2 oxidative addition When these bonding interactions with H2 become strong enough (e.g. with a very basic metal center), the H-H bond is broken resulting in formation of a dihydride complex. H H H H CO PR 3 R 3P W PR 3 PR3 for CO R 3P W PR 3 OC R 3P CO CO W(0) W(II) This reaction is precisely a concerted oxidative addition (of H2). The metal d orbitals weaken the H-H bond to the point where it no longer exists and is instead replaced by two new W-H bonds. H H H H PR 3 PR 3 In the process, tungsten is R 3P W PR 3 R 3P W PR 3 oxidized by two electrons. R 3P R 3P CO CO ©2022 Halawisi,KFU 11 Organometallic Chemistry CHE Other σ-bonds The example with the dihydrogen complex can be generalized to any σ-bond. X Y X Y X Y X Y ox. add. M M M M M is oxidized by 2 electrons, total electron count is increased by 2. In most all cases, concerted oxidative addition will begin with coordination of a σ-bond to the metal center. Note that oxidative addition of H2 is more facile o.a. of a C-H bond, which is more facile than o.a. of a C-C bond. ©2022 Halawisi,KFU steric and electronic factors. 12 Organometallic Chemistry CHE Polar Oxidative Addition Pathways Oxidative addition of aryl halides A very important example of concerted oxidative addition is the reaction of an aryl halide with a Pd(0) phosphine complex. C6H5 PPh PPh C6H5Br Ph3P Pd Br Pd(PPh3)4 Pd(PPh3)3 Pd(PPh3)2 18 e− 16 e− 14 e− Ph3P Pd(0) complexes do not react via 18-electron complexes and must dissociate phosphine prior to oxidative addition. Ar-X is concerted oxidative addition ©2022 Halawisi,KFU 13 Organometallic Chemistry CHE 2. SN2 mechanism The common SN2 mechanism observed in organic chemistry can also serve as a pathway for oxidative addition with transition metals. CH3 CH3 N CH3 N N CH3 Pt + H 3C I CH3 + Pt Pt I N CH3 N N CH3 CH3 I In this example, CH3I adds to Pt(II) to give the Pt(IV) trimethyl complex. The methyl and iodide groups occupy trans positions in the resulting octahedron, in contrast to concerted o.a. where they end up cis. Alkylation reactions of nucleophilic metal complexes proceed by the SN2 mechanism. As in organic chemistry, oxidative addition by SN2 proceeds with inversion of configuration at the electrophile and is accelerated in polar media. ©2022 Halawisi,KFU 14 Organometallic Chemistry CHE Example: The reaction of Vaska’s compound with CH3I: (M complex can behave as nucleophile attacking R-X ©2022 Halawisi,KFU 15 Organometallic Chemistry CHE 3. Single electron transfer and radical processes In contrast to the concerted and SN2 mechanisms, which are both two electron processes, oxidative addition can also occur through single electron steps. The products formed via these mechanisms may be identical to those formed by the two electron processes. Radical reactions are more common with first row transition metals, which in general have a greater tendency to undergo single electron transfer (SET). However, heavier metals can still undergo oxidative addition by radical processes. Radical traps and probes are a very useful means of determining if a reaction proceeds through a radical mechanism. Several single electron transfer pathways exist for oxidative addition including radical chain mechanisms and outer sphere electron transfer. ©2022 Halawisi,KFU 16 Organometallic Chemistry CHE Example: Metal complexes with an odd number of d electrons, including Co(II) and Rh(II), are good candidates for radical oxidative additions. + Br2 2 Br Mn(CO)5 (CO)5Mn−Mn(CO)5 17 e− Mn undergoes a one-electron change in oxidation state. ©2022 Halawisi,KFU 17 Organometallic Chemistry CHE Example It is also possible for the same complex to display different mechanisms of oxidative addition depending upon the nature of the bond being activated. PCy 2 PCy 2 PCy 2 N Co N2 + N Co Cl + N Co PCy 2 PCy 2 PCy 2 Cl PCy 2 PCy 2 O N Co N2 + N Co H Ph PCy 2 PCy 2 H O ©2022 Halawisi,KFU 18 Organometallic Chemistry CHE Summary of oxidative addition reactions Oxidative addition entails the formal insertion of a metal into a σ-bond with concomitant oxidation of the metal by two electrons. Three general pathways for this reaction exist: 1. Oxidative addition by a concerted mechanism Typically occurs with coordinatively unsaturated complexes containing < 18 electrons. Results in a cis product with retention of configuration. Usually observed with 2nd and 3rd row metals. Usually observed with H2, C-H bonds, C-C bond, and aryl halides. 2. Oxidative addition by SN2 pathways Common for nucleophilic metal centers (typically anionic) Can result in a trans product; always with inversion of configuration. Common for alkyl iodides and other strong electrophiles (acids). 3. Oxidative addition by single electron transfer pathways Common for 1st row metals. Observed for coordinatively and electronically saturated complexes. ©2022 Halawisi,KFU 19 Organometallic Chemistry CHE Reductive elimination Reductive elimination is the microscopic reverse of oxidative addition. As such, many of the factors influencing the rates of oxidative addition will also apply to reductive elimination. Oxidative addition and reductive elimination are typically reversible processes so thermodynamics will dictate whether the reaction proceeds in the forward or reverse direction. ‡ X LnM Y X X sigma complex LnM LnM Y Y ΔG red. elimination reactants ox. addition formal 2 electron reduction C.N. decreased by 2 products ΔGº reaction coordinate ©2022 Halawisi,KFU 20 Organometallic Chemistry CHE Mechanisms of reductive elimination The different mechanisms for reductive elimination are identical to those for oxidative addition. Concerted reductive eliminations, stepwise reductive eliminations involving charged intermediates, and radical processes can all be observed with different metal complexes. Several general trends for reductive elimination reactions are as follows: Reductive elimination reactions are typically faster with more electron-poor complexes. First row transition metals undergo reductive elimination faster than 2nd and 3rd row complexes. Ligand dissociation to give 3-coordinate (in the case of square planar complexes) or 5-coordinate (in the case of octahedral complexes) species usually precedes reductive elimination. Steric hinderance about M generally promotes reductive elimination. ©2022 Halawisi,KFU 21 Organometallic Chemistry CHE Concerted reductive elimination cis-Elimination –For concerted reductive elimination processes, the ligands undergoing elimination must be cis with respect to one another. –C-H reductive elimination is generally much more facile than C-C reductive elimination. –Reductive eliminations involving sp2-hybridized carbons (aryl or vinyl) are usually faster than those involving sp3-hybridized carbons. –supporting ligands (e.g. phosphines) play a crucial role in determining rates of reductive elimination with bulkier, less electron-rich ligands typically giving rise to faster rates. ©2022 Halawisi,KFU 22