Agostic Bonding in Pincer Complexes of Ruthenium

Abstract

High-yield preparations have been developed for 16-e square-pyramidal complexes of ruthenium(II) RuCl(CO)[2,6-(CH2PBut2)2C6H3] (1), RuHCl[1,3-(CH2PBut2)2C6H4] (2), and RuHI[1,3-(CH2PBut2)2C6H4] (3). NMR and X-ray studies have revealed strong η2 CH bonding in the Ru[1,3-(CH2PBut2)2C6H4] fragment of 2 and 3. These complexes show stretched agostic C−H bonds, elongated ipso-C−Ru distances, reduced 1JCH couplings, and large IPR shifts in 1H NMR. A consistent picture has emerged of a stronger agostic bonding and greater C−H bond activation in the η2 C−H trans to chloride in 2. This finding is attributed to stronger π-donor properties of chloride compared to iodide and increased back-donation into the antibonding orbital of the η2 C−H. Analysis of the X-ray and NMR T1 data has also indicated the presence of an attractive interaction, dihydrogen bonding, between the agostic C−H hydrogen and neighboring hydride in 2 and 3.