The Original CoII Heteroscorpionates Revisited: On the EPR of Pseudotetrahedral CoII

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

Chlorocobalt complexes with pyridylethyl-derived diazacycloalkanes

With cobalt(II) chloride, some piperazine- and homo-piperazine-derived ligands yield tetra- or penta­coordinate complexes. Observed variations in coordination number are ascribed as being related to chloride solvophobicity. Optical spectra are presented, while magnetism measurements indicate governance of the magnetism by zero-field splitting of the cobalt ion.

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) 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)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.