A Triad of Variable-Valent Rhenium Aldimine and Amide Systems Interrelated by Successive Oxygen Atom Transfer

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Abstract
The title systems are Re(V)OCl(3)(RA), 1, Re(III)(OPPh(3))Cl(3)(RA), 2, and Re(IV)(OPPh(3))Cl(3)(RB), 3, where RA is a 2-pyridinecarboxaldimine, p-RC(6)H(4)N=CHC(5)H(4)N, and RB(-) is the corresponding 2-picolinamide, p-RC(6)H(4)NC(=O)C(5)H(4)N(-) (R = H, Me, OMe, Cl). Controlled reaction of ReOCl(3)(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 Ce(4+) or H(2)O(2) furnishes 3. The X-ray structure of 1 (R = Me) has revealed meridional ReCl(3) 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 ReCl(3)N(aldimine) plane. The couples 2(+)/2 (E(1/2) approximately 0.3 V vs SCE), 3(+)/3 (E(1/2) approximately 1.3 V), and 3/3(-) (E(1/2) approximately -0.5 V) are observable electrochemically. The conversion 1--> 2 follows a second-order rate law with a large and negative entropy of activation ( approximately -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(+) + H(2)O --> 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, approximately -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 ReOCl(3)(MeA) are as follows: empirical formula C(13)H(12)Cl(3)N(2)ORe; crystal system triclinic; space group P&onemacr;; a = 7.136(4) Å, b = 8.329(5) Å, c = 14.104(9) Å, alpha = 73.88(5) degrees, beta = 76.35(5) degrees, gamma = 85.64(5) degrees; V = 782.5(8) Å(3); Z = 2.