Electronic aspects of the phosphine-oxide → phosphinous acid tautomerism and the assisting role of transition metal centers

Abstract

The H3P(O) → H2P(OH) tautomerism is addressed by experimental and DFT approaches. The process, disfavored for the free molecule, is easier over metal fragments of the type {CpRuIIL2}n (L = uncharged or anionic phosphine ligand), with an energy barrier reduced to one fourth. The free H3P(O) molecule is a very weak acid and hardly a proton migrates intra-molecularly towards the oxo atom, as expected for classic acid-base reactions. Rather, some electron density of the highly covalent P–H bond remains anchored to the H atom at least up to the TS with the barrier originated from the electronic repulsion with the approached O lone pair. Beyond TS, the H atom transforms into a proton after having released its electron portion at the P atom (lone pair). The calculations show the experimentally undetected intermediate [CpRu(PR3)2(H)(H2PO)]n, at which the metal has induced a P–H oxidative addition. Consistent behaviors are found for all the molecules Hn(OH)3−nP(O) (n = 3, 2, 1), whereas some anomalies have been experimentally observed with the anionic TPPMS coligands [TPPMS = PPh2(m-C6H4SO3)−], used to favor the chemistry in water. In particular: i) the reaction with H3P(O) indicates that the product [CpRu(TPPMS)2{H2P(OH)}]− exists in two isomeric forms; ii) the tautomerization of H(OH)2P(O) is uniquely inhibited. Ad-hoc DFT calculations indicate that the features are attributable to the strong H-bonding networks between the sulphonate substituent and OH group(s) and water as well.