2-(4',4'-Dimethyl-3',4'-dihydrooxazol-2'-yl)phenol: some first-row transition metal complexes of this naturally occurring binding group

Abstract

The synthesis and characterization of monomeric Mn^III, Co^II, Ni^II, Cu^II, and Zn^II phenolates derived from a biologically relevant ligand, 2-(4',4'-dimethyl-3',4'-dihydrooxazol-2'-yl)phenol (1), are described. The addition of Mn(OAc)₂·4H₂O to an ethanolic solution of 1 affords an unexpected 1:3 complex [Mn(OR){O(Ox)}₂] (2) [R=C₆H₄CONHC(Me₂)CH₂OH, Ox=2-(4,4-dimethyl-2-oxazolinyl)phenyl] in which one of the five-membered oxazoline rings is partially hydrolyzed. The complexes [M{O(Ox)}₂] [M=Co (3), Ni (4), Cu (6), or Zn (7)] were prepared in good yield by the reactions of 1 with the corresponding metal acetates. Novel formation of the six-coordinate Ni^II complex 5 with neutral acetic acid was observed during the crystallization process. The complexes were characterized by elemental analysis, magnetic susceptibility, as well as EPR, ¹H NMR, electronic, and mass spectral techniques. Complexes 2, 3, 5, 6, and 7 were characterized by single crystal X-ray diffraction studies. Complex 2 is six-coordinate whereas complexes 3, 6, and 7 are four-coordinate with two oxazoline bidentate ligands. The geometry around the metal center is found to be distorted tetrahedral for all of the four-coordinate complexes. Complex 5 is octahedral with four coordination sites occupied by the two bidentate oxazoline ligands and the other two are occupied by two neutral acetic acid molecules. The hydroxy groups of the coordinated acetic acid are hydrogen-bonded to the phenolate oxygen atoms of the oxazoline ligands. The variable-temperature ¹H NMR spectroscopic studies of complex 7 indicate that the interconversion between the (M) and (P) helix is slow at low temperatures.