Syntheses, Structures, and Electronic Interactions of Dicyanamide/Tricyanomethanide-Bridged Binuclear Organometallic Complexes

Dicyanamide-bound mononuclear compounds Cp(dppe)FeN(CN)2 (3) and Cp(PPh3)2RuN(CN)2 (4) were isolated in
high yields by the reactions of Cp(dppe)FeCl (1) and Cp(PPh3)2RuCl (2), respectively, with excess sodium
dicyanamide. Compounds 3 and 4 are excellent precursors for the design of dicyanamide-bridged binuclear complexes
[{Cp(dppe)Fe}2N(CN)2](SbF6) (5) and [{Cp(PPh3)2Ru}2N(CN)2](SbF6) (6) by the incorporation with 1 and 2,
respectively. Controlling oxidation of 5 with ferrocenium hexafluorophosphate afforded the mixed-valence compound
[{Cp(dppe)Fe}2N(CN)2](PF6)2 (5a) which exhibits a broad absorption band in the near-infrared region (centered at
1500 nm, ε = 750 cm-1 M-1) due to the intervalence charge transfer of Robin and Day class II mixed-valence
system. Tricyanomethanide-bound mononuclear compounds Cp(dppe)FeC(CN)3 (7) and Cp(PPh3)2RuC(CN)3 (8)
were prepared by the same methods as 3 and 4 using potassium tricyanomethanide as the starting material
instead. The tricyanomethanide-bridged binuclear complexes [{Cp(dppe)Fe}2C(CN)3](CF3SO3) (9) and [{Cp(PPh3)2Ru}2C(CN)3](SbF6) (10) were prepared by the reactions between 7 and 1 and between 8 and 2, respectively.
Cyclic voltammograms of the dicyanamide/tricyanomethanide-bridged binuclear complexes showed stepwise reversible
one-electron oxidation waves with the potential separation of the two redox couples in the range 0.14−0.25 V,
indicating the demonstrably electronic communication is operative between the organometallic components through
a dicyanamide/tricyanomethanide spacer with metal···metal distances more than 7.8 Å. Furthermore, the electronic
coupling transmitted by the tricyanomethanide is appreciably greater than that by the dicyanamide. The complexes
3−10 were characterized by elemental analysis, IR, UV−vis, 1H and 31P NMR, and ES-MS. The crystal structures
of 3 and 5−9 were determined by X-ray crystallography.