A novel series of rhenium-bipyrimidine complexes: synthesis, crystal structure and electrochemical properties

Four novel rhenium complexes of formula [ ReCl4( bpym)] (1), [ ReBr4( bpym)] ( 2) PPh4[ ReCl4( bpym)] ( 3) and NBu4[ReBr4(bpym)] ( 4) ( bpym= 2,2'- bipyrimidine, PPh4 = tetraphenylphosphonium cation and NBu4 = tetrabutylammonium cation), have been synthesized and their crystal structures determined by single-crystal X- ray diffraction. The structures of 1 and 2 consist of [ ReX4( bpym)] molecules held together by van der Waals forces. In both complexes the Re(IV) central atom is surrounded by four halide anions and two nitrogen atoms of a bpym bidentate ligand in a distorted octahedral environment. The structures of 3 and 4 consist of [ ReX4( bpym)]- anions and PPh4+ ( 3) or NBu4+ ( 4) cations. The coordination sphere of the Re(III) metal ion is the same as in 1 and 2, respectively. However, whereas the Re - X bonds are longer the Re -N bonds are shorter than in 1 and 2. This fact reveals that the bpym ligand forms a stronger bond with Re( III) than with Re( IV) resulting in a stabilisation of the lower oxidation state. [ ReX4( bpym)] complexes are easily reduced, chemically and electrochemically, to the corresponding [ ReX4( bpym)](-) anions. A voltammetric study shows that the electron transference is a reversible process characterized by formal redox potentials of + 0.19 V ( 1) and + 0.32 V ( 2) vs. NHE, in acetonitrile as solvent.