Rhenium Chemistry of Azooximes: Oxygen Atom Transfer, Azoimine Chelation, and Imine−Oxime Contrast

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.