Hydrazine-Hydrazide-Linked Covalent Organic Frameworks for Water Harvesting.

We report a postsynthetic strategy and its implementation to make covalent organic frameworks (COFs) with irreversible hydrazide linkages. This involved the synthesis of three 2D and 3D hydrazine-linked frameworks and their partial oxidation. The linkage synthesis and functional group transformation-hydrazine and hydrazide-were evidenced by 15N multi-CP-MAS NMR. In addition, the isothermal water uptake profiles of these frameworks were studied, leading to the discovery of one hydrazine-hydrazide-linked COF suitable for water harvesting from air in arid conditions. This COF displayed characteristic S-shaped water sorption profiles, a steep pore-filling step below 18% relative humidity at 25 °C, and a total uptake capacity of 0.45 g g-1. We found that even small changes made on the molecular level can lead to major differences in the water isotherm profiles, therefore pointing to the utility of water sorption analysis as a complementary analytical tool to study linkage transformations.

Introduction

Multistep organic synthetic methods have been successfully translated to the solid state.[1−4] This practice has become especially attractive for the synthesis of new covalent organic frameworks (COFs).[5−8] Their hallmark features, porosity and crystallinity, allow chemical transformations to be done on the framework and enable rigorous analysis of concurrent structural changes through diffraction and spectroscopic techniques.[5,6]

The synthesis of COFs relies on an initial reversible step to guide the covalent assembly of organic building units into extended and crystalline structures.[9−12] This step can be followed by one or more postsynthetic transformations, leading to structural complexity and function of the framework.[5,8,10,11] This has been demonstrated for a number of imine-linked COFs,[13−15] but hydrazide moieties have not been reported yet.

Here, we report three 2D and 3D hydrazide-linked COFs following postsynthetic oxidation of the hydrazine-linking units (Scheme ). Considering the rather hydrophilic nature of the hydrazine and hydrazide linkages, we chose to study the effect of the synthetic transformations with water vapor sorption analysis.[16,17] This not only allowed us to explore new sorbents for water harvesting from air but also provided us with additional insight into the micro- and macroscopic changes occurring on the framework.

Scheme 1

Synthetic Strategy for Hydrazide-Linked COFs

This strategy was applied to a molecular model system and two 2D (hcb, sql) and one 3D (dia) COFs.

Our synthetic strategy toward hydrazide-linked COFs is based on the reversible formation of the hydrazine-linkage, followed by postsynthetic oxidation to the hydrazide (Scheme ). We first demonstrated the feasibility of this route on a molecular model system. Then, we explored its transferability to the solid state by synthesizing two 2D COFs with honeycomb (hcb) and square-lattice (sql) topologies and one 3D COF with diamond (dia) topology. In total, six COFs were synthesized, four of which were hitherto unreported, and comprised of hydrazine and hydrazide linkages—the latter presents a linkage that has not been reported in COF chemistry yet. The COFs were analyzed by 15N multiple cross-polarization magic-angle spinning spectroscopy (multi-CP-MAS NMR),[18] giving a quantitative readout of the chemical transformations. Fourier-transform infrared (FT-IR), solid- and solution-state nuclear magnetic resonance (NMR) spectroscopies, elemental analysis (EA), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and nitrogen sorption substantiated the structural and compositional integrity of the frameworks.

Results and Discussion

To show the viability of our postsynthetic strategy, we first synthesized a hydrazine molecular model compound, 1,2-benzylidenehydrazine, from benzaldehyde and hydrazine with quantitative yield (Scheme ).[19] The hydrazine compound was subsequently oxidized to afford the hydrazide molecular model using previously reported conditions for the conversion of imines to amides in COFs (see Supporting Information, section S2).[5]

To demonstrate the transferability from solution to the solid state, we first synthesized two 2D and one 3D hydrazine-linked COFs (Scheme ). AB-COF[20,21] was produced from trigonal-planar 1,3,5-benzenetrialdehyde (BTA) and linear hydrazine (Hy), and crystallized in the hcb topology with the reticular formula [(BTA)2(Hy)3]hydrazine. Py-COF-2D[22] is comprised of the square-planar 1,3,6,8-tetrakis(p-formylphenyl)pyrene (TFPPy) unit and Hy and crystallized in the sql topology with the reticular formula [(TFPPy)(Hy)2]hydrazine. Lastly, COF-670-hydrazine was synthesized from the tetrahedral 4,4′,4″,4‴-methanetetrayltetrabenzaldehyde (MTBA) and Hy and crystallized in the dia topology with the reticular formula [(MTBA)(Hy)2]hydrazine (Scheme ). While AB-COF and Py-COF-2D have been previously reported,[20,21] a hydrazine-linked 3D COF, COF-670-hydrazine, has not been reported to date.

Scheme 2

Overview of the Synthesis of (a) 2D hcb, (b) 2D sql, and (c) 3D dia Hydrazide-Linked COFs

The synthesis of hydrazine-linked COFs is detailed in the Supporting Information.

We first explored the oxidation of the hydrazine-linked AB-COF by subjecting it to our reported oxidation conditions[5] involving the use of NaClO2 as oxidant in an aqueous acidic medium in the presence of 2-methyl-2-butene over 3 days to obtain COF-480-hydrazide (see Supporting Information, section S2). FT-IR spectroscopic traces of the product indicated attenuation of the imine band at 1625 cm–1 corresponding to the hydrazine linkages and an emerging band at 1674 cm–1, which was assigned to the carbonyl stretching vibration of the hydrazide moiety (see Supporting Information, section S3). The solid-state 15N multi-CP-MAS NMR spectroscopy,[18] performed on a 50% 15N-labeled sample, further corroborated the transformation with a new peak at 171.7 ppm. However, no quantitative conversion was observed, despite prolonged reaction times or an increased amount of oxidation reagent (see Supporting Information, section S2). This led us to the conclusion that only partial oxidation was achieved—an observation that we further substantiated with an 15N NMR experiment on the hydrazide model system (see Supporting Information, section S4). Multi-CP-MAS NMR on the 15N-labeled COF samples allowed us to quantify the conversion, which amounted to 9.2% (see Supporting Information, section S4). Here, the resonances corresponding to the functionalities were fit to Gaussian line shapes, and their relative areas were compared. This analysis correlated well with the atomic composition determined by EA (see Supporting Information, section S2). TGA under nitrogen and air flow revealed an onset in decomposition for the pristine and the oxidized samples at around 380 °C.

PXRD analysis of the microcrystalline powder confirmed retention of crystallinity after oxidation and did not reveal additional diffraction peaks associated with hydrolyzed starting materials or additives (see Supporting Information, section S6). The PXRD pattern of AB-COF shows high-intensity peaks at 6.8°, 11.8°, and 26.4° 2θ values, associated with the (100), (12̅0), and (002) lattice planes. After oxidation, the broad peak indexed as the lattice plane (002) shifted to a higher 2θ value (26.6°), resulting from changes in the interlayer stacking of the structure.

N2 sorption analysis at 77 K of the pristine AB-COF and COF-480-hydrazide demonstrated permanent porosity and revealed a Type I isotherm, indicating a micropore structure. The Brunauer–Emmett–Teller (BET) surface areas were calculated to be 1209 and 989 m2 g–1, respectively. The pore size distribution, estimated from the N2 isotherms and calculated by density functional theory (DFT) using the cylinder geometry, indicated comparable pore sizes of around 11 Å in diameter (see Supporting Information, section S7).

To gain further insight into the impact of the chemical transformations on the water sorption behavior, we measured the water sorption isotherms at 15, 25, and 35 °C (Figure a,b). AB-COF displays an S-shaped water sorption isotherm with a steep pore-filling step at around 23% relative humidity (RH) at 25 °C (Figure a). The maximal uptake at P/Psat = 0.95 reaches a total uptake capacity of 0.45 g g–1. After oxidation, the water sorption isotherm of COF-480-hydrazide substantially shifts to lower RH with a steep pore-filling step at 18%—thus extending the RH range at which COF-480-hydrazide can operate to more arid conditions. Importantly, the COF preserves its total water uptake capacity of around 0.45 g g–1 at P/Psat = 0.95 (25 °C).

Figure 1

Water vapor sorption analysis of AB-COF (a) and COF-480-hydrazide (b) at different temperatures (15, 25, and 35 °C). P: water vapor pressure. Psat: saturation water vapor pressure at the given temperature. Water cycling stability test of 230 adsorption–desorption cycles of COF-480-hydrazide (c) subjected to an isobaric (1.7 kPa water vapor) temperature-swing adsorption between 30 and 85 °C, corresponding to 40% and 3% RH, respectively. The full data set is reported in the Supporting Information, section S8.

Furthermore, the water sorption isotherm profiles are retained at 15 and 35 °C, indicating robust sorption behavior under varying temperatures. The increased water uptake capacity at lower RH upon oxidation indicated stronger water–framework interactions during the pore-filling step.[23,24] To evaluate the strength of these interactions, the isosteric heat of water adsorption (Qst) was calculated with the Clausius–Clapeyron equation using the water isotherms at 15, 25, and 35 °C. The Qst was estimated to be 49 kJ mol–1 for the pristine AB-COF and 50.5 kJ mol–1 for the oxidized COF-480-hydrazide. A ΔQst = 1.5 kJ mol–1 upon oxidation likely results from the stronger hydrogen-bonding donor and acceptor sites in the framework, given that both COFs show comparable pore diameters of around 11 Å.

It is noteworthy that even such a small conversion (9.2%) significantly shifted the water uptake to lower RH and resulted in an increase in water affinity to the framework. Thus, COF-480-hydrazide extracts water from air at even lower humidity compared to the pristine hydrazine-linked COF, AB-COF.[21]

To test the long-term stability of COF-480-hydrazide over multiple water adsorption–desorption cycles, we subjected the COF to an isobaric (1.7 kPa water vapor) temperature-swing adsorption between 30 and 85 °C, corresponding to 40% and 3% RH, respectively. The water uptake working capacity under these conditions was 0.33 g g–1. After 230 adsorption–desorption cycles, we observed a reduction of the working capacity by 7% (Figure c). This is attributed to partial hydrolysis of the sample upon long-term exposure to water vapor at an elevated temperature.

Next, we pursued the oxidation of the hydrazine-linked Py-COF-2D to COF-482-hydrazide. Noteworthy, partial oxidation of the hydrazine to the hydrazide linkage was already observed during the synthesis of the Py-COF-2D— even under inert reaction conditions (N2 atmosphere; see Supporting Information, section S2). This phenomenon was mentioned by Lotsch and co-workers,[25] and in our work, the observation was substantiated by FT-IR spectroscopy and 15N multi-CP-MAS NMR spectroscopy on a 50% 15N-labeled sample (see Supporting Information, sections S3 and S4). Presumably, both steric and electronic effects contribute to the more facile oxidation of Py-COF-2D as compared to AB-COF.

For further oxidation, we applied comparable conditions as described for AB-COF. Similarly, we observed enhanced attenuation of the characteristic imine band at 1625 cm–1 in the FT-IR spectroscopic traces and a relative increase of the carbonyl band at 1674 cm–1. The solid-state 15N multi-CP-MAS NMR spectroscopy on a 50% 15N-labeled sample showed an enhanced peak at 125.3 ppm, further substantiating successful oxidation. The conversion of the hydrazine- to the hydrazide-linkage amounted to 26.3% conversion (see Supporting Information, section S4). TGA under nitrogen and air flow revealed a comparable onset in decomposition for the pristine and oxidized samples at around 500 °C.

PXRD analysis of the microcrystalline powder of Py-COF-2D and COF-482-hydrazide indicated retention of crystallinity after oxidation (see Supporting Information, section S6). Furthermore, the PXRD traces indicated a shift of the highest intensity peaks from 4.5° 2θ, associated with the (11̅0) lattice plane, for the hydrazine-linked COF to 4.6° 2θ for the hydrazide COF. This observation correlates well with the anticipated changes in unit cell parameters upon oxidation (see Supporting Information, section S6).

N2 sorption analysis at 77 K of Py-COF-2D and COF-482-hydrazide demonstrated permanent porosity and revealed a Type I isotherm, indicating a micropore structure. The calculated BET surface areas amounted to 1850 m2 g–1 for the hydrazine-linked Py-COF-2D and 1700 m2 g–1 for COF-482-hydrazide. The reduction in specific surface area is in line with the weight increase upon oxidation. The pore size distribution, calculated from the N2 adsorption isotherms using the above-described parameters, indicated two kinds of pores with diameters of 15.9 and 18.5 Å for Py-COF-2D and 15.4 and 18.5 Å for the oxidized COF-482-hydrazide.

The pristine Py-COF-2D displayed an S-shaped water sorption isotherm with a steep pore-filling step at around 52% RH and a total uptake 0.75 g g–1 at 25 °C (Figure ). Upon oxidation, the isotherm profile significantly shifted to 42% RH with a slightly lower total uptake of 0.70 g g–1 at 25 °C. The shift of the pore-filling step to lower RH upon oxidation is in line with our previous observation in AB-COF and presumably stems from the incorporation of the more polar hydrazide functionalities into the framework.

Figure 2

Overlay of water isotherms of Py-COF-2D and COF-482-hydrazide at 25 °C. P: water vapor pressure. Psat: saturation water vapor pressure.

This is one of the first examples where postsynthetic transformations on COFs enabled a shift of the pore-filling step of the water isotherms to lower RH.[26,27] The difference in the Qst between the pristine Py-COF-2D and COF-482-hydrazide, calculated from the water sorption isotherms at 15, 25, and 35 °C, amounted to ΔQst = 2 kJ mol–1 (Qst = 45 and 47 kJ mol–1, respectively). A comparable ΔQst of the pristine sample and the oxidized hydrazide sample was also observed for AB-COF and COF-480-hydrazide.

Finally, to demonstrate postsynthetic oxidation of hydrazine- to hydrazide-linked COFs in 3D, we synthesized COF-670-hydrazine from the tetrahedral MTBA and linear hydrazine building units (Scheme c and Figure a). The structure crystallized in the dia topology with six-fold interpenetration (see Supporting Information, section S6). Similar to our observation with Py-COF-2D, partial oxidation of the hydrazine to hydrazide units in COF-670-hydrazine was already observed at this stage (see FT-IR and 15N spectroscopic traces, sections S3 and S4 in the Supporting Information). This confirmed the susceptibility of the 3D COF to oxidation. Upon further oxidation, we observed enhanced attenuation of the characteristic imine band at 1625 cm–1 in the FT-IR spectroscopic traces and a more intensive band at 1674 cm–1 (see Supporting Information, section S3). Successful oxidation was further confirmed through 15N solid-state CP-MAS NMR spectroscopy on a 50% 15N-labeled sample before and after oxidation. The 15N NMR peaks at 127.8 ppm intensified upon oxidation, and the conversion of the hydrazine- to the hydrazide-linkage amounted to 13.4% (Figure b and Supporting Information, section S4). TGA under nitrogen and air flow revealed an increase in thermal stability upon oxidation of the hydrazine-linked COF. Here, COF-670-hydrazine and COF-670-hydrazide showed an onset in thermal decomposition at around 350 °C.

Figure 3

Structural models of the six-fold framework of COF-670-hydrazine (a) and the oxidation of COF-670-hydrazine to COF-670-hydrazide (b). Overlay of PXRD patterns of COF-670-hydrazine modeled structures (from one-fold to seven-fold) and its experimental PXRD pattern (c). PXRD patterns of simulated COF-670-hydrazine compared to the experimental ones of COF-670-hydrazine and COF-670-hydrazide (d).

PXRD analysis of the microcrystalline powder of the hydrazine-linked COF and comparison with structural models of varying degrees of interpenetration allowed us to assign the degree of interpenetration to six-fold (Figure c). After oxidation, PXRD indicated retention of the crystallinity (Figure d and Supporting Information, section S6). Furthermore, the PXRD traces indicated a small shift of the highest intensity peaks at 9.4° 2θ, associated with the (020) lattice plane, for the hydrazine-linked COF to 9.5° 2θ for the hydrazide-linked COF. This observation correlated well with the anticipated changes in unit cell parameters upon oxidation (see Supporting Information, section S6). We also observed shifts to higher 2θ values for the designated reflections occurring between 13° and 25°.

N2 sorption analysis at 77 K of the two samples demonstrated permanent porosity and revealed a Type I isotherm, indicating a micropore structure. The calculated BET surface areas amounted to 1209 m2 g–1 for the hydrazine-linked COF-670-hydrazine and 699 m2 g–1 for COF-670-hydrazide. The pore size distributions, calculated from the N2 adsorption isotherms using the above-described parameters, indicated overall retention of the pore diameters of 6 Å after the oxidation (see Supporting Information, section S7).

The pristine COF-670-hydrazine displayed a water sorption isotherm with a slightly sloped pore-filling step at around 43% RH and a total uptake of 0.33 g g–1 at 25 °C (see Supporting Information, section S8).

Conclusion

We synthesized a series of 2D and 3D hydrazine-hydrazide-linked COFs by applying a postsynthetic strategy. This involved first the synthesis of the hydrazine-linked frameworks, followed by oxidation to afford the hydrazine-hydrazide-linked structures, in which the synthesis and partial transformation from hydrazine to hydrazide were further substantiated by 15N multi-CP-MAS NMR. In addition, we subjected the frameworks to isothermal water sorption analysis. This not only allowed us to discover new structures capable of water harvesting from desert air but also provided us with additional insight into the micro- and macroscopic changes occurring in the framework. Importantly, we found that even small modifications of the molecular structures of the frameworks can lead to impactful differences in the water isotherm profiles. Overall, this study expands the scope of COF chemistry to include crystallization of irreversible linkages and shows that these new systems can extract water from low-humidity air.