Chlorocobalt complexes with pyridylethyl-derived diazacycloalkanes

Abstract

Syntheses are described for the blue/purple complexes of cobalt(II) chloride with the tetradentate ligands 1,4-bis[2-(pyridin-2-yl)ethyl]piperazine (Ppz), 1,4-bis[2-(pyridin-2-yl)ethyl]homopiperazine (Phpz), trans-2,5-dimethyl-1,4-bis[2-(pyridin-2-yl)ethyl]piperazine (Pdmpz) and tridentate 4-methyl-1-[2-(pyridin-2-yl)ethyl]homopiperazine (Pmhpz). The CoCl2 complexes with Ppz, namely, {μ-1,4-bis[2-(pyridin-2-yl)ethyl]piperazine}bis[dichloridocobalt(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 tetrahedrally coordinated CoCl2 units. Co2(Ppz)Cl4 and {dichlorido{4-methyl-1-[2-(pyridin-2-yl)ethyl]-1,4-diazacycloheptane}cobalt(II) [CoCl2(C13H21N3)] or Co(Pmhpz)Cl2, crystallize in the monoclinic space group P21/n, while crystals of the pentacoordinate monochloro chelate 1,4-bis[2-(pyridin-2-yl)ethyl]piperazine}chloridocobalt(II) perchlorate, [CoCl(C18H24N4)]ClO4 or [Co(Ppz)Cl]ClO4, are also monoclinic (P21). The complex {1,4-bis[2-(pyridin-2-yl)ethyl]-1,4-diazacycloheptane}dichloridocobalt(II) [CoCl2(C19H26N4)] or Co(Phpz)Cl2 (P\overline{1}) is mononuclear, with a pentacoordinated CoII 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 pentacoordinate 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.