Synthesis and Electrochemistry of 2,2′-Bipyridyl Complexes of Dioxorhenium(V)

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 C_2υ. symmetry. Very recent X-ray investigations (to be reported elsewhere) have now confirmed the cis assignment.