College of Chemistry

The dehydrogenation of alkanes to alkenes is an appealing strategy for upgrading abundant hydrocarbons, yet it is constrained by the inherent challenge of cleaving two inert C(sp³)-H bonds with selectivity and without overoxidation. We report a cooperative photocatalytic dehydrogenation of unactivated cycloalkanes under visible light irradiation enabled by a new dinuclear uranyl complex supported by an oxidatively stable germanotungstate, [NBu ⁿ ₄]₈[(UO₂)₂(GeW₁₀O₃₄(µ₂-OH)₂)₂]·(CH₃)₂CO (1). The uranyl complex catalytically converts cyclooctane to cyclooctene under ambient conditions with a TON per molecule of 44 and 9,10-dihydrophenanthrene to phenanthrene with a TON of 73 per molecule, using 1 mol% 1 in MeCN solution, under 427 nm irradiation, using [S₂O₈]^2- or chloranil (C₆Cl₄O₂) as a sacrificial oxidant. The value of preorganization of two uranyl centers to yield the product of double hydrogen atom abstraction (HAA) is discussed in comparison with uranyl nitrate, [UO₂(NO₃)₂(H₂O)₂]·4H₂O, the most widely studied uranyl photocatalyst, which is inactive for this reaction. A direct hydrogen atom transfer (d-HAT) mechanism with two HAA processes is proposed as trans-di-deuterated substrate (9S, 10S)-9,10-dihydrophenanthrene-9,10-d₂ (C₁₄H₁₀D₂) exclusively forms d₁ phenanthrene from abstraction of an H and a D atom from the same face.

Solute-intercalation-induced phase separation creates spatial heterogeneities in host materials, a phenomenon ubiquitous in batteries, hydrogen storage, and other energy devices. Despite many efforts, probing intercalation processes at the atomic scale has been a significant challenge. By utilizing liquid-phase transmission electron microscopy (TEM), we study hydrogen (de)intercalation in palladium nanocrystals as a model system and have achieved unprecedented atomic-resolution imaging of hydrogen intercalation wave dynamics. Our observations reveal that intercalation wave mechanisms, instead of shrinking-core mechanisms, prevail at ambient temperature for palladium nanocubes ranging from ∼60 nm down to ∼10 nm. Systematic image analysis uncovers the atomic evolution of the hydrogen intercalation wave, transitioning from nonplanar and inclined boundaries to those closely aligned with {100} planes. Our kinetic Monte Carlo simulations demonstrate that the observed intercalation wave dynamics correspond to sorption pathways minimizing the lattice mismatch strain at the phase boundary. Unveiling the atomic intercalation pathways holds profound implications for engineering intercalation-mediated devices and advancements in energy sciences.

Metal-organic frameworks with coordinatively unsaturated metal sites (open metal sites) capable of engaging in orbital interactions with π-acidic gases are of interest for enabling ambient-temperature gas separations, such as hydrogen isotope separations. In view of the weakly π-acidic nature of H₂, we sought to strengthen π-backbonding-mediated H₂ adsorption through pore confinement effects. Toward that end, we synthesized and characterized the ultramicroporous metal-organic framework Cu_xZn_5-xCl_4-yH_z(bbta)₃ (Cu^IZn-MFU-4; H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole), featuring π-basic trigonal pyramidal Cu^I sites that reside within 7 Å of one another at their closest. Gas adsorption measurements reveal an H₂ adsorption enthalpy of -38 kJ/mol, exceeding that of the larger-pore analog (Cu^IZn-MFU-4l; -33 kJ/mol) and representing the strongest H₂ adsorption yet achieved in a metal-organic framework. The stronger H₂ adsorption in Cu^IZn-MFU-4 is attributed to a combination of pore confinement effects and the increased σ-accepting nature of the Cu^I sites caused by a more electron-withdrawing bbta^2- linker, as supported by structural, spectroscopic, and computational evidence. With the strongest H₂ adsorption, equilibrium isotope effects in Cu^IZn-MFU-4 lead to a D₂/H₂ selectivity (as estimated by ideal adsorbed solution theory) of 1.35 even at 298 K, approaching the values reported below 200 K for conventional porous materials.

The homogeneous conversion of ambient dinitrogen to amine products via the N₂ reduction reaction (N₂RR) remains a prized yet challenging feat for d-block complexes and is scarcely reported for f-block complexes. New, mononuclear Ti^IV and Zr^IV aryloxide complexes Ti(DP)₂ (1Ti), Zr(DP)₂ (1Zr), and DP = [2-(OC₆H₂-2-^tBu,4-Me)₂CHPh] produce up to 51 eq. and up to 7.0 eq. of HN(SiMe₃)₂ per Ti/Zr, from N₂, K⁰, weak acid, and chlorotrimethylsilane. Complex 1Ti exhibits more than double the activity toward N₂-silylation of any previously reported Ti N₂RR catalyst and can also catalyze the formation of up to 19 eq. of NH₃, a new feature in early metal N₂RR chemistry. The mononuclear 1Zr is the most active Zr catalyst for N₂-silylation to date. [KSm(DP)₂(THF)₃] (1Sm), the mononuclear analogue of our previously reported dinuclear A-Sm complex, displays only stoichiometric N₂-silylation due to its vulnerability to deleterious side reactions. DFT calculations confirm the catalysis can proceed via a monomeric Ti complex with a terminally bound, activated N₂, agreeing with experimental ¹H DOSY NMR measurements; the N-H bond is formed first, directing the catalyst selectivity. The isolable reduction product [K₃(THF)Ti(DP)(DP^-)(N₂)] is also an active catalyst, and an intermediate in the calculated cycle.

Chlorosilanes are cheap and abundant raw materials as crucial building blocks in silicon chemistry, yet the metal-mediated activation and functionalization of Si–Cl bonds typically require precious metal sources due to their thermodynamic inertness. Herein, we report the stoichiometric, facile activation, and hydrogenolysis of chlorosilanes mediated by a series of low-valent NHC–Ni (NHC = N-heterocyclic carbene) complexes. Treatment of a Ni(0) complex (IPr)­Ni­(η6-toluene) (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazole-2-ylidene) with chlorosilanes (R1R2SiCl2, R1 = Cl, R2 = Cl, Me, Ph, or R1 = R2 = Me, Et, Ph, 4-MePh) rapidly afforded di-Ni­(I) complexes with a bridging silyl ligand ([(IPr)­Ni]2(μ-SiR1R2Cl)­(μ-Cl), 1 R1,R2 ) in high yields. Use of a bulkier chlorosilane, Ph2SiCl2, allowed the isolation of the mono-Ni­(II) silyl complex (IPr)­Ni­(SiPh2Cl)Cl (2 Ph ) as an intermediate generated via Si–Cl oxidative addition, which underwent comproportionation with (IPr)­Ni­(η6-toluene) to form 1 Ph,Ph in nearly quantitative yield. Interestingly, 1 R1,R2 was found to react with H2 at room temperature to form mono- or di-hydrosilanes in moderate to high yields, and the product selectivity was found to be highly dependent on the identity of substituents on Si. These results demonstrate a novel example of facile Si–Cl activation and hydrogenolysis mediated by low-valent mono- and dinuclear NHC–Ni complexes under mild conditions.

Atomic structures of nanomaterials are inherently dynamic and continuously reshaped through interactions with chemical species and external stimuli. Such dynamics are further amplified as the size and dimensionality of nanomaterials decrease. Despite advances in analytical methods, it remains challenging to capture the structural dynamics of nanomaterials in reactive environments with both atomic spatial resolution and commensurate temporal resolution. Here, we directly visualize atomic-scale dynamics of gold (Au) nanocrystals in reactive liquid environments with millisecond-speed liquid-cell electron microscopy (EM) and deep-learning denoising. We uncover reversible fluctuations in the local crystallinity of Au nanocrystals dependent on the surrounding chemical environment. These transient fluctuations, driven by interactions at nanocrystal-liquid interfaces, critically influence the dissolution kinetics and grain boundary relaxation. By overcoming the spatiotemporal limitations in conventional liquid-cell EM, our findings provide insights into how transient nanoscale structures dictate the stability and reactivity of nanomaterials.

Artificial metalloenzymes (ArMs) expand the suite of synthetically valuable, new-to-nature biocatalytic reactions. Integrating these enzymes into biosynthetic pathways enables reactions not found in nature to occur in living cells with the intermediates or products of the metabolic pathways. However, the integration of reactions catalyzed by ArMs into complex metabolic pathways is constrained by the lack of methods to assemble these ArMs in organisms that are commonly used for metabolic engineering. We report the assembly of an iridium-containing artificial metalloenzyme (Ir-ArM) in Streptomyces albus, a Gram-positive bacterial chassis widely used for the heterologous expression of natural products. In this engineered organism, the Ir-ArM assembles in the cytoplasm and catalyzes abiological carbene transfer to the unactivated, disubstituted double bond of an exogenously added terpene with turnover numbers (TONs) that are two times higher than those for the same reaction catalyzed within E. coli cells harboring Ir-ArM and 20 times higher than the TONs for the same reaction catalyzed by the purified holoprotein itself.

Cu-based catalysts are uniquely capable of C-C coupling during electrochemical CO₂ reduction (CO₂R), yet further mechanistic understanding remains hampered by the lack of spectroscopically resolved descriptors that demonstrate how surface adsorbates emerge and evolve within their catalytic environment. Here, we correlate in situ surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared absorption spectroscopy (SEIRAS) to resolve the potential-dependent dynamics during CO₂R on Cu nanograin catalysts. By building on previous benchmarking of low overpotential performance and nanograin structural evolution, we offer a diagnostic framework linking vibrational signatures to catalytic function, unveiling which species appear, persist, and turnover as the electrified surface and interfacial environment evolve under bias. The onset of linear CO is marked below -0.45 V, coincident with persistent adsorbed *OH/*O domains beyond the CO₂R onset. In this context, Cu nanograins serve as a platform to dissect contributions of adsorbate coverage. By carefully dissecting the potential dependence of emergent twin-defect/step CO stretch bands (P1-P2), alongside the prototypical terrace-site CO stretch band (P3), we provide important context for interpreting coupled spectroscopic trends driven by coverage effects and resolve this for the evidently complex nanograin morphology. Together, these observations highlight the intertwined roles of surface stabilization and interfacial flux in steering multicarbon product formation. By directly linking vibrational signatures to catalytic behavior, this work aims to bridge the gap between observation and control and help guide toward a predictive framework of fine-tuned selectivity for CO₂R.

The uranyl dication ([UO₂]²⁺) is a highly active photocatalyst for the functionalization of inert C_sp3-H bonds by direct hydrogen atom abstraction (HAA). However, photocatalysis by the uranyl ion remains underexplored. Most reports are limited to reactions catalyzed by simple uranyl salts, such as uranyl nitrate [UO₂(NO₃)₂·6H₂O] (U^NO3). We report a set of uranyl tris(benzoate) complexes 1-R containing strongly coordinating and tunable equatorial ligands that resist photodamage and control access to the oxo groups. These catalyst variants with appropriate aryl substituents undergo catalytic reactions at C-H bonds by HAA. The selectivity and reactivity of this step depend on the ligand framework and are distinct from that of U^NO3 or other photoactive oxo complexes, such as decatungstate, that lack ancillary ligands. Finally, consistent with the strong, stable axial U-O bond, reaction with exogenous radical acceptors outcompetes radical rebound, enabling C-C and C-N bond formation from the alkyl radical intermediate. Regioselective alkylation and functionalization of a broad range of substrates results, and this photocatalysis shows that modulation of equatorial ligands on [UO₂]²⁺ can influence the reactivity and selectivity of photocatalytic C-H bond functionalization.

Pathways and structural dynamics of phase transformations impact performance of materials in energy and information storage technologies. Palladium hydride (PdH_x) nanocrystals are an ideal model system for studying solute-induced phase transformations, where elastic energy from lattice mismatch between α-PdH_x and β-PdH_x phases is often considered a key to determining the transformation pathways. α/β-PdH_x interfacial elastic energy is affected by the confined geometry of a nanocrystal. However, how nanocrystal geometry influences phase transformation pathways is largely unknown. Using in situ liquid phase transmission electron microscopy, we directly visualize hydrogenation in Pd nanocrystals with two geometries, a nanocube and a hexagonal nanoplate. Both follow similar sequences of an initially curved nucleus, interface flattening, and reverse-stage nucleation; however, their evolving α/β-PdH_x interfaces exhibit geometry-dependent crystallographic alignments. In nanocubes, {100}-aligned configurations conform to static elastic energy ordering, representing a pathway that maintains a local mechanical equilibrium, whereas nanoplates display both {110}- and {211}-aligned interfaces. Theoretical simulations show that geometry determines the accessibility of alternative phase transformation pathways as the system is driven far from equilibrium during hydrogenation. These findings identify geometry as a fundamental parameter for directing phase transformation pathways, offering design principles for accessing atypical configurations and improving properties of intercalation-based devices.