Hybrids by Cluster Complex-Initiated Polymerization - ACS Publications

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Hybrids by Cluster Complex-Initiated Polymerization Xiaoyan Tu, Gary S. Nichol, Pei Keng, Jeffrey Pyun, and Zhiping Zheng* Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States S Supporting Information *

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norganic−organic hybrids offer great opportunities to integrate the desirable properties of the two distinct realms and to create entirely new properties not exhibited by either due to the compositional, functional, and organizational synergism between the inorganic and organic components.1 An intriguing example is the superparamagnetic hybrid obt ained by co ploym erizing eth yl acrylate with Mn12O12(acrylate)16,2 a complex featuring the celebrated superparamagnetic metal cluster core of Mn12O12. This work provides an encouraging example of how the properties of an inorganic component with interesting properties may be integrated with the propitious characteristics of organics to develop useful and processable materials. Another example is the cluster-cored metallodendrimer emanating from the biologically significant [Fe4S4]2+ cluster core.3 The kinetics and potential of the cluster-based redox processes have been found to be tunable by the dendritic ligands (dendrons), suggesting potential applications of such hybrids for information storage. Great potentials notwithstanding, there exist a number of issues.4 First, oxide or oxoalkoxide clusters are generally sensitive to water and other types of nucleophiles, thus limiting the choice of polymerization methods including reaction conditions, in addition to the concern of their structural integrity. Second, most of these clusters possess multiple reactive groups, leading to high degree of cross-linking and complicating the characterization of the resulting hybrids. Lastly, in terms of realizing functional materials, the high expectation is still limited by the small number of available clusters with useful electronic, optical, or magnetic properties. The use of hexanuclear clusters of the general formula [M6Xn]z (M, transition metal; X, halide or chacogenide; n = 6 or 12; z, charge of the cluster core) potentially addresses the aforementioned issues as these clusters, typically synthesized by high-temperature solid-state reactions, maintain their structural integrity under conventional reaction conditions yet quite amenable to synthetic manipulations.5 The existence of multiple metal sites allows for site-specific functionalization, facilitating the systematic modification of properties by varying the number and type of the ligands. Hybrids featuring clusters of this type include [Mo6Cl8]4+/poly(4-vinylpyridine) and poly(vinylimidazole),6,7 [Mo6S8] and [W6S8]/polystyrene,8 [Mo6Cl8]4+ cored-metallodendrimers,9 and the more recent mesogenic hybrids containing the cluster cores of [Mo6Br8]4+ and [Re6Se8]2+.10 Our own efforts along this line of interest have relied on the use of the [Re6Se8]2+ cluster core (Figure 1).11 The 24-electron face-capped hexanuclear core can be viewed as an octahedron of rhenium enclosed in a cube formed by the eight face-capping Se ligands, with six additional terminal ligands. The cluster core © 2012 American Chemical Society

Figure 1. From left to right: structure of the [Re6Se8]2+ cluster core shown with terminal ligands L and the molecular structures of cationic cluster solvates12 [Re6 Se 8(PEt 3 )5(CH3 CN)]2+ (P5 N) and cis[Re6Se8(PEt3)4(CH3CN)2]2+ (cis-P4N2).

undergoes a reversible, one-electron oxidation event, with the potential dependent upon the nature and number of the terminal ligands.12 The parent and oxidized clusters exhibit markedly different absorption characteristics, and more intriguingly, both are luminescent; the luminescence is dependent on both the oxidation state and coordination environment of the cluster.13 These findings suggest the possibility of altering the absorption and emission characteristics of the clusters by changing electrochemical conditions. Electrochromic and electroluminescent materials may thus be envisioned. The initial success of utilizing this cluster system has been demonstrated by the cluster-cored metallodendrimers using Fréchet-type dendrons derived from gallic acid.14 We found that the nature of the focal coordinating group of the dendron profoundly affects the electronic structure of the cluster core, giving rise to intriguing photophysical properties of the hybrids. In principle, it should be possible to tune such properties via the use of various combinations of dendrons. This effort would require the cluster precursors be modified to accommodate the different numbers and types of polymeric ligands. In other words, the availability of appropriately designed and systematically altered cluster building blocks is key to establishing the relationship between the structure of the building blocks and the property of the hybrids. Materials with controlled structure, composition, dimension, and properties can thus be realized. Toward the goal of synthesizing compositionally and structurally tunable cluster-containing hybrids, we have previously prepared a cluster complex [Re6Se8(PEt3)5(4vinylpyridine)](SbF6)2 in which a bifunctional 4-vinylpyridine ligand coordinates the cluster core via Re−N (pyridyl) bonding while leaving the vinyl group available for further chemical transformations.15 Copolymerization of the complex with Received: January 5, 2012 Revised: February 9, 2012 Published: February 22, 2012 2614

dx.doi.org/10.1021/ma300024a | Macromolecules 2012, 45, 2614−2618

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Figure 2. Syntheses of cluster complexes (2 and 3) with polymerization-initiating ligand 1 and the polymerization of methyl methacrylate (MMA) initiated by 2 with the pure organic initiator 1 in a control experiment.

Figure 3. Comparative 1H NMR studies of complexes P5N, 1, and 2 (from top to bottom; *CDCl3 solvent peak).

[Re6Se8(PEt3)4(PyBr)2](SbF6)2 (3), have been prepared, and polymerization of methyl methacrylate (MMA) using 2 as the initiator has been studied (Supporting Information). The synthesis of the organic initiator 1 was achieved by acylation of pyridine-4-methanol with α-bromoisobutyryl bromide (Figure 2). Originally obtained using CH2Cl2 as solvent produced a complex mixture, from which pure product was obtained by chromatographic separation. Characterization by 1H NMR spectroscopy was straightforward: Upon acylation

styrene resulted in a waxy solid, displaying an orange color characteristic of the parent cluster but contrasting morphologically with the highly crystalline precursor complex. In this Note, we report our preliminary studies by employing sitedifferentiated cluster complexes as initiators for controlled radical polymerization.16 Two cluster complexes featuring respectively one and two potentially polymerization-initiating ligands, pyridin-4-ylmethyl 2-bromo-2-methylpropanoate (1, PyBr), [Re 6 Se 8 (PEt 3 ) 5 (PyBr)](SbF 6 ) 2 (2) and cis2615


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the broad −OH signal of the starting pyridine-4-methanol at 3.32 ppm disappeared, while the −CH2− proton resonance sizably shifted from 4.72 to 5.19 ppm of the product. The product yield was significantly improved when the reaction was carried in a hexanes/THF mixture, but chromatographic separation was still necessary. Compound 1 can be stabilized up to 6 months as a 20% solution in hexanes when stored under dry N2 at 4 °C; gradual decomposition was observed upon standing as a solid under ambient conditions. Subsequent ligand substitution reaction of site-differentiated nitrile solvates12 [Re6Se8(PEt3)5(CH3CN)](SbF6)2 (P5N) and cis-[Re6Se8(PEt3)4(CH3CN)2](SbF6)2 (cis-P4N2) with 1 was unexceptional, affording cluster-complex initiators 2 and 3 in essentially quantitative yields. The formation of the targeted complex 2 is clearly revealed by comparative NMR studies: The 1 H NMR spectrum of 2 shows significant downfield shift of the pyridyl α-H from 8.59 ppm in 1 to 9.18 ppm upon coordination to the Lewis acidic cluster core, but shifts of other signals are not as obvious due to their remoteness to the cluster core (Figure 3). The distinct relative integration of 4:1 in the 31P NMR indicates that the pentakis substitution of the cluster core was maintained upon ligand exchange (Figure 4). The identity of complex 2 was further verified by using X-ray diffraction analysis. Its crystal structure is shown in Figure 5.

Figure 5. Crystal structure of 2 with partial labelings. Counterions, ethyl groups of PEt3 ligands, and H atoms are omitted for clarity. Anisotropic displacement ellipsoids at the 50% probability level.

carried out by following standard procedures. Analysis by sizeexclusion chromatography showed that the number-averaged molecular weight (Mn) of the polymer is 10 309 amu with a polydipersity of 1.18 (Figure 6). Integration of the 1H NMR of 4 yielded an averaged initiator-to-monomer ratio of 1:88; the initiator efficiency is thus 94% (based on 97% conversion).

Figure 6. SEC analysis of polymer 4 using THF as mobile phase and with a RI detector. The SEC molar masses were calibrated against low polydispersity linear polystyrene standards. Figure 4. Comparative (bottom).

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These values suggested that the initiation by 1 was effective and that the polymerization was nicely controlled. The polymerization of MMA using the cluster-based initiator 2 was then carried out under otherwise the same conditions as when the organic initiator 1 was used. SEC analysis of the product mixture showed two fractions, one with Mn of 24 716 amu and the other, 7930 amu (Figure 7a). Since the cluster absorbs at ca. 240 nm, a UV detector was also used to monitor the presence of cluster in the product. It has been found that only the lower molecular weight fraction has noticeable UV absorption at 240 nm. These observations may suggest that (1) the initiating power of 1 is mitigated when bound to the cluster and (2) decomposition of cluster-complex initiator 2 may have occurred under the polymerization conditions. The former may be rationalized in terms of the electronic influences upon coordination to the Lewis acidic core. As for the latter, both 2,2′-dipyridyl and DMF are potentially competing for coordination with the cluster, thus destabilizing the cluster-

P NMR studies of P5N (top) and 2

The metric values of bond distances and angles (Supporting Information) defining the Re6Se8(PEt3)5 moiety are comparable to its precursor [Re6Se8(PEt3)5(CH3CN)](SbF6)2 and other structurally characterized cluster complexes containing the same structural motif.11c,12 The Re−N bond of 2.211(17) Å and the pyridyl C−C bond distances averaged at 1.39 Å fall in the respective ranges reported for analogous cluster complexes with pyridyl coordination.12 The ability of 1 to initiate controlled radical polymerization was first tested in order to generate a reference point for the evaluation of the cluster complex 2 in a similar capacity. Under conditions typically applied for atom transfer radical polymerization,16 a mixture containing 1 and MMA in a molar ratio of 1:100 and 1.1 equiv of Cu(I)-2,2′-bipyridyl catalyst in dimethylformamide (DMF) was heated at 70 °C for 90 min. Isolation and purification of the polymeric product (4) were 2616


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In summary, two [Re6Se8]2+ core-containing cluster complexes with a ligand capable of initiating controlled radical polymerization have been prepared, and the potential of using such cluster complexes to create cluster-containing hybrids was demonstrated by the polymerization of MMA with one of these complexes as initiator. The present results validate our original goal of using such cluster complexes in making functional hybrid materials, but the performance of the cluster complex initiator is somewhat compromised when compared with the initiator ligand itself, possibly due to the decomposition of the complex as a result of the competitive coordination by the solvent and the ancillary ligand of the Cu(I) catalysts used in the ATRP process. Indeed, significant improvement was achieved by substituting 2,2′-dipyridyl for pentamethyldiethylenetriamine (PMDEA) as ligand for the Cu(I) catalysts and dimethylformamide (DMF) for acetonitrile (CH3CN) as solvent; both are less coordinating toward the cluster core. We will extend this chemistry with the use of other complexes equipped with different types, numbers, and/or spatial arrangement of the initiator groups for the generation of polymeric hybrids with structurally distinct architectures.16b The potential applications of such hybrids, for example, as radiographic contrast agents,17 redox-active liquid crystals,18 and phosphorescent organic light-emitting materials,19 will also be pursued.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization details and crystal data and structure refinement parameters of compound 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (520) 626-6495; Fax (520) 621-8407.

Figure 7. SEC analyses of hybrids obtained by using 2 as initiator and under two different sets of conditions: (a) ratio [2]:[CuCl]:[2,2′bipy]:[MMA] = 1:1:2:200; 20% DMF, 70 °C, 6 h (top); (b) ratio [2]: [CuCl]:[PMDEA]:[MMA] = 1:1.5:2:200; 30% CH3CN, 70 °C, 1.5 h (bottom; hybrid 5 in Figure 2).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF (CHE-0750530) for financial support of this work.

complex initiator and effectively reducing its effectiveness in polymerizing MMA. As a possible solution to minimize competitive coordination of the cluster core and thus the decomposition of 2, pentamethyldiethylenetriamine (PMDEA) was used in place of 2,2′-dipyridyl as ligand for the Cu(I) catalyst. For the same consideration, CH3CN was used in place of DMF as solvent. Polymerization under such conditions reached the viscous state after 1.5 h with a monomer conversion of 81% compared to 6 h when the combination of 2,2′-dipyridyl/DMF was used. The SEC trace showed a predominant product with Mn of 11 366 amu and a polydispersity index of 1.21, accompanied by a minor fraction (about 1.6%) with Mn of 44 822 amu and a PDI of 1.02 (Figure 7b). The lower-molecular-weight fraction displays a strong UV absorption at 240 nm, characteristic of the [Re6Se8]2+ core, whereas the higher-molecular-weight fraction does not show any significant absorption associated with the presence of the cluster. On the basis of these results, it is reasonable to conclude that with a less competitive ligand for Cu(I) and a less coordinating solvent, the polymerization reaction can be improved.

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Hybrids by Cluster Complex-Initiated Polymerization - ACS Publications

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