title: Metal-complexes as ligands to generate asymmetric homo- and heterodinuclear M(A)(III)M(B)(II) species: a magneto-structural and spectroscopic comparison of imidazole-N versus pyridine-N.
abstract: Ten hetero- and homodinuclear M(A)(III)M(B)(III) complexes, 1-10, containing the metal centers Fe(III)Zn(II) (1), Fe(III)Cu(II) (2), Fe(III)Ni(II) (3), Fe(III)Fe(II) (4), Fe(III)Mn(II) (5), Cr(III)Ni(II) (6), Cr(III)Zn(II) (7), Ga(III)Ni(II) (8), Co(III)Fe(II) (9), and Mn(III)Mn(II) (10) are described. The tridentate ligation property of the divalent metal complexes tris(1-methylimidazole-2-aldoximato)metal(II) with three facially disposed pendent oxime O-atoms has been utilized to generate the said complexes. Complexes were characterized by various physical methods including MS, IR, UV-vis, Mossbauer and EPR spectroscopy, cyclic voltammetry (CV), variable-temperature (2-290 K) magnetic susceptibility, and X-ray diffraction techniques. Binuclear complexes 1-10 contain three oximato anions as bridging ligands and are isostructural in the sense that they all contain a metal(III) ion, LM(A)(III) (L = 1,4,7-trimethyl-1,4,7-triazacyclononane), in a distorted octahedral environment M(A)(III)N(3)O(3) and a second six-coordinated divalent metal ion, M(B)(II), in a trigonally distorted M(B)(II)N(6) geometry. A comparison of the cyclic voltammograms of the complexes with those of similar systems reveal both ligand-centered and metal-centered redox processes. Complexes 2, 3, 5, and 6 display antiferromagnetic exchange coupling of the neighboring metal centers in the order Fe(III)Mn(II) (5) < Fe(III)Ni(II) (3) < Fe(III)Cu(II) (2) whereas Fe(III)Ni(II) (3) > Cr(III)Ni(II) (6). On the contrary, complex 10, containing high-spin Mn(III) and Mn(II) centers, exhibits ferromagnetic coupling yielding a "high-spin" molecule with an S(t) = (9)/(2) ground state. X-band electron paramagnetic resonance (EPR) spectroscopy for 6, Cr(III)Ni(II) and 3, Fe(III)Ni(II) has been used to establish the electronic ground state in great detail and to complement the magnetic susceptibility measurements. Moreover, computational results have been included to compare the sigma-bonding character of the nitrogen lone pair in imidazole-containing ligand and the analogous pyridine-containing oxime.
authors.affiliation: Max-Planck-Institut fur Bioanorganische Chemie, Stiftstrasse 34-36, 45470 Mulheim an der Ruhr, Germany.
authors.forename: Biplab
authors.initials: B
authors.lastname: Biswas
authors.forename: Sunita
authors.initials: S
authors.lastname: Salunke-Gawali
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Vinzenz
authors.initials: V
authors.lastname: Bachler
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Phalguni
authors.initials: P
authors.lastname: Chaudhuri
mesh_terms: D018360:Crystallography, X-Ray; D007093:Imidazoles / Q000737:chemistry*; D008024:Ligands; D008280:Magnetics; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011725:Pyridines / Q000737:chemistry*
chemical_list: D007093:Imidazoles; D008024:Ligands; D008670:Metals; D011725:Pyridines; C029899:imidazole; C023666:pyridine