Heterogeneous C-H Functionalization in Water via Porous Covalent Organic Framework Nanofilms: A Case of Catalytic Sphere Transmutation.

Heterogeneous catalysis in water has not been explored beyond certain advantages such as recyclability and recovery of the catalysts from the reaction medium. Moreover, poor yield, extremely low selectivity, and active catalytic site deactivation further underrate the heterogeneous catalysis in water. Considering these facts, we have designed and synthesized solution-dispersible porous covalent organic framework (COF) nanospheres. We have used their distinctive morphology and dispersibility to functionalize unactivated C-H bonds of alkanes heterogeneously with high catalytic yield (42-99%) and enhanced regio- and stereoselectivity (3°:2° = 105:1 for adamantane). Further, the fabrication of catalyst-immobilized COF nanofilms via covalent self-assembly of catalytic COF nanospheres for the first time has become the key toward converting the catalytically inactive homogeneous catalysts into active and effective heterogeneous catalysts operating in water. This unique covalent self-assembly occurs through the protrusion of the fibers at the interface of two nanospheres, transmuting the catalytic spheres into films without any leaching of catalyst molecules. The catalyst-immobilized porous COF nanofilms' chemical functionality and hydrophobic environment stabilize the high-valent transient active oxoiron(V) intermediate in water and restricts the active catalytic site's deactivation. These COF nanofilms functionalize the unactivated C-H bonds in water with a high catalytic yield (45-99%) and with a high degree of selectivity (cis:trans = 155:1; 3°:2° = 257:1, for cis-1,2-dimethylcyclohexane). To establish this approach's "practical implementation", we conducted the catalysis inflow (TON = 424 ± 5) using catalyst-immobilized COF nanofilms fabricated on a macroporous polymeric support.


Introduction
Direct and selective functionalization of inert C-H bonds as a latent functional group in water is a demanding endeavour, [1][2][3][4][5] although enzymes do the same transformation under ambient conditions. [6][7][8] However, most enzymes are expensive and lose their catalytic activity with a slight alteration of reaction conditions. 9 A few homogeneous catalysts and catalytic methods have already been developed to functionalize inert C-H bonds in water. [10][11][12][13][14] For example, μ-nitrido diiron phthalocyanine and porphyrin complexes were found to catalyze the oxidation of CH 4 in water. 15 However, selective oxidative functionalization of complex molecules, analogous to the chemical processes that occur in all living systems, with appreciable catalytic activity and selectivity in water, is exceptionally challenging. Water is economical and green, and pharmaceutical and biologically relevant molecules are generally only soluble in water/polar media. [16][17] Hence, a challenging new area of catalysis research should be to develop a general protocol where most of the catalytic processes are performed in water. However, many synthetically useful catalysts will get excluded in such cases because of their loss of catalytic activity or intolerance towards the water. Additionally, most organic substrates are insoluble in water. A pragmatic solution to this problem would be to use ordered organic hydrophobic porous supports that could entrap both the catalyst and substrate in close proximity to signi cantly enhance the reaction rate. This concept of integrating one or more catalytically active functionalities and substrates into a solid organic host material would mimic enzymes such as cytochrome P450, in which the hydrophobic active site holds both the heme cofactor and the substrate in close proximity. Whereas this strategy seems easy, it poses problems because of the challenges associated with designing such crystalline yet hydrophobic and porous organic host structures.
We envisaged that catalyst encapsulation within a chemically stable porous covalent organic framework (COF) could be a strategy to provide a hydrophobic microenvironment around the active site so that the catalytic process could be performed in the aqueous media. Covalent organic frameworks (COFs) are a new class of crystalline materials, with highly ordered organic building blocks and discrete nano-pores. [18][19][20][21][22] The organic building units in COFs, stacked into in nite 1D columns, are ideal for docking catalysts and providing a hydrophobic microenvironment around the active site. [23][24][25] Hence, we designed a nanospherical formulation of a two-dimensional covalent organic framework with predesigned functionality.
These nano-spheres can be used to immobilize catalysts via non-covalent interactions. Simultaneously, the material's porous nature would allow free diffusion of the substrate and the products formed after the reaction.
The rst catalyst that was immobilized is a member of the fth-generation biuret modi ed TAML, (Et 4 N) 2 [Fe III (Cl)bTAML], which catalyzes the selective hydroxylation of C-H bonds with very high regioselectivity (3° C-H over 2° C-H) and stereoretention. 26,27 This bioinspired catalyst typically activates oxo transfer reagents, such as mCPBA, NaOCl, or O 2 , to oxidize alkane bonds and alkenes to their corresponding oxidized products. Herein, we present a unique strategy for concerted heterogeneous catalysis by generating a catalytically active FebTAML complex within the inter-crystallite voids of the COF nano-spheres. Furthermore, noting the FebTAML immobilized COF nano-spheres' inability to functionalize C-H bonds in water, we decided to transmute the catalyst loaded COF nano-spheres into thin-lms. We could fabricate such self-standing COF thin-lms, as these crystalline and porous COF nano-spheres 28 undergo mesoscale ″ ber protrusion". We took advantage of this exceptional phenomenon to arrest (Et 4 N) 2 [Fe III (Cl)bTAML] molecules during the nano-spheres ® thin-lm transmutation. It is fascinating that the immobilized catalyst molecules remained entrapped despite such dynamic transmutation and culminated into highly fecund catalytic COF thin-lms. To the best of our knowledge, this is the rst example of the selective C-H functionalization of the water-insoluble hydrophobic substrates in pure water using the catalyst immobilized self-standing thin-lms. We believe that the organic substrates preferentially partitioned inside the COF lm, allowing water-insoluble substrate oxidation in water. Finally, we performed in ow catalysis using catalyst immobilized COF thinlms on a macroporous solid polymeric support. After the 60 th cycle, this in ow catalysis showcased the e cacy of the system, with 72% catalytic yield and an excellent [~355] turnover number.

Results And Discussion
The TpDPP COF nano-spheres were synthesized by the reaction between the 0.03 mmol triformylphloroglucinol (Tp), and 0.045 mmol 3,8-diamino-6-phenylphenathridine (DPP) in the presence of a catalytic amount (10-15mL) of tri uoroacetic acid ( gure 1a and section S-2). The reaction conditions rst led to the direct nucleation of the organic building blocks, followed by spatiotemporal growth. The formation of the organic framework structure of the COF nano-spheres could be ascertained by the intense PXRD peak at [3.65±0.06] (2θ) imputed to their (100) plane diffractions (Figure 1d). 29 The experimental diffraction pattern manifests good agreement with the slipped-AA stacking models¢ simulated powder diffraction pattern. Additionally, the Pawley re nement between the simulated slipped-AA model and the experimental powder diffraction pattern using the Re ex Plus module of the Material Studio ( Figure S2) showed good agreement (R p = 4.5%, R wp = 5.4%). The N 2 adsorption analysis revealed a surface area (S BET ) of 686 m 2 g -1 with a uniform pore size (~1.9 nm) distribution ( Figure S8 and S9).  The high surface area and defects within the COF nano-spheres made them a promising candidate for the physisorption of catalytically active (Et 4 N) 2 [Fe III (Cl)bTAML] molecules within the condensed space. In enzyme catalysis, the amide functionalities of the enzyme backbone or the charged amino acid side chains can in uence the active sites' electronic properties through long-range interactions. 33 We envisaged that the hydrogen bonding between the rst coordination sphere amide functionality (-C=O) of the (Et 4 N) 2 [Fe III (Cl)bTAML] molecules and the free -NH 2 of the COF nano-spheres has a signi cant effect on the catalytic reactivity and selectivity. Such increased reactivity towards oxidation reactions upon binding Lewis acids to the ligand carbonyl in the related oxo Mn(V)-TAML complexes has been reported earlier. 34,35 The Raman spectra of the (Et 4 N) 2 [Fe III (Cl)bTAML] catalyst obtained upon 785 nm excitation revealed the presence of an Am I (-C=O stretch) band at 1616 cm -1 and an Am II (C-N stretch) band at Am II (C-N stretch) band blue-shifted to 1573 cm -1 , and a new peak appeared at 1562 cm -1 ( Figure S31). This shift in -C=O and C-N stretch bands indicates the presence of N─H···O hydrogen bonding between the free amine of the COF nano-sphere and the amide carbonyl of the immobilized catalyst. 36,37 The XPS analysis of the (Et 4 N) 2 [Fe III (Cl)bTAML] catalyst revealed the deconvoluted C 1s spectra, which indicates the presence of two distinct peaks at 286.1 and 287.1 eV corresponding to the amide carbon and the diamide carbon, respectively ( Figure S32-33). While immobilized inside the COF nano-sphere, these two peaks appeared at almost similar regions (286.6-286.9 eV) due to the hydrogen bonding and could not be appropriately resolved. 38 1.0 mg of solid COF nano-spheres can immobilize ~95% (Et 4 N) 2  . We deliberately formulated COF nano-spheres for catalyst immobilization because of their adsorption capacity, by considering a similar quantity of highly crystalline COF powder 39 and lms ( Figure S12). TpDPP COF powder (1565 m 2 g -1 ) and thin-lm (1265 m 2 g -1 ) do have a higher surface area compared to the nano-spheres (686 m 2 g -1 ). However, the catalyst adsorption capacity of the COF nano-spheres (94.8%) is 2-3 times higher than the COF powder (47%) and COF thin-lms (21%).
Furthermore, the COF nano-spheres' ability to transmute into thin-lms gave us another avenue to fabricate COF thin-lms with a higher amount of immobilized catalyst. The dispersible COF nano-spheres achieved 90% of their absorption e ciency within 90 seconds. The maximum uptake of the (Et 4 N) 2 [Fe III (Cl)bTAML] catalyst by the as-synthesized TpDPP COF nano-spheres was found to be 378.9 mg g -1 . We have performed the identical absorption experiment with another iron-based electrophilic homogeneous catalyst [Fe II (S, S-PDP)](SbF 6 ) 2 , which is a cationic complex and also known to catalyze the hydroxylation reaction of C-H bonds ( Figure S16-17 (Figure 3c).
Substrates having activated benzylic C-H bonds such as ethylbenzene (EB), diphenylmethane (DPM), dihydroanthracene (DHA), and xanthene were explored. Primarily ketone products were formed for all these substrates with high conversion and yield (Fig. 3f-h). The oxidation of 3 º C-H bonds in simple hydrocarbons was then extended to natural product derivatives. For example, cedryl acetate, a natural product derivative of cedrol found in essential oil, having a rigid structure with ve 3° C-H bonds, affords a single hydroxylated product in 70% yield (Figure 3h). Ambroxide, a naturally occurring terpenoid used in perfumery, undergoes exclusive oxidation at the alpha ethereal C-H bond predominantly among many other electronically and sterically accessible secondary and tertiary C-H bonds (Figure 3h). Finally, the use of alkenes as substrates led to the corresponding epoxides' formation in high yields (60-99%) ( Figure  3g). During these catalytic reactions, the products formed parallel those that have been found under homogeneous conditions using Fe-bTAML analogs and NaOCl. We have previously demonstrated 40 Figure S34) manifested a rhombic S= ½ species with g = 2.00, 1.93, and 1.73, which indicated that the same oxoiron(V) intermediate was generated inside the COF nano-spheres, which con rms that the overall mechanism of oxidation remained unchanged upon immobilization of the catalyst.
Although the overall mechanism of oxidation remains unchanged, several aspects are worth noting. The via the "non-rebound" pathway leading to an additional free radical auto-oxidation pathway. In fact, during the homogeneous oxidation of adamantane in air, 1-adamantanol is formed along with other products such as 2-adamantanol and 2-adamantanone (9-10%), which signi cantly reduces the regioselectivity of 3° C-H bond oxidation over 2° C-H bonds.  41,42 Such regioselectivity could be due to the total shutdown of the free-radical pathway since the hydrophobic scaffold of the COF nano-spheres creates an appropriate binding pocket for both the anionic macrocyclic catalyst (via ionic interactions) and the substrate (via predominantly hydrophobic interactions). Consequently, the carbon-centered radical formed upon C-H abstraction does not escape to the bulk to react with O 2 , thereby initiating a freeradical process that would signi cantly reduce the selectivity. Therefore, the oxidation mechanism solely involves the "rebound" of the carbon-centered radical with the Fe(IV)-OH to form the hydroxylated product with high regioselectivity and stereo-retention. 43 Additionally, the prototype Fe-bTAML complex under homogeneous conditions displays poor turnover numbers for catalytic oxidation of hydrocarbons since it undergoes acid or base induced demetallation with chemical oxidants (e.g., mCPBA or NaOCl) (pH=12.6) ( Figure S42). We need to i) modify the head part of the Fe-bTAML by introducing a -NO 2 group to increase its robustness to hydrolytic degradation, 44,45 and ii) use sodium phosphate buffer to maintain the pH of the reaction to achieve a high turnover number. In this case, high catalytic e ciency was observed with the prototype Fe-bTAML encapsulated inside the COF nano-spheres. Besides, no further use of base was required for maintaining the pH. The [Fe III (Cl)bTAML] 2-@2X TpDPP COF nano-spheres could be recycled up to 4 cycles without altering the activity and selectivity of the catalyst (Figure 3d).
The improved reactivity of (Et 4 N) 2  Hence, we decided to construct TpDPP COF thin-lms from the catalyst immobilized TpDPP COF nanospheres via covalent self-assembly. Individual TpDPP COF nano-spheres undergo covalent self-assembly in the water-DCM bilayer or even while drop-casted on top of any support, and eventually form uniform COF nano-lms. The COF nano-lms are more crystalline and porous than COF nano-spheres and do not undergo any further transmutation in water. The covalent self-assembly driven by the free amine and aldehyde functionality of the COF nano-spheres occurs via a unique, dynamic process previously unheard of. The attractive capillary forces and convective transport of the COF nano-spheres drive their selfassembly. Consequently, when two separate spheres come in contact, they start reacting via a reversible Schiff base reaction, which initiates the bers/threads' protrusion at their interface (Figure 3a-i and section S-2). The distribution of bers enhances with time to accomplish the formation of a COF nano-lm of thicknesses of 250-270 nm after three days (Figure 5e-f and S44). It is quite fascinating that the probability/chance of immobilized (Et 4 N) 2 [Fe III (Cl)bTAML] molecules escaping from the COF nanospheres to the bulk during the covalent self-assembly was nulli ed entirely by the non-covalent interactions of the catalyst molecules with the COF binding pockets. The dynamic transmutation from the nano-spheres ® nano-bers ® nano-lms did not affect the immobilized catalyst molecules and culminated in highly fecund catalytical COF thin-lms. The EDX analysis con rmed uniform distribution of (Et 4 N) 2 [Fe III (Cl)bTAML] within COF nano-spheres, nano-lms, and every intermediate phase during this transmutation process ( Figure S37). We have monitored the spheres ® bers ® lm transmutation with confocal imaging to ascertain the immobilized catalysts' enduring stability during the covalent selfassembly (Figure 4j-m and section S-15). The confocal imaging revealed the uniform adsorption and distribution of (Rhodamine B) dye molecules inside the COF nano-spheres and COF bers, which manifested their high porosity and adsorption capacity. These (Et 4 N) 2 [Fe III (Cl)bTAML] molecules are con ned within the hydrophobic pockets of the COF thin-lm. Because of the hydrophobicity of the [Fe III (Cl)bTAML] 2-@TpDPP COF thin-lms (contact angle 123.2°), only the hydrophobic substrates could contact the active catalytic sites but not the water molecules (Figure 5c and S41). The wt% loading of the (Et 4 N) 2 [Fe III (Cl)bTAML] catalyst within the COF nano-spheres and the COF thin-lms were found to be 23 wt% and 19 wt%, respectively (Section S-7). The ICP-MS analysis of the catalyst immobilized COF nanospheres and COF lm showcased the percentage of metal (Fe) loading to be 4.5 and 3.6 wt% respectively.
We could achieve the heterogenous C-H functionalization in water for the very rst-time using (Et 4 N) 2 [Fe III (Cl)bTAML]immobilized COF nano-lms. To a dispersion of [Fe III (Cl)bTAML] 2-@TpDPP COF thin-lm in water was added adamantane and NaOCl (0.1 M, 250 eq. added iteratively), and the reaction was allowed to proceed for 1.5 hrs. Analysis of the products showed the formation of 1-adamantanol with 45% yield (47% conversion) and 141:1 (3°:2°) selectivity Figure 5a). With cis-dimethyl cyclohexane as the substrate, cis dimethyl cyclohexanol (yield 75%) was formed as the predominant product with 257:1 (3°:2°) regioselectivity and 155:1 (cis:trans) stereoretention from cis-dimethyl cyclohexane ( Figure   5a). After the reaction, the [Fe III (Cl)bTAML] 2-@TpDPP COF thin-lm was recovered easily by centrifugation. The same thin-lm was used for four more subsequent reactions, and the yield and selectivity were unchanged even after the fourth cycle (Figure 5d). We believe that the cisdimethylcyclohexane preferentially partitions inside the COF nano-spheres in the COF lm, and this allows the reaction of water-insoluble substrate oxidation in water. We could not achieve more than 65% conversion in each cycle, probably due to the substrate's similar binding a nity and the product inside the COF lm in water. C-H bonds of other substrates such as cis-decahydroanthracene, ethylbenzene (EB), diphenylmethane (DPM), and ambroxide were also oxidized using the [Fe III (Cl)bTAML] 2-@TpDPP COF thin-lm/NaOCl in water with high catalytic yield and excellent selectivity (Figure 5a-b and section S -19). Under optimized conditions, for cis-decahydroanthracene, 3° hydroxylated products were obtained in 83% yield with 99% stereoretention without the formation of any 2° hydroxylated products. Only keto products were found for the functionalization of EB, DPM, and ambroxide with 99% (acetophenone), 99% (benzophenone), and 91% (sclareolide) yields, respectively (Figure 5a-b and section S-19).
We performed a proof-of-concept ow catalysis 46,47 using catalyst immobilized COF thin-lms on a macroporous solid polymeric support to establish the generality of this C-H functionalization approach (Figure 5g and  of NaOCl (0.06 M). After the 60 th cycle, this in ow catalysis showcased the 72% catalytic yield of 1adamantanol with an excellent turnover number of 355±5 (Figure 5h).

Conclusion
In summary, we have designed and synthesized crystalline TpDPP COF nano-spheres and utilized their distinct morphology and dispersibility to create a bridge between homogeneous and heterogeneous catalysis. We have successfully immobilized an anionic macrocyclic catalyst, [Fe III (Cl)bTAML] 2-, inside the porous COF nano-spheres, which was facilitated via N─H···O hydrogen bonding between the nanospheres and the complex. We have conducted the functionalization of unactivated C-H bonds using the dispersed heterogeneous catalyst, resulting in increased conversion, yield, and a high degree of regioselectivity. This stems from the ability of the cavity inside the nano-spheres to co-localize both the substrate and the catalyst in close proximity. We took advantage of the mesoscale ″ ber protrusion" phenomenon of these porous nano-spheres and fabricated self-standing COF thin-lms. We could arrest (e-f) The C-H hydroxylation was performed using the catalytic COF nano-spheres in acetonitrile and catalytic COF thin-lm in water. (g-j) The sequence of transformation from catalytic COF nano-spheres to catalytic COF thin-lm (via covalent self-assembly at the DCM-water interface) is demonstrated via SEM imaging.