Modifying Electronic Communication in Dimolybdenum Units by Linkage Isomers of Bridged Oxamidate Dianions

Reactions of Mo₂(O₂CCH₃)(DAniF) ₃, DAniF = N, N′-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr]^2-, Ar = C₆H₅ and p-anisyl, give pairs of isomeric compounds where the [Mo₂] units are bridged by the oxamidate anions. For the α isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the β isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the ΔE_1/2 for the oxidation of each of the Mo₂ units is significantly better for the β isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the β isomers but not the α isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the α isomers and the relative energy differences between the α and β isomers.