Chuan-De Wu

Single-Crystal to Single-Crystal Cross-Linking of an Interpenetrating Chiral Metal–Organic Framework and Implications in Asymmetric Catalysis†

Metal–organic frameworks (MOFs) have received extensive interest in the past decade because of their interesting properties. MOFs exhibit exceptional gas-uptake capacity owing to their extreme porosity. The ability to incorporate desired functional groups allows MOFs to perform in a variety of applications, such as catalysis, chemical sensing, and drug delivery. In particular, MOFs represent ideal candidates as heterogeneous catalysts by simultaneously imparting porosity and introducing catalytic sites into MOFs. Unlike other catalyst heterogenization strategies, the MOF-derived heterogeneous catalysts can remain singlecrystalline, thus providing a unique opportunity for detailed structural interrogation and therefore delineating the relationships between the MOF structures and their catalytic activities and selectivities. Moderate hydrolytic and thermal stabilities of most MOFs limit the scope of reactions that can be heterogeneously catalyzed by MOFs. We have recently focused our efforts on the design of chiral porous MOFs for heterogeneous asymmetric catalysis as most asymmetric catalytic reactions are carried out under mild conditions in aprotic solvents. In order for chiral MOFs to be useful asymmetric catalysts, they must possess large nanometer-scale open channels for the facile transport of sterically demanding substrates and products. We have recently synthesized isoreticular chiral MOFs based on tetracarboxylate bridging ligands derived from 1,1’-bi-2-naphthol (BINOL) and copper paddle-wheel secondary building units (SBUs), and observed the remarkable dependence of the enantioselectivities of the addition of Et2Zn to aromatic aldehydes on the MOF openchannel sizes. In order to expand the scope of applications of such chiral tetracarboxylate ligands, and to further understand the relationships between framework structures and catalytic activities, we have used these ligands in combination with other metal-connecting points or metal-cluster SBUs. Herein we report the synthesis and characterization of two interpenetrating chiral MOFs [Zn2(L)(dmf)(H2O)]·2EtOH·4.3DMF·H2O (1, where L is (R)-2,2’-diethoxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate) and [Zn2(L’)(dmf)(H2O)]·2EtOH·4.3DMF·4 H2O (2, L’= (R)-2,2’-dihydroxy-1,1’-binaphthyl-4,4’,6,6’-tetrabenzoate), which are constructed from dizinc SBUs and chiral tetracarboxylate ligands. More importantly, we observed unprecedented single-crystal to single-crystal crosslinking of the two interpenetrating networks in 2 by Ti(OiPr)4 to lead to intermolecular [Ti(BINOLate)2] complexes that exhibit modest enantioselectivity in catalyzing the addition of diethylzinc to aromatic aldehydes to afford chiral secondary alcohols. This result provides unambiguous structural identification of an immobilized homogeneous catalyst and has significant implications in rational design of MOF-based heterogeneous asymmetric catalysts. MOF 1 was synthesized by heating a mixture of ZnI2 and (R)-H4L in DMF/EtOH at 90 8C for five days, while MOF 2 was produced by heating ZnI2 and (R)-H4L’ in DMF/EtOH at 100 8C for one week (Scheme 1). The formulae of 1 and 2 were established by single-crystal X-ray diffraction studies, NMR analysis, and thermogravimetric analysis (TGA).

A chiral porous 3D metal–organic framework with an unprecedented 4-connected network topology

A novel homochiral 3D metal-organic framework [CdL2(H2O)2][ClO4]2.2DMF.3EtOH.5/3H2O, 1, (L =(R)-6,6-dichloro-2,2-diethoxy-1,1-binaphthyl-4,4-bipyridine) exhibits an unprecedented 4-connected network topology owing to the cis- configuration of the Cd coordination and possesses permanent porosity as demonstrated by TGA, XRPD, and CO2 adsorption isotherm studies.

Homochiral porous solids based on 1D coordination polymers built from 46-membered macrocycles

Six homochiral coordination polymers 1-6 based on an enantiopure elongated and bent bipyridine ligand were synthesized and characterized by single-crystal X-ray diffraction studies. The framework structures of all six compounds were built up from similar 1D polymeric chains composed of 46-membered metallomacrocycles. Four distinct packing patterns were observed for this family of coordination polymers. With the exception of 1, the anions do not coordinate to the metal centers and reside in the open channels. Single-crystal X-ray diffraction studies show that the structures of these coordination polymers are sensitive to the anions even though they do not coordinate to the metal centers. The framework structures are somewhat tolerant of the change of metal centers and their local coordination environments. Gas sorption measurements on 1 suggest that chiral porous solids can be obtained with the present 1D coordination polymeric building blocks.