Molecular Insight into Fluorocarbon Adsorption in Pore Expanded Metal-Organic Framework Analogs.

The rapid growth in the global energy demand for space cooling requires the development of more efficient environmental chillers for which adsorption-based cooling systems can be utilized. Here, in this contribution, we explore sorbents for chiller use via a pore-engineering concept to construct analogs of the 1-dimensional pore metal-organic framework MOF-74 by using elongated organic linkers and stereochemistry control. The prepared pore-engineered MOFs show remarkable equilibrium adsorption of the selected fluorocarbon refrigerant that is translated to a modeled adsorption-based refrigeration cycle. To probe molecular level interactions at the origin of these unique adsorption properties for this series of Ni-MOFs, we combined in situ synchrotron X-ray powder diffraction, neutron powder diffraction, X-ray absorption spectroscopy, calorimetry, Fourier transform infrared techniques, and molecular simulations. Our results reveal the coordination of fluorine (of CH2F in R134a) to the nickel(II) open metal centers at low pressures for each Ni-MOF analog and provide insight into the pore filling mechanism for the full range of the adsorption isotherms. The newly designed Ni-TPM demonstrates exceptional R134a adsorption uptake compared to its parent microporous Ni-MOF-74 due to larger engineered pore size/volume. The application of this adsorption performance toward established chiller conditions yields a working capacity increase for Ni-TPM of about 400% from that of Ni-MOF-74, which combined with kinetics directly correlates to both a higher coefficient of performance and a higher average cooling capacity generated in a modeled chiller.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction.

Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu 2 NPh 2 )(15 NH3 )][BArF 4 ], (PtBu 2 NPh 2 =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; ArF =3,5-(CF3 )2 C6 H3 ), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N-H bonds and the formation of an N≡N bond was achieved by coupling H+ and e- transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6-tri-tert-butylphenoxyl radical (t Bu3 ArO. ) as the H atom acceptor. Employing an excess of t Bu3 ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15 N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond-forming step in the catalytic cycle.

Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo-NH3 Complex.

We report ammonia oxidation by homolytic cleavage of all three H atoms from a [Mo-NH3]+ complex using the 2,4,6-tri-tert-butylphenoxyl radical to yield a Mo-alkylimido ([Mo═NR]+) complex (R = 2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one). Chemical reduction of [Mo═NR]+ generates a terminal Mo≡N nitride complex upon N-C bond cleavage, and a [Mo═NH]+ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from [Mo-NH3]+ and formation of [Mo═NR]+.

Nickel and iron pincer complexes as catalysts for the reduction of carbonyl compounds.

The reductions of aldehydes, ketones, and esters to alcohols are important processes for the synthesis of chemicals that are vital to our daily life, and the reduction of CO2 to methanol is expected to provide key technology for carbon management and energy storage in our future. Catalysts that affect the reduction of carbonyl compounds often contain ruthenium, osmium, or other precious metals. The high and fluctuating price, and the limited availability of these metals, calls for efforts to develop catalysts based on more abundant and less expensive first-row transition metals, such as nickel and iron. The challenge, however, is to identify ligand systems that can increase the thermal stability of the catalysts, enhance their reactivity, and bypass the one-electron pathways that are commonly observed for first-row transition metal complexes. Although many other strategies exist, this Account describes how we have utilized pincer ligands along with other ancillary ligands to accomplish these goals. The bis(phosphinite)-based pincer ligands (also known as POCOP-pincer ligands) create well-defined nickel hydride complexes as efficient catalysts for the hydrosilylation of aldehydes and ketones and the hydroboration of CO2 to methanol derivatives. The hydride ligands in these complexes are substantially nucleophilic, largely due to the enhancement by the strongly trans-influencing aryl groups. Under the same principle, the pincer-ligated nickel cyanomethyl complexes exhibit remarkably high activity (turnover numbers up to 82,000) for catalytically activating acetonitrile and the addition of H-CH2CN across the C═O bonds of aldehydes without requiring a base additive. Cyclometalation of bis(phosphinite)-based pincer ligands with low-valent iron species "Fe(PR3)4" results in diamagnetic Fe(II) hydride complexes, which are active catalysts for the hydrosilylation of aldehydes and ketones. Mechanistic investigation suggests that the hydride ligand is not delivered to the carbonyl substrates but is important to facilitate ligand dissociation prior to substrate activation. In the presence of CO, the amine-bis(phosphine)-based pincer ligands are also able to stabilize low-spin Fe(II) species. Iron dihydride complexes supported by these ligands are bifunctional as both the FeH and NH moieties participate in the reduction of C═O bonds. These iron pincer complexes are among the first iron-based catalysts for the hydrogenation of esters, including fatty acid methyl esters, which find broad applications in industry. Our studies demonstrate that pincer ligands are promising candidates for promoting the first-row transition metal-catalyzed reduction of carbonyl compounds with high efficiency. Further efforts in this research area are likely to lead to more efficient and practical catalysts.

Mechanistic studies of ammonia borane dehydrogenation catalyzed by iron pincer complexes.

A series of iron bis(phosphinite) pincer complexes with the formula of [2,6-((i)Pr2PO)2C6H3]Fe(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-((i)Pr2PO)2-4-(MeO)C6H2]Fe(PMe2Ph)2H (3) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes 1-3 release 2.3-2.5 equiv of H2 per AB in 24 h. Among the three iron catalysts, 3 exhibits the highest activity in terms of both the rate and the extent of H2 release. The initial rate for the dehydrogenation of AB catalyzed by 3 is first order in 3 and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (k(NH3BH3)/k(ND3BD3) = 3.7) is the product of individual KIEs (k(NH3BH3)/k(ND3BH3) = 2.0 and k(NH3BH3)/k(NH3BD3) = 1.7), suggesting that B-H and N-H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing trans PMe3 or PMe2Ph (relative to the hydride) by AB. Loss of NH3 from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H2 release at the late stage of the dehydrogenation reaction.

Iron-based catalysts for the hydrogenation of esters to alcohols.

Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent.

Anti-cancer evaluation of carboxamides of furano-sesquiterpene carboxylic acids from the soft coral Sinularia kavarattiensis.

The chemical investigation of soft coral Sinularia kavarattiensis is described. It yielded furano-sesquiterpene carboxylic acids 1 and 2 and their methyl esters 3 and 4. Semi-synthesis of furano-sesquiterpene carboxylic acid 1 gave amide derivatives 5-12. Structures of all the compounds were established by IR, NMR and mass spectral analysis. Interestingly all compounds are selectively potent on leukemia cell line. All these compounds were screened for cytotoxic activity against five human cancer cell lines (leukemia, prostate, lung, breast and cervix). Among these compounds 9 and 10 showed promising activity against leukemia and prostate cancer cell lines.

Probing the inner space of resorcinarene molecular capsules with nitroxide guests.

In quarantine: Nitroxide spin probes are encapsulated by hexameric resorcinarene molecular capsules in dichloromethane solutions (see picture). A substantial reduction in the tumbling rates occurs upon encapsulation of two cationic probes and one neutral probe. As the molecular volume of the probe increases, the tumbling rate of the probe reflects the overall tumbling rate of the entire supramolecular assembly.

Preparation, characterization, and electrochemical properties of a new series of hybrid dendrimers containing a viologen core and Frechet and Newkome dendrons.

Five new 4,4'-bipyridinium (viologen) core dendrimers containing a Frechet (Fn, n = 1-3, first to third generation) and a Newkome (Nn, n = 1-3) dendron linked to each of the termini of the viologen residue were prepared and characterized. These macromolecules (FnNn) were prepared according to synthetic methodology already developed by our group. The electrochemical behavior of these dendrimers is characterized by the stepwise reduction of the viologen nucleus (V(2+)/V(+) and V(+)/V). The recorded half-wave potentials are affected by dendron growth in the three surveyed solvent media (dichloromethane, tetrahydrofuran, and acetonitrile). The size of the Newkome dendron has a more pronounced effect on the half-wave potentials than the size of the Frechet dendron. However, increasing the size of the Frechet dendron diminishes the magnitude of the cathodic potential shifts resulting from Newkome dendron growth. The largest dendrimers investigated (F1N3 and F2N3) exhibit quasi-reversible voltammetric behavior. The diffusion coefficients of these molecules were also determined using pulse gradient stimulated echo NMR techniques.