Inorganic Chemistry, Vol.55, No.12, 5780-5786, 2016
Photocatalytic Hydroxylation of Benzene by Dioxygen to Phenol with a Cyano-Bridged Complex Containing Fe-II and Ru-II Incorporated in Mesoporous Silica-Alumina
Photocatalytic hydroxylation of benzene to phenol was achieved by using 02 as an oxidant as well as an oxygen source with a cyano-bridged polynudear metal complex containing Fe-II and Ru-II incorporated in mesoporous silica-alumina Fe(H2O)(3)](2)[Ru(CN)(6)]@sAl-MCM-41). An apparent turnover number (TON) of phenol production per the monomer unit of [Fe(H2O)(3)](2)[Ru(CN)(6)] was 41 for 59 h. The cyano-bridged polynudear metal complex, [Fe(H2O)(3)](2)[Ru(CN)(6)], exhibited catalytic activity for thermal hydroxylation of benzene by H2O2 in acetonitrile (MeCN), where the apparent TON of phenol production reached 393 for 60 h. The apparent TON increased to 2500 for 114 h by incorporating [Fe(H2O)(3)](2)[Ru(CN)(6)] in sAl-MCM-41. Additionally, [Fe(H2O)(3)](2)[Ru(CN)(6)] acts as a water oxidation catalyst by using [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) and Na2S2O8 as a photosensitizer and a sacrificial electron acceptor as evidenced by O-18-isotope labeling experiments. Photoirradiation of an O-2-saturated MeCN solution containing [Fe(H2O)(3)](2)[Ru(CN)(6)]@Al-MCM-41 and scandium ion provided H2O2 formation, where photoexcited [Ru(CN)(6)](4-) moiety reduces 02 as indicated by laser flash photolysis measurements. Thus, hydroxylation of benzene to phenol using molecular oxygen photocatalyzed by [Fe(H2O)(3)](2)[Ru(CN)(6)] occurred via a two-step route; (1) molecular oxygen was photocatalytically reduced to peroxide by using water as an electron donor, and then (2) peroxide thus formed is used as an oxidant for hydroxylation of benzene.