Over the past decade, metal-organic frameworks (MOFs) have provided an excellent platform for engineering functional materials through judicious choices of the constituent building blocks. Numerous MOFs have been synthesized, and some of them have been explored for potential applications such as gas storage, [1] chemical sensing, [2] catalysis, [3] biomedical imaging, [4] and drug delivery. [5] Catalytic MOFs having imbedded, well-defined active sites are of particular interest owing to their utility as recyclable and reusable catalysts. Because of their highly ordered and typically crystalline structures, MOF catalysts can in principle be characterized by X-ray diffraction methods to provide precise structural information on the catalytic active sites, thus allowing the delineation of catalyst structure-function relationships. Herein we report the first observation of the actuation of a MOF catalyst through a reversible single-crystal to singlecrystal reduction process.
Among many strategies for synthesizing catalytic MOFs, direct incorporation of catalytically competent building blocks into the MOF frameworks has recently emerged as a powerful approach toward building highly active and selective solid catalysts. [3, 7] Motivated by excellent asymmetric catalytic activities exhibited by many homogeneous metal/salen complexes [where an archetypical chiral salen ligand is (R,R)-1,2cyclohexanediamino-N,N'-bis(3-tert-butyl-salicylidene)], [8] MOFs containing metal/salen building blocks have attracted a great deal of recent interest. [9] Whereas some of the chiral metal/salen-based MOFs have shown promise in chiral recognition and separation, two manganese/salen-derived MOF systems have recently been shown to be excellent asymmetric alkene epoxidation catalysts. [11] In this work, a pair of interpenetrated and non-interpenetrated chiral MOFs (CMOFs) of the primitive cubic unit (pcu) topology were constructed from redox active ruthenium/salen-based bridg-ing ligands and [Zn 4 (m 4 -O)(O 2 CR) 6 ] secondary building units (SBUs). These CMOFs showed the first example of reversible single-crystal to single-crystal reduction/reoxidation behavior. Although a few examples of single-crystal to single-crystal oxidation of MOFs were reported, none of these redox reactions were demonstrated to be reversible. [12] In contrast, the reduction of a MOF was recently elucidated by a Rietveld analysis of powder X-ray diffraction data. We report here that upon single-crystal to single-crystal reduction, catalytically inactive Ru III -based CMOFs were activated to form Ru II -based MOF catalysts for the asymmetric cyclopropanation of styrene and other substituted alkenes with very high diastereo-and enantioselectivities (d.r. = 7:1 and ee = 91 %). The catalytic activity of the CMOFs is catenation dependent: the non-interpenetrated CMOF is highly active whereas the interpenetrated CMOF is nearly inactive. We also show that the CMOFs maintain their crystallinity, and less than 0.01 % of the ruthenium/salen catalyst leached into the solution after the catalytic reaction.
The enantiopure ruthenium(II) complex, [Ru(L-Me 2 )(py) 2 ] (py = pyridine), where L-Me 2 is the methyl ester of (R,R)-(À)-N,N'-(methyl-3-carboxyl-5-tert-butylsalicylidene)-1,2-cyclohexanediamine, was prepared by a metathesis reaction between the potassium salt of L-Me 2 and [{RuCl 2 (pcymene)} 2 ]. Saponification of [Ru(L-Me 2 )(py) 2 ] and subsequent acidification with dilute hydrochloric acid and air oxidation resulted in the Ru III /salen-derived dicarboxylic acid, [Ru(L-H 2 )(py) 2 ]Cl. The diamagnetic complex [Ru(L-Me 2 )(py) 2 ] was characterized by 1 H and 13 C NMR spectroscopy, whereas the paramagnetic [Ru(L-H 2 )(py) 2 ]Cl was characterized by MS and single-crystal X-ray diffraction (Figure ). Solvothermal reactions of [Ru(L-H 2 )(py) 2 ]Cl with Zn-(NO 3 ) 2 •6 H 2 O in DBF/DEF/EtOH (DBF = dibutylformamide, DEF = N,N-diethylformamide) or in DEF/DMF/ EtOH (DMF = N,N-dimethylformamide) at 80 8C afforded, after 36 hours, dark-green, cuboid single crystals of the twofold interpenetrated CMOF 1 with the formula {Zn 4 (m 4 -O)[(Ru(L-H 2 )(py) 2 Cl] 3 }•(DBF) 7 •(DEF) 7 , and the non-interpenetrated CMOF 2 with the formula {Zn 4 (m 4 -O)[(Ru(L-H 2 )(py) 2 Cl] 3 }•(DEF) 19 •(DMF) 5 •(H 2 O) 17 (Figure 1 b). Compound 1 crystallizes in the R32 space group, with the asymmetric unit containing two [Ru(L)(py) 2 ]Cl ligands and two-thirds of a Zn 4 (m 4 -O) cluster composed of two Zn atoms of full occupancy and two Zn atoms of one-third occupancy, as well as two O atoms of one-third occupancy. As expected, the carboxylate groups from six adjacent [Ru(L)(py) 2 ]Cl ligands coordinate to the four Zn centers to form [Zn 4 (m 4 -