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Synthetic polymers to promote cooperative Cu activity for O2 activation: Poly vs. Mono Srinivas Thanneeru, Nicholas Milazzo, Aaron Lopes, Zichao Wei, Alfredo M Angeles-Boza, and Jie He J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019
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Journal of the American Chemical Society
Srinivas Thanneeru,a Nicholas Milazzo,a Aaron Lopes,a Zichao Wei,a Alfredo M. Angeles-Boza,a Jie He a,b* aDepartment
of Chemistry, and bPolymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, United States Supporting Information Placeholder ABSTRACT: We report polymer-promoted cooperative catalysis of Cu for oxygen activation. A series of random copolymers containing dipicolylamine as binding motifs are designed to coordinate type-3 Cu sites. The Cu-copolymers show a 6-8-fold activity enhancement, compared to the molecular complex of Cu with an identical coordination site. Michaelis-Menten analysis demonstrates that the kinetic enhancement results from flexible polymer-promoted cooperative catalysis among multi-Cu sites despite the imposed thermodynamic barrier. These observations provide guidance for the bio-inspired design of metallopolymers as soluble catalysts with high activity.
There is tremendous interest in polymer-supported metal catalysts due to their potential applications in various chemical transformations. Polymers as supports to metal ions, in particular precious metal catalysts, offer a valuable way to recover and recycle expensive catalysts.1 Insoluble polymer resins are among the most popular to separate the catalysts. Alternatively, by incorporating binding sites in individual polymers, these hybrid catalysts can be endowed with a variety of unexpected functionalities, e.g., responsiveness, not found in classical metal catalysts alone.2 Notably, synergistic properties arising from the hybridization of polymer and metal catalysts have rarely been reported in the literature except for dendritic catalysts.3 Copper (Cu)-bound metalloenzymes play an important role in biological O2 metabolism.4 Considerable effort has been devoted to the synthesis of Cu complexes as mimics of the active site of type3 Cu enzymes such as tyrosinase.5 The geometrical organization of type-3 Cu sites and the distance of adjacent Cu sites play a key role in their activity. Dinuclear6 or multinuclear7 Cu complexes have been found to show higher activity than mononuclear ones due to the close proximity of the catalytic sites that promote more effective binding and activation of O2.6b, 8 For example, trinuclear Cu complexes show remarkable kinetic enhancement in electrochemical oxygen reduction9 and oxidative DNA cleavage.10 As such, incorporating Cu sites into flexible polymer chains should promote localized cooperative behavior among adjacent Cu sites. In this communication, we demonstrate the use of synthetic polymers to promote Cu-based dioxygen activation. Linear random copolymers of poly(N,N’-dimethylacrylamide-co-2-hydroxy-3-(dipicolylamino) propyl methacrylate) (P(DMA-co-GMADPA)) were designed to incorporate type-3 Cu sites through Cu-dipicolylamine (DPA) coordination. The enzyme-like activity of these Cu-polymers using O2 as the oxidant for the oxidation of ascorbic acid (AA) and 3,5-di-tert-butylcatechol (DTBC) was studied. Compared to
the monomeric catalyst with an identical coordination environment, polymeric catalysts are ca. 6-8 times more active due to the enhanced intramolecular cooperativity of the Cu ions within the polymers. The design and synthesis of Cu-containing polymers is shown in Figure 1a. The monomer, 2-hydroxy-3-(dipicolylamino) propyl methacrylate (GMADPA), has a DPA moiety as the binding motif to coordinate one Cu2+ ion. The syntheses of DPA and GMADPA is detailed in the SI (Figure S1). The random copolymers of P(DMA-co-GMADPA) were synthesized using reversible addition fragmentation chain transfer polymerization as shown in Figure 1a. A small library of copolymers with different mole fractions of GMADPA were prepared by varying the feed ratios of DMA and GMADPA monomers. The average number of repeat units was characterized by proton nuclear magnetic resonance spectroscopy (1H NMR, Figure S2). A summary of the copolymers with different ratios of GMADPA is given in Figure 1b.
Figure 1. (a) Synthesis and the chemical structures of P(DMA-coGMADPA). (b) Summary of the different copolymers of P(DMAco-GMADPA). Note * The repeat unit numbers and the mole fraction of the monomers were estimated using NMR spectroscopy as given in SI. The coordination of Cu2+ ions with the copolymers was confirmed by UV-vis titration with Cu(NO3)2 as the titrant (Figure S3). Using P(GMADPA92) as an example, the characteristic absorption peak around 670 nm was observed corresponding to the d-d transition band of N-coordinated Cu2+ when adding Cu(NO3)2.11 The peak intensity showed a continuous increase with the addition of Cu(NO3)2 and reached a plateau. The binding of Cu2+ to DPA was estimated to be 1:1 by plotting the absorbance at 676 nm vs. the
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equivalents of Cu2+ ions relative to the mole of DPA ligands, consistent with previous reports.12 Since the binding motif of the copolymers and the GMADPA monomer is identical, the coordination number of Cu2+ to DPA, as well as the coordination environment of Cu2+ ions, is similar regardless of the molecular state of the ligands. The same coordination number and UV-vis absorption were seen for the Cu-GMADPA complex. The Cu concentration in all Cu-polymers was measured using the standard curve of the CuGMADPA complex to ensure the accuracy of Cu concentration in the final polymer solutions (Figure S4).
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polymer plays a key role in controlling the proximity of adjacent Cu sites. When the concentration of the Cu sites is low, e.g., 6.3% for Cu-P1, the polymeric catalyst is less efficient with a k = 0.0124 min-1, compared to that of Cu-GMADPA (see Table S1). When increasing the concentration of the Cu sites to > 10 mol% relative to DMA units, the cooperative catalysis is clearly seen as the polymeric catalysts become more active. The maximum k of 0.17 min1 is reached for Cu-P4 with 49.7 mol% of GMADPA. Since the concentration of polymer catalysts is
