CHM 303 Organometallic Chemistry Past Paper PDF 2021

CHM 303: ORGANOMETALLIC CHEMISTRY Dr. Segun A. Ogundare Lecture content: Classification of organometallic compounds. Preparation, structure and reactions including abnormal behaviour of organometallic compounds. Synthetic utility of organometallics. Generation and detection of free-radicals from o...

CHM 303: ORGANOMETALLIC CHEMISTRY Dr. Segun A. Ogundare Lecture content: Classification of organometallic compounds. Preparation, structure and reactions including abnormal behaviour of organometallic compounds. Synthetic utility of organometallics. Generation and detection of free-radicals from organometallics. Classification of ligands. Electron rule, bonding. Preparation of organo-transition metal compounds. Reactions and structures of organometallics. Definition, classification and nomenclature of organometallic compounds An organometallic compound contains at least one metal-carbon (M-C, M=C or M≡C). The chemistry of organometallic compounds is the study of compounds containing metal(s) and organic group(s) in which there is at least one bond existing between the carbon atom(s) of the organic group(s) and the metal(s). There several compounds containing metals and organic groups but the chemistry of organometallic compounds is mostly related to those compounds in which there is at least a metal-carbon bond. Many of these compounds occur naturally and they are of biological significance. Examples are: Cobalamin cofactors (vitamin B12). There are other compounds not captured by definition of organometallic chemistry that are included in the study of organometallic compounds. Examples are: metal carbonyls, metal nitroyls, metal hydrides and others. The first organometallic compound was reported by Ziese in 1827 but the structure was not established until about 100 years later by another Chemist. The compound was prepared by bubbling ethylene gas into a solution of potassium teterachloropalatinate(II). The X-ray diffraction analysis of the compound indicated presence of three chloride ions and ethylene occupying the corners of a square-planar geometry round the platinum ion. K2PtCl4 + H2O + C2H4 → K[PtCl3(C2H4)] + KCl In 1890, Mond reported the synthesis of Ni(CO) 4, which became relevant for the commercial purification of nickel. After the report of Mond in 1890, several metal carbonyls (Fe2(CO)9, Mo(CO)6, Ru(CO)5) were prepared. Ni + 4CO → Ni(CO)4 Page 1 of 21 An attempt to synthesize a compound called fulvanene led to the preparation of ferrocene by Kealy and Pauson in 1951. The structure of ferrocene revealed that the metal ion was sandwiched between to cyclopentadienyl groups. Other related compounds have been reported leading to the chemistry of metallocenes. The reaction for the synthesis of ferrocene was carried out in dry diethyl ether using C5H5MgBr and FeCl3. The discovery of ferrocene introduced the chemistry of organometallic compounds. 2C5H5MgBr + FeCl3 → (C5H5)2Fe One of the methods of classifying organometallic compounds is based on the number of carbon atom of the organic group that is directly bonded to the metal. The number of carbon atoms of the organic group that is directly linked to the metal is indicated by the Greek letter eta (η) with a numerical superscript (n) denoting the number of the carbon atoms. This symbol (η n) is referred to as hapticity or hapto number. Page 2 of 21 Note that symbol may be used without the superscript if all the carbon atoms of the Organic group are directly bonded to the metal. This is acceptable to describe ferrocene and other related compounds (metallocene) in which all the carbon atoms of the Organic group are directly attached to the metal. Similarly, note that for the carbonyl there is no need to indicate or include hapticity since the ligand has only one carbon atom but the number of the ligands must reflect in the nomenclature the compound. Example: Fe(CO)5: iron pentacarbonyl The Effective Atomic Number (EAN) rule or the 18-electron rule The effective atomic number rule also known as the 18-electron rule states that stable organometallic compounds should have 18 electrons in their outermost (valence) shell. This rule was first proposed by Sidgwick and extended later by Bailey. Therefore, it is also referred to as sidgwick-Bailey's rule. Although there are many exceptions to this rule, it still provides useful guides to the chemistry of many organometallic complexes. The 18-electrons in the valence shell of the metal will occupy ns, np and (n-1)d orbitals a total of 9 orbitals, which can accommodate two electrons each giving a total of 18 electrons for a complete shell. This makes an organometallic complex to be kinetically stable. By this definition, the metal should be a transition metal and n must be greater than 4. Page 3 of 21 If there are less than 18 electrons in the valence shell of the metal, an empty low-lying orbitals into which electrons may be promoted will be available. This will lead to decomposition on slight heating. If there are more than 18 electrons, the excess electron will move to the antibonding orbitals which will reduce the stability by decreasing the bond order due to interaction with bonding electrons. Note that a = a single non-degenerate orbital, e = double degenerate orbitals and t = triple degenerate orbitals. 1 indicates symmetrical to the plane of reflection and 2 indicates unsymmetrical to the plane of reflection. Gerade (g) implies symmetrical to center of inversion and ungerade (u) implies unsymmetrical to center of inversion. Application of 18-electron rule The 18-electron rule helps to predict the stability of organometallic complexes and also helps to predict the most probable structure of newly synthesized organometallic complexes. The application of the 18-electron rule involves the determination of electrons in the valence shell of the metal in zero oxidation state. This is added to the electrons contributed by the ligands and if the complex is positively charged the Lost electron must be removed or deducted from the total Page 4 of 21 electrons. However, if the complex is negatively charged the gained electrons must be added to the total number of electrons. If the compound or complex has 18 electrons in the valence shell of the metal it is considered stable and on heating such complex it will not decompose at relatively low temperature. However, if the number of electrons in the valence shell of a metal is less than or greater than 18, the complex is unstable and may decompose on heating except those that have a special stability associated with 16-electron Square planar complexes. Note that the second row and third row transition metals under these group we'll have the same number of valence electron: nickel, palladium and platinum will have 10 valence electron is zero station state. Similarly, iron osmium and ruthenium will have eight electrons in valence shell in zero oxidation state. The number of electrons donated by the ligands depend on the nature of the ligand and the nature of the bonding below are some examples: Page 5 of 21 Questions i. Using the above given values predict the stability of the following organometallic compounds based on effective atomic number rule. Page 6 of 21 ii. Predict the most probable structures for the complexes given below if the effective atomic number rule is obeyed.1. [(C7H7)Co(CO)3] 2. [Ni(η5-C5H5)(NO)] iii. Draw the most probable structure for [IrCl(PPh3(NO)(CO)]Cl if the number of valence electrons is 16 in the metal. Limitations of the effective atomic number (18-electron) rule There are no known cases in which the meta center in organometallic compounds do not have 18 electrons in their valence shell and they are very stable. Examples include: CH 3TiCl3, (CH3)2NbCl3, W(CH3)2, [Rh(CO)2Cl]2, which are: 8, 10, 12 and 16 electron systems respectively. The geometry adopted by these complexes is square planner. Another example is: [Ni(P(cyclohexyl)3)2(C2H4)]. The bulkiness of bonded ligands imposes steric effect on the approach of incoming ligand. this can also impose limitations with regards to the number of ligand that can be accommodated by the metal and limit the number of electrons in the valence shell. Transition metal clusters The term metal cluster is applicable to transition metal complexes containing at least three metals bonded by metal to metal bonds. Cluster is defined as a finite group of metal atoms bonded together by entirely or to a significant extent by bonds between the metal atoms. Bond(s) may also exist between the metal in the cluster and other associated organic groups. The First metal cluster was synthesized in high-yield by reaction between osmium oxide and carbon monoxide. Page 7 of 21 X-ray crystallographic study of the compound showed that each osmium is surrounded by 4 carbonyls (terminally bonded) and the three osmium atoms are connected by metal-metal Bond. The name of the compound formed is Triosmium dodecacarbonyl. The heating of triosmium dodecacarbonyl will give a mixture of cluster products which can be separated using chromatography technique. During reactions of metal clusters, it is very important to activate the starting material to prevent unwanted or unexpected products this is because the reaction occurs at high temperature and pressure under which several modifications can occur. To prevent such occurrence, the starting material is activated for example by reacting with hydrogen. Properties of transition metal clusters 1. They have high nuclearity (having more than two metal centres) 2. The show properties similar to those of metal carbonyls. 3. They are mostly stable solids with crystalline structure. 4. They are very important in homogeneous and heterogeneous catalysis Page 8 of 21 To predict the structure of metal clusters the general 18-electron rule is employed. Number of M-M bonds = (the total number of bonding electrons - available electrons)/2 Bonding electron = 18 x number of metal centres present in each cluster since each metal must have 18 electrons in its valence shell to be stable. Available electrons are those in the valence shell of the metal and those donated by the ligand examples are shown below. Question: Predict the structures of Rh4(CO)12 and Fe3(CO)12. Generation and detection of free radicals from organometallic (a relationship between main group and organometallic chemistry). A free radical is an atom or group of atoms bearing unpaired electron(s) that can take part in chemical bonding. Examples: H*, Cl*, HO*, Page 9 of 21 H* + H* → H-H F* + F* → F-F Cl* + Cl* → Cl-Cl Organometallic compounds also possess similar chemistry. It is important to Note that the bonding in the above occurs in order for the free radicals to attain stability or have stable electronic configuration after the nearest noble gas. Example chlorine radical shares one electron with each other to have stable electronic configuration similar to argon core [Ar]. Similarly, a 17-electron organometallic specie can form a dimer to attain 18 electrons through formation of metal-metal Bond examples are shown below: *Mn(CO)5 + *Mn(CO)5 → (CO)5Mn-Mn(CO)5 *Co(CO)4 + *Co(CO)4 → (CO)4Co-Co(CO)4 3:Fe(CO)4 → Fe3(CO)12 *H + *Cl → H-Cl 2*H + (CO)4Co-Co(CO)4 → 2HCo(CO)4 2*H + :S → H2S 2*H + :Fe(CO)4 → H2Fe(CO)4 Preparation of organometallic compounds Organometallic compounds are prepared using different methods. Some of these methods will be highlighted. i. Reaction of metals and alkyl/ aryl halides This technique is most suitable for highly reactive metals. 2Li + C3H7Cl → LiC3H7 + LiCl 2Na + C6H5Cl → NaC6H5 + NaCl CH3Br + Mg → CH3MgBr (Grignard reagent) Pb (Na/Hg) + 4C2H5Cl → Pb(C2H5)4 + 4NaCl + Hg (Na/Hg) helps to aid the reaction 2Zn/Cu + 2C3H7I → Zn(C3H7) + ZnI2 + 2Cu Cu as catalyst ii. Transfer of alkyl/aryl groups 4NaC6H5 + SiCl4 → Si(C6H5)4 + 4NaCl 2Al(C2H5)3 + 3ZnCl2 → 3 Zn(C2H5)2 + 2AlCl3 Page 10 of 21 iii. Reaction of Grignard reagents for new organometallic compounds 3C6H5MgBr + SbCl3 → Sb(C6H5)3 + 3MgBrCl 2CH3MgBr + HgCl2 →Hg(CH3)2 + 2MgBrCl iv. Reaction of metals and alkenes 2Al + 3H2 + 6C2H4 → 2Al(C2H5)3 2CH3Cl + Si → (CH3)2SiCl2 Classification of Ligands in Organometallic Chemistry Compounds classified as organometallic are those that contain at least one metal-carbon bond, though in practice, complexes containing several other ligands similar to CO in their bonding, such as NO and N2, are frequently included in the discussion of organometallic compounds. Other π-acceptor ligands like phosphines, often occur in organometallic complexes, and their chemistry may be studied in association with the chemistry of organic ligands. In addition, dihydrogen, H2, participates in important aspects of organometallic chemistry and deserves consideration. Examples of these and other nonorganic ligands as appropriate are included in the discussion of organometallic chemistry. Many ligands are known to bond to metal atoms through carbon. Carbon monoxide (CO) forms a great number of organometallic compounds called metal carbonyls, other related diatomic molecules (NO, N2, H2) also bond directly to metal centers and are also classified as ligands. Many organic molecules such as saturated, unsaturated, conjugated and cyclic π-system hydrocarbons also form numerous organometallic complexes. Similarly, some organic frameworks are known to interact with metal centers by formation of double or triple bonds. These classes of ligands lead to formation of carbene (metal-carbon double bond) and carbyne (metal-carbon triple bond) complexes. Carbonyl (CO) ligand Organometallic complexes with CO as ligand are very abundant. They are among the early compounds used in the description and classifications of organometallics. CO serves as the only ligand in binary carbonyls such as Ni(CO)4, W(CO)6, and Fe2(CO)9 or, more commonly, in Page 11 of 21 combination with other ligands, both organic and inorganic. CO may bond to a single metal or it may serve as a bridge between two or more metals. CO can sometimes bond through oxygen as well as carbon. This phenomenon was first noted in the ability of the oxygen of a metal-carbonyl complex to act as a donor toward Lewis acids such as AlCl3, with the overall function of CO serving as a bridge between the two metals. Many examples are now known in which CO bonds through its oxygen to transition metal atoms, with the C-O-metal arrangement generally bent. This class of carbonyl is sometimes called isocarbonyls. Page 12 of 21 Nitrosyl (NO) ligand NO (nitrosyl) ligand is isoelectronic with CO and can also behave as both electron donor and π- acceptor. It can serve as a terminal or bridging ligand. However, terminal NO has two common coordination modes, linear (like CO) and bent. Complexes containing bridging nitrosyl ligands are also known, with the neutral bridging ligand. One NO complex, the nitroprusside ion, [Fe(CN)5(NO)]2-, has been widely used as a vasodilator in the treatment of high blood pressure. Its therapeutic effect is a consequence of its ability to release its NO ligand; the NO itself acts as the vasodilating agent. Two compounds containing only a metal and NO ligands are known, Cr(NO)4 and Co(NO)3. In recent years, several dozen compounds containing the isoelectronic NS (thonitrosyl) ligand have been synthesized. Infrared data have indicated that, like NO, NS can function in linear, bent, and bridging modes. In general, NS is similar to NO in its ability to act as a π-acceptor ligand; the relative π-acceptor abilities of NO and NS depend on the electronic environment of the compounds being compared. Page 13 of 21 Hydride ligand The term hydride ligand suggests Hδ- and is consistent with the charge distribution expected for an H atom attached to an electropositive metal centre. However, the properties of H ligands depend on environment and in many organometallic complexes, hydrido ligands behave as protons, being removed by base or introduced by treatment with acid. Hydride ligands can adopt terminal, bridging or (in metal clusters) interstitial modes of bonding. A localized 2c-2e M-H bond is an appropriate description for a terminal hydride, delocalized 3c- 2e or 4c-2e interactions describe µ-H and µ3-H interactions respectively, and a 7c-2e interaction is appropriate for an interstitial hydride in an octahedral cage. Dihydrogen (H2) ligand Although complexes containing H2 molecules coordinated to transition metals had been proposed for many years, the first structural characterization of a dihydrogen complex did not occur until 1984, when Kubas and coworkers synthesized M(CO)3(PR3)2(H2) (M = Mo, W, R = cyclohexyl, isopropyl). Subsequently, many H2 complexes have been identified, and the chemistry of this ligand has developed rapidly. The bonding between dihydrogen and a transition metal involves the σ electrons in H2 being donated to a suitable empty orbital on the metal (such as a d orbital or hybrid orbital), and the empty σ* orbital of the ligand (H2) can accept electron density from an occupied d orbital of the metal. The result is an overall weakening and lengthening of the H-H bond in comparison with free H2. Page 14 of 21 π-Bonded organic ligands Unsaturated hydrocarbons are known to coordinate with metal centres. The first organometallic compound to be reported was synthesized in 1827 by Zeise and characterized to contain CH2=CH2, an unsaturated hydrocarbon, directly bonded to Pt centre. A large number of these compounds are now know with different organic frameworks. Included in this categories are the metallocenes such as ferrocene, where the metals are sandwiched between two cyclopentadienyl ring. Wide varieties of Organometallic compounds are known with various organic ligands some of which are shown in the Table below. Synthetic utility of organometallic Alkane synthesis using R2CuLi Organolithium cuprates, R2CuLi, react with alkyl halides forming a new C-C, giving alkanes. Primary alkyl iodides make the best substrates otherwise elimination can be a problem. The R group of the cuprate can also be aryl or vinyl. The R' group in the halide can also be aryl or vinyl. (CH3)2CuLi + C2H5Cl → C3H5CH3 + CH3Cu + LiCl (C2H5)2CuLi + C6H5Cl → C6H5CH3 + C2H5Cu + LiCl Reactions of RLi and RMgX with Esters usually in Et2O followed by H3O+ leading to synthesis of alcohol Page 15 of 21 Synthesis of Cyclopropanes using RZnX (The Simmons-Smith reaction) Reaction type: 1. Oxidation-Reduction, 2. Addition This is the most important reaction involving an organozinc reagent. Also known as the Simmons-Smith reaction. Iodomethyl zinc iodide is usually prepared using Zn activated with Cu. The iodomethyl zinc iodide reacts with an alkene to give a cyclopropane. The reaction is stereospecific with respect to to the alkene. Substituents that are trans in the alkene are trans in the cyclopropane. Reaction type: Nucleophilic Acyl Substitution then Nucleophilic Addition Carboxylic esters, R'CO2R'', react with 2 equivalents of organolithium or Grignard reagents to give tertiary alcohols. The tertiary alcohol contains 2 identical alkyl groups. The reaction proceeds via a ketone intermediate which then reacts with the second equivalent of the organometallic reagent. Since the ketone is more reactive than the ester, the reaction cannot be used as a preparation of ketones. Page 16 of 21 Step 1: The nucleophilic C in the organometallic reagent adds to the electrophilic C in the polar carbonyl group of the ester. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex. Step 2: The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, this produces a ketone as an intermediate. Step 3: The nucleophilic C in the organometallic reagent adds to the electrophilic C in the polar carbonyl group of the ketone. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex. Step 4: This is a simple acid/base reaction. Protonation of the alkoxide oxygen creates the alcohol product from the intermediate complex. Page 17 of 21 Reactions of RC≡CM with Aldehydes and Ketones usually in Et 2O followed by H3O+ leading to synthesis of alcohol with acetylide functional group Reaction type: Nucleophilic Addition Acetylide reagents react with the carbonyl group, C=O, in aldehydes or ketones to give Alcohols. The substituents on the carbonyl dictate the nature of the product alcohol. Addition to methanal (formaldehyde) gives primary alcohols. Addition to other aldehydes gives secondary alcohols. Addition to ketones gives tertiary alcohols. The acidic work-up converts an intermediate metal alkoxide salt into the desired alcohol via a simple acid base reaction. Synthesis of acetic acid Reaction type: The Monsanto Acetic Acid Process The Monsanto acetic acid process is the major commercial production method for acetic acid. Methanol, which can be generated from synthesis gas ("syn gas", a CO/H2 mixture), is reacted Page 18 of 21 with carbon monoxide in the presence of a catalyst to afford acetic acid. In essence, the reaction can be thought of as the insertion of carbon monoxide into the C-O bond of methanol, i.e. the carbonylation of methanol. The catalysts The catalyst system has two components, iodide and rhodium. Almost any source of Rh and I- will work in this reaction as they will be converted to the actual catalyst, [Rh(CO)2I2]- under the reaction conditions. The role of iodide is simply to promote the conversion of methanol to methyl iodide, the species which then undergoes reaction with the Rh metal catalyst: The catalytic cycle Once methyl iodide has been generated, the catalytic cycle begins with the oxidative addition of methyl iodide to [Rh(CO)2I2]-. Coordination and insertion of carbon monoxide leads an intermediate 18-electron acyl complex which can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst: Page 19 of 21 Notice that there are two catalytic cycles going in this reaction, the metal complex and the iodide catalytic cycles. The acetyl iodide produced in the lower cycle is then hydrolysed in the upper one to give acetic acid. This hydrolysis produces HI which can then convert more methanol to iodide and continue the cycle: Olefin Metathesis The olefin metathesis reaction (the subject of 2005 Nobel Prize in Chemistry) can be thought of as a reaction in which all the carbon-carbon double bonds in an olefin (alkene) are cut and then rearranged in a statistical fashion: If one of the product alkenes is volatile (such as ethylene) or easily removed, then the reaction shown above can be driven completely to the right. Likewise, using a high pressure of ethylene, internal olefins can be converted to terminal olefins. There are a wide variety of variants on this reaction as is discussed below. Mechanism The commonly accepted mechanism for the olefin metathesis reaction was proposed by Chauvin and involves a [2+2] cycloaddition reaction between a transition metal alkylidene complex and the olefin to form an intermediate metallacyclobutane. This metallacycle then breaks up in the opposite fashion to afford a new alkylidene and new olefin. If this process is repeated enough, eventually an equilibrium mixture of olefins will be obtained. Page 20 of 21 Such cycloaddition reactions between two alkenes to give cyclobutanes is symmetry forbidden and occurs only photochemically. However, the presence of d-orbitals on the metal alkylidene fragment breaks this symmetry and the reaction is quite facile. Catalysts Titanocene-based catalysts. Reaction of Cp2TiCl2 with two equivalents of AlMe3 to yield Cp2Ti(μ-Cl)(μ-CH2)AlMe2, commonly called Tebbe's Reagent. In the presence of a strong base such as pyridine, the reagent is functionally equivalent to "Cp 2Ti=CH2". Grubbs has shown that these Ti complexes undergo stoichiometric Wittig-like reactions with ketones, aldehydes and other carbonyls to form the corresponding methylene derivatives. The mechanism of this reaction is identical to that of the olefin metathesis reaction except that the final step is not reversible. Polymerization of Acetylenes When an alkyne is reacted with an alkylidene, a [2 + 2] cycloaddition occurs as with olefins, a metallacyclobutene is formed instead of a metallacyclobutane. If this metallacycle opens in a productive fashion, the result is a growing polymer chain: This reaction works well with 2-butyne or terminal alkynes. Polymerization of terminal acetylenes is complicated by the potential for the R group to insert alpha or beta with respect to the metal. It is extremely challenging to always get a beta insertion and generate a polymer with reproducible properties. Page 21 of 21