A triad of variable-valent rhenium aldimine and amide systems interrelated by successive oxygen atom transfer

Abstract

The title systems are Re^VOCl₃(RA), 1, Re^III(OPPh₃)Cl₃(RA), 2, and Re^IV(OPPh₃)Cl₃(RB), 3, where RA is a 2-pyridinecarboxaldimine, p-RC₆H₄N=CHC₅H₄N, and RB^- is the corresponding 2-picolinamide, p-RC₆H₄NC(=O)C₅H₄N^- (R = H, Me, OMe, Cl). Controlled reaction of ReOCl₃(PPh₃)₂ with RA affords 1, which is converted to 2 upon reaction with PPh₃. The oxidation of 2 in aqueous media by Ce⁴⁺ or H₂O₂ furnishes 3. The X-ray structure of 1 (R = Me) has revealed meridional ReCl₃ geometry, the oxo atom lying trans to the pyridine nitrogen of chelated MeA. The metal atom is displaced by 0.35 Å toward the oxo atom from the ReCl₃N(aldimine) plane. The couples 2⁺/2 (E_1/2 0.3 V vs SCE), 3⁺/3 (E_1/2 1.3 V), and 3/3^- (E_1/2 ~-0.5 V) are observable electrochemically. The conversion 1 → 2 follows a second-order rate law with a large and negative entropy of activation (~-40 eu). The reaction is proposed to proceed via nucleophilic attack by the phosphine on Re=O; the observed effects of R and phosphine variation on the rate are consistent with this. In the conversion of 2 to 3, the active species is 2⁺, the stoichiometry of the reaction being 32⁺ + H₂O → 22 + 3 + 3H⁺. The amide oxygen in 3 originates from the water molecule. The reaction follows a second-order law, and the entropy of activation is large and negative, ~-30 eu. The rate-determining addition of water to the aldimine function is believed to afford an α-hydroxy amine intermediate which undergoes induced electron transfer and associated changes, affording 3. Crystal data for ReOCl₃(MeA) are as follows: empirical formula C₁₃H₁₂Cl₃N₂ORe; crystal system triclinic; space group P↑; a = 7.136(4) Å, b = 8.329(5) Å, c = 14.104(9) Å, α = 73.88(5)°, β = 76.35(5)°, γ = 85.64(5)°; V = 782.5(8) ų; Z = 2.