Cu2O Nanoparticles Supported on a Phenol-Pyridyl COF as

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Energy, Environmental, and Catalysis Applications 2

Cu/CuO Nanoparticles Supported on a Phenol-Pyridyl COF as Heterogeneous Catalyst for the Synthesis of Unsymmetrical Diynes via Glaser-Hay Coupling Debanjan Chakraborty, Shyamapada Nandi, Dinesh Mullangi, Sattwick Haldar, Chathakudath Prabhakaran Vinod, and Ramanathan Vaidhyanathan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02860 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Cu/Cu2O Nanoparticles Supported on a Phenol-Pyridyl COF as Heterogeneous Catalyst for the Synthesis of Unsymmetrical Diynes via Glaser-Hay Coupling Debanjan Chakraborty,⊥§ Shyamapada Nandi,⊥ Dinesh Mullangi,⊥ Sattwick Haldar,⊥§ Chathakudath P. Vinod,†* Ramanathan Vaidhyanathan⊥§* ⊥Department

of Chemistry, Indian Institute of Science Education and Research, Pune 411008, India. for Energy Science, Indian Institute of Science Education and Research, Pune 411008, India. †CSIR-NCL Catalysis and Inorganic Chemistry Division, Pune, India. §Centre

Keywords: Covalent Organic Framework, Copper nanoparticles, Heterogeneous catalysis, Glaser-Hay heterocoupling, Unsymmetrical diynes. ABSTRACT: Covalent Organic Frameworks (COFs) are a new class of porous crystalline polymers with a modular construct that favors functionalization. COF pores can be used to grow nanoparticles (nPs) with dramatic size-reduction, stabilize them as dispersions and provide excellent nP access. Embedding substrate binding sites in COFs can generate host-guest synergy leading to enhanced catalytic activity. In this report, Cu/Cu2O nPs (2-3nm) are grown on a COF, which is built by linking a phenolictrialdehyde and a triamine through Schiff bonds. Their micropores restrict the nP to exceptionally small size (~2-3nm) and the porewalls decorated with strategically positioned hydrogen-bonding phenolic groups anchor the substrates via hydrogen-bonding, while the basic pyridyl sites serve as cationic species to stabilize the [CuclusterCl2]2- type reactive intermediates. This composite catalyst shows high activity for Glaser-Hay heterocoupling reactions, an essential 1,3-diyne yielding reaction with widespread applicability in organic synthesis and material science. Despite their broad successes in homocoupled products, preparation of unsymmetrical 1,3-diynes is challenging due to poor selectivity. Here our COF-based Cu catalyst shows elevated selectivity towards hetero coupling product(s) (Cu nP loading 0.0992 mol%; TOF: ~45-50; TON: ~175-190). The reversible redox activity at the Cu centers has been demonstrated by carrying out XPS on the frozen reactions, while the crucial interactions between the substrates and the binding sites in their optimized configurations have been modeled using DFT methods. This report emphasizes the utility of COFs in developing heterogeneous catalyst for a truly challenging organic heterocoupling reaction.

INTRODUCTION: Covalent Organic Frameworks (COF), as metal-free porous crystalline polymer has gained immense attention.1-8 Owing to their chemically tunable modular structure and pore architecture, COFs have expanded their horizons into diverse applications starting from gas storage and separation,9-11 sensing,12-14 energy storage and conversion,15-28 proton conduction29-31, optics32-36 and catalysis.37-49 An effective strategy in many cases has been the inclusion of inorganic clusters within the COF pores to introduce novel functions.37-39 COF pores functionalized with heteroatom can stabilize smallsized nano particles, while their micro/meso pores enable unhindered access to these nP based active sites. Such COF-nP composites, as heterogeneous catalysts, bring obvious advantages such as recyclability, easy handling, scalability, most importantly, their activity can be tuned via a 'by-design' strategy.16,18,37-49 The lability of the binding groups forms a vital aspect in catalysis; the same ligand can be made to bind stronger or weaker with the reactive sites depending on what entity they are appended to (e.g., a self-standing pyridyl vs. the one embedded on a graphene framework).50-52 Similarly, heteroatoms integrated into the COF framework can stabilize

catalytic intermediates by adopting different protonation states and can serve as labile metal-binding sites. Unsymmetrical 1,3-diynes are an important class of intermediates in organic synthesis and key motifs in many natural products and functional materials.53-56 Partial reduction and metal-assisted electronic manipulations at these diyne cores unlocks diverse chemistry.57-59 Thus developing methodologies for the synthesis of novel diynes, in particular, unsymmetrical diynes is rewarding. Though homogeneous catalysts60-68 have been systematically developed for the synthesis of homocoupled 1,3-diynes through the Glaser-Hay coupling, attempts to make unsymmetrical 1,3-diynes by coupling two different aryl or alkyl substrates have been marred by poor selectivity.69-83 Very few heterogeneous Cubased catalysts have been developed, but they give predominantly homocoupling products.84-91 Recently, Yin and co-workers reported a Cu-based homogeneous catalyst, wherein [CuCl4]2- units generated by adding the metallic copper to N1,N1,N2,N2-tetramethylethylenediamine (TMEDA) in chloroform catalyzes the heterocoupling of alkynes quite effectively.92

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Figure 1. (A) The three-dimensional structure of 1 showing uniform 1-D channels along the y-axis. (B) AAA.. stacking of the -stacked eclipsed layers. (C) View of a single channel showing the periodic lining of the channels by the hydrogen bonding hydroxyl and basic pyridyl sites (shown in cyan color), and the channel dimensions.

A notable aspect of this catalyst is that the source of the chloride at the active metal complex is from the chloroform solvent and the amine counterbalances this catalytically active anionic complex. Drawback: the entire process is homogeneous, which makes the catalyst non-recoverable; the Cu powder gets consumed during the process. However, inspired by this chemistry and understanding the value for developing Cu-based heterogeneous catalyst for selective Glaser-Hay heterocoupling, here we have employed COF as a porous matrix to disperse the reactive copper nanoparticles. This Cu@COF is used as a heterogeneous catalyst for GaserHay heterocoupling. Interestingly, it works as good as the best known homogeneous catalyst92 and with advantageous recyclability. A thorough analytical study of the multiplecycled sample and the supernatants substantiates the structural integrity of the catalyst. The COF's role in determining the selectivity from its pore size and shape is validated from DFT studies. The DFT energies advocates the coexistence of the hetero substrates with the [CuclusterCl2]2- species within the pores to be more favorable than the homo substrates. The catalyzed reaction proceeds even in the absence of TMEDA, suggesting that this pyridyl-functionalized COF can provide a cationic environment. These are crucial roles played by the pore-environment. The high chemical stability of the COF and its strong interactions with the Cu centers renders the catalyst to be highly recyclable which can be made in grams. To the best of our knowledge, this is the first report of a Cu@COF-based heterogeneous catalyst for selective Gaser-Hay heterocoupling. RESULTS & DISCUSSIONS Synthesis: IISERP-COF9 has been synthesized via the Schiff base condensation of Triformyl phenol and 4,4',4''-(pyridine2,4,6-triyl)trianiline in dichlorobenzene-butanol solvent mixture at 120°C for 3 days (Figures S1-S3 and scheme S1). The isolated solid was purified by Soxhlet extraction in

dimethyl formamide (DMF) and tetrahydro furan (THF) under reflux conditions. The crystalline nature of the COF has been confirmed from Powder X-Ray pattern (PXRD) (Figure S4). This phenolic trialdehyde is much easier to synthesize in gram scale quantities with high yields compared to the phlorogluinol analogue. Structure solution: A 2D structure made up of hexagonal layers consistent with the experimentally observed powder Xray pattern was modeled using the Material Studio program (Figure 1). For the structure solution, we followed a similar routine as in our earlier work.16-18 In brief, the XCELL program was used to index the powder X-ray diffraction (PXRD) pattern and to identify the best possible space group as being P6 (FOM: >20). Following this, the model was constructed in a hexagonal cell. However, the presence of asymmetrically positioned pyridyl nitrogen and the phenolic group lowers the hexagonal symmetry to triclinic. This triclinic structure was geometry optimized using tight binding density functional theory (DFTB) algorithm embedded in the Materials Studio. In the optimized geometry the structure adopts a Pm space group (Initial cell in Pm: a = 18.66; b = 18.66; c = 3.51; = 120.0°, Table S1). The experimental PXRD pattern refined against this model using a Pawley routine yielded an excellent fit (Figure 2A, Refined cell: a = 18.23; b = 17.93; c = 3.51; = 117.74°; Rp = 2.44 and wRp = 3.15). The relative intensities of the experimental PXRD pattern matched simulated PXRD pattern of the eclipsed form more closely compare to the staggered configuration (Figure S4). The eclipsed configuration has uniform 1-D pores of ~13.5Å (factoring the van der Waal radii of the atoms), which agree well with the experimentally determined pore size. Because of the geometry of the Schiff bonded monomers, the active sites, hydroxyl (h-bond site) and the pyridyl (basic site) are proximally positioned at angles that favor potential interactions with the metal atoms individually or conjointly (Figure 1B).


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Figure 2: (A) Fit from the Pawley refinement using the COF's PXRD. (B) Comparison of the 77K N2 adsorption isotherms of IISERPCOF9 and Cu@COF. The BET surface area and the pore size obtained from NLDFT for the neat COF are shown. (C &D) Chemical stability of the IISERP-COF9 from PXRD and 77K N2 adsorption studies. (E) HRTEM image of the Cu@COF showing the lattice fringes due to Cu and Cu2O. The inset shows the particle size distribution from HRTEM. (F) The XPS spectrum of the Cu@COF showing the peaks corresponding to Cu(0) and Cu(I) and the '' indicates the satellite peaks corresponding to higher oxidation states of copper.

Bulk characterization: Thermo Gravimetric Analysis (TGA) shows that the COF is thermally stable up to 350°C (Figure S5). Variable temperature PXRD also supports the high thermal stability of the IISERP-COF9 (Figure S6). Solid state 13C magic angle spinning NMR spectra reveals the characteristics peaks of the COF (Figure S7). The broad peaks in the range of δ = 114 ppm to 124 ppm appear due to the aromatic carbons. Peak from the imine bonds of the Schiff linkages occur at δ = 156 ppm. The –OH groups in the triformyl phenol undergo reversible proton transfer and ketoenol tautomerism which has been established by Ning et al.93 We find that when dried the solid exists predominantly in keto-form, while in protic solvents they adopt the enolic form. Here the peak corresponding to δ = 197 ppm indicates that the –OH groups are tautomerized to keto form at ambient temperature. The observation corroborates well with the InfraRed (IR) spectra of the COF (keto: 1614 cm-1 aromatic –C-H: ~2900 cm-1; -C=C-N: 1435 cm-1, C=N: 1510 cm-1) (Figure S8). Importantly, from the IR spectra of the evacuated sample (exists in keto form) we detect the presence of both C=N and C-N, which is from the Schiff bonds stabilized by the tautomerization of the hydroxyl positioned beta to it. If there had been a phloroglucinol core,17,94 one would observe only the C-N band for the keto form. The permanent porosity of the COF is confirmed by N2 isotherm at 77 K. It exhibits a type-I adsorption isotherm with a rapid nitrogen uptake at low partial pressures (Figure 2B). The COF possess very high Brunauer– Emmet–Teller (BET) and Langmuir surface area, 1172 m2/g and 1503 m2/g, respectively (Figure S9). A Non Local Density Functional Theory (NLDFT) fit yielded a pore size of 13 Å,

which agrees well with the eclipsed model. The COF is quite stable to boiling in water as well as to harsh acidic and basic conditions, as confirmed from the PXRD and the 77K N2 adsorption isotherms (Figure 2C and D, S10). The COF loses some of its crystallinity when treated with 6N NaOH. Surprisingly, despite the presence of only a single hydroxyl group as compared to the three in the phloroglucinol analogue,17,94 the keto-enol tautomerism stabilizes the Schiff bonds in the COF. As an indirect corollary, this would mean that the hydrolysis of an unstabilized Schiff bonds (no hydroxyl containing) in an oligomeric or polymeric Schiffbonded framework happens via a mechanism that involves the aromatic rings with multiple meta-positioned Schiff bonds (Figure S11). However, establishing this requires further probing. Field Emission Scanning Electron Microscopy (FESEM) images of the COF reveals its spherical morphology.(Figures S12-S14) The AFM image of a sample prepared by sonicating for 30mins in THF also showed the presence of isolated spherical particles (Figure S15). Nevertheless, it can be seen from the Field Emission Transmission Electron Microscopy (FETEM) that these spheres are made up of aggregated COF flakes (Figure S16). Cu@COF characterizations: Encouraged by the richness of accessible pyridyl and hydroxyl groups within the COF, which are known to bind to Cu in its metallic or low-oxidation state, we decided to utilize this COF for growing "capping agentfree" Cu nanoparticles. At first, the Cu2+ ions are adsorbed into the THF suspension of the COF. These pore-confined Cu2+ species are then reduced with ascorbic acid at 80°C.95 But the reduction of Cu (II) can result in Cu(0) and/or Cu(I) species.



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Figure 3: Schematic illustration of the catalytic reaction with detailed substrate scope. % yield, TON and TOF (in h-1) have been presented. Color code: Pink and Cyan ellipses- alkyne bonds linked to different functional units. Green ellipse- alkyne bonds in the heterocoupled products.

Also, prolonged exposure of the composite to air may partially oxidize Cu(0) to Cu(I). The PXRD pattern of the Cu@IISERP-COF9 (Cu@COF) showed only the peaks corresponding to COF (Figure S17). No reflections for Cu were observed suggesting the Cu species to be extremely small-sized nanoparticles or amorphous. The N2 isotherm of the Cu@COF measured at 77K shows negligible nitrogen uptake suggesting that the nanoparticles are dispersed in the COF (Figure 2B). The Cu@COF has spherical morphology similar to the neat COF as observed from the FESEM and FETEM images (Figures S18-S19). The Energy Dispersive X-ray (EDX) analysis indicated the Cu loading to be at 6.3 weight percent, which agrees well with the 7 wt% determined from the Inductively Coupled Plasma (ICP) analysis (Figure S20). While the elemental mapping revealed the homogeneous dispersion of Cu into the COF and also shows that they remain as isolated clusters (Figure S21-S22). To identify the phase of the Cu species embedded in the COF, high-resolution TEM (HRTEM) images of the Cu@COF were recorded. The observed lattice fringes indexed to metallic Cu and Cu(I) oxide. The d-spacing of 0.212 nm corresponds to the (111) plane of the cubic Cu nP (ICSD-4349326) and the d-spacing of 0.236 nm is from the (111) plane of the Cu2O nano particle (ICSD 063281). So, the Cu nP were not amorphous. A statistical estimate of the particle size distribution from HRTEM images collected at two different magnifications display the Cu nP having a size range of 2 to 3nm (Figure 2E, S23). The richness of metal-binding groups in the micropore lining could be key to stabilizing such small-sized Cu nPs. Such small-sized Cu nPs is difficult to achieve even with the use of capping agents or stabilizers.96 The prominent characterization of the nano particles has been done by X-Ray

Photo Electron Spectroscopy (XPS). The Cu 2p XPS spectra exhibits peaks at 932.3, 933.9, 952 and 953.8 eV, which are assigned to the Cu 2p3/2 and Cu 2p½ levels of the metallic Cu and Cu(I) from Cu2O (Figure 2F).97-101 Additionally, two shake-up peaks appear at 944 eV and 964 eV. The shake-up peaks or satellite peaks represent the higher oxidation states of Cu.97-101 The N 1s spectra show two peaks at 398.5 and 400.3 eV 398.5 eV attributed to the weak Cu-N interaction between Schiff nitrogen and Cu, while the peak at 400.3 is assigned for C=N bonds in the COF (Figure S24).102,103 To further demonstrate the composition of the nanoparticles, the freshly prepared Cu@COF was burnt under dry Ar atmosphere at 600°C. This produces nano particles coated with carbon from the pyrolyzed COF. This carbon composite was quickly transferred to an air-sensitive PXRD holder equipped with a Kapton window and evacuated before subjecting to X-ray diffraction. The PXRD of this pyrolyzed product showed characteristic peaks for the Cu (ICSD-4349326) and Cu2O nano particles (ICSD 063281, Figure S25). Glaser-Hay catalysis: The Cu@COF can be seen as a copper powder dispersed in an insoluble COF matrix- this makes it equivalent to a solid Cu powder, which, in the presence of a suitable amine, is known to be an excellent catalyst for Glaserhay coupling.92 However, can it give the activity and the selectivity towards heterocoupled products?- Something that has been evading the other heterogeneous Cu catalysts.84-91 The catalytic reaction was carried out in CHCl3-dioxane solvent system with an optimal catalyst loading of 0.0992 mol%. The reaction was performed in the open air and it completed within four hours. The catalyst was easily removed from the reaction mixture by centrifugation and the product mixture was characterized using a range of techniques. Extensive substrate scope


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Figure 4. (A & B) Cyclic voltammammogram of the COF vs. the Cu@COF showing the presence of redox activity in both. For neat COF the reversible redox peaks correspond to the imine bond (R1/O1), keto bond (R2/O2) and pyridyl ring (R3/O3). The CV plot of the Cu@COF displays the reversible formation of the Cu(0)-Cu(I)-Cu(II)-Cu(III)-Cu(0) species. Note that the voltage at which the redox peaks appear for the COF vs. Cu@COF are significantly different and many redox peaks in the COF's CV are missing in the Cu@COF. (C-F) XPS spectra from Cu@COF sample isolated at different stages of the catalyzed reaction. (G) Mechanism of the heterocoupling reaction at the copper cluster surface. (H) IR spectra of the catalyst's extracts from the arrested reactions showing no change in the functional groups of the COF. (I) Cl 2p XPS of isolated sample during catalysis confirming Cu-Cl species.

was studied. The average percent yield of the hetero coupled products is around 70% which is quite remarkable (Figure 3). Turn Over Number (TON) for the products is greater than 150 and Turn Over Frequency (TOF) has been calculated for each of the product and this lies around 45 h-1 which are quite reasonable for non-noble metal based heterogeneous catalyst. The reason for not being able to achieve exceptionally high TON ( >1000) could be due to the fact that catalytically active species could be forming only on the exposed facets of the Cu/Cu2O clusters, which leaves the atoms in the core of the clusters as non-participants or the nP do not fill the entire pore but just plugs the entry of the pore,46 which lowers the active site/COF ratio and hence the TON. Reactivity can be improved if the nanoparticles of < 1 nm can be made. The catalyst is effective for substrates containing Electron Withdrawing Groups (EWG) and Electron Donating Groups (EDG). Different aliphatic alkynes with primary, secondary and tertiary propargyl alcohols have been used for catalytic conversion. Each of these combinations has produced the hetero-coupled product with good yields, selectivities and significant TON, TOF (Figure 3 and Tables S2-S6). Mechanistic studies: To identify the different oxidation states adopted by the copper sites during these catalytic reactions, we carried out Cyclic Voltammetry studies. For this, the sample (Cu@COF) was coated on a Toray paper and used as the working electrode, while Pt and Ag/AgCl were used as the counter and reference electrodes, respectively. The electrolyte

was a saturated solution of tetrabutyl ammonium hexafluorophosphate in acetonitrile.35 From the CV (Figures 4A and B), the reversible oxidation of Cu(0)/Cu(I) to Cu(II) and Cu(III) could be observed.104,105 Access to such multiple copper valency is vital to achieving synergetic effects during the catalysis.68 Also, a comparison of the CV of the neat COF with that of the Cu@COF shows some marked differences. The redox activity of the neat COF is most probably due to the protonation/oxidations at the pyridyl sites106,107 and the ketoenol tautomerism at the phenol sites,35 which is assisted by the protonation at the Schiff bond nitrogens (see figure 4).14 Notably, the potentials associated with the reduction and oxidation events are distinctly different when the Cu is loaded into the sample. For example, the redox peak occur at -0.38V, +0.128. +0.71, +1.11 (redn.), 0.54, -0.70, -1.128, -1.44V (oxdn.) for the COF. Notably, most of them do not emerge in the Cu@COF's CV plot, but new peaks appear (-0.65, -0.33, +0.63, -0.47, -0.72V). This points that there are prominent interactions between the Cu nP and the COF modulating the redox activity and thereby the catalytic activity. To further support the mechanism with respect to the oxidation state changes at the Cu centers, we have performed a detailed XPS study on the aliquots isolated at different stages of the reaction. The reaction mixture was frozen at different time intervals and the isolated solid was mounted onto the XPS sample holder inside the glove box



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loss in activity (Figure 5A) and the PXRD and IR confirmed the retainment of crystallinity and functional integrity (Figures 5B and C) while the XPS of the spent catalyst displays the recovery of the catalyst to its original form (Figure 5D, S102S109). Investigating favorability for heterocoupling in the COF pores using DFT studies: To rationalize why this Cu@COF favors heterocoupled products and verify the ability of hydroxyl and pyridyl groups to act as active sites, we have docked the substrates along with the catalytically active species, namely a Cu cluster with halides as labile ligands on them (derived from ref.92), within the COF pores. The copper cluster employed was a 3 x 3 x 3 copper nanocluster derived from cubic copper lattice.108 The charge balance was maintained by protonating the pyridyl centers of the COF. The existence of halides in the solid catalyst,

Figure 5. (A) The catalyst recyclability over 5 cycles established using the formation of 1a in Figure 3. Comparison of the (B) PXRD patterns and (C) IR spectra of the spent catalysts with the as-made. (D) A comparison of the XPS spectra of as-made Cu@COF with the post-catalysis Cu@COF sample.

and analyzed without exposing to air (Figure S101). Initially in the presence of TMEDA and CHCl3, the XPS spectrum of the sample contains shake-up peaks (940.5 and 944.5 eV Figure 4C) which indicates that Cu is present in a higher oxidation state which is due to the oxidative addition at the Cu center and conversion of Cu(0)/Cu(I) to Cu(II). When two different alkynes were added to the reaction mixture, they undergo ligand exchange most probably with the chlorides coordinated to the Cu(II) centers forming a copper diacetylide complex (Figure 4G). Further, the Cu(II) species undergoes oxidation to Cu(III) in the presence of air as explained by Su et al.,92 which is a critical intermediate that assists the formation of hetero-coupled product. Finally, the Cu(III) readily undergoes reductive elimination and the desired heterocoupled product forms (Figure 4G).92 The XPS spectra of the subsequent intermediates show a significant difference in the shake-up peaks (Figures 4C-F). No shake-up peak is observed beyond three hours of the reaction which indicates the reduction of the oxidation state of the Cu species to 0 or +1 (Figure 4F). Another important feature from the XPS spectra of the extracted aliquots is the shift in the Cu 2p binding energy with time (Figures 4C-F). After 60 minutes into the reaction, the Cu 2p3/2 and Cu 2p½ appear at 933.8 and 953.8 eV. But after 300 minutes, the peaks are slightly red-shifted to 932.4 and 952.2 eV which again is a mark of oxidation state change of Cu species supported on the COF. Also, the IR spectra of these extracts collected at different time intervals did not show any appreciable change in peaks positions or intensities confirming the structural integrity (Figure 4H). We believe, confining the reactants and the Cu-cluster within the COF's functionalized nanopores is favoring them to interact mutually. Catalyst recyclability and spent-catalyst analysis: The coupling reaction did not proceed when the supernatant extracted by centrifuge was employed as the catalyst-cumsolvent. ICP analysis of this supernatant revealed that no notable Cu was present. This confirms the lack of leaching and indirectly the true heterogeneous nature of the catalyzed reaction. The Cu@COF could be recycled for 5 cycles without

Figure 6. (A) Optimized geometries for the homo and hetero substrates showing the associated energies. (B) Shows the column of -stacked alkynes and the column of the propargyl alkyne ordered by hydrogen-bonding between their hydroxyl groups with the phenolic groups lining the COF pores. The uniformly spacing of these COF-substrate interactions are crucial in establishing a lower energy configuration and this could explain why the hetero products get favored by the COF pores.

extracted at different time intervals of the reaction, was confirmed from EDAX and XPS (Figures 4H, S109). A comparison of the relative energies obtained from a geometry optimization carried out using a CASTEP routine (Figure 6A), reveals that the co-existence of hetero substrates with the catalyst in the pore has the lowest energy (relative energy: homo = +9.3 kcal/mol vs. hetero = -7.1 kcal/mol). We verified experimentally that the reaction proceeds even in the absence of TMEDA, but the Cu@COF had to be protonated using formic or acetic acid as it is not a strong enough base like TMEDA hence cannot abstract the proton from the reaction medium (i.e. CHCl3). Removal of TMEDA and supplying the required cation from the heterogeneous catalyst does bring additional atom efficiency and minimizes complexity. Most importantly it minimizes the steric crowding by the TMEDA within the limited space available within the Cu@COF nanopores. In a nutshell, the hydroxyl, pyridyl and the pore-


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environment all play a crucial role in the heterocoupling (Figure 6B). CONCLUSION: Coupling two inactivated alkynes is challenging; more complexity lies in selective heterocoupling of alkynes to form unsymmetrical products. Here we have developed a COF that has H-bonding hydroxyl and protonatable pyridyl-lined pores. These functionalized pores, when loaded with small Cu/Cu2O nanoparticles, work cooperatively to stabilize Cu-halide intermediates derived from the chloroform solvent. Such species are known to catalyze Glaser-Hay coupling through redox cycles at the Cu center.92 Control experiments coupled with DFT studies elicit the role of COF in providing a cationic environment to balance labile anionic copper-halide intermediates and suggests the presence of favorable substrateCOF hydrogen bond interactions. The CV studies indicate the electronic synergy between the COF and the redox active copper sites. Our findings encourage the application of COF in the designed-development of recyclable heterogeneous catalysts for some crucial organic synthesis.

ASSOCIATED CONTENT Supporting Information. Materials and methods, PXRD, TGA, IR, Adsorption studies, Microscopic studies (FESEM, FETEM, HRTEM, AFM), XPS, NMR, HRMS, GC characterization, Solid state UV and calculation details.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. P. V.). *E-mail: [email protected] (R. V.). ORCID Ramanathan Vaidhyanathan: 0000-0003-4490-4397

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank IISER-Pune for support and the funding by “DSTNanomission under the Thematic Unit Program” (EMR/2016/003553). DC thanks DST-Inspire for financial support. We thank Ms. Meghamala Sarkar, IISER, Pune, for her help with the Gas Chromatography measurements.

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Cu/Cu2O Nanoparticles Supported on a Phenol-Pyridyl COF as Heterogeneous Catalyst for the Synthesis of Unsymmetrical Diynes via Glaser-Hay Coupling Debanjan Chakraborty, Shyamapada Nandi, Dinesh Mullangi, Sattwick Haldar, Chathakudath P. Vinod, Ramanathan Vaidhyanathan


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Cu2O Nanoparticles Supported on a Phenol-Pyridyl COF as

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