Tom Autrey

Iridium Catalyzed Dehydrogenation of Substituted Amine Boranes: Kinetics, Thermodynamics and Implications for Hydrogen Storage.

Dehydrogenation of amine boranes is catalyzed efficiently by the iridium pincer complex (kappa (3)-1,3-(OP ( t )Bu 2) 2C 6H 3)Ir(H) 2 ( 1). With CH 3NH 2BH 3 (MeAB) and with AB/MeAB mixtures (AB = NH 3BH 3), the rapid release of 1 equiv of H 2 is observed to yield soluble oligomeric products at rates similar to those previously reported for the dehydrogenation of AB catalyzed by 1. Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry. The experimental heats of reaction are compared to results from computational studies.

Hydrogen release studies of alkali metal amidoboranes.

A series of metal amidoboranes LiNH(2)BH(3) (LAB), NaNH(2)BH(3) (SAB), LiNH(Me)BH(3) (LMAB), NaNH(Me)BH(3) (SMAB), KNH(Me)BH(3) (PMAB), and KNH((t)Bu)BH(3) (PBAB) were synthesized by solution phase methods, and the thermal release of H(2) in the solid state was studied. The mechanism of hydrogen release from metal amidoboranes, a metal ion assisted hydride transfer, is very different than the mechanism of hydrogen release from the parent compound ammonia borane (AB). On the basis of the observed trends in reaction rates of H > Me > (t)Bu, K > Na > Li, and the kinetic isotope effect, the mechanism of hydrogen release from MAB compounds was found to proceed through a bimolecular mechanism involving the intermediacy of a MH (M = Li, Na, or K). The non-volatile products formed from MABs are significantly different than the products formed after hydrogen release from AB. The boron containing products resulting from the release of 1 equiv of hydrogen from the metal amidoboranes were characterized by MAS (11)B NMR spectroscopy and found to contain both BH(3) and sp(2) hybridized BH groups, consistent with the general structure MN(R)=BHN(R)MBH(3).

Bis-BN Cyclohexane: A Remarkably Kinetically Stable Chemical Hydrogen Storage Material

A critical component for the successful development of fuel cell applications is hydrogen storage. For back-up power applications, where long storage periods under extreme temperatures are expected, the thermal stability of the storage material is particularly important. Here, we describe the development of an unusually kinetically stable chemical hydrogen storage material with a H2 storage capacity of 4.7 wt%. The compound, which is the first reported parental BN isostere of cyclohexane featuring two BN units, is thermally stable up to 150 °C both in solution and as a neat material. Yet, it can be activated to rapidly desorb H2 at room temperature in the presence of a catalyst without releasing other detectable volatile contaminants. We also disclose the isolation and characterization of two cage compounds with S4 symmetry from the H2 desorption reactions.

Automated gas burette system for evolved hydrogen measurements.

This paper reports a simple and efficient gas burette system that allows automated determination of evolved gas volume in real time using only temperature and pressure measurements. The system is reliable and has been used successfully to study the hydrogen release kinetics of ammonia borane thermolysis. The system is especially suitable for bench scale studies involving small batches and potentially rapid reaction kinetics.

A new angle into time-resolved photoacoustic spectroscopy: A layered prism cell increases experimental flexibility

A new pulsed photoacoustic calorimetry cell that uses transmission of light through a pair of dovetail prisms is discussed. The layered prism cell (LPC) combines the enhanced time-resolution capabilities of the “layered” front-face irradiation geometry with the zero-background and broadband flexibility of the classical cuvette geometry. This work provides a phenomenological description of photoinduced pressure changes to yield an analytical expression to calculate the magnitude of the photoinduced acoustic pressure wave in a series of solvents. The mechanical to electrical conversion efficiency for an ultrasonic transducer coupled to the LPC is presented to provide a comparison of the experimentally observed photoinduced acoustic signal amplitudes to the empirically calculated acoustic signal amplitudes. An analysis of the background signals due to absorption and electrostriction of the media provides insight into the issues of sensitivity and limitations of pulsed photoacoustic experiments. The LPC provi...

Measurement of select radical processes in hydrocarbon pyrolysis

Abstract Methods for the measurement of absolute and relative rates of reactions of organic free radicals related to hydrocarbon pyrolysis in the condensed phase are illustrated. Examples are provided of methods for the determination of absolute rate expressions. The absolute rate expressions are employed in competition kinetic experiments that measure the ratio of an unknown rate constant to the known basis rate constant, yielding the desired competing reaction rate. Examples of use of the absolute and competition kinetic methods to measure rates of select free radical reactions of interest to hydrocarbon pyrolysis are illustrated. Methods for the measurement of absolute rate expressions for reactions of organic free radicals that will be covered include kinetic laserflash optical spectroscopy (KLS), kinetic electron spin resonance spectroscopy (KESR), and kinetic laserflash time resolved infrared spectroscopy (TRIR).

Listening to Colloidal Silica Samples: Simultaneous Measurement of Absorbed and Scattered Light Using Pulsed-Laser Photoacoustics

Laser photoacoustic spectroscopy (LPAS) has been used to simultaneously measure scattered light and absorbed light with the use of a single piezoelectric transducer detector. Samples of Ludox™ colloidal silica, with and without added potassium chromate, were illuminated with pulses of 308 or 355 nm light. Signals were measured with a 1 MHz ultrasonic transducer clamped to the side of the cuvette. The resulting oscilloscope tracing shows a pattern of signals originating from light scatter and light absorption that is consistent with experimental geometry and the speeds of light and sound, and linear with incident laser pulse energy. The absorbed light signals are independent of the distance of the laser beam from the transducer, while the scattered light signals are strongly distance-dependent. The absorbed light signals are more indicative of true sample absorbance than are readings from a standard spectrophotometer. A spectrophotometer sums contributions from scattering and absorbance to give an optical density reading, while LPAS separates scattering from absorbance to give quantitative information on each function. Scattered light and fluorescent light behave very similarly in the LPAS technique. Our experiments were done with well-characterized colloidal silica samples, but the technique is readily extended to larger, more heterogeneous, and photochemically complex biological and environmental samples.

Mechanistic investigations of iron/sulfur-catalyzed bond scission in aromatic hydrocarbons. A catalytic hydrogen atom transfer step involving a late transition state

Abstract The mechanism of carbon-carbon bond scission by ‘iron sulfide’ catalysts is explored through the use of substituted diphenylmethane model compounds. The reaction kinetics are shown to be first order in model compound with the methylene-arene carbon-carbon bond adjacent to the most substituted arene being selectively cleaved. These results are consistent with the catalyst acting as a hydrogen atom transfer agent.

Mechanistic studies of hydrogen release from solid amine borane materials

Ammonia borane (NH 3 BH 3 ) is a molecular solid with a high volumetric and gravimetric density of hydrogen. We report room temperature structural data which shows how the freely rotating NH 3 and BH 3 groups allow a N-H…H-B dihydrogen bond in which hydrogen atoms on adjacent molecules are separated by only 1.90A. The initial decomposition of ammonia borane at 80-100°C into (NH 2 BH 2 ) n and H 2 has been studied by in-situ nmr spectroscopy and kinetic studies using isotopic substitution. The reaction proceeds by a bimolecular pathway involving a [NH 3 BH 2 NH 3 ] + BH 4 − intermediate with an activation energy of 136kJmol −1 .

Heterolysis of H2 Across a Classical Lewis Pair, 2,6-Lutidine-BCl3: Synthesis, Characterization, and Mechanism

We report that 2,6-lutidine⋅trichloroborane (Lut⋅BCl3 ) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut⋅BHCl2 . The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut⋅BCl3 was calculated to react with H2 and form the ion pair, [LutH(+) ][HBCl3 (-) ], with a barrier of ΔH(≠) =24.7 kcal mol(-1) (ΔG(≠) =29.8 kcal mol(-1) ). Metathesis with a second molecule of Lut⋅BCl3 produced Lut⋅BHCl2 and [LutH(+) ][BCl4 (-) ]. The overall reaction is exothermic by 6.0 kcal mol(-1) (Δr G°=-1.1). Alternate pathways were explored involving the borenium cation (LutBCl2 (+) ) and the four-membered boracycle [(CH2 {NC5 H3 Me})BCl2 ]. Barriers for addition of H2 across the Lut/LutBCl2 (+) pair and the boracycle BC bond are substantially higher (ΔG(≠) =42.1 and 49.4 kcal mol(-1) , respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle BN bond is comparable (ΔH(≠) =28.5 and ΔG(≠) =32 kcal mol(-1) ). Conversion of the intermediate 2-(BHCl2 CH2 )-6-Me(C5 H3 NH) to Lut⋅BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3 . Intramolecular protodeboronation, which could form Lut⋅BHCl2 directly, is prohibited by a high barrier (ΔH(≠) =52, ΔG(≠) =51 kcal mol(-1) ).