Synthesis, structure and redox behaviour of facial [Re^IIIL(PPh₃)Cl₃] and its stereoretentive conversion to [Re^IVL′(PPh₃)Cl₃] via metal promoted aldimine → amide oxidation (L = pyridine-2-aldimine; L′ = pyridine-2-carboxamide)

Abstract

The reaction of mer-[Re^IIIL(OPPh₃)Cl₃] 2 with PPh₃ in benzene has afforded bluish violet fac-[Re^IIIL(PPh₃)Cl₃] 1, where L is the Schiff base 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 (σ- and π-donor), PPh₃ (σ-donor and π-acceptor) and L (σ-donor and π-acceptor). The cyclic voltammetric E1/2 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-[Re^IVL′(PPh₃)Cl₃] 3 which is also obtainable via oxidation by dilute nitric acid (L′ is a monoanionic pyridine-2-carboxamide). Complex 3 displays Re^IV–Re^III and Re^V–Re^IV couples near –0.2 V and 1.4 V respectively 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 configured RePN₂Cl₃ coordination spheres. The average Re–Cl distance is lower by 0.06 Å in 3 due to contraction of the metal radius upon oxidation. The Re–P length is however larger by 0.1 Å in 3 signifying a weakening of Re–P back-bonding.