DOI: 10.1002/chem.201103075 Dioxygen Activation of a Trinuclear Cu I Cu I Cu I Cluster Capable of Mediating Facile Oxidation of Organic Substrates: Competition between O-Atom Transfer and Abortive Intercomplex Reduction Suman Maji, Jason C.-M. Lee, Yu-Jhang Lu, Chang-Li Chen, Mu-Cheng Hung, Peter P.-Y. Chen, Steve S.-F. Yu, and SunneyI. Chan* [a] Introduction Copper proteins and enzymes present in biological systems perform a variety of physiological functions including elec- tron transfer, dioxygen transport, and oxidase/oxygenase ac- tivities. [1] Multicopper active sites have been identified in such enzymes as laccase, [2] ascorbate oxidase, [3] and the par- ticulate methane monooxygenase. [4] The design and synthesis of multicopper model complexes are important not only as enzyme active-site models but also in potential industrial applications towards the function- alization of hydrocarbons and in fuel cells. [5] Self-assembled multicopper clusters have been reported but many of these complexes are unstable due to the ease of dissociation of the ligands. [6] Multidentate ligands [7] or macrocyclic ligands [8] have been used to develop more stable tricopper complexes but most of these clusters are not activated by O 2 at all. Recently, this laboratory has reported several tricopper clusters that are capable of mediating facile O-atom transfer to simple organic substrates. [9] These tricopper complexes are based on the ligands (L) 3,3’-(1,4-diazepane-1,4-diyl)- bis(1-{[2-(dimethylamino)ethyl]ACHTUNGTRENNUNG(methyl)amino}propan-2-ol) (7-Me) or 3,3’-(1,4-diazepane-1,4-diyl)bis(1-{[2-(diethyl- ACHTUNGTRENNUNGamino)ethyl]ACHTUNGTRENNUNG(ethyl)amino}propan-2-ol) (7-Et), which con- tain six neutral amines and two hydroxyl groups that are ca- pable of trapping three copper ions simultaneously. [Cu I Cu I Cu I (L)] 1 + complexes are readily prepared by treat- ing [CuACHTUNGTRENNUNG(MeCN) 4 ](X) (3 equiv; X = BF 4  , ClO 4  ) in anhy- drous acetonitrile (MeCN) with [7-Me] 2 ACHTUNGTRENNUNG[Na 2 ] 2 + (1 equiv) or [7-Et] 2 ACHTUNGTRENNUNG[Na 2 ] 2 + (1 equiv). Interestingly, these [Cu I Cu I Cu I (L)] 1 + complexes react with dioxygen to form an intermediate capable of facile O-atom insertion across the central C  C bonds in benzil and 2,3-butanedione at ambient Abstract: The dioxygen activation of a series of Cu I Cu I Cu I complexes based on the ligands (L) 3,3’-(1,4-diazepane- 1,4-diyl)bis(1-{[2-(dimethylamino)- ACHTUNGTRENNUNGethyl]ACHTUNGTRENNUNG(methyl)amino}propan-2-ol) (7- Me) or 3,3’-(1,4-diazepane-1,4-diyl)bis- ACHTUNGTRENNUNG(1-{[2-(diethylamino)ethyl]ACHTUNGTRENNUNG(ethyl)- ACHTUNGTRENNUNGamino}propan-2-ol) (7-Et) forms an in- termediate capable of mediating facile O-atom transfer to simple organic sub- strates at room temperature. To eluci- date 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 8C 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 spectropho- tometry. At both 80 and 55 8C, the mixing of the starting [Cu I Cu I Cu I (L)] 1 + complex (1) with O 2 -saturated propio- nitrile (EtCN) led to a bright green so- lution consisting of two paramagnetic species: the green dioxygen adduct [Cu II Cu II (m-h 2 :h 2 -peroxo)Cu II (L)] 2 + (2) and the blue [Cu II Cu II ACHTUNGTRENNUNG(m-O)Cu II (L)] 2 + species (3). These observations are consistent with the initial formation of [Cu II Cu II ACHTUNGTRENNUNG(m-O) 2 Cu III (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 subse- quent oxygenation of the partially oxi- dized [Cu II Cu I Cu I (L)] 2 + (5) to form the green dioxygen adduct 2. Assignment of 2 to [Cu II Cu II (m-h 2 :h 2 -peroxo)- Cu II (L)] 2 + is consistent with its reactiv- ity with water to give H 2 O 2 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 ob- servations, the development of an oxi- dation catalyst based on the tricopper system requires consideration of the following design criteria: 1) rapid di- oxygen chemistry; 2) facile O-atom transfer from the activated cluster to substrate; and 3) a suitable reductant to rapidly regenerate complex 1 to ac- complish efficient catalytic turnover. Keywords: bioinorganic chemistry · copper · enzyme models · N,O ligands · O  O activation [a] Dr. S. Maji, Dr. J.C.-M. Lee, Y.-J. Lu, Dr. C.-L. Chen, M.-C. Hung, Dr. P.P.-Y. Chen, Prof. S.S.-F. Yu, Prof. S.I. Chan Institute of Chemistry Academia Sinica Nankang, Taipei 11529 (Taiwan) Fax: (+ 886) 2-27831237 E-mail: sunneychan@yahoo.com Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103075. Chem. Eur. J. 2012, 18, 3955 – 3968 # 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3955 FULL PAPER