New FeII and CuII Complexes Bearing Azathia Macrocycles - Catalyst Precursors for Mild Peroxidative Oxidation of Cyclohexane and 1-Phenylethanol

The new mononuclear Fe-II and Cu-II complexes [FeCl2(L-1,L-3)] [L-1 (1), L-3 (4)], [Cu(OTf)(2)(L-1,L-2)] [L-1 (2), L-2 (3)] and [Cu(OTf) (L-3)(H2O)](OTf) (5) were generated by the treatment of iron(II) chloride or copper(II) triflate in THF solution at ambient temperature with the 14-membered N2S2 macrocycles 6,7,13,14,15,16,17,18-octahydrodibenzo(e,m)-1,4,8,11-dithia-diazacyclotetradecine (L-1), 8,11-dimethyl-5,6,7,8,9,10,16,17-octahydrodibenzo(e,m)-1,4-dithia-8,11-diazacyclotetradecane (L-2), and a new nine-membered NS2 macrocycle bearing a pendant 2-methylpyridyl arm 7-(2-methylpyridyl)aza-1,4-dithiacyclononane (L-3). Complexes 1-5 were characterized by IR spectroscopy, ESI-MS(+), elemental analysis and single-crystal X-ray diffraction (for 2, 4 and 5). The oxidation catalytic properties of 1-5 were evaluated in two model reactions: (i) the mild oxidation of cyclohexane to cyclohexanol and cyclohexanone by H2O2 in MeCN and (ii) the solvent-free oxidation of 1-phenylethanol to acetophenone by tBuOOH under low microwave (MW) irradiation power (10 W). The FeII complexes 1 and 4 exhibited the highest efficiency in both homogeneous oxidation systems, leading to maximum product yields (based on substrate) of up to 21 and 92% in the oxidation of cyclohexane and 1-phenylethanol, respectively. In the latter transformation, markedly high values of TONs (up to 1200) and TOFs (up to 4200 h(-1)) were also attained.