Exploring and expanding the Fe-terephthalate metal–organic framework phase space by coordination and oxidation modulation

The synthesis of phase pure metal–organic frameworks (MOFs) – network solids of metal clusters connected by organic linkers – is often complicated by the possibility of forming multiple diverse phases from one metal–ligand combination. For example, there are at least six Fe-terephthalate MOFs reported to date, with many examples in the literature of erroneous assignment of phase based on diffraction data alone. Herein, we show that modulated self-assembly can be used to influence the kinetics of self-assembly of Fe-terephthalate MOFs. We comprehensively assess the effect of addition of both coordinating modulators and pH modulators on the outcome of syntheses, as well as probing the influence of the oxidation state of the Fe precursor (oxidation modulation) and the role of the counteranion on the phase(s) formed. In doing so, we shed light on the thermodynamic landscape of this phase system, uncover mechanistics of modulation, provide robust routes to phase pure materials, often as single crystals, and introduce two new Fe-terephthalate MOFs to an already complex system. The results highlight the potential of modulated self-assembly to bring precision control and new structural diversity to systems that have already received significant study.


S3.1. General Conditions for Synthesis
Either iron(II) chloride tetrahydrate or iron(III) chloride hexahydrate (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added, for modulated samples acetic acid (1-50 mmols) was also added. The jar was capped and sonicated until the solids dissolved before heating in an isothermal oven at 120 °C for either 24 or 72 hours. After allowing to cool to room temperature, the suspension was separated by centrifuging. The supernatant was decanted and fresh DMF (20 mL) was added before centrifuging again. This was repeated three times, before repeating the procedure another three times with DCM (20 mL). The sample was then dried overnight in a desiccator under vacuum.
The naming system for these samples is FeCl2-AAx(T,t) and FeCl3-AAx(T,t), where 'x' equals the number of molar equivalents of acetic acid (AA) added, 'T' is the synthesis temperature, and 't' is the synthesis time. S8 Figure S4. Stacked PXRD patterns of FeCl2-AAx(120°C,24h) where 'x' is the number of molar equivalents of acetic acid (AA) used in the synthesis. A minor phase/impurity is marked with an asterisk for x = 30. Figure S5. Stacked PXRD patterns of FeCl2-AAx(120°C,72h), where 'x' is the number of molar equivalents of acetic acid (AA) used in the synthesis. S10 Figure S6. Stacked PXRD patterns of FeCl3-AA40(120°C,24h) compared to the predicted patterns for MOF-235(Fe) S6 and MIL-88B(Fe) closed, confirming the formation of MIL-88B(Fe). S7 Some minor differences in the position of Bragg reflections are apparent, due to the sample not fully closing to the structure previously predicted. This phenomenon occurs throughout our characterisation of MIL-88B(Fe) samples, possibly due to adsorption of ambient moisture prior to measurement, but the presence of the three main Bragg reflections with characteristic intensities allows assignment of phase as MIL-88B(Fe).  c) S14 Figure S11. Comparison of the PXRD pattern of FeCl2-AA0(120°C,24h) with the predicted pattern for MIL-101(Cr). S4 The predicted PXRD pattern for MIL-101(Cr) is used, as there is no freely available data for MIL-101(Fe), and the two MOFs are isostructural.

S4.1. General Conditions for Synthesis
Either iron(II) chloride tetrahydrate or iron(III) chloride hexahydrate (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added; for modulated samples acetic acid (30 mmol) was also added. The jar was capped and sonicated until the solids dissolved, before heating in an isothermal oven at either 120 °C or 150 °C for either 2, 4, 24, 48, 72, 120 or 168 hours. After allowing to cool to room temperature, the suspension was separated by centrifuging. The supernatant was decanted and fresh DMF (20 mL) was added before centrifuging again. This was repeated three times, before repeating the procedure another three times with DCM (20 mL). The sample was then dried overnight in a desiccator under vacuum. The naming system for these samples is FeCl2-AAx(T,t) and FeCl3-

S4.2. Single-Crystal Synthesis of MIL-53(Fe)-DMF
FeCl2·4H2O (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added. The jar was capped and sonicated until the solids dissolved, before heating at 150 °C in an isothermal oven. After 3 days, the jar was removed and allowed to cool to room temperature. Yellow needle-shaped crystals were evident and the DMF was   (1 or 2 molar equivalents) as modulator, using either FeCl2·4H2O or FeCl3·6H2O as starting material, compared to the predicted PXRD pattern of MIL-53(Fe).

S5.1. General Conditions for Synthesis
The iron precursor (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added, for modulated samples acetic acid (1-50 mmols) was also added. The jar was capped and sonicated until the solids dissolved before heating in an isothermal oven at 120 °C for either 24 or 72 hours. After allowing to cool to room temperature, the suspension was separated by centrifuging. The supernatant was decanted and fresh DMF (20 mL) was added before centrifuging again. This was repeated three times, before repeating the procedure another three times with DCM (20 mL). The sample was then dried overnight in a desiccator under vacuum. The naming system used is Fe(counterion)-AAx(T,t) where 'x' equals the number of molar equivalents of acetic acid (AA) added, 'T' is the synthesis temperature, and 't' is the synthesis time.

S5.2. Single-Crystal Synthesis of [Fe(DMF)(BDC)]
Fe(BF4)2·6H2O (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added. The jar was capped and sonicated until the solids dissolved before heating at 150 °C in an isothermal oven. After 3 days, the jar was removed and allowed to cool to room temperature. Yellow polyhedral crystals were evident and the DMF was decanted and replaced several times with fresh DMF, in which the crystals were kept for further analysis.

S5.3. Single-Crystal Synthesis of [Fe3O(DMF)3(BDC)3][BF4]
Fe(BF4)2·6H2O (1 mmol) and terephthalic acid (1 mmol) were added to a 50 mL Pyrex reagent jar and DMF (10 mL) was added along with acetic acid (40 mmol). The jar was capped and sonicated until the solids dissolved before heating at 120 °C in an isothermal oven. After 24 hours, the jar was removed and allowed to cool to room temperature. Orange hexagonal-shaped crystals were evident and the DMF was decanted and replaced several times with fresh DMF, in which the crystals were kept for further analysis. Elemental analysis suggests the formula

S5.4. Single-Crystal Synthesis of MIL-88B(Fe)
FeSO4·7H2O (0.05 mmol) and terephthalic acid (0.05 mmol) were added to a 25 mL Pyrex reagent jar and DMF (4 mL) was added along with acetic acid (0.2 mL). The jar was capped and sonicated until the solids dissolved, before heating at 120 o C in an isothermal oven. After 24 hours, the jar was removed and allowed to cool to room temperature. Orange hexagonal rod-shaped crystals were evident and the DMF was decanted and replaced several times with fresh DMF, in which the crystals were kept for further analysis.

S5.5. Single-Crystal Synthesis of Fe-BDC-Br
FeSO4·7H2O (0.05 mmol) and 2-bromoterephthalic acid (0.05 mmol) were added to a 25 mL Pyrex reagent jar and DMF (4 mL) was added along with acetic acid (0.2 mL). The jar was capped and sonicated until the solids dissolved, before heating at 120 o C in an isothermal oven.
After 24 hours, the jar was removed and allowed to cool to room temperature. Dark red blockshaped crystals were evident and the DMF was decanted and replaced several times with fresh DMF, in which the crystals were kept for further analysis.

S5.6. Bond Valence Sum (BVS) Calculations
Bond valence sum calculations were carried to confirm the oxidation states of the Fe centres in MIL-88B(Fe) and Fe-BDC-Br using the equations given by O'Keeffe et al. S9 This method uses the Fe-O bond lengths around each metal centre to obtain an approximate valence, with an error of about 14%. S9 The bond lengths, bond valences, and valence sums for each Fe atom are given in Tables S1-3. The BVS equations used are given below: vij is the valence of a bond between 2 atoms i and j Rij is the valence parameter for a bond between atoms i and j dij is the bond length between atoms i and j b is a universal constant equal to 0.37 Å S10 Vi is the valence of an atom i The Rij value for Fe-O = 1.745 Å S11  Symmetry codes: (i) x, -y+1/2, z; (ii) -x+1, y, z; (iii) -x+1, -y+1/2, z. Symmetry codes: (i) x, -y+1/2, z.

DFT Calculations
To understand the relative stability of MIL-88B(Fe) (acs) and MIL-88B(Fe) (snw), we performed density functional theory (DFT) calculations. To obtain a more accurate description of the Fe 2+ /Fe 3+ ions with unpaired electrons, we have used a hybrid DFT functional. All DFT calculations have been performed using the CP2K code (version 7.1), which uses a mixed Gaussian/plane-wave basis set. S12, S13 We employed double- polarization quality Gaussian basis sets S14 and a 400 Ry plane-wave cutoff for the auxiliary grid, in conjunction with the Goedecker-Teter-Hutter pseudopotentials. S15, S16 All DFT calculations were performed in the -point approximation with sufficiently large supercells. Total energy calculations and structural optimizations, including both atomic coordinates and cell parameters, were performed under periodic boundary conditions at the hybrid DFT level using the PBE0 exchange and correlation functional, S17, S18 which has 25% Hartree-Fock exchange (HFX), with Grimme's D3 van der Waals correction (PBE0+D3). S19 The HFX calculations were significantly accelerated by using the auxiliary density matrix method (ADMM) S20 and a truncated Coulomb potential, with which the HFX energy becomes zero beyond a pre-defined real-space cutoff radius. A convergence threshold of 1.0×10 -6 Hartree was used for the selfconsistent field cycle, and structural optimizations were considered to have converged when the maximum force on all atoms falls below 4.5 × 10 −4 Hartree/Bohr.