Chiral MOFs

Chiral MOFs have to cope with two kinds of challenges when they are used as catalysts. First, in those cases where chirality is induced via sp carbon centers, the incorporation of such centers may confer a degree of flexibility to the framework that eventually impairs the stability after desolvation. Such problems can be circumvented, for instance, using mixed ligand systems where a rigid nonchiral backbone ensures the stability of the structure [82], MOFs with large and flexible chiral ligands... 

There are a couple of reports from Pal and coworkers on chiral MOFs capable of enantiospecific inclusion of chiral rotamers [79,80], Although this type of work is worthwhile from a theoretical point of view, in practical sense the desorbed rotamer will racemize immediately through rotation around the C-C bond, making isolation of optically pure rotamers impossible. 

The same group also reported that when cadmium nitrate or perchlorate salts were used in the preparation of the MOF, two different MOF structures can be obtained with the same chiral binaphthyl ligand [130]. This diversity in crystal structures seemed to arise from the participation of the anion accompanying Cd " " in the structure. The chiral MOF prepared from nitrate acts as an efficient heterogeneous catalyst for the room-temperature asymmetric addition of diethylzinc to a series of aromatic aldehydes, with ee values up to 90% at 100% substrate conversion. Conversely, the MOFs derived from the perchlorate salt were inactive under the same conditions. 

MIL-101, and HKUST-1, were synthesized by this method. Ion thermal synthesis refers to the use of ionic liquids as reaction solvents to synthesize MOFs. Several MOFs have been synthesized by using ionic liquids as solvents. For example, Lin et al. [39] reported a chiral MOF, which was synthesized by employing the chiral ionic liquid. 

MOFs have also been widely explored in catalysis, starting from the earlier state of MOF development [21]. For example, in 2000, Seo et al. [54] reported homo-chiral MOF, POST-1 (Zn3(tL3-0)(L3i-H)6), which exhibited an interesting catalytic activity for an asymmetric chemical reaction based on the special organic groups on the pore surface of the MOF. Following that, the asymmetric catalysis promoted by metal ions on a MOF framework was reported in 2001 by Lin s group [55]. 

Some other naturally chiral carboxylate ligands were also selected to construct chiral MOFs. For example, assemblies of Zn " and Cd " with o-camphoric acid in combination with some semi-rigid N-donor auxiliary ligands, such as 1,3- and l,4-bis(imidazol-l-ylmethyl)benzene and 4,4 -dipyridylsulfide, result in the formation of chiral MOFs [M(l,3-bimb)(D-ca)] (M = Zn, Cd), [Zn(l,4-bimb)(D-ca)] and a noncentrosymmetric MOF [Cd2(dpys)(D-ca)2(H20)2] H2O , in which the ID Zn (Cd )/D-ca chains are linked by N-donor ligands to form either a 2D network or a ladder-like structure [21]. Preliminary experimental results indicate that they exhibit SHG intensities ca. 0.3, 0.1, 0.3, and 0.8 times, respectively, as large as that of urea. In fact, many MOFs with nonaromatic chiral ligands are... 

Lin and coworkers reported the chiral MOF [Cd3Cl6(pybinol)3] where pybinol is the pyridine-decorated chiral binol ligand (/ )-6,6 -dichloro-2,2 -dihydroxy-1,1 -binaphthyl-4,4 -bipyridine shown in Chart 3. The... 

Spherical shape and narrow particle size distribution are the two key factors for a good HPLC stationary phase. Very recently, Zhang et al. present a chiral MOF with an average particle size of 5 pm for the HPLC separation of alcohol, ketone, flavone, phenol, base, and amide racemates. Ten racemates were well-separated on a 25-cm long MOF column with excellent selectivity. The stereoselectivity likely resulted from the interaction of the analytes with the inner pore space of MOF, which has the most appropriate size and steric fit. Besides, the dispersion, dipole-dipole, and hydrogen-bonding forces which come from the mobile phase may also play significant roles in chiral separation. These results show that chiral MOFs are practicable for HPLC enantioseparation. 

Xue et al. reported a new 3D chiral MOF [67], using a flexible dicarboxylate ligand, which exhibits an unprecedented, temperature-induced SC-SC transformation involving release of bridging aqua molecules and hehc-ity conversion of the hehcal chains in two directions within the MOF (Figure 65). 

This approach is exemplified by the protonation of a chiral MOF and its use in the enantioselective methanolysis of d5-2,3-epoxybutane [100] (Reaction 4) ... 

Nevertheless, photoactive MOFs also show unique photocatalytic properties that other materials cannot compete with, especially in organic synthesis applications. MOFs create the opportunity to combine photocatalyst with organocatalyst. One example is the chiral MOF, namely, Zn-PYIl, which exhibits high selectivity for photocatalytic asymmetric a-alkylation of aldehydes, as demonstrated in Fig. 4.13c. The Zn-PYll has also been synthesised via a PSM process of the parent MOF Zn-BClPl (top of Fig. 4.13c), which has been synthesised via solvothermal reaction from L-N-tert-butoxycarbonyl-2-(imidazole)-l-pyrrolidine (l-BCIP) [58]. The key point of the PSM process is the removal of the protective tert-butoxycarbonyl (Boc) moiety to expose active sites, which are likely to be the N — H of pyrrolidine of the L-BCIP molecules that is located within the channels according to dye adsorption test. This has been realised by microwave irradiation in dry lV,lV-dimethyl-formamide solution. The activated Zn-PYIl shows a high reaction efficiency (74 % in yield) and excellent enantioselectivity (92 % ee) in photocatalytic a-alkylation of aliphatic aldehydes compared to that of other MOFs. 

Scheme 10.6 Chemical structures of chiral ligands or chiral catalysts (Lj-Ljj) incorporated in to chiral MOFs through either direct incorporation or post-synthesis modification for asymmetric catalysis.

Post-synthetic modification can also be used for the synthesis of chiral MOFs via introducing chiral catalysts into the open coordination site of metal nodes of achiral MOFs [4S]. After the removal of the coordinated water molecules, two... 

Scheme 10.7 Schematic representation of the formation process of the photoactive chiral MOF and Zn-PYI. Reprinted with permission from Ref. [47]. Copyright 2012 American...

Using the post-synthetic modification method, aspartic add (L2 Scheme 10.6) [49] and chiral proline (L5 in Scheme 10.6) [50] were incorporated into MOFs. These chiral MOFs exhibited low-to-moderate enantioselectivity in the asymmetric catalysis, such as the methanolysis of ds-2,3-epoxybutane and asymmetric aldol reactions. 

Recently, Cui and coworkers [101] reported that Co(Salen) (L25 in Scheme 10.6) incorporated in chiral MOFs also could go through bimolecular reaction pathways for the HKR of racemic epoxides with up to 99.5% ee. Crystal structure analysis suggests that the MOP structure brought Co(Salen) units into a highly dense arrangement and close proximity which could enhance the bimetallic cooperative interactions. The same bimolecular activation process in Co(Salen)-based MOP has also been found by lin and coworkers [102]. 

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