organometallics
DESCRIPTION
Lecture notesTRANSCRIPT
Professor Claire J. Carmalt
E-mail: [email protected]
Office number: room 135
Office Hours: Tues 1-2 pm; Wed 1-2 pm
CHEM3101 Moodle Site: handouts, lecture notes, self-study questions, revision and weblinks
CHEM3101 Organometallic Chemistry
Suggest reading: Standard inorganic textbooks provide useful material for this course: “Inorganic Chemistry”, Shriver and Atkins, OUP, 5th Edition “Inorganic Chemistry”, Catherine Housecroft and Alan Sharpe, Prentice Hall, 4th Edition F A Cotton, G. Wilkinson, C A Murillo and M Bochmann, Advanced Inorganic Chemistry, 6th ed, 1999, Wiley-Interscience. Further Reading: Cheap guides to organometallic chemistry of transition metals may be worth considering as alternatives: M Bochmann, Organometallics 1: Complexes with Transition Metal-Carbon sigma-Bonds M Bochmann, Organometallics 2: Complexes with Transition Metal-Carbon π-Bonds, Oxford Chemistry Primers, 1994
Overview – A brief overview of organometallics including definitions/ applications. Classification of ligands based on numbers of electrons donated by neutral ligands and on hapticity (hapto-nomenclature). Electron counting and considerations in reaction mechanisms, including catalysis. Ligand Types Structure and bonding in organometallics containing simple ligands – Dihapto-alkene, trihapto-allyl, tetrahapto-butadiene and tetrahapto-cyclobutadiene. Fluxionality – fast and slow motions involving organic ligands within complexes. Reactivity – mechanisms: oxidative addition and reductive elimination, C-H bond activation, insertions of alkenes into M-H bonds, beta-elimination, agostic (C-H-M) bonding. 2
Synopsis
s- and p-block: • These have major use in organic synthesis (RLi and RMgCl), catalysis (Ziegler
Natta polymerisation AlR3) and as precursors to semiconductors, e.g. GaAs
• s-block: many have ionic bonding (i.e. NaR) but some have covalent character, e.g. Li, Be and Mg – covalency; p-block – covalent
• M-C bonds are usually weaker than M-O or M-X bonds.
e.g. B-C (365) < B-Cl (465) < B-O (526) (kJmol-1)
Important for organic synthesis, especially when trying to form C-C bonds
e.g. M-CR3 + R’3C-X à M-X + R3C-CR’3
• Average M-C decrease down group e.g. P-C (276) > As-C (229) > Sb-C (214) > Bi-C (141) (kJmol-1)
This is due to poorer overlap between carbon valence orbitals and those of the metal as the primary quantum number increases.
• Unstable with respect to oxidation and susceptible to hydrolysis
• Some are pyrophoric 3
Survey of organometallics of the s- and p-block
s-block structures
p-block structures Generally organic ligands are σ-bonded and terminal (i.e. BR3, SiR4, SbR3, BiR3. Sometimes multicentre alkyl-bridges are formed e.g. Al2Me6.
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s- and p-block
5 More information about the characteristics of elements can be found: www.webelements.com
Introduction - organometallics
What do we want to achieve?
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Introduction
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Introduction
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Introduction
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Introduction
Important in many catalytic reactions For example the hydroformylation of a lkenes produces a ldehydes on a megatonne scale per annum. Important reaction as aldehydes are easily converted into secondary products, e.g. alcohols which are used in detergents, fragrances and other products.
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Introduction
!
COCo
H
OCOC
COCo
H
OCOC
R
COCo
CO
OCOC
CO
R
Co
CO
OCOC
COCo
H
OCOC CO
CO
H2
R
CHO
1
2
3
5
7CH2=CHR
COCo
CO
OC
R
4
Co
H
OC
COOC
6
H
O
R
O
R
Oxidation states and d-electron count
Carbon-based fragments are generally soft and thus the metal is in a low oxidation-state, however, it is possible to have a wide-range of metal oxidation-states in some instances – especially when ligands such as cyclopentadienyl (C5H5
-) are involved – these have both donor and acceptor properties
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d-block organometallics
Sometimes it is not obvious what charge a ligand carries – we can try and figure this out from a consideration of a related neutral molecule by successive removal of protons.
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d-block: oxidation states and d-electron count
Occasionally it is difficult to resolve the issue of ligand charges and thus metal oxidation state. Example: C7H7 ligand – since cyclic ligands will tend to become aromatic – (4n + 2)π electrons – is it best considered as (C7H7)+ i.e. n = 1 or (C7H7)3- i.e. n = 2?? This would imply oxidation-states of 0 and +4 respectively in the compounds below. Compare to the neutral C6H6 ligand.
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d-block: oxidation states and d-electron count
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d-block: ligand binding modes
For simple organic ligands such as alkyl groups there is essentially only a single binding mode (not quite true), however, many other ligands have a range of binding modes and this can affect the number of electrons the ligand donates to a metal centre.
Both σ- and π-complexation are found
The 18-electron rule (effective atomic number – EAN) is the cornerstone of organometallic chemistry – assumes that all nine valence orbitals of the transition element (s + 3p + 5d) are filled - determine the valence electron count (VEC) of a metal in a complex - majority of organometallic compounds characterised by VECs of 18. In this course we will use the covalent (neutral) model.
There are different conventions to count the electrons in a complex. • Ionic model: remove all of the ligands from the metal and, if necessary, add
the proper number of electrons to each ligand to bring it to a closed valence shell state.
• Covalent model: remove all of the ligands from the metal, but rather than take them to a closed shell state, we do whatever is necessary to make them neutral.
Example: [ClRh(PPh3)3] Covalent Ionic Rh Cl- 3 PPh3 ––––––––––––––––––––––––––––––– Total:
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d-block: valence electron counts
The d-electrons from the metal
The number of d-electrons from the atom is calculated considering the group of the periodic table the metal belongs to (see the table below). For instance Ti is in group 4
• Number of electrons donated by the ligands to the metal centre (according to covalent model) – see table
• Metal-metal multiple bonds - These are counted as donating one electron per bond to each metal but are ignored in the calculation of oxidation state.
• Charges: Each negative charge is counted as adding an electron to the VEC. Each positive charge is counted as an electron subtracted from the VEC.
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d-block: valence electron counts
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d-block: valence electron counts
Oxidation states: We remove all ligands from the metal and consider 1-, 3-, 5- and 7-electron (i.e., odd number) donor ligands to be negatively charged and the rest (i.e., ones donating even numbers of electrons) neutral. The charge (oxidation state) on the metal must balance the A compound that satisfies the 18-electron rule is coordinatively saturated. Example (covalent model): [RuCl2(CO)2(PPh3)2]: Ru is in group 8 so it has 8 d-electrons (d8). The Cl ligands
are both 1-electron donors. The CO and PPh3 ligands are 2-electron donors:
The oxidation state of the Ru centre can be worked out by splitting up the complex into the two 1-electron donating Cl ligands (two negative charges) and the neutral CO and PPh3 ligands. This leaves the metal with a 2+ charge. [RuCl2(CO)2(PPh3)2] is a neutral complex:
Ligands:
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d-block: VEC and oxidation state
Further Example: [Co(C5H5)(C6H6)]2+ Co is in group 9, the Cp ligand (C5H5
-) is a 5-electron donor and the benzene ligand donates 6 electrons. Two electrons must be deducted due to the double positive charge of the complex.
[Co(C5H5)(C6H6)]2+ is a doubly positively-charged complex. Ligands:
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d-block: VEC and oxidation state
Major exceptions to the 18-electron rule The 18-electron rule is valid most of the time but, unfortunately, not always. Common exceptions: A. d8 square planar system (16 electrons) Complexes of Rh(I), Ir(I), Pd(II) and Pt(II). As it can be seen in the diagram, the energy of the d orbital changes due to the ligands presence and the system is more stable leaving an empty d orbital
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d-block: VEC and oxidation state
! !
B. Steric Effects – Phosphine Complexes Phosphine ligands have the general formula PR3, where R= alkyl, aryl, etc. A way to measure the steric effect of the phosphine complexes is given by the value of the Tolman cone angle θ. The higher the value of θ, the greater will the steric effect and the higher the probability that the complex
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Depending on the size/shape of R, the complex with a metal M will be more stable with 4, 3 or two ligands.
d-block: VEC and oxidation state
A visualization of the Tolman cone angles of phosphines and phosphates
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d-block: VEC and oxidation state
C. Steric effects – early transition metals • Many early transition metals (group 4-7) often do not follow the 18-electron rule
because it is not possible to pack the required • However, the electron count will be maximised if possible, as shown in the
equilibrium below:
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An 18-electron arrangement is not obtained; however, Ti achieves 16 electrons.
d-block: VEC and oxidation state
D. Reaction intermediates Most of reactions involving organometallic complexes take place with changes in the number of electrons Occasionally these species can be isolated; however, they are just intermediates, with a very short lifetime.
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d-block: VEC and oxidation state
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Examples continued: Oxidation/reduction
d-block: VEC and oxidation state
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d-block: VEC and oxidation state
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d-block: VEC and oxidation state
Types of Ligands:
There are many different types of ligands that bind through carbon.
Two important points to remember: • the number of atoms the ligand uses to
• the number of electrons the ligand Note: many ligands can bond in various modes (variable hapticity) using differing numbers of carbon atoms. This will result in a different number of electrons being donated to the metal. These factors are important in determining the electron count at the metal centre, which influences structure and reactivity.
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d-block: types of ligands
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Monohapto (1-electron donors)
Alkyl ligands are the most common monohapto ligands.
• The Greek letter eta (η) is used to denote hapticity, which is best described as the number of donor atoms used by the ligand to coordinate to the metal.
• With s-block metals, bridging alkyls are known. • The Greek letter mu (µ) is used to denote the number of centres bridged.
• Only the first bonding mode of alkyls is common in d-block compounds. The
others are found mainly in compounds of the s-block metals.
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d-block: monohapto ligands
Examples of metal alkyl complexes:
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d-block: monohapto ligands
Some other common monohapto 1-electron donor ligands are:
Monohapto (2-electron donors) The most common of these ligands is the carbonyl group CO. The second most common are carbene or alkylidene ligands.
• The general term for these ligands is carbene but alkylidene is also used. • If one of more substituents on the carbon is a heteroatom such as O, S, N, the
ligand is usually called a and if the substituents are carbon based the term is used.
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d-block: monohapto ligands
LnM CER
HLnM C
R
HA B
Fischer Schrock
E = O, S, N; R = alkyl, aryl R = H, alkyl, aryl
Metal carbenes are used as catalysts in olefin metathesis (an organic reaction that involves the redistribution of fragments of alkene (olefins) by the scission and regeneration of C=C bonds. Applications include: synthesis of pharmaceutical drugs, production of propylene (plastics) and the manufacturing of high-strength materials.
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d-block: monohapto ligands
There are two classes of carbene – Fischer-type and Shrock-type carbenes.
A: Fischer-type carbenes – have one or more stabilizing heteroatom substituent B: Shrock-type carbenes – the carbon forming the bond with the metal is
bonded to two other carbon or hydrogen atoms.
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LnM CER
HLnM C
R
HA B
Fischer Schrock
E = O, S, N; R = alkyl, aryl R = H, alkyl, aryl
Metals in low oxidation states and the high electron density at the metal centre is stabilized by π-acceptor ligands such as CO, which can receive this electron density into their orbitals.
Metals in high oxidation states with π-donating ligands to compensate for the low electron density at the metal centre
d-block: monohapto ligands
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Formation of a Fischer-type carbene complex:
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Preparation of Schrock-type carbene:
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d-block: monohapto ligands
Note: Although both Fischer and Schrock carbenes contain a metal-carbon double bond, the nature and reactivity of these carbene ligands is usually quite different. This is due to the polarity of the bonds and the differing electron richness of the metal centres.
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d-block: monohapto ligands
Fischer carbenes have a vacant p-orbital on the carbon atom into which they can accept electron density from the metal and also makes the carbon susceptible to attack by nucleophiles.
Schrock alkylidenes have a half-filled p-orbital on the carbon atom and so can donate electron density to the metal (and this also explains why the carbon is nucleophilic and is attacked by electrophiles).
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d-block: monohapto ligands
Note: when calculating oxidation states, it is important to remember that if the carbene is attached to a metal with a low oxidation state (Fischer-type), then it is counted as a If the carbene is attached to a metal in a high oxidation state (Schrock type) then the carbene is often counted as having a 2- charge
The electrophilicty of the carbon atom in Fischer carbenes is indicated by the resonance form A: !
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Example of the reactivity of Fischer carbenes is the reaction with Li reagents:
d-block: monohapto ligands
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Schrock carbenes are more reactive than Fischer carbenes - the carbon atom reacts as a nucleophile.
!cf. phosphonium ylides Ph3P=CH2 + O=CR2 →
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Monohapto (3-electron donors)
The most common ligand in this class are the carbynes or alkylidynes. The main characteristic of the complex is the presence of a metal-carbon triple bond. These complexes are normally obtained by “reduction” of carbenes, both Fischer- and Schrock-type carbenes.
d-block: monohapto ligands
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Fischer carbene
Schrock carbene
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1,5-cyclooctadiene (cod) ligands are common in organometallic chemistry. Examples: Synthesis of alkene complexes
d-block: dihapto ligands
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Dihapto (2-electron donors) Alkene and alkyne complexes are common examples of 2-electron donors (Note: alkyne ligands can also act as 4 electron donors – see later).
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d-block: dihapto ligands
Alkenes are typically relatively weakly coordinating ligands. They are extremely important substrates for catalytic reactions.
Examples of synthetic routes to alkene complexes:
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Alkene • The behaviour of alkenes as ligands is similar to the one of carbon monoxide
CO, the model to describe it is known as • For CO, there is a 2-electron σ-donation from a lone pair on the carbon into an
empty metal d-orbital making the metal electron rich. • At the same time, a filled metal d-orbital can interact with the empty π* orbital
on the carbonyl ligand, passing electron density back onto the ligand (π-backbonding). The σ-donation and the π-backbonding have a synergistic effect; it means that the combined effect of the two bonds is higher than the sum of the two individual effects.
• A similar mechanism can be used to describe the bonds in the complexes with
alkenes as ligands. We see a from the C=C π orbital with concomitant into an empty π* orbital on the alkene. There is synergistic effect because the greater is the σ-donation to the metal, the greater the π-backbonding to the alkene.
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d-block: dihapto ligands
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This is represented in the Molecular Orbital diagrams below.
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d-block: dihapto ligands
