Visible‐Light Promoted C–O Bond Formation with an Integrated Carbon Nitride–Nickel Heterogeneous Photocatalyst

Abstract Ni‐deposited mesoporous graphitic carbon nitride (Ni‐mpg‐CNx) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross‐coupling reactions. The dual catalytic Ni‐mpg‐CNx is demonstrated for C–O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good‐to‐excellent yields (60–92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron‐withdrawing, ‐donating and neutral groups. The heterogeneous Ni‐mpg‐CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

S4 filtration sequentially washed with acetonitrile, hot water, boiling ethanol, boiling methanol, boiling acetone, and dried under vacuum.
The vial was crimped by a septum-aluminum cap and purged with N2 atmosphere via "vacuum and N2 refill" cycles (x3). Then, degassed alcohol (2 mL) as a solvent/substrate was added to vial with a syringe needle. The reaction mixture was then shaken rapidly using an orbital stirrer and irradiated with a blue LEDs based photoreactor ( = 447 ± 20 nm, 1.03 W @ 700mA per LED) at 40 °C ( Figure S15). The reaction was monitored by TLC. Upon completion of the reaction, the reaction mixture was purified by flash column chromatography.

General procedure for the NMR analysis during reaction optimization
Upon completion of the reaction, the reaction mixture was diluted with 1 mL of acetonitrile for better solubility of products and any side products. To the reaction mixture, 1,3,5-trimethoxy benzene (0.05 mmol) was added as an internal standard. Then, 200 μL of the resulting solution was transferred into a vial diluted it with 1 mL of deuterated acetonitrile. 700 μL of this solution was then submitted for NMR analysis. To get better resolution of the peaks in the NMR spectra 64 or 128 scans were performed. ( Figure S16)

General procedure for the kinetic reactions
All catalytic reactions were conducted in a 10 mL septum-capped vial under vigorous stirring using an orbital stirrer and irradiating at 447 nm for 420 min under nitrogen atmosphere at 40 ºC, unless otherwise indicated. Catalytic photoreductions were performed in MeOH (2 mL) as reaction solvent mixture, 4-bromoacetophenone as substrate (200 mM, 0.4 mmol), Ni-mpg-CNx (10 mg) and NaOH as base (1.2 equivalents, 240 mM). A 447 nm LED photoreactor was S5 employed as light source. 500 l aliquots of the rection were taken along the reaction time and 1,3,5-Trimethoxybenzene (100 l, 160 mM final concentration) was added as internal standard after the aliquot extraction and the reaction was quenched by adding 2 mL of DCM and 1 mL H2O. The crude reaction mixtures were purified by extraction with DCM (1 x 2 mL), the organic layer was passed through a MgSO4. The solvent from the resulting organic solution was evaporated using a rotary evaporator and subjected to 1 H-NMR analysis to determine the conversion of 4-bromoacetophenone and the yield of the desired coupling product respectively.
All 1 H-NMR conversions and yields reported are an average of at least two runs.

General procedure for the recovery of the material after catalytic studies
After completion of the reaction, reaction mixture was transferred into tube and centrifuged at 7000 rpm for 10 mins. The recovered material was washed with water and acetone and centrifuged again. At the end of the process, more than 90% of the material can be recovered. The external quantum efficiency (EQE) was determined using simulated solar light simulator (LOT LSN 254) equipped with a monochromator (LOT MSH 300). Sample was irradiated at 447 nm at a light intensity (I) of I ~ 6.40 mW cm −2 (exact intensity was checked after the experiment). For the calculation, we have assumed one photon is required to generate one coupling product although the Ni catalytic cycle is self-sustained. EQE was calculated using the following equation: where, nproduct is number of moles of coupling product, NA is Avogadro's number, h is Planck's constant, c is speed of light, tirr is reaction time, I is the intensity of light, λ is the wavelength of incident light, and A is cross-sectional area of irradiation.

Coupling products characterization 4-Methoxybenzonitrile
The reaction was completed in 5 hours and the desired product (white solid, 80% yield) was obtained by flash chromatography (10% ethyl acetate/hexane). 1

4-Methoxybenzamide
The reaction was completed in 23 hours and the desired product (white powder, 78% yield) was obtained by flash chromatography (20% ethyl acetate/hexane). 1

1-(tert-butyl)-4-methoxybenzene
The reaction was run for 72 hours and the desired product NMR yield is 42% with rest unreacted starting material. 1

3-Methoxyquinoline
The reaction was completed in 14 h hours and the desired product (colorless oil, 66% yield) was obtained by flash chromatography (30 % ethyl acetate/hexane). 1

4-ethoxybenzonitrile
The reaction was completed in 18 hours and the desired product (white powder, 75% yield) was obtained by flash chromatography (10% ethyl acetate/hexane). 1

Kinetic studies
The activation parameters of the C-O coupling reaction by the dual Ni-mpg-CNx photocatalytic system were obtained through kinetic analysis of the formation of 4-methoxy acetophenone (3) during initial reaction times in a range of temperatures between 293 K and 323 K ( Figure S11).
The reaction rate at a given temperature was approximated as the initial rate of formation of 3 quantified by means of 1 H-NMR. Then, the experimental data obtained from the kinetics S12 experiments (Table S5) was fitted to the Eyring equation (Eq. 1) and represented in Figure   S12: [2,3] = − ∆ ‡ 1 + The results obtained from the Eyring analysis are summarized in Table S6.
Since the value of ΔS ‡ is related to the rate determining step in a reaction. [4] The negative value of it suggest that entropy decreases upon achieving the transition state, which often indicates an associative mechanism in which two reaction partners form a single activated complex, this is agreement with an associative mechanism and the first order observed regarding the substrate.

Computational studies
DFT calculations have been performed with the Gaussian09 software package. [5] First, geometry optimizations and frequency calculations of the ground state structure of a simplified model for the Ni-mpg-CNx system have been performed at the B3LYP/6-31G* level of theory (see Scheme S1a). [6][7][8] Solvent effects (MeOH) are considered with the polarizable continuum model PCM-SMD of Truhlar and coworkers. [9][10][11][12] Additionally, the energy of the geometry optimized molecules was refined by single point calculation with the 6-311+G** basis set [13][14][15] for all atoms.
Scheme S1. Simplified model systems used for the computational studies.
Standard reduction potentials (E°) have been evaluated through the Nernst equation in standard state conditions using the Standard Hydrogen Electrode ( ) as reference following the equation 5: where is the free energy change associated with reduction at standard conditions, n is the number of electrons involved in the redox reaction, F is the Faraday constant and is the free energy change associated with the reduction of a proton (-4.28 eV). [16] S14 Finally, a 2-layer ONIOM (own n-layered integrated molecular orbital and molecular mechanics) method developed by Morokuma and co-workers [12,17,18]  Additionally, the energy of the geometry optimized molecules at the ONIOM B3LYP/6-31G* level was refined by single point calculation with the 6-311+G** basis set [13][14][15] for all atoms.
The optimized structures for Ni II -mpg-CNx and Ni I -mpg-CNx show the same distorted square planar type coordination of the Ni centre within the mpg-CNx layer ( Figure S18).
All energies given in this work are referred to Gibbs energies G in kcal·mol -1 , unless otherwise noted. The nature of the stationary points was established by frequency calculations in the solvent-phase, in which minima have no imaginary frequencies.

Redox potentials
The

S23
High resolution Ni 2p, N 1s and C 1s XPS spectra of Ni-mpg-CNx (as synthesized, black trace), same material after photocatalysis (red trace), and the material obtained after performing catalysis with NiCl2 (salt)/mpg-CNx (blue trace).
High resolution N1s XPS spectra of mpg-CNx and Ni-mpg-CNx. The raw data (open circles) was fitted to the components using casaXPS software. The dark grey solid line corresponds to the overall fitted result. Deconvolution of the main XPS peak for unmodified mpg-CNx shows three peaks at 397.0 eV (red), 398.6 eV (green) and 399.6 eV (yellow). Ni-mpg-CNx shows an additional peak ta 397.9 eV which can be attributed to pyridinic N coordinated to Ni 2+ .

S24
High resolution C1s XPS spectra of mpg-CNx and Ni-mpg-CNx. The raw data (open circles) was fitted to the components using casaXPS software. The dark grey solid line corresponds to the overall fitted result.

S27
Eyring plot and linear fit of the kinetic data in Table S5. Light