Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal-Organic Framework.

Significant advances have been made in the synthesis of chemically selective environments within metal-organic frameworks, yet materials development and industrial implementation have been hindered by the inability to predictively control crystallite size and shape. One common strategy to control crystal growth is the inclusion of coordination modulators, which are molecular species designed to compete with the linker for metal coordination during synthesis. However, these modulators can simultaneously alter the pH of the reaction solution, an effect that can also significantly influence crystal morphology. Herein, noncoordinating buffers are used to independently control reaction pH during metal-organic framework synthesis, enabling direct interrogation of the role of the coordinating species on crystal growth. We demonstrate the efficacy of this strategy in the synthesis of low-dispersity single-crystals of the framework Co2(dobdc) (dobdc4-= 2,5-dioxido-1,4-benzenedicarboxylate) in a pH 7-buffered solution using cobalt(II) acetate as the metal source. Density functional theory calculations reveal that acetate competitively binds to Co during crystallization, and by using a series of cobalt(II) salts with carboxylate anions of varying coordination strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography to show that crystal morphology has a direct impact on guest diffusional path length for the industrially important hydrocarbon m-xylene. Together, these results provide molecular-level insight into the use of modulators in governing crystallite morphology and a powerful strategy for the control of molecular diffusion rates within metal-organic frameworks.

Cooperative carbon capture and steam regeneration with tetraamine-appended metal-organic frameworks.

Natural gas has become the dominant source of electricity in the United States, and technologies capable of efficiently removing carbon dioxide (CO2) from the flue emissions of natural gas-fired power plants could reduce their carbon intensity. However, given the low partial pressure of CO2 in the flue stream, separation of CO2 is particularly challenging. Taking inspiration from the crystal structures of diamine-appended metal-organic frameworks exhibiting two-step cooperative CO2 adsorption, we report a family of robust tetraamine-functionalized frameworks that retain cooperativity, leading to the potential for exceptional efficiency in capturing CO2 under the extreme conditions relevant to natural gas flue emissions. The ordered, multimetal coordination of the tetraamines imparts the materials with extraordinary stability to adsorption-desorption cycling with simulated humid flue gas and enables regeneration using low-temperature steam in lieu of costly pressure or temperature swings.

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.

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,ß-unsaturated ketones and hydrogenation of substituted alkene and carbonyl compounds.

Chiral metal-organic frameworks with tunable open channels as single-site asymmetric cyclopropanation catalysts.

A pair of interpenetrated and non-interpenetrated chiral metal-organic frameworks with the same catalytic sites but different open channel sizes catalysed asymmetric cyclopropanation of substituted terminal alkenes with excellent diastereoselectivities (up to 9.6) and enantioselectivities (up to >99%).

Autocatalytic O2 cleavage by an OCO3 trianionic pincer Cr(III) complex: isolation and characterization of the autocatalytic intermediate [Cr(IV)]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).

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.

A series of isoreticular chiral metal-organic frameworks as a tunable platform for asymmetric catalysis.

Metal-organic frameworks, built by bridging metal ions with organic linkers, represent a new class of porous hybrid materials with attractive tunability in compositions, structures and functions. In particular, the mild conditions typically employed for their synthesis allow for the functionalization of their building blocks, and thus the rational design of novel materials. Here we demonstrate the systematic design of eight mesoporous chiral metal-organic frameworks, with the framework formula [LCu2(solvent)2] (where L is a chiral tetracarboxylate ligand derived from 1,1'-bi-2-naphthol), that have the same structures but channels of different sizes. Chiral Lewis acid catalysts were generated by postsynthesis functionalization with Ti(OiPr)4, and the resulting materials proved to be highly active asymmetric catalysts for diethylzinc and alkynylzinc additions, which converted aromatic aldehydes into chiral secondary alcohols. The enantioselectivities of these reactions can be modified by tuning the size of the channels, which alters the diffusion rates of the organic substrates.

Catalytic aerobic oxidation by a trianionic pincer Cr(III)/Cr(V) couple.

Aerobic oxidation that incorporates both O atoms into a substrate (PPh(3)) is achieved by employing a Cr(III)/Cr(V)[triple bond]O catalytic couple. A terphenyl trianionic pincer ligand stabilizes a high oxidation state Cr(V)[triple bond]O complex, and both the reduced (Cr(III), IR/X-ray) and oxidized (Cr(V)[triple bond]O, electron paramagnetic resonance/IR/X-ray) participants in the catalytic cycle are characterized.