C−C Bond Formation between the μ‐Alkylidyne Ligands in a Diruthenium Bis(μ‐alkylidyne) Complex; σ‐ and π‐Aromaticity of the Ru 2 C 2 Core

Zn‐free μ‐ethylidyne‐μ‐propylidyne complex, [(Cp*Ru) 2 (μ‐CMe)(μ‐CEt)] ( 2 b ; Cp*=C 5 Me 5 ) was synthesized by the treatment of [(Cp*Ru) 2 (μ‐CMe)(μ‐CEt)(μ‐ZnCl 2 )] ( 1 b ) with ethylenediamine. Similarly, bis(μ‐ethylidyne) complex [Cp*Ru(μ‐CMe)] 2 ( 2 a ) was synthesized from [Cp*RuCl 2 ] 2 . Although the bis(μ‐alkylidyne) complexes 2 a , b adopted a coordinatively unsaturated 32‐electron configuration, these complexes were unreactive toward acetylene, ethylene, and H 2 . Adaptive natural density partitioning (AdNDP) analysis suggested that the enhanced stability of 2 a and 2 b was due to the dual aromaticity of the planar Ru 2 C 2 core, viz. σ‐ and π‐aromaticity. On the other hand, complexes 2 a and 2 b reacted with π‐acidic CO to yield a μ‐η 2 :η 2 ‐alkyne complex, [(Cp*Ru) 2 (μ‐η 2 :η 2 ‐MeCCR)(μ‐CO)] ( 6 a , R=Me; 6 b , R=Et) via C−C bond formation between the μ‐alkylidyne ligands, along with Ru−Ru bond formation. Dearomatization of the Ru 2 C 2 core is crucial for this transformation. Complex 6 a further reacted with CO in the presence of methanol to yield [Cp*Ru(CO)(μ‐CO)] 2 ( 7 ) by liberating 2‐butyne.