The fac diastereoisomer of tris(2-pyridinealdoximato)cobalt(III) and a cationic cobalt(III) complex containing both the neutral and anionic forms of the ligand: Synthetic, structural and spectroscopic studies

The study of the general reaction system Co(NO3)2 center dot 6H(2)O/Ln(NO3)(3)center dot 6H(2)O (Ln = Tb, Dy, Ho, Er)/2-pyridinealdoxime (paoH)/(Bu4NOH)-N-n in MeCN under aerobic conditions provided access to fac-[Co-III(pao)(3)] (4). Starting from Co(II) sources, i.e. in the absence of Ln(III) ions, the known [Polyhedron 28 (2009) 1638] mer diastereoisomer, mer-[Co-III(pao)(3)] (3), could only be isolated. The crystallization of 4 is probably due to the formation of a mixed Co-III/Ln(III) species in solution, which already possesses the fac-{Co-III(pao)(3)} unit. The fac isomer 4 is converted readily to the mer-isomer 3, while the opposite transformation was not achieved under the reaction conditions employed. Complex [Co-III(pao)(2)(paoH)](ClO4) (5) was prepared in good yield from the aerobic reaction of Co(ClO4)2 center dot 6H(2)O and excess of paoH in MeOH. Treatment of 5 in MeCN/MeOH with one equivalent of base gives 3. The structures of the new complexes 4 center dot MeCN and 5 have been determined by single-crystal X-ray crystallography. The distorted octahedral Co-III ion in 4 is coordinated by three N,N'-bidentate chelating ligands; the molecule is the facial diastereoisomer. The distorted octahedral Co-III center in the cation of 5 is coordinated by four nitrogen atoms of two pao(-) ligands and two nitrogen atoms of one neutral paoH molecule; the disposition of the donor atoms around the metal center is meridional. The cations [Co-III(pao)(2)(paoH)](+) form centrosymmetric dimers in the crystal lattice as a consequence of O(oxime)-H center dot center dot center dot O- (oximate) H bonds; the dimers form tetramers through weak C(oxime)-H center dot center dot center dot O(oxime) H-bonding interactions. Metric parameters show that the deprotonated oximato group of the pao(-) ligands in 4 center dot MeCN and 5 is better described as C = N = 0. Solid-state UV-Vis and IR data are discussed in terms of the low-spin octahedral stereochemistry of Co-III and the nature of bonding, respectively. The (NMR)-N-1 spectrum of 4 center dot MeCN in CDCl3 provides strong evidence of the fac -> mer transformation in solution. (C) 2014 Published by Elsevier Ltd.