title: Structural, magnetic, and redox diversity of first-row transition metal complexes of a pyridine-based macrocycle: well-marked trends supported by theoretical DFT calculations.
abstract: A series of first-row transition metal complexes with 15-membered pyridine-based macrocycle (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene = L) was prepared ([M(II)(L)Cl2], where M = Mn, Co, Ni, Zn (1, 3, 4, 6); [Fe(III)(L)Cl2]Cl (2), [Cu(II)(L)Cl]Cl (5)) and thoroughly characterized. Depending on the complexated metal atom, the coordination number varies from 7 (Mn, Fe, Co), through 5 + 2 for Ni and 4 + 1 for Cu, to 5 for Zn accompanied by changes in the coordination geometry from the pentagonal bipyramid (1-4) to the square pyramid (5 and 6). Along the series, the metal-oxygen distances were prolonged in such manner that their bonding character was investigated, apart from X-ray structural analysis, also by ab initio calculations (Mayer's bond order, electron localization function), which confirmed that, in 4 and 5, two and one oxygen donor atoms are semicoordinated, respectively, and one and two oxygen atoms are uncoordinated in 5, and 6, respectively. On the basis of the temperature variable magnetic susceptibility measurements, 1 and 2 behave as expected for 3d(5) high-spin configuration with negligible zero-field splitting (ZFS). On the other hand, a large axial ZFS (D(Co) ≈ 40 cm(-1), D(Ni) ≈ -6.0 cm(-1)) was found for 3 and 4, and rhombic ZFS (E/D ≈ 0.15) for 4. Antiferromagnetic exchange coupling was observed for 4 and 5 (J(Ni) = -0.48 cm(-1), and J(Cu) = -2.43 cm(-1), respectively). The obtained results correlate well with ab initio calculations of ZFS parameters as well as J-values, which indicate that the antiferromagnetic exchange is mediated by hydrogen bonds. The complexes were also investigated by cyclic voltammetry in water or acetonitrile. A quasi-reversible couple Mn(II)/Mn(III) at 1.13/0.97 V, an almost reversible couple Fe(II)/Fe(III) at 0.51/0.25 V, and a one-step/multistep reduction/oxidation of Cu(II) complex 5 at -0.33 V/0.06-0.61 V were detected.
authors.affiliation: Department of Inorganic Chemistry, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, 17. listopadu 12, CZ-771 46 Olomouc, Czech Republic.
authors.forename: Bohuslav
authors.initials: B
authors.lastname: Drahoš
authors.affiliation: Department of Inorganic Chemistry, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, 17. listopadu 12, CZ-771 46 Olomouc, Czech Republic.
authors.forename: Radovan
authors.initials: R
authors.lastname: Herchel
authors.affiliation: Department of Inorganic Chemistry, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, 17. listopadu 12, CZ-771 46 Olomouc, Czech Republic.
authors.forename: Zdeněk
authors.initials: Z
authors.lastname: Trávníček