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A Novel Synthetic Approach to Asymmetric Salen, Dihydrosalen and Tetrahydrosalen Ligands: Structures and O2-Activating Properties of Their Nickel(II) and Cobalt(II) Complexes

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Böttcher, A., Elias, H., Eisenmann, B., Hilms, E., Huber, A., Kniep, R., et al. (1994). A Novel Synthetic Approach to Asymmetric Salen, Dihydrosalen and Tetrahydrosalen Ligands: Structures and O2-Activating Properties of Their Nickel(II) and Cobalt(II) Complexes. Zeitschrift für Naturforschung Section B-A Journal of Chemical Sciences, 49(8), 1089-1100.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0019-BA81-6
Abstract
A synthetic procedure is described for the preparation of the tetradentate N2O2 ligands H-2[H-4]L(1) = 6-(2-hydroxyphenyl)-2,5-diaza-3,3-dimethyl-1-(2-hydroxy-3-t-butyl-5-meth ylphenyl)heptane, H-2[H-2]L(1) = 6-(2-hydroxyphenyl)-2,5-diaza-3,3-dimethyl-1-(2-hydroxy-3-t-butyl-5-meth ylphenyl-1-heptane, and H(2)L(1) = 6-(2-hydroxyphenyl)-2,5-3,3-dimethyl-1-(2-hydroxy-3-t-butyl-t-methylphen yl)-1,5-heptadiene, which are asymmetric derivatives of the ligands tetrahydrosalen, dihydrosalen, and salen. complexes Ni[H-4]L(1), Ni[H-2]L(1), NiL(1), CoL(1), Ni[H-4]L(2) (([H-4]L(2))(2-) = anion of H-2[H-4]L(2) = n,n'-bis(2-hydroxy-3-t-butyl-5-methylbenzyl)-trans-(S,S)-1,2-diaminocycl ohexane), and NiL(2) were prepared, characterized (VIS and IR absorption, magnetic moment) and subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. Single crystal X-ray structure anaylses were carried out for Ni[(H)2]L(1) (monoclinic, P2(1)/n; a = 8.926(4), b = 29.324(7), c = 8.411(4) Angstrom; beta = 95.3(1)degrees; Z = 4), CoL(1) (monoclinic, C2/c; a = 25.389(2), b = 18.139(2), c = 10.179(1) Angstrom; beta = 112.227(6); Z = 8), and Ni[H-4]L(2).acetone (tetragonal, P4(1)2(1)2; a = 13.928(3), c = 33.698(5) Angstrom; Z = 8). In all the three complexes, the N2O2-metal coordination core is square-planar. The skeleton of the tetradentate ligand is more or less twisted. The planar cobalt(II) comples CoL(1) is a low spin d7 system with mu(exp) = 2.02 BM at 298 K, whereas the planar complexes NiL(1), Ni[H-2]L(1), and Ni[H-4]L(2) are diamagnetic (mu(exp) = 3.04 BM), which points to octahedral coordination. In aerated acetone solution, the tetrahydrosalen-type complex Ni[H-4]L(1) activates dioxygen and one C-N bond is oxidatively dehydrogenated. The VIS spectrum of the dihydrosalen-type complex formed is virtually identical with that of the aldimine complex Ni[H-2]L(1). In the presence of dioxygen, Co[H-4]L(1) and Co[H-2]L(1) are readily oxidized to CoL(1) in acetone solution.