Dissymmetrical <i>trans-</i>Ethynyl-Butadiynyl Adducts on a Diruthenium Core: Synthesis, Characterization, and Selective Deprotection

Reactions between the ethynyl complexes (4,0)-[Ru2(ap)4](C2SiR3) (ap is 2-anilinopyridinate, R = iPr (1a) and CH3 (1b)) and LiC4SiMe3 result in the formation of dissymmetrical ethynyl-butadiynyl adducts trans-(4,0)-(Me3SiC4)[Ru2(ap)4](C2SiR3) (R = iPr (2a) and CH3 (2b)). Treating 2b with K2CO3 in MeOH/THF leads to trans-(HC4)[Ru2(ap)4](C2SiMe3) (3b) and trans-(HC4)[Ru2(ap)4](C2H) (4) in 86% and ca. 10% yields, respectively, and the former can be quantitatively converted to 4 using NaOH. Treating 2a with NaOH in MeOH/THF yields trans-(HC4)[Ru2(ap)4](C2SiiPr3) (3a) only. Single-crystal structural analysis of 2a and 3b revealed that the Ru−Ru unit and the axial alkynyl ligands are approximately collinear in both molecules. Rich redox chemistry was revealed for all the compounds through voltammetric study: compound 1 exhibits reversible one-electron oxidation and reduction, and compounds 2−4 exhibit one one-electron oxidation and two one-electron reductions. All the ethynyl-butadiynyl adducts (2−4) exhibit an intense charge-transfer absorption of λmax around 1035 nm, revealing a HOMO−LUMO gap of 1.20 eV.