Relative stability of half-sandwich η⁶-benzene Ru(II) complexes of tridentate (2-pyridyl)alkylamine ligands of varying chelate ring-size: nucleophilic addition of hydride ion onto the benzene ring

Abstract

A full account of half-sandwich complexes of ruthenium(II) having three-legged "piano-stool" geometry supported by tridentate (2-pyridyl)alkylamine ligands is presented. Reaction of the dimer [{(η⁶-C₆H₆)RuCl(μ-Cl)}₂] with N-methyl-N,N-bis(2-pyridylmethyl)amine (MeL) in CH₃OH in the presence of NH₄PF₆ affords the complex [(η⁶-C₆H₆)Ru(MeL)][PF₆]₂ (1). A similar reaction with N-methyl-N,N-bis(2-pyridylethyl)amine (MeL), however, affords a non-organometallic Ru(III)-dimeric complex [(MeL)₂Ru₂^III(μ-O)(μ-Cl)Cl₂][PF₆](5) (the composition of this complex has been established by physicochemical method). Nucleophilic addition reaction on 1 with NaBH₄ leads to the isolation of a cyclohexadienyl complex [(η⁵-C₆H₇)Ru(MeL)][PF₆] (3). The molecular structure of 1·2CH₃CN, 3, and previously reported cyclohexadienyl complex [(η5-C₆H₇)Ru(MeL)][PF₆] (4) [MeL = N-methyl-[(2-pyridyl)ethyl(2-pyridyl)-methyl]amine], obtained from the reaction between NaBH₄ and previously reported "piano-stool" complex [(η⁶-C₆H₆)Ru(MeL)][PF₆]₂ (2), has been confirmed by X-ray crystallography. Solution-state structure of new complexes 1 and 3 has been elucidated by their ¹H NMR spectra in CD₃CN. The behavior of complex 3 has been investigated with the aid of two-dimensional ¹H NMR spectroscopy, as well. An attempt has been made to provide a rationale for the effect of supporting tridentate N-donor ligand [MeL, MeL, and MeL], varying in the chelate ring-size on (i) the relative stability of half-sandwich η⁶-benzene Ru(II) complexes and (ii) the electrophilicity of Ru(II)-coordinated benzene ring on the nucleophilic addition reactions.