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

The title systems are (ReOCl3)-O-V(RA), 1, Re-III(OPPh(3))Cl-3(RA), 2, and Re-IV(OPPh(3))Cl-3(RB), 3, where RA is a 2-pyridinecarboxaidimine, p-RC(6)H(4)N=CHC5H4N, and RB(-) is the corresponding 2-picolinamide, p-RC(6)H(4)NC(=O)C5H4N- (R = H, Me, OMe, Cl). Controlled reaction of ReOCl3(PPh(3))(2) with RA affords 1, which is converted to 2 upon reaction with PPh(3). 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 Angstrom toward the oxo atom from the ReCl3N(aldimine) plane. The couples 2(+)/2 (E(1/2) similar to 0.3 V vs SCE), 3(+)/3 (E(1/2) similar to 1.3 V), and 3/3(-) (E(1/2) similar to -0.5 V) are observable electrochemically. The conversion 1 --> 2,follows a second-order rate law with a large and negative entropy of activation (similar to-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, similar to-30 eu. The rate-determining addition of water to the aldimine function is believed to afford an alpha-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(1)over bar>; a = 7.136(4) Angstrom, b = 8.329(5) Angstrom, c = 14.104(9) Angstrom, alpha = 73.88(5)degrees, beta = 76.35(5)degrees, gamma = 85.64(5)degrees; V = 782.5(8) Angstrom(3); Z = 2.