[acs symposium series] metal-containing and metallosupramolecular polymers and materials volume 928...

Chapter 28

Neutral and Cationic Cyclopentadienyliron Macromolecules Containing Azo Dyes

Alaa S. Abd-El-Aziz * and Patrick O. Shipman

Department of Chemistry, The University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canada

*Corresponding author: [email protected]

Abstract

A number of classes of organoiron polymers containing azo dyes pendent to, or in their backbone have been prepared. The first class includes cationic organometallic polyethers, polythioethers and polynorbornenes incorporating aryl and hetaryl azo dyes. These organometallic polymers were synthesized via nucleophilic aromatic substitution reactions or ring opening metathesis polymerization. The resultant polymers were all brightly colored and displayed excellent solubility in polar organic solvents. Thermal analysis indicated that the polymers were thermally stable with decomplexation of the metal moiety at approximately 235 °C. Photolytic demetallation of the polymers resulted in the decoordination of the cationic cyclopentadienyliron moieties and the formation of organic polymers. The organic polymers displayed lower glass transition temperatures than their cationic organoiron analogues. The second class of polymers is ferrocene based polymers with pendent cationic cyclopentadienyliron moieties. The reaction of arene complexes containing azo dye chromophores and terminal hydroxyl groups with 1,1'-ferrocenedicarbonyl chloride gave rise to triiron complexes with a substituted ferrocene center and two terminal arene complexes containing chloro groups. Nucleophilic aromatic substitution polymerization of these triiron complexes with O- and S-containing nucleophiles followed by photolytic demetallation led to the isolation of neutral ferrocene based polymers containing azo dyes in the backbone. Electrochemical studies of these complexes showed the reduction of the cationic iron moieties at -1.42 V , while the neutral iron species were oxidized at 0.89 V . UV-visible studies showed absorption at 419 nm and a bathochromic shift to 530 nm with the addition of H C l .

© 2006 American Chemical Society 401

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In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

mailto:[email protected]

402

Background

Due to the large number of potential applications for organometallic macromolecules, there has been tremendous growth in this field of research over the past two decades.1"4 Organometallic macromolecules can be used as electrocatalysts, chemical sensors, and photoactive molecular devices. Based on the nature of the organic ligand as well as the metallic moiety, organometallic macromolecules can be divided into a number of different classes. Metals can be either σ- or π-bonded to the carbon skeleton of the polymer backbone or the side chains. The most widely studied type of organometallic polymer is that which incorporates metallocenes into their structures.5*9

Cationic cyclopentadienyliron chloroarene complexes have been utilized in the synthesis of novel classes of organoiron monomers and polymers.10*15

Sequential nucleophilic aromatic substitution reactions of dichlorobenzene complexes with hydroquinone has given rise to organoiron oligomers and polymers with well-defined molecular weights and molecular weight distributions.16 A1 arge v ariety ο f ρ oly(aromatic e thers) h ave b een s ynthesized using this methodology, including those containing up to 35 cyclopentadienyliron moieties pendent to the alternating arene rings in the backbone.17 Nucleophilic aromatic substitution polymerizations of dichloroarene complexes with O- and S-containing nucleophiles allowed for one-step syntheses of cationic cyclopentadienyliron polyethers and polythioethers.1 8 , 1 9 Ring opening metathesis polymerization (ROMP) of cationic organoiron norbornenes using Grubbs' catalyst has allowed for the production of polynorbornenes with pendent cationic cyclopentadienyliron moieties.20

Azo dye containing polymers, commonly referred to as photoresponsive polymers are interesting for numerous reasons. One reason that stands out is the cis-trans isomerism of the azo dye. 2 1" 2 4 It is thought, and in some cases proven that this isomerism in conjunction with the special environment of the polymer matrix, can control the chemical and physical properties of the polymer, such as conductivity, glass transition temperature, and metal ion capture ability. 2 4

Incorporation of azo dyes into polymers can occur by two main methodologies; in the first method, the azo dye is incorporated through a diffusion or adsorption pathway into a pre-made polymer.2 4 The second method uses a reactive azo dye, which can be reacted to form either the side chains or the backbone of the polymer.2 4 The incorporation of azo dyes into organometallic polymers is relatively new. These polymers are particularly interesting due to the combination of the physical properties of the azo dyes with those of the organometallic complex. , 0 » 2 1 * 2 4

This chapter wil l provide an overview of recent developments in the field of organoiron polymers containing azo dyes. Synthetic routes to obtain these

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polymers, their physical and chemical properties, and spectral studies will also be discussed.

Linear Organometallic Polymers Containing Azo Dyes as Side Chains

Aryl Azo Dye Containing Organoiron Polymers:

Recently, the synthesis of the first examples of cationic organoiron macromolecules with azo dyes incorporated in their side chains has been reported.2 5'2 7 The synthesis of the valeric acid based diiron complex (1), followed by the condensation reaction of the bimetallic complex with a number of azo dyes containing terminal hydroxyl groups (2a-c) allowed for the formation of the cationic diiron complexes with pendent azo dyes (3a-c) (Scheme 1).

Scheme 1

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Nucleophilic aromatic substitution polymerization reactions of these azo dye functionalized complexes with a number of O- and S-containing nucleophiles has led to the formation of polymeric materials with pendent azo dyes in very good yields (Scheme 2). Polymers 4-6 exhibited excellent solubility in polar organic solvents and were found to be thermally stable with the decoordination of the cyclopentadienyliron moiety between 220-240 °C and the degradation of the backbone starting at 450 °C. Differential scanning calorimetry (DSC) showed that the polymers 4a-c exhibited glass transition temperatures (T g) that ranged from 110 to 125 °C. 2 5

Photolysis of these polymers in an acetonitrile/dimethylfonnamide mixture allowed for decomplexation of the cationic cyclopentadienyliron moieties, which yielded the organic polyethers or polythioethers with the azo dyes intact (7a-c -9a-c). Due to the interaction of the cationic organoiron moieties with the gel

Scheme

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405

permeation chromatography (GPC) column, the molecular weights of the metallated polymers were estimated from the molecular weights of their corresponding demetallated analogues. M w's for the metallated polymers 4-6 ranged from 13 400 to 31 600 with polydispersities from 1.2 to 2.6. Polymers 7-9 exhibited much lower glass transition temperatures than those of their organometallic analogues. The glass transition temperature for polymers 9a-c was between 34 and 64 °C and was between 110-122 °C for polymers 7a-c and 8a-c.

UV-visible studies indicated that the polymers exhibited similar U V spectra to their respective monomers. The peaks obtained in various organic solutions of varying pH are indicative of the n-> π* and π-> π* transitions of the azo dye. 2 6

These polymeric materials displayed bathochromic shifts when the R group was altered to a more electron withdrawing group. The spectra of 4a-c are shown in Figure 1 as an example. The XmM for the polymers with a weakly electron withdrawing group ranged from 418 to 420 nm, whereas polymers containing a highly electron withdrawing group displayed between 451 to 454 nm. Bathochromic shifts were also seen with changing pH, thus increasing the pH of the solution showed a shift from 418 to 520 nm for polymer 4a due to the formation of the azonium ion.

The second class of linear organometallic polymers is organoiron polynorbornenes functionalized with aryl azo dyes.2 6 Preparation of monomers 14a, b was accomplished via the reaction of azo dyes 10a,b with the organoiron complex (11) followed by a condensation reaction with 5-norbornene-2-carboxylic acid (13) as describe in Scheme 3. ROMP of these monomers using Grubbs' catalyst in dichloromethane led to die isolation of substituted polynorbornenes (15a, b) in very good yields.

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350 400 450 500 550 600 Wavelength (nm)

Figure J: UV-visible spectra for organoiron polymers 4α-κ

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These brightly colored polymers (15a, b) showed λ , ^ values between 420 and 430 ran in D M F solutions with a bathochromic shift upon addition of HC1 solutions to 518 nm. Polymers 15af b displayed excellent solubility in polar organic solvents and had molecular weights between 16 800 and 32 000 with a polydispersity index between 1.10 and 1.16. Electrochemical studies of polymers 15a, b showed that the cationic cyclopentadienylironarene complexes

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408

underwent a reversible reduction between -1.2 to -1.4 V , with the production of the nineteen-electron iron complex from the eighteen-electron complex. The cathodic and anodic peak potentials occurred at -1.51 and -1.19 V respectively (Figure 2).

- ΐ76 - Γ. 2 -1 ! 4 -1~ 6 -1." 8 E(V)

Figure 2: Cyclic Voltammogram of Polymer 15a.

Thermogravimetric analysis (TGA) showed that the cationic organoiron polynorbornenes 15a, b were thermally stable, with the cleavage of the cyclopentadienyliron moiety and partial decomposition of the side chains starting between 225-231 °C. Degradation of the backbone occured between 400-450 °C. Organic azo dye functionalized polynorbornenes (16a, b) were isolated by the photolytic cleavage of the cyclopentadienyliron moiety of the organoiron polymers. Complete thermal data for these polymers are listed in Table 1.

a= C H 3

b = C H 2 C H 3 16a, b

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Table 1. Thermal analysis of azo dye containing organoiron polynorbornenes and their organic analogues.

Polymer First Weight Loss

Second Weight Loss

Tg°C

T°C % T°C %

15a 226 12 450 25 178

15b 225 16 437 36 172

16a 230 12 400 31 147

16b 231 23 424 23 145

Polymers 16a, b exhibited lower glass transition temperatures than those for their metallated analogues. For example polymer 16a showed a glass transition temperature of 147 °C, whereas polymer 15a exhibited a glass transition temperature of approximately 178 °C.

Hetaryl Azo Dye Containing Organoiron Polymers:

New research has delved into azo dyes containing heterocyclic rings due to the color enhancement and increased spectral range of these dyes. 2 3 , 2 4 Azo dyes based on thiazol rings are of particular interest since thiazole ring systems have shown antibacterial and anti-inflammatory activity.2 3 The first examples of organoiron polynorbornenes containing hetaryl azo dyes based on benzothiazole (17,18) have recently been reported.27

The synthesis of organoiron polynorbornenes containing hetaryl azo dyes followed the same methodology as previous organoiron polynorbornenes containing azo dyes. UV-visible studies showed that polymer 18 exhibited a λ , ^ at 423 nm and polymer 17 displayed a λ ^ χ at 520 nm. Upon the addition of an HC1 solution, the polymers underwent a bathochromic shift to 522 and 608 nm respectively. The UV-visible spectrum for polymer 17 is shown in Figure 3 as an example.

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Wavoknglh (tun)

Figure 3: UV-visible spectrum ofpolymer 17, a) before addition of HCl solution, b) after addition of HCl.

Electrochemical studies at low temperatures showed that these polymers exhibited the reversible reduction process of the cationic iron centers at -1.08 V . The weight average molecular weights of polymers 17 and 18 were 24 500 to 40 900, respectively. Thermal analysis showed that polymers 17 and 18 possessed glass transition temperatures of 146 and 161 °C respectively.27

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Ferrocene Based Polymers Containing Azo Dyes in the Backbone

Nucleophilic aromatic substitution reactions of p-dichloro-arenecyclopentadienyliron complex (19) with azo dyes possessing terminal phenolic and alcoholic groups (20a, b) gave rise to organoiron azo dye complexes (21a, b) with terminal alcoholic groups. These complexes 21a, b were further reacted with Ι,Γ-ferrocenedicarbonyl chloride (22) to yield monomers 23a, b which contain a bridging ferrocene unit, two backbone azo dye units, and two pendent chloro functionalized cyclopentadienyliron units (Scheme 4). 2 8

è HO - C K «

19 20a, b

a: R'=H, R= C H 2 C H 3

b: R'=CH 3 , R=CH 2 CH 3

κ 21a, b

22

R

Fe+ ο

Scheme 4

23a, b

Reaction of the monomers 23a, b with various S- or O-containing dinucleophiles led to the synthesis of organometallic polymers (24-26) as described in Scheme 5. These brightly colored organoiron polymers (24-26) showed good solubility in polar organic solvents and exhibited weight averaged molecular weights between 11 000 to 16 000. Thermogravimetric analysis showed that polymers 24-26 were thermally stable with cleavage of the cationic cyclopentadienyliron moiety between 210-300 °C.

Neutral ferrocene based polymers containing azo dyes in the backbone (27-29) were formed through the photolysis of their corresponding cationic organoiron polymers (Scheme 5). Polymers 27-29 exhibited lower glass

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transition temperatures (85 to 92 °C) than those of their cationic analogues polymers 24-26 (126 to 164 °C). Electrochemical studies of the cationic polymers 24-26 d isplayed two redox processes. The first process showed the oxidation of the neutral iron centers at 0.89 V , while the second corresponded to the reduction of the cationic iron center at -1.42V. UV-visible studies of these polymers (27-29) showed that the polymers displayed a KmiX similar to those of the corresponding starting azo dye material. Polymer 27a, as an example, showed a XmàK at 419 nm in a D M F solution. Upon addition of H C l , a bathochromic shift occurred at a λ™ χ of 530 nm due to the protonation of the azo group (Figure 4).

Organoiron polymers containing azo dyes in the backbone have also been prepared via condensation reactions of Ι,Γ-ferrocenedicarbonyl chloride (22) with disubstituted cyclopentadienylironarene complexes (30a, b) as shown in Scheme 6.

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4 1 3

400.0 500-0 600.0 Wavelength (n»)

Figure 4: UV-visible spectrum ofpolymer27a: a) before addition of HCl solution, b) after addition of HCl.

31a, b

Scheme 6

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Conclusion

A n efficient route to the synthesis of aryl and hetaryl azo dye containing organoiron polymers h as been developed. These polymers incorporate the azo dye chromophores into the backbone or the side chains. These polymers possess many different properties, which could be useful for numerous applications. While it is clear that the area of organometallic polymers containing azo dyes is still being explored, there have been significant steps taken in the study of these unique and interesting materials.

References

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[acs symposium series] metal-containing and metallosupramolecular polymers and materials volume 928...

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