The rhenium(V) bipyridyl (bpy) complexes [(bpy)(py)2Re(O)2](C1O4) and [(bpy)(3-pySO3)2Re(O)2](Na)-2H2O have been prepared by reaction of 2,2′-bipyridine with trans-[(py)4Re(0)2](Cl) and trans-[(3-pySO3)4Re(O)2](Na)3in methanol or methanol/water solutions (py = pyridine, 3-pySO3= 3-pyridinesulfonate). Despite the seemingly trivial change in coordination environment, the bpy-containing complexes were found to behave very differently from the corresponding tetrapyridyl complexes. For example, the pH-independent formal potential for reduction of Re(VI) to Re(V) was lowered by some 600 mV. On the other hand, the pH-dependent potential for the Re(V/III) couple increased by ca. 200 mV upon bpy coordination. The V/III couple also became considerably more reversible (kinetically) in its electrochemical response. The Re(III/II) potential was also shifted in the positive direction. At high pH’s, the V/III and III/11 couples coalesced to yield the first example of three-electron redox behavior in rhenium chemistry. By analogy with known dioxoosmium complexes, both the thermodynamic and kinetic electrochemical effects were attributed to a relative stabilization of the inaccessible Re(IV) oxidation state following bpy coordination. It was proposed that the destabilization is induced by trans to cis isomerization. The cis-dioxo configuration has only rarely been seen in d2coordination chemistry, and never for hexacoordinate rhenium. Nevertheless, circumstantial evidence from IR, NMR, and Raman experiments supported the assignment. Unfortunately, proof was not possible by these methods since both cis- and trans-(bpy)(py)2Re(O)2+would possess C2υ. symmetry. Very recent X-ray investigations (to be reported elsewhere) have now confirmed the cis assignment.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry