Equilibration of ortho- and para-hydrogen by homogeneous hydrogenation catalysts in solution; A test for the reversibility of hydrogen addition using Raman spectroscopy

para-Enriched hydrogen is converted into the ortho-para equilibrium mixture by tris(triphenylphosphine)rhodium chloride in toluene solution. The rotational Raman effect can be used to show that this equilibration occurs during the course of hydrogenation of cyclohexene by the above catalyst. The methanol solvate formed by reduction of 1,4-bis(diphenylphosphino)butanebicyclo2.2.1] heptadienerhodium tetrafluoroborate also catalyses the ortho-para equilibration of hydrogen, in common with related cationic rhodium complexes. No equilibration occurs, however, during the course of hydrogenation of (Z)-α-benzamidocinnamic acid by cationic chelate rhodium complexes, including that derived from the chiral ligand (R,R)-1,2-bis(o-methoxyphenylphenyl)ethane (DIPAMP), thus demonstrating that addition of hydrogen to rhodium is irreversible.