5818
Macromolecules 1999, 32, 5818-5828
Synthesis and Properties in Solution of Rodlike, 2,2′:6′,2′′-Terpyridine-Based Ruthenium(II) Coordination Polymers Steffen Kelch and Matthias Rehahn* Polymer-Institut, Universita¨ t Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany Received February 23, 1999; Revised Manuscript Received June 5, 1999
ABSTRACT: Rodlike ruthenium(II) coordination polymers have been prepared via the conversion of 4,4′′bis(2,2′:6′,2′′-terpyridine)-2′,5′-dihexyl-p-terphenyl and an appropriately reactive ruthenium(III) species in 1-butanol/DMA. The diamagnetic polymers readily dissolve in polar organic solvents, and their homogeneous constitution could thus be proven and their degrees of polycondensation could be estimated to be Pn g 30, using 1H and 13C NMR spectroscopy. The Pd-catalyzed polycondensation of bis[4′-(pbromophenyl)-2,2′:6′,2′′-terpyridine]ruthenium(II) tetrafluoroborate and 2,5-dihexylbenzene-1,4-diboronic acid, on the other hand, which should lead to a constitutionally identical polymer, only resulted in oligomers under all tested conditions. The intrinsic viscosity [η] of the high-molecular-weight polymers (determined in 0.02 M NH4PF6/DMA) is of the order of 300 mL g-1. In salt-free DMA solution and thus at low ionic strengths, the polymers display the characteristic polyelectrolyte behavior in viscosity experiments which becomes more and more pronounced with increasing Pn. The UV-vis absorption spectra recorded in DMA solution do not show any evidence of intramolecular electronic interactions of the metal centers along the polymer chains: on the contrary, the coordination polymers behave like sequences of electronically independent ruthenium(II) complexes.
Introduction Soluble, constitutionally well-defined transition-metal coordination polymers, that is, polymers whose main chains are held together by transition-metal complexes and thus by coordinative bonds, are of considerable current interest because of their unusual electronic, photooptical, magnetic, and catalytic properties.1-11 Moreover, such self-assembled multinuclear complexes are key compounds of modern supramolecular chemistry and have been shown, for example, to serve as useful building blocks for well-defined multilayer systems.12,13 Last but not least, if coordination polymers became available that (i) are sufficiently stable in solution, (ii) have a conformationally rigid backbone, and (iii) contain charged metal complexes, these polymers could develop into important model systems for the exploration of polyelectrolyte behavior: the quantitative understanding of the properties in a solution of common flexiblechain polyelectrolytes is a difficult task since both the chain conformation and the strength of the intermolecular Coulomb interactions are functions of the ionic strength and since the actual contribution of the respective parameter to the observed properties is hard to determine. Rigid-chain polyelectrolytes, on the other hand, will retain their chain conformation regardless of the respective ionic strength. Hence, only the intermolecular Coulomb interactions are variables here, and this kind of polymeric electrolyte thus allows a more reliable comparison of experimental data with theoretical predictions.14-21 For this reason, several naturally occurring as well as synthetic organic polyelectrolytes of rigid-chain conformation have been studied during recent decades.22-25 The potential of transition-metal coordination polymers in this field, on the other hand, has been widely ignored up to now because of the lack of appropriate systems. Only in 1994, Chen and Archer reported for the first time on the synthesis and the * Corresponding author. E-mail
[email protected].
properties in solution of some charged Schiff-base coordination polymers.26 Two years later, the first soluble, high-molecular-weight ruthenium(II) coordination polyelectrolytes 1a-d were developed in our group.27 All these polymers have randomly coiled main chains and display the typical polyelectrolyte behavior in saltfree solutionsdespite the fact that the former Schiffbase systems still have a certain chain flexibility while the latter polymers 1a-d have a ribbonlike and thus quite rigid backbone. To further deepen the knowledge of the structure-property relationship of such rather unusual coordination polyelectrolytes and to make available more experimental information needed for the development of a powerful theoretical model that can also quantitatively describe their properties in solution, we are interested in the synthesis of further stiff-chain ruthenium(II) coordination polymers which, however, have a rigid-rod-like backbone instead of a coiled one. Taking into account the (pseudo)octahedral coordination geometry of ruthenium-polyimine complexes, rodlike coordination polyelectrolytes should result when appropriately designed bis-tridentate chelating monomers are catenated by ruthenium(II) centers.5,12,28-31 Thus, polyelectrolytes of the general structure 2 were our target structures provided that bridging units Y can be found which ensure (i) rigidity, (ii) rodlike shape, and (iii) a solubility of the polymers that is high enough for the analysis of their constitution, their degrees of polymerization, and their properties in solution. In the present paper, we discuss the specific advantages of using 2′,5′-dihexyl-p-terphenylene as the bridging unit Y. Moreover, we present the results of two competitive synthetic pathways developed for the preparation of the thus chosen target polymer 2a and provide a first insight into its properties in solution. Strategy For the selection of an appropriate bridging unit Y and thus for establishing the constitution of the desired
10.1021/ma990266u CCC: $18.00 © 1999 American Chemical Society Published on Web 08/19/1999
Macromolecules, Vol. 32, No. 18, 1999
Tpy-Based Ru(II) Coordination Polymers 5819
Figure 1. Formulas of the coiled coordination polymers 1a-d and their rodlike counterparts 2a-d.
coordination polyelectrolyte 2, we had to take into consideration geometric, synthetic, and solubility aspects. First, to ensure the rodlike shape of 2, the two 2,2′:6′,2′′-terpyridine (tpy) moieties of the ligand monomers required for its synthesis must be connected via their 4′-positions. The unit Y serving as the bridge might be either a direct bond (leading to polymer 2b) or a subunit which itself has a conformationally rigid-rodlike shape such as a p-(oligo)phenylene. The latter would lead to polymers such as 2c or 2d (Figure 1). For synthetic as well as for solubility reasons, we decided to use a 4,4′′-p-terphenylene bearing two solubilizing n-hexyl side chains attached to the central phenylene moiety as the bridging unit. The synthesis of the thus chosen target polymer 2a was expected to be possible, on one hand, via a “complex-formation strategy” using 4,4′′-bis(2,2′:6′,2′′-terpyridine)-2′,5′-dihexyl-p-terphenyl (3) and RuCl3‚3H2O (4) as the monomers (Scheme 1, route A). On the other hand, a constitutionally identical polymer 2a should be available via Pd-catalyzed polycondensation of bis[4′-(p-bromophenyl)-2,2′:6′,2′′-terpyridine]ruthenium(II) tetrafluoroborate (5a) and 2,5dihexylbenzene-1,4-diboronic acid (6) and hence via a “ligand-formation strategy” (Scheme 1, route B).
Scheme 1
Results and Discussion Monomer Synthesis. The key monomer required for the planned polymer synthesis according to route A, 4,4′′-bis(2,2′:6′,2′′-terpyridine)-2′,5′-dihexyl-p-terphenyl (3), was prepared in a two-step procedure according to Scheme 2. In the first step, p-bromobenzaldehyde (7) and o-acetylpyridine (8) were heated in a mixture of acetamide and ammonium acetate.32 The intermediate, 4′-(p-bromophenyl)-2,2′:6′,2′′-terpyridine (9a), was obtained in about 20% yield from this process and subsequently treated with 2,5-dihexylbenzene-1,4-diboronic acid (6)33 using a palladium complex as catalyst. Monomer 3 was obtained from this latter conversion in almost quantitative yields when the reaction was performed in a heterogeneous mixture of toluene and 1 M aqueous
Na2CO3, using 1 mol % of [Pd(PPh3)4] as the precursor complex of the catalytically active Pd species.33-35 Bis[4′-(p-bromophenyl)-2,2′:6′,2′′-terpyridine]ruthenium(II) tetrafluoroborate (5a), on the other hand, was the required starting material for the synthesis of polymers 2a according to route B. It was obtained as a deep red solid in nearly quantitative yields via conversion of 9a with the appropriately reactive ruthenium(III) complex 4* which was prepared by refluxing RuCl3‚ 3H2O (4) and a 3-fold molar excess of AgBF4 in acetone.36 The constitution of monomers 3, 5a, and 6 as well as of intermediate 9a was verified using 1H and 13C NMR spectroscopy. The purity of monomers 3, 5a, and 6 was estimated to be >98% (1H NMR).
5820 Kelch and Rehahn Scheme 2
Model Investigations. Model studies were carried out on low-molecular-weight reference compounds to develop optimized reaction conditions for the planned polymer syntheses via routes A and B. For route A, conditions must be found which (i) ensure the nearly quantitative formation of the [Ru(tpy)2]2+ motif and (ii) reliably prevent the formation of defect structures such as A or B (Figure 2) in which the terpyridine ligands act as bidentate ligands only and thus will bend the polymer main chains and destroy the rodlike shape of 2a. For route B, on the other hand, a coupling medium had to be found which ensures (i) the solubility of both monomers as well as of the formed products, (ii) practically quantitative aryl-aryl bond formation, and (iii) inertness of the ruthenium(II) complexes. To find out the optimized conditions for the formation of the [Ru(tpy)2]2+ motif, first, 2,2′:6′,2′′-terpyridine (9b, Z ) H) was converted with RuCl3‚3H2O (4) under various conditions which were known from the literature to give [Ru(tpy)2]X2 derivatives in good yields (Scheme 3). A nearly quantitative formation of model complex 5b was achieved when the conversion of 9b and 4 was carried out in a mixture of DMF and water. In this case, neither the 1H nor the 13C NMR spectra showed any evidence of remaining starting materials or of side products in which, for example, the 2,2′:6′,2′′terpyridine ligands act as bidentate ligands only. Unfortunately, this very convenient procedure proved to
Figure 2. Possible defect structures that may lower the persistence length of polymers 2.
Macromolecules, Vol. 32, No. 18, 1999 Scheme 3
be unappropriate for the homogeneous formation of other model complexes such as 5c (Z ) Cl) or 5a (Z ) C6H4Br). Just after the transformation of RuCl3‚3H2O (4) into the activated complex 4*, all absorptions observed in the 1H and 13C NMR spectra of representative product mixtures obtained via conversion of 4* and 9c/9a in 1-butanol/DMA-solution could be assigned to the desired complexes 5c/5a. As an example, Figure 3A displays the aromatic region of a typical 1H NMR spectrum of complex 5a obtained in this manner. On the basis of this result, ruthenium(III) derivative 4* and 1-butanol/DMA were chosen as the metal monomer and solvent system, respectively, for the following polymer syntheses according to route A. The second series of model reactions was performed to assess whether the Pd-catalyzed polycondensation reaction of the bis(p-bromophenyl)-substituted ruthenium(II) complex 5a and benzenediboronic acid derivative 6 (Scheme 1, route B) is an alternative to the synthesis of polymer 2a via route A. In analogy to recent studies on the Pd-catalyzed polycondensation of ferrocene-containing monomers,37 monomer 5a was treated with 2 equiv of benzeneboronic acid 10 in the presence of various Pd catalysts, solvents, and bases (Scheme 4). Again, the success of the respective experiment was
Figure 3. Aromatic regions of the 1H NMR spectra of (A) the bis(p-bromophenyl)-substituted monomer 5a and (B) a representative product mixture obtained via Pd-catalyzed condensation according to Scheme 4, recorded in DMSO-d6 at room temperature. The signal assignment is according to the numbering specified in Scheme 2. See text for details.
Macromolecules, Vol. 32, No. 18, 1999 Scheme 4
determined by analyzing the NMR spectra of representative product mixtures. It soon became evident that the low solubility of the charged ruthenium complex 5a in toluene or THF and thus in solvents that are wellknown to give top results elsewhere (g98% conversion)33-35 prevented their use for the present condensation reaction. Therefore, further reaction media were tested which give somewhat lower yields in general (
