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

The cationic 16e complexes [RuCp*(eta(2)(N,N)-Me2NCH2CH2N(CH2CHMe2)(2))](+) (1) and [RuCp*(eta(2)(N,N)-Me2NCH2CH2NCH2CH2OCH2CH2](+) (2) as well as the 18e complex [RuCp*(eta(6)-C6H5-N(Me)NCH2CH2NMe2)](+) (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 EPI 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 sigma skeleton of the chelating diamine (NN) ligand tin contrast to the P analogs) and further to the high pi donor strength of Cp* (relative to parent Cp). Possible ligand rearrangements to increase the chemical reactivity are analyzed. (C) 1999 Elsevier Science S.A. All rights reserved.