Joseph M. Falkowski

Isoreticular Chiral Metal-Organic Frameworks for Asymmetric Alkene Epoxidation: Tuning Catalytic Activity by Controlling Framework Catenation and Varying Open Channel Sizes

A family of isoreticular chiral metal-organic frameworks (CMOFs) of the primitive cubic network topology was constructed from [Zn(4)(μ(4)-O)(O(2)CR)(6)] secondary building units and systematically elongated dicarboxylate struts that are derived from chiral Mn-Salen catalytic subunits. CMOFs 1-5 were synthesized by directly incorporating three different chiral Mn-Salen struts into the frameworks under solvothermal conditions, and they were characterized by a variety of methods, including single-crystal X-ray diffraction, PXRD, TGA, and (1)H NMR. Although the CMOFs 1 vs 2 and CMOFs 3 vs 4 pairs were constructed from the same building blocks, they exhibit two-fold interpenetrated or non-interpenetrated structures, respectively, depending on the steric sizes of the solvents that were used to grow the MOF crystals. For CMOF-5, only a three-fold interpenetrated structure was obtained due to the extreme length of the Mn-Salen-derived dicarboxylate strut. The open channel and pore sizes of the CMOF series vary systematically, owing to the tunable dicarboxylate struts and controllable interpenetration patterns. CMOFs 1-5 were shown to be highly effective catalysts for asymmetric epoxidation of a variety of unfunctionalized olefins with up to 92% ee. The rates of epoxidation reactions strongly depend on the CMOF open channel sizes, and the catalytic activities of CMOFs 2 and 4 approach that of a homogeneous control catalyst. These results suggest that, although the diffusion of bulky alkene and oxidant reagents can be a rate-limiting factor in MOF-catalyzed asymmetric reactions, the catalytic activity of the CMOFs with large open channels (such as CMOFs 2 and 4 in the present study) is limited by the intrinsic reactivity of the catalytic molecular building blocks. The CMOF catalysts are recyclable and reusable and retain their framework structures after epoxidation reactions. This work highlights the potential of generating highly effective heterogeneous asymmetric catalysts via direct incorporation of well-defined homogeneous catalysts into framework structures of MOFs.

Privileged Phosphine-Based Metal–Organic Frameworks for Broad-Scope Asymmetric Catalysis

A robust and porous Zr metal-organic framework (MOF) based on a BINAP-derived dicarboxylate linker, BINAP-MOF, was synthesized and post-synthetically metalated with Ru and Rh complexes to afford highly enantioselective catalysts for important organic transformations. The Rh-functionalized MOF is not only highly enantioselective (up to >99% ee) but also 3 times as active as the homogeneous control. XAFS studies revealed that the Ru-functionalized MOF contains Ru-BINAP precatalysts with the same coordination environment as the homogeneous Ru complex. The post-synthetically metalated BINAP-MOFs provide a versatile family of single-site solid catalysts for catalyzing a broad scope of asymmetric organic transformations, including addition of aryl and alkyl groups to α,β-unsaturated ketones and hydrogenation of substituted alkene and carbonyl compounds.

Actuation of Asymmetric Cyclopropanation Catalysts: Reversible Single‐Crystal to Single‐Crystal Reduction of Metal–Organic Frameworks

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, chemical sensing, catalysis, biomedical imaging, and drug delivery. 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. 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)], MOFs containing metal/salen building blocks have attracted a great deal of recent interest. 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. 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 bridging ligands and [Zn4(m4-O)(O2CR)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. 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-based CMOFs were activated to form Ru-based MOF catalysts for the asymmetric cyclopropanation of styrene and other substituted alkenes with very high diastereoand 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(LMe2)(py)2] (py = pyridine), where L-Me2 is the methyl ester of (R,R)-( )-N,N’-(methyl-3-carboxyl-5-tert-butylsalicylidene)-1,2-cyclohexanediamine, was prepared by a metathesis reaction between the potassium salt of L-Me2 and [{RuCl2(pcymene)}2]. Saponification of [Ru(L-Me2)(py)2] and subsequent acidification with dilute hydrochloric acid and air oxidation resulted in the Ru/salen-derived dicarboxylic acid, [Ru(L-H2)(py)2]Cl. The diamagnetic complex [Ru(LMe2)(py)2] was characterized by H and C NMR spectroscopy, whereas the paramagnetic [Ru(L-H2)(py)2]Cl was characterized by MS and single-crystal X-ray diffraction (Figure 1a). Solvothermal reactions of [Ru(L-H2)(py)2]Cl with Zn(NO3)2·6 H2O 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 {Zn4(m4O)[(Ru(L-H2)(py)2Cl]3}·(DBF)7·(DEF)7, and the non-interpenetrated CMOF 2 with the formula {Zn4(m4-O)[(Ru(LH2)(py)2Cl]3}·(DEF)19·(DMF)5·(H2O)17 (Figure 1b). [15] 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 Zn4(m4-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 [Zn4(m4[*] J. M. Falkowski, C. Wang, S. Liu, Prof. W. Lin Department of Chemistry, CB#3290, University of North Carolina, Chapel Hill, NC 27599 (USA) E-mail: wlin@unc.edu Homepage: http://www.chem.unc.edu/people/faculty/linw/ wlindex.html

Heazlewoodite, Ni3S2: A Potent Catalyst for Oxygen Reduction to Water under Benign Conditions

Electrodeposited thin films and nanoparticles of Ni3S2 are highly active, poison- and corrosion-resistant catalysts for oxygen reduction to water at neutral pH. In pH 7 phosphate buffer, Ni3S2 displays catalytic onset at 0.8 V versus the reversible hydrogen electrode, a Tafel slope of 109 mV decade(-1), and high faradaic efficiency for four-electron reduction of O2 to water. Under these conditions, the activity and stability of Ni3S2 exceeds that of polycrystalline platinum and manganese, nickel, and cobalt oxides, illustrating the catalytic potential of pairing labile first-row transition metal active sites with a more covalent sulfide host lattice.

Autocatalytic O2 Cleavage by an OCO3– Trianionic Pincer CrIII Complex: Isolation and Characterization of the Autocatalytic Intermediate [CrIV]2(μ-O) Dimer

Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = [2,6-((t)BuC(6)H(3)O)(2)C(6)H(3)](3-), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4).

CHAPTER 11:Asymmetric Catalysis with Chiral Metal Organic Frameworks

Metal organic frameworks (MOFs) have been demonstrated in the last decade to be competent catalysts for asymmetric transformations. In a few years, asymmetric catalysts based on MOFs have rapidly evolved from simple Lewis acid catalysts to complex and robust systems that exhibit enhanced activity, unique selectivity, enzyme‐like behavior, and a high degree of recyclability. This chapter provides a view of the current state of the art in asymmetric catalysis by MOFs.