chem 153 organotransition metal chemistry · · 2012-08-30organotransition metal chemistry...

M.C. White, Chem 153 Structure & Bonding -1- Week of September 17, 2002

Organotransition Metal Chemistry Organotransition Metal Chemistry (MCW definition): Transition metal mediated reactions that solve (or have potential to solve) challenging problems in the synthesis of organic molecules.

Coordination Chemistry:The chemistry of transition metal complexes that havenoncarbon ligands (Werner complexes). Classification applies to the catalyst and all reaction intermediates.

Organometallic Chemistry:The chemistry of transition metal complexes that have M-C bonds (organometallic complexes). Classification applies to the catalyst and/or reaction intermediates.

RuH3CCN NCCH3

H3CCN

(PF6-)

R

R

RuNCCH3

(PF6-)

R

Trost enyne cycloisomerization catalyst

Trost JACS 2002 (124) 5025.

+

+

proposed intermediate

PPh3

PdPh3P PPh3

Ph3P

Suzuki cross-coupling catalyst

B(OH)2 N

OTf

CO2Me

Ph3PPd

Ph3P

N CO2Me


N

CO2Me

de Lera Synthesis 1995 285.

OTiIV

RORO

OTiIV

O O

O

R'(O)C

R'OR

OR'

OR

OH t-BuOOH, 4Å MS

OTiIV

ROOR

OTiIV

O O

R'(O)C

CO2RO

OR'

O

O

t-Bu

R

R

OHRO

Sharpless JACS 1987 (109) 5765.

Sharpless titanium-tartrateepoxidation catalyst

CH2Cl2, -20oC

70-90% yield94->98% ee


C(O)R'

C(O)R'


Cross-Couplings

Tandem Heck

IO

OH

TBSO O

O

H

OTBS

AcOPd

Ph3P OAc

PPh310mol%

Ag2CO3, THF, reflux

82%

Overman JOC 1993 (58) 5304.

Nobel Prize in Chemistry 2010: Richard Heck, Ei-ichi Negishi, Akira Suzuki

Nicolaou's Rapamycin Synthesis: Note* last step!!!

O

I

OCH3

OHO

NOH

OO

HOMe

OHO

OH

OMe

O

I+

SnBu3

Bu3Sn

ClPdII

Cl NCCH3

NCCH3

20 mol%

(i-Pr)2NEtDMF, THF25oC, 24h

28%

O

OCH3

OHO

NOH

OO

HOMe

OHO

OH

OMe

O

Nicolaou JACS 1993 (115) 4419.


Reactive Site Selectivity in Multifunctional Molecules

HO

MeO

O OH

N

O

H

O

OO

OH

OMe

OMe

OH

HO

MeO

O OH

N

O

H

O

OO

OH

OMe

OMe

O

HO

MeO

O OH

N

O

H

O

OO

OH

OMe

OMe

O

H

H H

FK 506

Ru

PPh3

PPh3

Cl

Cl Ph

H

10 mol%CH2Cl2, rt, 22h

49%E:Z ; 1:1

Schreiber JACS 1997 (119) 5106.

No protecting groups used! The majority of the mass recovered after reaction termination was unreacted starting material.

Nobel Prize in Chemistry 2005: Yves Chauvin, Robert Grubbs, Richard Schrock


Asymmetric Catalysis Nobel Prize in Chemistry 2001: William S. Knowles, Ryoji Noyori, K. Barry Sharpless

Wilkinson : Investigations into the reactivity of (PPh3)RhCl uncovered its high activity as a homogeneous hydrogenation catalyst. This was the 1st

homogeneous catalyst that compared in rates with heterogeneouscounterparts (e.g. PtO2).

RhPh3P

Ph3P

Cl

PPh3

H2 (1 atm)

Wilkinson J.Chem. Soc. (A) 1966 1711.

MeO

AcO

CO2H

NHAc

RhP

P OMe

OMe

+

BF4- H2

MeO

AcO

CO2H

NHAcH

95% ee, 100 % yield

MeO

AcO

CO2H

NH2H

H3O+

L-DOPA

cat.

The Monsanto Process

W. Knowles: Replacement of achiral PPh3 ligands with non-racemicphosphines ((-)-methylpropylphenylphosphine, 69%ee) demonstrated that a chiral transition metal complex could transfer chirality to a non-chiralsubstrate during hydrogenation.

CO2H RhPr(Ph)(Me)P

Pr(Ph)(Me)P

Cl

P(Me)(Ph)PrCO2H

Electronically tuning the metal center and using a C2 symmetric, bidentatechiral phosphine ligand led to highly enantioselective hydrogenations ofenamides (very good substrates for asymmetric hydrogenations). TheMonsanto Process (1974) that resulted is the 1st commercialized asymmetric synthesis using a chiral transition metal complex. Asymmetrichydrogenation is the key step in the industrial synthesis of L-DOPA (a rareamino acid used to treat Parkinson's disease).

H2 (1 atm)

**

*

15 % eeKnowles Chem. Commun. 1968, 1445.

Royal Swedish Academy of Sciences:www.kva.se

The Transition Metals

* d electrons in group3 are readily removedvia ionization, those ingroup 11 are stable and generally form part ofthe core electronconfiguration.

valence (d) electron count:

for complexed transitionmetals: the (n)d levels arebelow the (n+1)s and thus getfilled first. note that group # =d electron count

OC FeCOCO

CO

CO

3d8

K Sc Ti V Cr Mn Fe Co Ni Cu Zn

Rb Y Zr Nb Mo Tc Ru Rh Pd Ag Cd

Cs Hf Ta W Re Os Ir Pt Au Hg

Na

B

Al

Ga

In

Tl

Li Ne

Ar

Kr

Xe

Rn

H He

3 4 5 6 7 8 9 10 11 12

13

1 18

4s23d2 4s23d3

3d4 3d5

5s24d2

4d4

5s14d4

4d5

4s13d5

3d6

5s14d5

4d6

6s25d2

5d4

6s25d3

5d5

6s25d4

5d6

4s23d5 4s23d6

3d7 3d8

4s23d7

3d9

4s23d8

3d10

5s24d5

4d7

5s14d7

4d8

5s14d8

4d9

5s04d10

4d10

6s25d5

5d7

6s25d6

5d8

6s25d7

5d9

6s15d9

5d10

Transition metals (d-block metals):elements that can have a partially filled dvalence shell. Typically group 4-10 metals.*

EARLY LATE

La


N

NNFeII

Cl Cl

3d6

for oxidized metals, subtract the oxidation state from the group #.

Fe 4s23d6

for free (gas phase)transition metals: (n+1)s isbelow (n)d in energy (recall: n = principal quantum #).

Electron Counting

Ph2PCORh

P

HOC

OO

O

To determine ligand charges, create an ionic model by assigning each M-L electron pair to the moreelectronegative atom (L). This should result instable ligand species or ones known as reactionintermediates in solution.

COOC

P

POO

O

RhI

H

Ph2

-1

neutral (0)

Step 1: Determine the oxidation state of the metal.To do this, balance the ligand charges with an equalopposite charge on the metal. This is the metal's formal oxidation state.

Co

Rh

Ir

9

3d9

4d9

5d9

RhI = d8

Step 3: Determine the electron count of the complexby adding the # of electrons donated by each ligand to the metal's d electron count.

COOC

P

POO

O

RhI

H

Ph2

2e-

2e-

ligands: 10e-metal: 8 e-complex: 18 e-


Step 2: Determine the d electron count. Recall: subtract the metal's oxidation state from its group #.

η1-Ligands Hapticity (ηx): The number of atoms (x) in the ligand binding

to the metal

V

OO OR

O

O

t-Bu

VO

OOR

Ot-Bu

O

η1-alkyl peroxo

terminal oxo η2-alkyl peroxo

Proposed intermediates in VO(acac)2 catalyzed directed epoxidation of allylic alcohols.

Sharpless Aldrichimica Acta 1979 (12), 63.

Bridging ligands (µ): the ligand bridges 2 or more metals

FeN

O FeN

NCl

Cl

NN

N

N

N

Nishida Chem. Lett. 1995 885.

linear µ-oxo


MX

M

M

RO

M

MO

M

η1 ligands (monodentate):

H (hydride)CH3 (alkyl)COX (halides)µ-X (bridging)

OR (terminal alkoxide)µ-OR (bridging)

OR2 (ether)O2 (superoxide)O (terminal oxo)µ-O (bridging)

PR2 (phosphide)PR3 (phosphine)NR2 (amide)NR3 (amine)iminesnitrilesNO (nitrosyl )

Formal charge

# of e-donated

-1-1 0-1-1

-1

-1

0-1-2-2

-1 0-1 0 0 0+1

22224

2

4

2244

2222222

(2/metal)

(2/metal)

(2/metal)

linear

Electron Counting M.C. White, Chem 153 Structure & Bonding -8- Week of September 17, 2002

Ph3PRh

Cl PPh3

PPh3 Ph3PRhI

Cl PPh3

PPh3

PRu

PH2N

NH2

Cl

Cl

Ar2

Ar2

O

O

PRuII

P N

N

Cl

Cl

O

OAr2

Ar2

H2

H2

N

NPd

Me

Me N

NPdII

Me

Me

NFe

N NOTf

N

OTf

NFeII

N NOTf

N

OTf

PPh3

PPh3Pd

Ph3PPh3P

PPh3

PPh3Pd0

Ph3PPh3P

Wilkinson's catalyst(Ph3P)3RhCl

ligands: 8e-metal: d8, 8e-complex: 16 e-


Noyori hydrogenationcatalyst

Brookhart polymerizationcatalyst precursor


Olefin dihydroxylation catalyst



Palladium "tetrakis" triphenylphosphinecross coupling catalyst

Noyori JACS 1998 (120) 13529. Que JACS 2001 (123) 6722.

Brookhart JACS 1995 (117) 6414.

M

M

R M

H

M

O

O M

η1-coordinationFormal charge

# of e-donated

-1

-1

-1

-1

-1

-1

2

2

2

2

2

2

η1-aryl

η1-alkenyl

η1-alkynyl

η1-Cp (cyclopentadienyl)

η1-acetate

M

η1-allyl

M

M

R HM

M

O

OM

0

0

0

-1

-1

-1

6

2

2

6

4

4

η6-arene

η2-alkene

η2-alkyne

η5-Cp (cyclopentadienyl)

η2-acetate

ηx-coordinationFormal charge

# of e-donated

M

η3-allyl

= M M

Unsaturated Ligands M.C. White, Chem 153 Structure & Bonding -9- Week of September 17, 2002

Electron Counting II M.C. White, Chem 153 Structure & Bonding -10- Week of September 17, 2002

HIr

H O

O

P(Cy)3

P(Cy)3

CF3H

IrIIIH O

O

P(Cy)3

P(Cy)3

CF3


Crabtree's dehydrogenationcatalyst

RhH

HMe3P

Cp*

RhIIIH

HMe3P


Bergman: direct observation

of C-H-> C-M

ZrClCl

Brintzinger catalyst

ZrIVCl

Cl


RuS

RuS

CH3

CH3

Cl

ClRuIII

SRuIII

S

CH3

CH3

Cl

Cl

Ru-Ru bond = 2 e-note: metal oxidation state doesn't change

Hidai catalyst forpropargylic substition

ligands: 12 e-metal: d5, 5e-Ru 2: 1 e-complex: 18 e-

Ru 1ligands: 12 e-metal: d5, 5e-Ru 1: 1 e-complex: 18 e-

Ru 2

Hidai JACS 2002 (124) 7900Brintzinger JOMC 1985 (228) 63.

Crabtree JACS 1987 (109) 8025. Bergman OM 1984 (3) 508.

Weakly Coordinating Counterions

The least coordinating anion: hexahalocarboranes (CB11H6X6

-)

Strem: Silver hexabromocarborane (Ag+CB11H6Br6

-) 1g = $653

Strauss Chem. Rev. 1993 (93) 927. Reed Acc. Chem. Res. 1998 (31) 133.

Fe ClCl

N

N

Me

Me

N

N

CH3CNFe NCCH3

NCCH3

N

N

Me

Me

N

N

(SbF6-)2

2 equiv. Ag+SbF6-

2+

Jacobsen JACS 2001 (123) 7194.

Common weakly coordinating counterions used in organotransition metal catalysis to generate cationic catalysts: Weakly coordinating anions generally

have: 1. low charge, 2. high degree ofcharge delocalization (i.e. noindividual atom has a highconcentration of charge), 3. steric bulk.

SynthesisMetathesis: Ag (I) halide abstraction. Most general approach for the introduction of weakly coordinating counterions.

Protonolysis


note: neutral solventreplaces L- in rxn.

TfO-< ClO4- < BF4

- < PF6- < SbF6

- < BAr'4 (B[3,5-C6H3(CF3)2]4)

More weakly coordinating

N

NAr

Ar

NiMe

MeEt2O

N

NAr

Ar

NiMe

OEt2

H+(OEt2)2 BAr'4-

+

(BAr'4-)

Brookhart JACS 1999 (121) 10634.

Electron Counting III M.C. White, Chem 153 Structure & Bonding -12- Week of September 17, 2002

Ir

N

P(Cy)3(PF6

-) IrI

N

P(Cy)3 PF6

BPh3

Rh+

BPh3

RhI

RuH3CCN NCCH3

H3CCN

(PF6-) RuII

CH3CN NCCH3CH3CN

PF6

Fe ON

N

Me

Me

N

N

(SbF6-)3Fe

N

N

N

NO O

Me

Me

+

Crabtree's catalystsfor hydrogenations

+

weakly coordinating anion does not contribute to theelectron count for complex

ligands: 8 e-metal: d8, 8e-complex: 16 e-

review: Crabtree Acct. Chem. Res. 1979 (12) 331.

COD = 1,5-cyclooctadiene

NBD = norbornadiene

"Zwitterionic complex"used in hydroformylations

1st synthesis: Schrock and Osborn Inorg. Chem. 1970 (9) 2339.hydroformylation: Alper Chem. Comm. 1993, 233.



1st synthesis: Mann OM 1982 (1) 485.catalytic enyne cycloisomerizations: Trost JACS 2002 (124) 5025.

+ +

Jacobsen JACS 2001 (123) 7194.ligands: x e-metal: dx, 5e-complex: x e-

Fe 1ligands: x e-metal: dx, xe-complex: x e-

Fe 2

3+

epoxidation catalyst

Question:

Common Geometries for TM Complexes M.C. White, Chem 153 Structure & Bonding -13- Week of September 17, 2002

CN = 6, ML6:

LM

L L

L

L

L

90o, cis180o, trans

octahedral

CN = 5, ML5:

L

L

ML

L

109.5o

CN = 4 ,ML4:

tetrahedral

Leq M

Leq

Leq

Lax

Lax120o

90o

trigonal bipyramidal

CN = 2, ML2:

M LL

180o

CN = 3, ML3

L ML

L

linear

trigonal planar

120o

Coordination number (CN):The number of ligands (L) bonded to the same metal (M).

Sterics. to a 1st approximation, geometry of TM complexesdetermined by steric factors(VSEPR -valence shell electronpair repulsion). The M-L bondsare arranged to have themaximum possible seperationaround the M.

LM

L L

L

90o, cis

square planar

L

ML Lba sal

Lba sal

Lapical

square pyramidal

180o, trans

~90o

~90-100o

Electronics: d electron count combined with the complex electron count must beconsidered when predicting geometries forTM complexes with non-bonding delectrons. Often this leads to sterically lessfavorable geometries for electronic reasons (e.g. CN = 4, d8, 16 e- strongly preferssquare planar geometry) .

M LL

L90o

T-shaped


Ligand sterics

P

RR

R

M

θ 2.28 Åaverage of Ni-P bondlengths obtained from crystal data

Tolman Chem. Rev. 1977, 77, 313.

N

RR

R

M

θ 2.2 Åaverage of Pd-N bond lengths obtained fromcrystal data

Trogler JACS 1991, 113, 2520.

∗θ values measured using strain-free CPK model ofM(L). For ligands with many internal degrees offreedom, the values do not account for distortions in geometry due to contacts with other atoms in thecomplex. Very valuable as a relative scale.

PH3

PF3

P(OMe)3

PMe3

PCl3

Ph2P PPh2

PPhMe2

PEt3PPh2MePPh2EtPPh2PrPPh3

PPh2CyPPhCy2

PCy3

P(t-Bu)3

P(o-tol)3

P(mesityl)3

Ligands Cone angle*θ (ο)

87104107118124

125

127132136140140145153161170182194212

Ligands

NH3

NMe3,quinuclidine, NMe2EtNMeEt2NEt3

NPr3

NPh3

NEt2PhNBz3

N(i-Pr)3

Cone angle*θ (ο)

94132

145150160166170210220

others

HMeCOCp

759095136

phosphines 3o amines

M.C. White/ Q. Chen Chem 153 Structure & Bonding -15- Week of September 17, 2002

Effect of ligand sterics on coordination number

Pd(PtBu2Ph)2

Bond length (Å) Pd-P1: 2.251 Bond angle (o) P1-Pd-P1a: 176.51

Otsuka JACS, 1976 (98)5850.

Although Pd(PtBu2Ph)2 is coordinatively unsaturated electronically, the steric bulk of PtBu2Ph ligands prevents additional ligands from coordinating to the metal.

Generalizations about CN:Low CN favored by:1. Low oxidation state (e- rich) metals.2. Large, bulky ligands.

High CN favored by:1. High oxidation state (e- poor) metals.2. Small ligands.

Pd0 P(tBu)2PhPh(But)2P

176.51o

ligands: 4 e-metal: d10, 10 e-complex: 14 e-

Transition Metal Valence Orbitals

� 18 electron rule: upper limit of 18 e- can be accomodated w/out using antibonding molecular orbitals (MO's).

dz2 dx2-y2 dxy dxz dyz

(n)d orbitals

� dz2 and dx2-y2 orbital lobes located on the axes� dxy, dxz, and dyz lobes located between the axes

� orbitals oriented 90o with respect to each other creating unique ligand overlap possibilities

pz px py

(n+1)p orbitalsz

y

x

(n+1)s orbital

s


� 9 Valence Orbitals: upper limit of 9 bonds may be formed. In most cases a maximum of 6 σ bonds are formed and the remaining d orbitals are non-bonding. It's thesenon-bonding d orbitals that give TM complexes many of their unique properties.


M H

Pure σ-donors

Hydride Alkyl

M C

R

RR

3o Amines

M N

R

RR

σ-bonding

M

z

y

x

L

Best Overlap

M

z

y

x Worst overlap

L

best shape complementarity

t2g

eg

LUMOσ*

Metal d orbitals

ligand σ-bondingorbitals

MO Description of σ−bonding in an octahedral complex

Conclusion:The energy of the LUMO is directly affected by M-L σ bond strength. Weak bonds willhave low-lying LUMO's making the metalmore electrophilic.

σ

HOMO

M.C. White, Chem 153 Structure & Bonding -18- Week of September 17th, 2002

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd

Cs Ba Hf Ta W Re Os Ir Pt Au Hg

Na

Be

Mg

B C

Al

Ga

In

Tl

Si

Ge

Sn

Pb

Li N O

P

As

Sb

Bi

S

Se

Te

Po

F

Cl

Br

I

At

H

La*

1

2

3 4 5 6 7 8 9 10 11 12

13 14

EARLY (EM) LATE (LM)

15 16 17

2.2

1.0

0.9

0.8

0.8

0.8

1.6

1.3

1.0

1.0

0.9

1.3

1.2

1.1

1.5

1.3

1.6

1.6

1.5

1.6

2.1

2.3

1.6

1.9

1.9

1.8

2.2

2.2

1.9

2.3

2.2

1.9

2.2

2.3

1.9

1.9

2.5

1.7

1.7

2.0

2.0

1.6

1.8

1.6

1.6

2.5

1.9

2.0

1.8

1.9

3.0

2.2

2.2

2.0

2.0

3.4

2.6

2.5

2.1

2.0

4.0

3.1

2.9

2.6

2.2

increasing electronegativityincreasing electronegativity

increasing electronegativity


TRANSITION METALS (TM)

Periodic table trends:electronegativity

The electronegativity of theelements increases substantially as in progressing from left toright (EM to LM) across theperiodic table.

Whereas the electronegativity of main group elementsincreases in progressing up a column, that of the TMincreases in progressing down.

Co

HN H

H

H

N HH

H NHH

HN

HH

H

NHH HN

HH

Electrostatic Model

3+Co

HN HH

H

N HHH N

HH

HN

HH

H

NHH HN

HH

Covalent Model

3-The most accurate description ofσ-bonding in TM complexes liessomewhere in between the 2 extremes anddepends in large part on the relativeelectronegativities of the metal and ligands

Pauling The Nature of the Chemical Bond, 3rd Ed.;1960



Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd


Na

Be

Mg

B C

Al

Ga

In

Tl

Si

Ge

Sn

Pb

Li N O

P

As

Sb

Bi

S

Se

Te

Po

F

Cl

Br

I

At

H

La*

1

2

3 4 5 6 7 8 9 10 11 12

13 14

EARLY (EM) LATE (LM)

15 16 17

2.2

1.0

0.9

0.8

0.8

0.8

1.6

1.3

1.0

1.0

0.9

1.3

1.2

1.1

1.5

1.3

1.6

1.6

1.5

1.6

2.1

2.3

1.6

1.9

1.9

1.8

2.2

2.2

1.9

2.3

2.2

1.9

2.2

2.3

1.9

1.9

2.5

1.7

1.7

2.0

2.0

1.6

1.8

1.6

1.6

2.5

1.9

2.0

1.8

1.9

3.0

2.2

2.2

2.0

2.0

3.4

2.6

2.5

2.1

2.0

4.0

3.1

2.9

2.6

2.2

increasing electronegativityincreasing electronegativity



TRANSITION METALS (TM)

Electronegativity II

Ionic bonding is greater when orbitals of unequal electronegativities interact. M-L σ-bonding inelectropositive metals (e.g. early metals) hassignificant ionic character.

Covalent bonding is greater when orbitals of similar electronegativities interact. Therefore, M-L σ-bonding in electronegative metals (e.g. late metals) is primarilycovalent in nature.

ZrIV

H

Cl + ZrIV

O

Cl

R HOEt

Schwartz's reagent

Labinger ACIEE 1976 (15) 333.

HMLn H + MLn

adds H-Zr across alkenes and alkynes(hydrozirconation). incompatible withmost carbonyls b/c of hydridic properties.

O

REtO

(easier to break heterolytically)

intermediate in catalytichydroformylation

of alkenes

HMLn H�+ �MLn

RhI

O

HO

OREtO2C

OREtO2C

OC

HO

Leighton JACS 2001 (123) 11514.

(easier to break homolytically)


σ-bonding EB EI + EC

bondingenergy

ionicbonding

covalentbonding

Co

HN H

H

H

N HHH N

HH

HN

HH

H

NHH HN

HH

Electrostatic Model

3+Co

HN HH

H

N HHH N

HH

HN

HH

H

NHH HN

HH

Covalent Model

3-

Bond strength in polarized M-L bonds resultsfrom a gain in covalent and ionic bonding energy. The degree to which each type of bondinginfluences bond strength is highly dependent onthe relative electronegativities of the metal andligands.

€

∝

€

∝

€

∝

€

∝

€

∝

€

∝

€

∝

€

∝

Ionic bonding is greater when elements of high and opposite chargeinteract. Differences in charge are paralleled in differences inelectronegativities. Large differences in electronegativity favor strong ionic bonding. M-L σ-bonding in early metals has significant ioniccharacter.

M

L

M+

L-incr

easi

ng io

niza

tion

pote

ntia

l (ε)

EI

EI (εM-εL)

EI −(QMQL)Q = charge density

− (QMQL) − (εM-εL)

Fleming Frontier Orbitals and Organic Chemical Reactions, 1976.Pauling The Nature of the Chemical Bond, 3rd. Ed.; 1960.

Electrostatic Model: Ionic Bonding

HOMO

LUMO

Covalent bonding is greater when orbitals of similar energiesinteract. The energy of atomic orbitals is inversely proportional tothe element's electronegativity (i.e. the orbital energy of anelectronegative element is lower than that of a electropositiveelement). Small differences in electronegativity favor strongcovalent bonding. M-L σ-bonding in late metals has a high degree of covalent bonding.

M

LEI

ECML σ

ML σ∗

ECorbital overlap

incr

easi

ng e

nerg

y

(εM-εL)

ECorbital overlap

(EM-EL)

EM1εM

EL1εL

Covalent Model

HOMO

LUMO


Periodic table trends II: hard/soft

La*

1

2

3 4 5 6 7 8 9 10 11 12

13 14

EARLY (EM)

LATE (LM)


Rb Y Zr Nb Mo Tc Ru Rh Pd Ag Cd


Na

Be

Mg

B C

Al

Ga

In

Tl

Si

Ge

Sn

Pb

Li

15 16

N O

P

As

Sb

Bi

S

Se

Te

Po

17

F

Cl

Br

I

At

H2.2

1.0

0.9

0.8

0.8

0.8

1.6

1.3

1.0

1.0

0.9

1.3

1.2

1.1

1.5

1.3

1.6

1.6

1.5

1.6

2.1

2.3

1.6

1.9

1.9

1.8

2.2

2.2

1.9

2.3

2.2

1.9

2.2

2.3

1.9

1.9

2.5

1.7

1.7

2.0

2.0

1.6

1.8

1.6

1.6

2.5

1.9

2.0

1.8

1.9

3.0

2.2

2.2

2.0

2.0

3.4

2.6

2.5

2.1

2.0

4.0

3.1

2.9

2.6

2.2

increasing electronegativity/decreasing orbital energy


increasing electronegativity/decreasing orbital energy


HARDelectrophile

SOFTelectrophile

SOFTnucleophile

HARDnucleophile

Hard nucleophiles (ligand): have a low energy HOMOwith high charge density (negative charge).Hard electrophiles (metal) : have a high energy LUMOwith high charge density (positive charge).Hard (L) - Hard (M) interaction: is predominantly ionic in character. It is favorable because of strong Coulombicattraction.

Soft nucleophiles (ligand): have a high energy HOMO with low charge density.Soft electrophiles (metal) : have a low energy LUMOwith low charge density.Soft (L) - Soft (M) interaction: is predominantly covalent in character. It is favorable because of small ΔE between the HOMO of the ligand and the LUMO of the metal (EM-EL).

Fleming Frontier Orbitals and Organic Chemical Reactions, 1976.


Ir-X bond dissociation enthalpies for (η5-Me5C5)(PMe3)Ir(X)2

X

Ph

Vy

H

Pentyl

Me

Cy

Neopentyl

DIr-X (kcal/mol)

82

74

71

58

56

52

48

IrIIIX

XMe3P

Bergman's C-H activationcomplex

M-C Bond Strengths M-C Bond Strength Trends: the trends in M-C σ bond strengths generally parallel those found in H-C σ bond strengths.

90

100

110

120

40 50 60 70 80 90

Cy

Neopentyl

Me

Pentyl

Vy H

Ph

D(Ir-X) kcal/mol

D(H

-X) k

cal/m

ol

sp C-M > sp2 C-M > sp3 C-M 1o C -M > 2o C-M > >> 3o C-M

Bergman Polyhedron 1988 (7) 1429.

As in C-H σ bonding, there is a general trend towards weaker M-Cwith increased substitution. Large deviations occur when the alkylgroup is very bulky or when it is methyl. Bulky ligands likeneopentyl are thought to destabilize the M-C bond because of steric hinderance, making it much weaker than the correlation wouldpredict. There is a strong thermodynamic preference to form thesterically less hindered M-C bond.

As in C-H σ bonding, an increase in % s character of the carbonstrengthens the M-C σ bond because of better orbital overlap. The correlation between C-H and M-C (C = aryl, vinyl) BDE's is notperfect with M-C bonds being stronger than predicted because ofπ-bonding with the metal.


M L

z

y

x

σ-bonding

M

z

y

x

L

π-bonding

σ and π bonding in ML6

dxy dxz dyz

Six valence metal orbitals that participate in σ-bonding inan octahedral complex along the x,y, and z axes.

dz2 dx2-y2

pz px py

z

y

x

s

Three valence metal orbitals that may participate in π-bonding inan octahedral complex with ligands that have orbitals of matching symmetry (i.e. p, d, π, π*).


σ and π donors

M

z

y

x Best overlap

M

z

y

x Worst overlap

t2g

HOMO

Δ

LUMO

t2g

eg*

LUMOσ*

π

π∗

HOMO

σ−complex ligand π-bondingorbitals

MO Description for M-L π-donor system in an octahedral complex

M

σ-bonding: Lsp2 -> Mdσπ-donation: Lp -> Mdπ

Cl

or I-, Br-, F-

Halides

M OR

Alkoxides

M NR

R

1o, 2o Amines



OO-

N N

O- -O

acac (acetylacetonate)

other π-donors

salen

Cp

benzene

Conclusion:The energy of the HOMO is directly affected by M-L πbonding. Ligand to metal π donation increases the energyof the HOMO making the metal more basic. π-donorligands stabilize electron poor, high oxidation state metals. Very prevalent for early TM complexes (low d electroncount) and less so for late TM (high d electron count).

Oxidation state formalism Electroneutrality principle (Pauling): "stable complexes are those with structures such that each atom has only a small electric charge." Stable M-L bond formation generally reduces the positive charge on the metal as wellas the negative charge and/or e- density on the ligand. The result is that the actual charge on the metal is notaccurately reflected in its formal oxidation state.

Pauling The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.

Sharpless JACS 1987 (109) 1279.

The "18 electron rule" often fails for early transition metals. Formal oxidation state is not an accurate description of electron density at the metal. Low oxidation state, early TM complexes are stabilizedvia π-donation (i.e. a shifting of electron density from π-donor ligands to the metal). This in partaccounts for the extreme oxophilicity of early TM.


N N

O OMnIII

Clt-Bu

t-Bu

t-Bu

t-Bu

Jacobsen epoxidation catalystMn (salen)

ligands: 10e-metal: d4 ,4e-complex: 14 e-

VIV

O

OO

OO

VO(acac)2 "vanadium acac"epoxidation catalyst

ligands: 12 e-metal: d1 ,1e-complex: 13 e-

Sharpless titanium-tartrateepoxidation catalyst

self-assemblingdimer based oncrystal structure.

ligands: 12 e-metal: d0 ,0e-complex: 12 e-

OTiIV

RORO

OTiIV

O O

O

R'(O)C

R'OR

OR'

OR

C(O)R'


σ and π acceptors

Conclusion:Metal to ligand π donation (π backbonding) lowers theenergy of the HOMO making the metal less basic.π-backbonding stabilizes electron rich, low oxidation state metals. Very prevalent in late TM complexes.

HOMO

Δ

LUMO

t2g

eg*

t2g*

LUMOσ*

π

HOMO

π∗

Δ

ligand π-bonding orbitals

MO Description for M-L π -acceptor system in an octahedral complex

σ−complex

LUMO

M C OC

CM

σ-bonding: Ln -> Mdσπ-backbonding: Md π -> Lπ*

H

H

M

σ-bonding: L π -> Mdσπ-backbonding: Md π -> Lπ*

σ-bonding: L σ-> Mdσπ-backbonding: Md π -> Lσ*

P

Rationalization of M -> P backbonding iscontroversial. The classic picture envokes a Mdπ -> P 3d interaction. Quantummechanical calculations indicate that P-Xσ* orbitals play a major role.Hybridization of phosphorus 3d and P-Rσ* resulting in π-acceptor orbitals hasbeen envoked.

M

Orpen Chem. Comm. 1985 , 1310.Braga Inorg. Chem. 1985 , 2702.

N N NN

CH3CN, NO, N2, CN-

R'

R N N R

R'

bpy phen


π-backbonding

P(t-Bu)3PCy3P(i-Pr)3P(NMe2)3PMe3PPhMe2PBz3PPh2MePPh3PPh2(OEt)P(p-C6H4Cl)3PPh(OEt)2P(OEt)3PH3PCl3PF3

PhosphorusLigand (L) CO v, cm-1

2056

20592062206420652066206720692072207320742077208320972111

Tolman Chem. Rev. 1977 (77) 313.

CO stretching frequencies measured forNi(CO)3L where L is PR3 ligands ofdifferent σ-donor abilities. Free CO vibrates at 2143 cm-1.

The increase in electron density at the nickel from phosphine σ-donation isdispersed through the M-L π system via π-backbonding. Much of the electron density is passed onto the CO π* and is reflected in decreased v(CO) IRfrequencies which corresponds to weaker CO bonds.

P

R

R

RNiCO

Recall: Band position in IR is governed by :1. force constant of the bond (f) and 2. individual masses of the atoms (Mx and My).Stronger bonds have larger force constants than weaker bonds.

v = 12πc

f(MxMy)/(Mx+My)

1/2

π-acids: effect on the metal M.C. White, Chem 153 Structure & Bonding -28- Week of September 17th, 2002

N

NNiII

C

N

N

Ni0 +24oC

no reaction without π acid

OH

H OH

π-acid

Yamamoto JACS 1971 (93)3350.

CO's render the electron rich Cr metal electrophilic via strong π-backbonding. Complexation ofbenzene with the electrophilic Cr(CO)3 fragment withdraws electon density from the aromatic ring activating it towards nucleophilic attack.

CNNC

CNNC

F3C

N

FF

FF

F

NO2

other π-acids

Acrolein is thought to act as a π-acid, withdrawing electron density from theNi(II) complex via π-backbonding and promoting elimination of the diethylfragment to reduce the metal.

OC

CoIOC CO

CO

H acidic

pka < 1 H2O

Norton JACS 1987 (109) 3945.

CrCO

CrCO

Cr(0), d6, 18e-

OCOC

OCOC

LDA

MeOMeO

NCO

(±)-Acorenone B

π-acid

Semmelhack JACS 1980 (102) 5926

NC


C

CM

Dewar-Chatt-Duncanson Model

Olefin-metal bonding is thought tooccur via a 2-way donor-acceptormechanism that involves σ-donationfrom the bonding π-electrons of theolefin to empty σ orbitals of the metal and π-backbonding from the metal tothe empty π* orbitals of the olefin. Both interactions are important in forming astable M-olefin complex

olefin-metal complexes

The balance of electron flow can be shifted predominantly in one direction dependent on the electronic properties of themetal. If the metal is electron withdrawing, M-L σ-bondingpredominates and withdraws electron density from theπ-bond of the olefin. This results in the induction of a δ+charge on the olefin that activates it towards nucleophilicattack.

LPdII

Cl

Cl

R

OH2

σ donation>>π-backbonding

OH2

RLPdII

Cl

Cl

δ+

intermediates in Wacker oxidation (commercial production of acetaldehyde) Bercaw JACS 1983 (105) 1136.

Takaya OM 1991 (10) 2731.

Powerful take-home message: the appropriate metal complex can invert the chemical behavior of an alkene.

If the metal is electron donating (i.e. low oxidation state metals like Pd(0),Ni(0), Pt(0)) π-backbonding predominates and the metal alkene complexbegins to approach a metallocyclopropane structure. In complexes involvingelectropositive metals in low oxidation states, the metallocyclopropanecarbons are rendered nucleophilic as evidenced by their reaction withelectrophiles (i.e. aldehydes). Cp2Ti metallocyclopropane is a stable complex, crystal obtained by Bercaw.

CpTiII

Cp

RH

H H

δ-

δ-

R

CpTiII

Cp

H

R'CHO

CpTiIV

Cp

O

R

R'

π-backbonding >>σ donation

note: convention is to not change formal oxidation state of the metallocyclopropane.

M.C. White/M.W. Kanan Chem 153 Structure & Bonding -30- Week of September 17th, 2002

*Cp

TiII

*CpH

H

H

H

Bercaw JACS 1983 (105) 1136

Ph3P

Pt0

Ph3P

Cheng Canadian J. Chem. 1972 (50) 912.

Metallocyclopropanes

chem 153 organotransition metal chemistry · · 2012-08-30organotransition metal chemistry...

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