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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


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

PF3 weakest donor (strongest acceptor)

PMe3 strongest donor (weaker acceptor)


The number of active bands

as determined by group theory


Typical reactions of metal carbonyls

Ligand substitution:

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

Migratory insertion:


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


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


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


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


Synthesis of metal hydride complexes

Characterize these kinds of reactions.


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


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


Bridging metal hydrides

Anti-bonding

Non-bonding

4-e ligand

2-e ligand

bonding


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)


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)


Effects of coordination on the C=C bond

C=C bond is weakened (activated) by coordination


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


Reactions of metal-olefin complexes


Metal alkyl, carbene and carbyne complexes


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)


Reactions of transition-metal alkyls

Blocking kinetically favorable pathways allows isolation of stable alkyls


Metal-carbene complexes

L ligand

Late metals

Low oxidation states

Electrophilic

X2 ligand

Early metals

High oxidation states

Nucleophilic


Fischer-carbenes


Schrock-carbenes

Synthesis

Typical reactions

Compare to Wittig

+ olefin metathesis (we will speak more about that)


Grubbs carbenes

Excellent catalysts for olefin metathesis


Metal cyclopentadienyl complexes

Metallocenes

(“sandwich compounds”)

Bent metallocenes

“2- or 3-legged

piano stools”


Homogeneous catalysis:

an important application of organometallic compounds

Catalysis in a homogeneous liquid phase

Very important fundamentally

Many synthetic and industrial applications


Fundamental reaction of organo-transition metal complexes


Combining elementary reactions


Completing catalytic cycles

Olefin hydrogenation

(reductive elimination)


Completing catalytic cycles

Olefin isomerization

b-H elimination

no net reaction

b-H elimination resulting in C=C bond migration


Completing catalytic cycles

Olefin isomerization


Completing catalytic cycles

Olefin hydrogenation


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


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


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


reductive elimination

CO insertion

Olefin hydrogenation

(reductive elimination)

What else could happen if CO is present?

R behaves as H did


Olefin hydroformylation


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)


Catalytic synthesis of polyolefin


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


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


Catalytic synthesis of polyolefin

“Kaminsky catalysts”

Vacant site

Coordinated alkyl

Electrophilic metal center

Soluble (homogeneous) catalyst, structural rigidity determines tacticity


Polymerization mechanism


Schrock catalyst

Grubbs catalyst

Olefin metathesis

The Nobel Prize 2005 (Chauvin, Schrock, Grubbs)


The metathesis mechanism (Chauvin, 1971)


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