polymers containing nickel(ii) complexes of goedken's macrocycle: optimized synthesis and...

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While there has been impressive progress in the metal-lopolymer fi eld as a whole, there are relatively few exam-ples of synthetic strategies that afford well-defi ned nickel-containing polymers. For example, Manners and co-workers have expanded their ring-opening polymerization methods to include tricarba[3]nickelocenophanes, affording paramagnetic polymers (e.g., 1 ). [ 9 ] Wang and co-workers have prepared soluble nickel-polyyne copolymers that exhibit nonlinear optical properties (e.g., 2 ), [ 10 ] and Yamamoto and co-workers have prepared electrochemi-cally active copolymers based on nickel–salophen com-plexes and 9,9-dioctylfl uorene (e.g., 3 ). [ 11 ]

Metallopolymers based on transition–metal complexes of macrocyclic ligands, including porphyrins (e.g., 4 ) and phthalocyanines have received signifi cant attention due to their unique properties. [ 12 ] However, the metal com-plexes involved are often produced by low-yielding and time-consuming synthetic routes. The nickel complex of 4,11-dihydro-5,7,12,14-tetramethyldibenzo[b,i]-[1,4,8,11]tetraazacyclotetradecine ( aka , Geodken’s macrocycle) 5 exhibits unusual and potentially very useful electrochem-ical properties and can be produced via simple synthetic pathways in large quantities (>10 g) and high yield (ca.

The synthesis and characterization of a new class of nickel-containing polymers is described. The optimized copolymerization of alkyne-bearing nickel(II) complexes of Goedken’s macrocycle (4,11-dihydro-5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine) and brominated 9,9-dihexylfl uorene produced polymers with potential application as func-tional redox-active materials. The title polymers exhibit electrochemically reversible, ligand-centered oxidation events at 0.24 and 0.73 V versus the ferrocene/ferrocenium redox couple. They also display exceptional thermal stability and inter-esting absorption properties due to the presence of the mac-rocyclic nickel(II) complexes and π-conjugated units incor-porated in their backbones.

Polymers Containing Nickel(II) Complexes of Goedken’s Macrocycle: Optimized Synthesis and Electrochemical Characterization

Joseph A. Paquette , Ethan R. Sauvé , Joe B. Gilroy*

J. A. Paquette, E. R. Sauvé, Prof. J. B. Gilroy Department of Chemistry and the Centre for Advanced Materials and Biomaterials Research (CAMBR) , The University of Western Ontario , 1151 Richmond St. N. , London , Ontario , Canada, N6A 5B7 E-mail: [email protected]

1. Introduction

Metallopolymers (or metal-containing polymers) have attracted signifi cant attention due to their potential appli-cations as functional materials. [ 1,2 ] Metallopolymers gain their unique properties through the combination of the desirable properties of transition metals (e.g., redox, cata-lytic, magnetic, preceramic, and optical) and the process-ability and fi lm-forming characteristics of polymers. As a result, metallopolymers have shown utility as, for example, the functional component of redox-active capsules [ 3 ] and responsive surfaces, [ 4 ] photonic crystal displays, [ 5 ] antimi-crobial surfaces, [ 6 ] nanostructured magnetic materials, [ 7 ] and photovoltaic cells. [ 8 ]

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87%) from inexpensive starting materials. [ 13 ] Although this complex has garnered signifi cant interest from coordination chemists for over 5 decades, [ 14 ] very little has been reported with respect to its incorporation into polymers with the only existing examples produced in small quantities by electropolymerization. [ 15 ] A polymer-substituted derivative of complex 5 has also been used to template ladder-like nanostructures. [ 16 ] In this com-munication, we report the synthesis and characterization of a new class of nickel-containing polymers produced by microwave-induced palladium-catalyzed Sonogashira cross-coupling reactions.

2. Results and Discussion

2.1. Synthesis

In order to access the desired polymerizable macro cyclic nickel(II) complex, two equivalents of 4-[(trimethylsilyl)ethynyl]-benzoyl chloride [ 17 ] was fi rst combined with macrocycle 5 to afford complex 6 in 96% yield (Figure 1 a, S1 and S2, Supporting Information). The trimethylsilyl-protecting groups were removed by treating complex 6 with potassium carbonate to afford monomer 7 in 90% yield (Figure 1, 2 , and S3, Supporting Information). Single crystals suitable for X-ray diffraction were grown via slow evaporation of a saturated dichloromethane solution of 7 (Figure 1 b,c). In the solid state, monomer 7 adopts a saddle-like geometry, although it must be noted that the 4-ethynylbenzoyl substituents are expected to rotate freely in solution and their orientation in the solid state likely results due to crystal-packing effects. The average C C and C N bond lengths within the N C(Me) CR C(Me)

N ligand backbones are 1.414 and 1.333 Å respectively,

and are between the lengths of typical single and double bonds. [ 18 ] The nickel(II) ion is coordinated by four nitrogen atoms with an average bond distance of 1.8550 Å in a square planar coordination environment described by angles of N1 Ni N2 94.35(6), N2 Ni N3 85.97(6), N3 Ni N4 93.93(6), N1 Ni N4 85.69(6)°. The torsion angles between the planes defi ned by N1 C2 C3 C4 N2 and N3 C16 C17 C18 N4 and the plane defi ned by the nickel-bound nitrogen atoms (N1 N2 N3 N4) are 28.29 and 29.76°, respectively.

Bromine-substituted 9,9-dihexylfl uorene 8 was pre-pared according to a published procedure, [ 19 ] and was chosen as a co-monomer for 7 as it has been widely used in the synthesis of highly soluble π-conjugated poly-mers possessing interesting and useful electronic and optical properties. [ 20 ] In order to optimize the copoly-merization of monomers 7 and 8 (Figure 1 a), several microwave reactions were attempted (see section 2.2 for polymer characterization). The mass balance for these reactions was typically made up of highly soluble oli-gomers, soluble polymers, and an unidentifi ed insol-uble gel (see below). Initially, three solvent combina-tions were screened for potential use in the production of polymer 9 (Table 1 , runs 1−3a). The polymers pro-duced after 30 min of microwave irradiation at 100 °C in toluene/diisopropylamine (DIPA) and dimethylfor-mamide (DMF)/DIPA had lower molecular weights than the polymer produced from a mixture of DMF/DIPA con-taining a small quantity of water. Using the latter sol-vent combination, the effect of catalyst loading on the production of polymer 9 was studied (runs 4 and 5). In both cases, an increase in catalyst loading resulted in a decrease in polymer yield and molecular weight, perhaps due to the presence of an increased number of active oli-gomeric/polymeric species in solution.



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Shorter and longer reaction times (runs 6 and 7) resulted in polymers with similar molecular weight distributions, but decreased yields compared to those isolated for run 3. Longer reaction times led to the for-mation of increased quantities of insoluble materials, while shorter reaction times led to increased quanti-ties of oligomeric species. When the temperature was decreased to 75 °C (run 8), no reaction was observed. When the temperature was increased to 125 °C (run 9) an increased fraction of insoluble material was observed and the yield of soluble polymer was greatly diminished.

Upon completion of these experiments, we con-cluded that the optimal conditions for the production of polymer 9 were those employed in run 3a. The reaction was performed in triplicate to demonstrate reproduc-ibility (runs 3a−c). The average isolated yield, number-average molecular weight ( M n ), and PDI for the isolated polymers was 57%, 10 900 Da, and 2.27, respectively (Figure S4, Supporting Information). The mass bal-ance for these reactions was made up of 12% oligomers

(Figure S5 and S6, Supporting Information) and 30% dark-green insoluble solid (gel). [ 21 ] The thermal and spectro-scopic properties of purifi ed polymer 9 produced under these conditions are discussed below.

2.2. Characterization

The 1 H NMR spectra of monomers 7 and 8 and polymer 9 are shown in Figure 2 a. The latter spectrum confi rmed the formation of polymer species 9 as the signal attributed to the alkyne moiety disappeared ( δ 3.31), the signals arising from the aromatic portion of monomer 8 (δ 7.54−7.45) shifted (δ 8.25−7.58), and the spectrum for polymer 9 was broadened relative to those of monomers 7 and 8 .

Thermal gravimetric analysis illustrated the high thermal stability of polymer 9 as it did not lose mass until a temperature of 316 °C was reached. Above this tempera-ture, the polymer decomposed smoothly to a magnetically responsive black powder in a char yield of 32% at 1000 °C (Figure S7, Supporting Information). Differential scanning



Figure 1. a) Synthesis of nickel-containing polymer 9 , b) side view, and c) top view of the solid-state structure of monomer 7 . Ellipsoids are shown at 50% probability. Hydrogen atoms and co-crystallized solvent molecules omitted for clarity. Selected bond lengths (Å): N1 C2 1.335(2), N1 C35 1.417(2), N1 Ni 1.8543(15), N2 C4 1.336(2), N2 C29 1.4156(19), N2 Ni 1.8590(13), N3 C16 1.333(2), N3 C34 1.4140(19), N3Ni 1.8533(15), N4 C18 1.329(2), N4 C40 1.414(2), N4 Ni 1.8533(13), C2 C3 1.411(2), C3 C4 1.411(2), C16 C17 1.418(2), C17 C18 1.414(2). Selected bond angles (°): N1 Ni N2 94.35(6), N2 Ni N3 85.97(6), N3 Ni N4 93.93(6), N1 Ni N4 85.69(6).


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calorimetry of polymer 9 did not reveal a glass transi-tion in the temperature window between −50 °C and 300 °C (Figure S8, Supporting Information). The absence of a glass transition may be attributed to the potential interdigitation of the pendant hexyl chains within the polymer backbone.

The UV–vis absorption spectra of monomers 7 and 8 and polymer 9 are presented in Figure 2 b. The spectrum of mon-omer 8 was composed only of high-energy absorptions with a maximum absorption at 281 nm ( ε = 24 600 M −1 cm −1 ) while monomer 7 possesses absorption maxima at 262 nm ( ε = 76 900 M −1 cm −1 ), 389 nm (ε = 31 475 M −1 cm −1 ), and 582 nm ( ε = 6525 M −1 cm −1 ). The high-energy absorptions for monomers 7 and 8 have previously been assigned to intra-ligand π→π* transitions, [ 22 ] while the low-energy transition observed for monomer 7 has previously been attributed to charge transfer from the highest occupied ligand molec-ular orbital to the lowest empty d orbital of the nickel(II) ion. [ 22 ] A similar absorption maximum (588 nm, ε = 6000 M −1 cm −1 ) was observed for polymer 9 . Ligand-based absorption maxima were also observed at 275 nm ( ε = 50 850 M −1 cm −1 ) and 378 nm ( ε = 100 750 M −1 cm −1 ), the latter being assigned to a π→π* transition associ-ated with the π-conjugated organic spacer [-CO-benzene-alkyne-(9,9-dihexylfl uorene)-alkyne-benzene-CO-] pro-duced during the polymerization reaction. [ 22,23 ] Nearly identical spectral features were observed for a model com-pound 10 synthesized from mono-brominated 9,9-dihexyl-fl uorene and monomer 7 (Figure S9–S11), demonstrating that polymer 9 does not possess a signifi cant degree of

extended π-conjugation. We believe the intensity of the π→π* transitions found at 378 nm for polymer 9 has increased relative to monomer 7 as a result of the instal-lation of 9,9-dihexylfl uorene spacer, and thus a drastic change in the π-systems appended to the nickel(II) com-plex of Goedken’s macrocycle (an increase from 12 π elec-trons to 36 π electrons). We attribute the lack of extended



Table 1. Reaction conditions for the production of polymer 9 (optimized conditions shown in bold).

Run a) Solvent Time [min]

Temp. [°C]

Catalyst [%]

Yield [%]

M n b) [Da]

PDI b) Gel yield c) [%]

Pd(PPh 3 ) 4 CuI

1 Toluene/DIPA 30 100 2.5 5.0 58 6775 1.72 22

2 Dry DMF/DIPA 30 100 2.5 5.0 50 5825 1.65 22

3a DMF/DIPA/H 2 O 30 100 2.5 5.0 56 10 100 2.37 29

3b DMF/DIPA/H 2 O 30 100 2.5 5.0 59 12 050 2.43 30

3c DMF/DIPA/H 2 O 30 100 2.5 5.0 56 10 500 2.00 30

4 DMF/DIPA/H 2 O 30 100 5.0 10.0 39 8175 2.10 30

5 DMF/DIPA/H 2 O 30 100 10.0 20.0 34 5550 1.85 65

6 DMF/DIPA/H 2 O 15 100 2.5 5.0 19 8525 2.60 15

7 DMF/DIPA/H 2 O 45 100 2.5 5.0 25 12 175 2.42 48

8 DMF/DIPA/H 2 O 30 75 2.5 5.0 – – – –

9 DMF/DIPA/H 2 O 30 125 2.5 5.0 15 5125 1.53 51

a) Compounds 7 and 8 (1:1 ratio) were combined with catalytic amounts of Pd(PPh 3 ) 4 and CuI. The reagents were dissolved in a degassed solvent mixture containing 2 mL DMF/H 2 O (100:1) and 1 mL DIPA in a glove box. The greaseless reaction vessel was sealed and heated in a microwave reactor. b) The number-average molecular weight ( M n ) and polydispersity index (PDI) of each polymer were determined by con-ventional-calibration gel permeation chromatography analysis relative to monodisperse polystyrene standards. c) In addition to the iso-lated polymer, a dark-green, insoluble gel formed during each reaction. The remaining mass balance was made up by oligomeric species.

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π-conjugation to the orthogonal orientation of the plane defi ned by the π-conjugated spacer described above and the plane defi ned by the macrocyclic nickel complex adopted due to steric interactions between the methyl sub-stituents of the ligand and the carbonyl functionality.

Cyclic voltammetry studies of polymer 9 in dichlo-romethane revealed two reversible oxidation waves at potentials 0.24 and 0.73 V relative to the ferrocene/ferro-cenium redox couple (Figure 2 c). The oxidation behavior observed is closely related to the electrochemical proper-ties of parent complex 5 , which has been shown to oxi-dize in a stepwise fashion to a ligand-centered radical cation and dication. [ 15a , 24 ] Crucially, by derivatizing the 6 and 13 positions of the macrocyclic framework, we have precluded electropolymerization, which may allow for future exploitation of the redox properties observed for

polymer 9 in materials-based applications. [ 23,25 ] Monomer 7 and model complex 10 are each oxidized twice at poten-tials of 0.25, 0.76 V and 0.24, 0.75 V, respectively (Figure 2 c and S12, Supporting Information). For each compound, the observed current responses for each oxidation wave were confi rmed as one-electron processes through com-parison with the current generated by equimolar quanti-ties of ferrocene.

3. Conclusion

By exploiting the chemistry of nickel(II) complexes of Goedken’s macrocycle, we have produced alkyne-func-tionalized metal-containing monomers suitable for micro-wave-induced Sonogashira cross-coupling polymerization



Figure 2. a) 1 H NMR spectra recorded in CDCl 3 (asterisks denote residual CHCl 3 and H 2 O), b) UV–vis absorption spectra recorded in CH 2 Cl 2 . For polymer 9 , the molar absorptivity was calculated based on the molar mass of the repeating unit of the polymer backbone (987.9 g mol −1 ). c) Cyclic voltammograms acquired at a scan rate of 250 mV s −1 in a CH 2 Cl 2 solution containing 1 × 10 −3 M analyte and 0.1 M tetrabutylam-monium hexafl uorophosphate as supporting electrolyte.


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reactions. Thorough characterization of polymers derived from these nickel complexes and 9,9-dihexylfl uorene has allowed us to demonstrate the potential utility of a new family of nickel-containing metallopolymers with inter-esting and potentially useful electrochemical properties. Future work in this area will focus on the application of our potentially versatile synthetic approach to allow for the incorporation of different π-conjugated spacers and transition–metal complexes of Goedken’s macrocycle to produce diverse libraries of functional metallopolymers.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements: Financial support from the University of Western Ontario and the National Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. The authors also thank Dr. Jacquelyn T. Price for solving the solid-state structure presented.

Received: September 3, 2014 ; Revised: October 2, 2014 ; Published online: ; DOI: 10.1002/marc.201400500

Keywords: electrochemistry ; macrocycles ; metallopolymers ; nickel(II) ; redox chemistry

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