Nitric Oxide (NO) as a Reagent for Topochemical Framework Transformation and Controlled NO Release in Covalent Organic Frameworks

Covalent organic frameworks (COFs) have emerged as versatile platforms for the separation and storage of hazardous gases. Simultaneously, the synthetic toolbox to tackle the “COF trilemma” has been diversified to include topochemical linkage transformations and post-synthetic stabilization strategies. Herein, we converge these themes and reveal the unique potential of nitric oxide (NO) as a new reagent for the scalable gas-phase transformation of COFs. Using physisorption and solid-state nuclear magnetic resonance spectroscopy on 15N-enriched COFs, we study the gas uptake capacity and selectivity of NO adsorption and unravel the interactions of NO with COFs. Our study reveals the clean deamination of terminal amine groups on the particle surfaces by NO, exemplifying a unique surface passivation strategy for COFs. We further describe the formation of a NONOate linkage by the reaction of NO with an amine-linked COF, which shows controlled release of NO under physiological conditions. NONOate-COFs thus show promise as tunable NO delivery platforms for bioregulatory NO release in biomedical applications.


Fourier-Transform Infrared Spectroscopy
Fourier-transform infrared (FT-IR) spectra were recorded on a PerkinElmer UATR Two in attenuated total reflection (ATR) geometry equipped with a diamond crystal.
The samples were prepared dry onto a copper lacey carbon grid (Plano). Images were recorded with a TVIPS TemCam-F216 CMOS camera. The program EM-Menu 4.0 Extended was used for analysis.

X-ray Powder Diffraction
X-ray powder diffraction (PXRD) measurements were performed on a Stoe Stadi-P diffractometer in Debye-Scherrer geometry with Cu-Kα1 radiation equipped with a Ge(111) primary monochromator. The glass capillaries (1 mm in diameter) were spun during data collection for an improved particle statistics.
Pawley refinements of the different COF structures were performed using TOPAS V6. Model structures created by Material Studio were used for the Pawley refinements with fixed atomic coordinates. The peak profile of the XRPD patterns was described by applying the fundamental parameter approach as implemented in TOPAS. The background was modeled by Chebychev polynomials. The microstructure of the different COFs was modeled using microstrain (Lorentzian and Gaussian components).

Nuclear Magnetic Resonance Spectroscopy
Solid state nuclear magnetic resonance spectra (ssNMR) were recorded on a Bruker Avance III 400 MHz spectrometer (magnetic field 9.4 T). For ssNMR spectroscopy, the samples were packed in ZrO2 rotors, and spun in a Bruker WVT BL4 double resonance MAS probe. The spinning rate was 12-14 kHz in 13 C measurements, and 6 kHz in 15 N experiments. A standard cross-polarization sequence with a ramped contact pulse was used for both nuclei. The duration of contact pulse was 2 ms for 13 C and 4 ms for 15 N. A total of 4096-8192 scans were routinely accumulated in 13 C experiments, and 80000 scans in the experiments with 15 N. All the measurements were performed under conditions of high-power broadband proton decoupling (SPINAL 64) with the spectral conditions being optimized for the shortest relaxation delay by measuring 1 H T1 relaxation time. Chemical shifts were referenced relative to tetramethylsilane in 13 C (δiso = 0.0 ppm) and relative to nitromethane in 15 N (δiso = 0.0 ppm), with solid glycine as the secondary reference (δiso [ 15 N] = -347.54 ppm).

UV/VIS Spectroscopy
Diffuse reflectance UV-Vis spectra were collected on a Cary 5000 spectrometer referenced to barium sulfate as reference.

Sorption
Sorption measurements were performed on a Quantachrome Instruments Autosorb iQ MP. BET suface areas and pore size distributions were calculated from argon isotherms recorded at 87 K using the quenched solid-state density functional theory (QSDFT) for cylindrical pores in carbon model for argon at 87 K. CO2 and NO isotherms were measured at 273, 288 and 298 K. [1]

Explanation of fittings and selectivity calculations based on ideal adsorption solution theory (IAST)
The NO and CO2 isotherms were fitted with a dual-site Langmuir-Freundlich model ( Figure S36 and S37). n is the adsorbed gas amount (mmol g -1 ), p is the pressure in the bulk gas phase (bar), qsat is the saturation amount (mmol g -1 ), b is the Langmuir-Freundlich parameter (bar), α is the Langmuir-Freundlich exponent (dimensionless) for two adsorption sites A and B. [2] = , 1 + + , 1 + The IAST selectivities SIAST were calculated with the IAST equation. SIAST is the selectivity (dimensionless), q is the adsorbed amount (mmol g -1 ), and p is the partial pressure (bar). [

Synthesis of 15 N Enriched TTI-COF
TTI-COF was synthesized following a literature procedure. [3] Into a 10 mL Biotage microwave vial, The vial was capped and placed in an aluminum heating block that was preheated to 120°C. Under stirring at 500 rpm the mixture was kept at 120°C for 72h. After cooling to room temperature the solid was isolated by filtration, washed with acetone, isopropanol, and methanol before subjecting it to a Soxhlet extraction with MeOH for 24h. The MeOH soaked solid was then activated by scCO2 drying to obtain TTI-COF (67.6 mg, 77 %).

Synthesis of 15 N Enriched TTT-COF
TTT-COF was synthesized following a literature procedure. [5]

Synthesis of 15 N Enriched TT-Imide-COF
TT-Imide-COF was synthesized following a literature procedure. [6] A Schlenk tube was charged with the

Exposure of COFs to NO
The COFs were exposed to nitric oxide via the NO adsorption measurements on the Quantachrom Autosorb IQ3 system. All analytics on the post-NO materials were performed after the performance of all NO sorption experiments, including seven succeeding NO adsorption/desorption isotherms at 298 K and three isotherms at 298 K, 288 K and 273 K, respectively, to ensure full reaction of the frameworks with the gas.

NO release experiments
The NO release experiment was conducted by suspending 10 mg rTTI-COF-NO in 5 ml 0.1 M PBS buffer solution. The temperature was kept constant at 37 °C during the observation period and the mixture was shaken permanently to avoid precipitation of the COF. To determine the concentration of NO in the mixture, a Griess reagent kit for nitrite determination (G-7921) produced by Probes has been used.
The conversion of the released nitric oxide to nitrite appears in situ by atmospheric oxygen: The formed nitrite is detected by using the Griess reagent through the formation of an azo-dye following the reaction: For each measurement, 500 μl sample were taken and centrifuged. 300 μl of the supernatant were mixed with 100 μl Griess reagent and diluted with 2.6 ml water. After 30 min dwelling time, the nitrite concentration was measured by UV/VIS spectroscopy using a serial dilution as reference.                       TT-Imide-COF, NO, 288K  TTT-COF, NO, 288K  TTI-COF, NO, 288K rTTI-COF, NO, 288K Figure S36. NO adsorption and desorption isotherms of TTI-, rTTI-, TTT-, and TT-Imide-COF at 288 K, after the cycling experiments. The reversibility of the isotherms indicates a process that can be described by physisorption. Figure S37. CO2 isotherms of (a) TT-Imide-COF and (b) TTT-COF before (blue) and after NO exposure (red). Filled circles represent adsorption and empty circles desorption isotherms. (c) IAST selectivity of TTT-COF-NO and TT-Imide-COF-NO for a binary CO2/NO (50/50) gas mixture during adsorption (filled circles) and desorption (empty circles). For a binary CO2/NO (50/50) gas mixture, we found a selectivity towards CO2 over the whole pressure range during adsorption up to 100 kPa for both COFs. The selectivities first increase until around 50 kPa before slightly declining to an almost identical value of 2.5 at 100 kPa. However, applying IAST to the desorption instead of the adsorption branch, in both COFs, the selectivity changes in favor of NO for low pressures below 30 kPa. In our opinion, this practice is a better representation of the thermodynamic equilibrium state due to the unusual and broad hysteresis of the pure NO isotherm.   rTTI-COF rTTI-COF-NO after NO release rTTI-COF-NO Figure S40. Argon adsorption isotherms at 87 K of rTTI-COF-NO (red) and rTTI-COF-NO after NO release (blue). Figure S41. High-resolution N1s XPS spectra of rTTI-COF (top) and rTTI-COF-aNO (bottom). The N1s spectrum of the rTTI-COF was fitted with two peaks at 397.2 eV (triazine) and 400.7 eV (amine). The N1s spectrum of the rTTI-COF shows a significant decrease of the amine-shoulder correlating to the transformation of the secondary amine. It was fitted with three peaks at 396.8 eV (triazine), 401.0 and 404.7 eV (NONOate). [8,9] The low intensity of the NONOate signals is suspected to be the result of low stability of NONOates under strong X-rays.