Parahydrogen Induced Polarization PHIP

The study of detailed chemical reaction mechanisms in homogeneous catalysis requires the identification and characterization of reaction intermediates. However, limitations arise due to both the short life time (transient type) and the low concentration of such species [203]. 

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Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

The mechanism of H2 addition to Cp 2Ta(=CH2)H has been studied using parahydrogen-induced polarization (PHIP), C labeling, and comparison to the related complex Cp 2Ta(=CH2)Me. The reaction of />ara-enriched H2 with labeled Cp 2Ta(= CH2)H leads to polarized resonances for both Cp 2TaH3 and Cp 2Ta(CH3)H2, where the polarized resonances correspond to the two hydride hgands. The results thus support a pathway of H2 addition to a Ta(lll) intermediate Cp 2TaCH3, rather than addition directly across the Ta=C bond. ... 

Lanthanide chelates have been used as aids in studying the products of hydrogenation reactions monitored through parahydrogen-induced polarization (PHIP). Carrying out... 

Theory of Parahydrogen-induced Polarization (PHIP) and Its Mechanistic Significance 430... 

Proved examples, which can be viewed to involve initial intramolecular heterolysis of an H2 ligand, are rare. Unstable but IR-detectable species such as M(CO)4(norbotnadiene)(H2) are proposed intermediates for photocata-lytic hydrogenation of norbornadiene to norbornene and nortricyclene involving direct reaction of H2 ligands. Evidence for direct transfer of to co-bound substrates is limited, but parahydrogen induced polarization (PHIP) provides crucial NMR evidence. The Mo-norbornadiene (NBD) complex in Scheme 4 forms an unstable H2 complex on photolysis that is proposed to directly transfer H2 to NBD to form norbornene (NBN). ... 

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