μ-Peroxo Dicobalt Complexes Containing an Unsymmetrical Dinucleating Ligand : Synthesis, Characterization, and Oxygen Affinity

A new unsymmetric dinucleating ligand, Htpdb (Htpdb = 1,4-bis[bis(2-pyridylmethyl)amino]-2-butanol), was synthesized. This is a hybrid of Htpdp (1,3-bis[bis(2-pyridylmethyl)amino]-2-propanol) and Htmdp (1,5-bis[bis(2-pyridylmethyl)amino]-3-pentanol). Its dinuclear cobalt(II,II) complexes, [Co2(tpdb)(RCOO)](ClO4)2·nH2O (RCOO = CH3COO (1) and C6H5COO (2)), were prepared. Their electronic spectra indicate that 1 and 2 have a five-coordinate structure with a bridging carboxylato group. The complexes reacted reversibly with molecular oxygen in acetonitrile to form dioxygen complexes [Co2(tpdb)(RCOO)(O2)]2+. The structure of a dioxygen complex [Co2(tpdb)(CH3COO)(O2)](PF6)2·CH3CN·0.5H2O (3) was determined by X-ray crystallography. 3 cystallizes in orthorhombic space group Pmn21 with a = 23.034 (6), b = 17.352 (2), c = 10.355 (2) Å, and Z = 4. Two cobalt atoms are triply bridged by dioxygen, acetato, and alkoxo groups. Two cobalt atoms are triply bridged by dioxygen, acetato, and alkoxo groups. P(O2)1/2 of 1 is 720 Torr in acetonitrile at 20 °C; this value is in between those of the corresponding tpdp and tmdp complexes. The bridging skeleton of the dinucleating ligands contributes significantly to the oxygen affinities of the complexes. The complex 2, [Co2(tpdb)(C6H5COO)]2+, underwent an oxidative degradation in methanol–water mixture (1 : 1) in the presence of C6H5COONa to produce a dinuclear mixed valence complex [Co2(tpdb)(C6H5COO)2](ClO4)2·H2O (4).