Rhenium chemistry of azooximes: oxygen atom transfer, azoimine chelation, and imine-oxime contrast

Abstract

The concerned azooximes (L¹OH, 1) are of type p-X-C₆H⁴C(N₂Ph)(NOH) (X = H, Me, Cl). The reaction of [Re(MeCN)Cl₃(PPh₃)₂] with [Ag(L¹OH)(L¹O)] in cold dichloromethane-acetonitrile solvent has furnished the green colored ionized azoimine complex [ReV(O)Cl(PPh₃)₂(L¹)](PF₆), 2. In effect L¹O^- has undergone oxidative addition, the oxygen atom being transferred to the metal site. Upon treatment of [ReV(NPh)Cl₃(PPh₃)₂] with L¹OH in solution, the neutral azoimine complex [ReV(NPh)Cl₃(L¹H)], 3, resulted due to the spontaneous transfer of the oxime oxygen atom to a PPh₃ ligand, which is eliminated as OPPh₃. In contrast, the oxime of 2-acetylpyridine (L²OH, 4) did not undergo oxygen atom transfer and simply afforded the imine-oxime complex [ReV(NC₆H₄Y)Cl₂(PPh₃)(L²O)], 5, upon reacting with [ReV(NC₆H₄Y)Cl₃(PPh₃)₂] (Y = H, Me, Cl). The spectral and electrochemical properties of 2, 3, and 5 and the structures of three representative compounds are reported. In the cation of 2 (X = H) the two PPh₃ ligands lie trans to each other and the equatorial plane is defined by the five-membered azoimine chelate ring and the oxo and chloro ligands. The oxo ligand which forms a model triple bond (Re-O length 1.616(6) Å) lies cis to the imine-N atom. In 3 (X = Cl) the ReCl₃ fragment has meridional geometry and the imido nitrogen lies trans to the imine nitrogen of the planar azoimine chelate ring. In 5·H₂O (Y = Me), the Cl, oximato-N, and P atoms define an equatorial plane and the pyridine-N lies trans to the imido-N. The water of crystallization is hydrogen bonded to the oximato oxygen atom (O···O, 2.829(5) Å). Reaction models in which chelation of the azooxime precedes oxygen atom transfer are proposed on the basis of oxophilicity of trivalent rhenium, Lewis acid activity of pentavalent rhenium, electron withdrawal by the azo group, and observed relative disposition of ligands in products.