Tautomerism of hydrophosphoryl compounds and their features as ligands in metal complex catalysis. Quantum-chemical simulations by the density functional method

The density functional method (gradient-corrected nonempirical functional PBE, basis TZ2p) was used to perform a large-scale study of the mechanism of tautomerization of hydrophosphoryl compounds RR′P (H)O ? RR′POH (R,R′ = Alk, Ar, OR, NR₂). It was shown that intramolecular proton transfer in this rearrangement is forbidden (activation barriers 43.3–60 kcal mol^?1), and, in the absence of carrier molecules, it occurs as synchronous transfer of two protons in fairly strong dimeric associates (2.50–10.5 kcal mol^?1) formed due to O-H···O, O-H···P, and C-H···O hydrogen bonding. The process involves six-membered transition states with activation barriers of 5–15 kcal mol^?1. The contribution of tunneling into the rate constants at 300–400 K, according to estimates in terms of the reaction-path Hamiltonian formalism, reaches 20–40% and increases as the temperature decreases. The mechanism of ethylene hydroformylation in a model complex of a hydrophosphoryl compound with Pt(II) [(H₂PO)₂H]Pt(PH₃)(H)] was considered to reveal factors responsible for the high efficiency of such complexes in the reaction studied. It was found that the key stages of the catalytic cycle involve reversible proton migration in the ?PH₂OH··· O=P chain of the quasi-chelate ring, which provides fine tuning of the electron distribution in the catalytic node and thus functions as a molecular switcher.