Nuclear magnetic resonance studies of homogeneous catalysis using parahydrogen: Analysis of nuclear singlet–triplet mixing as a diagnostic tool to characterize intermediates

Abstract

The 1H–NMR polarization spectra of the products after homogeneous hydrogenations with para- or orthohydrogen (the PHIP spectra) convey significant information about the course of the reaction. Mixing of the nuclear singlet and triplet states of the two protons derived from dihydrogen during the reaction cycle has been found to modify the PHIP signal pattern of the product molecule characteristically. The intermediate coordination of the two dihydrogen protons at the catalyst is proposed to be responsible for this behavior. A parallel to the singlet–triplet mixing of electronic states according to the well-known CIDNP phenomenon is outlined. The homogeneous hydrogenation of 2,5-dihydrofuran to tetrahydrofuran using Wilkinson’s catalyst serves as an example.

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)

“…difference between the two parahydrogen protons on reaction intermediate as reported by Bargon et al 16 or to chemical shift anisotropy relaxation mechanism. Therefore a different process from that previously reported must be involved.…”

Investigating Catalytic Processes with Parahydrogen: Evolution of Zero-Quantum Coherence in AA′X Spin Systems

“…[12] and [13], the creation of polarization signals on the hetero nucleus X can be understood. The application of a pulse along the y axis to the hetero nucleus X with a flip angle ␣ transforms the two-spin order terms (I 1z Ϫ I 2z )I 3z with a scaling factor of sin ␣ into single-quantum antiphase terms of the form (I 1z Ϫ I 2z )I 3x .…”

“…This was proven by showing that the structure of the polarization spectra depends crucially on the catalyst and substrate used (13 ). Hence, the PHIP phenomenon also reveals information about hydrogenation intermediates with a lifetime much too short to be directly detectable by NMR spectroscopy.…”

“…It was, however, quickly realized that this effect only occurs if the two former parahydrogen nuclei are strongly coupled in the product molecule. The S/T 0 mixing was analyzed for hydrogenations into symmetric substrates, that is, into substrates where only the natural content of 13 C isotopes breaks the symmetry of parahydrogen. In these substrates, the relevant spin systems were of AAЈX character (14 ).…”

Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: a parahydrogen based investigation

“…[17][18][19][20] Workers have also used this method to examine not only a range of heterogeneous reactions [21][22][23][24][25] but some that don't involve a metal centre. [26][27][28][29] The PHIP approach has therefore developed substantially from the early starting point of Weitekamp, 30, 31 Eisenberg 32,33 and Bargon, 34,35 as illustrated in several reviews. 11,12 In order to understand the physical basis of PHIP, p-H 2 needs to be recognised as simply dihydrogen that exists in the anti-symmetric magnetic state that is represented by the nuclear wave function  { .…”