p-Tolylimido rhenium(v) complexes with phenolate-based ligands: synthesis, X-ray studies and catalytic activity in oxidation with tert-butylhydroperoxide

Abstract

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.