Utilization of a Mixed-Ligand Strategy to Tune the Properties of Cuboctahedral Porous Coordination Cages.

Ligand functionalization has been thoroughly leveraged to alter the properties of paddlewheel-based coordination cages where, in the case of ligand-terminated cages, functional groups are positioned on the periphery of synthesized cages. While these groups can be used to optimize solubility, porosity, crystal packing, thermal stability toward desolvation, reactivity, or optical activity, optimization of multiple properties can be challenging given their interconnected nature. For example, installation of functional groups to increase the solubility of porous cages typically has the effect of decreasing their porosity and stability toward thermal activation. Here we show that mixed-ligand cages can potentially address these issues as the benefits of various functional groups can be combined into one mixed-ligand cage. We further show that although ligand exchange reactions can be employed to obtain mixed ligand copper(II)-based cages, direct synthesis of mixed-ligand products is necessary for molybdenum(II) paddlewheel-based cages as these substitutionally inert clusters are resistant to ligand exchange. We ultimately show that highly soluble, highly porous, and thermally stable cuboctahedral cages are isolable by this strategy.

Elaboration of Porous Salts.

A large library of novel porous salts based on charged coordination cages was synthesized via straightforward salt metathesis reactions. For these, solutions of salts of oppositely charged coordination cages are mixed to precipitate MOF-like permanently porous products where metal identity, pore size, ligand functional groups, and surface area are highly tunable. For most of these materials, the constituent cages combine in the ratios expected based on their charge. Additional studies focused on the rate of salt metathesis or reaction stoichiometry as variables to tune particle size or product composition, respectively. It is expected that the design principles outlined here will be widely applicable for the synthesis of new porous salts based on a variety of charged porous molecular precursors.

Gas Storage in Porous Molecular Materials.

Molecules with permanent porosity in the solid state have been studied for decades. Porosity in these systems is governed by intrinsic pore space, as in cages or macrocycles, and extrinsic void space, created through loose, intermolecular solid-state packing. The development of permanently porous molecular materials, especially cages with organic or metal-organic composition, has seen increased interest over the past decade, and as such, incredibly high surface areas have been reported for these solids. Despite this, examples of these materials being explored for gas storage applications are relatively limited. This minireview outlines existing molecular systems that have been investigated for gas storage and highlights strategies that have been used to understand adsorption mechanisms in porous molecular materials.

Stabilizing Porosity in Organic Cages through Coordination Chemistry.

The number of studies concerning the permanent porosity of molecular materials, especially porous organic cages (POCs) and porous coordination cages (PCCs), have increased substantially over the past decade. The work presented here outlines novel approaches to the preparation of porous molecular structures upon metalation of nonporous, amine-based organic cages. Reduction of the well-known CC3 and CC1 imine-based POCs affords nonporous, highly flexible amine cages. These materials can be endowed with significant levels of structural rigidity via post-synthetic metalation of their ethylenediamine-type binding pockets. The hybrid metal-organic cages accessed through this approach combine aspects of POC and PCC chemistry, with structures of this type providing a potentially promising new direction for the design and development of porous molecular materials with tunability in overall charge, metal cation, porosity, and solubility.

Synthesis and Characterization of an Isoreticular Family of Calixarene-Capped Porous Coordination Cages.

Functionalization of permanently porous coordination cages has been used to tune phase, surface area, stability, and solubility in this promising class of adsorbents. For many cages, however, these properties are intricately tied together, and installation of functional groups, for example, to increase solubility often leads to a decrease in surface area. Calixarene-capped cages offer the advantage in that they are cluster-terminated cages whose solid-state packing, and thus surface area, is typically governed by the nature of the capping ligand rather than the bridging ligand. In this work we investigate the influence of ligand functionalization on two series of isoreticular Ni(II)- and Co(II)-based calixarene-capped cages. The two types of materials described are represented as octahedral and rectangular prismatic coordination cages and can be synthesized in a modular manner, allowing for the substitution of dicarboxylate bridging ligands and the introduction of functional groups in specific locations on the cage. We ultimately show that highly soluble cages can be obtained while still having access to high surface areas for many of the isolated phases.

Synthesis and Functionalization Reactivity of Fe-Thiocarbonyl and Thiocarbyne Complexes.

The remarkable catalytic transformation of CO to liquid hydrocarbons by Fe and Co catalysts in the industrial Fischer-Tropsch process motivates interest in developing well-defined systems to model aspects of this chemistry. One of the most interesting potential intermediates in this chemistry is a terminally-bound, first row metal carbide, yet a molecular model of this species remains elusive. With this in mind, we targeted the synthesis of highly-activated Fe-thiocarbonyl complexes, as prospective precursors to S-functionalization, C-S bond cleavage, and carbide generation. Herein, we describe the synthesis of three Fe-CS complexes that can be alkylated to generate a series of terminal Fe-carbynes that could be characterized across three oxidation states. Strategies to access C-S bond scission from these species are discussed, including limitations that, thus far, have precluded the generation of a terminal Fe-carbide for this system.

Dihydrogen Adduct (Co-H2 ) Complexes Displaying H-Atom and Hydride Transfer.

The prototypical reactivity profiles of transition metal dihydrogen complexes (M-H2 ) are well-characterized with respect to oxidative addition (to afford dihydrides, M(H)2 ) and as acids, heterolytically delivering H+ to a base and H- to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H-atom or hydride transfer from the σ-H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S= 1 / 2 and an anionic S=0 Co-H2 adduct, both supported by a trisphosphine borane ligand (P3 B ). The thermally stable metalloradical, (P3 B )Co(H2 ), serves as a competent precursor for hydrogen atom transfer to t Bu3 ArO⋅ . What is more, its anionic derivative, the dihydrogen complex [(P3 B )Co(H2 )]1- , is a competent precursor for hydride transfer to BEt3 , establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M-H2 complexes known.

Inversion of Configuration at the Phosphorus Nucleophile in the Diastereoselective and Enantioselective Synthesis of P-Stereogenic syn-Phosphiranes from Chiral Epoxides.

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (SN 2) or racemization (SN 1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P-stereogenic syn-phosphiranes. DFT studies suggested that the novel stereochemistry results from acid-promoted tosylate dissociation to yield an intermediate phosphenium-bridged cation, which undergoes syn-selective cyclization.

CO Reduction to CH3OSiMe3: Electrophile-Promoted Hydride Migration at a Single Fe Site.

One of the major challenges associated with developing molecular Fischer-Tropsch catalysts is the design of systems that promote the formation of C-H bonds from H2 and CO while also facilitating the release of the resulting CO-derived organic products. To this end, we describe the synthesis of reduced iron-hydride/carbonyl complexes that enable an electrophile-promoted hydride migration process, resulting in the reduction of coordinated CO to a siloxymethyl (LnFe-CH2OSiMe3) group. Intramolecular hydride-to-CO migrations are extremely rare, and to our knowledge the system described herein is the first example where such a process can be accessed from a thermally stable M(CO)(H) complex. Further addition of H2 to LnFe-CH2OSiMe3 releases CH3OSiMe3, demonstrating net four-electron reduction of CO to CH3OSiMe3 at a single Fe site.

Synthesis, characterization, and polymerization of capped paddlewheel porous cages.

Although paddlewheel-based structures are common among permanently porous metal-organic materials, suitable strategies for the isolation of metal node-terminated, capped paddlewheel-based cage structures remain limited. We explored the use of chelating dicarboxylate ligand derivates (esp) for the isolation of trimesate-linked cages, Mo12(btc)4(esp)6, that are structural analogs of the small octahedral pore of HKUST-1. The porosity of these novel cages is appreciably higher than that of previously reported structures of this type. We also demonstrate that pillaring the isolated cage with DABCO generated an amorphous polymer that featured exceptional thermal stability and enhanced porosity.

Structure and redox tuning of gas adsorption properties in calixarene-supported Fe(ii)-based porous cages.

We describe the synthesis of Fe(ii)-based octahedral coordination cages supported by calixarene capping ligands. The most porous of these molecular cages has an argon accessible BET surface area of 898 m2 g-1 (1497 m2 g-1 Langmuir). The modular synthesis of molecular cages allows for straightforward substitution of both the bridging carboxylic acid ligands and the calixarene caps to tune material properties. In this context, the adsorption enthalpies of C2/C3 hydrocarbons ranged from -24 to -46 kJ mol-1 at low coverage, where facile structural modifications substantially influence hydrocarbon uptakes. These materials exhibit remarkable stability toward oxidation or decomposition in the presence of air and moisture, but application of a suitable chemical oxidant generates oxidized cages over a controlled range of redox states. This provides an additional handle for tuning the porosity and stability of the Fe cages.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials.

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm3/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.

O-Functionalization of a cobalt carbonyl generates a terminal cobalt carbyne.

Despite efforts toward extending multiple bonding motifs to late metal systems, examples of late transition metal carbynes remain scarce. Herein, we describe the synthesis of a series of L3Co(CO) complexes supported by a trisphosphine ligand framework, with the most reduced of these complexes being amenable to O-functionalization. This transformation provides access to the second reported example of a terminal Co-carbyne complex, in this case stabilized in a pseudotetrahedral geometry (i.e., L3Co[triple bond, length as m-dash]C-OSiR3). Its geometry makes its electronic structure suitable for comparison to structurally-related examples of terminal Co-imido and oxo species.

Electrophile-promoted Fe-to-N2 hydride migration in highly reduced Fe(N2)(H) complexes.

One of the emerging challenges associated with developing robust synthetic nitrogen fixation catalysts is the competitive formation of hydride species that can play a role in catalyst deactivation or lead to undesired hydrogen evolution reactivity (HER). It is hence desirable to devise synthetic systems where metal hydrides can migrate directly to coordinated N2 in reductive N-H bond-forming steps, thereby enabling productive incorporation into desired reduced N2-products. Relevant examples of this type of reactivity in synthetic model systems are limited. In this manuscript we describe the migration of an iron hydride (Fe-H) to Nα of a disilylhydrazido(2-) ligand (Fe[double bond, length as m-dash]NNR2) derived from N2 via double-silylation in a preceding step. This is an uncommon reactivity pattern in general; well-characterized examples of hydride/alkyl migrations to metal heteroatom bonds (e.g., (R)M[double bond, length as m-dash]NR' → M-N(R)R') are very rare. Mechanistic data establish the Fe-to-Nα hydride migration to be intramolecular. The resulting disilylhydrazido(1-) intermediate can be isolated by trapping with CN t Bu, and the disilylhydrazine product can then be liberated upon treatment with an additional acid equivalent, demonstrating the net incorporation of an Fe-H equivalent into an N-fixed product.