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 serie s of aminopyridine ligands that range from tri- to heptadentate is describe d. The ligands include the tripod ligand tris(2-((2-pyridylmethyl)amino)eth yl)ami TREN-pyr, and the following linear ligands: (2-pyridylmethyl)(2-(2-( (2-((2-pyridylmethyl)amino)ethyl)amino)ethyl)amine, TRIEN-pyr; bis(2-((2-py ridylmethyl)amino)ethyl)amine, DIEN-pyr; (2-pyridylmethyl)(2-((2-pyridylmet hyl)amino)ethyl)amine EN-pyr; bis(2-pyridylmethyl)amine, AM-pyr; and methyl (2-((-pyridylmethyl)amino)ethyl) amine, MeEN-pyr. The following methods wer e used to determine the binding geometries of the nickel(II) complexes in t he solid, solution, and gas phases: magnetic susceptibility measurements, a bsorption spectroscopy, electrochemistry, and analyzing the gas phase ion-m olecule reactions in a mass spectrometer. The linked five-membered chelate character of the linear ligands appear to have imposed high-spin, octahedra l 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-coordina te complexes in the gas phase. In contrast, the rest of the complexes (TRIE N-pyr, DIEN-pyr, and EN-pyr) were found to have lower coordination numbers in the gas phase (five-, five-, and four-coordinate, respectively). The pot entially 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 d oes appear to be more effective than the hexadentate ligand TRIEN-pyr at ma intaining the six-coordinate geometry in the gas phase. (C) 2000 Elsevier S cience S.A. All rights reserved.