Synthesis, structure, and catalytic properties of V^IV, Mn^III, Mo^VI, and U^VI complexes containing bidentate (N, O) oxazine and oxazoline ligands

Abstract

Synthesis of seven complexes containing oxazoline {[(L₁)₂V=O] (4), [(L₁)₂MoO₂] (5), [(L₁)₂UO₂] (6); HL₁ (1) [HL₁=2-(4',4'-dimethyl-3'-4'-dihydroxazol-2'-yl)phenol]}, chiral oxazoline {[(L₂)₂UO₂] (7); HL₂ (2) [HL₂=(4'R)-2-(4'-ethyl-3'4'-dihyroxazol-2'-yl)phenol]}, and oxazine {[(L₃)₂V=O] (8), [(L₃)₂Mn(CH₃COO⁻)] (9), [(L₃)₂Co] (10); HL₃ (3) [HL₃=2-(5,6-dihydro-4H-1,3-oxazolinyl)phenol]} and their characterization by various techniques such as UV-vis, IR, and EPR spectroscopy, mass spectrometry, cyclic voltammetry, and elemental analysis are reported. The novel oxazine (3) and complexes 4, 5, 8 and 9 were also characterized by X-ray crystallography. Oxazine 3 crystallizes in the monoclinic system with the P2₁/n space group, complexes 4 and 9 crystallize in the monoclinic system with the P2₁/c space group, and complexes 5 and 8 crystallize in the orthorhombic system with the C222₁ space group and the P2₁2₁2₁ chiral space group, respectively. The representative synthetic procedure involves the reaction of metal acetate or acetylacetonate derivatives with corresponding ligand in ethanol. Addition of Mn(OAc)₂·4H₂O to an ethanol solution of 3 gave the unexpected complex Mn(L₃)₂·(CH₃COO⁻) (9) where the acetate group is coordinated with the metal center in a bidentate fashion. The catalytic activity of complexes 4-9 for oxidation of styrene with tert-butyl hydroperoxide was tested. In all cases, benzaldehyde formed exclusively as the oxidation product.