Yuri A. Ustynyuk

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.

The structures of dimers of several types of dimethylphosphinous acid (CH₃)₂POH and dimethylphosphine oxide (CH₃)₂P(O)H and dimers of the corresponding perfluorinated derivatives (CF₃)₂POH and (CF₃)₂P(O)H were studied in detail by density functional theory with the PBE gradient-corrected functional and the TZ2p basis set. Fairly strong dimeric associates (2.50–10.5 kcal/mol) were shown to form thanks to O-H···O, O-H···P, and C-H···O H-bonds and dipole-dipole interactions of polar phosphoryl groups P → O of two monomer molecules. The existence of C-H···O and the absence of P-H···O H-bonds in (CH₃)₂P(O)H dimers was substantiated by an AIM (atoms in molecules) analysis of their structures according to Bader. The reaction coordinates were calculated for synchronous transfer of two protons in (CH₃)₂POH and (CF₃)₂P(O)H dimers. Both rearrangements were shown to occur via symmetrical six-membered planar transition states with activation barriers of less than 20 kcal/mol, which was much lower than for intramolecular transfer in the corresponding monomers (47 kcal/mol for the (CH₃)₂P(O)H → (CH₃)₂POH pair). The tautomeric transitions between the phosphinous acid and phosphine oxide forms observed experimentally in nonpolar media under mild conditions in the absence of molecules that could act as proton carriers were shown to proceed as bimolecular reactions with the intermediate formation of the corresponding dimers.

The geometry, electron density distribution, dipole moments, energy and nodal properties of frontier orbitals, and relative stability of tautomeric forms were studied using density functional theory (PBE, TZ2p basis set) for a representative series of the RR’P(H)O (IIIa) ⇆ RR’POH (IIIb) (R, R’ = H, CH₃, CF₃, C₆H₅, C₆F₅, N(CH₃), OCH₃, and N(CF₃)₂ in various combinations) hydrophosphoryl compounds and their complexes with water molecules formed with the participation of H-bonds of various types. Electron donor substituents were shown to stabilize the oxide form (IIIa), and electron acceptor substituents, the phosphinuos acid form (IIIb). The induction and resonance effects were found to be approximately equally significant, which was in agreement with the available experimental data. Oxides IIIa with donor substituents R and R’ formed the strongest H-bonds with water molecules coordinated at the phosphoryl group oxygen atom, which additionally stabilized these forms compared with their tautomers IIIb. At the same time, phosphinuos acids RR’POH (IIIb) with acceptor substituents more readily acted as H-bond donors with respect to water molecules and formed coordination bonds with water oxygens, which shifted tautomeric equilibria toward them. Intramolecular hydrogen transfer was shown to be forbidden in tautomeric equilibria. A description of these equilibria therefore required using other models with the participation of proton donor molecules as carriers or hydrophosphoryl compound dimers.

The quantum-chemical simulation (DFT, PBE, TZ2p basis set) of the mechanism of catalytic hydrogenation of compounds containing R_nX → O semipolar bonds (R_nX = N₂, Me₂S, C₅H₅N, Ph₃P) on the Wilkinson catalyst (Ph₃P)₃RhCl with para-hydrogen showed that this process proceeds with retention of proton nuclear spin correlation, which enables a principal possibility to synthesize para-H₂O, i.e. the nuclear spin isomer of water with antiparallel proton spins, using this route.

A new 2,6-diamidopyridinedipyrromethane hybrid macrocycle (1) has been synthesized that shows a high selectivity for dihydrogen phosphate and hydrogen sulfate relative to nitrate in acetonitrile solution as judged from UV-vis spectrophotometric titrations; this leads to the suggestion that this or related systems might find use in nuclear waste remediation applications requiring the selective removal of hydrogensulfate from nitrate-rich waste mixtures.