Synthesis, structure and redox behaviour of facial [(ReL)-L-III(PPh3)Cl-3]and its stereoretentive conversion to [(ReL)-L-IV '(PPh3)Cl-3] via metal promoted aldimine -> amide oxidation (L = pyridine-2-aldimine; L ' = pyridine-2-carboxamide

The reaction of mer-[(ReL)-L-III(OPPH3)Cl-3] 2 with PPh3 in benzene has aff orded bluish violet fac-[(ReL)-L-III(PPh3)Cl-3] 1, where L is the Schiff ba se of pyridine-2-carbaldehyde and the substituted aniline p-XC6H4NH2 (X = H , Me, OMe or Cl). Geometrical preferences are rationalized in terms of the electronic nature of the ligands OPPh3 (sigma- and pi- donor), PPh3 (sigma- donor and pi-acceptor) and L (sigma-donor and pi-acceptor). The cyclic volt ammetric E-1/2 values of 1 lie near 0.6 V (Re-IV-Re-III) and -0.6 V (Re-III -Re-II). Electrooxidation of 1 at 1.0 V vs. SCE in wet acetonitrile affords yellow fac-[(ReL)-L-IV'(PPh3)Cl-3] 3 which is also obtainable via oxidatio n by dilute nitric acid (L' is a monoanionic pyridine-2-carboxamide). Compl ex 3 displays Re-IV-Re-III and Re-V-Re-IV couples near -0.2 V and 1.4 V res pectively reflecting superior stabilization of the rhenium(IV) state by the amide ligand. The X-ray structures of two representative complexes of type 1 and 3 have revealed the presence of severely distorted and facially conf igured RePN2Cl3 coordination spheres. The average Re-Cl distance is lower b y 0.06 Angstrom in 3 due to contraction of the metal radius upon oxidation. The Re-P length is however larger by 0.1 Angstrom in 3 signifying a weaken ing of Re-P back-bonding.