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Basic inorganic chemistry part 2 organometallic chemistry

The document provides an overview of organometallic chemistry including: - Key concepts such as the 18 electron rule, metal carbonyls, and sandwich compounds. - Important discoveries such as Zeise's salt (first transition metal organometallic compound) and ferrocene (first sandwich compound). - Industrial applications of organometallic catalysts in homogeneous catalysis including hydroformylation and hydrocarbon conversions. - Methods for counting electrons in organometallic complexes using the neutral atom and oxidation state methods.

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Basic Inorganic Chemistry For B.Tech & B.Sc I yr Organometallic Chemistry ( 4 lectures) The 18 electron rule, Metal Carbonyls and sandwich compounds, Unique reactions of organometallics and their use in explaining homogeneous catalysis Prepared by Prof. Anil J. Elias, IIT Delhi
Organometallic Chemistry An area which bridges organic and inorganic chemistry A branch of coordination chemistry where the complex has one or more metal-carbon bonds The metal-ligand interactions are mostly  acid type M-C bond can be a  type or  type bond Pb H2 C H3C CH2 H3C H2C H2C CH3 CH3 C C C C M M  donation back donation  from to *  *
Traditional chemists do not agree for classifying metal cyanide complexes as organometallic 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 a main group, transition, lanthanide, or actinide metal atom (or atoms). Following 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. What all compounds are considered as organometallic? C always more electronegative compared to M
Zeise’s Salt- The first transition metal organometallic compound W C Zeise, Danish pharmacist, I789- I847 ‘The breakthrough, the isolation of a pure, crystalline compound came when Zeise added potassium chloride 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 K2PtCl4 + C2H5OH K[(C2H4)PtCl3]. H2O + KCl Also father of the chemistry of mercaptans R-SH Discovery 1827 Structure ~ 150 years later
Frankland coined the term “Organometallic” Edward Frankland 1825-1899 Student of Robert Bunsen (Bunsen burner fame!). Prepared diethyl zinc while trying to make ethyl radicals. 3 C2H5I + 3 Zn  (C2H5)2Zn + C2H5ZnI + ZnI2 First  bonded Organometallic Compound- Diethyl zinc 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.
Metal carbonyls Ludwig Mond 1839-1909 Father of Metal Carbonyl Chemistry Founder of Imperial Chemical Industry, England The Mond process of Nickel purification NiO + H2 ( from Syn gas) 200 °C Impure Ni ( Fe and Co) + H2O excess CO 50 -60 °C Ni(CO)4 (g) bp 42 °C 220- 250 °C Ni(s) + 4 CO Ni(CO)4, Fe(CO)5, Co2(CO)8, Mo(CO)6 1890 1891 1910 1890-1930 textbooks ‘Mond nickel company’ was making over 3000 tons of nickel in 1910 with a purity level of 99.9% Mo(CO)6
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. The Grignard Reagent François Auguste Victor Grignard 1871- 1935
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 Bent Sandwich Half Sandwich Triple decker & polycyclic (5-C5H5)2Fe (6-C6H6)2Cr
Ferrocene: Pathbreaking discovery of a sandwich compound Miller, Tebboth and Tremaine Kealy and Pauson Fe + Cp FeCl3 + CpMgBr Fe H H H H Fe+2 G. Wilkinson E. O. Fischer R. B. Woodward 1973 Nobel Prize ‘sandwich compounds’ 1965 Nobel Prize ‘art of organic synthesis’ A new type of organo-iron compound, Nature 1951 Dicyclopentadienyl iron, J. Chem. Soc., 1952 Pauson Kealy Ferrocene Wilkinson, Rosenblum, Whitney, Woodward, J. Am. Chem. Soc., 1952 Fe expected fulvalene
Ferox Gas & Diesel Fuel Additive is a catalyst that is an eco-friendly fuel additive and horsepower booster. It allegedly increases mileage from between 10 and 20% while also significantly reducing harmful emissions. Ferrocene: Fuel additive, smoke suppressant and chiral catalyst precursor Ferrocene powder Ferrocene crystals
First organometallics in homogeneous catalysis- The Hydroformylation (1938) Otto Roelen Pioneer in Industrial homogeneous catalysis (1897-1993) First Industrial plant- hydroformylation C CH2 R H CO, H2 CH CH2 HC R O H HCo(CO)4 200 bar, 110°C O H O O O O diethylhexylphthalate [DEHP] Plasticizer detergents
Hydrogenation Methanol to acetic acid process Olefin polymerization and oligomerization Organometallic catalysts in industrial synthesis : Three Nobel Prizes 2000, 2005 and 2010 RHC CH2 + H2 RCH2CH3 CH3OH + CO CH3COOH * * n * * n * * n Isotactic polypropylene Syndiotactic polypropylene Atactic polypropylene n C4-C8 40% C10- C18 40 % C20 & > 20 %
18 electron rule : How to count electrons 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 favouring 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 The hapto symbol, , with a numerical superscript, provides a topological description by indicating the connectivity between the ligand and the central atom. For example, if all the five carbon atoms of a cyclopentadienyl moiety are equidistant from a metal atom, we term it as 5-cyclopentadienyl The symbol  indicates bridging normally we have 2 and rarely 3 bridging Examples: 2-CO, 3-CO, 2-CH3, 2-H, 2-Cl, , 3-Cl, 2-OR, 2-PR2, 2-NR2 Examples: 1-R, 1-Ar 2-C2R4 1-allyl, 3-allyl, 4- Cb, 5-Cp, 6-C6H6 8-C8H8 2-C60, 5- R5C60.
Ligand Neutral atom Oxidation state Ligand Neutral atom Oxidation state Electron contributi on Formal charge Electron contribu tion Formal charge 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) X = halogen 3 4 –1 -alkyl 1 2 –1 -amido (M–(NR2)–M 3 4 –1 -phosphido (M–(PR2)–M 3 4 –1 -alkoxide (M–(OR)–M 3 4 –1 Methods of counting: Neutral atom method & Oxidation state method
Ru CO PPh3 PPh3 neutral atom method oxidation state method Ru 8 6 (Ru +2) 3 - allyl 3 4 2 PPh3 4 4 CO 2 2 charge -1 16 16 not required Fe N Me Me Fe 8 6 (Fe +2) 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
Easy way to remember ligand electron contribution for neutral atom counting method Electron contribution 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
Determine the total valence electrons (TVE) in the entire molecule (that is, the number of valence electrons of the metal plus the number of electrons from each ligand and the charge); say, it is A. Subtract this number from n × 18 where n is the number of metals in the complex, that is, (n × 18) – A; say, it is B. (a) B divided by 2 gives the total number of M–M bonds in the complex. (b) A divided by n 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. How to determine the total number of metal - metal bonds Fe Fe Fe Co Co Co Co Molecule TVE (A) (18 × n) – A (B) Total M–M bonds (B/2) Bonds per metal Basic geometry of metal atoms Fe3(CO)12 48 54 – 48 = 6 6/2 = 3 48/3 = 16; 2 Co4(CO)12 60 72 – 60 = 12 12/2 = 6 60/4 = 15; 3 [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 Fe2(CO)9 34 36 – 34 = 2 2/2 = 1 34/2 = 16; 1 Fe–Fe
The following organometallic compounds are stable and has a second row transition metal at its centre. Find out the metal and its oxidation state Problem solving
A few worked out examples Understanding metal –metal bond electron count become easier if you compare and see how octet is attained by each Cl atom of Cl2
• 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 bonding of organometallic clusters of moderate to big sizes (6 Metal atoms and above) are considered. • The rule is not applicable to organometallic compounds of main group metals as well as to those of lanthanide and actinide metals. Exceptions to the 18 electron rule
CO CO Ni OC CO CO OC Fe CO CO CO CO Cr OC OC CO CO CO CO Mn OC OC CO CO OC Mn OC CO CO CO OC Co OC OC OC Co O C CO CO CO C O Os Os Os OC OC CO CO OC CO CO CO CO CO CO Ir Ir Ir Ir OC OC OC OC CO CO CO CO CO CO OC OC Coordination number around the metal normally remains six or lesser. 17 electron species such as Mn(CO)5, Co(CO)4 dimerize to gain 18 electrons V(CO)6 does not dimerize. Metal carbonyls
Why study metal carbonyls ? Simplest of 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 A range of metal carbonyls are used as catalysts in Chemical Industry Hydroformylation Alkene to Aldehyde Methanol to Acetic acid Process MeOH + HI MeI + H2O MeI CO [Rh(CO)2I2] C O I H3C C O I H3C H2O C O OH H3C C CH2 R H CO, H2 CH CH2 HC R O H HCo(CO)4
2s 2s 2p 2p CO O C HOMO LUMO      * *  * 32.4 ev 19.5 ev 15.9 ev 10.7 ev The highest occupied molecular orbital (HOMO) of CO is weakly antibonding (compared with the O atomic orbitals) and is an MO which is carbon based. Secondly, the * antibonding orbital which is the lowest unoccupied molecular orbital (LUMO) is also of comparatively lower energy which makes it possible to interact with metal t2g orbitals for  bonding. There exists a strong back bonding of metal electrons to the  * antibonding orbitals of CO Molecular Orbital diagram of CO Why does CO bind a metal through its less electronegative carbon atom than its more electronegative oxygen ? What makes it a good  acceptor ?
Counting the electrons helps to predict stability of metal carbonyls. But it will not tell you whether a CO is bridging or terminal
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a compound positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Functional groups are identified based on vibrational modes of the groups such a stretching, bending etc. Different vibrational modes absorb characteristic frequencies of IR radiation. An infrared spectrophotometer is an instrument that passes infrared light through a molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. Absorption of radiation lowers the percentage transmittance value. Infrared Spectroscopy- A spectro-analytical tool in chemistry
Infrared Spectroscopy- Spectra of Metal Carbonyls Mn OC OC CO CO OC Mn OC CO CO CO OC Fe OC OC OC Fe O C CO CO CO C O O C The range in which the band appears decides bridging or terminal . The number of bands is only related to the symmetry of the molecule bridging terminal terminal
M C O M M C O terminal bridging  2 M M M C O bridging  3 2120-1850 cm-1  CO 1850-1700 cm-1 1730-1620 cm-1 Cr OC OC CO CO CO CO Fe Fe Fe OC Fe OC CO CO Cp Cp Cp Cp 1620 cm-1 2018, 1826 cm-1 2000 cm-1 Terminal versus bridging carbonyls
Variation in CO (cm–1) of the first row transition metal carbonyls free CO 2143 Ni(CO)4 2057 Co(CO)4 - 1890 Co2(CO)8 2044(av, ter) [Fe(CO)4]2- 1815 Fe(CO)5 2030 [Mn(CO)4]3- 1600,1790 Mn(CO)6 + 2098 Mn2(CO)10 2013 (av) [Cr(CO)4]4- 1462,1657 Cr(CO)6 2000 V(CO)6 ¯ 1860 V(CO)6 1976 Ti(CO)6 2- 1747 As the electron density on a metal centre increases, more -backbonding to the CO ligand(s) takes place. This weakens the C–O bond further as more electron density is pumped into the empty * anti-bonding carbonyl orbital. This increases the M–C bond order and reduces the C-O bond order. That is, the resonance structure M=C=O becomes more dominant. M C O M C O  CO Higher  CO Lower Factors which affect CO stretching frequencies More back bonding 1.Charge on the metal 2. Effect of other ligands
Other spectator ligands: Phosphines PR3 CO, (cm–1) (cm–1)  CO wrt P(t-Bu)3 PR3 CO, (cm–1) (cm–1)  CO wrt P(t-Bu)3 P(t-Bu)3 2056.1 0.0 PPh2(C6F5) 2074.8 18.7 PCy3 2056.4 0.3 P(OEt)3 2076.3 20.2 P(i-Pr)3 2059.2 3.1 P(p-C6H4-CF3)3 2076.6 20.5 PEt3 2061.7 5.6 P(OMe)3 2079.5 23.4 P(NMe2)3 2061.9 5.8 PH3 2083.2 27.1 PMe3 2064.1 8.0 P(OPh)3 2085.3 29.2 PBz3 2066.4 10.3 P(C6F5)3 2090.9 34.8 P(o-Tol)3 2066.6 10.5 PCl3 2097.0 40.9 PPh3 2068.9 12.8 PF3 2110.8 54.7 PPh2H 2073.3 17.2 P(CF3)3 2115.0 58.9 PR3 Ni OC CO CO Lowest CO stretching frequency Most donating phosphine best donor Highest CO stretching frequency Least donating phosphine best  acceptor
Effect of different co-ligands on CO (cm-1) of Mo(CO)3L3 Complex (fac isomers)  CO cm–1 Mo(CO)3(PF3)3 2090, 2055 Mo(CO)3(PCl3)3 2040, 1991 Mo(CO)3[P(OMe)3]3 1977, 1888 Mo(CO)3(PPh3)3 1934, 1835 Mo(CO)3(NCCH3)3 1915, 1783 Mo(CO)3(dien)* 1898, 1758 Mo(CO)3(Py)3 1888, 1746 With each negative charge added to the metal centre, the CO stretching frequency decreases by approximately 100 cm–1. The better the  donating capability of the other ligands on the metal, more electron density given to the metal, more back bonding (electrons in the antibonding orbital of CO) and lower the CO stretching frequency. Mo L L CO CO CO L Effect of a ligands trans to CO More back bonding = More lowering of the C=O bond order = More lower  CO stretching frequency
Synthesis of Metal Carbonyls Direct carbonylation Reductive carbonylation
W(CO)6 + PPh3 h W(CO)5(PPh3) + CO Fe(CO)5 + h Fe(CO)3 + 2CO Reactions of Metal Carbonyls Photochemical substitution Co2(CO)8 + 2Na 2 Na[Co(CO)4] Fe(CO)5 + Na/Hg Na 2Fe(CO)4 Mn2(CO)10 + 2Na 2 Na[Mn(CO)5] Reduction : Carbonyl anions V(CO)6 + Na Na[V(CO)6] Oxidation : Iodocarbonyls Mn2(CO)10 + I2 2 Mn(CO)5I Fe(CO)5 + I2 Fe(CO)4I2 In the presence of UV radiation a monodentate ligand displaces only one CO unit
Reactions of Metal Carbonyls W OC OC CO CO CO OC RLi ether W OC OC CO CO C OC [Me3O]BF4 R OLi W OC OC CO CO C OC R OCH3 Fischer Carbene Nucleophilic addition to CO Fe OC O C C O Fe CO Fe OC C C Fe CO 2 AlEt3 O O Al Al Et Et Et Et Et Et Carbenes are catalysts for olefin metathesis Mn OC OC CO CO Me OC CO high pressure Mn OC OC CO CO C OC O Me Migratory insertion of CO
Mn2(CO)10 + 2 Na 2 NaMn(CO)5 NaMn(CO)5 + CH3I CH3Mn(CO)5 CH3Mn(CO)5 + CO CH3C(O)Mn(CO)5 ( migratory insertion) CH3C(O)Mn(CO)5 + PPh3 hv CH3C(O)Mn(CO)4PPh3 Or at step 3 direct reaction with acyl chloride instead of MeI. Step 1 other reducing agents e.g. AlEt3 can also be used. 2 Mn(OAc)2 + 4 Na + 10 CO Mn2(CO)10 + 4 NaOAc high temp high pressure Give a scheme for the synthesis of Mn(CO)4(PPh3)[C(O)CH3] starting from Manganese acetate, Mn(OAc)2. Problem solving - synthesis
Metal- Sandwich compounds Hapticity of sandwich compounds varies from 1-8
Why metal – sandwich compounds are important? 1. Transition metal/ metal ion embedded inside an organic matrix: Makes a metal ion soluble even in hydrocarbon solvents. E.g. Ferrocene is soluble in hexane while Fe2+ as such is not. Outcome: a hydrocarbon soluble additive/catalyst 2. Coordination to an electropositive metal often changes the reactivity and electronic properties of the  system bound to it (benzene vs ferrocene) 3. A stericially protected metal site where a wide range of catalytic applications are possible on the. e.g alkene polymerization 4. Metal sandwich compounds are excellent substrates to make planar chiral compounds. Applications as chiral catalysts in asymmetric catalysis Fe Y X Fe Y X Planr chirality: Non- super-imposable mirror images
Cyclopentadienyl (Cp) • Cyclopentadienyl (Cp) the most important of all the polyenyl ligands • It gets firmly bound to the metal • generally inert to nucleophilic reagents. • used as a stabilising ligand for many complexes. M M M    most common Least common (5-Cp)(3-Cp)W(CO)2 •Neutral cyclopentadiene (C5H6) is a weak acid with a pKa of around 15 •Deprotonated with strong base or alkali metals to generate the anionic Cp
Synthesis of Cp (C5H5-) based sandwich compounds FeCl2 + 2 C5H6 + 2 Et2NH Cp2Fe + 2 [Et2NH2]Cl RuCl3(H2O)n + 3C5H6 + 3/2 Zn Cp2Ru + C5H8 + 3/2 Zn2+ 2 C5H6 + 2 KOH + Tl2SO4 H2O 2 CpTl + K2SO4 + H2O CpTl + Mn(CO)5Cl CpMn(CO)3 + TlCl + 2 CO (poisonous) H H H H 180°C H H Na 2 NaCp MCl2 + 2 NaCp Cp2M [ M = V, Cr, Mn, Fe, Co] Solvent: THF, DME, Liquid NH3 etc + H2 cracking dicyclopentadiene CpTl based chemistry is not practiced nowadays due to toxicity
Ferrocene: synthesis Fe Fe + 2 (R3NH)Cl FeCl2 + 2 R3N + H2 FeCl2 + 2 C5H6 + 2 R3N Cp2Fe + 2(R3NH)Cl Lab Synthesis FeCl2 + 2 NaCp Cp2Fe
Reactions of Ferrocene Ferrocene undergoes electrophilic substitution reactions. Many of its reactions are faster than similar reactions of benzene Necessary requirement: The electrophile should not be oxidizing in nature Fe Fe I2 I3 The oxidized Cp2Fe+, ferrocenium cation, will repel the electrophile away. Therefore direct nitration, halogenation and similar reactions cannot be carried out on ferrocene. Acetylation 3.3 x 106 times faster than benzene Fe Fe H3C(O)C Fe C(O)CH3 C(O)CH3 90 % 90 % Ac2O/ H3PO4 60 min, 50 °C CH3C(O)Cl AlCl3(1:2:2) Fe C(O)CH3 C(O)CH3 traces FeCp2 + HBF4.OEt2 p- benzoquinone Et2O [FeCp2][BF4] FeCp2 + NH4PF6 H2O/Acetone [FeCp2][PF6] FeCl3
Fe Fe Fe HgCl Hg(OAc)2 Hg(OAc) LiCl Br, I derivatives Br2/I2 Chloromercuration (hazardous) 109 times faster than benzene Fe Fe HCHO/R2NH H2 C H3PO4 NR2 Mannich reaction Does not happen with benzene; only with bromobenzene Lithiation reaction Fe Fe Li Fe Li t-BuLi n-BuLi TMEDA Li N N (3:2 adduct) Does not happen with benzene; only with phenols/anilines
dppf [1]ferrocenophane Lithiation and 1,1’-di-lithiation – access to range of new derivatives Fe Fe Fe HOOC Fe I Cl3Si (BuO) 3 B SiCl4 1/8 S8 Fe SLi Li CO2/H+ I2 H+ Fe (HO)2B NaCN Fe CN
AJELIAS L2-S22 Si Fe Me Me Fe Si Me Me n 130 °C M. Wt: 3.4 X 105 Polymers with ferrocene in the backbone Cr 3 CrCl3 + 2 Al + AlCl3 + 6 C6H6 3 AlCl4 Na2S2O4 KOH Cr Bisbenzene chromium: Prepared by Fischer and Hafner
Problem solving - synthesis Starting from ferrocene show minimum number of steps for preparing 1,1’- ferrocene dicarboxylic acid
Unique reactions in organometallic chemistry • Oxidative Addition • Reductive Elimination • Migratory Insertion •  - Hydrogen Elimination
When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction LnM + XY Ln(X)(Y)M dn dn-2 • availability of nonbonded electron density on the metal, • two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated, • a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. Requirements for oxidative addition Oxidative addition OX state of metal increases by 2 units Coordination number increases by 2 units 2 new anionic ligands are added to the metal Rh Ph3P Ph3P PPh3 Cl Rh Ph3P H PPh3 Cl H PPh3 H2 oxidative addition Rh+1 Rh+3
Examples of Oxidative addition : Cis or trans ? Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans
An important step in many homogeneous catalytic cycles Rh Ph3P Ph3P PPh3 Cl Rh Ph3P H PPh3 Cl H PPh3 H2 oxidative addition Rh+1 Rh+3 Ir I CO I CO Ir I CO I CO CH3 I CH3I Ir+1 Ir+3 Pd PPh3 Ph3P Pd Ph3P Br PPh3 Pd0 Pd+2 Br Hydrogenation of alkenes- Wilkinson catalyst Methanol to acetic acid conversion- Cativa process Pd catalyzed Cross coupling of Ar-B(OH)2 and Ar-X – Suzuki Coupling The more electron rich the metal, more easy is the oxidative addition Often the first step of the mechanism
Oxidative addition involving C-H bonds and cyclo/ortho metallation Agostic interaction This type of reactions help to activate unreactive hydrocarbons such as methane – known as C-H activation
Pt P Ph2 Ph2 P CH3 CH3 CH3 CH3 reductive elimination Pt P Ph2 Ph2 P CH3 CH3 + H3C CH3 165 °C, days Reductive elimination • a high formal positive charge on the metal, • the presence of bulky groups on the metal, and • an electronically stable organic product. Cis orientation of the groups taking part in reductive elimination is a MUST Almost the exact reverse of Oxidative Addition Factors which facilitate reductive elimination Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable  bond and leave the metal Pt4+ Pt2+
Final step in many catalytic cycles Hydroformylation ( conversion of an alkene to an aldehyde) Sonogashira Coupling (coupling of a terminal alkyne to an aryl group Cativa Process (Methanol to Acetic acid)
Migratory Insertion Y M X [M-Y-X] + L Y M L X d n d n Mn OC OC CO CO OC CH3 Mn OC OC C CO OC Ph3P + PPh3 O CH3 No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates
1, 2-migratory insertion Rh CO Ph3P H Ph3P R Rh OC Ph3P CH2CH2R PPh3 Rh OC Ph3P CO PPh3 CH2CH2R Rh OC Ph3P CCH2CH2R PPh3 O 1, 1-migratory insertion M A X B M A B X M X A B M A B X 1, 1 - migratory insertion 1, 2 - migratory insertion Types of Migratory Insertion
Stability of  Bonded alkyl groups as ligands Pt Et3P Br PEt3 Br EtMgBr Pt Et3P H3CH2C PEt3 Br Heat Joseph Chatt 1962 - 68 Pt Et3P H PEt3 Br + CH2 CH2 poor yields unstable Pressure (RCH2CH2)3Cr(THF)3 stable only at -40 °C RCH2CH3 + RCH=CH2 + H2 + Metal hydride RT Stable only at -78 °C CH3CH3CH2CH2Cu[P(n-Bu)3] RT CH3CH2CH2CH3 + CH3CH2CH=CH2 + H2 + Cu + P(n-Bu)3 + H2 + 0.1% octane CrCl3(THF)3 RCH2CH2MgBr + [R = PhCH2] Pt Ph3P Ph3P Pt Ph3P Ph3P + + 60 °C, sealed tube cis PtCl2(PPh3)2 n - BuLi Pt Ph3P Ph3P Why does some  bonded alkyl complexes decompose readily? Pt Ph3P Ph3P CH3 CH3 60 °C sealed tube No decomposition
-Hydride elimination C C H H H M H H C C H H H M H H C C H H H M H H Beta-hydride elimination is a reaction in which an alkyl group having a  hydrogen,  bonded to a metal centre is converted into the corresponding metal-bonded hydride and a  bonded alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. mechanism Can either be a vital step in a reaction or an unwanted side reaction No change in the formal oxidation state of the metal
-hydrogen elimination does not happen when • the alkyl has no -hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2) • (ii) the -hydrogen on the alkyl is unable to approach the metal (as in C≡CH) • the M–C–C–H unit cannot become coplanar Pt Et3P Et3P C C C C H H B Pt Ph3P Ph3P C Pt Ph3P Ph3P D Pt Ph3P Ph3P A SiMe3 SiMe3 Select the most unstable platinum  complex from the given list. Justify your answer No -H -H unable to MCCH unit will not be approach Mcoplanar
Classify the following reactions as oxidative addition, reductive elimination, (1,1 / 1,2)migratory insertion, - H elimination, ligand coordination change or simple addition (a) [RhI3(CO)2CH3]  {RhI3(CO)( solvent)[C(O)CH3]} (b) Ir(PPh2Me)2(CO)Cl + CF3I  Ir(I)(CF3)(PPh2Me)2(CO)Cl (c) TiCl4 +2 Et3N  TiCl4 (NEt3)2 (d) HCo(CO)3(CH2=CHCH3) + CO  CH3CH2CH2Co(CO)4 Problem solving Step 1. determine the oxidation state of the metal in reactant and product Step 2. count the electrons for reactant and product Step 3. see if any ligand in the reactant has undergone change
Homogeneous catalysis using organometallic Catalysts uncatalyzed catalyzed Reactants Products Gibbs Energy stable intermediate Gibbs energy of activation A catalyst typically increases the reaction rates by lowering the activation energy by opening up pathways with lower Gibbs free energies of activation (G). Heterogeneous Homogeneous
Homogeneous versus Heterogeneous Catalysis Parameter Heterogeneous Homogeneous Phase Gas/solid Usually liquid/ or solid soluble in the reactants Required temperature High Low ( less than 250°C) Catalyst Activity Low High Product selectivity Less (often mixtures) More Catalyst recycling Simple and cost effective Expensive and complex Reaction mechanism Poorly understood Reasonably well understood Product separation from catalyst Easy Elaborate and sometimes problematic Fine tuning of catalyst Difficult Easy
Heterogeneous Catalyst- Catalytic Converter of a Car Platinum and Rhodium Platinum and Palladium Chemistry at the molecular level – Poorly understood Home assignment : See Youtube video ‘Catalysis’
Comparing different catalysts; Catalyst life and Catalyst efficiency TON is defined as the amount of reactant (in moles) divided by the amount of catalyst (in moles) times the percentage yield of product. A large TON indicates a stable catalyst with a long life. It is the number of passes through the catalytic cycle per unit time (often per hour). Effectively this is dividing the TON by the time taken for the reaction. The units are just time–1 . A higher TOF indicates better efficiency for the catalyst Turnover Number (TON) Turnover Frequency (TOF) AJELIAS L7-S16
Wilkinson’s Catalyst for alkene hydrogenation RhCl3 (H2O)3 + CH3CH2OH + 3 PPh3 RhCl(PPh3)3 + CH3CHO + 2HCl + 3H2O Wilkinson’s catalyst: The first example of an effective and rapid homogeneous catalyst for hydrogenation of alkenes, active at room temperature and atmospheric pressure. Square planar 16 electron d8 complex (Ph3P)3RhCl Discovered by G Wilkinson as well as by R Coffey almost at the same time (1964–65) AJELIAS L7-S17
Conventional Catalytic cycle for hydrogenation with Wilkinson’s catalyst Rh P P P Cl H Rh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition 1, 2 -migratory insertion reductive elimination R CH2 R RCH2CH3 P P alkene coordination R P = PPh3 14e The first step of this catalytic cycle is the cleavage of a PPh3 to generate the active form of the catalyst followed by oxidative addition of dihydrogen.
Conventional Catalytic cycle for hydrogenation with Wilkinson’s catalyst Rh P P P Cl H Rh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition 1, 2 -migratory insertion reductive elimination R CH2 R RCH2CH3 P P alkene coordination R P = PPh3 14e The first step of this catalytic cycle is the cleavage of a PPh3 to generate the active form of the catalyst followed by oxidative addition of dihydrogen.
Rh P P P Cl Rh P H P Cl H P H Rh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition H2 oxidative addition 1, 2 -migratory insertion reductive elimination P (due to trans effect of H ) R CH2 R RCH2CH3 P P alkene catalytic cycle for hydrogenation Kinetic studies have shown that the dissociation of PPh3 from the distorted square planar complex RhCl(PPh3)3 in benzene occurs only to a very small extent (k = 2.3 × 10–7 M at 25°C), and under an atmosphere of H2, a solution of RhCl(PPh3)3 becomes yellow as a result of the oxidative addition of H2 to give cis- H2RhCl(PPh3)3. The trans effect is the labilization (making unstable) of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence: H2O, OH− < NH3 < py < Cl− < Br− < I−, < PR3, CH3− < H−, NO, CO
R > > > > > R R R R R R R R R > R R R R • Cis alkenes undergo hydrogenation more readily than trans alkenes •Internal and branched alkenes undergo hydrogenation more slowly than terminal ones, and Relative reactivity of alkenes for homogenous catalytic hydrogenation
Catalyst 25°C, 1 atm H2 Turnover frequency (TOF) in h–1 for hydrogenation of alkenes Wilkinson’s catalyst 650 700 13 NA Schrock–Osborn catalyst 4000 10 NA NA Crabtree’s catalyst 6400 4500 3800 4000 Rh Ph3P Ph3P PPh3 Cl Rh PPh3 PPh3 + PF6 Schrock-Osborn's catalyst Ir PCy3 N + PF6 Crabtree's catalyst Wilkinson's catalyst Fine tuning of a catalyst: hydrogenation catalysts which are more efficient than Wilkinsons catalyst The cationic metal center is relatively more electrophilic than neutral metal center and thus favours alkene coordination.
Hydrogenation with Crabtree’s catalyst The di-solvated form of the active catalyst generated by the removal of COD [after it gets hydrogenated and leaves] favors coordination of sterically bulky alkenes as well. Ir PCy3 N PF6 Ir PCy3 N PF6 H H 16e 18e Ir PCy3 N PF6 H 16e Ir S PCy3 N PF6 16e Ir S S PCy3 N PF6 16e oxidative addition migratory insertion   reductive elimination solvent coordination repeat of cycle with cyclooctene di-solvated active form of catalyst H2 This mechanism is only for understanding not for the exam
Factors which have been found to improve the efficiency (better TOF) of transition metal catalysts for hydrogenation • Making a cationic metal center : makes catalyst electrophillic for alkene coordination • Use of ligands (eg. Cyclooctadiene) which will leave at the initial stages of the cycle generating a di-solvated active catalyst : facilitates binding of even sterically hindered alkenes • Use of chelating biphosphines: Cis enforcing: reduces steric hindrance at the metal centre Cis enforcing
Problem solving- fill in the blanks 1,2 Migr. Insertion 1,1 Migr. Insertion Oxidative addition

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In this document
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Introduction to organometallic chemistry, focusing on the metal-carbon bond, classifications, and definitions.

Pioneers like Zeise and Frankland in organometallic chemistry were crucial in defining compounds and their properties.

The significance of metal carbonyls, their discovery, and key reactions notably involving Ludwig Mond and related processes.

Introduction of hapto-ligands and sandwich compounds, highlighting the structural aspects through examples like ferrocene.

Ferrocene's properties as a fuel additive, its utility, and its historical context in homogeneous catalysis.

Detailed explanation of the 18-electron rule for stability in metal complexes and methods for electron counting.

Discussion of exceptions to the 18-electron rule in various transition metal complexes.

Metal carbonyls as catalysts in significant chemical processes are emphasized, particularly in spectroscopic and industrial contexts.

Molecular orbital diagrams explaining CO bonding with metals and electron counting limitations.

Infrared spectroscopy utilized to analyze metal carbonyls and interpret bridging vs terminal carbonyls.

The influence of metal charge and co-ligands on the stretching frequencies of CO in complexes.

Methods of synthesizing metal carbonyls and their various reactions including implications in complex synthesis.

Characteristics and importance of metal sandwich compounds, particularly in catalytic applications.

Details on the synthesis of ferrocene and its reactivity in various chemical transformations.

Key reactions such as oxidative addition, reductive elimination, and their roles in catalytic cycles.

Exploration of catalysis involving organometallic catalysts, focusing on transition metal mechanisms.

Comparison between homogeneous and heterogeneous catalysis in terms of efficiency, selectivity, and challenges.

Metrics such as turnover number (TON) and turnover frequency (TOF) to evaluate catalyst performance.

Factors enhancing catalyst efficiency in hydrogenation reactions and related problem-solving approaches.

Basic inorganic chemistry part 2 organometallic chemistry

  • 1.
    Basic Inorganic ChemistryFor B.Tech & B.Sc I yr Organometallic Chemistry ( 4 lectures) The 18 electron rule, Metal Carbonyls and sandwich compounds, Unique reactions of organometallics and their use in explaining homogeneous catalysis Prepared by Prof. Anil J. Elias, IIT Delhi
  • 2.
    Organometallic Chemistry An areawhich bridges organic and inorganic chemistry A branch of coordination chemistry where the complex has one or more metal-carbon bonds The metal-ligand interactions are mostly  acid type M-C bond can be a  type or  type bond Pb H2 C H3C CH2 H3C H2C H2C CH3 CH3 C C C C M M  donation back donation  from to *  *
  • 3.
    Traditional chemists donot agree for classifying metal cyanide complexes as organometallic 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 a main group, transition, lanthanide, or actinide metal atom (or atoms). Following 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. What all compounds are considered as organometallic? C always more electronegative compared to M
  • 4.
    Zeise’s Salt- Thefirst transition metal organometallic compound W C Zeise, Danish pharmacist, I789- I847 ‘The breakthrough, the isolation of a pure, crystalline compound came when Zeise added potassium chloride 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 K2PtCl4 + C2H5OH K[(C2H4)PtCl3]. H2O + KCl Also father of the chemistry of mercaptans R-SH Discovery 1827 Structure ~ 150 years later
  • 5.
    Frankland coined the term “Organometallic” EdwardFrankland 1825-1899 Student of Robert Bunsen (Bunsen burner fame!). Prepared diethyl zinc while trying to make ethyl radicals. 3 C2H5I + 3 Zn  (C2H5)2Zn + C2H5ZnI + ZnI2 First  bonded Organometallic Compound- Diethyl zinc 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.
  • 6.
    Metal carbonyls Ludwig Mond1839-1909 Father of Metal Carbonyl Chemistry Founder of Imperial Chemical Industry, England The Mond process of Nickel purification NiO + H2 ( from Syn gas) 200 °C Impure Ni ( Fe and Co) + H2O excess CO 50 -60 °C Ni(CO)4 (g) bp 42 °C 220- 250 °C Ni(s) + 4 CO Ni(CO)4, Fe(CO)5, Co2(CO)8, Mo(CO)6 1890 1891 1910 1890-1930 textbooks ‘Mond nickel company’ was making over 3000 tons of nickel in 1910 with a purity level of 99.9% Mo(CO)6
  • 7.
    He was thestudent 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. The Grignard Reagent François Auguste Victor Grignard 1871- 1935
  • 8.
    Hapto ligands andSandwich 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 Bent Sandwich Half Sandwich Triple decker & polycyclic (5-C5H5)2Fe (6-C6H6)2Cr
  • 9.
    Ferrocene: Pathbreaking discoveryof a sandwich compound Miller, Tebboth and Tremaine Kealy and Pauson Fe + Cp FeCl3 + CpMgBr Fe H H H H Fe+2 G. Wilkinson E. O. Fischer R. B. Woodward 1973 Nobel Prize ‘sandwich compounds’ 1965 Nobel Prize ‘art of organic synthesis’ A new type of organo-iron compound, Nature 1951 Dicyclopentadienyl iron, J. Chem. Soc., 1952 Pauson Kealy Ferrocene Wilkinson, Rosenblum, Whitney, Woodward, J. Am. Chem. Soc., 1952 Fe expected fulvalene
  • 10.
    Ferox Gas &Diesel Fuel Additive is a catalyst that is an eco-friendly fuel additive and horsepower booster. It allegedly increases mileage from between 10 and 20% while also significantly reducing harmful emissions. Ferrocene: Fuel additive, smoke suppressant and chiral catalyst precursor Ferrocene powder Ferrocene crystals
  • 11.
    First organometallics inhomogeneous catalysis- The Hydroformylation (1938) Otto Roelen Pioneer in Industrial homogeneous catalysis (1897-1993) First Industrial plant- hydroformylation C CH2 R H CO, H2 CH CH2 HC R O H HCo(CO)4 200 bar, 110°C O H O O O O diethylhexylphthalate [DEHP] Plasticizer detergents
  • 12.
    Hydrogenation Methanol to aceticacid process Olefin polymerization and oligomerization Organometallic catalysts in industrial synthesis : Three Nobel Prizes 2000, 2005 and 2010 RHC CH2 + H2 RCH2CH3 CH3OH + CO CH3COOH * * n * * n * * n Isotactic polypropylene Syndiotactic polypropylene Atactic polypropylene n C4-C8 40% C10- C18 40 % C20 & > 20 %
  • 13.
    18 electron rule: How to count electrons 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 favouring 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 The hapto symbol, , with a numerical superscript, provides a topological description by indicating the connectivity between the ligand and the central atom. For example, if all the five carbon atoms of a cyclopentadienyl moiety are equidistant from a metal atom, we term it as 5-cyclopentadienyl The symbol  indicates bridging normally we have 2 and rarely 3 bridging Examples: 2-CO, 3-CO, 2-CH3, 2-H, 2-Cl, , 3-Cl, 2-OR, 2-PR2, 2-NR2 Examples: 1-R, 1-Ar 2-C2R4 1-allyl, 3-allyl, 4- Cb, 5-Cp, 6-C6H6 8-C8H8 2-C60, 5- R5C60.
  • 14.
    Ligand Neutral atom Oxidation stateLigand Neutral atom Oxidation state Electron contributi on Formal charge Electron contribu tion Formal charge 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) X = halogen 3 4 –1 -alkyl 1 2 –1 -amido (M–(NR2)–M 3 4 –1 -phosphido (M–(PR2)–M 3 4 –1 -alkoxide (M–(OR)–M 3 4 –1 Methods of counting: Neutral atom method & Oxidation state method
  • 15.
    Ru CO PPh3 PPh3 neutral atom method oxidation state method Ru8 6 (Ru +2) 3 - allyl 3 4 2 PPh3 4 4 CO 2 2 charge -1 16 16 not required Fe N Me Me Fe 8 6 (Fe +2) 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
  • 16.
    Easy way toremember ligand electron contribution for neutral atom counting method Electron contribution 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
  • 17.
    Determine the totalvalence electrons (TVE) in the entire molecule (that is, the number of valence electrons of the metal plus the number of electrons from each ligand and the charge); say, it is A. Subtract this number from n × 18 where n is the number of metals in the complex, that is, (n × 18) – A; say, it is B. (a) B divided by 2 gives the total number of M–M bonds in the complex. (b) A divided by n 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. How to determine the total number of metal - metal bonds Fe Fe Fe Co Co Co Co Molecule TVE (A) (18 × n) – A (B) Total M–M bonds (B/2) Bonds per metal Basic geometry of metal atoms Fe3(CO)12 48 54 – 48 = 6 6/2 = 3 48/3 = 16; 2 Co4(CO)12 60 72 – 60 = 12 12/2 = 6 60/4 = 15; 3 [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 Fe2(CO)9 34 36 – 34 = 2 2/2 = 1 34/2 = 16; 1 Fe–Fe
  • 18.
    The following organometalliccompounds are stable and has a second row transition metal at its centre. Find out the metal and its oxidation state Problem solving
  • 19.
    A few workedout examples Understanding metal –metal bond electron count become easier if you compare and see how octet is attained by each Cl atom of Cl2
  • 20.
    • Square planarorganometallic 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 bonding of organometallic clusters of moderate to big sizes (6 Metal atoms and above) are considered. • The rule is not applicable to organometallic compounds of main group metals as well as to those of lanthanide and actinide metals. Exceptions to the 18 electron rule
  • 21.
    CO CO Ni OC CO CO OC Fe CO CO CO CO Cr OC OC CO CO CO CO Mn OC OC CO CO OC Mn OC CO CO CO OC Co OC OC OC Co O C CO CO CO C O Os OsOs OC OC CO CO OC CO CO CO CO CO CO Ir Ir Ir Ir OC OC OC OC CO CO CO CO CO CO OC OC Coordination number around the metal normally remains six or lesser. 17 electron species such as Mn(CO)5, Co(CO)4 dimerize to gain 18 electrons V(CO)6 does not dimerize. Metal carbonyls
  • 22.
    Why study metalcarbonyls ? Simplest of 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 A range of metal carbonyls are used as catalysts in Chemical Industry Hydroformylation Alkene to Aldehyde Methanol to Acetic acid Process MeOH + HI MeI + H2O MeI CO [Rh(CO)2I2] C O I H3C C O I H3C H2O C O OH H3C C CH2 R H CO, H2 CH CH2 HC R O H HCo(CO)4
  • 23.
    2s 2s 2p 2p CO O C HOMO LUMO      * *  * 32.4ev 19.5 ev 15.9 ev 10.7 ev The highest occupied molecular orbital (HOMO) of CO is weakly antibonding (compared with the O atomic orbitals) and is an MO which is carbon based. Secondly, the * antibonding orbital which is the lowest unoccupied molecular orbital (LUMO) is also of comparatively lower energy which makes it possible to interact with metal t2g orbitals for  bonding. There exists a strong back bonding of metal electrons to the  * antibonding orbitals of CO Molecular Orbital diagram of CO Why does CO bind a metal through its less electronegative carbon atom than its more electronegative oxygen ? What makes it a good  acceptor ?
  • 24.
    Counting the electronshelps to predict stability of metal carbonyls. But it will not tell you whether a CO is bridging or terminal
  • 25.
    Infrared (IR) spectroscopyis one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a compound positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Functional groups are identified based on vibrational modes of the groups such a stretching, bending etc. Different vibrational modes absorb characteristic frequencies of IR radiation. An infrared spectrophotometer is an instrument that passes infrared light through a molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. Absorption of radiation lowers the percentage transmittance value. Infrared Spectroscopy- A spectro-analytical tool in chemistry
  • 26.
    Infrared Spectroscopy- Spectraof Metal Carbonyls Mn OC OC CO CO OC Mn OC CO CO CO OC Fe OC OC OC Fe O C CO CO CO C O O C The range in which the band appears decides bridging or terminal . The number of bands is only related to the symmetry of the molecule bridging terminal terminal
  • 27.
    M C O M M C O terminal bridging 2 M M M C O bridging  3 2120-1850 cm-1  CO 1850-1700 cm-1 1730-1620 cm-1 Cr OC OC CO CO CO CO Fe Fe Fe OC Fe OC CO CO Cp Cp Cp Cp 1620 cm-1 2018, 1826 cm-1 2000 cm-1 Terminal versus bridging carbonyls
  • 28.
    Variation in CO(cm–1) of the first row transition metal carbonyls free CO 2143 Ni(CO)4 2057 Co(CO)4 - 1890 Co2(CO)8 2044(av, ter) [Fe(CO)4]2- 1815 Fe(CO)5 2030 [Mn(CO)4]3- 1600,1790 Mn(CO)6 + 2098 Mn2(CO)10 2013 (av) [Cr(CO)4]4- 1462,1657 Cr(CO)6 2000 V(CO)6 ¯ 1860 V(CO)6 1976 Ti(CO)6 2- 1747 As the electron density on a metal centre increases, more -backbonding to the CO ligand(s) takes place. This weakens the C–O bond further as more electron density is pumped into the empty * anti-bonding carbonyl orbital. This increases the M–C bond order and reduces the C-O bond order. That is, the resonance structure M=C=O becomes more dominant. M C O M C O  CO Higher  CO Lower Factors which affect CO stretching frequencies More back bonding 1.Charge on the metal 2. Effect of other ligands
  • 29.
    Other spectator ligands:Phosphines PR3 CO, (cm–1) (cm–1)  CO wrt P(t-Bu)3 PR3 CO, (cm–1) (cm–1)  CO wrt P(t-Bu)3 P(t-Bu)3 2056.1 0.0 PPh2(C6F5) 2074.8 18.7 PCy3 2056.4 0.3 P(OEt)3 2076.3 20.2 P(i-Pr)3 2059.2 3.1 P(p-C6H4-CF3)3 2076.6 20.5 PEt3 2061.7 5.6 P(OMe)3 2079.5 23.4 P(NMe2)3 2061.9 5.8 PH3 2083.2 27.1 PMe3 2064.1 8.0 P(OPh)3 2085.3 29.2 PBz3 2066.4 10.3 P(C6F5)3 2090.9 34.8 P(o-Tol)3 2066.6 10.5 PCl3 2097.0 40.9 PPh3 2068.9 12.8 PF3 2110.8 54.7 PPh2H 2073.3 17.2 P(CF3)3 2115.0 58.9 PR3 Ni OC CO CO Lowest CO stretching frequency Most donating phosphine best donor Highest CO stretching frequency Least donating phosphine best  acceptor
  • 30.
    Effect of differentco-ligands on CO (cm-1) of Mo(CO)3L3 Complex (fac isomers)  CO cm–1 Mo(CO)3(PF3)3 2090, 2055 Mo(CO)3(PCl3)3 2040, 1991 Mo(CO)3[P(OMe)3]3 1977, 1888 Mo(CO)3(PPh3)3 1934, 1835 Mo(CO)3(NCCH3)3 1915, 1783 Mo(CO)3(dien)* 1898, 1758 Mo(CO)3(Py)3 1888, 1746 With each negative charge added to the metal centre, the CO stretching frequency decreases by approximately 100 cm–1. The better the  donating capability of the other ligands on the metal, more electron density given to the metal, more back bonding (electrons in the antibonding orbital of CO) and lower the CO stretching frequency. Mo L L CO CO CO L Effect of a ligands trans to CO More back bonding = More lowering of the C=O bond order = More lower  CO stretching frequency
  • 31.
    Synthesis of MetalCarbonyls Direct carbonylation Reductive carbonylation
  • 32.
    W(CO)6 + PPh3 hW(CO)5(PPh3) + CO Fe(CO)5 + h Fe(CO)3 + 2CO Reactions of Metal Carbonyls Photochemical substitution Co2(CO)8 + 2Na 2 Na[Co(CO)4] Fe(CO)5 + Na/Hg Na 2Fe(CO)4 Mn2(CO)10 + 2Na 2 Na[Mn(CO)5] Reduction : Carbonyl anions V(CO)6 + Na Na[V(CO)6] Oxidation : Iodocarbonyls Mn2(CO)10 + I2 2 Mn(CO)5I Fe(CO)5 + I2 Fe(CO)4I2 In the presence of UV radiation a monodentate ligand displaces only one CO unit
  • 33.
    Reactions of MetalCarbonyls W OC OC CO CO CO OC RLi ether W OC OC CO CO C OC [Me3O]BF4 R OLi W OC OC CO CO C OC R OCH3 Fischer Carbene Nucleophilic addition to CO Fe OC O C C O Fe CO Fe OC C C Fe CO 2 AlEt3 O O Al Al Et Et Et Et Et Et Carbenes are catalysts for olefin metathesis Mn OC OC CO CO Me OC CO high pressure Mn OC OC CO CO C OC O Me Migratory insertion of CO
  • 34.
    Mn2(CO)10 + 2Na 2 NaMn(CO)5 NaMn(CO)5 + CH3I CH3Mn(CO)5 CH3Mn(CO)5 + CO CH3C(O)Mn(CO)5 ( migratory insertion) CH3C(O)Mn(CO)5 + PPh3 hv CH3C(O)Mn(CO)4PPh3 Or at step 3 direct reaction with acyl chloride instead of MeI. Step 1 other reducing agents e.g. AlEt3 can also be used. 2 Mn(OAc)2 + 4 Na + 10 CO Mn2(CO)10 + 4 NaOAc high temp high pressure Give a scheme for the synthesis of Mn(CO)4(PPh3)[C(O)CH3] starting from Manganese acetate, Mn(OAc)2. Problem solving - synthesis
  • 35.
    Metal- Sandwich compounds Hapticityof sandwich compounds varies from 1-8
  • 36.
    Why metal –sandwich compounds are important? 1. Transition metal/ metal ion embedded inside an organic matrix: Makes a metal ion soluble even in hydrocarbon solvents. E.g. Ferrocene is soluble in hexane while Fe2+ as such is not. Outcome: a hydrocarbon soluble additive/catalyst 2. Coordination to an electropositive metal often changes the reactivity and electronic properties of the  system bound to it (benzene vs ferrocene) 3. A stericially protected metal site where a wide range of catalytic applications are possible on the. e.g alkene polymerization 4. Metal sandwich compounds are excellent substrates to make planar chiral compounds. Applications as chiral catalysts in asymmetric catalysis Fe Y X Fe Y X Planr chirality: Non- super-imposable mirror images
  • 37.
    Cyclopentadienyl (Cp) • Cyclopentadienyl(Cp) the most important of all the polyenyl ligands • It gets firmly bound to the metal • generally inert to nucleophilic reagents. • used as a stabilising ligand for many complexes. M M M    most common Least common (5-Cp)(3-Cp)W(CO)2 •Neutral cyclopentadiene (C5H6) is a weak acid with a pKa of around 15 •Deprotonated with strong base or alkali metals to generate the anionic Cp
  • 38.
    Synthesis of Cp(C5H5-) based sandwich compounds FeCl2 + 2 C5H6 + 2 Et2NH Cp2Fe + 2 [Et2NH2]Cl RuCl3(H2O)n + 3C5H6 + 3/2 Zn Cp2Ru + C5H8 + 3/2 Zn2+ 2 C5H6 + 2 KOH + Tl2SO4 H2O 2 CpTl + K2SO4 + H2O CpTl + Mn(CO)5Cl CpMn(CO)3 + TlCl + 2 CO (poisonous) H H H H 180°C H H Na 2 NaCp MCl2 + 2 NaCp Cp2M [ M = V, Cr, Mn, Fe, Co] Solvent: THF, DME, Liquid NH3 etc + H2 cracking dicyclopentadiene CpTl based chemistry is not practiced nowadays due to toxicity
  • 39.
    Ferrocene: synthesis Fe Fe +2 (R3NH)Cl FeCl2 + 2 R3N + H2 FeCl2 + 2 C5H6 + 2 R3N Cp2Fe + 2(R3NH)Cl Lab Synthesis FeCl2 + 2 NaCp Cp2Fe
  • 40.
    Reactions of Ferrocene Ferroceneundergoes electrophilic substitution reactions. Many of its reactions are faster than similar reactions of benzene Necessary requirement: The electrophile should not be oxidizing in nature Fe Fe I2 I3 The oxidized Cp2Fe+, ferrocenium cation, will repel the electrophile away. Therefore direct nitration, halogenation and similar reactions cannot be carried out on ferrocene. Acetylation 3.3 x 106 times faster than benzene Fe Fe H3C(O)C Fe C(O)CH3 C(O)CH3 90 % 90 % Ac2O/ H3PO4 60 min, 50 °C CH3C(O)Cl AlCl3(1:2:2) Fe C(O)CH3 C(O)CH3 traces FeCp2 + HBF4.OEt2 p- benzoquinone Et2O [FeCp2][BF4] FeCp2 + NH4PF6 H2O/Acetone [FeCp2][PF6] FeCl3
  • 41.
    Fe Fe Fe HgCl Hg(OAc)2 Hg(OAc) LiCl Br, Iderivatives Br2/I2 Chloromercuration (hazardous) 109 times faster than benzene Fe Fe HCHO/R2NH H2 C H3PO4 NR2 Mannich reaction Does not happen with benzene; only with bromobenzene Lithiation reaction Fe Fe Li Fe Li t-BuLi n-BuLi TMEDA Li N N (3:2 adduct) Does not happen with benzene; only with phenols/anilines
  • 42.
    dppf [1]ferrocenophane Lithiation and 1,1’-di-lithiation– access to range of new derivatives Fe Fe Fe HOOC Fe I Cl3Si (BuO) 3 B SiCl4 1/8 S8 Fe SLi Li CO2/H+ I2 H+ Fe (HO)2B NaCN Fe CN
  • 43.
    AJELIAS L2-S22 Si Fe Me Me Fe Si Me Me n 130°C M. Wt: 3.4 X 105 Polymers with ferrocene in the backbone Cr 3 CrCl3 + 2 Al + AlCl3 + 6 C6H6 3 AlCl4 Na2S2O4 KOH Cr Bisbenzene chromium: Prepared by Fischer and Hafner
  • 44.
    Problem solving -synthesis Starting from ferrocene show minimum number of steps for preparing 1,1’- ferrocene dicarboxylic acid
  • 45.
    Unique reactions inorganometallic chemistry • Oxidative Addition • Reductive Elimination • Migratory Insertion •  - Hydrogen Elimination
  • 46.
    When addition ofligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction LnM + XY Ln(X)(Y)M dn dn-2 • availability of nonbonded electron density on the metal, • two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated, • a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable. Requirements for oxidative addition Oxidative addition OX state of metal increases by 2 units Coordination number increases by 2 units 2 new anionic ligands are added to the metal Rh Ph3P Ph3P PPh3 Cl Rh Ph3P H PPh3 Cl H PPh3 H2 oxidative addition Rh+1 Rh+3
  • 47.
    Examples of Oxidativeaddition : Cis or trans ? Homonuclear systems (H2, Cl2, O2, C2H2) Cis Heteronuclear systems (MeI) Cis or trans
  • 48.
    An important stepin many homogeneous catalytic cycles Rh Ph3P Ph3P PPh3 Cl Rh Ph3P H PPh3 Cl H PPh3 H2 oxidative addition Rh+1 Rh+3 Ir I CO I CO Ir I CO I CO CH3 I CH3I Ir+1 Ir+3 Pd PPh3 Ph3P Pd Ph3P Br PPh3 Pd0 Pd+2 Br Hydrogenation of alkenes- Wilkinson catalyst Methanol to acetic acid conversion- Cativa process Pd catalyzed Cross coupling of Ar-B(OH)2 and Ar-X – Suzuki Coupling The more electron rich the metal, more easy is the oxidative addition Often the first step of the mechanism
  • 49.
    Oxidative addition involvingC-H bonds and cyclo/ortho metallation Agostic interaction This type of reactions help to activate unreactive hydrocarbons such as methane – known as C-H activation
  • 50.
    Pt P Ph2 Ph2 P CH3 CH3 CH3 CH3 reductive elimination Pt P Ph2 Ph2 PCH3 CH3 + H3C CH3 165 °C, days Reductive elimination • a high formal positive charge on the metal, • the presence of bulky groups on the metal, and • an electronically stable organic product. Cis orientation of the groups taking part in reductive elimination is a MUST Almost the exact reverse of Oxidative Addition Factors which facilitate reductive elimination Oxidation state of metal decreases by 2 units Coordination number decreases by 2 units 2 cis oriented anionic ligands form a stable  bond and leave the metal Pt4+ Pt2+
  • 51.
    Final step inmany catalytic cycles Hydroformylation ( conversion of an alkene to an aldehyde) Sonogashira Coupling (coupling of a terminal alkyne to an aryl group Cativa Process (Methanol to Acetic acid)
  • 52.
    Migratory Insertion Y M X [M-Y-X] + L Y M L X dn d n Mn OC OC CO CO OC CH3 Mn OC OC C CO OC Ph3P + PPh3 O CH3 No change in the formal oxidation state of the metal A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand) The groups undergoing migratory insertion must be cis to one another These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates
  • 53.
    1, 2-migratory insertion Rh CO Ph3P H Ph3P R Rh OC Ph3P CH2CH2R PPh3 Rh OC Ph3P CO PPh3 CH2CH2R Rh OC Ph3PCCH2CH2R PPh3 O 1, 1-migratory insertion M A X B M A B X M X A B M A B X 1, 1 - migratory insertion 1, 2 - migratory insertion Types of Migratory Insertion
  • 54.
    Stability of Bonded alkyl groups as ligands Pt Et3P Br PEt3 Br EtMgBr Pt Et3P H3CH2C PEt3 Br Heat Joseph Chatt 1962 - 68 Pt Et3P H PEt3 Br + CH2 CH2 poor yields unstable Pressure (RCH2CH2)3Cr(THF)3 stable only at -40 °C RCH2CH3 + RCH=CH2 + H2 + Metal hydride RT Stable only at -78 °C CH3CH3CH2CH2Cu[P(n-Bu)3] RT CH3CH2CH2CH3 + CH3CH2CH=CH2 + H2 + Cu + P(n-Bu)3 + H2 + 0.1% octane CrCl3(THF)3 RCH2CH2MgBr + [R = PhCH2] Pt Ph3P Ph3P Pt Ph3P Ph3P + + 60 °C, sealed tube cis PtCl2(PPh3)2 n - BuLi Pt Ph3P Ph3P Why does some  bonded alkyl complexes decompose readily? Pt Ph3P Ph3P CH3 CH3 60 °C sealed tube No decomposition
  • 55.
    -Hydride elimination C C H H H M H H C C H H H M H H C C H H H M H H Beta-hydrideelimination is a reaction in which an alkyl group having a  hydrogen,  bonded to a metal centre is converted into the corresponding metal-bonded hydride and a  bonded alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. mechanism Can either be a vital step in a reaction or an unwanted side reaction No change in the formal oxidation state of the metal
  • 56.
    -hydrogen elimination doesnot happen when • the alkyl has no -hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2) • (ii) the -hydrogen on the alkyl is unable to approach the metal (as in C≡CH) • the M–C–C–H unit cannot become coplanar Pt Et3P Et3P C C C C H H B Pt Ph3P Ph3P C Pt Ph3P Ph3P D Pt Ph3P Ph3P A SiMe3 SiMe3 Select the most unstable platinum  complex from the given list. Justify your answer No -H -H unable to MCCH unit will not be approach Mcoplanar
  • 57.
    Classify the followingreactions as oxidative addition, reductive elimination, (1,1 / 1,2)migratory insertion, - H elimination, ligand coordination change or simple addition (a) [RhI3(CO)2CH3]  {RhI3(CO)( solvent)[C(O)CH3]} (b) Ir(PPh2Me)2(CO)Cl + CF3I  Ir(I)(CF3)(PPh2Me)2(CO)Cl (c) TiCl4 +2 Et3N  TiCl4 (NEt3)2 (d) HCo(CO)3(CH2=CHCH3) + CO  CH3CH2CH2Co(CO)4 Problem solving Step 1. determine the oxidation state of the metal in reactant and product Step 2. count the electrons for reactant and product Step 3. see if any ligand in the reactant has undergone change
  • 58.
    Homogeneous catalysis usingorganometallic Catalysts uncatalyzed catalyzed Reactants Products Gibbs Energy stable intermediate Gibbs energy of activation A catalyst typically increases the reaction rates by lowering the activation energy by opening up pathways with lower Gibbs free energies of activation (G). Heterogeneous Homogeneous
  • 59.
    Homogeneous versus HeterogeneousCatalysis Parameter Heterogeneous Homogeneous Phase Gas/solid Usually liquid/ or solid soluble in the reactants Required temperature High Low ( less than 250°C) Catalyst Activity Low High Product selectivity Less (often mixtures) More Catalyst recycling Simple and cost effective Expensive and complex Reaction mechanism Poorly understood Reasonably well understood Product separation from catalyst Easy Elaborate and sometimes problematic Fine tuning of catalyst Difficult Easy
  • 60.
    Heterogeneous Catalyst- CatalyticConverter of a Car Platinum and Rhodium Platinum and Palladium Chemistry at the molecular level – Poorly understood Home assignment : See Youtube video ‘Catalysis’
  • 61.
    Comparing different catalysts;Catalyst life and Catalyst efficiency TON is defined as the amount of reactant (in moles) divided by the amount of catalyst (in moles) times the percentage yield of product. A large TON indicates a stable catalyst with a long life. It is the number of passes through the catalytic cycle per unit time (often per hour). Effectively this is dividing the TON by the time taken for the reaction. The units are just time–1 . A higher TOF indicates better efficiency for the catalyst Turnover Number (TON) Turnover Frequency (TOF) AJELIAS L7-S16
  • 62.
    Wilkinson’s Catalyst foralkene hydrogenation RhCl3 (H2O)3 + CH3CH2OH + 3 PPh3 RhCl(PPh3)3 + CH3CHO + 2HCl + 3H2O Wilkinson’s catalyst: The first example of an effective and rapid homogeneous catalyst for hydrogenation of alkenes, active at room temperature and atmospheric pressure. Square planar 16 electron d8 complex (Ph3P)3RhCl Discovered by G Wilkinson as well as by R Coffey almost at the same time (1964–65) AJELIAS L7-S17
  • 63.
    Conventional Catalytic cyclefor hydrogenation with Wilkinson’s catalyst Rh P P P Cl H Rh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition 1, 2 -migratory insertion reductive elimination R CH2 R RCH2CH3 P P alkene coordination R P = PPh3 14e The first step of this catalytic cycle is the cleavage of a PPh3 to generate the active form of the catalyst followed by oxidative addition of dihydrogen.
  • 64.
    Conventional Catalytic cyclefor hydrogenation with Wilkinson’s catalyst Rh P P P Cl H Rh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition 1, 2 -migratory insertion reductive elimination R CH2 R RCH2CH3 P P alkene coordination R P = PPh3 14e The first step of this catalytic cycle is the cleavage of a PPh3 to generate the active form of the catalyst followed by oxidative addition of dihydrogen.
  • 65.
    Rh P P P Cl Rh P H P Cl H P HRh P P H Cl Rh P H H Cl P H2C Rh P P H Cl Cl Rh P P H2 oxidative addition H2 oxidative addition 1, 2 -migratory insertion reductive elimination P (due to trans effect of H ) R CH2 R RCH2CH3 P P alkene catalytic cycle for hydrogenation Kinetic studies have shown that the dissociation of PPh3 from the distorted square planar complex RhCl(PPh3)3 in benzene occurs only to a very small extent (k = 2.3 × 10–7 M at 25°C), and under an atmosphere of H2, a solution of RhCl(PPh3)3 becomes yellow as a result of the oxidative addition of H2 to give cis- H2RhCl(PPh3)3. The trans effect is the labilization (making unstable) of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence: H2O, OH− < NH3 < py < Cl− < Br− < I−, < PR3, CH3− < H−, NO, CO
  • 66.
    R > > >> > R R R R R R R R R > R R R R • Cis alkenes undergo hydrogenation more readily than trans alkenes •Internal and branched alkenes undergo hydrogenation more slowly than terminal ones, and Relative reactivity of alkenes for homogenous catalytic hydrogenation
  • 67.
    Catalyst 25°C, 1 atmH2 Turnover frequency (TOF) in h–1 for hydrogenation of alkenes Wilkinson’s catalyst 650 700 13 NA Schrock–Osborn catalyst 4000 10 NA NA Crabtree’s catalyst 6400 4500 3800 4000 Rh Ph3P Ph3P PPh3 Cl Rh PPh3 PPh3 + PF6 Schrock-Osborn's catalyst Ir PCy3 N + PF6 Crabtree's catalyst Wilkinson's catalyst Fine tuning of a catalyst: hydrogenation catalysts which are more efficient than Wilkinsons catalyst The cationic metal center is relatively more electrophilic than neutral metal center and thus favours alkene coordination.
  • 68.
    Hydrogenation with Crabtree’scatalyst The di-solvated form of the active catalyst generated by the removal of COD [after it gets hydrogenated and leaves] favors coordination of sterically bulky alkenes as well. Ir PCy3 N PF6 Ir PCy3 N PF6 H H 16e 18e Ir PCy3 N PF6 H 16e Ir S PCy3 N PF6 16e Ir S S PCy3 N PF6 16e oxidative addition migratory insertion   reductive elimination solvent coordination repeat of cycle with cyclooctene di-solvated active form of catalyst H2 This mechanism is only for understanding not for the exam
  • 69.
    Factors which havebeen found to improve the efficiency (better TOF) of transition metal catalysts for hydrogenation • Making a cationic metal center : makes catalyst electrophillic for alkene coordination • Use of ligands (eg. Cyclooctadiene) which will leave at the initial stages of the cycle generating a di-solvated active catalyst : facilitates binding of even sterically hindered alkenes • Use of chelating biphosphines: Cis enforcing: reduces steric hindrance at the metal centre Cis enforcing
  • 70.
    Problem solving- fillin the blanks 1,2 Migr. Insertion 1,1 Migr. Insertion Oxidative addition