Metallo-supramolecular polymers : on the way to new
materials
Citation for published version (APA):
Eschbaumer, C. (2001). Metallo-supramolecular polymers : on the way to new materials. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR544067
DOI:
10.6100/IR544067
Document status and date: Published: 01/01/2001
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–
prof.dr. U.S. Schubert en
prof.dr. E.W. Meijer
Cover: The cover shows the symbols of Munich, the home of the author, and part of the Alps. The “Frauenkirche” and St Peter´s are displayed on the front, the Church of Holy Spirit and the tower of the Munich city hall together with the summit of the “Zugspitze” on the back. The molecular modeling picture represents a [2 × 2] grid system. A sketch of a polymer, based on non-covalent forces, is displayed on the front.
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Eschbaumer, Christian
Metallo-supramolecular polymers : on the way to new materials / by Christian Eschbaumer. – Eindhoven : Technische Universiteit Eindhoven, 2001.
Proefschrift. – ISBN 90-386-2702-5 NUGI 813
Trefwoorden: supramoleculaire chemie / niet-covalente organometallische polymeren;
synthese / heterocyclische verbindingen; terpyridinen / coördinatieverbindingen; overgangsmetaalionen
Subject headings: supramolecular chemistry / non-covalent organometallic polymers; synthesis / heterocyclic compounds; terpyridines / coordination compounds; transition metal ions
Copyright 2001, C. Eschbaumer Omslag: Christian Eschbaumer Druk: Universiteitsdrukkerij, TUE
–
On the Way to New Materials
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
Rector Magnificus, prof.dr. M. Rem, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen op
donderdag 8 maart 2001 om 16.00 uur
door
Christian Eschbaumer
beiden die Vereinigung bilden. Von allen Bändern ist aber dasjenige das schönste, welches zugleich sich selbst und die durch dasselbe verbundenen Gegenstände möglichst zu einem macht.
Platon aus “Timaeus”
CHAPTER 1: SUPRAMOLECULAR CHEMISTRY MEETS POLYMER CHEMISTRY 1
1.1 SUPRAMOLECULAR CHEMISTRY: ON THE WAY TO NEW MATERIALS 1
1.2 METALLO-SUPRAMOLECULAR POLYMERS 5
1.2.1 INTRODUCTION 5
1.2.2 MACROMOLECULES BEARING BIPYRIDINE UNITS 6
1.2.2.1 Dendrimers with Bipyridine Units 6
1.2.2.2 Polymers with Bipyridine Units in the Side-Chain 10
1.2.2.3 Polymers with Bipyridine Units in the Polymer Backbone 20
1.2.2.4 Bipyridine-containing Resins and Beads 36
1.2.3 MACROMOLECULES BEARING TERPYRIDINE UNITS 39
1.2.3.1 Dendrimers with Terpyridine Units 39
1.2.3.2 Polymers with Terpyridine Units in the Side-Chain 46
1.2.3.3 Polymers with Terpyridine Units in the Polymer Backbone 48
1.3 AIM AND SCOPE OF THIS THESES 55
1.4 REFERENCES 56
CHAPTER 2: SYNTHESIS OF SUPRAMOLECULAR LIGANDS 61
2.1 INTRODUCTION 62
2.2 SYNTHESIS OF “LEGO” BUILDING BLOCKS 67
2.3 PLAYING THE GAME: SUPRAMOLECULAR LIGANDS - FUNCTIONALIZATION AND
CHARACTERIZATION 74
2.4 CONCLUSION 83
2.5 EXPERIMENTAL 83
2.6 REFERENCES 91
CHAPTER 3: METALLO-SUPRAMOLECULAR COORDINATION ARRAYS 93
3.1 INTRODUCTION 94
3.2 SELF-ASSEMBLY OF METALLO-SUPRAMOLECULAR ARRAYS 94
3.3 CHARACTERIZATION OF METALLO-SUPRAMOLECULAR ARRAYS 97
3.3.1 CHARACTERIZATION IN SOLUTION I: UV/VIS-SPECTROSCOPY 97
3.3.2 CHARACTERIZATION IN SOLUTION II: NMR-SPECTROSCOPY 100
3.3.3 CHARACTERIZATION IN SOLUTION III: ANALYTICAL ULTRACENTRIFUGE 103 3.3.4 CHARACTERIZATION IN SOLID STATE I: X-RAY STRUCTURE ANALYSIS 110
3.3.5 CHARACTERIZATION IN SOLID STATE II: MALDI-TOF MASS SPECTROMETRY 116
3.3.6 CHARACTERIZATION IN SOLID STATE III: ELECTRON SPIN RESONANCE
3.5 EXPERIMENTAL SECTION 125
3.6 REFERENCES 130
CHAPTER 4: DESIGNED METALLO-SUPRAMOLECULAR POLYMERS 133
4.1 INTRODUCTION 134 4.2 SUPRAMOLECULAR POLYMERS WITH TERPYRIDINE UNITS IN THE POLYMER-BACKBONE 135 4.3 UNSYMMETRICAL-FUNCTIONALIZED METAL-COMPLEXING POLYMER 137 4.4 TELECHELIC METAL-COMPLEXING POLYMERS 142 4.5 NON-COVALENT BLOCK COPOLYMERS 148 4.6 CONCLUSION 154 4.7 EXPERIMENTAL 155 4.8 REFERENCES 159 CHAPTER 5: APPLICATIONS 161
5.1 CONSTRUCTION OF EXTENDED 2-DIMENSIONAL STRUCTURES 162 5.2 EXTENDED ARRAYS WITH ORDERED MONOLAYERS 163 5.3 LAYER BY LAYER SELF-ASSEMBLY 168 5.4 ATOMIC FORCE MICROSCOPY 176 5.5 CONCLUSION: 181 5.6 EXPERIMENTAL 182 5.7 REFERENCES 183 SUMMARY 185 ZUSAMMENFASSUNG 187 DANKSAGUNG 189 CURRICULUM VITAE 191 LIST OF PUBLICATIONS 193
1111
Supramolecular Chemistry
meets
Polymer Chemistry
Summary
This chapter provides an introduction to the research described in this thesis. Starting with a short overview on supramolecular chemistry a review on bipyridine and terpyridine containing polymers and their corresponding metal complexes is given. As a special class of macromolecules, bipyridine and terpyridine containing dendrimers are discussed as well.
1.1 Supramolecular Chemistry: On the Way to new Materials
Since 1987, when J.-M. Lehn, C.J. Pederson and D.J. Cram were honored with the Nobel prize for their results in selective host-guest chemistry [1-6], Supramolecular Chemistry has become a well-known concept and a major field of today’s research. Starting with the development and design of crown ethers, spherands and cryptands, modern Supramolecular Chemistry represents the generation of well-defined structures by self-assembly processes [7] (similar to the systems known from nature [8,9]). Molecular recognition is the core of Supramolecular Chemistry. It is defined as selection and binding of a substrate molecule by a receptor molecule. The information about the structure of the resulting supermolecule, which is characterized by its thermodynamic and kinetic stability, is stored in the structure of the substrate and read out by the receptor. The binding sites are characterized by their electronic properties, their size, shape, number and arrangement in the receptor framework as well as their eventual reactivity that may allow the combination of binding with other processes.
Lehn divided Supramolecular Chemistry into two broad overlapping areas [7]: (1) Supermolecules are well-defined, discrete oligomolecular species that result from the intermolecular association of a few components following a built-in “Aufbauplan” based on molecular recognition; (2) supramolecular assemblies are polynuclear entities that result from the spontaneous association of a large undefined number of components into a specific phase having more or less well-defined microscopic organization and macroscopic characteristics.
The field of Supramolecular Chemistry can be subdivided based on the kind of building principle that is used: (1) Organic supramolecular systems are built up by non-covalent interactions like hydrogen-bonding, electrostatic or van der Waals interactions. There is a large number of examples especially utilizing hydrogen bonding. One of the most prominent examples is the barbituric acid / 2,4,6-triaminopyrimidine motif shown in Figure 1.1 [10,11]. (2) Inorganic supramolecular systems are well-defined structures built up by organic ligands and metal ions. The components are being held together by non-covalent metal ligand interactions. This field, called metallo-supramolecular chemistry, was initiated among others by E.C. Constable and J.P Sauvage. During recent years a broad variety of structures has been built [12-16]. A prominent example are double-helical bipyridine)-copper(I) complexes yielded by a spontaneous self-organization of 6,6'-disubstituted oligo(2,2'-bipyridines) and copper(I) ions (Figure 1.2) [17].
Figure 1.1: Lehn´s barbituric acid / 2,4,6-triaminopyrimidine motif [10,11]. N N N NCu N N N NCu O O N N N NCu N N N NCu N N N NCu O O O O N N N NCu N N N NCu O O N N N NCu N N N NCu O O O O N N N NCu N N N NCu O O N N N NCu O O N N N NCu N N N NCu O O O O N N O N N O N N n Cu+ or Ag+ 2+ 3+ 4+ 5+ n = 0 to 3
Figure 1.2: Double helical structures formed by self-assembly of oligo(bipyridines) and Cu(I) ions
[17].
Some of the supramolecular systems described, in particular the metallo-supramolecular entities, could represent new materials with interesting properties, e.g. magnetic [18], optical [19], electrochemical [20,21], catalytic [22] ones, etc. However, there are several drawbacks preventing the systems from commercial use. Common synthetic pathways do not allow the production of those materials on a large scale, and the quantities synthesized rarely exceed some hundreds of milligrams. Additionally, these molecules often are in a crystalline state and therefore not suitable for many potential applications. Industrial demands on the other hand require easy and stable processing with adjustable properties, e.g. with respect to solubility or thermal behavior. A new approach into this direction is the blending of supramolecular systems with traditional polymers. Using a relatively large polymer part in
the resulting supramolecular entity it is possible to produce, on a large scale, materials combining the new and interesting features and structures of supramolecular species with well-known polymeric properties. Due to their polymeric nature, these materials can for example be spin-coated to thin films or used in bulk, thus reducing technical processing to common techniques in polymer-engineering. By altering the polymer part, the macroscopic properties of the material can be changed dramatically, e.g. from rigid to elastic, from hydrophobic to hydrophilic or from solid to liquid.
During the last decade several groups have published many results on supramolecular polymers, see e.g. [23-27]. Nevertheless, only few of them combined this topic with “traditional” polymer chemistry. A recent example was presented by Brigitte Folmer et al. from E.W. Meijer’s group [25]. They described stable polymers built up from low-molecular weight poly(ethylene/butylene) with telechelic 2-ureido-4[1H]-pyrimidinone endgroups (Figure 1.3). The endgroups bind each other by quadruple hydrogen bonds resulting in supramolecular polymers with high molecular weight. Since the polymerization is reversible, a strong temperature dependence of the melt viscosity due to depoly-merization was found. In the case of the poly(ethylene/butylene) with 2-ureido-4[1H]-pyrimidinones endgroups the melt viscosity dropped from 2 × 106 Pa × s at 40°C to 0.4 × 106 Pa × s at 50°C.
Figure 1.3: Supramolecular polymers based on quadruple hydrogen bonding. Left sketch of the polymerization on basis of hydrogen-bonds; right: the materials obtained [25].
The objective of this thesis was the design of polymers with special organic ligands capable of the formation of stable metallo-supramolecular units. Complexation results in further polymerization based on non-covalent interactions. The use of metal ions and coordinative bonds not only adds reversibility to the polymerization but has the additional advantage that, by simple changing the type of metal ions, the chemical and physical properties of the materials can be changed without synthetic expenditure. Besides preparation and investigation of the non-covalent polymers, a major part of this work deals with the design, synthesis and characterization of suitable organic ligands and the investigation of their complexation characteristics. Before describing own results, an overview will be given on macromolecules, dendrimers and polymers bearing 2,2'-bipyridine and 2,2':6',2''-terpyridine units and on the corresponding metal complexes. Polymers containing the bipyridine analog phenanthroline will not be considered here, since the chemistry of phenanthrolines is rather different to the chemistry used in this thesis. An overview on macromolecules bearing phenanthroline can be found in [28]. For metal-containing polymers in general see, e .g., [29-31,28].
1.2 Metallo-Supramolecular Polymers
1.2.1 Introduction
The supramolecular entities considered in this chapter can be divided into two classes, depending on the supramolecular units being used (Figure 1.4). Each of them will be described in an own section. In the first section macromolecules bearing 2,2'-bipyridine units will be reviewed. Since we will consider 2,2'-bipyridines only, we shall call them in the following just bipyridines and adopt the abbreviation bpy for these ligands. Bipyridines are known since 1888 when F. Blau synthesized the first bipyridine iron complex [32]. One year later it was again Blau who synthesized and analyzed bipyridine by dry distillation of copper picolinate [33]. Since the molecule consists of two identical parts no directed coupling procedure is required for its synthesis. Therefore unsubstituted and symmetrically substituted, in particular 4,4'-functionalized bipyridines are accessible in good yields. Apart from this, bipyridine metal complexes (especially ruthenium complexes) have very interesting photochemical and other properties, with applications in solar energy conversion [22]. For those reasons, scientists started in the early 1970´s to insert bipyridine units into polymers, thin films and membranes. A large part of the review is therefore devoted to this type of experiments.
N N R R N N N R R R
Figure 1.4: 2,2'-Bipyridine and 2,2':6',2''-terpyridine.
The chemistry of 2,2':6',2''-terpyridines is much younger than that of the 2,2'-bipyridines (again we do not consider other types than 2,2':6',2''-terpyridines and therefore we will call them just terpyridine and adopt the abbreviation tpy for them). Terpyridine was isolated for the first time by Morgan and Burstall in the 1930s [34,35]. The molecule contains three nitrogen atoms and can therefore act as a tridentate ligand. It is well-known that it forms stable complexes chelating a broad variety of transition metal ions [36,37]. This property has been widely used in analytical chemistry [37]. Nevertheless, it took almost 30 years longer for the first example of a terpyridine- than for the bipyridine-containing polymer to be published. This is due to differences in the synthetic accessibility: In contrast to bipyridine synthesis, the built-up of terpyridines and especially of substituted terpyridines is rather difficult. Macromolecules bearing 2,2':6',2''-terpyridine units will be dealt with in the second part of this review. Synthetic approaches to terpyridine units will be discussed in detail in Chapter 2.
As an elegant special case of macromolecules we will separately discuss, within each section, metallo-dendrimers built up from either bipyridine or terpyridine units or from both species. For further information regarding metallo-dendrimers with other complex building units than bi- and terpyridines see, e.g., [38,39].
1.2.2 Macromolecules bearing Bipyridine Units
1.2.2.1 Dendrimers with Bipyridine Units
In 1999 Georg R. Newkome and coworker published a very detailed review on metallodendrimers, which represent a very special case of macromolecules [39]. There have been many publications about macromolecules with bipyridines being attached only to the outermost sphere of the dendrimer to saturate the free coordination sites of a metal complex (see e.g. [38,39]). In this review only selected dendrimers are considered. We will divide this part into two sections. In the first section dendrimers with bipyridine metal complexes within its core are discussed. The second section deals with molecules containing either complexed or free bipyridines at the surface.
The work of Newkome and coworkers is to be placed just in between these two sections [40]. They incorporated an 5,5''-diamino-functionalized bipyridine via ester binding into the backbone of two molecules (Figure 1.5). Addition of a bis(bipyridine)ruthenium(II) complex resulted in systems in which tris(bipyridine)ruthenium(II) complexes are bound within the inner sphere of the dendrimer, but do not act as branching units. These systems consist of the same core, but differ in their surfaces. The molecules have been characterized by electrochemical means, NMR and UV/vis spectroscopy as well as MALDI-TOF mass spectrometry.
Figure 1.5: Newkome´s dendrimers contain metal complexes within the inner sphere of the molecule [40].
Dendrimers containing a bipyridine metal core are rare. In 1991 Paul D. Beer et al. reported the first dendritic system with a ruthenium core and 6 benzo- or aza-crown ether units on the surface (Figure 1.6) [41]. Linked by alkynyl or vinyl spacers, the systems are electropolymerizable to produce films capable of recognizing alkali and alkaline-earth metal guest cations (utilizing spectroscopic methods). The ligand centered absorption band and the metal-ligand charge transfer (MLCT) emission maximum of the monomer as well as of the polymer film was shifted to lower wavelength in presence of those ions. 14 nm (from 686 to 670 nm) are given as a typical value for the shift of a monomeric complex.
Ru2+ O O O O O N N O O O O O O O O O O N N O O O O O O O O O O N N O O O O O Na Na Na Na Na Na
Figure 1.6: Alkali and alkaline-earth metal guests can be recognized by Beer`s system [41].
Vögtle, De Cola and Balzani et al. used a convergent strategy as well. They have reported about five different types of dendrimers up to the third generation with bipyridine ruthenium complexes being the core of these molecules [42-44]. The incorporation of bipyridine complexes aimed at the design of photoactive and redox-active dendrimers. Subsequently, the photophysical and electrochemical properties have been studied. The large dendritic complexes exhibit more intense emission and a longer excited-state lifetime than common [Ru(bpy)3] complexes. E.g., for the dendrimer with 54
peripherical methylester units, the excited-state lifetime in aerated acetonitrile solution is longer than 1 µs. The structures of the dendritic ruthenium complexes have been deduced using NMR spectroscopy and MALDI-TOF mass spectrometry. A typical example is shown in Figure 1.7.
Another example for a dendrimer with a tris(bipyridine)ruthenium(II) core comes from the group of E.C. Constable [45,46]. This molecule contains additional terpyridine ruthenium complexes and is therefore discussed in detail in the corresponding Chapter 1.2.3.1.
In 1997 V. Marvaud and D. Astruc presented aromatic dendrimers containing either benzene or [Fe(II)(K5-C
5H5)(K6-C6R6) as core and pyridine, bipyridine or terpyridine and corresponding ruthenium
complexes building up the branches [38]. For constructing of the molecules they used a divergent approach (see Figure 1.8). Characterization of molecules with up to 12 positive charges was performed, besides by NMR spectroscopy, by MALDI-TOF and electrospray mass spectrometry. Further evidence for the presence of terminal Ru-complexes was given by electrochemical measurements where the set of Ru(II)-Ru(III) redox system gives a single and reversible anodic wave at +0.82 and +0.79 V (vs.Fc-Fc+).
Some innovative work in this direction has been done by H.D. Abruña´s group [47-49]. Their poly(amidoamine)-based dendrimers contain pendant bipyridine or terpyridine ruthenium complexes and will be discussed in detail in Chapter 1.2.3.1.
Unusual structures are presented by R.J. Puddephatt, S. Achar, V.J. Catalano et al. [50-55]. For all syntheses they chose a divergent route to form organoplatinum or mixed organoplatinum-palladium dendrimers. In the most recent work up to 12 ferrocene units were incorporated [50]. The dendrimers were built up converting a four coordinate, square planar platinum(II) center to the six coordinate, octahedral platium(IV) center by oxidative addition of an aryl bromide to the repeat unit. The obtained molecules are new types of dendrimers in which transition metals are present in every layer of the dendrimer except in the core. Figure 1.9 shows an organoplatinum dendrimer with four arms.
Pt N N Br N N Pt Pt N N N N Pt N N Br Pt N N Br Pt N N Br Br Br Pt N N Br N N Pt Pt N N N N Pt N N Br Pt N N Br Pt N N Br Br Br Pt N N Br N N Pt Pt N N N N Pt N N Br Pt N N Br Pt N N Br Br Br Pt N N Br N N Pt Pt N N N N Pt N N Br Pt N N Br Pt N N Br Br Br Pt Pt Pt Pt Br Br Br Br
1.2.2.2 Polymers with Bipyridine Units in the Side-Chain
Starting in 1980, Masao Kaneko presented studies on polymers with covalently bound bipyridines [56,57]. They aimed at the preparation of a heterogeneous photocatalyst for the decomposition of water by solar irradiation. Kaneko used radically polymerized poly(styrene) as the polymer backbone. The poly(styrene) was brominated, lithiated and finally bipyridylated in different molar ratios (see Figure 1.10). The resulting modified polymers were characterized by IR- and UV/vis-spectroscopy. First investigations concerning the photochemical reduction of methyl viologen were carried out and later studied in detail [58]. The authors found three components for the lifetime W of the excited state of Ru(bpy3)2+* ranging from 7 to 474 ns. The rate constant k1 for the transition from the excited state
complex to methylviologen MV2+ was determined and found to be around 108 L/mols as in monomeric
complexes. CH2CH n Br2, FeCl3 n-m CH2CH CH2CH Br m n-BuLi benzene CCl4 x CH2CH CH2CH CH2CH Li Br y z bpy THF / benzene CH2CH x CH2CH y CH2CH z Br N N Ru(bpy)2Cl2 x y z CH2CH CH2CH CH2CH Br N N N N N N Ru PS-bound bpy 23%
Figure 1.10: M. Kaneko presented the first polymer with pendant tris(bipyridine)ruthenium complexes [56,57].
As a next step, M. Kaneko et al. prepared p-aminostyrene-N-vinylpyrrolidone copolymers with pendant tris(bipyridine)ruthenium(II) complexes [59]. Again, the polymer backbone was synthesized first. Afterwards it was reacted with 2,2'-bipyridine-4,4'-dicarboxylic acid to form the corresponding amides (Figure 1.11). Ruthenium complexes were obtained by subsequent reaction of the polymer with [Ru(bpy)2CO3].
In 1982 Kaneko et al. also started with the copolymerization of 4-methyl-4'-vinylbipyridine with styrene [60], followed by analogous work on a variety of other co-monomers like acrylic acid, methyl methacrylate or N-vinylpyrrolidone [61-65]. Again efforts were made to characterize the photoreaction of the excited species of the corresponding polymer pendant ruthenium complex with methyl viologen. In DMF/water mixtures (9/1) a lifetime W of 430 ns and a rate constant of 108 1/mols were found. In
photo-excited state were investigated. The quenching of the photo-photo-excited state has been dealt with in several subsequent papers, the latest in 1998 [61-63,65].
CH CH2 NH CH CH2 N O C O N N COOH x y CH CH2 x-z y NH C O N N COOH CH CH2 NH CH CH2 N O C O N N COOH Ru z (bpy)2 [Ru(bpy)2]CO3 x = 0.14, y = 0.86, z = 0.096
Figure 1.11: Kaneko´s p-aminostyrene-N-vinylpyrrolidone copolymers with pendant tris(bi-pyridine)ruthenium(II) complexes [59].
Another contribution aiming at the photochemical conversion of solar energy was made by the group of K. Sumi [66]. As shown in Figure 1.12 a) and b), they synthesized 6-vinylbipyridine and 4-methyl-4'-vinylbipyridine and used free radical polymerization to prepare the corresponding homopolymers. While the polymer prepared from 4-methyl-4'-vinylbipyridine was insoluble due to cross-linking, the other polymer was soluble in common organic solvents. The solubility changed after complexation with different transition metal chlorides. The polymer based on 6-vinylbipyridine became soluble in water and insoluble in THF, the cross-linked polymer was found to swell to a voluminous gel after addition of metal ions. In a second paper an alternative preparation of the same monomers was described [67]. For the synthesis of the 4-methyl-4'-vinylbipyridine, Pd-catalyzed coupling of pyridine-N-oxide was used instead of copper catalysis (see Figure 1.12 b) and c)). Interestingly in this approach the poly(4-methyl-4'-vinylbipyridine) was soluble in common organic solvents. The photophysical properties of the polymers with pendant tris(bipyridine)ruthenium(II) complexes were studied in detail [67]. N N 1) CH3Li 2) KMnO4 N N CH2O C5H10NH N N C2H4OH NaOH N N N NH2 N Br Cu N N N N C2H4OH N N CH2O C5H10NH NaOH CH2O C5H10NH N N N N C2H4OH N N N O N NaOH + Pd-Pt/C a) b) c)
Figure 1.12: Furue et al. synthesized vinyl-substituted bipyridines as monomers for radical polymerization [66,67].
First investigations on the use of bipyridine metal complexes as templates for polymeric matrices were carried out by Neckers and Gupta in 1982 [68]. Polymers were derived by polymerization of
divinylbenzene and metal complexes of 4'-methyl-4-vinylbipyridine in variable quantities. The idea was to prepare a polymer from metal-chelated monomers, to remove the metal ion, and then to measure the selectivity of the prepared polymer for the metal ion of the template (Figure 1.13). This system showed some template effect but on too small scale to be of practical application.
Bipyridine
polymer
Metal-free
polymer
Metal absorbed in
polymer + metal in solution
Residue polymer
metal complex
Filtrate
free metal ion
Metal
analysis
1) Washed with 8N HCl 2) Washed with deionized water 3) Dried
Filtered
Treated with metal salt in acetate buffer (pH 4.6) or in methanol shaken for 24 h.
Figure 1.13: Procedure for the investigation of template effects of polymers imprinted with metal complexes [68].
In 1982 Kelly, Tinnemans and coworkers reported on copolymers of 6-styrene-bipyridine with styrene, methyl methacrylate or maleic anhydrid [69]. Besides ruthenium complexes, tungsten polymer complexes were formed. The photo- and electrochemical properties of these complexes were investigated. Interestingly, in contrast to Kaneko´s results, the authors found that the rate of production of methylviologen MV+ in presence of the Ru(II) complexes was only slightly greater than in their
absence suggesting that the different sites of attachment might be responsible for the results.
In 1984, C.-H. Fischer investigated the preparation of bipyridines with long-chain substituents [70]. Starting from 4,4'-dimethylbipyridine after a deprotonation step with lithium diisopropylamide (LDA) bromododecane or poly(vinylbenzylchloride) was added to yield the corresponding substituted bipyridine.
In their research towards polymeric catalytic systems for the hydrolysis of organophosphate esters, C.G. Pitt et al., used a different synthetic approach to the monomer, 4-methyl-4'-vinylbipyridine [71]. Starting with commercially available 4,4'-dimethylbipyridine they applied a N-oxide route via the mono-chloro and the mono-(methylenetriphenyl)phosphoniumchloride to yield the mono-vinylene functionalized bipyridine. Homopolymerization yielded a white non-cross-linked product soluble in organic solvents. Derivatization of commercially available poly(ethyleneimine), poly(vinyl alcohol) and poly(4-vinylpyridine) was also used to incorporate the bipyridine into the polymer side chain [72,71]. In later work, a variety of polymeric amines was presented (see Figure 1.14) and the catalytic activity of the polymer metal complexes in the solvolysis of organophosphorus esters was studied [73]. The greatest catalytic activity was exhibited by copper(II) complexes of polymers containing the
bipyridine group. At pH 7.6 and at a concentration of 3.7 × 10-3 M, these catalysts reduced the half-life
of diisopropyl fluorophosphate from 800 to 9 min. CH2CH N N n CH2CH CH2CH N N 0.97 0.03 CH2CH2NH CH2CH2N N N 0.93 0.07 0.44 CH2CH N N C2H5 CH2CH N N 0.56 Br -0.22 0.78 CH2CH2NH CH2CH2N N N CH2CH OH CH2CH O N N 0.97 0.03 CH2CH N CH2CH N R Br -CH2CH N CH2 N N Cl -1-x-y x y a) R = Ethyl: x = 0.30; y = 0.31 b) R = Ethyl: x = 0.16; y = 0.47 c) R = Ethyl: x = 0.48; y = 0.47 d) R = Hexyl: x = 0.27; y = 0.40 e) R = Dodecyl: x = 0.26; y = 0.35
Figure 1.14: Pitt et al. investigated polymeric amines and their complexes as catalysts for the hydrolysis of organophoshate esters [73,72,71].
Thomas J. Meyer´s group synthesized a series of redox active polymers containing chromophores absorbing in the visible region of light ([tris(bipyridine)ruthenium(II)] derivatives) or organic energy transfer reagents (derivatized anthracenes) [74]. They used the nucleophilic displacement of Cl- from
poly(m(p)-(chloromethyl)styrene-stat-styrene) with alkoxide or carboxylate nucleophiles, under concomitant formation of ester and ether linkages (Figure 1.15). In case of pendant bipyridine complexes, the corresponding unsymmetrically functionalized tris(bipyridine)ruthenium(II) complex was used as a nucleophile. In later work, tris(bipyridine)ruthenium(II) complexes or tris(bi-pyridine)osmium(II) complexes were attached to a polystyrene backbone. Polymers with Ru(II) and Os(II) complexes were prepared as well. The type and length of the linkage as well as the molar ratios of the polystyrene and the pendant complexes were varied [75-80]. The authors found a large difference between the ether and amide linkage in the ability to promote intra-strand energy transfer.
In later works these polymers have been studied in view of their electrogenerated chemiluminescence in SiO2 sol-gel/polymer composites [81,82]. The sols were spin-coated onto an ITO electrode. While
MLCT absorption spectra were found to be similar to those in solvents with Omax(abs) = 458 nm,
emission in dry films is blue shifted compared to those in water or ethanol. Emission lifetimes in solutions and films vary between 294 and 648 ns.
N N N N NH C O N N Os N N N N NH C O N N CONEt2 Ru CONEt2 CONEt2 Et2NOC Et2NOC x y x y y x N N N N O N N Ru y x N N N N C O N N Ru NH
Figure 1.15: Thomas J. Meyer´s group studied the energy transfer in polystyrene bearing pendant bipyridine ruthenium or osmium complexes [75-80,74].
A new partially cross-linked polymer based on poly(N-vinylcarbazole-vinyl alcohol) with bipyridine ruthenium luminophores as new material for electroluminescence was reported by Farah and Pietro in 1999 [83]. Poly(N-vinylcarbazole-vinyl alcohol) was cross-linked in a post-polymer modification by reaction with butyllithium and 5,5'-dibromomethyl-bipyridine. The resulting moiety was reacted with
bis(bipyridine)ruthenium to get the corresponding polymer ion complex shown in Figure 1.16.
Studying the thermal and photochemical behavior it was found that the material was thermally stable up to 300°C and exhibited light emission at 590 nm upon radiation at the maximum absorption at 400 nm. It could be worthwhile to optimize the reaction conditions of the bipyridine synthesis since 3.4% yield in a simple coupling procedure are not the state of the art.
CH CH2 CH CH2 N N CH2 CH2 O O CH CH2 CH CH2 N N x y y x Ru(bpy2)Cl2 x y y x N N N N CH CH2 CH CH2 N N CH2 CH2 O O CH CH2 CH CH2 N N Ru
Figure 1.16: Pietro et al. prepared a new photoactive polymer-ruthenium complex [83].
A different approach to materials for electroluminescence was made by W.K. Chan and C.T. Wong [84]. They reported polymers based on poly(phenylenevinylene) with pendant ruthenium bipyridine or terpyridine complexes (see Figure 1.65). This work is discussed in chapter 1.2.3.2.
Newkome and Yoneda prepared homopolymers from 5-vinyl-6,6'-dimethylbipyridine [85], a monomer not commonly used (see Figure 1.17), and copolymers of the corresponding acrylate-functionalized bipyridine with styrene [86]. The polymer-metal complexes were formed and characterized by NMR
and IR spectroscopy .The polymeric Pd(II) complexes were found to be effective in the hydrogenation of simple olefins like (E)-cinnamic acid.
N N N N
COOCH3
Figure 1.17: Newkome´s rather uncommon monomers [86,85].
Polystyrene-bound 2,2'-bipyridine metal complexes were synthesized by the group of Lei and characterized by IR, X-ray, and photoelectric spectroscopy, by thermogravimetry, differential thermal analysis, inductively coupled plasma atomic emission spectrometry, and elemental analysis. The metal complexes were covalently attached to the polymer via the bipyridine. Free coordination sites were saturated with bipyridine, oxime or phenanthroline units (see Figure 1.18). The complexes were found to be catalysts for the oxidation of alkylbenzenes and cyclohexene in the presence of molecular oxygen and in the absence of solvent. Reaction rates of 3.57 × 104 to 3.83 × 105 mLmin-1mol-1 and
selectivity to ketone and alcohol ranging from 79% to about 100% were found. The alcohol/ketone molar ratio of the products can be adjusted by employing different additives or by varying the pH of the reaction [87-91]. n m CH2CH CH2CH N N Co CH2CH CH2CH m n N N N N Co N N Co N O CH2CH CH2CH m n H CH2CH CH2CH m n N N N N Co
Figure 1.18: Lei et al. prepared polymeric catalyst for the oxidation of cyclic alkenes [91].
A polymer-bound palladium catalyst for the synthesis of acetylene-terminated resins was synthesized from palladium diacetate and single crystalline poly(ethylene) containing pendant bipyridine-units by D.L. Trumbo and C.S. Marvel [92]. Both the polymer and the polymer palladium complex were characterized by elemental analysis. The authors found that the synthesized materials were unsuitable as supported catalysts for the planned reaction due to leaching, non-recycleability and low yields.
The preparation of a series of copolymers of 2-hydroxyethyl methacrylate and vinylpyridine or 4-methyl-4'-vinylbipyridine was reported by J.D. Miller and A.L. Lewis [93-97]. Adding a solution of FeSO4 or CuCl2 to such polymers led to the formation of a hydrogel of cross-linked polymers. The
mechanism and the kinetics of the formation and the influence of the metal ions have been studied.
Chujo et al. also published results concerning the formation of hydrogels [98,99]. This group utilized a poly(oxazoline) containing pendant bipyridine units. Addition of Co(III) and Fe(II) ions gave rise to non-covalent cross-linking and a swelling of the polymer. The Fe(II) gel was stable for several days at ambient temperature but turned soluble in hot water within 30 min. The stability of the Co(III) gel was in between the stability of a Ni(II) and a Fe(II) gel. After reduction to Co(II) the polymer-ion complex was too labile to be handled as hydrogel in water (see Figure 1.19).
CoII(bpy) 3 - e -+ e- Co III(bpy) 3 - Kinetically Labile - Thermodynamically Stable (K1 = 5.8, E2 = 11.24, E3 = 15.9) - Kinetically Inert - Thermodynamically Stable
Redox Potential: [MII/III(bpy)
3] Co: -0.30 V (vs SCE) Fe: +0.91 V (vs SCE) L: bpy L L L L L L Co2+ L L L L L L Co2+ L L L L L L Co3+ L L L L L L Co3+ L L L Polymer Polymer Gel Oxidation
Reduction Reduction Oxidation
Diluted or Swollen Concentrated Fast Co2+ / bpy = 1 / 3 Complex [Polymer]: Low [Polymer]: High
Figure 1.19: A thermally as well as oxidatively reversible hydrogel designed by Chujo et al. [98,99].
Stimuli-responsive polymer gels as novel materials with sensor, actuator and processor functions were in the focus of scientific interest of Ryo Yoshida et al. [100-102]. As shown in Figure 1.20 they used the temperature-responsive N-isopropylacrylamide to be copolymerized with bis(bipyridine)(4-methyl-4'-vinyl-bipyridine)ruthenium(II). Immersing the system in a CeIV/CeIII solution induced the oscillatory Belousov-Zhabotinsky reaction [103] and as a result the gel swelled and deswelled periodically at a constant temperature without external stimuli. Depending on the composition of the surrounding, the chemical energy is transduced into mechanical energy and vice versa to drive the polymer gel oscillation with a period of about 5 min.
x y O NH CH CH2 CH CH2 N N N N N N Ru CH2CH O NH CH2 NH O CH2CH
Figure 1.20: Self-oscillating gels based on N-isopropylacrylamide and copolymerized bipyridine-ruthenium complexes [101].
Using a similar material, the tris(bipyridine)ruthenium(II) complex of N-isopropylacrylamide copolymerized with 2-(4-(4'-methylbipyridyl))ethylacrylate, T. Miyashita and coworkers studied the electron transfer quenching of the complex by methyl viologen as a function of the temperature [104]. They found the rate constant to be 4-5 times higher in the globular state than in the coil state. The result is in contrast to a similar experiment with a pyrene-labeled probe. This could mean that in the globular state the hydrophilic ruthenium complexes are placed at the interface of the hydrophobic polymer matrix and are therefore more easily accessible for the methyl viologen quencher (Figure 1.21). N N N N N N Ru CH2CH CH2 C O NH R CH C O O CH2 CH2 x y R = CH(CH3)2 C(CH3)2CH2CH3 (CH2)11CH3
Figure 1.21: Left: a thermo-responsive polymer with pendant ruthenium complexes; right: in the globular state the hydrophilic ruthenium complexes are placed at the surface of an the pyrene units within the polymer matrix [105,106,104,107].
In later work Miyashita´s group formed Langmuir-Blodgett films of these materials and investigated the photoelectrochemical response of a monolayer prepared on ITO [108,106,107]. Upon light irradiation in the presence of an electron donator they found an anodic photocurrent of typically 20 nA.
26 chelating copolymers for heavy metal ion sorption were recently described by R.A. Bartsch´s group [109]. The copolymers were prepared by free radical copolymerization of chelating monomers like 4-methyl-4'-vinyl-bipyridine, 2-vinylpyridine, or 4-vinylpyridine with a dimethacrylate crosslinker (Figure 1.22). The bipyridine-containing polymers were found to exhibit most efficient metal ion sorption. The polymers were highly selectivity for Cu(II) over Co(II) and Ni(II) and for Hg(II) over Cd(II). H2C C C O CH3 O (CH2)2 O C O C CH3 CH2 EGDMA DEGDMA H2C C C O CH3 O (CH2)4 O C O C CH3 CH2 H2C C C O CH3 O (CH2)6 O C O C CH3 CH2 TEGDMA CHDMA O C O C CH3 CH2 H2C C C O CH3 O CH2 CH2 O C O C CH3 CH2 H2C C C O CH3 O C CH3 CH3 H2C C C O CH3 O CH2CHCH2 O C O C CH3 CH2 OH BPDMA GDMA TMDMA H2C C C O CH3 O (CH2CH2O)2 O C O C CH3 CH2 H2C C C O CH3 O (CH2CH2O)3 O C O C CH3 CH2 HMDMA
Figure 1.22: Bartsch et al. prepared 26 new copolymers for metal sorption, using different dimethacrylate cross-linking units and chelating monomers [109].
In 1995, Antonietti et al. presented microlatex dispersions with a surface of ca. 120 m2 per g polymer functionalized with bipyridine [110]. The polymers were synthesized by radical copolymerization of styrene and 10% (w/w) of 6'-methylbipyrid-6-ylmethyl methacrylate or 4-(6'-methylbipyrid-6-ylmethoxy)-butyl methacrylate (see Figure 1.23) and a cross-linking unit. Complexation experiments with transition metal ions revealed that most of the bipyridine units were located at the latex surface and were accessible in the binding process.
N N O
O
N N O O
O
Figure 1.23: Antonietti et al. used methacrylate-functionalized bipyridine ligands for preparation of metal complexing microlatex dispersions [110].
A system completely different to those shown above was presented by W. Edward Lindsell et al. in 1999 [111]. In their research on systems with nonlinear optical properties they prepared novel polydiacetylenes with bipyridine ligands in the side-chain (see Figure 1.24). The monomer was synthesized by reaction of 1,3-diynes, mono- or bis-functionalized with carboxylic acid, and 5-aminobipyridine to get the corresponding amides. Polymerization was achieved by UV or J-irradiation at room temperature, resulting in insoluble material. After adding metal ions the corresponding polymeric ruthenium(II) and molybdenum(0) complexes were soluble. Their linear absorption
coefficients were found to be in a range of 25.8 cm-1 for the Ru(II) complex and 33.2 cm-1 for the
Mo(0) containing polymer. The non-linear absorption coefficient was found to be in a range of –6.5 and –8.5 cmGW-1 respectively. n N N N H C(CH2)6 O N N (CH2)6C O N H C C C C Mo Mo (CO)4 (CO)4
1.2.2.3 Polymers with Bipyridine Units in the Polymer Backbone
T. Yamamoto and and coworkers published several papers on polymers with bipyridines in the polymer backbone [112-123]. In earlier approaches 5,5'-dihalogene substituted bipyridines and nickel(0) complexes as catalysts were reacted, resulting in rather low molecular weight polymers (Mw
= 3800; DP = 21) only partially soluble in formic acid [118,120,123]. In contrast to the uncomplexed polymer, metal containing oligomers (M = 1500) were found to be soluble in water [112]. Thew
soluble polymeric platinum or ruthenium complexes can act as photocatalysts for H2-evolution from
aqueous media [112,115]. Additionally they can transfer the photoactivated energy to other molecular parts of the systems and thus cause emission of light from the other molecular parts (Figure 1.25 left) [121]. Evidence for this energy transfer was found in the absorption at 350 nm assigned to the poly(bipyridine) unit and the fluorescence emission at 640 nm assigned to the Ru(II) complex. By changing to 6,6'-dihexyl-substituted bipyridines the degree of polymerization was increased to 65 (M = 21000); those polymers remained soluble [113,119]. Due to steric effects of the alkyl groups,w
S-conjugation was shortened in these systems. Copper complexes of polymers composed of 6,6'-dimethyl-substituted and 6,6'-dihexyl-substituted bipyridines were synthesized and investigated with respect to their optical and electrochemical properties [114]. In a film, vacuum-deposited on a glass substrate, poly(bipyridine) molecules were oriented parallel with the surface of the substrate. Coordination of a poly(bipyridine) as shown in Figure 1.25 (right) is proposed to explain such orientation [119]. N N N N Si O H Si O H n
glass substrate
Figure 1.25: Yamamoto et al. prepared poly(bipyridine) polymers; left: transfer of the photoactivated energy to other molecular parts; right: coordination of bipyridine units to Si-O-H groups [112-115,117-121,123].
In 1985, Richard E. Sassoon described a two electrolyte system for energy storage based on
tris(bipyridine)ruthenium compounds bound covalently to a poly(3,3-ionene) polyelectrolyte (see
Figure 1.26) and a N,N,N,N-tetraalkyl-p-phenylenediamine derivative linked to a similar polyelectrolyte [124]. The photochemical properties of the system were studied and, compared to the reverse electron-transfer reaction between Ru(bpy)3 and the cation radical of
N,N,N,N-tetraalkyl-p-phenylenediamine, an inhibition of more than 5 orders of magnitude was observed. However, the preparation of the polymers is neither described nor cited in the paper.
N N N N N N Ru (CH2)3 N (CH2)3 CH3 CH3 (CH2)3 N (CH2)3 CH3 CH3 x x
Figure 1.26: Sassoon´s photosensitizer bound covalently to a polyelectrolyte [124].
In 1997 Michael R. Wasielewski and Bing Wang published a hightly sophisticated approach to metal-ion sensitive polymers [125], choosing a partially conjugated polymeric system consisting of oligo(p-phenylenevinylene) segments that are connected covalently to bipyridines at their 5,5'-positions (see Figure 1.27). The polymerization was carried out by a Wittig-type reaction. Due to their transoid conformation, the free bipyridines have an dihedral angle of approximately 20° between the pyridine planes. As shown in Figure 1.27 two polymers with different segment ratios have been synthesized. The corresponding metal complexes have been formed and the optical behavior of the free polymer and the polymer-ion complex has been compared. Since upon metal complexation the twisted conformation is forced into a planar one and therefore the partially conjugated polymer is changed into a fully conjugated polymer, different physical properties are obtained (Figure 1.28). E.g., in the case of the polymer shown in Figure 1.27 left, addition of Pd(II) shifts the absorption maximum of the uncomplexed polymer from 455 nm to 564 nm. With different metal ions different absorption maxima were obtained, the shift ranging from 38 nm to 112 nm.
N N OR RO OR RO OR RO N N OR RO n n R = 2-ethylhexyl
Figure 1.27: Wasielewski and Wang synthesized partially conjugated ion sensitive polymers [125].
Chen et al. studied the effects of S-conjugation attenuation on the photophysics and exciton dynamics of these polymers in 1999 [126]. Compared to the homopolymers, they found blue shifts in absorption and emission spectra, spectral diffusion in stimulated emission and enhancement in photoluminescence quantum yields and lifetimes. The ionochromic effects and structures of the metalated polymers were recently published [127].
Figure 1.28: Chelating a metal ion forces the bipyridine from a twisted conformation into a planar one and enables fully conjugation of the polymer [125].
Yu et al. utilized Heck-coupling reactions to synthesize photorefractive copolymers based on p-phenylenevinylene and tris(bipyridine)ruthenium or tris(bipyridine)osmium complexes for non-linear optics [128,129]. A variety of polymers with slightly different substituents to enhance solubility was reported. I I H15C7 (CH2)4 N SO2Me N N N N I RO OR N N RO OR I Ru + + xn yn n (PF6-)2 (PF6-)2 H15C7 (CH2)4 N SO2Me N N N N RO OR N N RO OR Ru xn yn 80°C Pd(OAc)2 Tri(O-tolyl)phosphine Et3N
Figure 1.29: A typical polymer obtained by Heck-coupling of alkenes with aryl iodides [128].
While most polymers have been synthesized by Heck-coupling of iodo-substituted
co-monomers (Figure 1.29), there has been one approach utilizing Horner-Wadsworth-Emmons reaction with free bipyridine ligands and metal complexation after polymerization (Figure 1.30).
Figure 1.30: Horner-Wadsworth-Emmons reaction for the synthesis of photorefractive polymers [128].
E. Klemm and coworkers also used Heck-coupling procedure as shown in Figure 1.31 (A) for the preparation of conjugated organic polymers [130,131]. Reaction of 5,5'-dibromobipyridine and substituted phenyldiacetylenes provided conjugated polymers with rigid backbone and molecular weights up to 23000 g/mol. The polymers were characterized by NMR and UV/vis spectroscopy, GPC and vapor pressure osmometry. Fluorescence quantum yields between Ifl = 71 and 96 % make these
new compounds possible candidates for LED or laser dye applications. Polymeric metal complexes have not been prepared yet.
Conjugated polymers comprising the arylene-ethylene architecture as well have been published by Schanze et al. (Figure 1.31 (B)) [132-134]. Similar to Wasielewski and Wang [125] (Figure 1.27; page 21), the transoid bipyridine was forced into a planar conformation upon complexation and therefore conjugation in the polymer was extended. For that purpose, instead of chelating the polymer after polymerization, bipyridine rhenium(I) complexes were polymerized. The polymer-metal complexes were characterized by FTIR, UV/vis and nanosecond laser flash photolysis. Spectroscopic
studies revealed fluorescence at 435 nm (2.85 eV) in all polymers at 298 K. With increased mole fraction of Re(II) in the polymer the fluorescence was found to be quenched.
N N Br Br O O R R + n n Pd0 / CuI O O R R N N n R = (CH2)7CH3; (CH2)17CH3 I I O O R R + + Pd0 / CuI O O R R N N O O R R Re x y R = (CH2)17CH3 N N Re (CO)3Cl (CO)3Cl (A) (B)
Figure 1.31: Conjugated polymer based on bipyridine and aryldiacetylene (A) [130,131]; (B) [134].
In 1986 Evers and Moore were the first to prepare bipyridine-containing poly(benzobisoxazole)s and poly(benzobisthiazole)s [135]. They copolymerized bipyridine-4,4'-dicarboxylic acid chloride with terephthalic acid and 4,6-diamino-1,3-benzenediol dihydrochloride or 2,5-diamino-1,4-benzenedithol. The resulting polymers were investigated with respect to their thermooxidative stability by isothermal aging. At 316°C the polymers were essentially unaffected and displayed thermooxidative stability for 200 h. At 371°C a weight loss of approximately 60% was found after 200 h.
12 years later Chan et al. reported on the synthesis and physical properties of a series of poly(benzobisoxazole)s and poly(benzobisthiazole)s containing bipyridine derivatives [136,137]. These polymers were synthesized by polycondensation of bipyridine-5,5'-dicarboxylic acid or bipyridine-4,4'-dicarboxylic acid and diaminobenzenediols in poly(phosphoric acid) (see Figure 1.32). The polymers exhibit thermal stabilities up to 650°C in a nitrogen atmosphere and up to 590°C in air. Polymers containing the bipyridine-5,5'-dicarboxylic acid showed a lyotropic liquid crystal phase in methanesulfonic acid; however, the bipyridine-4,4'-dicarboxylic acid containing polymers did not show any anisotropy. Formation of polymeric ruthenium complexes increases the charge carriers mobilities two orders of magnitude to be of the order of 10-5 cm2V-1s-1.
N N HOOC COOH N N HOOC COOH NH2 H2N HO OH NH2 HX H2N XH N N O N O N n n N N X N N X NH2 HX H2N XH NH2 H2N HO OH O N O N N N X N N X N N n n X = O, S
Figure 1.32: Poly(benzobisoxazole)s and poly(benzobisthiazole)s containing bipyridine units in the backbone [137].
Rigid-rod benzobisazole polymers with main chain bipyridine-5,5'-diyl units were reported by Tan et
al. in 1999 [138]. Similar to Chan et al., poly(benzobisthiazole)s and poly(benzobisoxazole)s (Figure
1.33) were prepared as well. The polymers were doped with AgNO3 to enhance conductivity. In
unspoiled state a conductivity of 10-10 – 10-8 S/cm was found. This value was increased to 10-5 – 10-4
S/cm upon infiltration with AgNO3. 25 – 42 S/cm were reached when Ag+ was reduced to Ag0 by
NaBH4. N N ClOC COCl NH2 H2N HO OH NH2 H2N H2N NH2 N N O N O N n n N N S N N S N N N N N N n SH H2N HS NH2 x 2 HCl x 2 HCl x 2 HCl
Figure 1.33: Tan et al. prepared polymers similar to Chan´s (Figure 1.32) [138].
In more recent work Chan et al. published a broad variety of bipyridine-based polyamides and polyesters [139,140] (Figure 1.34). Polyamides were synthesized by Yamazaki´s method using pyridine and triphenyl phosphite. The resulting polymers based on bipyridine 5,5'-dicarboxylic acid showed higher viscosity and decomposition temperature (300 – 500°C) than polymers based on bipyridine 4,4'-dicarboxylic acid. Polyesters were synthesized by either alcoholysis of acid chlorides or transesterification.
Figure 1.34: Chan et al. prepared a broad variety of bipyiridine-based polyamids (a) and polyesters (b) [140].
The decomposition temperatures for the polyesters were found to be in a more narrow range of 320-440°C. Due to insolubility in common solvents the molecular weight of the polymers could not be determined by GPC measurements. Therefore static light scattering techniques have been used resulting in molecular weight values of Mw 20000 to 40000 g/mol for the polyamides. For incorporation of metal ions the polymers were reacted with bis(bipyridine)ruthenium complexes.
Thermotropic liquid crystalline polyesters with the bipyridine unit acting as mesogene were synthesized by Hanabusa et al. in 1989 (Figure 1.35) [141]. They form complexes with transition metal salts. Polyesters bearing an alkyl chain as co-component are smectic in the absence or at low concentrations of metal ions. A nematic mesophase was observed in the case of the aromatic polyesters. Two endothermic transitions at 114°C and 135°C, respectively, due to the transition from crystalline to liquid-crystalline and finally to isotropic state were observed regardsless of the kind and amount of metal ions present. The structural properties of a polyester-Cu(II)-complex were examined by ESR. q p n C O O(CH2)10O N N CH2O OCH2 C O C O C O N N CH2O OCH2 C O (CH2)n C O + CH3OOC(C6H4)COOCH3 CH3OOC(CH2)nCOOCH3 HO(CH2)10OH 1) KMnO4 2) EtOH, HCl 3) Red-Al N N CH2OH HOCH2 N N
Figure 1.35: Liquid-crystalline polyesters: The thermotropic behavior after addition of several molar ratios metal ions (FeCl2 or CuCl2) was studied [141].
Several years later Rubner et al. described the synthesis of a polyester similar to that shown by Hanabusa et al. [141] utilizing a modified Schotten-Baumann reaction of bis(hydroxymethyl)-bipyridine and dodecanedioyl dichloride in THF [142,143]. It is not obvious from the paper whether 4,4' or 5,5' bisfunctionalized bipyridine was used. However, since solubility of the resulting polymer was poor, they reacted bis(hydroxymethyl)bipyridine with Ru(bpy)2Cl2 and polymerized the resulting
Ru-complex with dodecanedioyl dichloride to obtain a polymer with an average molecular weight of about 5500 g/mol. Utilizing PF6 counterions they found the polymer to be soluble in acetone. In
contrast, when the counterions were changed to chloride, the new polymer was soluble only in polar solvents such as water or methanol. Light-emitting devices were fabricated from this materials utilizing either spin-coating technique or layer-by-layer self-assembly. However, the spin-coated devices were found to produce maximum luminance levels of 250 – 300 cd/m2 with an external
quantum efficiency of 0.2% photons/electron. Devices based on sequentially adsorbed layers of the polyester and poly(acrylic acid) exhibited a maximum light output of 40 - 50 cd/m2 and external
quantum efficiency in the 1 - 3% photons/electron range [142,143].
Polyamids and polyureas with acyclic and cyclic bipyridyl diamino structures in the polymer backbone were described by S. Pappalardo et al. in 1987 [144,145].
N N N N N N H H N N N N N N H H N N N N N N C O R C O n n N N N N N N C O R C O HOOC R COOH HOOC R COOH R = (CH2)6 N N H H N (CH2)6 H N H
Figure 1.36: Pappalardo et al. showed polyamides and polyureas with cyclic and acyclic bis(bipyridyl) structures [144,145].
While the acyclic polymer formed stable metal complexes with Co(II), Cu(II) or Ni(II), the cyclic structure exhibits special affinity towards Cu(II) (see Figure 1.36). The polymers were characterized by NMR and IR spectroscopy and their thermal behavior was studied. The decomposition rate was found to be maximal at temperatures above 400°C for the polyamides and between 300 and 400°C for the polyureas.
The first bipyridine containing polyureas have been reported by Zhang and Neckers in 1983 [146-148]. The polymers were prepared from 4,4'-diaminobipyridine and toluenediisocyanate. The metal-complexing abilities of the polymers, especially with cobalt, palladium and vanadium ions, were investigated with special attention to the catalytic activity in the hydrogenation and epoxidation of olefins and in aldol condensation. In addition to the soluble linear polymers, insoluble cross-linked moieties were prepared. In 1985 Neckers et al. reported on polyamides and their Pd(0) and Rh(III) complexes [149,150].
Polyimids consisting of 5,5'-diamino-bipyridine derivatives and aromatic tetracarboxylic acid anhydrides, designed to be more heat-resistant have been described by Kurita and Williams in 1973 [151-153]. This group synthesized similar polyamide-imides as well. The pyridine units of the bipyridine derivatives were separated in most cases by a hetero atom or a functional group like S, NH or SO2, but were also linked together directly.
N N X N N O H O O n N N X N O O N O O n
Figure 1.37: 5,5'-Diaminobipyridine derivatives were used for the preparation of polyimides and polyamide imides [152,153]; X = -, S, SO, SO2, NH, ...
Ten years after the first publication, Yang et al. described polyimides consisting exclusively of 2,2'-bipyridine (see Figure 1.37, left: X = -) [154]. They reacted 5,5'-diamino2,2'-bipyridine and three different aromatic dianhydrides, followed by imidization upon heating to obtain fairly thermostable polymers. E.g., 5% weight loss occurred in the range of 480 – 580°C. The polymers were reacted with several different transition metal ions and the catalytic activity of the polymeric complexes for the condensation of benzaldehyde with acetophenone was investigated. The degree of catalytic activity for different metal ions was found to be in the order Zn(II) > Co(II) > Cu(II). Yields of chalcone were obtained in the range of ca. 6 – 32 %.
Recently a Chinese group reported on novel polyimides (Figure 1.38) on a bipyridine basis [155]. In addition to the synthesis the group investigated the influence, in particular on the thermal stability, of the coordination of nickel malenonitriledithiolate on the copolymers. They found elevated glass transition in the range of 270 – 320°C and decomposition temperatures ranging from 370 – 520°C.
N N H2N NH2 NH2 H2N F3CCF3 O O O O O O xn + yn + n F3CCF3 N O O O O F3CCF3 N N O O O O N N N xn yn S Ni S CN CN H3N H3N yn xn S Ni S CN NC N N N F3CCF3 N O O O O F3CCF3 N N O O O O
Biswas and Majumdar published the reaction of pyromellitic dianhydride with unsubstituted bipyridine [156,157] in 1991. To our knowledge the presented structures are to be doubted and will not be considered any further.
In 1997 Kira et al. published the synthesis of poly(disilanylene-2,2'-bipyridine-5,5'-diyl) and poly(silylene-2,2'-bipyridine-5,5'-diyl) and their ruthenium complexes (Figure 1.39) [158]. The polymeric ruthenium complexes showed photoconductivity caused by metal-to-ligand charge transfer excitation. In a sandwich type cell with aluminum and ITO as top and bottom electrode at 6 V a photocurrent with a maximum at 470 nm of approximately 1.8 × 10-13 A was measured. The optical
properties of the polymeric metal complexes were studied two years later [159].
N Li Br N Si N Br Br R R m ClSi2Pr4Cl / (n-C6H13)2SiCl2 Et2O, -80°C m m N N N N N Si R R N Si N R R N Ru xn yn m = 2; R = Pr m = 1; R = n-C6H13 m = 2; R = Prm = 1; R = n-C6H13 x : y = 0.35 : 0.65 1) NiBr2(PPh3)2, Zn, Et4NBr 2) Ru(bpy)2(PF6)2
Figure 1.39: Kira et al. published V-S-alternating copolymers and their ruthenium complexes
[159,158].
A non-covalent polymerization of bis(bipyridine)alkanes was carried out by Ching and Elliot in 1999 [160]. Addition of iron and cobalt ions to spin-coated films of the monomers shown in Figure 1.40 resulted in highly cross-linked electroactive coordination polymers. Electrochemical characterization has been carried out with thin films. Charge-transport measurements using chronoamperometry have been conducted on films of the polymer iron(II) complex in which the alkyl linker consists of 2, 4, and 6 methylene groups. Values of D½C (D = charge-transfer diffusion coefficient; C = concentration of
redox sites) range from 9.5 × 10-8 to 2.3 × 10-8 mol/cm2s½ at 298 K and follow a trend of decreasing
D½C with increasing alkyl chain length for the 4-position-linked Fe(II) polymers. No such trend was
observed for the analogous 5-position-linked systems. Attempts to prepare polymer films with other metal ions like Ni(II), Cu(II) or Ru(II) failed.
N N
(CH2)n
N N N N N N
(CH2)m
n = 2, 4 m = 4, 6
Figure 1.40: Coordinative polymer films were prepared by adding metal ions to the shown ligands [160].
A special case of polymers containing bipyridine units in the polymer backbone are those containing exactly one bipyridine unit. Addition of metal ions to that moiety results in star-shaped polymers.
Different approaches to this structures are reported in literature. Murray et al. reacted bipyridine-4,4'-dicarboxylic acid chloride with poly(ethylenoxide) monomethylester (350) to give the tailed ligand 4,4'-di(poly(ethylenoxide) methyl ether)-bipyridine [161]. Cobalt perchlorate was added and transport, ionic conductivity and viscosity properties of the molten salt were studied. For LiClO4 (1.3 M), the
average diffusive jump rate was ca. 3 s-1, the average electron hopping rate was ca. 2 × 104 s-1, and the
rate of short-range motions of the hard metal complex core within its soft polyether shell was found to be ca. t 105 s-1.
Chujo et al. reported bipyridines substituted just on one side with poly(ethylenoxide)s or poly(propylene oxide) (Figure 1.41) [162-164]. Starting with 4,4'-dimethylbipyridine, reaction with LDA and subsequent addition of an D-alkyl ether-Z-tosylate polymer yielded the desired polymer containing a single bipyridine unit. Addition of metal ions such as Ni(II), Co(II) or Ru(III) yielded a star-shaped polymer metal complex.
N N N N N N N N N N N N Ru Coordination = polymer chain
Figure 1.41: Left: Chujo et al. prepared star-shaped polymers by addition of metal ions to mono bipyridine functionalized polymers; right: GPC analysis showed a significant shift in molecular weight after complexation [162,163].
The structures were confirmed by UV/vis spectroscopy and GPC analysis. It was found that the Ru-complexes were stable against sheer forces under the GPC conditions, while Ni(II) and Co(II) complexes dissociated. Following the current interest in metal clusters and colloids in the size range of 1 - 10 nm for catalytic use and as advanced materials in electronics (e.g. quantum dots), poly(ethylene-oxide) grafted palladium clusters were prepared in a first application [164].
A completely different approach towards star-shaped polymers was established at almost the same time by the groups of Cassandra L. Fraser and Ulrich S. Schubert [165-174]. They used halomethyl substituted bipyridine metal complexes as initiators for living cationic polymerization of 2-oxazolines. Polymer growth starts from the metal complex. Depending on the substitution pattern of the complex, several different structural types are accessible.
Figure 1.42: With different substitution patterns of the metal complex (upper row), different structural types of star-shaped molecules (lower row) are accessible [168].
To obtain a higher order of structural diversity, Schubert et al. not only used 4,4'-bipyridines but 5,5'-and 6,6'-functionalized bipyridines as well. In addition, the living character of the polymerization 5,5'-and the accessibility of suitable monomers, e.g. ethyloxazoline, phenyloxazoline, and 2-undecyloxazoline, allows the preparation of amphiphilic block copolymers (Figure 1.43).
N N N N N N H N H N R1 O R2 O R1 O R2 O Cu(I)L n n m m O O N N N N N N
Figure 1.43: Schubert and Hochwimmer designed this sophisticated amphiphilic molecule with two different metal binding sites [174].
Finally, by suitable choice of the termination reagent, more functionalities can be introduced into the molecules. In the case of Schubert´s molecule shown in Figure 1.43, an oxidation of the Cu(I) to Cu(II) would switch from the star-shaped molecule to an infinitely linear polymer since Cu(II) can not be chelated by 6,6'-substituted bipyridines but it is chelated in a very stable way by terpyridines. The principle of initiating polymerization by supramolecular metal complexes and ligands does not only work with oxazolines. U.S. Schubert et al. and C.L. Fraser et al., in two independent papers, recently used 6,6'-substituted bipyridine-copper(I) complexes [175] and 4-mono- and
4,4'-bissubstituted bipyridines [176] as initiator for the controlled radical polymerization of styrene (Figure
1.44). The Schubert group used the tris(4,4'-dimethylbipyridine)Cu(II) complex as a catalyst for polymerization, in 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidone as a solvent. The resulting polymers were characterized by GPC, UV/vis and MALDI-TOF-MS. Fraser´s group used either
Polymerization was carried out either in bulk or solution with anisole as solvent. The polymers were characterized by GPC, UV/vis and NMR.
N N Br Br Cu N N Cl Cl L CuCl bpy(C13H27)2 n 4 n [Cu(II)((4,4'-CH3)2bpy)3](PF6)2 Al[OCH(CH3)2]3 L N N Cu Br n nBr N N Cl n nCl A) B)
Figure 1.44: A) Schubert and Hochwimmer utilized metallo-supramolecular initiators and catalysts to obtain star-shaped polystyrene; B) Fraser and Wu used a functionalized bipyridine for initiation and CuCl / bipyridine as catalyst [175,176].
For a further extension of this method, Schubert and Hochwimmer used the so-called “co-initiator approach”, with hydroxy functionalized N-heterocyclic ligands as co-initiator and aluminium-alkoxides as efficient initiators for the ring-opening polymerization of H-caprolacton and lactid acid (Figure 1.45) [177].
The formation of well-defined structures by self-assembly is the major theme of supramolecular chemistry. Polymers with two bipyridine units connected by a short and flexible spacer belong to this area of research. In their monomeric form, these systems are already well-known in supramolecular chemistry (see Figure 1.2). Claus D. Eisenbach and coworkers prepared telechelic polymers consisting of bipyridine and bis(bipyridine) endgroups [178-188]. As shown in Figure 1.46, both types of polymers can be elongated on addition of Cu(I) or Ag(I) ions by formation of the corresponding metal complexes. N N HO AlEt3 N N O Al 3 O O N N O Al O O N N O O N N O O O O n m p
Figure 1.45: Schubert and Hochwimmer introduced metal binding units into biodegradble polymers [177].
The bis(bipyridine) functionalized polymers form helical structures similar to their monomeric analogs (see Figure 1.2). The resulting coordination polymers were characterized in detail with respect to their mechanical properties and morphological behavior. They were found to be microphase-separated systems in bulk, with nano to mesoscopic superstructures consisting of copper-bipyridine complex aggregates in a polyether matrix.
y 28 N N O N N N N N N O O O O O O O O O N N O N N O y O N N N N N N O O O O O O O N N 28 Cu(I) Cu(I)
= bipyridine-Cu(I) complex = polyoxytetramethylene (POTM)
Figure 1.46: Eisenbach and coworkers prepared ABA-copolymers. Addition of Cu(I) ions resulted in the formation of supramolecular (ABA)n-systems [178-188].
Hosseini et al. desribed a single-stranded helical coordination polymer (Figure 1.47) as a racemic mixture of left- and right-handed helices, obtained by self-assembly of an exo-ditopic ligand and silver [189,190]. The structure of the infinite network was established by single-crystal X-ray structural analysis. Although the structure of the coordination polymer was investigated, no information concerning the degree of polymerization is given.
Figure 1.47: Hosseni et al. formed a helical coordination polymer (B) by self-assembly of a bipyridine-based ligand ((A) right) [189,190].
In this field of supramolecular chemistry J. Fraser Stoddart et al. described poly(bis[2]catenane) containing a combination of covalent, mechanical and coordinative bonds (Figure 1.48) wherein the coordinative bonds are based on bipyridine silver interactions [191]. The macro-monomer was characterized by NMR and LSIMS (liquid secondary ion mass spectrometry) and the resulting polymer by GPC. The degree of polymerization was found to be 40, equivalent to a molecular weight of Mn 150000 g/mol.
Figure 1.48: Stoddart´s poly(bis[2]catenane) consists of covalent, mechanical and coordinative bonds [191].
