When oxidized, most amineruthenium(II) complexes undergo dehydrogenation of the amine to yield a coordinated imine or nitrile. To examine intermediates and explore new reaction pathways, we have designed complexes in which the amine ligand is geometrically constrained to prevent the final imine-forming step. Di-(2-picolyl)amine (dpic) is a tridentate amine ligand that can coordinate with meridional or facial geometry. Meridional coordination results in a nearly planar ligand that is ideally oriented for oxidative dehydrogenation to the imine. Facial coordination locks the ligand into a tetrahedral geometry at the aliphatic nitrogen, which should make imine formation impossible. Our goal was to prepare two di-(2-picolyl)amineruthenium(II) complexes: one in which the ligand has a meridional coordination, the other in which the ligand has a facial coordination. An ancillary ligand, 2,2′:6′,2′′-terpyridine (tpy) or tri-(2-pyridyl)amine (tpa), was first coordinated to the metal, yielding [RuIII(tpy)Cl3] and [RuIII(tpa)Cl3]; dpic was then substituted for the chloro ligands. The tpy ligand always coordinates meridionally and tpa always coordinates facially; thus, dpic has meridional coordination in [RuII(tpy)(dpic)]2+ and facial coordination in [RuII(tpa)(dpic)]2+. [RuII(tpa)(dpic)](PF6)2 was successfully prepared and its structure verified by 1H NMR and ESI-MS. Efforts to obtain the [RuII(tpy)(dpic)]2+ complex were unsuccessful. The difficulty experienced in synthesizing this complex is likely attributable to significant steric hindrance between the dpic NH hydrogen and one of the 6-pyridyl hydrogens on the tpy. DFT computations predict these two hydrogens to be separated by 1.94 Å. By comparison, the van der Waals radius for hydrogen is 1.2 Å. No significant steric hindrance for the dpic NH site exists in [RuII(tpa)(dpic)]2+.

Synthesis of ruthenium(II) complexes of di-(2-picolyl)amine

2019 Spring National Meeting

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