Strong electronic interaction between two dimolybdenum units linked by a tetraazatetracene

The large rigid dianion fluoflavinate, C14H8N42-, consisting of four fused and planar six-membered rings with four nitrogen donor atoms, has been used to link two metal-to-metal bonded and redox-active Mo-2(n+) units which are each locally bridged by three additional groups, collectively denoted [Mo-2]. In 1, the [Mo-2] units are Mo-2(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), and in 5, they are trans-Mo-2(DAniF)(2)(O2CCH3) groups, These [Mo-2](fluoflavinate)[Mo-2] compounds show three reversible one-electron oxidation steps, one more than all other [Mo-2](linker)[Mo-2] species known to date. The first two redox processes are metal-based, and the third one has been assigned to a ligand oxidation by comparison to that of paddlewheel compound 4 which contains only one dimolybdenum unit with a monoanionic fluoflavinate ligand. Chemical oxidations of 1 produce the singly- and doubly-oxidized species 2 and 3, respectively. All compounds have been characterized by X-ray crystallography and, as appropriate, by various techniques such as NMR, EPR, near-IR, and UV-vis. The fluoflavinate ligand strongly mediates electronic communication between the dimetal units, and the mixed valence species 2 can be described as electronically delocalized. Calculations at the DFT level using a variety of functionals support such an assignment and indicate that a strong transition in the NIR for the singly oxidized species can be assigned to the HOMO-1 to SOMO transition.