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

Reactions of the LCrIII unit with an in situ prepared M(PyA)3n- 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 [LCrIII(PyA)3M]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 CrIIINiII 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, CrN3O3, and a second six-coordinated metal ion M in a mostly trigonal prismatic MN6 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 CrIIIMII and CrIIICoIII 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 Å. 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.