An N,N′-diethylformamide solvent-induced conversion cascade within a metal–organic framework single crystal

Jing Chen a, Meng-Yao Chao a, Yan Liu a, Bo-Wei Xu a, Wen-Hua Zhang *a and David J. Young b
aCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China. E-mail:
bCollege of Engineering, Information Technology & Environment, Charles Darwin University, Darwin, Northern Territory 0909, Australia

Received 3rd April 2020 , Accepted 28th April 2020

First published on 1st May 2020

Crystals of a two-dimensional (2D) metal–organic framework (MOF) [Cd3(BTB)2(DEF)4]·2(DEF)0.5 (1; BTB = benzene-1,3,5-tribenzolate; DEF = N,N′-diethylformamide) immersed in a solution of trans-1,2-bis(4-pyridyl)ethylene (BPEE) yields an interpenetrated three-dimensional (3D) MOF of [Cd3(BTB)2(BPEE)(H2O)2]·(BPEE)·xSol (2). Crystals of MOF 2, in turn, undergo a cascade conversion when immersed in DEF, yielding [Cd3(BTB)2(BPEE)1.8(DEF)0.9(H2O)0.8xSol (3a) over 100 seconds and [Cd3(BTB)2(BPEE)2(DEF)2xSol (4) after one hour, before finally shuttling back to MOF 1 after six hours.

Dynamic transformations are ubiquitous in metal–organic frameworks, and understanding them is useful for elucidating the mechanisms of various important processes, including guest adsorption,1 catalysis,2 and sensing,3 in addition to the value of illuminating these responsive distortions for themselves.4 However, progressive snapshots of dynamic conversions are often challenging to obtain, and so we have an incomplete understanding of the reaction profile.1c,4f,5 Some intermediate states are amenable to synthetic manipulation such as in tandem MOF functionalization,6 multivariate linker installation,7 and breathing,8 and these processes have been elegantly described.

Competitive ligand/solvate binding to the cluster-based secondary building units (SBUs) often results in mixed MOFs in time-dependent ratios, which can be difficult to resolve and characterize. In some cases, spectroscopic differences are insignificant, such as when the difference between two MOFs is whether the solvent molecules are coordinated or encapsulated.9 In the present work, we describe a multi-step MOF conversion cycle involving the DEF-induced dynamic conversion cascade of a two-fold interpenetrated, three-dimensional (3D) MOF of [Cd3(BTB)2(BPEE)(H2O)2]·(BPEE)·xSol (2) (Scheme 1; BTB = benzene-1,3,5-tribenzolate; BPEE = trans-1,2-bis(4-pyridyl)ethylene; DEF = N,N′-diethylformamide), which was the product from the reaction of a two-dimensional (2D) MOF [Cd3(BTB)2(DEF)4]·2(DEF)0.5 (1) and BPEE in CHCl3 in a solid-solution transformation.10 The structure of MOF 2 features a linear trinuclear Cd3 cluster SBU inherited from 1 (Scheme 1 and Fig. 1a). The central Cd2+ of the trinuclear Cd3 SBU of 2 extends to a pair of identical Cd2+ ions via two bridging (μ-η11) and two bridging-chelating (μ-η12) carboxylates (Scheme 1 and Fig. 1b). It is further coordinated by a pair of trans-located H2O molecules. Each flanking Cd2+ ion is additionally coordinated by a chelating carboxylate and one N atom of the axial BPEE ligand.

image file: d0cc02420a-s1.tif
Scheme 1 Solid-state conversion cycle starting from the 2D MOF 1.

image file: d0cc02420a-f1.tif
Fig. 1 The Cd3 SBUs of MOFs 1 (a), 2 (b), 3a (c), 4 (d), and 5 (e) showing the coordination spheres around the Cd2+ centers. Colour legend: Cd (dark magenta), C (grey), O (light-red). The color of BPEE, DMF, DEF, and H2O ligands are set to orange for clarity reasons. The disordered ligands in 3a are further distinguished by orange and green.

A distinctive feature of MOF 2 is that in each cavity supported by the four Cd3 SBUs, there resides one additional BPEE guest molecule associated with two H2O molecules of the central Cd2+via a pair of O–H⋯N hydrogen bonds (O1w⋯N4i 2.92(5) Å; ∠O1w–H2w1–N4i 141.0°, i: x + 1, y − 1, z; O2w⋯N3 2.757(12) Å; ∠O2w–H1w2–N3 151.9°; Scheme 1 and Fig. 2). When crystals of MOF 2 were immersed in DEF solvent for one hour, we observed that [Cd3(BTB)2(BPEE)2(DEF)2xSol (MOF 4) was formed by the replacement of the two trans-located H2O solvates with one DEF and one BPEE molecule (Scheme 1 and Fig. 1d). It is notable that one of the flanking Cd2+ ions also associated with one additional DEF, presumably enabled by the structural distortion of this displacement, as well as by the relatively large radius of Cd2+. The BPEE ligands at the axial positions of the SBU remained attached at this stage, suggesting that the BPEE coordinated to the central Cd2+ came from a guest molecule in the pores. On extended exposure for six hours, the 3D interpenetrated MOF 2 completely converted back to 2D MOF 1 (denoted as MOF 1′), as evidenced by powder X-ray diffraction (PXRD) analysis (Fig. S1, ESI).

image file: d0cc02420a-f2.tif
Fig. 2 Top-down (a) and side (b) views of MOF 2, showing the accommodation of BPEE hydrogen-bonded to the aqua molecules of the framework skeleton. Colour legend: Cd (dark magenta), C (grey), O (light-red), H (light-green). The encapsulated BPEE ligands are distinguished by orange.

DEF and its methyl analog N,N′-dimethylformamide (DMF) are widely used solvents for the preparation of MOFs, while dipyridyl ligands are rarely replaced by these weakly nucleophilic amides.11 When this ligand exchange does occur, the driving force is the formation of thermodynamic products.4h Such a one-pot DEF-induced tandem replacement of a pair of trans-located H2O solvates (in MOF 2) by BPEE and DEF (in MOF 4), and ultimately a pair of trans-located DEF molecules (in MOF 1′) prompted our interest in the mechanism of this conversion pathway. Such a process presumably involves subtle interplay between aqua, BPEE, and DEF donors.

We next monitored the products from the solid-state reaction of MOF 2 with DEF by X-ray crystallography at different time intervals. No change was observed after immersion for 40 seconds (Tables S1 and S2, ESI). The structure contained BPEE solvate and could be refined with full occupancy. X-ray structure analysis after an extended immersing time of 100 seconds, indicated that the H2O ligands of the central Cd2+ were partly replaced by BPEE and DEF in a ratio of BPEE[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2, and DEF[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 0.4[thin space (1/6-em)]:[thin space (1/6-em)]0.6 (Table S2, ESI), giving rise to [Cd3(BTB)2(BPEE)1.8(DEF)0.9(H2O)0.8xSol (MOF 3a). Prolonging the reaction time to 120 seconds resulted in a BPEE[thin space (1/6-em)]:[thin space (1/6-em)]H2O ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]0 and DEF[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 0.6[thin space (1/6-em)]:[thin space (1/6-em)]0.4 (Table S2, ESI), viz. [Cd3(BTB)2(BPEE)2(DEF)1.1(H2O)0.4xSol (MOF 3b). After one hour, crystals of MOF 4, in which the two H2O ligands were fully replaced by one BPEE and one DEF, were isolated (Table S2, ESI). The structure of MOF 4 was retained after three hours of reaction (Table S1 and Fig. S2, ESI). However, the 2D MOF 1′ was obtained as the exclusive product after six hours.

The observation of the fast transformation of MOF 2 to MOF 4 and the higher site occupancy of BPEE relative to DEF at the trans-position suggests that step one of this solid-state transformation involves enthalpy-driven DEF replacement of coordinated aqua ligand. This DEF-for-aqua exchange simultaneously triggers the BPEE-for-DEF exchange because of the stronger coordination of BPEE to Cd2+. Association of BPEE with Cd2+ weakens bonding to the trans coordinated aqua, facilitating DEF displacement of this solvate. This trans effect is kinetic in nature and proceeds rapidly via lower activation energy.12 An entropic advantage might also dominate the conversion of 3D interpenetrated MOF 4 to 2D MOF 1. This transformation involves the disintegration of the 3D MOF into multiple 2D MOF sheets and the release of bulky BPEE into solution, versus the uptake of smaller DEF molecules.

We similarly treated MOF 2 with DMF which is comparable in donor strength to the more expensive DEF. Immersing MOF 2 in DMF for two hours gave [Cd3(BTB)2(BPEE)(DMF)(H2O)]·xSol (MOF 5), in which only one H2O ligand was replaced by DMF (Fig. 1e and Table S1, ESI), while the aqua ligand at the trans-position, as well as the coordination environment of the flanking Cd2+ remained unchanged. The structure of MOF 5 did not change over several hours, or indeed over several months (Fig. S3 and Tables S1, S2, ESI). These observations demonstrate the strikingly different reactivity of DMF and DEF in these solvent-mediated displacement reactions. We assume that this difference originates from the relative size of each amide. Coordinated DEF on the central Cd2+ is susceptible to substitution by BPEE due to its greater bulk that de-stabilize its coordination, despite marginally stronger electron donation. The less stable coordination of DEF is the key to its subsequent replacement by BPEE, followed by DEF-for-H2O exchange trans to the newly ligated BPEE on the way to the formation of MOF 4.

To observe this 123a/3b41′ conversion cycle visually, we prepared the orange-brown MOF [Cd3(BTB)2(AZOPY)(H2O)2xSol (MOF 6, AZOPY = 4,4′-azopyridine), which is topologically identical to MOF 2.10a Batches of MOF 6 were immersed in DEF and then filtered at different time intervals followed by flushing with CHCl3 to wash away surface adhesions. We found that the color of MOF 6 faded quickly within the first 16 minutes (Fig. 3). Crystals of 6 became almost colorless upon extended DEF exposure and converted back to the 2D MOF 1, as evidenced by powder X-ray diffraction analysis (Fig. S4, ESI). These color changes indicate that complete replacement of the encapsulated and coordinated dipyridyl ligand AZOPY by the (presumed) weakly coordinating solvent DEF is in fact rather fast. By contrast, when MOF 6 was immersed in DMF, the orange-brown color similarly faded, but much more slowly (Fig. S5, ESI) and was still observable after 96 hours. This result mirrors the relative efficacy of DEF versus DMF to remove BPEE from MOFs 4 and 5.

image file: d0cc02420a-f3.tif
Fig. 3 Crystals of MOF 6 immersed in DEF at different time intervals, showing the complete release of colored AZOPY.

The conversion cycle starting with MOF 1 involves a significant topological change from 2D → 3D → 2D, and so we investigated whether this molecular re-organization could be observed by a macroscopic change to the surface morphology using atomic force microscopy (AFM). A single-crystal of MOF 1 was loaded into a capillary tube and subjected to AFM analysis at three points in the conversion cycle, viz. stage 1 (MOF 1), stage 2 after BPEE displacement (MOF 2), and the final stage (MOF 1′) (Fig. S6 and S7, ESI). We employed PeakForce™ quantitative nanomechanical mapping (PF-QNM) to measure the roughness and elastic (Young's) modulus of each crystal. The roughness of each single crystal surface was similar (Fig. S7a–c, ESI), but their elastic moduli were quite different: MOF 1 (Ra = 1420 MPa, Rq = 2068 MPa), MOF 2 (Ra = 222 MPa, Rq = 282 MPa) and MOF 1′ (Ra = 1271 MPa, Rq = 1668 MPa) (Fig. S7d–f, ESIRa and Rq stand for the roughness average and the root mean square roughness). These results show that the physical surface properties of MOF 2 were significantly different to those of pristine MOF 1 and the final MOF 1′. We attribute this morphological change to the flexible interpenetrated structure of MOF 2 relative to the rigid, non-interpenetrated structures of MOF 1 and MOF 1′.

The retention of porosity from MOF 2 to MOF 4 was investigated by measuring the N2 (at 77 K) and CO2 (at 195 K) isotherms of MOF 4 as guided by thermogravimetric analysis (TGA) (Fig. S8, ESI). MOF 4 possessed a type I CO2 adsorption profile with steep uptake in the low-pressure region (Fig. S9, ESI) suggested the presence of open channels in the degassed phase.12 This CO2 isotherm displayed hysteresis upon desorption, indicating that there exists framework flexibility or strong framework–gas interactions.13 An activated sample of MOF 4 adsorbed up to 98.6 cm3 g−1 CO2 (4.42 mmol g−1, 19.5 wt% at standard temperature and pressure, STP). These data are unremarkable and comparable with the corresponding values for MOF 2 (87.5 cm3 g−1, 3.94 mmol g−1; 17.3 wt% at STP)10a,b and [Cd3(BTB)2(BPEE)(BIPY)2xSol (67.3 cm3 g−1, 3.0 mmol g−1; 13.3 wt% at STP, BIPY = 4,4′-bipyridyl).10b We observed no obvious absorption of N2 at 77 K in MOFs 2 or 4, presumably due to the larger kinetic size of N2 (3.64 Å) relative to CO2 (3.30 Å).14 It is notable that the PXRD of the activated sample of MOF 4 resembled the pre-activated patterns (Fig. S2, ESI), indicating the permanent porosity of MOF 4.

In summary, MOF 1 undergoes a post-synthetic conversion cycle from 2D (MOF 1) → 3D (MOF 2) → 3D (MOF 4) → 2D (MOF 1′) involving a sequence of ligand displacements facilitated by a weakly coordinating solvent DEF. This molecular reorganization is observable by a macroscopic change to crystal surface morphology and elastic modulus. However, resolving the reaction kinetics and displacement sequence of H2O, DEF, and BPEE requires substantial additional work. X-ray diffraction studies are not adequate to determine the behaviour of encapsulated solvates because of the disorder. This work has nonetheless uncovered the previously underestimated complexity of MOF conversions in the presence of seemingly benign solvents DMF and DEF and may serve as a caution to the community on the possibility of MOF reorganization induced by donor solvents (e.g. H2O, MeOH, EtOH, Me2CO, DMF, DEF) during material activation and in subsequent applications.

This work was financially supported by the National Natural Science Foundation of China (21871203 and 21671143).

Conflicts of interest

The authors declare no conflicts of interest.

Notes and references

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Electronic supplementary information (ESI) available: Experimental details, crystallographic refinement details, additional tables and figures. CCDC 1978516–1978522. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0cc02420a

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