Synthesis and Electronic Structure of Ru-2(Xap)(4)(Y-gem-DEE) Type Compounds: Effect of Cross-Conjugation

by Forrest, W. P.; Choudhuri, M. M. R.; Kilyanek, S. M.; Natoli, S. N.; Prentice, B. M.; Fanwick, P. E.; Crutchley, R. J.; Ren, T.

Reported in this Article are the preparation and characterization of a series of new Ru-2(II,III) compounds bearing one cross-conjugated sigma-geminal-diethynylethene ligand (gem-DEE), namely, Ru-2(Xap)(4)(Y-gem-DEE) (Xap = N,N'-anilinopyridinate (ap) or 2-(3,5-dimethoxy)anilinopyridinate (DiMeOap), and Y = (SiPr3)-Pr-i (1) or H (2)) and [Ru-2(ap)(4)](2)(mu-gem-DEE) (3). Compounds 1-3 were characterized by spectroscopic and voltamrnetric techniques as well as the single crystal X-ray diffraction study of 2a. The X-ray structural data of 2a and the spectroscopic/voltammetric data of compounds 1 and 2 indicate that the gem-DEE ligands are similar to simple alkynyls in their effects on the molecular and electronic structures of the Ru-2(Xap)(4) moiety. Similar to the previously studied [Ru-2(ap)(4)](2)(mu-C-2n) type compounds, dimer 3 exhibits pairwise 1e(-) oxidations and reductions, albeit the potential splits within the pair (Delta E-1/2) are significantly smaller than those of [Ru-2(ap)(4)](2)(mu-C-4). The electronic absorption spectra of the reduced and oxidized derivatives of la and 3 were determined using spectroelectrochemistry methods. No discernible intervalence charge transfer transition (IVCT) was detected in the near-IR spectrum for either 3(-) or 3(+), suggesting that the Ru-2-Ru-2 coupling in these mixed-valence states is weak. DFT calculations on a model compound of 3 yielded six singly occupied molecular orbitals (SOMOs), which have Ru-2 contributions similar to those previously calculated for the [Ru-2(ap)(4)](2)(mu-C-2n) type compounds. Among six SOMOs, SOMO-2 is the only one containing substantial d pi-pi(gem-DEE) character across the entire Ru-2-mu-gem-DEE-Ru-2 linkage, which explains the weakened Ru-2-Ru-2 coupling.

Inorganic Chemistry
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