Basic Organometallic Chemistry: Principles and Applications CYL slides PDF

Institute Core Course for M. Sc. Program Basic Organometallic Chemistry: Principles and Applications  What is Organometallic Chemistry? A branch of coordination chemistry where the complex has one or more metal-carbon bonds. Organometallic compounds could be transiti...

Institute Core Course for M. Sc. Program Basic Organometallic Chemistry: Principles and Applications  What is Organometallic Chemistry? A branch of coordination chemistry where the complex has one or more metal-carbon bonds. Organometallic compounds could be transition metal-based, main group-based or lanthanides/actinides-based. An area which bridges organic and inorganic chemistry.  What All Compounds are Considered as Organometallic? C always more electronegative compared to M The leading journals of the field define an "organometallic" compound as one in which there is a bonding interaction (ionic or covalent, localized or delocalized) between one or more carbon atoms of an organic group or molecule and the main group, transition, lanthanide, or actinide metal atom (or atoms). Following a longstanding tradition, organic derivatives of the metalloids such as boron, silicon, germanium, arsenic, and tellurium also are included in this definition. It is also understood that the element to which carbon is bound is more electropositive than carbon in organometallic chemistry.  Metal-Carbon Bond Polarity The value underneath the element symbols are Pauling electronegativities.  A Brief History of Organometallic Chemistry 1) Organometallic Chemistry has really been around for millions of years Naturally occurring Cobalimins contain Co—C bonds Vitamin B12  A Brief History of Organometallic Chemistry Zeise’s Salt- The first transition metal organometallic compound K2PtCl4 + C2H5OH K[(C2H4)PtCl3]. H2O + KCl Discovery 1827 W C Zeise, Danish Structure ~ 150 years later pharmacist, I789- I847 Also father of the chemistry of ‘The breakthrough, the isolation of a pure, crystalline compound came when Zeise added potassium chloride mercaptans R-SH to a concentrated PtCl4 /ethyl alcohol reaction solution and evaporated the resulting solution. Beautiful lemon yellow crystals, often one half inch or more in length were isolated. On longer exposure to air and light, they gradually became covered with a black crust. They contained water of hydration, which was lost when they were kept over concentrated sulfuric acid in vacuo or when heated to around 100°C. Chemists in those days often reported how the compounds that they had prepared tasted. Zeise described the taste of this potassium salt as metallic, astringent and long lasting.’ Dietmar Seyferth, Organometallics, 2001, 20, 2  A Brief History of Organometallic Chemistry First  bonded Organometallic Compound- Diethyl zinc 3 C2H5I + 3 Zn → (C2H5)2Zn + C2H5ZnI + ZnI2 Edward Frankland 1825-1899 Frankland coined the Student of Robert Bunsen (Bunsen burner fame!). Prepared diethyl term zinc while trying to make ethyl radicals. “Organometallic” As the early 1850s English chemist Edward Frankland described flasks exploding, throwing bright green flames across his lab, as he heroically distilled dialkylzinc compounds under an atmosphere of hydrogen.  A Brief History of Organometallic Chemistry Metal carbonyls 244px-Mocarbonyl The Mond process of Nickel purification 200 °C NiO + H2 ( from Syn gas) Impure Ni ( Fe and Co) + H2O excess CO 50 -60 °C 220- 250 °C Ni(s) + 4 CO Ni(CO)4 (g) bp 42 °C Mo(CO)6 Ni(CO)4, Fe(CO)5, Co2(CO)8, Mo(CO)6 Ludwig Mond 1890 1891 1910 ‘Mond nickel company’ was making over 3000 tons of nickel in 1910 with a purity 1839-1909 level of 99.9% Father of Metal Carbonyl Chemistry Founder of Imperial Chemical Industry, England 1890-1930 textbooks  A Brief History of Organometallic Chemistry The Grignard Reagent He was the student of Philippe Barbier (Barbier reaction [Zn]) He discovered the Grignard reaction [Mg] in 1900. He became a professor at the University of Nancy in 1910 and was awarded the Nobel Prize in Chemistry in 1912. François Auguste Victor Grignard 1871-1935  A Brief History of Organometallic Chemistry Hapto ligands and Sandwich compounds The hapto symbol, , with a numerical superscript, provides a topological description by indicating the number of carbon atoms at a bonding distance to the metal Sandwich (5-C5H5)2Fe (6-C6H6)2Cr Bent Sandwich Half Sandwich Triple decker & polycyclic  A Brief History of Organometallic Chemistry Ferrocene: Pathbreaking discovery of a sandwich compound H FeCl3 + CpMgBr Fe Kealy and Pauson H expected fulvalene Pauson Fe + Cp H Miller, Tebboth and Tremaine Fe+2 Kealy H A new type of organo-iron compound, Nature 1951 Dicyclopentadienyl iron, J. Chem. Soc., 1952 Ferrocene Fe 1973 Nobel Prize G. Wilkinson E. O. Fischer R. B. Woodward ‘sandwich compounds’ 1965 Nobel Prize ‘art of organic synthesis’ Wilkinson, Rosenblum, Whitney, Woodward, J. Am. Chem. Soc., 1952  A Brief History of Organometallic Chemistry First organometallics in homogeneous catalysis: The Hydroformylation (1938) R C CH2 H HCo(CO)4 CO, 200 bar, H2 110°C Otto Roelen R Pioneer in Industrial CH CH2 First Industrial plant- hydroformylation homogeneous catalysis (1897-1993) H HC O O H O O O O diethylhexylphthalate [DEHP] Plasticizer detergents  Nobel-Prize Winners Related to the Area and Rural Environment (1) Victor Grignard and Paul Sabatier (1912): Grignard Reagent (2) K. Ziegler, G. Natta (1963): Zieglar-Natta Catalyst (3) E. O. Fisher, G. Wilkinson (1973): Sandwich Compounds (4) K. B. Sharpless, R. Noyori (2001): Hydrogenation and oxidation (5) Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005): Metal- catalyzed alkene metathesis (6) Richard F. Heck, Akira Suzuki, Ei-ichi Negishi (2010): Palladium-catalyzed cross-couplings in organic synthesis. Basic Concepts in Organometallic Chemistry  Chelate Effect ▪ Ligands with more than one donor atom, such as ethylenediamine (NH2CH2CH2NH2, or “en”), can donate both lone pairs to form a chelate ring. ▪ The most favorable ring size is five, but six is often seen.  Chelate Effect ▪ Chelating ligands are much less easily displaced from a complex than are comparable monodentate ligands. Other multidentate Ligand: Pincer Ligand EDTA  The Trans Effect ▪ Pt(II) is four coordinate and adopts a square planar geometry. ▪ These complexes can react with incoming ligands, Li, to replace an existing ligand L in a substitution reaction. ▪ Where a choice exists between two possible geometries of the product, the outcome is governed by the trans effect. The Trans Effect past 5 years Proposal Cisplatin transplatin  The Trans Effect ▪ Ligands, Lt, that are more effective at labilizing a ligand trans to themselves past 5 years have a higher trans effect. Proposal ▪ The highest trans effect ligands either: (i) form strong σ bonds, such as Lt = H− or Me−, or (ii) are strong π acceptors, such as Lt = CO, C2H4, or One of the highest trans effect ligands of all, CF3−, falls into classes (i) and (ii).  The Trans Effect High Trans Effect Ligands Implies: Lengthen and weaken trans M–L bonds, as can be found from X-ray crystallography by an increase in the M–L distance. or In nuclear magnetic resonance (NMR) spectroscopy by a decrease in the M, L coupling. or In the IR (infrared) spectrum by a decrease in the ν(M–L) stretching frequency.  Types of Ligand ▪ Most ligands are Lewis bases and thus typically neutral or anionic, rarely cationic. ▪ Anionic ligands, often represented as X, form polar covalent M–X bonds. ▪ Broadly the ligands can be divided into three categories: (i) Neutral ligands, denoted by L (such as :CO or :NH3): Binds via lone-pair. (ii) π donors (such as C2H4): Bind via donation of a ligand π-bonding electron pair. (iii) σ donors (such as H2): Bind via donation of a ligand σ-bonding electron pair to the metal. 32  Types of Ligand ▪ Side-on binding of σ and π donors results in short bonding distances to two adjacent ligand atoms. ▪ This type of binding is represented as η2-C2H4 or η2-H2, where the letter η (often pronounced eeta) denotes the ligand hapticity, the number of adjacent ligand atoms directly bound to the metal. 34  Ambidentate Ligands ▪ Alternate types of electron pair are sometimes available for bonding. For example, aldehydes have both a C=O π bond and oxygen lone pairs. ▪ As π-bond donors, aldehydes bind side-on like ethylene, but as lone-pair donors, they can alternatively bind end-on. ▪ Thiocyanate, SCN¯, can bind via ‘N’ in a linear fashion, or via ‘S’, in which case the ligand is bent. 36  Ambidentate Ligands The Os(II) prefers to bind to the π acceptor aromatic C=C bond of aniline, not to the nitrogen. 37  Actor and Spectator Ligands ▪ Actor ligands associate, dissociate or react in some way. They are particularly important in catalytic reactions when they bind to the metal and engage in reactions that lead to the release of a product molecule. 40  Actor Ligands ▪ Actor ligands may allow the isolation of a stable material as a precursor to a reactive species only formed after the departure of the actor, that species either being too reactive to isolate or not otherwise easily accessible. ▪ A classic example is chelating 1,5-cyclooctadiene (cod) that binds to Rh(I) or Ir(I) in the [(cod)M(PR3)2]+ hydrogenation catalysts. Under H2, the cod is hydrogenated to free cyclooctane, liberating {M(PR3)2}+ as the active catalyst. 41  Spectator Ligands Spectator ligands remain unchanged during chemical transformations but still play an important role by tuning the properties of the metal to enhance desired characteristics. For example, in the extensive chemistry of [CpFe(CO)2X] and [CpFe(CO)2L]+ (Cp = cyclopentadienyl; X = anion; L = neutral ligand), the [CpFe(CO)2] fragment remains intact. The spectators impart solubility, stabilize Fe(II), and influence the electronic and steric properties of the complex. Apparently small changes in ligand can entirely change the chemistry. 42  Non-innocent Ligands In chemistry, a (redox) non-innocent ligand is a ligand in a metal complex where the oxidation state is not clear. Typically, complexes containing non-innocent ligands are redox active at mild potentials. The concept assumes that redox reactions in metal complexes are either metal or ligand localized, which is a simplification, albeit a useful one. 42  The Ligand Field ▪ The crystal field picture gives a useful qualitative understanding, but for a more complete picture, we turn to the more sophisticated ligand field theory (LFT), really a conventional molecular orbital, or MO, picture. ▪ In this model, we consider the s, the three p, and the five d orbitals of the valence shell of the isolated ion, as well as the six lone-pair orbitals of a set of pure σ-donor ligands in an octahedron around the metal. 30  The Ligand Field 3 orbitals 1 orbital 5 orbitals Total: 9 orbitals Total: 6 orbitals 31  The 18 Electron Rule ▪ Just as organic compounds follow the octet or eight valence electron rule, typical organometallic compounds tend to follow the 18e rule. ▪ This is also known as the noble-gas or effective atomic number (EAN) rule because the metals in an 18e complex achieve the noble-gas configuration. ▪ 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”. ▪ In general, the conditions favoring 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. 44  The 18 Electron Rule: Electron Counting Method Covalent Electron Counting Model: (1) (2) (3) Ionic Electron Counting Model : (1) (3) (2) 44 The 18 Electron Rule: Electron Donation of the Ligand Ligand Neutral Oxidation state Ligand Neutral Oxidation state atom atom Electron Formal Electron Formal contributi charge contribu charge on tion Carbonyl (M–CO) 2 2 0 Halogen ( M–X) 1 2 –1 Phosphine (M–PR3) 2 2 0 Alkyl (M–R) 1 2 –1 Amine (M–NR3 ) 2 2 0 Aryl (M–Ar) 1 2 –1 Amide (M–NR2 ) 1 2 –1 acyl (M–C(O)–R 1 2 –1 Hydrogen (M–H) 1 2 –1 1-cyclopentadienyl 1 2 –1 Alkene (sidewise) 2- 2 2 0 1-allyl 1 2 –1 Alkyne (sidewise) 2- 2 2 0 3-allyl 3 4 –1 2-C60 2 2 0 5-cyclopentadienyl 5 6 –1 Nitrosyl bent 1 2 –1 6-benzene 6 6 0 Nitrosyl linear 3 2 +1 7-cycloheptatrienyl 7 6 +1 Carbene (M=CR2) 2 4 –2 Carbyne (MCR) 3 6 –3 Alkoxide (M–OR) 1 2 –1 Thiolate (M–SR) 1 2 –1 -CO (M–(CO)–M) 2 2 0 -H 1 2 –1 -alkyne 4 4 0 -X (M–X–M) 3 4 –1 X = halogen -alkyl 1 2 –1 -amido 3 4 –1 (M–(NR2)–M -phosphido 3 4 –1 -alkoxide 3 4 –1 (M–(PR2)–M (M–(OR)–M 45 Easy Way to Remember Electron Contribution for Neutral Atom Counting Neutral terminal : CO, PR3, NR3 2 electrons Anionic terminal : X-, H-, R-, Ar-, R2N-, R2P-, RO- 1 electron Hapto ligands : 2-C2R4 2-C2R2, 4-C2R2 ,1-allyl, 3-allyl, 4- Cb, 5-Cp, 6-C6H6 7-C7H7 8-C8H8 2-C60, 5-R5C60 same as hapticity bridging neutral 2-CO, 3-CO 2 electrons Bridging anionic 2-CH3, 2-H ( no lone pairs) 1 electron Bridging anionic 2-Cl, , 2-OR, 2-PR2, 2-NR2 3 electrons (with 1 lone pair) 3-Cl( 2 l.p) 5 electrons Bridging alkyne 4 electrons NO linear 3 electrons NO bent ( l. p on nitrogen) 1 electron Carbene M=C 2 electron Carbyne MC 3 electron 46  The 18 Electron Rule: Examples neutral atom oxidation state method method CO Ru 8 6 (Ru +2) PPh3 Ru 3- allyl 3 4 2 PPh3 4 4 PPh3 CO 2 2 charge -1 not required 16 16 Me N Fe 8 6 (Fe +2) Me Fe 2 5-Cp 10 12 18 18 Neutral atom method: Metal is taken as in zero oxidation state for counting purpose Oxidation state method: We first arrive at the oxidation state of the metal by considering the number of anionic ligands present and overall charge of the complex Suggestion: Focus on one counting method till you are confident 47 The 18 Electron Rule: Examples 48 Metal-Metal Bonding and Electron Counting in Polynuclear Complexes Co2(CO)8 Re2(-Cl)2(CO)8 49  Metal-Metal Bonding and Electron Counting in Polynuclear Complexes TVE divided by M gives the number of electrons per metal. If the number of electrons is 18, it indicates that there is no M–M bond; if it is 17 electrons, it indicates that there is 1 M–M bond; if it is 16 electrons, it indicates that there are 2 M–M bonds and so on. Molecule TVE (18 × M) – Total M–M Bonds per Basic geometry (A) N bonds metal of metal atoms (B) (B/2) (A/M) Fe Fe Fe3(CO)12 48 54 – 48 = 6 6/2 = 3 48/3 = 16; 2 Fe Co4(CO)12 60 72 – 60 = 12 12/2 = 6 60/4 = 15; 3 Co Co Co Co [η5-CpMo(CO)2]2 30 36 – 30 = 6 6/2 = 3 30/2 = 15; 3 Mo≡Mo (4-C4H4)2Fe2(CO)3 30 36 – 30 = 6 6/2 = 3 30/2 = 15; 3 Fe≡Fe 50 Fe2(CO)9 34 36 – 34 = 2 2/2 = 1 34/2 = 17; 1 Fe–Fe  Exceptions of 18-Electron Rule Square planar organometallic complexes of the late transition metals (16e). Some organometallic complexes of the early transition metals (e.g. Cp2TiCl2, WMe6, Me2NbCl3, CpWOCl3). A possible reason for the same is that some of the orbitals of these complexes are too high in energy for effective utilization in bonding or the ligands are mostly  donors. Some high valent d0 complexes have a lower electron count than 18. Sterically demanding bulky ligands force complexes to have less than 18 electrons. The 18-electron rule fails when the bonding of organometallic clusters of moderate to big sizes (6 Metal atoms and above) is considered. The rule is not applicable to organometallic compounds of main group metals as well as to 51 those of lanthanide and actinide metals.  Problem Solving The following organometallic compounds are stable and has a second-row transition metal at its centre. Find out the metal and its oxidation state 51  Metal Carbonyls CO CO CO OC CO Mn(CO)5 and Co(CO)4 dimerizes CO Ni OC Fe Cr CO OC CO OC CO CO CO CO O CO OC CO C CO OC OC OC Mn Mn CO OC Co Co CO OC CO OC CO CO OC C O CO OC CO CO OC CO Ir OC CO Os CO OC CO OC CO Ir Ir OC CO Os Os V(CO)6 does not dimerize Ir CO OC CO CO OC OC CO CO CO  Metal Carbonyls Simplest organometallic compounds, where M-C  bonding is well understood. CO is one of the strongest  acceptor ligands. Back bonding ( bonding) and variation in electronic properties of CO can be monitored very efficiently by Infrared spectroscopy. Hydroformylation Alkene to Aldehyde Methanol to Acetic acid Process R A range of metal carbonyls C CH2 H MeOH + HI MeI + H2O are used as catalysts in CO, HCo(CO)4 CO H3C C I H2 MeI Chemical Industry [Rh(CO)2I2] O R CH CH2 H2O H3C C I H3C C OH H HC O O O  Metal Carbonyls  bond  back bond M C O M C O empty filled filled d empty * p or d orbital orbital orbital orbital Counting the electrons helps to predict the stability of metal carbonyls. But it will not tell you whether a CO is bridging or terminal.  Molecular Orbital of Metal Carbonyls * Why does CO bind a metal through its less electronegative carbon atom than its more electronegative oxygen? * LUMO The highest occupied molecular orbital (HOMO) of CO is 2p weakly antibonding (compared with the O atomic 10.7 ev  orbitals) and is an MO is carbon-based. HOMO What makes CO a good  acceptor? The * antibonding orbital, which is the lowest 2p 15.9 ev unoccupied molecular orbital (LUMO) is also of comparatively lower energy, which makes it possible to  interact with metal t2g orbitals for  bonding. There exists 2s * a strong back bonding of metal electrons to the * 19.5 ev antibonding orbitals of CO.  2s 32.4 ev C CO O