Novel Conjugated Polymers Containing Ferrocene
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OH
O Br
HCHO,HCl
DMSO/KOH
AcOH
OH
O
O
O
(EtO) 2P
O
P(OEt)3
Cl Cl
- EtCl
O
O
O
P(OEt) 2
1
O
n-B uLi/TM EDA
Fe
DMF
Fe
ethe r O
2
O ( EtO) 2 P
O O
KOtBu + O
O
Fe
THF
P(OEt) 2 O
1
2
O
Fe
soluble in THF, CHCl3, CH2Cl2, and toluene slightly soluble in Et2 O and hexane insoluble in alcohol and water
O
3
n
Scheme 1: Preparation of a processible conjugated polymer containing ferrocene
has several advantages compared to normal Wittig reactions. First of all, it is relatively easy to prepare alkyl dialkylphosphonate reagents from alkyl halides and they are usually very soluble in most organic solvents even they have multiple phosphonate sites, which is a main drawback of multifunctional Wittig reagents. When alkyl dialkyl-phosphonate reacts with aldehyde, it generates only trans double bond. And side product, salt of alkylphosphoric acid, can be easily removed by simple aqueous extraction. This type of reaction has been used to prepare short conjugated systems, and all trans configuration products have been easily prepared [10,11]. Long alkyl chains are introduced in central phenyl ring of bisphosphonate reagent (1) to increase the solubility of resulted polymers. Ferrocene-1,1’-dialdehyde (2) was prepared from ferrocene with a high yield. Organic part (bisphosphonate) and organometallic part (ferrocene-1,1’-dialdehyde) were coupled in the presence of potassium t-butoxide as a base in a refluxing THF solution. The resulted polymer (3) was precipitated by addition of excess amount of MeOH to the reaction mixture. As expected 3 is very soluble in various organic solvents such as THF, CHCl3, CH2Cl2, and toluene; slightly soluble in Et 2O and hexane; and insoluble in alcohol and water. In order to compare, a simple analog (4) of the conjugated polymer was also synthesized from bisphosphonate reagent and ferrocenealdehyde. (Scheme 2)
O (EtO) 2P
O O
KOtBu + O
O
2
Fe
THF
P(OEt) 2
1 O
Fe
Fe
O
4
Scheme 2: Preparation of simple analog (4) of conjugated polymer
RESULTS AND DISCUSSION UV-Vis. spectra of 3 and 4 showed absorption bands corresponding ferrocene and conjugated phenylene vinylene group. (Figure 1) UV-Vis. spectrum of 3 had a broader band in phenylene vinylene absorption region due to the longer conjugation length. 1HNMR spectra also confirmed the formation of polymeric compound 3. The peaks corresponding to free cyclopentadienyl(Cp) ring at 4.15 ppm for the model analog
A b s o r b a n c e
1.5 (a) (b ) (c)
1.0
0.5
0.0 3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e l e n g t h ( n m )
Figure 1. UV-Vis. spectra of (a) ferrocene, (b) compound 4, and (c) polymer 3 in THF
compound 4 disappears in polymeric compound 3, and all the other peaks were detected in exactly same chemical shifts and they were slightly broaden due to the inhomogeneous magnetic environment in the polymeric materials. (Figure 2) Cyclic voltammetry measurements showed the presence of ferrocene units, and polymeric compound 3 had a lower oxidation peak than ferrocene itself because ferrocene units in
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