PHIP Studies of Reaction Intermediates

While certainly useful, the information about the reaction obtained via PHIP of reaction products is inevitably rather limited. However, PHIP can provide a much deeper insight into the reaction mechanisms based on the possibility to detect short-Uved reaction intermediates. The symmetry of the pHj molecule is often broken early in the catalytic cycle, which can lead to the strong signal enhancement of reaction intermediates that are present in low concentrations and thus escape detection in conventional NMR studies. This made it possible in numerous studies to use PHIP to detect for the first time certain reaction intermediates and other species involved, and to establish their detailed structure based on the data about their chemical shifts and the spin-spin couplings of the pH2-derived hydrides with the magnetic nuclei of the ligands and sometimes the metal as well (e.g., Rh, Pt) in these complexes. 

In addition to the metal dihydrides and metal dihydrido substrate complexes, other mechanistically significant intermediates are sometimes detected. For instance, for Pd complexes, the intermediates formed upon incorporation of one hydride in the substrate (e.g., palladium vinyl intermediates) have been observed [56, 57). If the metal dihydride is observable in the presence of an alkene, further insight into the reaction mechanism can be provided using exchange-type pulse sequences to accomplish magnetization transfer from a hydride signal to the hydrogens of a product [27, 58). 

Activation of Hj and Structure and Dynamics of Metal Dihydride Complexes 

S = solvent. Adapted with permission from Ref. [59]. Copyright 2001 American Chemical Society. 

Structural characterization of the dihydrides of Rh and Ir phosphine complexes with properly modified 2D NMR methods (such as COSY, HSQC, HMQC, and NOESY) was reported, which, in particular, can address heteronuclear correlations involving such nuclei as and ° Rh [27,63]. Dynamic transformations can be addressed with ID and 2D NOESY/EXSY methods. The exchange between the dihydrides and pH2 can be studied efficiently as it leads to the permanent replenishment of hyperpolarization of the dihydride complex. Ligand (e.g., CO, PPhj) exchange has been addressed as well. 

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