Chlorocobalt complexes with pyridylethyl-derived diazacycloalkanes

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 CoCl 2 complexes with Ppz, namely, {μ-1,4-bis[2-(pyridin-2-yl)ethyl]piperazine}bis[dichloridocobalt(II)], [Co 2 Cl 4 (C 18 H 24 N 4 )] or Co 2 (Ppz)Cl 4 , 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 CoCl 2 units. Co 2 (Ppz)Cl 4 and {dichlorido{4-methyl-1-[2-(pyridin-2-yl)ethyl]-1,4-diazacycloheptane}cobalt(II) [CoCl 2 (C 13 H 21 N 3 )] or Co(Pmhpz)Cl 2 , crystallize in the monoclinic space group P 2 1 / n , while crystals of the pentacoordinate monochloro chelate 1,4-bis[2-(pyridin-2-yl)ethyl]piperazine}chloridocobalt(II) perchlorate, [CoCl(C 18 H 24 N 4 )]ClO 4 or [Co(Ppz)Cl]ClO 4 , are also monoclinic ( P 2 1 ). The complex {1,4-bis[2-(pyridin-2-yl)ethyl]-1,4-diazacycloheptane}dichloridocobalt(II) [CoCl 2 (C 19 H 26 N 4 )] or Co(Phpz)Cl 2 ( P \overline{1}) is mononuclear, with a pentacoordinated 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 pentacoordinate Co atoms in Co(Phpz)Cl 2 , [Co(Ppz)Cl] + and Co(Pmhpz)Cl 2 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)Cl 2 and Co(Phpz)Cl 2 , 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.