Versatile Chemical Transformations of Benzoxazole Based Ligands on Complexation with 3d-Metal Ions
Résumé
Two benzoxazoles derivative ligands were synthesized from the condensation of 3,5-di-tert-butyl-o-benzoquinone (DTBBQ) with ethanolamine or 1,3-diamino-2-hydroxypropane in methanol. Condensation of DTBBQ with ethanolamine gives the expected 5,7-di-tert-butyl-2-methylenhydroxylbenzoxazole (HL1) while with 1,3-diamino-2-hydroxypropane it gives (2-hydroxyethyl-2-{2,4-bis(1,1-dimethylethyl)-1-phenol-6 amino}-2{5,7-di-tert-butyl-benzoxazole}) (H2L2) with only one benzoxazole ring instead of the symmetric bis-benzoxazole derivative. The structure of HL1 and H2L2 were confirmed by NMR-spectroscopy and X-ray diffraction on a single crystal for HL1. The reaction of HL1 with CuCl2 gives a mononuclear [CuII(HL1)2Cl2] (1) complex for which the crystal structure shows that HL1 is preserved. In contrast, upon reaction with nickel(II), cobalt(II), and manganese(II) H2L2 is further oxidized and transformed in new ligands HL3 in mononuclear complexes [MII(L3)2] (M = NiII (2); M = CoII (3)) and H2L4 in tetranuclear complex [MnII4(HL4)4Cl4] (4) as found from the crystal structures of complexes 2–4. Electrochemical studies for complexes 2 and 3 evidence complicated redox properties. [MnII4(HL4)4Cl4] (4) has a cubane-like structure with a “4 + 2” fashion The magnetic susceptibility of 4 is well fitted considering one Mn---Mn interaction Ja(MnII-MnII) = −0.50(1) cm–1 with g = 2.00(7).