An isomeric pair of fluoride-bridged cyclic dimolybdenum triads

A pair of isomeric cyclic triads containing three quadruply bonded Mo2] units, Mo2(cis-DAniF)2]2+ (DAniF = N,N'-di-p-anisylformamidinate), bridged by six fluoride anions, has been synthesized and crystallographically characterized. For the alpha isomer, the three Mo2] units are oriented in two orthogonal directions. Two of them are structurally equivalent and parallel to each other, but oriented perpendicular to the third one. The beta isomer is a triangle with three geometrically identical Mo2] units, parallel to each other, as the vertices. Thus, the beta isomer possesses idealized D3h symmetry while the alpha isomer only has C2v symmetry. These two isomers do not interconvert in boiling THF or toluene or under irradiation with ultraviolet light, but oxidation of the alpha isomer first generates an alpha+ species that changes to beta+. The two isomers have very similar electrochemical behavior, both showing three reversible one-electron redox processes for the Mo2] centers and similar potential separations (DeltaE(1/2)). The first and second redox couples are well separated (ca. 390-410 mV), while the second and third ones are separated by only about 150 mV.