Influence of Bonding Mode of the Linkers in the Electronic Communication of Molecular Pairs Having Dimolybdenum Units Linked by Pseudohalides

Depending on conditions the reactions of [Mo2(cis-DAniF)2(NCCH3)4](BF4)2 (DAniF = N,N′-di-p-anisylformamidinate) with solutions containing thiocyanate anions lead to two compounds: a quadruple-bonded dinuclear species 1, (Bun4N)2[Mo2(cis-DAniF)2(NCS)4], and a molecular pair 3, [Mo2(cis-DAniF)2]2(μ1,3-NCS)4. The latter has a cuboidal structure having two (cis-DAniF)2Mo22+ units, [Mo2], with four thiocyanate groups bridging two [Mo2] units in an end-to-end fashion in which the N and S atoms serve as the bridging units. On the contrary, the structure of the cyanate isomer, [Mo2(cis-DAniF)2]2(μ1,1-NCO)4 (2), shows an end-on binding of the cyanate linkers. Various physical measurements of 2 and its oxidized species 2·PF6 indicate that there is strong electronic communication between the two dimetal cores. For 1, two reversible oxidation processes were observed in the cyclic voltammogram corresponding to the successive oxidation to 1+ and an uncommon 12+ species that has a triple-bonded Mo26+ core. DFT calculations indicate that the antibonding character between the Mo−Mo δ orbitals and thiocyanate p orbitals plays a very important role in elevating the HOMO δ orbital energy that allows formation of the dication. A selenium isomer of 3 was also studied. In both the thiocyanate and selenocyanate bridged dimers of dimers, in which the pseudohalide bridges bind the two dimetal units in an end-to-end fashion, long separations between the dimetal units are observed, and these generate very weak electronic interactions.