Dioxygen Activation of a Trinuclear CuICuICuI Cluster Capable of Mediating Facile Oxidation of Organic Substrates: Competition between O-Atom Transfer and Abortive Intercomplex Reduction

The dioxygen activation of a series of CuICuICuI complexes based on the ligands (L) 3,3′‐(1,4‐diazepane‐ 1,4‐diyl)bis(1‐{[2‐(dimethylamino)ethyl](methyl)amino}propan‐2‐ol) (7‐Me) or 3,3′‐(1,4‐diazepane‐1,4‐diyl)bis(1‐{[2‐(diethylamino)ethyl](ethyl)amino}propan‐2‐ol) (7‐Et) forms an intermediate capable of mediating facile O‐atom transfer to simple organic substrates at room temperature. To elucidate the dioxygen chemistry, we have examined the reactions of 7‐Me, 7‐Et, and 3,3′‐(1,4‐diazepane‐1,4‐diyl)bis[1‐(4‐methylpiperazin‐1‐yl)propan‐2‐ol] (7‐N‐Meppz) with dioxygen at −80, −55, and −35 °C in propionitrile (EtCN) by UV‐visible, 77 K EPR, and X‐ray absorption spectroscopy, and 7‐N‐Meppz and 7‐Me with dioxygen at room temperature in acetonitrile (MeCN) by diode array spectrophotometry. At both −80 and −55 °C, the mixing of the starting [CuICuICuI(L)]1+ complex (1) with O2‐saturated propionitrile (EtCN) led to a bright green solution consisting of two paramagnetic species: the green dioxygen adduct [CuIICuII(μ‐η2:η2‐peroxo)CuII(L)]2+ (2) and the blue [CuIICuII(μ‐O)CuII(L)]2+ species (3). These observations are consistent with the initial formation of [CuIICuII(μ‐O)2CuIII(L)]1+ (4), followed by rapid abortion of this highly reactive species by intercluster electron transfer from a second molecule of complex 1 to give the blue species 3 and subsequent oxygenation of the partially oxidized [CuIICuICuI(L)]2+ (5) to form the green dioxygen adduct 2. Assignment of 2 to [CuIICuII(μ‐η2:η2‐peroxo)CuII(L)]2+ is consistent with its reactivity with water to give H2O2 and the blue species 3, as well as its propensity to be photoreduced in the X‐ray beam during X‐ray absorption experiments at room temperature. In light of these observations, the development of an oxidation catalyst based on the tricopper system requires consideration of the following design criteria: 1) rapid dioxygen chemistry; 2) facile O‐atom transfer from the activated cluster to substrate; and 3) a suitable reductant to rapidly regenerate complex 1 to accomplish efficient catalytic turnover. Three does it better: Low‐temperature oxygenation of [CuICuICuI(L)]1+ leads to the formation of two oxidized tricopper complexes: a green dioxygen adduct and a blue species (see scheme), which were isolated and characterized as [CuIICuII(μ‐η2:η2‐peroxo)CuII(L)]2+ and [CuIICuII(μ‐O)CuII(L)]2+, respectively. The results are reminiscent of the abortive dioxygen chemistry observed for the particulate methane monooxy