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)2·6H2O/Ln(NO3)3·6H2O (Ln = Tb, Dy, Ho, Er)/2-pyridinealdoxime (paoH)/Bun4NOH in MeCN under aerobic conditions provided access to fac-[CoIII(pao)3] (4). Starting from Co(II) sources, i.e. in the absence of LnIII ions, the known [Polyhedron 28 (2009) 1638] mer diastereoisomer, mer-[CoIII(pao)3] (3), could only be isolated. The crystallization of 4 is probably due to the formation of a mixed CoIII/LnIII species in solution, which already possesses the fac-{CoIII(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 [CoIII(pao)2(paoH)](ClO4) (5) was prepared in good yield from the aerobic reaction of Co(ClO4)2·6H2O 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·MeCN and 5 have been determined by single-crystal X-ray crystallography. The distorted octahedral CoIII ion in 4 is coordinated by three N,N′-bidentate chelating ligands; the molecule is the facial diastereoisomer. The distorted octahedral CoIII 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 [CoIII(pao)2(paoH)]+ form centrosymmetric dimers in the crystal lattice as a consequence of O(oxime)–H⋯O− (oximate) H bonds; the dimers form tetramers through weak C(oxime)–H⋯O(oxime) H-bonding interactions. Metric parameters show that the deprotonated oximato group of the pao− ligands in 4·MeCN and 5 is better described as . Solid-state UV–Vis and IR data are discussed in terms of the low-spin octahedral stereochemistry of CoIII and the nature of bonding, respectively. The 1NMR spectrum of 4·MeCN in CDCl3 provides strong evidence of the fac → mer transformation in solution.

The missing facial diastereoisomer of tris(2-pyridinealdoximato)cobalt(III) and a complex containing both the neutral and anionic forms of 2-pyridinealdoxime have been prepared, structurally characterized and spectroscopically studied.Figure optionsDownload as PowerPoint slide