Tris(pyridinealdoximato)metal Complexes as Ligands for the Synthesis of Asymmetric Heterodinuclear CrIIIM Species [M = Zn(II), Cu(II), Ni(II), Fe(II), Mn(II), Cr(II), Co(III)]: A Magneto−Structural Study

Reactions of the LCr(III) unit with an in situ prepared M(PyA)(3)(n-) ion, where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane and PyA(-) is the monoanion of pyridine-2-aldoxime, yield heterodinuclear complexes of general formula [LCr(III)(PyA)(3)M](2+/3+) as perchlorate salts, where M = Cr(II) (1), Mn(II) (2), low-spin Fe(II) (3), Ni(II) (4), Cu(II) (5), Zn(II) (6), and low-spin Co(III) (7). These compounds contain three oximato anions as bridging ligands. The hexadentate ligand with the identical donor atoms, tris(2-aldoximato-6-pyridyl)phosphine, P(PyA)(3), has been employed to prepare a second Cr(III)Ni(II) species 8, whose magnetic properties differ significantly from those of 4. Complexes 1-8 have been characterized on the basis of elemental analysis, mass spectrometry, IR, UV-vis, Mössbauer, and EPR spectroscopies, and variable-temperature (2-295 K) magnetic susceptibility measurements. They are isostructural in the sense that they all contain a terminal Cr(III) ion in a distorted octahedral environment, CrN(3)O(3), and a second six-coordinated metal ion M in a mostly trigonal prismatic MN(6) geometry. The crystal structures of the perchlorate salts of 2-5, 7, and 8 have been determined by X-ray crystallography at 100 K. The structures consist of mixed-metal Cr(III)M(II) and Cr(III)Co(III) complexes with a geometry in which two pseudooctahedral polyhedra are joined by three oximato (=N-O(-)) groups, with an intramolecular Cr.M(Co) distance in the range of 3.4-3.7 A. The cyclic voltammograms of the complexes reveal ligand oxidation and reduction processes, and in addition, metal-centered oxidation processes have been observed. X-band EPR spectroscopy has been used to establish the electronic ground state of the heterodinuclear complexes. Analysis of the susceptibility data indicates the presence of weak exchange interactions, both ferro- and antiferromagnetic, between the paramagnetic centers. A qualitative rationale on the basis of the Goodenough-Kanamori rules is provided for the difference in magnetic behaviors.