Retrosynthesis of multi-component metal−organic frameworks

Crystal engineering of metal−organic frameworks (MOFs) has allowed the construction of complex structures at atomic precision, but has yet to reach the same level of sophistication as organic synthesis. The synthesis of complex MOFs with multiple organic and/or inorganic components is ultimately limited by the lack of control over framework assembly in one-pot reactions. Herein, we demonstrate that multi-component MOFs with unprecedented complexity can be constructed in a predictable and stepwise manner under simple kinetic guidance, which conceptually mimics the retrosynthetic approach utilized to construct complicated organic molecules. Four multi-component MOFs were synthesized by the subsequent incorporation of organic linkers and inorganic clusters into the cavity of a mesoporous MOF, each composed of up to three different metals and two different linkers. Furthermore, we demonstrated the utility of such a retrosynthetic design through the construction of a cooperative bimetallic catalytic system with two collaborative metal sites for three-component Strecker reactions.


Supplementary Methods
Materials and Instrument. All commercial chemicals were used without further purification unless otherwise mentioned. Powder X-ray diffraction (PXRD) was carried out with a BRUKER D8-Focus Bragg−Brentano X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178) at 40 kV and 40 mA. Single Crystal X-ray Diffraction (SC-XRD) was measured on a Bruker D8 Venture or D8 Quest diffractometer equipped with a Cu-Kα (λ = 1.54184 Å, graphite monochromated) or Mo-Kα sealed-tube X-ray source (λ = 0.71073 Å, graphite monochromated).
Elemental analyses for C, H and N were carried out on a German Elementary Vario EL III instrument. Thermogravimetric analyses (TGA) were conducted on a TGA-50 (SHIMADZU) thermogravimetric analyzer. Gas sorption measurements were conducted using a Micrometritics ASAP 2420 system at various temperatures. NMR data were collected on a Mercury 300 spectrometer. Ultraviolet-visible absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer. ICP-MS data were collected with a Perkin Elmer NexION 300D ICP-MS. Field-emission SEM images were collected on the FEI Quanta 600 field-emission SEM (America) at 20 kV.
Single Crystal X-ray Crystallography. All crystals were taken from the mother liquid without further treatment, transferred to oil and mounted into a loop for single crystal X-ray data collection. Diffraction was measured on a Bruker D8 Venture or D8 Quest diffractometer equipped with a Cu-Kα (λ = 1.54184 Å, graphite monochromated) or Mo-Kα sealed-tube X-ray source (λ = 0.71073 Å, graphite monochromated). The raw frame data were processed using SAINT and SADABS to yield the reflection data file. 1 The structure was solved using the charge-flipping algorithm, as implemented in the program SUPERFLIP 2 and refined by full-matrix least-squares techniques against Fo 2 using the SHELXL program 3 through the OLEX2 interface. 4 The hydrogen atoms from the linkers were placed geometrically and refined using a riding model. The refinement of the framework was performed by ignoring the contribution of the disordered solvent molecules. The region containing the disordered electron density was identified by considering the van der Waals radii of the atoms constituting the ordered framework. Platon SQUEEZE 5 was used and .fab file was created containing partial structure factors representing the SQUEEZE region. The appropriate partial structure factors were used for input to SHELXL with the ABIN instruction. The ABIN instruction reads h, k, l, A and B from the file name fab, where A and B are the real and imaginary components of a partial structure factor. 3 Refinement Details. For PCN201Cu, due to the weak diffractions at high Bragg angle, several atoms show unusual isotropic thermal parameters, thus several restraints were applied to ensure a reasonable refinement. In details, SIMU restraints were also used for organic ligands and partial metal center with large thermal motion (O2 C9 C10 C12 N2 C11 C1 C2 O1 C4 C3 C6 C5 C7 C8 N1 Ni1). FLAT was used to ensure the planarity of some part of ligand. DFIX was used to fix the C9-C10 to 1.55 Å, C10-C11 and C11-C12 to 1.35 Å. For PCN201Ni, due to the weak diffractions at high Bragg angle, several atoms show unusual isotropic thermal parameters, thus several restraints were applied to ensure a reasonable refinement. In details, SIMU restraints were also used for organic ligands with large thermal motion (O3 O4 O1 O2 C9 C10 C11 N2 C12 O1 C1 C2 C3 C4 C5 C6 C8 C7 N1). FLAT was used to ensure the planarity of some part of ligand (O2 C9 C10 C11 N2 C12). DFIX was used to fix the C12-C11, C0-C11 and N2-C12 to 1.35 Å, N2-C11 and C10-C12 to 2.35 Å, C9-C10 to 1.55 Å.
For PCN224INA, due to the weak diffractions at high Bragg angle, several atoms show unusual isotropic thermal parameters, thus several restraints were applied to ensure a reasonable refinement. In details, SIMU restraints were also used for organic ligands with large thermal motion (N2 C12 C11 C10 C9 O2). FLAT was used to ensure the planarity of some part of ligand (C10 N2 C12 C11). DFIX was used to fix the C12-C11, C10-C11, N2-C12 to 1.35 Å, N2-C11 and C10-C12 to 2.35 Å. Topology Analysis. To better understand the structure of PCN-202(Ni)-Hf, topology analyses were carried out using TOPOS 4.0 3 . The disordered DCDPS linker appears as a "tritopic" linker because of the 2-fold disorder. Topologically, the 12-connected metal clusters can be regarded as cuboctahedron nodes and tetratopic TCPP linkers can be viewed as square nodes. If the 2-fold disordered DCDPS linker is regarded as a 3-connected triangle node, the overall structure can be simplified into a 3,4,12,12 connected net with a point symbol of {4 24 .6 36 .8 6 }5{4 3 }12{4 4 .6 2 }6.
To eliminate the disorder of the DCDPS linker, we simulated an ordered structure of PCN-202(Ni)-Hf by reducing the space group from Im-3m to I-43m. The reduced space group with lower symmetry will eliminate the positional disorder of DCDPS linker by removing the mirror plan passing through its center. It should be noted that we attempted to refine the crystal structure of PCN-202(Ni)-Hf with a lower symmetry space group such as I-43m. However, the disorder is not eliminated, suggesting an inherent disorder of PCN-202(Ni)-Hf. In the simulated structure, each DCDPS linker is 2-connected to a Zr6 cluster and a Hf6 cluster. Therefore, each Zr6 cluster is 9 connected to 6 TCPP and 3 DCDPS respectively, while each Hf6 cluster is 12 connected to DCDPS linkers. Three pairs of DCDPS linkers bridges a pair of Zr6 and Hf6 cluster so that topologically they regarded as one edge. Consequently, Zr6 clusters are simplified into a 5-connected hexagonal pyramid nodes while Hf6 clusters are reduced into 4-connected tetrahedron nodes. The overall structure is simplified into a 4,4,7-connected net with point symbol of {4 4 .6 2 }6{4 6 .6 15 }4{6 6 }. Note that the topology of ordered structure is dependent on the space groups that are chosen to eliminate the disorder. Different topologies might result if other space groups are selected to simplify the structure. N2 Sorption Isotherm. Before gas sorption experiments, as-synthesized sample was washed with N,N-dimethylmethanamide (DMF) and immersed in acetone for 3 days, during which the solvent was decanted and freshly replenished three times. The solvent was removed under vacuum at 100 o C, yielding porous material. Gas sorption measurements were then conducted using a Micrometritics ASAP 2020 system. 1 H NMR spectroscopy. For 1 H NMR spectroscopy of digested MOF samples, the activated samples (around 5 mg) were dissolved with saturated K2CO3 aqueous solution (1 mL), neutralize by 10 M HCl aqueous solution, and dried in a 100 o C oven. The solid was then dissolved in about 0.5 mL DMSO-d6 for 1 H NMR analysis. For the catalytic reactions, the MOF catalyst was immediately separated from the reaction system after 10 min. About 100 µL of reaction supernatant was sampled, dissolved in 1 mL of dimethyl sulfoxide-d6 and then measured by 1 H NMR. It took about 20 min before the spectrum was recorded. SEM/EDX Analysis. Instrumental information of SEM/EDX: Images and analyses of