Main Group OM Part 1 2005

8/8/2019 Main Group OM Part 1 2005

1/49

Main-Group Organometallics

MH+ - CH-

carbanionic in character

susceptible to attack by electrophilessusceptible to nucleophilic attack

Stability M-C is weak compared to M-N, M-O or M-Hal p use for organometallics

in synthesis

M-Cbondenergies cover wide range

within a main-group

decrease with increasing

atomic number


2/49


Lability thermal decomposition Pb(C2H5)4 Pb + C2H5

p Pb, EtH, C2H4, C4H10.

F-elimination

R2C CR2

H

R2C CR2

H

R2C CR2H

E F

Facilitated by vacant orbital at metal to accommodate metal-hydrogen bond pair

(Group I-III)

Stabilisation by adduct formation of Lewis base e.g. (bipy)BeEt2

BeEt2 inflames, (bipy)BeEt2 stable 10-15 min in air


3/49


Reactivitytowards O2 and H2O highest for organometallics with free electron pair,low lying empty orbitals and/or high polarity of M-C

bond

InMe3 pyrophoric/hydrolysed vacant orbital on In, moderate bond polarity

SiMe4 inert/not hydrolysed Si shielded well, low bond polarity

eactivity of M-C bonds may also be controlled by use of sterically demanding

substituents, e.g., (Me3Si)3C, mesityl p kinetic stabilisation

c.f. Zn(CH3)2 Zn{C(SiMe3)3}2

Pyrophoric, explodes with water Stable in air, steam


4/49

Main Structural Types of Organometallic

Compounds

Li

1.0

Be

1.6

B

2.0

C

2.5

N

3.0

O

3.4

F

4.0

Na

0.9

Mg

1.3

Al

1.6

Si

1.9

P

2.2

S

2.6

Cl

3.1

K

0.8

Ca

1.0

Sc

1.3

Ti

1.5

V

1.6

Cr

1.6

Mn

1.6

Fe

1.8

Co

1.9

Ni

1.9

Cu

1.9

Zn

1.7

Ga

1.8

Ge

2.0

As

2.2

Se

2.6

Br

2.9

b

0.8

Sr

1.0

Y

1.2

Zr

1.3

Nb

1.6

Mo

2.1

Tc

1.9

u

2.2

h

2.3

Pd

2.2

Ag

1.9

Cd

1.7

In

1.8

Sn

1.8

Sb

2.0

Te

2.1

I

2.6

Cs

0.8

Ba

0.9

La

1.1

Hf

1.3

Ta

1.5

W

2.3

e

1.9

Os

2.2

Ir

2.2

Pt

2.3

Au

2.5

Hg

2.0

Tl

1.6

Pb

1.9

Bi

2.0

Po

2.0

At

2.2

metals with a strong tendency to form

alkyl- or aryl-bridged species;

covalent, multicenter bonds

metals that

form ionic

derivatives

transition metals

T-complexes tend to

predominate

metals and metalloids that

form volatile, covalent

organo derivatives

mainly M-C W-bonds

rarely M-C T-bonds

non-metals


5/49

Synthesis

DirectSynthesis 2 M + n X nM + MXn (or nMXn)

2 Li + C4H9Br C4H9Li + LiBr

Mg + C6H5Br C6H5MgBr

2 Na + Hg + 2 CH3Br (CH3)2Hg + 2 NaBr4 NaPb + 4 C2H5Cl (C2H5)4Pb + 3 Pb + 4 NaCl

Mixed Metal

Synthesis

not fore.g. Hg or Pb

Transmetallation M + M M + M

Zn + (CH3)2Hg (CH3)2Zn + Hg

favourable when M is higher in electrochemical

series than M

slow

2 Al + 3 MeCl Me3Al2Cl3


6/49

Synthesis

Metathesis M + MX X + M

Li Mg Al Zn

Electronegativity: 0.98 1.31 1.61 1.65

Si B As P

1.90 2.04 2.18 2.19

Li4(CH3)4 + SiCl4 4 LiCl + Si(CH3)4

Al2(CH3)6 + 2 BF3 2 AlF3 + 2 B(CH3)3

Hydrometallation M-H + C C C CHM M = B, Al, Si, Zr, e.g.

(C2H5)2AlH + C2H4 (C2H5)3Al

everse ofF-elimination


7/49

Reaction Pattern

Oxidation - potential reducing agents; for electropositive elements very strong reducing

agents- componds of electropositive metals have unfilled valence orbitals,

or readily dissociate to fragments with unfilled orbitals - pyrophoric

Nucleophilic(carbanion) Character

-organic group at electropositive

- metal has partial negative charge

- strong nucleophile and Lewis base

- most commonly used carbanion

reagents LiR and RMgX


8/49

Reaction Pattern

Protolysis Reaction

Ga Et

Et

Et

Ga O

Et

EtEt

CH3

H

Ga OCH3

Et

Et

CH3OH

- C2H6

Al2(CH3)6 + 6 C2H5OH 2 Al(OC2H5)3 + 6 CH4

Lewis Acidity presence of unoccupied orbitals on metal, electron-deficient

B(C6H5)3 + Li(C6H5) Li[B(C6H5)4]

Al2(CH3)6 + 2 N(C2H5)3 2 (CH3)3AlN(C2H5)3


9/49

Alkali Metal Organometallics Method of Synthesis

DirectSynthesis - organic halide + metal

Transmetallation - using Hg organyls

MetalExchange - PhLi + (CH2=CH)

4Sn 4 (CH

2=CH)Li + Ph

4Sn

(good yields of vinyllithium)

Metal-HalExchange - BunLi + PhX BunX + PhLi (practicable only for ArX; competingreaction Wurtz coupling)

MetallationofC-HAcid- Na + C5H6 C5H5Na + 1/2H2


10/49

Alkali Metal Organometallics Method of Synthesis

Forheavieralkalimetal organometallics widely used method metathesis of

organolithium reagent and an alkoxide

e.g., BunLi + KOBut LiOBut + BunK

in hydrocarbons, easy to separate the MR product, e.g. BunK

CH3CH2OCH2CH3 + KC4H9 C4H10 +

KOC2H5 + H2C=CH2

[CH-O-C2H5]

CH3

K

Ether Cleavage


11/49

Organolithium Structures

i

i

i

i

i

i i

i

i

i i

i

s

i grouor itals

O iagram for one of the four e c on s in R i

hh

ii

i

h

h

i

.O t

.O t

t O.

t O.Ten ency to form

oligomers through

multicentre on s

e i tetramer


12/49


Hexamer of BunLiButLi tetramer


13/49


LiR Solvent Aggregation

MeLi thf, Et2O

Me2CH2CH2NMe2

(tmeda)

tetramer

(Li4 tetrahedron

monomer, dimer

BunLi cyclohexane

Et2O

hexamer

tetramer

ButLi hydrocarbons

thf

tetramer

monomer

LiCH2Ph thf, Et2O monomer

LiC3H5 (allyl) Et2O

thf

columnar structure

dimer

Ph

Ph

Ph

Ph

Li

Li

Li

OEt2

OEt2

Et2O


14/49


(n- i)tme a

i-

Degree of association strongly epen ent on nature of solvent

Affects structures an reactivity by increasing polarity of i- bon

Rates of metallation by Ph i excee those of 3 i by factor of 0 , although

3- stronger base as Ph -

omplexation of i ; polarisation of i- bon ;

carbanionic character of butyl group increase


15/49

Reactions of Organolithium Compounds

Metallation ofC-H, N-H, O-Hacids R-Li + E-H R-H + E-Li

when E-H is stronger acid than R-H

H HLi

+

+ uHu

nLi

HHLi

unLi

+ uH

Reactions with Main-GroupandTransition-metalHalides

RLi + M-X M-R + LiX


16/49

Reactions of Organolithium Compounds

C NR C NLiR

R'

C NHR

R'

C OR

R'R'Li hydrolysis hydrolysis

C

O

NR'2

H R CH

OLi

NR'2 CO

H

RRLi hydrolysis

W

OC

OC CO

CO

CO

CO

W

OC

OC CO

CO

CO

C

R O-Li+

W

OC

OC CO

CO

CO

C

R OCH3

LiR [(CH3)3O]BF4

Additions to Multiple Bonds


17/49

Radical Anion Salts

Na + C10H8(thf) Na[C10H8](thf) sodium naphthalenide

+ ArH+

+ ArH . + ArH + + ArH -

( e5C5) SiBrK, anthracene

- KBrSi


18/49

Radical Anion Salts

-

-.

-

non lanar

nTelectr

ti r tic

pl r

( n+1) T-electr

pl r

( n+2) T-electr

r tic


19/49

Organomagnesium Compounds

DirectSynthesis Mg + RX RMgX(OR2)n

Transmetallation Mg + R2Hg R2Mg + Hg

MetallationR

-C|CH + EtMgBrR

-C|CMgBr + EtH

SchlenkEquilibrium 2 RMgX + 2 dioxane R2Mg + MgX2(dioxane)2


20/49


RX

Mg Mg Mg

RX R MgX

R RMgX

. .

.

Mgacti eMgCl2K, t f

C 17Mg %!C 17F

r.t.,

Formation

Acti ation of Mg: I2, CCl4, 1,2-dibromoet ane

Riecke magnesium:


21/49


2 R g R2Mg + Mg 2

I IIIII I

Mg Mg

R

R

Mg Mg

R R

sol ent it donor properties, usuall et er

Dominant forms: I et er solution of lo concentration

II in Et3N, it dioxane onl MgR2 in solution,precipitation ofMgCl2(dioxane)2

III and I in ig er concentration and it more basic t f

Schlenkequilibrium


22/49


Structure

Generally polymeric/oligomeric structures, where halide (2e2c) bridges are preferred

over2e3calkyl bridges

Exception: [(Me3Si)3C]Mg is monomeric

due to bulky substituents

Al

Mg

Al


23/49


Reactivity

RMgX + R'C|N R C R'

O

RMgX + R'CHO RR'CH-OH

Use in organic synthesis, e.g.

Alkylating/arylating reagents for main group and transition metals halides

l + Mg ( )

Mg

- Mg l

compared to LiR, RMg reagents - less reactive (do not form ate complexes)

- less reducing with transition metal halides


24/49


Magnesium-ate-complexes MxMgyR

z (M = group 1,2 and 13);

the less EP metal usually in ate-complex anion

MgMe2+ iMeEt2O

iMgMe3(Et2O)ntmeda

lithium magnesiate

Me i


25/49

Organometallics of Calcium, Strontium and Barium

Synthesis

Highly reactive due to predominantly ionic character of metal-ligand bond

increased lability complicates synthetic access; unstable and/or sparingly soluble

Transamination M[N(SiMe3)2]2 + 2 HR

MR

2 + 2 HN(SiMe3)2

Direct metallation 2 HR + activated M MR2 + H2

Transmetallation/ HgR2 + activated M MR2 + Hg

Metal exchange

2 LiR + M(OR)2 MR2 + 2 LiOR

M[N(SiMe3)2]2 + 2 LiR MR2 + 2 LiN(SiMe3)2

M[N(SiMe3)2]2 + 2 LiCH2Ph M(CH2Ph)2 + 2 LiN(SiMe3)2

MI2 + 2 LiCp* Cp*2M + 2 LiI


26/49


Most intensively studied are the cyclopentadienyl systems

Magnesocene is useful reagent for introduction of C5H5 groups

Cp2Mg Cp*2Ca

M E,

MCp*2

Mg 180

Ca 154

Sr 149Ba 148

M2+ HH

Sterically should be parallel, however explained

by polarisable ion model bending due to dipole induced

on large central cation (also Yb(II), Eu(I) analogues)

maximises electrostatic bonding


27/49


purelyW

-bonded ligands - scarce

[Ca{CH(SiMe3)2}2(1,4-dioxane)2] Ca[C(SiMe3)3]2

CCaC 150r

Lappert, 1991 Eaborn/Smith, 1997


28/49

Tris(trimethylsilyl)methylmagnesium and -calcium


29/49

Organoaluminium Compounds

Synthesis

Transmetallation 3 Ph2Hg + 2 Al 2 AlPh3 + 3 Hg

Metathesis (RLi orRMgX) AlCl3 + 3 ButLi AlBut3 + 3 LiCl

Hydroalumination 3 RCH=CH2 + AlH3.OEt2 (RCH2CH2)3Al

.OEt2

Direct synthesis 2Al + 3RX 2 R3Al2X3 sesquihalide

Properties

Alkyls are usually colourless liquids that react violently with air and water; short

chain lengths pyrophoric

Lewis acidic (6 valence electrons) marked effects on structure and reactivity


30/49


Applications

Aufbau reaction (growth reaction) - multiple insertion of ethylene into the Al-C bond

e.g. AlC2H5 + C2H4 p AlC4H10 etc (Ziegler)

- produces 1-alkenes and (after reaction with

dioxygen and hydrolysis) unbranched C16 C20

primary alcohols for detergent industry

Catalytic dimerisation of propene basis for production of isoprene

(-synthetic rubber)

Olefin polymerisation Ziegler-Natta low-pressure process with mixed

catalysts like Et3Al/TiCl4


31/49


CCatalytic dimerisation of propene

CH2=CHCH3Pr3Al Pr2AlCH2CHMePr

CH2=CHCH3

Pr2AlH + CH2=CMeCH2CH2CH3

cracking

CH2=CMeCH=CH2 + CH4

isoprene


32/49


Structure andbonding

Al Al

C

C

109.5 Al Al 120

C

C

C

Al

C

AlAl CAl Al

AlC

AlAl

(2e3c) (2e3c) (2e2c) (2e4c)+ +

a b

Al(sp3) Al(sp2)

Al

H3C

H3C

Al

CH3

CH3

H3C

CH3

75

260 pm

214 pm

123

197 pm

H

H

Al2(CH3)6

Al2Cl6 d(Al-Al) 340 pm with Al-X-Al bridges

rcov Al = 252 pm [rcov(Al) = 146 pm]


33/49


l l

a

l l76

270 pm

218 pm

115

196 pm

114 sp2 120

l l

c

spsp

109.5

Q- 6H5 Q- 6H5

Structure andbonding Al2(C6H5)6


34/49


Associationin solution Al-C-Al bridging persists in non-polar solvents with

fast Al-Me exchange

50 rC + 20 rC

0.5 6.5 0.3

a R = Me b R = Ph

Al

R

R

R

R

R

R

Al Al

R

R

R

R

R

R

Al Al

R

R

R

R

R

R

Al Al

R

R

R

R

R

R

Al

**

*

*

a

b


35/49


R2AlX and RAlX2 (X = halide) are most conveniently prepared by redistribution

of trialkyls and trihalides in correct stoichiometry. Reactions occur readily at RT.

2 R3Al + AlCl3 3 R2AlCl

R3Al + 2 AlCl3 3 RAlCl2

X

Al

X

Al

Me Me

Me Me

usually oligomers, formed by Al-X-Al bridging

(interaction with heteroatom lone pair favoured over Al-C-Al)

Al-Me-M bridges also formed with other acidic metal centres, e.g.

AlMe3 + Cp2Yb


36/49


Reactivity

Organoaluminium compounds are hardacids and readily form adducts with bases

such as thf and amines

Mes3

Al + thf

Reactions with protic reagents gives access to wide variety of organoaluminium

compounds

AlR3-n + nROH R3-nAl(OR)n


37/49


Formation of ate-complexes AlR3 + LiR Li[AlR4]

Carbalumination

Hydroalumination

H2C CHR

Et2Al Et

Et3Al(Et

3Al)

2

CH2=CHR H2C CHR

Et2Al Et

1/2

+

H2C CHR

Et2Al H


38/49

Organometallics of Ga, In and Tl

Synthesis

R3M may be prepared by same methods as forR3Al metathesis orRLi/MgX with MX3

- transmetallation with organomercurials

Halides RnMX3-n most readily prepared by redistribution reactions

Structure R3Ga and R3In monomeric Lewis acidity less than that of AlMe3Ga very important ion semiconductor industry more inflammable

than dimeric Me3Al

ReactivityLess reactive than aluminium compounds so possible to get e.g.Me2GaOH easily


39/49

Organometallics of Ga, In and Tl

X

GaX

Ga

R R

R R

InCl

In Cl

Cl In

[Me2InCl]n

in R2MX compound the larger In can adopt coordination numberes > 4

leads to coordination polymers in solid state


40/49

Group 14 Organometallics

E Thermal stability Bond energy

E(E-C) in kJ/mol

Bond length

d(E-C) in pm

Bond polarity

EH+ - CH-EN

C 358 154 2.5

Si 311 188 1.9

Ge 249 195 2.0

Sn 217 217 1.8

Pb 152 224 1.9

Common oxidation state of +4 with increasing stability of +2 as group is descended

In contrast to group 13 derivatives R4M derivatives show lower bond polarity of the E-C bond,

have an octet configuration and reactivity towards nucleophiles is diminished,

ie. ER4 species are usually water-stable and often air-stable


41/49

Group 14 Organometallics

Chlorination of ER4

species: for E = C and Si chlorination of organic part but for

E = Ge, Sn or Pb cleavage of the E-C bond

CEH

+ H-

u El

nucleophilic

attac

electrophilic

attac

Availability of empty nd orbitals at E renders associative

Mechanism of substitution possible by extending

the C to 5

Successive replacement ofR by more E groups X in

RnEX4-n increases affinity of E for attacking nucleophiles

Me4Sn inert towards H2O and [SnMe6]2- unknown, but

Me2SnCl2 hydrolyses and [Me2SnCl4]2- can be prepared


42/49

Organosilicon Derivatives

Preparation

Metathesis withR

Li,R

MgX orR

3Al SiX4 + nLiR

SiX4-nR

n + nLiX

3 SiX4 + 4 AlR3 3 SiR4 + 4 AlX3

Hydrosilylation (anti-Markovnikov) R3SiH + CH2=CHR R3SiCH2CH2R

Industrially, prepared by 2 RCl + Si/Cu R2SiCl2 (ca. 70%)

Organosilanes -properties

R4Si are H2O and O2 stable because of low bond polarity, heterolytic cleavage does

not occur readily

Si-C bonds are thermally stable and do not decompose before ca. 700C

Leads to use in silicon polymers from hydrolysis of chlorosilanes now a very mature

industry

Chlorosilanes are commonest precursor for wide range of organosilanes

Rochowprocess


43/49

Organosilicon derivatives

R2SiCl2 + H2O / catalyst p (R2SiO)n + ClR2Si(OSiR2)nOSiR2Cl

cyclics linear

D4 predominates

Mainly R = Me 106 tonnes/year

Thermal stability up to 350 rC Unzipping rate less when R = Ph

Low temperature coefficient of viscosity Tg < 120 rC

SiOSi ca 145 rC easy rotation

Good insulator

Hydrophobicity coatings, waxes, sealants

Many variations in pendant groups - copolymers

Si O

SiO

Si O

SiO


44/49

Organosilicon Derivatives

SelectedReactions of organohalosilanes

R i l

R iOH

R i H2 (R i)2NH

R i H (R i)2S

(R Si)2O

R SiR'

R SiH

R Si-SiR

R SiSRR SiOR'

R'Li

Li lH4

Li

RSNa

R'OH

H2S

NH3H2O

-H2O

-NH3

-H2S


45/49

Organo(germanium), -tin and (-lead) derivatives, RnMX4-n

Preparation ofR4M most commonly by metathesis of MX4 with RLi, RMgX and R3Al

derivatives

other methods similar to those employed for organosilanes

Preparation of the most important routes involve redistribution of tetraalkyl

RnMX4-n derivatives with tetrahalides; in contrast to group 13 derivatives

these reactions generally involve elevated temperatures

(ca. 170 C)

Structure/ R4Sn simple tetrahedral (sp3) coordination is encountered inproperties tetraalkyls which are air/moisture stable

R3SnX with more electronegative groups attached the Lewis acidity

increases and higher coordinate derivatives result from coordination

by bases in absence of external bases polymeric structures are

common

R

SnR

R

Me SnMe

MeCl

N

Me

Sn

MeMe

Me

Sn

MeMe

F

Me

Sn

MeMe

F

F

Me3SnF, 1 polymer with tbp Sn (5 coordinate)


46/49

Organotin Derivatives

R3Sn l

R3SnOH

R3SnN'R2

(R3Sn)2O

R3SiR'

R3SnH

R3Sn-SnR3

R3SnMn( O)5

R3SnOR'

R'Li

Li lH 4

Na

NaMn( O)5

R'OH, R3N

LiNR'2

H2O

R3Sn 5H5

Na 5H5

SelectedReactions of organohalostananes

ompounds containing R3S

n are toxic and must be handled with great care


47/49

Organotin Hydrides

RnSnX4-n RnSnH4-nLiAlH4, Et2O

HC

HC C

O

PhH

H2C

H2C C

O

PhH

+ Bun3SnH

1. cat. (Ph3P)4Pd,

thf, 20 C

2. H2O

Et3SnH + CH2=CH-CH=CH2 Et3SnCH2-CH=CH-CH3AIBN

in absence of Lewis acids or radical forming reagents polar double bonds like C=O or

C=N are not attackedwell-suited forchemoselective hydrogenation of activated C=C bonds

in presence of AIBN 1,4-addition to conjugated dienes occurs

Sn H + A B Sn A B HHydrostannation


48/49

Organotin Hydrides

R'3Sn-H R'3Sn. + H

.

R'3Sn. + R R'3Sn + R

.

R. + R'3SnH RH + R'3Sn.

(start)

(propagation)

Sn H + X Y Sn Y + HXHydrostannolysis

R3Sn-H + Me OOH R3SnOO Me + H2 H-

4 R3Sn-H + i(NR'2)4 (R3Sn)4Ti + 4 HNR'2 H+

2 R3Sn-H + R'2Hg (R3Sn)2Hg + 2 R'H H.

R3SnH transfer

R6Sn2R =Me, -10

R = , +100

-Hg

low polarity ofSn-H bond R3SnH may act as a source of H-, H+ or H.,

depending on nature of attacking agent


49/49

Organotin Hydrides

Me2 Br2 Me2

Br

Me2 H2

But3SnH

hY

But3SnH

hY

most convenient methods for conversion R-X to R-H

e.g., selective reduction of geminal dihalides in presence of other sensitive

groups

Documents

Main Group OM Part 1 2005