Single-Site Photocatalytic H2 Evolution from Covalent Organic Frameworks with Molecular Cobaloxime Co-Catalysts

Abstract

We demonstrate photocatalytic hydrogen evolution using COF photosensitizers with molecular proton reduction catalysts for the first time. With azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst, and TEOA donor, H₂ evolution rate of 782 μmol h^-1 g^-1 and TON of 54.4 has been obtained in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM analysis, and quantum-chemical calculations suggest an outer sphere electron transfer from the COF to the co-catalyst which subsequently follows a monometallic pathway of H₂ generation from the Co^III-hydride and/or Co^II-hydride species.

Scheme 1. Structures of N2-COF and the Cobaloxime Co-Catalysts Used in This Study

Schematic representation of photocatalytic H₂ evolution with N2-COF and Co-1 is shown on the left.

Figure 1

(a) H₂ evolution using N2-COF and Co-1 (see text for details) as well as N2-COF and metallic platinum (5 μL of 8 wt % H₂PtCl₆ solution in water) in the presence of TEOA, when irradiated with 100 mW cm^–2 AM 1.5 light. Control experiments in absence of either of the three components, with all other conditions being the same, show no H₂ evolution in 3 h. (b) H₂ evolution using optimized parameters, 5 mg of N2-COF dispersed in 10 mL of 4:1 ACN/H₂O solvent together with 100 μL of TEOA, 400 μL of a 2.48 mM solution of Co-1 in ACN, and 4.69 mM dmgH₂ at a final pH of 8. The reaction mixture is illuminated with 100 mW cm^–2 AM 1.5 light.

Figure 2

(a) H₂ evolution with N2-COF and different co-catalysts. The co-catalyst concentration is 0.1 mM in all measurements. All other conditions are the same including a pH of 10. (b) H₂ evolution with different COFs at pH 8. 5 mg COF sample has been used in all the measurements. All other conditions are the same. Rates are 233, 390, 163, and 100 μmol g^–1 h^–1 for COF-42, N2, N3 and N1 COFs, respectively. TON for the reaction with N2-COF is 10.89 at 6.5 h.

Figure 3

(a) ¹³C CPMAS NMR spectra of N2-COF under different conditions. No change in chemical shift in the COF signals is seen. Please see Figure S12 for peak assignments. (b) ATR-IR spectra of N2-COF under different conditions. Again, no shift in the frequencies of the bands is seen.

Figure 4

Constrained optimized geometry of (a) pore-diazene, (b) pore-diazene-90°, (c) surface-diazene, and (d) surface-triazine cobaloxime-COF models, obtained on the PBE0-D3/def2-SVP level of theory using the Turbomole program package.⁻ The surface-diazene and triazine models are for possible interactions on the surface of the COF microstructure. Other details of the calculations can be found in the Supporting Information. The dashed pink lines show the shortest Co–N distance obtained and are 4.197, 4.082, 2.792, and 3.00 Å, respectively, in panels a–d.

Figure 5

(a) Red trace: UV–vis spectra of the degassed photocatalytic reaction dispersion containing 2.5 mg of COF-42, 50 μL of TEOA and 200 μL of Co-1 (2.48 mM in ACN) in 5 mL 4:1 ACN/H₂O mixture at pH 8 illuminated with 100 mW cm^–2 AM 1.5 light. The reaction mixture was allowed to stand for 1 h after illumination before a spectrum was recorded. Blue trace: similar reaction conditions as before except at pH 10 of the reaction mixture and 5 equiv of externally added P(n-Bu)₃. The noise in the spectra is from the still suspended COF particles. (b) X-band EPR spectrum at 4K of the photocatalytic reaction dispersion containing COF-42 before and after illumination. The microwave frequencies are 9.47614 GHz in both cases. The reaction conditions are identical to those in Figure 5a. (c) H₂ evolution at 3 h after illumination under different [Co-1]. In all measurements, 5 mg of N2-COF and 100 μL of TEOA in 10 mL of 4:1 ACN/H₂O has been used. The reaction pH is 8.