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Metal carbonyls may be mononuclear or polynuclear PowerPoint PPT Presentation


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Metal carbonyls may be mononuclear or polynuclear . Characterization of metal carbonyls. IR spectroscopy. (C-O bond stretching modes). Effect of charge. u (free CO) 2143 cm -1. Lower frequency, weaker CO bond. Effect of other ligands. Increasing elec donating ability of phosphines.

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Metal carbonyls may be mononuclear or polynuclear

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Metal carbonyls may be mononuclear or polynuclear


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Characterization of metal carbonyls

IR spectroscopy

(C-O bond stretching modes)


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Effect of charge

u(free CO) 2143 cm-1

Lower frequency, weaker CO bond

Effect of other ligands

Increasing elec donating ability of phosphines

PF3 weakest donor (strongest acceptor)

PMe3 strongest donor (weaker acceptor)


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The number of active bands

as determined by group theory


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Typical reactions of metal carbonyls

Ligand substitution:

Always dissociative for 18-e complexes, may be associative for <18-e complexes

Migratory insertion:


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Metal complexes of phosphines

PR3 as a ligand

Generally strong s donors, may be π-acceptor

strong trans effect

Electronic and steric properties may be controlled

Huge number of phosphines available


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Metal complexes of phosphines

Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3 > PCl3 > PF3

Can be measured by IR using trans-M(CO)(PR3) complexes

Steric properties:

Rigid structures create chiral complexes

apex angle of a cone that encompasses

the van der Waals radii of the outermost

atoms of the ligand


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Typical reactions of metal-phosphine complexes

Ligand substitution:

presence of bulky ligands (large cone angles)

can lead to more rapid ligand dissociation

Very important in catalysis

Mechanism depends on electron count


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Metal hydride and metal-dihydrogen complexes

Terminal hydride (X ligand)

Bridging hydride (m-H ligand, 2e-3c)

Coordinated dihydrogen (h2-H2 ligand)

Hydride ligand is a strong s donor and the smallest ligand available

H2 as ligand involves -donation and π-back donation


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Synthesis of metal hydride complexes

Characterize these kinds of reactions.


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Characterization of metal hydride complexes

1H NMR spectroscopy

High field chemical shifts (d 0 to -25 ppm usual, up to -70 ppm possible)

Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz

Coupling between inequivalent hydrides J(H-H) = 1-10 Hz

Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans

IR spectroscopy

n(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging

n(M-H)/n(M-D) = √2

Weak bands, not very reliable


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Some typical reactions of metal hydride complexes

Transfer of H-

Transfer of H+

A strong acid !!

Insertion

A key step in catalytic hydrogenation and related reactions


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Bridging metal hydrides

Anti-bonding

Non-bonding

4-e ligand

2-e ligand

bonding


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Metal dihydrogen complexes

Characterized by NMR (T1 measurements)

Very polarized

d+, d-

back-donation to s* orbitals of H2

the result is a weakening and lengthening of

the H-H bond in comparison with free H2

If back-donation is strong, then the H-H bond is broken (oxidative addition)


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Metal-olefin complexes

2 extreme structures

sp3

sp2

Zeise’s salt

π-bonded only

metallacyclopropane

Net effect weakens and lengthens

the C-C bond in the C2H4 ligand (IR, X-ray)


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Effects of coordination on the C=C bond

C=C bond is weakened (activated) by coordination


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Characterization of metal-olefin complexes

IR

n(C=C) ~ 1500 cm-1 (w)

NMR

1H and 13C, d < free ligand

X-rays

C=C and M-C bond lengths

indicate strength of bond


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Reactions of metal-olefin complexes


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Metal alkyl, carbene and carbyne complexes


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Metal-alkyl complexes

Main group metal-alkyls known since old times

(Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903))

Transition-metal alkyls mainly from the 1960’s onward

Ti(CH3)6

W(CH3)6

PtH(CCH)L2

Cp(CO)2Fe(CH2CH3)6

[Cr(H2O)5(CH2CH3)6]2+

Why were they so elusive?

Kinetically unstable (although thermodynamically stable)


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Reactions of transition-metal alkyls

Blocking kinetically favorable pathways allows isolation of stable alkyls


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Metal-carbene complexes

L ligand

Late metals

Low oxidation states

Electrophilic

X2 ligand

Early metals

High oxidation states

Nucleophilic


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Fischer-carbenes


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Schrock-carbenes

Synthesis

Typical reactions

Compare to Wittig

+ olefin metathesis (we will speak more about that)


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Grubbs carbenes

Excellent catalysts for olefin metathesis


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Metal cyclopentadienyl complexes

Metallocenes

(“sandwich compounds”)

Bent metallocenes

“2- or 3-legged

piano stools”


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Homogeneous catalysis:

an important application of organometallic compounds

Catalysis in a homogeneous liquid phase

Very important fundamentally

Many synthetic and industrial applications


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Fundamental reaction of organo-transition metal complexes


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Combining elementary reactions


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Completing catalytic cycles

Olefin hydrogenation

(reductive elimination)


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Completing catalytic cycles

Olefin isomerization

b-H elimination

no net reaction

b-H elimination resulting in C=C bond migration


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Completing catalytic cycles

Olefin isomerization


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Completing catalytic cycles

Olefin hydrogenation


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Wilkinson’s hydrogenation catalyst

RhCl(PPh3)3

Very active at 25ºC and 1 atm H2

Very selective for C=C bonds

in presence of other unsaturations

Widely used in organic synthesis

Prof. G. Wilkinson won the Nobel Prize in 1973


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Other hydrogenation catalysts

[Rh(H)2(PR3)2(solv)2]+

With a large variety of phosphines

including chiral ones for enantioselective hydrogenation

RuII/(chiral diphosphine)/diamine

Extremely efficient catalysts for the enantioselective hydrogenation

of C=C and C=O bonds

Profs. Noyori, Sharpless and Knowles won the Nobel Prize in 2001


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Olefin hydroformylation

Cat:HCo(CO)4; HCo(CO)3(PnBu3)

HRh(CO)(PPh3)3; HRh(CO)(TPPTS)3

  • 6 million Ton /year of products worldwide

  • Aldehydes are important intermediates towards plastifiers, detergents


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reductive elimination

CO insertion

Olefin hydrogenation

(reductive elimination)

What else could happen if CO is present?

R behaves as H did


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Olefin hydroformylation


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Catalysts for polyolefin synthesis

  • Polyolefins are the most important products of organometallic catalysis

  • (> 60 million Tons per year)

    • Polyethylene (low, medium, high, ultrahigh density) used in packaging,

    • containers, toys, house ware items, wire insulators, bags, pipes.

    • Polypropylene (food and beverage containers, medical tubing, bumpers,

    • foot ware, thermal insulation, mats)


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Catalytic synthesis of polyolefin


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Catalytic synthesis of polyolefin

High density polyethylene (HDPE) is linear, d 0.96

“Ziegler catalysts”: TiCl3,4 + AlR3

Vacant site

Coordinated alkyl

Electrophilic metal center

Insoluble (heterogeneous) catalyst


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Catalytic synthesis of polyolefin

Isotactic polypropylene is crystalline

“Natta catalysts”: TiCl3 + AlR3

Vacant site

Coordinated alkyl

Electrophilic metal center

Insoluble (heterogeneous) catalyst, crystal structure determines tacticity


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Catalytic synthesis of polyolefin

“Kaminsky catalysts”

Vacant site

Coordinated alkyl

Electrophilic metal center

Soluble (homogeneous) catalyst, structural rigidity determines tacticity


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Polymerization mechanism


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Schrock catalyst

Grubbs catalyst

Olefin metathesis

The Nobel Prize 2005 (Chauvin, Schrock, Grubbs)


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The metathesis mechanism (Chauvin, 1971)


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