Investigating the Kinetics of Homogeneous Hydrogenation Reactions Using PHIP NMR Spectroscopy

Abstract

The combination of parahydrogen induced polarization (PHIP), kinetics and NMR spectroscopy yields a powerful analytical tool: quantitative in situ NMR spectroscopy. Two versions of PHIP NMR experiments are presented to investigate the kinetics of homogeneously catalyzed hydrogenations. The first method, an experimental variation of the ROCHESTER experiment (ROCHESTER = rates of catalytic hydrogenation estimated spectroscopically through enhanced resonances), allows one to determine the hydrogenation rate independently of relaxation and other sources of decay, e.g., subsequent chemical reaction steps. The second method named DYPAS (dynamic PASADENA spectroscopy) uses a variable delay between the end of the hydrogen-addition period and the detection pulse. In principle, all processes during this delay can be described by a set of coupled differential equations. Their solutions can be fitted to the experimental data by a least-squares optimization of the involved kinetic parameters. The DYPAS method can be used to determine the rates of formation as well as the rates of decomposition of stable intermediates and has been applied to the case of freshly hydrogenated and still catalyst-attached product molecules. We provide kinetic data for the formation and decomposition of these unusual product-catalyst complexes during the hydrogenation of different styrene derivatives with a cationic RhI catalyst containing a chelating diphosphine ligand. The kinetic measurements indicate that the rate of formation of the catalyst-attached product increases whereas the rate constant of its decomposition diminishes if the para position of the arene ring of styrene carries an electron-donating substituent. In the case of p-aminostyrene as the substrate, the detachment step turned out to be rate limiting for the catalytic cycle. With certain substituted styrenes and cationic RhI complexes containing chiral chelating diphosphine ligands, two geometrically different (diastereomeric) product-catalyst adducts can be discriminated via PHIP NMR spectroscopy. The associated alternative reaction pathways have been analyzed by applying the DYPAS method, which can also be used to investigate the mechanism of an asymmetric hydrogenation

Role of the reaction intermediates in determining PHIP (parahydrogen induced polarization) effect in the hydrogenation of acetylene dicarboxylic acid with the complex [Rh (dppb)]+ (dppb: 1,4-bis(diphenylphosphino)butane)

“…[5][6][7] The interest in this class of catalysts, in particular with chelating phosphine, namely [(P-P)Rh(I)(diene)], for parahydrogenation reactions is due to the high polarization level that can be achieved on both 1 H and heteronuclear signals. In the presence of these Rh(I) complexes, the hydrogenation reaction follows the so-called unsaturated route (Figure 1), 1 in which the substrate is coordinated to the metal first and then hydrogen is irreversibly transferred to the unsaturated molecule.…”

Kinetic Study of Propylene Hydrogenation over Pt/Al2O3 by Parahydrogen-Induced Polarization

“…During homogeneous hydrogenation processes, many catalysts produce dihydride complexes that are normally invisible in the NMR spectra due to their low concentration; however, these complexes may be revealed by PHIP. The strong PHIP signal enhancement also allows one to study the kinetics of homogeneous reactions [13][14][15][16]. In these studies, kinetic data were obtained in experiments based on a time delay between the introduction of parahydrogen into the system and the NMR measurement.…”

Polarization transfer efficiency in PHIP experiments

“…2a). 9 The time delay Dt is chosen large enough to allow the spins to realign before the next pulse application. The total molar amount of product n tot is the sum of hp molecules P* and thermally polarized (tp) molecules P (n tot (t) = n P* (t) + n P (t)).…”

“…PHIP has become a valuable tool for the analysis of hydrogenation kinetics and reaction mechanisms 9,10 and the application for molecular imaging [11][12][13] is emerging.…”