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III. Synthesis of Polymers with Metal-Metal Bonds along their Backbones

III. Synthesis of Polymers with Metal-Metal Bonds along their Backbones

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Synthesis of Polymers with Metal-Metal Bonds along their Backbones



261



illustrates the reaction of a metal-metal bonded dialcohol with hexamethylene

diisocyanate (HMDI) to form a polyurethane.15



O

O C

C

HOCH2CH2



Mo



CO



Mo



OCN CH2

CH2CH2OH



C

O C C

O

O



6



NCO



Dibutyltin diacetate (cat.)

p-dioxane, 26°C



(3)

O

O C

C

OCH2CH2



Mo

C

O C C

O O



Mo



CO



O

O

CH2CH2OCNH(CH2)6NHC

n



This step-growth polymerization strategy is general, and a number of

metal-metal bond-containing polymers have been made from monomers containing functionalized cyclopentadienyl ligands.13 In principle, step polymers

could be synthesized from monomers containing other derivatized ligands, but

in practice not many other ligands have been derivatized for this purpose.

The comparatively weak metal-metal bonds (e.g., DMo-Mo 32 kcal mol21)

pose problems for the synthesis of the polymers. In particular, the relative

weakness of the metal-metal bonds makes them more reactive than the bonds

found in standard organic polymers, thus under many standard polymerization

reaction conditions, metal-metal bond cleavage would result. For example,

metal-metal bonds react with acyl halides to form metal halide complexes.

Therefore, the synthesis of polyamides using metal-metal bonded “diamines”

and diacyl chlorides would simply lead to metal-metal bond cleavage rather than

polymerization. Likewise, metal-metal bonded complexes are incompatible with

many Lewis bases because the Lewis bases cleave the metal-metal bonds in disproportionation reactions.32 This type of reactivity thus rules out many standard

condensation polymerization reactions in which bases are used to neutralize any

acids produced. Similar reasons prevent the use of acyl chlorides in the synthesis

of polyamides. Polymerization strategies must therefore be carefully designed to

avoid cleaving the metal-metal bond during the polymerization process.

A sample polymerization reaction, showing the synthesis of a polyurethane, is shown equation 3. Using similar synthetic strategies, various polyurethanes, polyureas (eq. 4), and polyamides (eq. 5) have been synthesized.14À17

Note that the step polymers in these reactions have a metal-metal bond in every

repeat unit. Copolymers are straightforwardly synthesized by adding appropriate difunctional organic molecules into the reaction mixture (e.g., eq. 6).



262



Polymers with Metal-Metal Bonds as Models



ϩ

NOϪ

3 H3N CH2CH2



OCN(CH2)6NCO

(CO)3Mo—Mo(CO)3



K2CO3, toluene

H2O, 23°C



CH2CH2NH3ϩNO3Ϫ



(4)

HNCH2CH2

(CO)3Mo—Mo(CO)3



O

O

CH2CH2NHCNH(CH2)6NHC



O



ϩ

NOϪ

3 H3N CH2CH2



(CO)3Mo—Mo(CO)3

CH2CH2NH3ϩNO3Ϫ



O



n



O

O



O



O



n O



O



THF, 3 Eq. TEA



(5)

O

O C

C



O

CHNCH2CH2



Mo



CO



Mo



C

O C C

O O



HO



O O

C C

CO

Mo Mo



OC C C

O O



O

CH2CH2NHC(CH2)n

n ϭ 4, 8



m



OH ϩ HO(CH2)4OH ϩ OCN(CH2)6NCO



DBTA

THF



(6)

O

O

O(CH2)4OϪCNH(CH2)6NHC



O

m



O O

C C

CO

Mo Mo

C

O C C

O O



O

O

O CNH(CH2)6NHC

n



Yet another step-growth synthesis strategy is to react the difunctional

dimer molecules with prepolymers. Equation 7 shows an example of this technique.16 (As received from the manufacturer, prepolymers are often ill-defined



Synthesis of Polymers with Metal-Metal Bonds along their Backbones



263



materials. In this instance, analysis of the prepolymer sample showed it to

contain, on average, three tolyl isocyanate end groups; Mn was B 2000.)

CH3



CH3



O

O

NHCO(C2H4O)nCNH



OCN



O O

C C



NCO



HOCH2CH2



CO



Mo



OC C C

O

O



CH3



O



Mo



O

(C2H4O)nCNH



CH2CH2OH



DBTA (cat.), THF, 48°C, 8h, dark



NCO



(7)

CH3



CH3



O

O

NHCO(C2H4O)nCH2CH2OCNH



O

CNH



O

NHCOCH 2 CH 2



CH3



O



CH2CH2O

(CO)3Mo—Mo(CO)3



O

NHCOCH 2 CH 2



O

(C2H4O)nCNH



(CO)3Mo —



n



1



Again, copolymers can likewise be synthesized by using prepolymers and

another organic difunctional molecule (eq. 8).33

CH3



CH3

O

N C O

H



O C N



ϩ



HO



CH2CH2O



O

CH2CH2O n C N

H



O O

C C

N C O ϩ



HO



Mo Mo

OC C C

O O



CO

OH



DBTA

m



H

THF, 50°C



(8)

CH3

O H

C N



CH3

H O

O H

N C O CH2CH2O C N

n



H O

N C



O R' O

p



2

O O

C C

R' ϭ



Mo Mo

OC C C

O O



CO



or



CH2CH2O



m–1



CH2CH2



In another example of a step-growth polymerization reaction that yielded

polymers containing metal-metal bonds, Moran and co-workers reported the



264



Polymers with Metal-Metal Bonds as Models



synthesis of a number of polysiloxanes that contain Fe-Fe bonds in their

backbones. Their syntheses start with the derivatized Cp2Fe2(CO)4 dimer:34

O

C



CH3

Me2N Si

CH3



Fe



O

C



CH3

Si NMe2

CH3



Fe

C

O



C

O



Reaction of this molecule with disilanols gave siloxanes (eqs. 9 and 10).



Me2N



CH3

Si

CH3



CH3

Me2N Si

CH3



O

C

Fe



O

C

Fe



C

O



C

O



O

C

Fe



CH3

Si NMe2

CH3



O

C



CH3

Si NMe2

CH3



Fe

C

O



C

O



Ph

HO Si OH

Ph



CH3

Si

CH3



O

C

Fe

C

O



O

C



CH3 Ph

Si O Si O

CH3 Ph



Fe

C

O



(9)

n



HOSiMe2O(-SiMe2O-)nMe2SiOH



(10)

CH3

Si

CH3



O

C

Fe

C

O



O

C

Fe



C

O



CH3 CH3 CH3 CH3

Si O Si O Si O Si O

CH3 CH3 CH3 CH3

n



m



The step-growth polymerization strategy used to incorporate metal-metal

bonded units into polymers can also be used to incorporate metal clusters into

polymers. One of only a few examples of this type of reactivity is shown by

equation 11.35 It is noteworthy that metal clusters also undergo photochemical

reactions.36,37 These reactions should also occur when the clusters are incorporated into polymer backbones. If polymers containing metal clusters can be

shown to have unusual properties or applications (photochemical or otherwise), then the synthesis of these polymers will likely burgeon in coming years.



HO



Mo



OH



Mo



+



DBTA



O C N R N C O



Ir



p-dioxane, 40°C

Ir



(11)

O

O



Mo



Mo



O



O

NH R NH



Ir

Ir



n



Synthesis of Polymers with Metal-Metal Bonds along their Backbones



265



B. ADMET Polymerization

As noted above, the syntheses of polymers containing metal-metal bonds

is challenging because the metal-metal bonds are relatively weak and will not

stand up to many of the conditions typically used for the synthesis, isolation,

and purification of organic polymers. To avoid the problems associated with

normal step polymerization methods, new synthetic routes to these polymers

are being investigated. For example, ADMET polymerization (acyclic diene

metathesis polymerization; eq. 12) of α,ω-dienes is another step-growth method

that is potentially useful for the synthesis of polymers with metal-metal bonds

(eq. 13). First demonstrated by Wagener in 1990 with organic monomers,

ADMET is a versatile method used to synthesize a broad range of organic

polymers with varying functionality and intriguing architectures.38À41

R



R



n



ϩ n C2H4



(12)



R'

[Cat]

RLnM



MLnR



RLnM



m R'



MLnR



(13)

n



Overall, ADMET polymerization is an advantageous strategy for synthesizing transition metal-containing polymers because of the functional group

tolerance of the metathesis catalysts employed, the mild conditions under

which it operates, the wide range of accessible architectures, and the precise

structural control afforded by the method.

One strategy to obtain organometallic α,ω-dienes for ADMET is to substitute phosphine ligands containing terminal alkene substituents onto metalmetal bonded dimers.42 An example of one such dimer is shown in equation 14.

Ph2P



CO

OC

OC



Mo



Mo



CH2Cl2



CO ϩ 2 PPh2

6



OC



Mo



Mo



OC



6

CO



(14)



CO

PPh2

6

3



Unfortunately, no ADMET polymerization of complex 3 was achieved

with Grubbs generation 1, Grubbs generation 2, or Schrock’s catalysts.42

Control experiments showed that the lack of reactivity of the Grubbs catalysts

toward Cp2Mo2(CO)4(Ph2P(CH)6CH 5 CH2)2 (1) is not related to catalyst

decomposition by the phosphine or to deactivation due to Ph2P

(CH)6CH 5 CH2 coordination on the catalyst. Presumably, the inability of

complex 3 to polymerize is due to its sterically demanding nature. For example,

in the step polymerization of related complexes reported by Humphrey and

Lucas,35 the length of the alkyl chain between the complex and the reacting

group was found to significantly effect the extent of polymerization. Specifically, they found that methylene chain lengths of at least 10 carbons were

required before step polymerization reactions could occur in materials with



266



Polymers with Metal-Metal Bonds as Models



Cp-substituted reactants. It is not clear why such long chain lengths are required to

alleviate steric interactions in the Humphrey-Lucas molecules, but similar steric

effects may be acting in the polymerizations of complex 3, where two molecules of 3

and a catalyst molecule all need to interact to carry out the polymerization.

Nevertheless, despite this one setback, ADMET remains an attractive method for

synthesizing new polymers with metal-metal bonds along their backbones.



C. Chain-Growth Polymers

Few chain growth polymers with metal-metal bonds have been reported.

The general synthetic route to these materials is to substitute a ligand on a metalmetal bonded dimer with a polymerizable olefin. In the case of Cp2M2(CO)n-type

molecules (M 5 Mo, W, Fe), it is difficult to synthesize a dimer that has only

one substituted Cp ring, and hence both Cp rings are substituted with polymerizable olefins. This leads to cross-linked polymers with metal-metal bonds in

the chain. Examples of this reactivity are shown in Scheme 4.43

An example of a chain-growth polymer that uses an olefin-substituted

ligand other than cyclopentadienyl is shown in equation 15.44 The electrons in

this reduction reaction are provided electrochemically. This polymer is particularly interesting because the metal-metal bond can be reformed by electrochemical reduction following photochemical cleavage in the presence of CCl4

radical trap (Scheme 5). It was demonstrated that the reversibility of the Re-Re

bond cleavage (and similar reactivity in other systems) may be useful in

applications involving reversible imaging.



N

OC

O



C



N

Re

C

O



Cl



N



2eϪ

(CO)3Re

N



N

Re(CO)3



(15)



N



IV. PHOTOCHEMICAL REACTIONS OF

THE POLYMERS IN SOLUTION

The photochemistry in solution of the polymers with metal-metal bonds in

their backbones is qualitatively similar to the reactions of the discrete metal-metal



Photochemical Reactions of the Polymers in Solution



O



CH2

N CH

O

O

H 2C

C C

F

e

Fe

HC

C C

O

O

n



CH2

CH3CO2 CCH3

H2C



CH



O

C



HC



CH2



n



HC N

H2C

O

m



O

C

CH

Fe Fe

C C

CH2

O

O

CH3C CO2CH3

H2C

m



CH2ϭC(CH3)CO2CH3



N

O



267



O

C



O

C

Fe Fe

C C

O

O



CH2ϭCHCN



CH2



CH2



CH



NC CH



O

H2C OC C

Fe Fe

HC

C C

O

O

n



O

H2C OC C

F

e

Fe

HC

C C

O

O

n



CH

CH2

HC CN

H2C

m



CH

CH2

HC

H2C

m



SCHEME 4. Synthesis of chain-growth polymers containing metal-metal bonds along

their backbones.



N

(CO)3Re

N



N



N

Re(CO)3



h

CCl4



Ϫ



ϩ2

Ϫ2Cl



N

(CO)3Re Cl

N



N



Cl Re(CO)3



N



SCHEME 5. Photochemical cleavage of the Re-Re bond and electrochemical formation of the Re-Re bond in poly[(vbpy)Re(CO)3]2.



bonded dimers in solution.15,45,46 Irradiation of metal-metal bonded complexes

into their lowest energy absorption band ( 500 nm) generally leads to one of four

fundamental types of reactivity:15,45,46

1. The metal radicals produced by photolysis react with radical traps to form

monomeric complexes (e.g., eq. 16).



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