Chloro-cobalt complexes with pyridyl-ethyl-derived di-aza-cyclo-alkanes

Syntheses are described for the blue/purple complexes of cobalt(II) chloride with the tetra-dentate ligands 1,4-bis-[2-(pyridin-2-yl)eth-yl]piperazine (Ppz), 1,4-bis-[2-(pyridin-2-yl)eth-yl]homopiperazine (Phpz), trans-2,5-dimethyl-1,4-bis-[2-(pyridin-2-yl)eth-yl]piperazine (Pdmpz) and tridentate 4-methyl-1-[2-(pyridin-2-yl)eth-yl]homopiperazine (Pmhpz). The CoCl₂ complexes with Ppz, namely, {μ-1,4-bis-[2-(pyridin-2-yl)eth-yl]piperazine}bis-[di-chlorido-cobalt(II)], [Co₂Cl₄(C₁₈H₂₄N₄)] or Co₂(Ppz)Cl₄, and Pdmpz (structure not reported as X-ray quality crystals were not obtained), are shown to be dinuclear, with the ligands bridging the two tetra-hedrally coordinated CoCl₂ units. Co₂(Ppz)Cl₄ and {di-chlorido-{4-methyl-1-[2-(pyridin-2-yl)eth-yl]-1,4-di-aza-cyclo-hepta-ne}cobalt(II) [CoCl₂(C₁₃H₂₁N₃)] or Co(Pmhpz)Cl₂, crystallize in the monoclinic space group P2₁/n, while crystals of the penta-coordinate mono-chloro chelate 1,4-bis-[2-(pyr-id-in-2-yl)eth-yl]piperazine}chlorido-cobalt(II) perchlorate, [CoCl(C₁₈H₂₄N₄)]ClO₄ or [Co(Ppz)Cl]ClO₄, are also monoclinic (P2₁). The complex {1,4-bis-[2-(pyridin-2-yl)eth-yl]-1,4-di-aza-cyclo-hepta-ne}di-chlorido-cobalt(II) [CoCl₂(C₁₉H₂₆N₄)] or Co(Phpz)Cl₂ (P ) is mononuclear, with a penta-coordinated Co^II ion, and entails a Phpz ligand acting in a tridentate fashion, with one of the pyridyl moieties dangling and non-coordinated; its displacement by Cl^- is attributed to the solvophobicity of Cl^- toward MeOH. The penta-coordinate Co atoms in Co(Phpz)Cl₂, [Co(Ppz)Cl]⁺ and Co(Pmhpz)Cl₂ have substantial trigonal-bipyramidal character in their stereochemistry. Visible- and near-infrared-region electronic spectra are used to differentiate the two types of coordination spheres. TDDFT calculations suggest that the visible/NIR region transitions contain contributions from MLCT and LMCT character, as well as their expected d-d nature. For Co(Pmhpz)Cl₂ and Co(Phpz)Cl₂, variable-temperature magnetic susceptibility data were obtained, and the observed decreases in moment with decreasing temperature were modelled with a zero-field-splitting approach, the D values being +28 and +39 cm^-1, respectively, with the S = 1/2 state at lower energy.