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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Dose Calculation on KV Cone Beam CT Images: An Investigation of the Hu-Density Conversion Stability and Dose Accuracy Using the Site-Specific Calibration

Precise calibration of Hounsfield units (HU) to electron density (HU-density) is essential to dose calculation. On-board kV cone beam computed tomography (CBCT) imaging is used predominantly for patients' positioning, but will potentially be used for dose calculation. The impacts of varying 3 imaging parameters (mAs, source-imager distance [SID], and cone angle) and phantom size on the HU number accuracy and HU-density calibrations for CBCT imaging were studied. We proposed a site-specific calibration method to achieve higher accuracy in CBCT image-based dose calculation. Three configurations of the Computerized Imaging Reference Systems (CIRS) water equivalent electron density phantom were used to simulate sites including head, lungs, and lower body (abdomen/pelvis). The planning computed tomography (CT) scan was used as the baseline for comparisons. CBCT scans of these phantom configurations were performed using Varian Trilogy{sup TM} system in a precalibrated mode with fixed tube voltage (125 kVp), but varied mAs, SID, and cone angle. An HU-density curve was generated and evaluated for each set of scan parameters. Three HU-density tables generated using different phantom configurations with the same imaging parameter settings were selected for dose calculation on CBCT images for an accuracy comparison. Changing mAs or SID had small impact on HU numbers. For adipose tissue, the HU discrepancy from the baseline was 20 HU in a small phantom, but 5 times lager in a large phantom. Yet, reducing the cone angle significantly decreases the HU discrepancy. The HU-density table was also affected accordingly. By performing dose comparison between CT and CBCT image-based plans, results showed that using the site-specific HU-density tables to calibrate CBCT images of different sites improves the dose accuracy to {approx}2%. Our phantom study showed that CBCT imaging can be a feasible option for dose computation in adaptive radiotherapy approach if the site-specific calibration is applied.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2024

Support size regulated ruthenium-sulfoacid-nitrogen sites intensify cellulose hydrogenolysis to 1,2-propylene glycol

Rational design of metal-acid-base multifunctional catalysts for upgrading cellulose to 1,2-propylene glycol (1,2-PG) is of great significance for building a sustainable world. However, it is time-consuming and tedious to regulate metal-acid-base sites to balance major reactions to render a high 1,2-PG yield. We herein report support size simultaneously regulated ruthenium-sulfoacid-nitrogen (Ru–SO 3 H–N) sites for cellulose hydrogenolysis to high yield 1,2-PG. Originated from the depolymerization and reassembly of zinc-1,3,5-benzenetricarboxylic acid (ZnBTC) fiber with zeolitic imidazolate framework (ZIF-8) in water, 2-methylimidazole infiltrated nanorod (ZnBTC(mIM)) with a varied aspect ratio was fabricated by varying the feed ratio of ZIF-8/ZnBTC. Upon being pyrolyzed, sulfonated and impregnated with Ru ions, the supported sites were tailored in terms of Ru single-atom/nanocluster ratio, SO 3 H acidity and N basicity. Further, the elaborately fabricated catalyst delivers 32.3% yield of 1,2-PG, corresponding to a high productivity of 67.71 mol h -1 g Ru -1 and a large turnover number of 34193, two and three orders of magnitude higher than those by using other Ru-containing catalytic systems for cellulose hydrogenolysis. The excellent performance can be attributed to optimized electronic and molecular structure of Ru–SO 3 H–N sites that can improve rate-determining cellulose hydrolysis/fructose hydrogenolysis, pivotal glucose isomerization with others to proceed at a matched rate. This study opens a new avenue to facilely tailor the metal-acid-base sites by rational design of size controlled supporting matrix.

1,2-Propylene glycol↗
DOE OSTI2012

Novel bis-(−)-nor-meptazinol derivatives act as dual binding site AChE inhibitors with metal-complexing property

The strategy of dual binding site acetylcholinesterase (AChE) inhibition along with metal chelation may represent a promising direction for multi-targeted interventions in the pathophysiological processes of Alzheimer's disease (AD). In the present study, two derivatives (ZLA and ZLB) of a potent dual binding site AChE inhibitor bis-(−)-nor-meptazinol (bis-MEP) were designed and synthesized by introducing metal chelating pharmacophores into the middle chain of bis-MEP. They could inhibit human AChE activity with IC{sub 50} values of 9.63 μM (for ZLA) and 8.64 μM (for ZLB), and prevent AChE-induced amyloid-β (Aβ) aggregation with IC{sub 50} values of 49.1 μM (for ZLA) and 55.3 μM (for ZLB). In parallel, molecular docking analysis showed that they are capable of interacting with both the catalytic and peripheral anionic sites of AChE. Furthermore, they exhibited abilities to complex metal ions such as Cu(II) and Zn(II), and inhibit Aβ aggregation triggered by these metals. Collectively, these results suggest that ZLA and ZLB may act as dual binding site AChEIs with metal-chelating potency, and may be potential leads of value for further study on disease-modifying treatment of AD. -- Highlights: ► Two novel bis-(−)-nor-meptazinol derivatives are designed and synthesized. ► ZLA and ZLB may act as dual binding site AChEIs with metal-chelating potency. ► They are potential leads for disease-modifying treatment of Alzheimer's disease.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Determination of the protease cleavage site repertoire—The RNase H but not the RT domain is essential for foamy viral protease activity

In contrast to orthoretroviruses, the foamy virus protease is only active as a protease-reverse transcriptase fusion protein and requires viral RNA for activation. Maturation of foamy viral proteins seems to be restricted to a single cleavage site in Gag and Pol. We provide evidence that unprocessed Gag is required for optimal infectivity, which is unique among retroviruses. Analyses of the cleavage site sequences of the Gag and Pol cleavage sites revealed a high similarity compared to those of Lentiviruses. We show that positions P2' and P2 are invariant and that Gag and Pol cleavage sites are processed with similar efficiencies. The RNase H domain is essential for protease activity, but can functionally be substituted by RNase H domains of other retroviruses. Thus, the RNase H domain might be involved in the stabilization of the protease dimer, while the RT domain is essential for RNA dependent protease activation. - Highlights: • Unprocessed Gag is required for optimal infectivity of foamy viruses. • Positions P2 and P2' are invariant in the foamy viral cleavage sites. • The RNaseH domain is essential for protease activity. • The RNaseH domains of other retroviruses support foamy viral protease activity.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Analysis of the molecular determinants for furin cleavage of the spike protein S1/S2 site in defined strains of the prototype coronavirus murine hepatitis virus (MHV)

We analyzed the spike protein S1/S2 cleavage of selected strains of a prototype coronavirus, mouse hepatitis virus (MHV) by the cellular protease furin, in order to understand the structural requirements underlying the sequence selectivity of the scissile segment. The probability of cleavage of selected MHV strains was first evaluated from furin cleavage scores predicted by the ProP computer software, and then cleavage was measured experimentally with a fluorogenic peptide cleavage assay consisting of S1/S2 peptide mimics and purified furin. We found that in vitro cleavability varied across MHV strains in line with predicted results—but with the notable exception of MHV-A59, which was not cleaved despite a high score predicted for its sequence. Using the known X-Ray structure of furin in complex with a substrate-like inhibitor as an initial structural reference, we carried out molecular dynamics (MD) simulations to learn the modes of binding of the peptides in the furin active site, and the suitability of the complex for initiation of the enzymatic cleavage. We identified the 3D structural requirements of the furin active site configuration that enable bound peptides to undergo cleavage, and the way in which the various strains tested experimentally are fulfilling these requirements. We find that despite some flexibility in the organization of the peptide bound to the active site of the enzyme, the presence of a histidine at P2 of MHV-A59 fails to properly orient the sidechain of His194 of the furin catalytic triad and therefore produces a distortion that renders the peptide/complex structural configuration in the active site incompatible with requirements for cleavage initiation. The Ser/Thr in P1 of MHV-2 and MHV-S has a similar effect of distorting the conformation of the furin active site residues produced by the elimination of the canonical salt-bridge formed by arginine in P1 position. This work informs a study of coronavirus infection and pathogenesis with respect to the function of the viral spike protein, and suggests an important process of viral adaptation and evolution within the spike S1/S2 structural loop.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Oxysterol-binding protein-related protein (ORP) 6 localizes to the ER and ER-plasma membrane contact sites and is involved in the turnover of PI4P in cerebellar granule neurons

Oxysterol-binding protein (OSBP)-related proteins (ORPs) are conserved lipid binding proteins found in organisms ranging from yeast to mammals. Recent findings have indicated that these proteins mainly localize to contact sites of 2 different membranous organelles. ORP6, a member of the ORP subfamily III, is one of the least studied ORPs. Using approaches in molecular cell biology, we attempted to study the characteristics of ORP6 and found that ORP6 is abundantly expressed in mouse cultured neurons. Deconvolution microscopy of cultured cerebellar granular cells revealed that ORP6 is localized to the endoplasmic reticulum (ER) and ER–plasma membrane (PM) contact sites, where it co-localized with extended synaptotagmin2 (E-Syt2), a well-known ER–PM contact site marker. E-Syt2 also co-localized with ORP3, another subfamily III member, and ORP5, a subfamily IV member. However, ORP5 does not distribute to the same ER-PM contact sites as subfamily III members. Also, the co-expression of ORP3 but not ORP5 altered the distribution of ORP6 into the processes of cerebellar neurons. Immunoprecipitation demonstrated binding between the intermediate region of ORP6 and ORP3 or ORP6 itself. Additionally, the localization of ORP6 in the PM decreased when co-expressed with the intermediate region of ORP6, in which the pleckstrin homology (PH) domain and OSBP-related ligand binding domain (ORD) are deleted. Over-expression of this intermediate region shifted the location of a phophtidylinositol-4-phosphate (PI4P) marker from the Golgi to the PM. Knockdown of ORP6 resulted in the same shift of the PI4P marker. Collectively, our data suggests that the recruitment of ORP6 to ER–PM contact sites is involved in the turnover of PI4P in cerebellar granular neurons.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2010

Site-Specific Proteomics Approach for Study Protein S-Nitrosylation

Here we present a novel and robust method for the identification of protein S-nitrosylation sites in complex protein mixtures. The approach utilizes the cysteinyl affinity resin to selectively enrich S-nitrosylated peptides reduced by ascorbate followed by nanoscale liquid chromatography tandem mass spectrometry. Two alkylation agents with different added masses were employed to differentiate the S-nitrosylation sites from the non-Snitrosylation sites. We applied this approach to MDA-MB-231 cells treated with Angeli’s salt, a nitric oxide donor that has been shown to inhibit breast tumor growth and angiogenesis. A total of 162 S-nitrosylation sites were identified and an S-nitrosylation motif was revealed in our study. The 162 sites are significantly more than the number reported by previous methods, demonstrating the efficiency of our approach. Our approach will further facilitate the functional study of protein S-nitrosylation in cellular processes and may reveal new therapeutic targets.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Systematic Perturbations of Binuclear Non-heme Iron Sites: Structure and Dioxygen Reactivity of de Novo Due Ferri Proteins

We report that DFsc (single-chain due ferri) proteins allow for modeling binuclear non-heme iron enzymes with a similar fold. Three 4A → 4G variants of DFsc were studied to investigate the effects of (1) increasing the size of the substrate/solvent access channel (G4DFsc), (2) including an additional His residue in the first coordination sphere along with three additional helix-stabilizing mutations [3His-G4DFsc(Mut3)], and (3) the three helix-stabilizing mutations alone [G4DFsc-(Mut3)] on the biferrous structures and their O 2 reactivities. Near-infrared circular dichroism and magnetic circular dichroism (MCD) spectroscopy show that the 4A → 4G mutations increase coordination of the diiron site from 4-coordinate/5-coordinate to 5-coordinate/5-coordinate, likely reflecting increased solvent accessibility. While the three helix-stabilizing mutations [G4DFsc(Mut3)] do not affect the coordination number, addition of the third active site His residue [3His-G4DFsc(Mut3)] results in a 5-coordinate/6-coordinate site. Although all 4A → 4G variants have significantly slower pseudo-first-order rates when reacting with excess O 2 than DFsc (~2 s ₋1 ), G4DFsc and 3His-G4DFsc(Mut3) have rates (~0.02 and ~0.04 s ₋1 ) faster than that of G4DFsc(Mut3) (~0.002 s ₋1 ). These trends in the rate of O 2 reactivity correlate with exchange coupling between the Fe(II) sites and suggest that the two-electron reduction of O 2 occurs through end-on binding at one Fe(II) rather than through a peroxy-bridged intermediate. Finally, UV–vis absorption and MCD spectroscopies indicate that an Fe(III)Fe(III)-OH species first forms in all three variants but converts into an Fe(III)-μ-OH-Fe(III) species only in the 2-His forms, a process inhibited by the additional active site His ligand that coordinatively saturates one of the iron centers in 3His-G4DFsc(Mut3).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Site-Selective Atomic Layer Deposition at Thermally Generated Surface Oxygen Vacancies on Rutile TiO 2

Unique atomic arrangements (i.e., defects) on material surfaces may exhibit distinctive reactivity that affords opportunities for selective surface chemistry exclusively at those sites. While theoretically appealing, site-selective atomic layer deposition (ALD) processes remain largely unrealized experimentally. Here we provide evidence for site-selective ALD that exclusively targets oxygen vacancies thermally generated at the surface of TiO 2 single crystals. High-resolution in situ ellipsometry reveals the nucleation behavior of dimethylaluminum isopropoxide (DMAI) and water at process conditions precisely tuned for selective hydroxylation of surface defects. An island growth model for nucleation and atomic force microscopy (AFM) imaging are consistent with a site-selective growth mechanism that depends on the surface oxygen vacancy density. This approach to direct ALD reactions at defective surface sites may provide future opportunities for targeted remediation of electronically imperfect interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Site-Dependent Fluctuations Optimize Electronic Energy Transfer in the Fenna–Matthews–Olson Protein

Light absorbed by light-harvesting antennae is transferred to the reaction center (RC). The excitation energy transfer (EET) to the RC is known to proceed with nearly perfect quantum yield. However, understanding of EET is still limited at the molecular level. Here in this paper, we examine the dynamics in the Fenna–Matthews–Olson (FMO) protein by developing an efficient molecular dynamics simulation that can properly describe the electronic properties of bacteriochlorophylls. We find that the FMO protein consists of sites with heterogeneous fluctuations extending from fast to slow modulation. We also find that efficient EETs are facilitated by site-dependent fluctuations that enhance the resonance condition between neighboring sites with large site energy differences and circumvent exciton trapping on the pathway to the RC. Knowledge of site-dependent fluctuations is an important component of understanding optimization of EET in photosynthetic systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Selective Hydroxylation of In 2 O 3 as A Route to Site-Selective Atomic Layer Deposition

We report that several atomic layer deposition (ALD) processes are now known that leverage the distinct surface chemistry of two disparate substrates (e.g., metal vs oxide) to realize markedly different ALD nucleation rates in an approach referred to as area-selective ALD. In contrast, few ALD processes have been identified that allow selective reaction at distinct surface sites of a single material surface, in a process that might be called site-selective ALD (SS-ALD). We describe one potential strategy to discriminate among several distinct surface sites on bixbyite In 2 O 3 to achieve site selectivity. Using density functional theory, we predict the discriminant hydration and hydroxylation of In 2 O 3 terrace and step-edge sites that depend strongly on the substrate temperature at low water coverage. Infrared measurement of surface hydroxyls on In 2 O 3 nanoparticles supports the predicted temperature dependence. The in situ examination of MgO ALD nucleation also shows results consistent with the predicted temperature dependence of In 2 O 3 hydroxylation. Together, these findings suggest that a selective hydroxylation approach may be a viable route to SS-ALD on In 2 O 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

DFT Study of Closed and Open Sites of BEA, FAU, MFI, and BEC Zeolites Substituted with Tin and Titanium

DFT with long-range corrections and ONIOM along with a polarizable-continuum model were used to analyze zeolites BEA, FAU, MFI, and BEC substituted with Sn and Ti. The preferential substitution sites for Ti and Sn in the different frameworks are reported. The Lewis acidities were measured through the NH 3 binding energies and through the charge transfer of NH 3 upon adsorption. The deprotonation energies of the open sites, which are proportional to the Brønsted acidities, and the hydrolysis energies are also reported. We also present the properties of BEA with a single and a double Sn-substitution to compare the active sites obtained with two methods commonly employed for the synthesis of Sn–BEA. Among the zeolites analyzed in this study, Sn–BEA with a double Sn-substitution has the highest Lewis acidity. The formation of open sites through the hydrolysis of Sn–BEA, Sn–FAU, and Ti–FAU is energetically favorable, but it is not favorable in MFI or Ti–BEA. On the basis of the deprotonation energies, the open sites of Sn–BEA have a strong Brønsted acidity, comparable to Al–BEA or Al–MFI. We also demonstrate that the VDW forces in the binding energies of NH 3 on MFI are more significant than in the other zeolite frameworks and that these forces decrease with increasing pore size.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Competitive Binding of Ethylene, Water, and Carbon Monoxide in Metal–Organic Framework Materials with Open Cu Sites

Typically, metal–organic frameworks (MOFs) with open metal sites (OMS) are understood to have selectivity in olefin/paraffin separations because of π–π interactions between olefin double bonds and OMS. One concern associated with these separations is that other species that potentially bind to OMS may also be present in feed streams, causing competition for these sites. We used density functional theory (DFT) to assess the binding energy of ethylene, water, and carbon monoxide on a set of more than 60 MOFs with open Cu sites in the form of Cu dimers. A useful observation from our results is that the relative binding energies of pairs of molecules (e.g., ethylene and water) can be calculated accurately from calculations that hold the MOF structure rigid and only relax the positions of the adsorbing molecules. These kinds of calculations are far more numerically efficient than calculations that relax all degrees of freedom in the system, so this observation will be useful in future efforts to screen larger collections of materials. A second observation is that the binding energies of each molecule in the 60 MOFs are quite similar to the binding energies in CuBTC, an exemplar MOF with open Cu sites in the form of Cu dimers. Analysis of the variations that do exist in the binding energies among materials points to possible avenues for controlling either the absolute binding energies or the relative binding energies of species associated with OMS in these materials. The third observation is that two unusual MOFs can bind ethylene more strongly than water because of a dual-site binding mechanism in which an ethylene molecule can interact simultaneously with both dimers while the smaller water molecule interacts primarily with a single OMS. This observation suggests a possible avenue for developing other MOFs in which the binding energy of ethylene is higher than that of water.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding the Unique Reactivity of Cu for Electrochemical CO 2 Reduction with a 3-Site Model

Cu-based catalysts for the electrochemical reduction of CO 2 and CO exhibit a perplexingly unique reactivity toward multicarbon based products compared to other studied electrocatalysts. Here, in this study, we use insights gained from a recent phenomenological 3-site microkinetic model and grand-canonical density functional theory calculations to clarify the importance of an underemphasized aspect critical to Cu’s unique reactivity: a population of so-called “reservoir” sites. Using model Cu surface motifs, we discuss how these types can be represented by undercoordinated structural defects like step edges and grain boundaries which form a network of highly anisotropic migration channels. These pathways are found to be amenable for feeding *CO over time to reactive sites like Cu adatoms more active toward C–C coupling. These results highlight an often overlooked aspect of catalyst optimization: reservoir site engineering, which exploits surface mobility and presents an equally important avenue for electrocatalyst engineering oversimply maximizing active site densities.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Dirac Nodal Line in Hourglass Semimetal Nb 3 SiTe 6

Glide-mirror symmetry in nonsymmorphic crystals can foster the emergence of novel hourglass nodal loop states. Here, we present spectroscopic signatures from angle-resolved photoemission of a predicted topological hourglass semimetal phase in Nb 3 SiTe 6 . Linear band crossings are observed at the zone boundary of Nb 3 SiTe 6 , which could be the origin of the nontrivial Berry phase and are consistent with a predicted glide quantum spin Hall effect; such linear band crossings connect to form a nodal loop. Furthermore, the saddle-like Fermi surface of Nb 3 SiTe 6 observed in our results helps unveil linear band crossings that could be missed. Additionally, in situ alkali-metal doping of Nb 3 SiTe 6 also facilitated the observation of other band crossings and parabolic bands at the zone center correlated with accidental nodal loop states. Overall, our results complete the system's band structure, help explain prior Hall measurements, and suggest the existence of a nodal loop at the zone center of Nb 3 SiTe 6 .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Formation of (Rh–Fe)–FeO x Complex Sites Enables Methanol Synthesis from CO 2

Here, we addressed the challenges of designing catalysts for selective CO 2 hydrogenation by incorporating oxide Fe species onto Rh nanoparticles. Nanoscopic FeO x domains created a “reverse catalyst” structure (i.e., a metal oxide supported on a metal) that increased the density of interfacial sites compared to traditional supported catalysts. The contact between the metal nanoparticle and the oxide overlayer induced the formation of a surface Rh-Fe alloy that stabilize methoxy groups while suppressing hydrogenolysis to methane. Sites at FeO x -metal interfaces interact with CO 2 sevenfold stronger than sites on metal surfaces, show larger energy barriers to cleave the C-O bonds, and offer a barrierless pathway for hydrogenation of methoxy species to methanol. Consequently, the multifunctional sites over FeO x /Rh-Fe catalysts highlight and meet the requirements of a selective methanol catalyst: strong interaction with CO 2 to ensure high density of transition states; metal sites to activate and make hydrogen available to surface intermediates; and high energy barriers for C-O bond cleavage to form carbides. These synthesis and catalytic chemistries, demonstrated for Rh-Fe-FeO x interfaces, enable us to overcome the limitations to the design of methanol production catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Activation of Surface ReO x Sites on Al 2 O 3 Catalysts for Olefin Metathesis

The nature of activated surface ReOx sites and surface reaction intermediates for supported ReOx/Al2O3 catalysts during propylene self-metathesis were systematically investigated for the first time using in situ spectroscopy (Raman, UV–vis, XAS (XANES/EXAFS) and IR). In situ Raman spectroscopy reveals that olefins selectively interact with the surface dioxo ReO4 sites anchored at acidic alumina hydroxyls. In situ UV–vis indicates that surface Re5+ and some Re6+ sites form, and in situ XAS indicates a reduction in the number of Re=O bond character in the propylene self-metathesis reaction environment, especially as the temperature is increased. The appearance of oxygenated products during propylene activation supports the conclusion that catalyst activation involves removal of oxygen from the surface rhenia sites (pseudo-Wittig mechanism). Isotopic CD3CD=CD2 → CH3CH=CH2 switch experiments demonstrate the presence of surface Re=CD2 and Re=CDCD3 reaction intermediates, with the surface Re=CD2 species being the most abundant reaction intermediate. In situ IR spectroscopy indicates the presence of significant surface propylene π complexes on alumina and rhenia sites of the catalyst, which complicates analysis of surface reaction intermediates during propylene self-metathesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Tuning Zr 6 Metal–Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts

The Zr 6 nodes of the metal–organic frameworks (MOFs) UiO-66 and UiO-67 are metal oxide clusters of atomic precision and can be used as catalyst supports. The bonding sites on these nodes—that is, hydrogen-bonded H 2 O/OH groups on UiO-67 and non-hydrogen-bonded terminal OH groups on UiO-66—were regulated by modulation of the MOF syntheses. Ir(C 2 H 4 ) 2 (C 5 H 7 O 2 ) complexes reacted with these sites to give site-isolated Ir(C 2 H 4 ) 2 complexes, each anchored to the node by two Ir–Onode bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the ν CO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order HY zeolite << UiO-66 < UiO-67 (= NU-1000) < ZrO 2 < MgO. The sharpness of the IR ν CO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO 2 ≈ UiO-67 ≈ NU-1000 < MgO < UiO-66 << HY zeolite. The reactivity of supported Ir(CO) 2 complexes with C2H4 to form Ir(C 2 H 4 )(CO) and Ir(C 2 H 4 ) 2 (CO) is influenced by the support electron-donor properties, with the reactivity increasing in the order MgO = ZrO 2 = NU-1000 (not reactive) < UiO-66 < UiO-67 << HY zeolite. Density functional theory calculations characterizing the complexes supported on NU-1000, UiO-66/67, and HY zeolite concur with the use of the calculated ν CO bands as indicators of electron-donor properties of the supported metal catalysts. Our calculations also show that the reactivity of the supported Ir(CO) 2 complexes with C 2 H 4 is correlated with the electron-donor properties of the iridium center. Lastly, the supported Ir(C 2 H 4 ) 2 samples are precatalysts for ethylene hydrogenation and ethylene dimerization, with the activity for each reaction increasing with increasing electron-withdrawing strength of the support.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗