Cationic 16-electron half-sandwich ruthenium complexes containing asymmetric diamines: Understanding the stability and reactivity of coordinatively unsaturated two-legged piano stool complexes

Article abstract of DOI:10.1016/S0020-1693(98)00386-7

The cationic 16e complexes [RuCp*(η²(N,N)-Me₂NCH₂CH ₂N(CH₂CHMe₂)₂)]⁺ (1) and [RuCp*(η²(N,N)-Me₂NCH₂CH ₂NCH₂CH₂OCH₂CH₂] ⁺ (2) as well as the 18e complex [RuCp*(η⁶-C₆H₅-N(Me)NCH ₂CH₂NMe₂)]⁺ (3) have been synthesized as the BAr′₄ (Ar′=3,5-C₆H₃(CF₃)₂) salts in one-pot reactions starting from [RuCp*(Cl)]₄. For 1, the X-ray crystal structure has also been determined showing the absence of any agostic interactions between ruthenium and the C-H bonds of the diamine ligand, and only minor deviations from the planar geometry despite the bulky diamine ligand. Based on EH model calculations, the extraordinary kinetic inertness of the planar 16e [Cp*Ru(NN)]⁺ structure is traced to a high HOMO-LUMO gap deriving from through-bond coupling through the intervening σ skeleton of the chelating diamine (NN) ligand (in contrast to the P analogs) and further to the high π donor strength of Cp* (relative to parent Cp). Possible ligand rearrangements to increase the chemical reactivity are analyzed.