Gas, solution, and solid state coordination environments for the nickel(II) complexes of a series of aminopyridine ligands of varying coordination number

The synthesis and characterization of the nickel(II) complexes from a series of aminopyridine ligands that range from tri- to heptadentate is described. The ligands include the tripod ligand tris(2-((2-pyridylmethyl)amino)ethyl)amine, TREN-pyr, and the following linear ligands: (2-pyridylmethyl)(2-(2-((2-((2-pyridylmethyl)amino)ethyl)amino)ethyl)amine, TRIEN-pyr; bis(2-((2-pyridylmethyl)amino)ethyl)amine, DIEN-pyr; (2-pyridylmethyl)(2-((2-pyridylmethyl)amino)ethyl)amine, EN-pyr; bis(2-pyridylmethyl)amine, AM-pyr; and methyl(2-((-pyridylmethyl)amino)ethyl)amine, MeEN-pyr. The following methods were used to determine the binding geometries of the nickel(II) complexes in the solid, solution, and gas phases: magnetic susceptibility measurements, absorption spectroscopy, electrochemistry, and analyzing the gas phase ion-molecule reactions in a mass spectrometer. The linked five-membered chelate character of the linear ligands appear to have imposed high-spin, octahedral geometry on the complexes in the condensed phases. The tripod ligand TREN-pyr and the two tridentate ligands, AM-pyr and MeEN-pyr, form six-coordinate complexes in the gas phase. In contrast, the rest of the complexes (TRIEN-pyr, DIEN-pyr, and EN-pyr) were found to have lower coordination numbers in the gas phase (five-, five-, and four-coordinate, respectively). The potentially heptadentate tripod ligand TREN-pyr does not appear to confer any unusual properties on the Ni(II) ion in the solution or solid phases, but does appear to be more effective than the hexadentate ligand TRIEN-pyr at maintaining the six-coordinate geometry in the gas phase.