A n acid-stable N A D H model compound, 10-methyl-9,lO-dihydroacridine (AcrH,) and 1 ,I '-dimethylferrocene [Fe(C,H,Me),] are oxidized by dioxygen in the presence of HCIO, in acetonitrile and water. The acid-catalysed two-electron and four-electron reductions of dioxygen by AcrH, and [ Fe( C,H,Me),],respectively, take place more efficiently in the heterogeneous systems using solid acid catalysts [alumina, silica-alumina, HY-type zeolite, and solid superacids (ZrO, and Fe,O, treated with H2S04)] activated by calcination at various temperatures. Mechanisms of both the homogeneous and heterogeneous reduction of dioxygen by AcrH, and [ Fe( C,H,Me),] are discussed.Catalytic reduction of dioxygen has been extensively studied in electrochemical systems. '
,However, little is known of the catalytic reduction of dioxygen by mild chemical reductants such as reduced nicotinamide adenine dinucleotide (NADH) which plays a vital role as the electron source in the respiratory chain., In the absence of an appropriate catalyst, triplet dioxygen is unreactive towards NADH or the model compounds because of spin restriction. Thus, in order to achieve the efficient reduction of dioxygen by NADH or the model compounds it is required to remove the spin restriction.We have previously reported that an acid-stable NADH model compound, 10-methyl-9,lO-dihydroacridine (AcrH,), reduces a series of aromatic aldehydes as well as p-benzoquinone derivatives in the presence of perchloric acid (HCIO,)., The reactivity of the acid catalyst may be improved by using appropriate heterogeneous catalysts, since the acid strength, as well as the acidity on the surface of solid acids, can be finely tuned by choosing an appropriate solid acid catalyst and by means of activation, eg., the calcination t e m p e r a t ~r e . ~~' In this study, we report the successful reduction of dioxygen by both a two-electron reductant (AcrH,) and a one-electron reductant [Fe(C,H,Me),] in the homogeneous and heterogeneous systems by using HCIO, and solid acid catalysts, respectively. The acid-catalysed electron transfer pathways of dioxygen, which are spin-allowed processes, in the reduction of dioxygen by AcrH, as well as by [Fe(C,H,Me),] are discussed based on the Marcus electron-transfer theory.8