p-Tolylimido rhenium(v) complexes with phenolate-based ligands: synthesis, X-ray studies and catalytic activity in oxidation with tert-butylhydroperoxide†
The reactions of mer-[Re(p-NTol)X3(PPh3)2] (X = Cl, Br) with chelating phenolate-based ligands (2-(2-hydroxy-5-methylphenyl)benzotriazole (HL1), 2-(2-hydroxyphenyl)benzothiazole (HL2) or 2-(2-hydroxyphenyl)benzoxazole (HL3)) afforded a series of p-tolylimido rhenium(V) complexes cis- or trans-(X,X)-[Re(p-NTol)X2(L)(PPh3)]·yMeCN (where X = Cl, Br; L = L1, L2, L3 and y = 0–2) and [Re(p-NTol)X(L)(PPh3)2]Z·pPPh3 (where X = Cl, Br; Z = ReO4, PF6; L = L1, L2, L3 and p = 0 or 1). The reported compounds were characterized by elemental analysis, FT-IR, NMR (1H, 13C and 31P) and X-ray crystallography. Interestingly, the halide ions of [Re(p-NTol)Cl2(L1)(PPh3)]·MeCN (1) and [Re(p-NTol)Cl2(L2)(PPh3)]·2MeCN (3) are in cis relative dispositions, whereas the complexes [Re(p-NTol)Br2(L)(PPh3)] (L1 for 2, L2 for 4 and L3 for 6) and [Re(p-NTol)Cl2(L3)(PPh3)] (5) were found to be trans-(X,X) isomers. The compounds [Re(p-NTol)X(L)(PPh3)2](PF6) (X = Cl, Br; L = L1 and L2) and [Re(p-NTol)X(L3)(PPh3)2](PF6)·PPh3 (X = Cl, Br) have been tested in oxidative catalysis. A few compounds exhibited very good catalytic properties in oxidation of alcohols with tert-BuOOH (TBHP) in acetonitrile solution at moderate temperatures. Complex [Re(p-NTol)Cl(L2)(PPh3)2]PF6 (13) is the catalyst of choice for oxidation of 1-phenylethanol to acetophenone (in 80% yield; turnover number attained 290 after 30 h) and cyclooctanol to cyclooctanone (in 88% yield). Notably lower activity has been found in the oxidation of alkanes with TBHP. Product distribution in the oxidation of methylcyclohexane indicates some steric hindrance around the reaction center.