Aaron B. League

Sintering-resistant Single-Site Nickel Catalyst Supported by Metal-Organic Framework

Developing supported single-site catalysts is an important goal in heterogeneous catalysis since the well-defined active sites afford opportunities for detailed mechanistic studies, thereby facilitating the design of improved catalysts. We present herein a method for installing Ni ions uniformly and precisely on the node of a Zr-based metal-organic framework (MOF), NU-1000, in high density and large quantity (denoted as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM is demonstrated to be an efficient gas-phase hydrogenation catalyst upon activation. The structure of the active sites in Ni-AIM is proposed, revealing its single-site nature. More importantly, due to the organic linker used to construct the MOF support, the Ni ions stay isolated throughout the hydrogenation catalysis, in accord with its long-term stability. A quantum chemical characterization of the catalyst and the catalytic process complements the experimental results. With validation of computational modeling protocols, we further targeted ethylene oligomerization catalysis by Ni-AIM guided by theoretical prediction. Given the generality of the AIM methodology, this emerging class of materials should prove ripe for the discovery of new catalysts for the transformation of volatile substrates.

A Hafnium-Based Metal–Organic Framework as a Nature-Inspired Tandem Reaction Catalyst

Tandem catalytic systems, often inspired by biological systems, offer many advantages in the formation of highly functionalized small molecules. Herein, a new metal-organic framework (MOF) with porphyrinic struts and Hf6 nodes is reported. This MOF demonstrates catalytic efficacy in the tandem oxidation and functionalization of styrene utilizing molecular oxygen as a terminal oxidant. The product, a protected 1,2-aminoalcohol, is formed selectively and with high efficiency using this recyclable heterogeneous catalyst. Significantly, the unusual regioselective transformation occurs only when an Fe-decorated Hf6 node and the Fe-porphyrin strut work in concert. This report is an example of concurrent orthogonal tandem catalysis.

Pyrene-Edged FeII4L6 Cages Adaptively Reconfigure During Guest Binding

Differential guest-binding behavior was observed between two pyrene-edged Fe4L6 cages, prepared from isomeric bis(4-aminophenyl)pyrene derivatives, 2-formylpyridine and iron(II). The cage based on a 1,6-pyrene scaffold possesses an enclosed cavity suitable for the encapsulation of large hydrophobic guests including fullerenes, polycyclic aromatic hydrocarbons, and large, structurally complex natural products such as steroids. Addition of the fullerenes C60 and C70 to the cage brought about a re-equilibration among the different cage diastereomers in order to maximize the binding affinity of the system. Density functional theory was employed to rationalize the experimentally observed energy differences for C60 binding within the cage diastereomers. In contrast, the cage isomer based on a 2,7-pyrene scaffold has a more porous cavity and did not show affinity for neutral hydrophobic guests.

Empirical and Theoretical Insights into the Structural Features and Host–Guest Chemistry of M8L4 Tube Architectures

We demonstrate a general method for the construction of M8L4 tubular complexes via subcomponent self-assembly, starting from Cu(I) or Ag(I) precursors together with suitable elongated tetraamine and 2-formylpyridine subcomponents. The tubular architectures were often observed as equilibrium mixtures of diastereomers having two different point symmetries (D2d or D2 ⇄ D4) in solution. The equilibria between diastereomers were influenced through variation in ligand length, substituents, metal ion identity, counteranion, and temperature. In the presence of dicyanoaurate(I) and Au(I), the D4-symmetric hosts were able to bind linear Au(Au(CN)2)2(-) (with two different configurations) as the best-fitting guest. Substitution of dicyanoargentate(I) for dicyanoaurate(I) resulted in the formation of Ag(Au(CN)2)2(-) as the optimal guest through transmetalation. Density functional theory was employed to elucidate the host-guest chemistries of the tubes.

Bridging Zirconia Nodes within a Metal–Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires

Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis, and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield heterobimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering resistance of these clusters during the hydrogenation of light olefins.

Water oxidation catalysis with ligand substituted Ru–bpp type complexes

A series of symmetric and non-symmetric dinuclear Ru complexes of general formula {[Ru(R2-trpy)(H2O)][Ru(R3-trpy)(H2O)](μ-R1-bpp)}3+ where trpy is 2,2′:6′,2′′-terpyridine, bpp− is 3,5-bis(2-pyridyl)-pyrazolate and R1, R2 and R3 are electron donating (ED) and electron withdrawing (EW) groups such as Me, MeO, NH2 and NO2 have been prepared using microwave assisted techniques. These complexes have been thoroughly characterized by means of analytical (elemental analysis), spectroscopic (UV-vis, NMR) and electrochemical (CV, SQWV, CPE) techniques. The single crystal X-ray structures for one acetate- and one chloro-bridged precursor have also been solved. Kinetic analysis monitored by UV-vis spectroscopy reveals the electronic effects exerted by the ED and EW groups on the substitution kinetics and stoichiometric water oxidation reaction. The catalytic water oxidation activity is evaluated by means of chemically (CeIV), electrochemically, and photochemically induced processes. It is found that, in general, ED groups do not strongly affect the catalytic rates whereas EW groups drastically reduce catalytic rates. Finally, DFT calculations provide a general and experimentally consistent view of the different water oxidation pathways that can operate in the water oxidation reactions catalyzed by these complexes.

Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LXZn catalysts (LX = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO2, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (Pm = 0.72-0.78) polylactide (PLA) with low dispersities: Đ = 1.06-1.17. In contrast to previous work with aluminum catalysts for which electronic effects of ligand substituents were significant (Hammett ρ = +1.2-1.4), the LXZn systems exhibited much less of an effect (ρ = +0.3). Density functional calculations revealed details of the initiation and propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.

Correction to “A Hafnium-Based Metal–Organic Framework as a Nature-Inspired Tandem Reaction Catalyst”

Supporting Information, CIF file. Upon deposition to the Cambridge Structural Database (CSD), it was noted that the hydrogen atoms on the porphyrin ring of the structure were modeled incorrectly. The appropriate changes have been made, and the structure has been refined to convergence. This does not affect the chemistry reported in the publication but has small effects in the final refinement values, and a new CIF file is provided. The corrected data have also been deposited to the CSD.