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

Reactions between the ethynyl complexes (4,0)-[Ru 2 ( ap ) 4 ](C 2 SiR 3 ) ( ap is 2-anilinopyridinate, R = i Pr ( 1a ) and CH 3 ( 1b )) and LiC 4 SiMe 3 result in the formation of dissymmetrical ethynyl-butadiynyl adducts trans -(4,0)-(Me 3 SiC 4 )[Ru 2 ( ap ) 4 ](C 2 SiR 3 ) (R = i Pr ( 2a ) and CH 3 ( 2b )). Treating 2b with K 2 CO 3 in MeOH/THF leads to trans -(HC 4 )[Ru 2 ( ap ) 4 ](C 2 SiMe 3 ) ( 3b ) and trans -(HC 4 )[Ru 2 ( ap ) 4 ](C 2 H) ( 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 -(HC 4 )[Ru 2 ( ap ) 4 ](C 2 Si i Pr 3 ) ( 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 λ max around 1035 nm, revealing a HOMO−LUMO gap of 1.20 eV.