1000-fold enhancement in proton conductivity of a MOF using post-synthetically anchored proton transporters

Pyridinol, a coordinating zwitter-ionic species serves as stoichiometrically loadable and non-leachable proton carrier. The partial replacement of the pyridinol by stronger hydrogen bonding, coordinating guest, ethylene glycol (EG), offers 1000-fold enhancement in conductivity (10−6 to 10−3 Scm−1) with record low activation energy (0.11 eV). Atomic modeling coupled with 13C-SSNMR provides insights into the potential proton conduction pathway functionalized with post-synthetically anchored dynamic proton transporting EG moieties.


Powder X-ray diffraction:
Powder XRDs were carried out using a Rigaku Miniflex-600 instrument and processed using PDXL2 software.

IR spectroscopy:
IR spectra were obtained using a Nicolet ID5 attenuated total reflectance IR spectrometer. The KBr pellets were used.

Solid State NMR spectroscopy:
Solid state NMR was recorded using an 800MHz Bruker NMR instrument.

Thermogravimetric Analysis:
Thermogravimetry was carried out on NETSZCH TGA-DSC system. The TGAs were done under N 2 gas flow (20ml/min) (purge + protective) and samples were heated from RT to 550⁰C at 2K/min.
• 2: Cd(C 8 O 4 H 4 )(C 5 NOH 5 ) A weight loss of 25.59 % from 350 to 380⁰C corresponds to loss of pyridinol. As inferred from TGA, material is not stable after pyridinol loss.

Impedance measurements:
The Bio-Logic (VMP-3) model was used to carry out the impedance measurement. The impedance spectra were recorded by applying range of frequency from 1 MHz to 100 mHz with an amplitude of 10 mV. Moreover, ESPEC chamber was used to maintain the temperature and humidity while measurement.

Single crystal structure determination:
Single-crystal data was collected on a Bruker SMART APEX four-circle diffractometer equipped with a CMOS photon 100 detector (Bruker Systems Inc.) and with a Cu Kα radiation (1.5418Å). The incident Xray beam was focused and monochromated using Micro focus (IµS). Crystal of 1, 2 and 3 were mounted on nylon Cryo loops with Paratone-N oil. Data was collected at 173(2) K for 1 and 3, and at 298K for 2.
Data was integrated using Bruker SAINT Software and was corrected for absorption using SADABS. Structure was solved by Intrinsic Phasing module of the Direct methods and refined using the SHELXTL 2014 software suite. All non-hydrogen atoms were located from iterative examination of difference Fmaps following which the structure was refined using least-squares method. Hydrogen atoms were placed geometrically riding mode (CIF file: CCDC 1041840-1041842).

Field Emission SEM:
Ultra Plus Field Emission Scanning Electron Microscope with integral charge compensator and embedded EsB and AsB detectors. Oxford X-max instruments 80mm 2 . (Carl Zeiss NTS, Gmbh), Imagin conditions: 2kV, WD=2mm, 200kX, Inlens detector. Figure s1: (a) PXRD comparison for 1 (simulated: black, as made: red) and 1_EG (blue); (b) PXRD comparison for 2; PXRD comparison for 3 (simulated: black, as made: red) and 3_EG (blue). The relatively broader peaks for 3 are most likely due to fluorescence from the sample and such broad peaks are known in lanthanide MOFs. Repeated synthesis produced very similar samples with the similar PXRD profiles.   Note the presence of pyridinols alternating up and down along the column. The dimers are connected by terephthalates to form the rhombic channels, which have not been shown for clarity. The separation between pyridinols are larger than in 1, however this opens up a lot of space for water molecules to come in as hydrogen bonding guest under the humid conditions. This could account for its higher proton conductivity than 1. Color code: Pink-Nd, blue-N, red-O, grey-C. Figure s5. a) Variable Temperature Impedance Spectra for 2 at 90% RH. b) Impedance spectrum for 3 at 90% RH. c) Variable Temperature Impedance Spectra for 3_EG at 90%RH and d) shows the equivalent circuit used to calculate resistance for 2, 3 and 3_EG.  Weight loss corresponding to loss of µ-2 bridging pyridinol happens in the temperature range of 320 to 450⁰C and from VTPXRD it can be seen that the crystallinity is maintained.

Discussion on the results from VT-PXRD experiments of 1:
The loss of pyridinol from 1 at 450⁰C would result in Mg with a highly unfavorable four-coordinated geometry inducing a severe structural strain. And one can expect the structure to rearrange. Accordingly, with the loss of pyridinol at 450⁰C, a phase change sets in and by 550⁰C a beta-phase crystallizes (Fig. s6). This essentially conveys that the Mg-terephthalate exists as a crystalline phase even upon the removal of the pyridinol. In order to verify if the phase formed by the complete loss of pyridinol corresponds to any of the known Mg terephtahlates, we indexed the beta phase and carried out a Pawley fit (Accelrys). This resulted in a triclinic cell for this high temperature beta phase (a= 12.006Å; b= 10.431Å; c= 10.699Å, α= 69.285º; = 111.284º; = 99.029º), which corresponds to a new Mg terephthalate phase (Fig. s8).

Figure s9.
Pawley refinement of the new beta phase formed by the transformation of the 1 at 550⁰C.

Discussion on higher relative conductivity in 2 and 3:
An alternate explanation to the higher conductivity observed for the as-made form of the Cd phase compared to the Mg and Nd analogues could lie in their coordination tendencies. In both 1 and 2, the metal centers are octahedrally coordinated. While this represents a saturated coordination for Mg, Cd can carry up to seven coordinations, offering one extra site for water to coordinate under humid conditions employed during the impedance measurements. And it is well known that coordinated water itself can act as an amphoteric center to not only to hydrogen bond but also to transfer proton across a channel. Again, Nd in 3 can also accommodate more coordinated water molecules, but the relatively little difference in conductivity between 3 and 1 suggests that this effect contributes much lesser to the overall conductivity compared to the acidity differences in pyridinol arising from the hardness of the metal centers.

Solid State NMR Experiments for 1 and 1_EG:
We used solid state NMR (SSNMR) to obtain some insights into the mobility associated with the framework components, particularly the EG and PyOH. For this purpose we have compared 1, 1_EG and a 'control' which is Mg-terephthalate with no pyridinol but having ethylene glycol linking two different metal centers in a μ-2 fashion via both its oxygen (1_C). The control represents the most rigid form of the EG and the carbons from such EG molecules should be equivalent. On the other hand the EG in 1_EG phase has been proposed to bind to the Mg centers only via one of its oxygen, which leaves the other -OH group pendant.

C CPMAS NMR of 1:
The 13 C CPMAS NMR of 1 (Figure 3c) owing to its high crystallinity showed well resolved peaks corresponding to both terephthalate and pyridinol. An important feature that was observed from this NMR was the presence of pyridinol in two forms, one wherein the pyridyl nitrogens seem to be protonated (chemical shifts: 139, 137, 121ppm) and other wherein the proton most probably resides on the -OH group bound to the metal (chemical shift: 183, 173, 129ppm). This explains the multiple peaks observed in the NMR (Figure 3c).
The region of focus is the 55-75ppm. This corresponds to peaks from EG. As can be seen from figure 3c, there is a prominent peak at 64ppm in the case of 1_EG, which is missing in 1. A closer look at the peak corresponding to the EG shows that it is not a singlet, but appears to be a quartet. A simple peak fit suggests the presence of four peaks (chemical shifts: 62, 64, 65, 67ppm;  2 = 0.99561). This indicates the presence of different coordination modes for the EG (Scheme s1). However, it does not rule out the possibility of a free EG. To gain further confidence on this coordination modes of EG, we carried out a proton decoupled 13 C MAS NMR of the EG loaded and it could be seen that the relative intensity of the 64ppm peak was significantly higher compared to the peak observed from the 13 C CPMAS NMR of the same phase (Figure 3c). This would mean the mobility associated with EG is relatively higher than those associated with the framework components.

Scheme s1. Different coordination modes possible for EG binding with Mg
Now to further rule out the possibility of a free EG trapped in the MOF, we need to consider few other logics based on the inferences. Considering that the structure is quite dense, the most likely option for the EG to be accommodated would require replacement of some of the PyOH units. This EG would have to bind to Mg to maintain its coordination and prevent the collapse of the structure. Also, following the postsynthetic EG loading, the sample was subjected to aggressive washing with ethanol and methanol, to ensure that any free EG is removed. The distance between the two magnesium centers required for accommodation of the ethylene glycol is ~5.9 Å whereas the available space in 1 is 3.6 Å. This compound has not been reported in the literature. Color code: Orange-Mg, red-O, Grey-C. Right: 13 C NMR for 1_C showing the peak for EG appearing as a singlet, which suggests the presence of only one type of coordination mode for the EG molecules.
To further support this, we prepared a model compound, MgTpEG, wherein the EG are monodentately coordinated to two different Mg centers via both its -OH groups (figure s15). The 13 C CPMAS NMR experiments on this indicated a clear singlet for the EG peak as compared to the multiplets observed in the case of 1-EG (figure s16). When the 13 C CPMAS is compared to the 13 C MAS hpdec, it is clearly seen that the relative intensity and the broad line shapes corresponding to the EG are comparable, suggesting that the EG are highly bonded in this sample in comparison to the EG in 1_EG (Figure 3c).
If the EG had been loaded as an extra-framework guest, given the extremely similar size and shape of the pores in 1 and 2 (Figure 1), both should have shown the tendency to load. However, just the Mg shows loading and there is a good chance that the EG is not loading merely as an extra-framework guest, but in addition takes up the coordination. These observations together suggest the presence of monodentately coordinating terminally bound EG in 1_EG to be the most likely case.

Synthesis of 1_C:
This compound was synthesized hydrothermally using the approximate molar ratios Mg(NO 3 ) 2 . 6 H 2 O, terephthalic acid and 4-hydroxypyridine (1:1). 0.1g Magnesium nitrate and 0.065 g terephthalic acid was stirred in 5 ml ethylene glycol for 15 minutes at RT. The contents were then sealed in an autoclave and heated at 140⁰C for 48 hours. It was slowly cooled down to room temperature. Product containing thin platy crystals was collected by filtration using methanol and acetone.