Insertion of Unsaturated Species Into M-C Bonds in Cyclometallated Complexes - Lab Report Example

Department Insertion of unsaturated species into M-C bonds in cyclometallated complexes from CH activation with the help of MCp* complexes (M = Ir, Rh, Ru) Professor name Date Introduction N- containing cyclometallated compounds have been focused on in the last few decades because of their interesting reactivity in the C-C bond forming reactions. Cyclometallated compounds which incorporate internal imine functionality are proposed as intermediates in a number of organic transformations. Cyclometallated metal complexes which include a nitrogen donor atom are known to possess excellent photoreduction property and reactivity towards C-H bond functionalization reactions, and have also been screened recently as catalysts for the transfer hydrogenation of ketones and imines. (Begman 2007) C-H activation C-H activation can be defined as reactions involving cleavage of an unreactive C-H bond of alkanes, arenes or alkyl chains by transition metal complexes to form products with the M-C bond.(Pekke 2007) Scheme 1: C-H activation Bond ( Pekke 2007) Blaser( 2004) observes that the plentiful supply of alkanes in oil and natural gas stirs up a desire to exlore their use as chemical feedstocks for the catalytic synthesis of organic molecules. However, alkanes are one of the most chemically inert organic chemicals known. For instance, methane is one of the most common but least reactive molecules in nature. Selective and efficient transformation of hydrocarbons into functionalized molecules such as alcohols, ketones and acids is of great industrial importance. The potential use of alkanes has attracted a lot of interest in the search for metal complexes that can activate C-H bonds in saturated hydrocarbons due to the fact that alkanes would be a much better feedstock economically for the organic chemical industry. ( Blaser, 2004) Cross Coupling Reactions Cross-coupling reactions have been developed as a versatile tool for carob carbon bond formations.( Davies 2006). Beucage goes further and makes an observation that there is a growing importance of these reactions due to their mild reaction conditions, making them compatible with a wide variety of functional groups. Therefore, these reactions are necessary in the advanced stage during total synthesis of complex chemicals such as drugs, vitamins, flavours, fragrances, agro chemicals and performance materials. Cross coupling reactions are classified according to the nature of the organometallic substrate applied and often named after their discoverers. The important examples include the following: 1. Suzuki reaction 2. Stille reaction 3. Negishi reaction 4. Heck reaction 5. Sonogashira reactions ( Davies 2006 & Beucage 1999). Thagathar explains that Heck reactions do not involve organometals as substrates and hence may not be considered as cross coupling reactions, but its alkyne version, especially the Sonogashira reaction which involves the use of catalytic amounts of CuI salts is usually deemed as cross coupling reaction. The most common catalyses for cross coupling reactions include Ni(O) and Pd(O)( Thagathar 2006). Pd Catalyst R-X + R’-M R-R’ + M-X R,R’+Aryl, vinyl,benzyl,allyl,alkyl X=Cl. Br,I, sulfonate M=B, Mg,Zn,Sn e.t.c Scheme 2: Definition of cross coupling Reactions (Thagathar 2006) Scheme 3:The generally accepted catalytic cycle of cross coupling reactions (Chen et al 2000). The catalytic cycle as explained by Engel (2002) has three processes namely: a. Oxidative oxidation of an electrophile, typically an organic halide(mostly iodides and bromides, rarely chlorides) or triflate to a Pd(O) species to form the corresponding Pd(II)(organyl( complex. b. Transmetallation of an (organometallic) nucleophilic coupling together with the starting Pd(O) complex that can re-enter the catalytic cycle. (Engel 2002). Suzuki Cross Coupling Reactions The coupling of aryl halides with arylboronic acids, the Suzuki reactions as shown in Scheme 4 below has gained a lot of popularity because of the fact that 1) it is compatible with the presence of electrophilic functional groups, 2) many c=boron compounds are stable and have lower toxicity compared to other organometallic reagents, 3)manu arylboronic acids are commercially available, 4) the inorganic product of the reaction can be easily eliminated in water, and 5) the reaction conditions tolerate aqueous media, which renders elimination of the boron containing reaction products easier.(Hull, Anani & Sanford 2006) Scheme 4: Suzuki Cross coupling reactions: (Hartwig 2002) Suzuli-Miyaura coupling reactions rely a great deal on the increased reactivity and stability of the Pd catalyst by use of increasing efficacious supporting ligands, some of which include phosphine-based, although a variety of others, including N-heterocyclic(NHC),have been employed. Heck Coupling Reactions The Heck reaction can be defined as the palladium-catalysed coupling of alkenyl or aryl (sp2) halides or triflates with alkenes to yield products which formally result from the substitution of a hydrogen atom in the alkene coupling partner. Unlike the other C-C coupling reactions that involve a polar addition, the Heck reaction tolerates almost any sensitive functionality e.g. unprotected amino, hydroxyl, aldehyde, ketone, carboxy, ester, cyano and nitro groups. Scheme 5: The Heck Coupling Reaction (Hartwig 2002) The reaction is typically performed with 1-5mol% of a palladium catalyst along with a phosphine ligand in the presence of a suitable base. Both the phosphines and their palladium complexes are prone to decomposition and an excess of phosphine is required. This leads to a reduction in the rate of reaction which in turn requires a higher Pd loading and increases the cost of large scale processes. Sonogashira Coupling This is an improvement of the other cross coupling reactions by the addition off co-catalytic CuI salts to the reaction mixture. The Sonogashira reaction is straightforward and widely applied methods to synthesize arylalkynes and conjugated enynes. Basically, the coupling is carried out in the presence of a catalytic amount of a palladium complex as well as copper iodide in an amine based solvent to obtain a good yield. Scheme 6: SonoGashira coupling Reaction (Hartwig 2002) Negishi Coupling Reaction This reactions use less reactive organometallic reagents to common functional groups including Al, Zr and Zn. The organolithium rragents are prepared from the selective deprotonation or lithium-halogen exchange. Scheme 7: Negishi Coupling Reaction (Hartwig 2002) Stille Coupling Reaction This uses organotin compounds( called Stannanes) as organometallic components. It represents hal of all current cross-coupling reactions but due to their high toxicity, stannanes tend to be replaced more and more with organozinc and organocarbon compounds. The reaction may be carried out intramolecularly and with alkynyl stannanes instead of the more usual aryl or vinyl stannanes to form medium-sized rings. Scheme 8: Stille coupling Reaction (Hartwig 2002) C-H Activation By metal Complexes Intramolecular and intermolecular C-H bond Activation This is a very common reaction in organometallic chemistry. Dick & Sanford( 2006) define C-H bond activation as a reaction that involves the cleavage of a C-H bond in the ligand that is linked to the metal center through an item like nitrogen or phosphorous. This process is known as orthometallation or cyclometallation albeit it is not restricted to “ortho” protons. An example of orthometallation with H/D exchange of the hydrogen or the deuterium ligand and another involving cyclometallation with HX.( Dick & Sanford 2006) Scheme 9: Intramolecular and intermolecular C-H bond activation (Dick & Sanford 2006) Five Stages of C-H Activation Oxidative addition Oxidative addition of C-H bonds across a metal center is typical for electron-rich, low valent complexes of a late transition metal. The reactive species, as noted by Godula and Sames (2006), is a coordinatively unsaturated intermediate generated thermally or photochemically from a suitable precursor. The bonds formed will be much more prone to functionalization than the unreactive C-H bond.( Godula and Sames 2006) Scheme 10: Oxidative stage of C-H bond activation(Godula and Sames 2006) Sigma-bond metathesis This is a reversible reaction that occurs for halkyl or hydride complexes of early transition metal complexes according to the equation below, where an alkyl fragments interchange or exchange of hydrogen and alkyl fragment occurs. Scheme 11: Sigma bond metathesis stage of C-H bond activation (Godula and Sames 2006) Homolytic or radical activation This involves the reversible breaking of C-H bond with the attachment of the two fragments into two separate metal centers. Scheme 12: Homolytic/ radical activation stage of C-H bond activation(Godula and Sames 2006) 1,2 Addition This is the addition of a C-H bond into an M=X bond where X can be a heteroatom containing ligand or alkylidene. Scheme 13: 1,2 addition stage of C-H bond activation(Godula and Sames 2006) Electrophilic addition This occurs in a strongly polar medium such as water or an anhydrous strong acid. It involves the use of an electrophilic metal center to break the C-H bond. Scheme 14: Electrophilic addition stage of C-H bond activation(Godula and Sames 2006) The carbon-hydrogen bond is the un-functional group. Its unique position in organic chemistry is well illustrated by the standard representation of organic molecules: the presence of C-H bonds is indicated by the absence of any other bond. Advantages of C-H Activation Although C-H bonds have a low bond energy, they are the most difficult bonds to activate. The chemical inertness of alkanes is as a result of several factors. Alkanes have lower proton affinities and acidity and so are held together by strong C-H and C-C bonds to ensure no empty orbitals of low energy or filled orbitals of high energy that could readily participate in a chemical reaction. Activation of C-H bonds is much more established because of the kinetic advantage associated with the prior π-coordination of the arene ring to the metal center; a route that is unavailable to alaknes. Jones and Perutz found out and reported the formation of an equilibrium mixture of [CpRh(PMe3){η2-C6H4(CF3)2}] and [CpRh(PMe3){C6H3(CF3)2}(H)] on irradiation of a solution of [CpRh(PMe3)(C2H4)] in 1,4-C6H4(CF3)2 (Yu, Giri and Chen 2006) Scheme 15: C-H bond activation: (Saaby et al 2002) Acetylene C-H bonds are strong but terminal alkynes are acidic and the hydrogen can be removed as a proton by a strong base. Several metal complexes in low oxidation state can activate C-H bonds in acetylenes via oxidative addition. (Ye & Mc Kervey 1994). Scheme 16: Further illustration of the C-H activation (Labinger and Bercaw 2002) Photo dissociation of Co or H2 initiated by UV from group 9 organometallic compounds of the typeCp*MLL’ (where M = Ir, Rh; L = CO, PMe3; L’ = CO, H2) have been known to produce 16-electron Cp*ML fragments that are able to activate the otherwise inert C-H bonds in hydrocarbon solvents and methane to form hydro alkyl species Cp*ML(R)(H) (Steffen et al 2005) Scheme 17: Photo dissociation of metal complexes (Labinger and Bercaw 2002) The rhodium and iridium complexes Cp*M(PMe3)(H)2 show selectivity in C-H bonding activation in different molecules(intermolecular selectivity) and for different types of C-H bonds in the same molecule (intramolecular selectivity). Saunders ( 1997) noted that the rate of attack of the metal center on a certain C-H bond depends on steric effects and C-H acidities more than bond energies. For the complexes, activation of benzene is preferred over alkenes, smaller cycloalkanes are preferred over larger cycloalkanes. In the normal alkanes, the primary C-H bond activation is preferred over secondary C-H bonds. The selectivity shown is as follows: Tp*Rh > Cp*Rh > Cp*Ir. This is true for the activation of C-H bonds in the same molecule. With acyclic alkanes, the rhodium complex inserts into the primary C-H bonds whilst both the internal and terminal C-H activation is observed with the iridium complex. The activation of C-H bonds of alkanes using Cp*Ir(PMe3)(H)2 has received an extension to functionalized organic molecules. Under photochemical activation, alcohols or amine showed C-H activation instead of X-H activation. With methanol and ethanol, products from subsequent transformation of the initial species are obtained. ( Saunders 1997). Scheme 18: Photochemical activation of C-H bonds (Labinger and Bercaw 2002) Rhodium and Iridium complexes with quinolyl-functionalized Cp ligands are known to have a predefined geometry so that the nitrogen donor atom is in a suitable position for coordination to the metal center. Because nitrogen donors do not bind strongly to electron-rich metals in low states of oxidation: only hemilable bonding is expected. Due to the close proximity of the donor to the metal center, it is always possible to attain stabilization of the low co-ordinate species. This ensures prevention of the active species. The quinolyl-Cp rhodium complex G1 has been synthesized by the reaction of F with with [(C2H4)2RhCl]2. Chelation of the nitrogen donor on the quinolyl side arm to the rhodium metal center in G1 can be achieved photochemically through loss of an ethane molecule. This reaction is reversible in the dark resulting in liberated ethane. (Mc Quade et al 2000) Scheme 19: The reversible C-H activation bond reaction (Labinger and Bercaw 2002) The activation of C-H bonds remains as an important goal in synthetic chemistry. As much as the activation of alkanes by cyclopentadienyl complexes of transition metals in group 9 have been reported by several groups, the metal complexes are coordinatively saturated and photoirradiation is required for ligand dissociation for reactive species generation. The lack of ligand ability also hinders functionalization of the activated hydrocarbon by prevent coordination of an additional reactant to the metal center. Transition metal complexes that contain a cyclopentadienyl ligand with a donor group tethered via a side-chain that can coordinate reversibly may be necessary. Complexes with metals in the +III oxidation state, are capable of forming tethered complexes but intramolecular coordination of the amino group to the metal center is not known for compounds of the type Cp*^ML2 (M = Co, Rh, Ir; L = CO or C2H4) where the metal state is in the +I oxidation state. The photochemical reaction of Cp*^Ir(CO)2 in cyclohexane therefore led to an interest. Infrared spectroscopy offers advantages of speed and high sensitivity, which is useful during monitoring of reactions by use of low concentration of metal complexes as catalyst. (Aizawa et al 2006) Insertion of alkynes/alkenes into M-C bonds Scheme 20: Insertion reactions on M-C bonds (Shilov 1997) Studies of the fundamental steps of insertion reactions of unsaturated molecules into metal-carbon bonds have revealed very important information about the exact role of the metal during the reactions. In addition, the cyclometalated complexes with different ancillary ligands have shown different reactivities and different mechanisms depending on the stereoelectronic properties of the unsaturated molecules employed. ( Shilov 1997). Insertion of alkenes into the M-C bond is very important. Propylene and ethylene insertion into titanium alkyls is the basis of Ziegler-Natta catalysis. Heterogeneous catalysts aspect is the main technology here but it is widely assumed that the principles and observations on homogenous systems are applicable to the solid state versions. ( Waltz and Hartwig 1997) Insertion reactions are chemical reactions where one chemical entity interposes itself into an existing bond of a second entity as shown below: A+B-C B-A-C In cases where a metal-ligand bond in a coordination complex is involved, these reactions are typically organometallic in nature and involved a bond between a transition metal and a carbon or hydrogen (Crabtree 2006). Ling et al (2010) found that the Ru-aryl bond was subject to insertion over the Ru-alkyl bond as observed by the reactions with ortho-matalated benzyl ruthenium compounds. There has however, been limited reports about the insertion reaction with organo-rhodium and organo-iridium complexes. Reaction with Ethylene Ling et al performed an experiment where the metal complexes were treated with an atmosphere of ethylene at room temperature. The yellow solutions of 1 and 3 turned pale yellow within 5 minutes, producing 1a and 3a respectively. whilst the orange solutions of 2 and 4 turned bright red within 5 minutes, producing 2a and 4a respectively. Scheme 21: Reaction of metal complexes with ethylene (Labinger and Bercaw 2002) It was observed that the coordination compounds 1a and 3a were unstable and decomposed very quickly to starting material without the protection of the ethylene atmosphere. No crystals of the products could be isolated to further confirm the structures. Concerning the mechanism of the reaction, both 1 and 3 are coordinatively saturated 18-electron complexes, and an open coordination site is required to form a 16-electron intermediate, which could then coordinate ethylene. The two possible sequences for producing coordinative unsaturation: (1) Nitrogen dissociation or (2) chloride dissociation to form a 16 electron iridium cation. In addition, it was also observed that the Rh-Cl bond is lengthened by 0.02 A after insertion products. (Ling et al 2010). Scheme 22: Insertion pathway for ethylene (Labinger and Bercaw 2002) Lastly, it was also observed that no further ethylene insertion occurred in the reactions of ethylene with 2 and 4 under the same reaction conditions even after a week. Reaction with propylene The reactions pf propylene (b) with 1-4 were conducted under the same conditions of ethylene and the following was observed: Bright red insertions were observed for the reaction of propylene with 2 and 4 at room temperature within 5 minutes. The product 2b from reaction with 2 with 2 is more reactive, making the reaction more not as clean as the reaction with ethylene. The X ray structure confirmed a propyl group instead of isopropyl group as observed by Pfeffer in the reaction of cycloruthenated. The varying results with rhodium compounds versus iridium compounds could be accounted for by a stronger binding of the olefin to iridium, lowering the energy of this complex relative to the insertion complex. (Ling et al 2010). Scheme 23: Reaction of Metal complexes with Propylene (Labinger and Bercaw 2002) References Aizawa, M., Yamada, T., Shinohara, H., Akagi, K. & Shirakawa, H.2006. Electrochemical fabrication of a polypyrrole-polythiophene p-n-junction diode. J. 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