Website: https://www.kpgcollege.org Email: [email protected] Subject Chemistry Paper No and Title CH-402: Organic Synthesis Module No and Title 5. Metallocenes, Nonbenzenoid Aromatic and Polycyclic Aromatic Compounds Module Tag PG-331 Content Writer:- Dr. Archana Chahar Asst. Professor and Head, Dept. of Chemistry, Kisan PG College, Simbhaoli TABLE OF CONTENTS: 1. General considerations of Metallocenes 2. Ferrocene 3. Chrysene 4. Azulene 1. General considerations of Metallocene Metallocene A metallocene is a compound typically consisting of two cyclopentadienyl − anions (C5H5 , abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to + [Cp2ZrCH3] catalyze olefin polymerization. Some metallocenes consist of metal plus two cyclooctatetraenide 2− 2− anions (C8H8 , abbreviated cot ), namely the lanthanocenes and the actinocenes (uranocene and others). Metallocenes are a subset of a broader class of compounds called sandwich compounds. In the structure shown below, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromatically stabilized. Here they are shown in a staggered conformation. Figure: General chemical structure of a metallocene compound, where M is a metal cation The first metallocene to be classified was ferrocene, and was discovered simultaneously in 1951 by Kealy and Pauson, and Miller et al. Kealy and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl salt with anhydrous FeCl3 but obtained instead the substance C10H10Fe. At the same time, Miller et al reported the same iron product from a reaction of cyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides. The structure of "C10H10Fe" was determined by Wilkinson et al. and by Fischer et al. These two were awarded the Nobel Prize in Chemistry in 1973 for their work on sandwich compounds, including the structural determination of ferrocene. They determined that the carbon atoms of the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that bonding occurred due to the metal d-orbitals and the π-electrons in the p- orbitals of the Cp ligands. This complex is now known as ferrocene, and the group of transition metal dicyclopentadienyl compounds is known as 5 metallocenes. Metallocenes have the general formula [(η -C5H5)2M]. Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of cyclopentadienide, metallocenes of many elements have been prepared.[5] One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado. Definition: The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe, systematically named bis(η5-cyclopentadienyl)iron(II). According to the IUPAC definition, a metallocene contains a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel planes with equal bond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η5, pronounced "pentahapto". There are exceptions, such as uranocene, which has two cyclooctatetraene rings sandwiching a uranium atom. In metallocene names, the prefix before the -ocene ending indicates what metallic element is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present. In contrast to the more strict definition proposed by IUPAC, which requires a d- block metal and a sandwich structure, the term metallocene and thus the denotation -ocene, is applied in the chemical literature also to non-transition metal compounds, such as barocene (Cp2Ba), or structures where the aromatic rings are not parallel, such as found in manganocene or titanocene dichloride (Cp2TiCl2). Some metallocene complexes of actinides have been reported where there are three cyclopendadienyl ligands for a monometallic complex, all three of them bound η5. Classification: 5 There are many (η -C5H5)–metal complexes and they can be classified by the following formulas: Formula Description 5 [(η -C5H5)2M] Symmetrical, classical 'sandwich' structure 5 [(η -C5H5)2MLx] Bent or tilted Cp rings with additional ligands, L Only one Cp ligand with additional ligands, L ('piano-stool' 5 [(η -C5H5)MLx] structure) Metallocene complexes can also be classified by type: 1. Parallel 2. Multi-decker 3. Half-sandwich compound 4. Bent metallocene or tilted 5. More than two Cp ligands 2. Ferrocene Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule consists of two cyclopentadienyl rings bound on opposite sides of a central iron atom. Ferrocene is a p-complex in which reactions between the d- orbitals of the Fe2+ metal centre with the p-orbitals of the two planar - cyclopentadienyl ligands (C5H5 ) form the metal-ligand bonds. Hence there is equal bonding of all the carbon atoms in the cyclopentadienyl rings to the central Fe2+ ion. It is an orange solid with a camphor-like odor, that sublimes above room temperature, and is soluble in most organic solvents. Ferrocene shows aromatic properties and is very stable. Figure: Ferrocene [Fe(η-C5H5)2] Synthesis 1. Via Grignard reagent The first reported syntheses of ferrocene were nearly simultaneous. Pauson and Kealy synthesised ferrocene using iron(III) chloride and a Grignard reagent, cyclopentadienyl magnesium bromide. Iron(III) chloride is suspended in anhydrous diethyl ether and added to the Grignard reagent. A redox reaction occurs, forming the cyclopentadienyl radical and iron(II) ions. Dihydrofulvalene is produced by radical-radical recombination while the iron(II) reacts with the Grignard reagent to form ferrocene. Oxidation of dihydrofulvalene to fulvalene with iron(III), the outcome sought by Kealy and Pauson, does not occur. 2. Gas-metal reaction The other early synthesis of ferrocene was by Miller et al., who reacted metallic iron directly with gas-phase cyclopentadiene at elevated temperature. 3. Cracking of dicyclopentadiene 4. An approach using iron pentacarbonyl was also reported. Fe(CO)5 + 2 C5H6(g) → Fe(C5H5)2 + 5 CO(g) + H2(g) 5. Via alkali cyclopentadienide More efficient preparative methods are generally a modification of the original transmetalation sequence using either commercially available sodium cyclopentadienide or freshly cracked cyclopentadiene deprotonated with potassium hydroxide and reacted with anhydrous iron(II) chloride in ethereal solvents. Modern modifications of Pauson and Kealy's original Grignard approach are known: Using sodium cyclopentadienide: 2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl Using freshly-cracked cyclopentadiene: FeCl2·4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O Using an iron(II) salt with a Grignard reagent: 2 C5H5MgBr + FeCl2 → Fe(C5H5)2 + 2 MgBrCl Even some amine bases (such as diethylamine) can be used for the deprotonation, though the reaction proceeds more slowly than when using stronger bases: 2 C5H6 + 2 (CH3CH2)2NH + FeCl2 → Fe(C5H5)2 + 2 (CH3CH2)2NH2Cl Direct transmetalation can also be used to prepare ferrocene from other metallocenes, such as manganocene: FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2 Reactions 1. With electrophiles Ferrocene undergoes many reactions characteristic of aromatic compounds, enabling the preparation of substituted derivatives. A common undergraduate experiment is the Friedel–Crafts reaction of ferrocene with acetic anhydride (or acetyl chloride) in the presence of phosphoric acid as a catalyst. Under conditions for a Mannich reaction, ferrocene gives N,N- dimethylaminomethylferrocene. Figure: Important reactions of ferrocene with electrophiles and other reagents 2. Synthesis of [Fe(η-C5H5)(η-C6H6)]PF6 Figure: Ligand exchange of ferrocene with benzene 3. Lithiation Ferrocene reacts with butyllithium to give 1,1′-dilithioferrocene, which is a versatile nucleophile. Tert-Butyllithium produces monolithioferrocene. Dilithioferrocene reacts with S8, chlorophosphines, and chlorosilanes. The strained compounds undergo ring-opening polymerization. 4. Protonation of ferrocene allows isolation of [Cp2FeH]PF6. 5. In the presence of aluminium chloride Me2NPCl2 and ferrocene react to give ferrocenyl dichlorophosphine, whereas treatment with phenyldichloro-phosphine under similar conditions forms P,P- diferrocenyl-P-phenyl phosphine. 6. Ferrocene reacts with P4S10 forms a diferrocenyl-dithiadiphosphetane disulfide. 7. Some transformations of dilithioferrocene. 8. The phosphine ligand 1,1'-bis(diphenylphosphino)ferrocene (dppf) is prepared from dilithioferrocene. 9. Reaction of [Fe(η-C5H5)(η-C6H6)]PF6 with Nucleophiles The reaction of the iron benzene p-complex with LiAlH4 and LiAlD4, sources of H and D- ions respectively. Arenes, for instance benzene, are more susceptible to attack by electrophiles than nucleophiles. However, associating with a metal often alters the reactivity of organic ligands. Thus, the reactivity of the benzene ligand in the iron p-complex towards the nucleophiles H- and D- is examined. 3. Chrysene Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula C18H12 that consists of four fused benzene rings. It is a natural constituent of coal tar, from which it was first isolated and characterized.