Cory Valente

Large-Pore Apertures in a Series of Metal-Organic Frameworks

Maximizing Molecular Pore Diameters Amorphous materials, such as activated carbon, can have pore diameters of several nanometers, but the synthesis of ordered structures with very large pore diameters is often thwarted by the creation of interpenetrating networks or difficulties in removing guest molecules. Deng et al. (p. 1018) avoided these problems in the synthesis of metal-organic frameworks (MOFs) with very large diameters (some exceeding 3 nanometers) by using a combination of short and very long linking groups. The compounds formed channels almost 10 nanometers in diameter that could be visualized by electron microscopy and that were large enough to accommodate protein molecules. Metal-organic frameworks with hexagonal channel pores up to almost 100 angstroms in diameter have been synthesized. We report a strategy to expand the pore aperture of metal-organic frameworks (MOFs) into a previously unattained size regime (>32 angstroms). Specifically, the systematic expansion of a well-known MOF structure, MOF-74, from its original link of one phenylene ring (I) to two, three, four, five, six, seven, nine, and eleven (II to XI, respectively), afforded an isoreticular series of MOF-74 structures (termed IRMOF-74-I to XI) with pore apertures ranging from 14 to 98 angstroms. All members of this series have noninterpenetrating structures and exhibit robust architectures, as evidenced by their permanent porosity and high thermal stability (up to 300°C). The pore apertures of an oligoethylene glycol–functionalized IRMOF-74-VII and IRMOF-74-IX are large enough for natural proteins to enter the pores.

The Development of Bulky Palladium NHC Complexes for the Most-Challenging Cross-Coupling Reactions

Palladium-catalyzed cross-coupling reactions enable organic chemists to form C-C bonds in targeted positions and under mild conditions. Although phosphine ligands have been intensively researched, in the search for even better cross-coupling catalysts attention has recently turned to the use of N-heterocyclic carbene (NHC) ligands, which form a strong bond to the palladium center. PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes. This Review shows the success of these species in Suzuki-Miyaura, Negishi, and Stille-Migita cross-couplings as well as in amination and sulfination reactions.

Pd‐Catalyzed Aryl Amination Mediated by Well Defined, N‐Heterocyclic Carbene (NHC)–Pd Precatalysts, PEPPSI

Pd-N-heterocyclic carbene (NHC)-catalyzed Buchwald-Hartwig amination protocols mediated by Pd-PEPPSI precatalysts is described. These protocols provide access to a range of hindered and functionalized drug-like aryl amines in high yield with both electron-deficient and electron-rich aryl- and heteroaryl chlorides and bromides. Variations in solvent polarity, base and temperature are tolerated, enhancing the scope and utility of this protocol. A mechanistic rationalization for base strength (pKb) requirements is also provided.

Metal–organic frameworks with designed chiral recognition sites

Linking struts containing Cram-like bisbinaphthyl[22]crown-6 with Zn(4)O(CO(2))(6) joints affords metal-organic frameworks with chiral recognition sites that are highly designed, ordered and placed in a precise manner throughout the entire crystal.

High yielding alkylations of unactivated sp3 and sp2 centres with alkyl-9-BBN reagents using an NHC-based catalyst: Pd-PEPPSI-IPr

High yielding, room temperature cross couplings of unactivated alkyl bromides and aryl bromides/chlorides with alkyl-9-BBN reagents has been achieved using an NHC-based catalyst (Pd-PEPPSI-IPr) via a general, functional-group tolerant and easily implemented protocol.

On the role of additives in alkyl–alkyl Negishi cross-couplings

An additives study for the alkyl-alkyl Negishi reaction using an NHC-Pd catalyst revealed that bromide salts promote coupling while the cation is mechanistically benign. A double titration revealed that the cross-coupling begins at a 1 : 1 ratio of LiBr : (n)BuZnBr, suggesting that a higher-order zincate, presumably Li(m)Zn((n)Bu)Br(3)((2-m)-), is the active transmetalating agent.

pH-Responsive mechanised nanoparticles gated by semirotaxanes

A [2]pseudorotaxane-based mechanised nanoparticle system, which operates within an aqueous acidic environment, has been prepared and characterised; this integrated system affords both water-soluble stalk and ring components in an effort to improve the biocompatibility of these promising new drug delivery vehicles.

Imprinting Chemical and Responsive Micropatterns into Metal–Organic Frameworks

Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals. Printing the MOFs with photochromic molecules and pH indicators generates stimuli-responsive micropatterns which change their appearance upon contact with specific chemicals (see picture), thus reporting the environmental “status” of the crystal.

Differentiating CBr and CCl Bond Activation by Using Solvent Polarity: Applications to Orthogonal Alkyl–Alkyl Negishi Reactions†

of two methods: A) fine-tuning the reaction conditions (i.e., temperature, catalyst, additives, etc.) for each reactive center or B) protecting group chemistry. Method A is commonly achieved by taking advantage of the differences in bond enthalpies (C I > C Br @ C Cl), and more recently has come to include the Caryl O bonds (i.e., aryl carboxylates, carbamates, carbonates, and sulfamates) that can be efficiently coupled in the presence of a Ni (but not Pd) catalyst, lending itself to orthogonal reaction strategies. Method B has been applied in the form of masked boronic acids, including pinacol esters, BF3K salts, [6] N-methyliminodiacetic acid (MIDA), and 1,8-diaminonaphthalene (dan)-borane derivatives. Strategies based on method B generally discount the possibility of one-pot reaction sequences due to the need for deprotection chemistry. However, bifunctional organodiborane linchpins possessing two of these boronic acid derivatives have been applied successfully in various orthogonal cross-coupling strategies to form aryl–aryl or aryl–vinyl motifs. Largely absent from the literature are examples of orthogonal alkyl–alkyl cross-couplings of two unactivated alkyl fragments; to our knowledge, only one example is known (Scheme 2, previous work). Kambe and co-workers showed in a single example that 1-bromo-6chlorohexane could undergo sequential Kumada–Tamao– Corriu cross-couplings using temperature as the orthogonal trigger in one-pot in the presence of a Cu catalyst. Examples of this type are rare as specially designed, highly active catalysts are typically required to couple unactivated alkyl fragments efficiently, with the trade-off that discrimination between Calkyl Br and Calkyl Cl bonds is now less chemoselective. For example, we have shown that NHC–Pd catalysts (NHC = N-heterocyclic carbene), generated in situ from either an imidazolium salt or a pre-catalyst, namely [Pd-PEPPSI-IPr] (1), can effectively couple both unactivated primary Calkyl Br and Calkyl Cl bonds in the Negishi reaction at room temperature. During the course of these studies, however, we discovered a unique property that allowed us to chemoselectively couple Calkyl Br bonds in the presence of Calkyl Cl bonds with alkylzinc reagents: that is, solvent polarity. This is a valuable attribute of this reaction, as it provides an opportunity for one-pot orthogonal alkyl–alkyl cross-couplings of bifunctional bromochloroalkanes by a solvent polarity “trigger” (Scheme 2). At its core, the chemoselectivity of these alkyl–alkyl Negishi cross-couplings depend on the ratio of dimethylimidazolidinone (DMI, e = 37.6) to tetrahydrofuran (THF, e = 7.5). Preliminarily, a competition experiment (Table 1) was designed to evaluate the chemoselectivity in the alkyl–alkyl Negishi reaction. The nBuZnBr for these studies was Scheme 1. General overview of orthogonal cross-coupling strategies.

Microcontact click printing for templating ultrathin films of metal-organic frameworks

The controlled growth of metal-organic frameworks (MOFs) over surfaces has been investigated using a variety of surface analytical techniques. The use of microcontact printing to prepare surfaces, patterned with regions capable of nucleating the growth of MOFs, has been explored by employing copper-catalyzed alkyne-azide cycloaddition (CuAAC) to pattern silicon wafers with carboxylic acids, a functional group that has been shown to nucleate the growth of MOFs on surfaces. Upon subjecting the patterned silicon surfaces to solvothermal conditions, MOF thin films were obtained and characterized subsequently by AFM, SEM, and grazing-incidence XRD (GIXRD). Large crystals (∼0.5 mm) have also been nucleated, as indicated by the presence of a bas-relief of the original pattern on one surface of the crystal, suggesting that it is possible to transfer the template surface pattern onto a single crystal of a MOF.