Modulating proton diffusion and conductivity in metal–organic frameworks by incorporation of accessible free carboxylic acid groups

The proton conductivity of barium-based MOFs MFM-510 to MFM-512 are analysed in relation to the absence and presence of free –COOH groups in the pores.


Proton conductivity measurements
Proton conductivity measurements were performed on a Solartron SI1260 Impedance analyser over a frequency range of 0.1 Hz to 1 MHz at an amplitude of 100 mV and 0 mV DC rest voltage. Conductivity measurements were carried out on pressed pellets of finely ground powder samples. Samples for conductivity measurements were prepared by grinding the sample (~0.1 g) into a homogeneous powder with a mortar and pestle, added to a standard 8 mm die, coated with a conductive silver paste to improve contact with the two blocking electrodes, and pressed at 5,000 kg for 5 minutes. Resultant pellets of 8 mm in diameter and a thickness of ~1.05 mm were placed in the electrochemical cell. Relative humidity of 99% was obtained using a Kambic KK-50 climatic chamber. The proton conductivity (σ, S cm -1 ) was calculated from the impedance data, using the following equation: σ = l/RS, where l and S are the thickness (cm) and cross-sectional area (cm 2 ) of the pellet respectively, and R, which was extracted from the impedance plots, is the total resistance of the sample (Ω). ZView software was used to analyse the impedance data.

Synthesis of (4-(ethoxycarbonyl)phenyl)boronic acid (1).
4-Boronobenzoic acid (20.00 g, 103 mmol) was added to a mixture of conc. H2SO4 and EtOH (500 mL). The reaction mixture was refluxed for 12 h. The resulting solution was filtered to remove any impurities and the filtrate concentrated to ca. 100 mL. A precipitate was obtained on addition of H2O and was collected by filtration, washed with excess H2O to pH 7 and dried under vacuum to afford the desired white product (13.26 1,5-Dibromo-2,4-dimethylbenzene (2.53 g, 9.5 mmol), 1 (4.47 g, 23 mmol) and K2CO3 (6.64 g, 48 mmol) were added to a mixture of toluene (200 mL) and H2O (50 mL). The resulting solution was heated to 60 o C and degassed for 15 mins under N2. A 1 M solution of P( t Bu)3 in toluene (3.4 mL, 3.22 mmol) and Pd2dba3 (1.06 g, 1.14 mmol) were added to the vigorously stirred reaction mixture which was then heated to 80 o C under N2.
Reaction completion was confirmed after 6 h by TLC (1:3 EtOAc/hexane) against starting materials. The resulting solution was filtered, H2O was added to the filtrate and the crude product was extracted with CH2Cl2.
3 (1.02 g, 2.54 mmol) and NaOH (2.69 g, 67 mmol) were stirred in t BuOH (50 mL) and H2O (100 mL). The solution was heated to 50 o C and KMnO4 (10.62 g, 67 mmol) was added gradually over 2 days until the reaction solution remained purple. On addition of 2/3 of KMnO4, the temperature was increased to 70 o C. Once the addition was complete, i PrOH (50 mL) was added and the reaction mixture heated at reflux for 2 h. The

S5
resulting suspension was filtered and washed with boiling H2O. The filtrate was concentrated to 50 mL and acidified to pH 1 with 12 M HCl (5 mL). The product was isolated by filtration to yield a white solid (0.8 g, 70%

Additional QENS analysis
The dynamics of the protons were probed using the neutron spectrometer IRIS at the ISIS Pulsed Neutron and Muon Source, Chilton, UK; a time-of-flight inverted-geometry crystal analyser spectrometer with diffraction capabilities. In the QENS measurements, neutrons scattered from the sample were energy-analysed by means of Bragg reflections from a single crystal array of pyrolytic graphite close to the backscattering geometry 2θB = 175 o , where θB is the Bragg angle of the analyser crystal, and were counted in a detector array covering 27 o < 2θ < 158 o yielding a wave vector range of 0.4 Å -1 to 1.8 Å -1 . In this study, IRIS was operated in the PG (002) configuration which provides an energy transfer window of -0.5 meV<ħω< 0.5 meV and an energy resolution ΔEres of 17.5 μeV. The powdered sample was loaded into an annular aluminium container having a suitable sample thickness to minimise multiple scattering effects. The QENS data were collected at temperatures between 248 K and 423 K with counting times of 6 h at each temperature. A Vanadium sample measured at 30 K was used for resolution.
The QENS data were treated presuming that a significant fraction of protons in MFM-512 move too slowly for the resolution of the spectrometer meaning they can be assumed to be immobile (p) (i.e. aromatic protons).
The mobile fraction (1-p) that is constrained to perform the molecular diffusion consists of the protons coming from the water molecules bound to the metal and the free hydroxy from the carboxylate. The function j1 the first order spherical Bessel function. Geometrical information of the molecular motions of active protons in MFM-512 were analysed via the elastic incoherent structure factor, EISF.

EISF = Ielastic / (Ielastic + IQENS)
where Ielastic and IQENS are the peak intensities of elastic and quasi-elastic scatterings, respectively. To obtain the elastic (Ielastic) and QENS (IQENS) intensities, the QENS data were fitted to resolution function convoluted with a Delta and a Lorentzian function as well as a flat background. The EISF data was fitted using the free diffusion inside a sphere model which showed good agreement with the experimental data allowing descriptions of localised diffusive motions to be made.