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

Dirghangi, Bimal Kumar ; Menon, Mahua ; Pramanik, Amitava ; Chakravorty, Animesh (1997) A triad of variable-valent rhenium aldimine and amide systems interrelated by successive oxygen atom transfer Inorganic Chemistry, 36 (6). pp. 1095-1101. ISSN 0020-1669

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Official URL: http://pubs.acs.org/doi/abs/10.1021/ic9611094

Related URL: http://dx.doi.org/10.1021/ic9611094


The title systems are ReVOCl3(RA), 1, ReIII(OPPh3)Cl3(RA), 2, and ReIV(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 (E1/2 0.3 V vs SCE), 3+/3 (E1/2 1.3 V), and 3/3- (E1/2 ~-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) Å3; Z = 2.

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