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

Abstract

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.