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Oxidation of tetranuclear copper(I) complexes [(Pip)nCuX]4; n=1 or 2, Pip=Piperidine and X=Cl or Br, with two equivalents of tetrachloro-1,2-benzoquinone (TClBQ) and sequentional oxidation with one equivalent of TClBQ followed by dioxygen in aprotic media: Models for intermediates in catalytic catechol–copper(II) systems

“…Since two piperidine ligands are coordinated to every copper center in [(Pip) 2 CuX] 4-Cat 2 and [(Pip) 2 CuX] 4 CatO, the piperidyl hydrogen of N-H moiety is free for one of those piperidine, while it is hydrogen-bonded for the other one with either halo, oxo or catecholato bridges [10]. The absence of carbonyl stretching vibrational bands in [(Pip) n CuX] 4-Cat 2 and [(Pip) n CuX] 4 CatO indicates that TClBQ is reduced to the corresponding catecholato during the oxidation of a copper(I) centers, i.e. acts as the twoelectron system upon oxidation of copper(I) centers in a cubane structure and such a conclusion eliminates the presence of any semiquinone [22].…”

“…Such spectra have been observed for all copper centers with a minimum of three halo ligands per each copper [23,24], suggesting that the above tetranuclear complexes have a cubane core structure, Scheme 1. The absorptivities for the bromocomplex in [(Pip) n CuBr] 4 Cat 2 and [(Pip) n CuBr] 4 CatO are, 4-10% and 40-50%, higher than the chloro analog, respectively. This shows that the absorptivities are much sensitive to the halo-bridging ligand in [(Pip) n CuX] 4 Cat 2 and [(Pip) n CuX] 4 CatO.…”

“…The absorptivities for the bromocomplex in [(Pip) n CuBr] 4 Cat 2 and [(Pip) n CuBr] 4 CatO are, 4-10% and 40-50%, higher than the chloro analog, respectively. This shows that the absorptivities are much sensitive to the halo-bridging ligand in [(Pip) n CuX] 4 Cat 2 and [(Pip) n CuX] 4 CatO. It is hard to detect the shift in the maxima due to the change of ligands because of the broadening of the maxima.…”

“…Oxidation of copper(I) complexes by dioxygen operated by k on ¼ Kk 2 [3][4][5][6]. For oxidation of halo-amine copper(I) complexes, the pre-equilibrium, K, between copper(I) center and dioxygen was negligible in all studied cases [7][8][9], except for [(TEED)CuCl] 2 [3] at low temperature.…”

“…For oxidation of halo-amine copper(I) complexes, the pre-equilibrium, K, between copper(I) center and dioxygen was negligible in all studied cases [7][8][9], except for [(TEED)CuCl] 2 [3] at low temperature. The existence of hydrogen bond association with the incoming dioxygen as in the case of [(DEED)CuBr] 2 [4] let K operate up to 30°C. High basicity of piperidine and its piperidyl hydrogen were behind (a) oxidation of [CuI] 4 core by increasing K, which was impossible in case of pyridine type ligands [10]; (b) increasing the rate of oxidation k on and (c) changing the reaction from endothermic as in pyridine type of ligands [7][8][9] to exothermic one due to promotion of pre-equilibrium K [3][4][5][6].…”