ORGANOMETALLIC CHEMISTRY 1 Introduction types and rationale 2

ORGANOMETALLIC CHEMISTRY 1. Introduction (types and rationale) 2. Molecular orbital (bonding) of CO, arrangement “in space” or ligand types (hapticity) 3. 16 and 18 electron rule (learning to count) 4. Synthesis, steric effects and reactivity - Wilkinsons catalyst (part 1) 5. Characterisation IR nmr etc. 6. Applications (oxidative addition b elimination)

What is organometallic chemistry? Chemistry: structures, bonding and properties of molecules. Organometallic compounds: containing direct metal-carbon bonds. Either s or p bonds can occur Main group: (Al. Me 3)2

v Structures s bonds and 3 c-2 e (or even 4 c-2 e) bonds Chem 210 v Synthesis the first M-C bond v Reactivity nucleophilic and basic auxiliaries in organic synthesis source of organic groups for transition metals

v Strong preference for s-donor groups but Cp is often p-bound (deceptively like with transition metals) Cp 2 Mg v Cp 2 Fe Electropositive metals: often 3 c-2 e or 4 c-2 e hydrides/alkyls (Me 3 Al)2 (Me. Li)4

As a Nucleophile v Addition to polar C=X bonds (C=O, C=N, CºN) v Substitution at sp 2 carbon (often via addition)

v Substitution at sp 3 carbon does occur but is far less easy and often has a multistep mechanism v Substitution at other elements: often easy for polar M-X bonds (Si-Cl, B-OMe)

As a base v More prominent in polar solvents think of free R- acting as base v Elimination mechanism can be more complex than this v Metallation chelate effect more important than inductive effect!

v b-hydrogen transfer mainly for Al: for more electropositive elements, deprotonation and nucleophilic attack are faster for less electropositive elements, often no reaction

Chemistry: structures, bonding and properties of molecules. Transition metal compounds

Some compounds do not contain metal-carbon bond, but they are usually included in the field of organometallic chemistry. They include: • Metal hydride complexes, e. g. • N 2 -complexes, e. g. • Phosphine complexes, e. g.

Exercise. Which of the following compounds is an organometallic compound? In general, metals in organometallic compounds include: • main group metals • transition metals • f-block metals In this course, transition metals are our main concern.

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 B 12

2) Zeise’s Salt synthesized in 1827 = K[Pt(C 2 H 4)Cl 3] • H 2 O Confirmed to have H 2 C=CH 2 as a ligand in 1868 Structure not fully known until 1975 3) Ni(CO)4 synthesized in 1890 4) Grignard Reagents (XMg. R) synthesized about 1900 Accidentally produced while trying to make other compounds Utility to Organic Synthesis recognized early on 5) Ferrocene synthesized in 1951 Modern Organometallic Chemistry begins with this discovery (Paulson and Miller) 1952 Fischer and Wilkinson

Nobel -Prize Winners related to the area: Victor Grignard and Paul Sabatier (1912) Grignard reagent K. Ziegler, G. Natta (1963) Zieglar-Natta catalyst E. O. Fisher, G. Wilkinson (1973) Sandwich compounds K. B. Sharpless, R. Noyori (2001) Hydrogenation and oxidation Yves Chauvin, Robert H. Grubbs, Richard R. Schrock (2005) Metalcatalyzed alkene metathesis

Common organometallic ligands

Why organometallic chemistry ? a). From practical point of view: * OMC are useful for chemical synthesis, especially catalytic processes, e. g. In production of fine chemicals In production of chemicals in large-scale reactions could not be achieved traditionally

* Organometallic chemistry is related to material sciences. e. g. Organometallic Polymers Small organometallic compounds: Precursors to films for coating (MOCVD) (h 3 -C 3 H 5)2 Pd -----> Pd film CH 3 C C-Au-C NMe -----> Au film Luminescent materials

* Biological Science. Organometallic chemistry may help us to understand some enzyme-catalyzed reactions. e. g. B 12 catalyzed reactions.

b). From academic point of view: * Organometallic compounds display many unexpected behaviors- discover new chemistry- new structures e. g. New reactions, reagents, catalysts, e. g. Ziegler-Natta catalyst, Wilkinson catalyst Reppe reaction, Schwartz's reagent Sharpless epoxidation, Tebbe's reagent

Types of bonds possible from Ligands Language: All bonds are coordination or coordinative Remember that all of these bonds are weaker than normal organic bonds (they are dative bonds) Simple ligands e. g. CH 3 -, Cl-, H 2 give s bonds p systems are different e. g. CO is a s donor and p acceptor Bridging ligands can occur two metals Metal-metal bonds occur and are called d bonds – they are weak and are a result of d-d orbital overlap

18 Electron Rule (Sidgwick, 1927) • OM chemistry gives rise to many “stable” complexes - how can we tell by a simple method • Every element has a certain number of valence orbitals: 1 { 1 s } for H 4 { ns, 3´np } for main group elements 9 { ns, 3´np, 5´(n-1)d } for transition metals s dxy px dxz py dyz pz dx 2 -y 2 dz 2

• Therefore, every element wants to be surrounded by 2/8/18 electrons – For main-group metals (8 -e), this leads to the standard Lewis structure rules – For transition metals, we get the 18 -electron rule • Structures which have this preferred count are called electron-precise • Every orbital wants to be “used", i. e. contribute to binding an electron pair The strength of the preference for electron-precise structures depends on the position of the element in the periodic table • For early transition metals, 18 -e is often unattainable for steric reasons - the required number of ligands would not fit • For later transition metals, 16 -e is often quite stable (square-planar d 8 complexes) • Addition of 2 e- from 5 th ligand converts complex to 5 CN 18 e- , marginally more stable

Predicting reactivity 14 e 16 e 18 e Most likely associative

Predicting reactivity 16 e 18 e 20 e (Sterics!) Most likely dissociative

N. B. How do you know a fragment forms a covalent or a dative bond? • • • Chemists are "sloppy" in writing structures. A "line" can mean a covalent bond, a dative bond, recognise/understand the bonding first Use analogies ("PPh 3 is similar to NH 3"). Rewrite the structure properly before you start counting. 1 e covalent bond 2 e dative bond "bond" to the allyl fragment 3 e Pd = Cl¾ = P® = allyl = 10 1 2 3 + ¾¾ e-count 16

"Covalent" count: (ionic method also useful) 1. Number of valence electrons of central atom. • from periodic table 2. Correct for charge, if any • but only if the charge belongs to that atom! 3. Count 1 e for every covalent bond to another atom. 4. Count 2 e for every dative bond from another atom. • no electrons for dative bonds to another atom! 5. Delocalized carbon fragments: usually 1 e per C (hapticity) 6. Three- and four-center bonds need special treatment 7. Add everything N. B. Covalent Model: 18 = (# metal electrons + # ligand electrons) - complex charge The number of metal electrons equals it's row number (i. e. , Ti = 4 e, Cr = 6 e, Ni = 10 e)

Hapto (h) Number (hapticity) For some molecules the molecular formula provides insufficient information with which to classify the metal carbon interactions The hapto number (h) gives the number of carbon (conjugated) atoms bound to the metal It normally, but not necessarily, gives the number of electrons contributed by the ligand We will describe to methods of counting electrons but we will employ only one for the duration of this module

The two methods compared: some examples N. B. like oxidation state assignments, electron counting is a formalism and does not necessarily reflect the distribution of electrons in the molecule – useful though Some ligands donate the same number of electrons Number of d-electrons and donation of the other ligands can differ Now we will look at practical examples on the black board

Does it look reasonable ? v Remember when counting: v Odd electron counts are rare v In reactions you nearly always go from even to even (or odd to odd), and from n to n-2, n or n+2. v Electrons don’t just “appear” or “disappear” v The optimal count is 2/8/18 e. 16 -e also occurs frequently, other counts are much more rare.

Exceptions to the 18 Electron Rule Zr. Cl 2(C 5 H 5)2 Zr(4) + [2 x Cl(1)] + [2 x C 5 H 5(5)] =16 Ta. Cl 2 Me 3 Ta(5) + [2+ x Cl(1)] + [3 x M(1)] =10 WMe 6 W(6) + [6 x Me(1)] =12 Pt(PPh 3)3 Pt(10) + [3 x PPh 3(2)] =16 Ir. Cl(CO)(PPh 3)2 Ir(9) + Cl(1) + CO(2) + [2 x PPh 3(2)] =16 What features do these complexes possess? • Early transition metals (Zr, Ta, W) • Several bulky ligands (PPh 3) • Square planar d 8 e. g. Pt(II), Ir(I) • σ-donor ligands (Me)

Alkyl ligands: Transition metal alkyl complexes important for catalysts e. g. olefin polymerization and hydroformylation thermodynamic Problem is their weak kinetic stability (Thermally fine: M-C bond dissociation energies are typically 40 -60 kcal/mol with 20 -70 kcal/mol) Simple alkyls are sigma donors, that can be considered to donate one or two electrons to the metal center depending on which electron counting formalism you use

Synthesis of Metal Alkyl Complexes 1. Metathetical exchange using a carbon nucleophile (R-). Common reagents are RLi, RMg. X (or R 2 Mg), Zn. R 2, Al. R 3, BR 3, and Pb. R 4. Much of this alkylation chemistry can be understood with Pearson's "hardsoft" principles

2. Metal-centered nucleophiles (i. e. using R+ as a reagent) Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example: Na. Fp + RX Fp-R + Na. X [Fp = Cp(CO)2 Fe] 3. Oxidative Addition. This requires a covalently unsaturated, lowvalent complex (16 e- or less). A classic example: 4. Insertion- To form an alkyl, this usually involves an olefin insertion. The simplest generic example is the insertion of ethylene into an M-X bond, i. e. M-X + CH 2 M-CH 2 -X

Carbonyl Complexes Bonding of CO Electron donation of the lone pair on carbon s This electron donation makes the metal more electron rich - compensate for this increased electron density, a filled metal d-orbital may interact with the empty p* orbital on the carbonyl ligand p-backbonding or p-backdonation or synergistic bonding Similar for alkenes, acetylenes, phosphines, and dihydrogen.

What stabilizes CO complexes is M→C π–bonding The lower the formal charge on the metal ion the more willing it is to donate electrons to the π–orbitals of the CO Thus, metal ions with higher formal charges, e. g. Fe(II) form CO complexes with much greater difficulty than do zero-valent metal ions For example Cr(O) and Ni(O), or negatively charged metal ions such as V(-I) In general to get a feeling for stability examine the charges on the metals

Syntheses of metal carbonyls Metal carbonyls can be made in a variety of ways. For Ni and Fe, the homoleptic or binary metal carbonyls can be made by the direct interaction with the metal (Equation 1). In other cases, a reduction of a metal precursor in the presence of CO (or using CO as the reductant) is used (Equations 2 -3). Carbon monoxide also reacts with various metal complexes, most typically filling a vacant coordination site (Equation 4) or performing a ligand substitution reactions (Equation 5) Occasionally, CO ligands are derived from the reaction of a coordinated ligand through a deinsertion reaction (Equation 6)

Synthesis of carbonyl complexes Direct reaction of the metal – Not practical for all metals due to need for harsh conditions (high P and T) – Ni + 4 CO Ni(CO)4 – Fe + 5 CO Fe(CO)5 Reductive carbonylation – Useful when very aggressive conditions would be required for direct reaction of metal and CO » Wide variety of reducing agents can be used – Cr. Cl 3+ Al + 6 CO Al. Cl 3 + Cr(CO)6 – 3 Ru(acac)3 + H 2 + 12 CO Ru 3(CO)12 +

N. B. From the carbonyl complex we can synthesize other derivatives

Main characterization methods: • Xray diffraction Þ (static) structure Þ bonding • NMR Þ structure en dynamic behaviour • EA Þ assessment of purity • (calculations) Useful on occasion: • IR • MS • EPR Not used much: • GC • LC

IR spectra and metal-carbon bonds The υCO stretching frequency of the coordinated CO is very informative Recall that the stronger a bond gets, the higher its stretching frequency M=C=O (C=O is a double bond) canonical structure Lower the υCO stretching frequency as compared to the M-C≡O structure (triple bond) Note: υCO for free CO is 2041 cm-1) [Ti(CO)6]2 - [V(CO)6]- [Cr(CO)6] [Mn(CO)6]+ [Fe(CO)6]2+ υCO 1748 1858 1984 2094 2204 cm-1 increasing M=C double bonding decreasing M=C double bonding

Bridging versus terminal carbonyls Bridging CO groups can be regarded as having a double bond C=O group, as compared to a terminal C≡O, which is more like a triple bond: ~ triple bond ~ double bond M M-C≡O C=O M terminal carbonyl bridging carbonyl (~ 1850 -2125 cm-1) (~1700 -1860 cm-1) Bridging CO between 1700 and 2200 cm-1 the C=O group in a bridging carbonyl is more like the C=O in a ketone, which typically has υC=O = 1750 cm-1

Bridging versus terminal carbonyls in [Fe 2(CO)9] terminal carbonyls bridging carbonyls

Summary 1. As the CO bridges more metal centers its stretching frequency drops – same for all p ligands – More back donation

2. As the metal center becomes increasingly electron rich the stretching frequency drops

Alkene ligands Dewar-Chatt-Duncanson model The greater the electron density back-donated into the p* orbital on the alkene, the greater the reduction in the C=C bond order Stability of alkene complexes also depends on steric factors as well An empirical ordering of relative stability would be: tetrasubstituted < trisubstituted < trans-disubstituted < cisdisubstituted < monosubstituted < ethylene

Alkyne ligands: Similar to alkenes Alkynes tend to be more electropositive-bind more tightly to a transition metal than alkenes -alkynes will often displace alkenes Difference is 2 or 4 electron donor sigma-type fashion (A) as we did for alkenes, including a pi-backbond (B) The orthogonal set can also bind in a pi-type fashion using an orthogonal metal d -orbital (C)

The back-donation to the antibonding orbital (D) is a delta-bond-the degree of overlap is quite small - contribution of D to the bonding of alkynes is minimal The net effect p-donation - alkynes are usually non-linear in TM complexes Resonance depict the bonding of an alkyne. I is the metallacyclopropene resonance form Support for this versus a simple two electron donor, II, can be inferred from the C-C bond distance as well the RC-C-R angles III generally does not contribute to the bonding of alkyne complexes.

Ally ligands: Allyl ligands are ambidentate ligands that can bind in both a monohapto and trihapto form The trihapto form can be expressed as a number of difference resonance forms as shown here for an unsubstituted allyl ligand: Important applications

Dihydrogen Ligands: Metal is more electropositive than hydrogen Hydrogen acts as a two electron sigma donor to the metal center. The complex is an arrested intermediate in the oxidative addition of dihydrogen How does this affect the oxidation state of the meta?

Dihydrogen complexes Bonding is “simple” a 3 C-2 electron bond. H 2 - neutral two electron sigma donor One could also describe a back-donation of electrons from a filled metal orbital to the sigma-* orbital on the dihydrogen

Electronic Attributes of Phosphines Like that of carbonyls As electron-withdrawing sigma-donating capacity decreases At the same time, the energy of the p-acceptor (sigma-*) on phosphorous is lowered in energy, providing an increase in backbonding ability. Therefore, range of each capabilities –tuning rough ordering -CO stretching frequency indicator- low CO stretching frequency- greater backbonding to M Experiments such as this permit us to come up with the following empirical ordering:

Cone Angle (Tolman) Steric hindrance: Phosphine Ligand Cone Angle A cone angle of 180 degrees effectively protects (or covers) one half of the coordination sphere of the metal complex PH 3 87 o PF 3 104 o P(OMe)3 107 o PMe 3 118 o PMe 2 Ph 122 o PEt 3 132 o PPh 3 145 o PCy 3 170 o P(t-Bu)3 182 o P(mesityl)3 212 o

You would expect a dissociation event to occur first before any other reaction -steric bulk (rate is first order -increasing size) This will also have an effect on activity for catalysts N. B. “flat” can slide past each other For example Wilkinson's catalyst (more later) Has a profound effect on the reactivity!

Reaction chemistry of complexes Three general forms: 1. Reactions involving the gain and loss of ligands a. Ligand Dissoc. and Assoc. (Bala) b. Oxidative Addition c. Reductive Elimination d. Nucleophillic displacement 2. Reactions involving modifications of the ligand a. Insertion b. Carbonyl insertion (alkyl migration) c. Hydride elimination (equilibrium) 3. Catalytic processes by the complexes Wilkinson, Monsanto Carbon-carbon bond formation (Heck etc. )

a) Ligand dissociation/association (Bala) • Electron count changes by -/+ 2 • No change in oxidation state • Dissociation easiest if ligand stable on its own (CO, olefin, phosphine, Cl-, . . . ) • Steric factors important

b) Oxidative Addition Basic reaction: • Electron count changes by +/- 2 (assuming the reactant was not yet coordinated) • Oxidation state changes by +/- 2 • Mechanism may be complicated The new M-X and M-Y bonds are formed using: • the electron pair of the X-Y bond • one metal-centered lone pair

One reaction multiple mechanisms Concerted addition, mostly with non-polar X-Y bonds H 2, silanes, alkanes, O 2, . . . Arene C-H bonds more reactive than alkane C-H bonds (!) Intermediate A is a s-complex Reaction may stop here if metal-centered lone pairs are not readily available Final product expected to have cis X, Y groups

Stepwise addition, with polar X-Y bonds – HX, R 3 Sn. X, acyl and allyl halides, . . . – low-valent, electron-rich metal fragment (Ir. I, Pd(0), . . . ) Metal initially acts as nucleophile – Coordinative unsaturation less important Ionic intermediate (B) Final geometry (cis or trans) not easy to predict Radical mechanism is also possible

Cis or trans products depends on the mechanism

c) Reductive elimination This is the reverse of oxidative addition - Expect cis elimination Rate depends strongly on types of groups to be eliminated. Usually easy for: • H + alkyl / aryl / acyl – H 1 s orbital shape, c. f. insertion • alkyl + acyl – participation of acyl p-system • Si. R 3 + alkyl etc Often slow for: • alkoxide + alkyl • halide + alkyl – thermodynamic reasons? We will do a number of examples of this reaction

Relative rates of reductive elimination Most crowded is the fastest reaction

Special case: Nucleophilic Attack on a Coordinated CO acyl anion Fisher carbene This is Fischer carbene It has a metal carbon double bond Such species can be made for relatively electronegative metal centers N. B. mid to late TMs Fischer carbenes are susceptible to nucleophilic attack at the carbon

Fischer carbenes act effectively as σ donors and p acceptors The empty antibonding M=C p orbital is primarily on the carbon making it susceptible to attack by nucleophiles Other type is called a Shrock carbene (alkylidene) Characteristic Fischer-type Schrock-type Typical metal (Ox. State) Middle to late T. M. Fe(0), Mo(0) Cr(0) Early T. M. Ti(IV), Ta(V) Substituents attached to carbene carbon At least one highly electronegative heteroatom H or alkyl Typical other ligands Good p acceptors Good s and p donors Electron count 18 10 -18

Nucleophilic displacement Ligand displacement can be described as nucleophilic substitutions O. M. complexes with negative charges can behave as nucleophiles in displacement reactions Iron tetracarbonyl (anion) is very useful

Modifications of the ligand a) Insertion reactions Migratory insertion! The ligands involved must be cis - Electron count changes by -/+ 2 No change in oxidation state If at a metal centre you have a s-bound group (hydride, alkyl, aryl) a ligand containing a p-system (olefin, alkyne, CO) the s-bound group can migrate to the p-system 1. CO, RNC (isonitriles): 1, 1 -insertion 2. Olefins: 1, 2 -insertion, b-elimination 1, 1 1, 2

1, 1 Insertion The s-bound group migrates to the p-system if you only see the result, it looks like the p-system has inserted into the M-X bond, hence the name insertion To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial) The reverse of insertion is called elimination Insertion reduces the electron count, elimination increases it Neither insertion nor elimination causes a change in oxidation state a- elimination can release the “new” substrate or compound

In a 1, 1 -insertion, metal and X group "move" to the same atom of the inserting substrate. The metal-bound substrate atom increases its valence CO, isonitriles (RNC) and SO 2 often undergo 1, 1 -insertion 1, 2 insertion (olefins) Insertion of an olefin in a metal-alkyl bond produces a new alkyl Thus, the reaction leads to oligomers or polymers of the olefin • polyethene (polythene) • polypropene

Standard Cossee mechanism Why do olefins polymerise? Driving force: conversion of a p-bond into a s-bond One C=C bond: 150 kcal/mol Two C-C bonds: 2´ 85 = 170 kcal/mol Energy release: about 20 kcal per mole of monomer (independent of mechanism) Many polymerization mechanisms Radical (ethene, dienes, styrene, acrylates) Cationic (styrene, isobutene) Anionic (styrene, dienes, acrylates) Transition-metal catalyzed (a-olefins, dienes, styrene)

b Hydride elimination (usually by b hydrogens) Many transition metal alkyls are unstable (the reverse of insertion) the metal carbon bond is weak compared to a metal hydrogen Bond Alkyl groups with β hydrogen tend to undergo β elimination M -CH 2 -CH 3 M - H + CH 2=CH 2 Two examples

A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product. To prevent beta-elimination from taking place, one can use alkyls that: Do not contain beta-hydrogens Are oriented so that the beta position can not access the metal center Would give an unstable alkene as the product

Catalysis (homogeneous) Reduction of alkenes etc.

The size of the substrate has an effect on the rate of reaction

Same reaction different catalyst

Alternative starting material

The Monsanto acetic acid process Methanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid Insertion of carbon monoxide into the C-O bond of methanol The catalyst system - iodide and rhodium Iodide promotes the conversion of methanol to methyl iodide, Methyl iodide - the catalytic cycle begins: 1. Oxidative addition of methyl iodide to [Rh(CO)2 I 2]2. Coordination and insertion of CO - intermediate 18 -electron acyl complex 3. Can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst

Catvia Process

Wacker process (identify the steps)

Identify the steps