Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate†

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2′-bipyridine, etc. We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

(Photo)catalytic hydrogen evolution reaction (HER) from water splitting is a promising clean energy production technique. COFs have been proved to be potential photocatalysts in promoting HER. [44][45][46][47][48][49] The system is typically comprised of COF photocatalyst, Pt nanoparticle cocatalyst, and a sacricial electron donor (e.g. ascorbic acid, triethanolamine) in an aqueous solution. Formic acid serves as an alternative high H 2 storage density reservoir (4.3 wt%). 50,51 The decomposition of HCOOH under basic conditions could release H 2 with high purity. However, some iridium-based homogeneous HCOOH dehydrogenation catalysts displayed low stability and were prone to deactivate via nanoparticle formation. 52,53 Immobilization of the catalyst on proper solid support is expected to extend the catalyst lifetime. 54 Taking all of these considerations together, in this work, we show that cyclometalation at the imine site leads to an iridacycle functionalized imine COF for the rst time (Fig. 1C), which exhibits fascinating performance in (photo)catalytic HER from aqueous formate solution.

Synthesis and characterization
Metallacycles of Schiff base ligands have been well explored for transition metals in organometallic chemistry. [55][56][57] Nevertheless, introducing such type of complexes into COFs is unprecedented. To explore this new binding mode, a pyrene-based imine COF (Py-1P) constructed from 1,3,6,8-tetrakis(4aminophenyl)pyrene and terephthalaldehyde was chosen as a model system because of its high crystallinity 58 and facile synthesis. 59 Iridium became the metal choice because of the various catalytic applications of iridacycles. 60,61 Metalation of Py-1P COF was carried out by reuxing with [Cp*IrCl 2 ] 2 (Cp*, pentamethylcyclopentadienyl) in methanol in the presence of NaOAc for 24 hours (Scheme 1, see ESI † for details).
The successful formation of the iridacycle functionalized COF Py-1P-Ir was conrmed by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state NMR characterization, and the retention of the framework crystallinity and porosity was examined by powder X-ray diffraction (PXRD) and N 2 adsorption-desorption isotherm analysis, respectively. Py-1P-Ir COF showed strong diffraction peaks at 2q ¼ 3.74, 5.36, 7.56, 11.40, 23.52 , which are slightly shied to low 2q direction (0.04 to 0.18 difference) compared to the diffraction pattern of the parent Py-1P COF ( Fig. 2A). This might result from the crystal cell expansion aer introducing the bulky Ir organometallic unit. FT-IR analysis revealed a new C]N vibration band at 1597 cm À1 , corresponding to the metal coordinated imines, in addition to the free ones at 1623 cm À1 for Py-1P-Ir COF and (Fig. 2B). The shiing of C]N vibration to lower wavenumber upon coordination to Ir was also observed for N-benzylideneaniline (L1) when forming the model iridacycle complex (L1-Ir, 1626 to 1582 cm À1 , Fig. S1 †). 62 The coordination of imine N to iridium was further proved by XPS analysis (Fig. S2 † and 2C). The N 1s XPS spectrum of Py-1P-Ir COF showed two subpeaks with binding energies of 399.0 and 400.0 eV, which were assigned to the free and coordinated imine nitrogen, respectively (Fig. 2C). The increase of N 1s binding energy upon coordination is consistent with that of the model complex L1-Ir (399.2 to 399.8 eV, Fig. S3 †). Solid-state crosspolarization/magic angle spinning (CP/MAS) 13 C NMR spectroscopy unambiguously conrmed the formation of the iridacycle in Py-1P-Ir COF. Three new signals at 173.7, 89.0, and 7.6 ppm appeared in the CP/MAS 13 C NMR spectrum of Py-1P-Ir COF (Fig. 2D). The broad peak at 173.7 ppm is assigned to the iridium bonded carbon and the imine carbon of the iridacycle, while the peaks at 89.0 and 7.6 ppm originate from the aromatic and methyl carbons of Cp* ring, respectively, which matches well with the 13 C NMR spectrum of the model complex L1-Ir (Fig. S4 †). Meanwhile, the relative intensity of the free imine carbon peak at 157.0 ppm of Py-1P-Ir COF decreased accordingly compared to that of Py-1P COF. N 2 adsorption experiment was carried out to investigate the porosity of Py-1P-Ir COF (Fig. 2E). The Brunauer-Emmett-Teller (BET) surface area of Py-1P-Ir COF is 972 m 2 g À1 , which is lower than 1960 m 2 g À1 of Py-1P COF ( Fig. S5 and S6 †). The pore size is decreased to 1.57 nm for Py-1P-Ir COF, compared to 2.19 nm for Py-1P COF, which is expected for the pore wall metalation (Fig. 2F). The iridium loading was determined to be 11.8 wt% by inductively coupled plasma mass spectrometry (ICP-MS), corresponding to ca. 20% imine metalation. Scanning electron microscope analysis revealed that Py-1P-Ir COF was composed of aggregates of nanometer-sized particles, similar to that of Py-1P COF (Fig. S7 †). Energy dispersive X-ray (EDX) analysis revealed the homogeneous distributions of Ir and Cl elements over the framework, and the Ir/Cl ratio is close to 1/1, as expected for the proposed structure (Fig. S8 †). Besides, the formation of the iridacycle was also successfully performed on an azine linked COF (see ESI † for details), demonstrating the generality of the cyclometalation modication of imine-based COFs.

Catalytic HER from formate
Iridium complexes have demonstrated their potential to be homogeneous catalysts for hydrogen production from formate. [50][51][52][53]63 The homogeneous distribution of the single-site iridacycle over the Py-1P-Ir COF with high surface area prompted us to test its capability in promoting formate decomposition to release H 2 . To our delight, Py-1P-Ir COF produced 96.3 mmol H 2 with a purity of 94% from 10 mL of 1.0 M HCOONa solution at 65 C in 6 hours (6.35 mmol catalyst based on Ir, Fig. 3A). In contrast, L1-Ir gave only 7.9 mmol H 2 with 70% purity under otherwise identical conditions. Py-1P COF was catalytically inactive, showing even lower H 2 production (0.8 mmol, 42%) than the control reaction (2.3 mmol, 65%). These results highlight the importance of the unique structure of Py-1P-Ir COF. Then, we set to investigate the inuence of reaction parameters on the hydrogen production efficiency catalyzed by Py-1P-Ir COF, including temperature, pH, and formate concentration.
As shown in Fig. 3B, Py-1P-Ir COF displayed higher efficiency at higher temperature. An amount of 250.0 mmol H 2 was obtained at 85 C, which is ca. 2.6 times of that obtained at 65 C. The high reactivity of Py-1P-Ir COF was manifested that 16.5 mmol H 2 was formed even at 25 C. In all the tested temperatures, the H 2 purity was no less than 90%. Lowering the pH was found to favor the reaction that 48.9 and 213.8 mmol H 2 were afforded at pH ¼ 5.92, 3.72 respectively, while further lowering the pH to 1.94 did not lead to higher H 2 production (200.2 mmol, Fig. 3C). The increased H 2 production came with compromised H 2 purity to ca. 70% under acidic conditions. The concentration of HCOONa on the HER showed a volcano-type effect (Fig. 3D). The H 2 yield increased to 163.2 mmol from the reaction of 2 M HCOONa but started to fall back to 137.5 mmol for 5 M HCOONa and 92.6 mmol for 5 M HCOONa respectively thereaer. The H 2 purities were all higher than 90% from the tested four HCOONa concentrations. The decreased reaction efficiency might be due to the decreased concentration of water, which is the proton source for H 2 production. Py-1P-Ir COF exhibited excellent stability under all the tested conditions. All the recovered COF samples preserved the crystallinity as shown by PXRD analysis (Fig. S20-S22 †). Interestingly, FT-IR and XPS indicate that the imine linkage was partially reduced (Fig. S20-S23 †). The imine bond reduction is likely mediated by an iridium hydride intermediate via an outer-sphere process. [64][65][66] The involvement of Ir in the imine reduction is supported by the fact that no imine reduction was observed in Py-1P COF during HER catalysis (Fig. S24 †) and the catalytic reduction of N-benzylideneaniline as an exogenous substrate in the presence of Py-1P-Ir COF was observed (Fig. S25 †). ICP-MS showed that the Ir concentrations in the reaction ltrates were all below 60 ppb (Table S2 †), demonstrating the heterogeneous catalysis nature of the reaction. The high stability of Py-1P-Ir COF allowed it to be recycled for at least another four runs (Fig. 4). Interestingly, an initial performance improvement was observed in the second cycle. This improved reactivity of Py-1P-Ir COF is attributed to the presence of more reactive Ir species in recovered material, in which the Cl ligand has been replaced (see mechanism discussion below). This is supported by the fact that no Cl element was detected by EDX in Py-1P-Ir COF aer one HER cycle (Fig. S26 †). The average H 2 production rate during the ve cycles was calculated to be 4626 mmol g À1 h À1 , corresponding to a TOF of 7.3 h À1 . The recovered Py-1P-Ir COF aer ve cycles was still crystalline (Fig. S28 †). No metal nanoparticle formation was observed by XPS (Fig. S29 †) and TEM (Fig. S30 †) analysis.
Pyrene-based COFs have exhibited excellent photophysical properties which benet photocatalysis. [67][68][69][70] Diffuse reectance UV-vis absorption spectrum of Py-1P-Ir COF displayed prominent absorption in 200-500 nm and a long tail to near-IR range (500-800 nm, Fig. 5). The incorporation of iridium complex into Py-1P COF did not change the absorption in 200-500 nm band but strongly improved the absorption ability in the 500-800 nm region. Encouraged by the excellent light absorption ability across the whole UV-visible region of Py-1P-Ir COF, we further     Table 1, only 0.7 mmol H 2 was detected in the absence of catalyst (entry 1). Py-1P-Ir COF catalyzed the generation of 84.5 mmol H 2 in high purity (93%) in 6 hours (entry 3), which is ca. 34 times higher than the molecular counterpart L1-Ir did (entry 2). The corresponding H 2 generation rate of 1358 mmol g À1 h À1 (TOF 2.2 h À1 ) is comparable to the typical COF photocatalyst/Pt cocatalyst/ sacricial electron donor system. 71 Py-1P COF failed to exert any catalytic effect showing the critical role of iridium for the reaction (entry 5, 0.6 mmol H 2 ). Under the dark condition, Py-1P-Ir COF produced 31.6 mmol H 2 in 92% purity, proving it to be a photocatalytic process (entry 4). A physical mixture of Py-1P COF and L1-Ir displayed lower H 2 production (19.5 mmol) and poorer selectivity (71% purity), highlighting the importance of covalent hybridization of Ir catalytic centers within the COF (entry 6). 72 The recovered Py-1P-Ir COF almost retained its crystalline structure as evidenced by PXRD analysis (Fig. S31 †). Similarly, reduction of the imine bond was observed (Fig. S31 †). ICP-MS analysis of the Py-1P-Ir COF catalyzed reaction solution revealed a low Ir concentration of 0.99 ppm, corresponding to 0.8% of the total Ir in Py-1P-Ir COF. Besides, the reaction ltrate only produced 2.0 mmol H 2 under the photocatalysis conditions (entry 7), demonstrating the heterogeneous catalytic process by Py-1P-Ir COF.
Based on literature reports, 52,53 a proposed mechanism is shown in Fig. 6A (Fig. S32 †). 52,73 To further support the proposed mechanism, we carried out DFT calculations. The computed Gibbs free energy prole of the prosed reaction pathway is shown in Fig. 6B. IM-2 is slightly lower (À1.56 kcal mol À1 ) in energy than IM-1, suggesting the rst step ligand exchange is feasible. The transformation of IM-2 to IM-3 via CO 2 extrusion is thermodynamically favored (DG ¼ 1.88 kcal mol À1 ) with a small energy barrier of 21.85 kcal mol À1 (TS1). This is consistent with our experimental ndings that the iridium hydride formed immediately at room temperature. The reaction of IM-3 with H 2 O to give IM-4 and H 2 is an energy demanding process (DG ¼ 18.07 kcal mol À1 ) with an energy barrier of 50.81 kcal mol À1 (TS2). In contrast, the reaction of IM-3 with HCOOH to give IM-2 and H 2 is energetically less demanding (DG ¼ 2.11 kcal mol À1 ) and the activation energy is also signicantly reduced to 28.68 kcal mol À1 (TS2 0 ). These results match well with the observed higher reactivity of the catalyst in lower pH conditions.

Conclusions
In summary, we identied a new metal binding mode of imine COFs where cyclometalation with iridium readily occurred. The iridium functionalized COF exhibited much enhanced catalytic efficiency in HER from an aqueous formate solution than the parent COF and the molecular counterpart in both thermal-and photo-triggered reactions. The easily available imine-based COFs, as well as various transition metals capable of forming

Conflicts of interest
There are no conicts to declare.