Versatile Chemical Transformations of Benzoxazole Based Ligands on Complexation with 3d-Metal Ions

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).