A chemical system is described that mimics to a significant extent a number of properties of the copper-containing dioxygen carrier hemocyanin (Hc), including the reversible binding of CO and O2 and major features of the UV-vis spectroscopy. A neutral dinucleating ligand, N4PY2, in which two tridenate PY2 units (PY2 = bis[2-(2-pyridyl)ethyl]amine) are connected by a tetramethylenealkyl chain, forms tetracoordinate dicopper(I) complexes, [Cu2(N4PY2)(CO)2]2+(1) and [Cu2-(N4PY2)(CH3CN)2]2+(1a), as well as the pseudotricoordinate complex [Cu2(N4PY2)]2+(2). Compounds 1 and 2 can be readily interconverted, indicating that 2 can bind carbon monoxide reversibly. Complexes 1a or 2, as ClO4−(i.e. 1a-(ClO4)2 or 2-(ClO4)2), PF6−, or BF4− salts (λmax350 nm (∊ 3500 M−1 cm−1)), can be oxygenated at-80 °C in dichloromethane to produce intensely brown-colored solutions of dioxygen complexes, 3, which are characterized by extremely strong and multiple electronic spectral absorptions (360 nm (∊ 14 000–18 700 M−1 cm−1), 458 (4500–6300), 550 (1200)) in the visible region. The reaction of 2 (or 1a) with O2 is reversible, and the application of a vacuum to the dioxygen adduct formed, 3, removes the bound O2 and regenerates the dicopper(I) complex, 2. This vacuum cycling can be followed spectrophotometrically over several cycles. In addition, saturating a -80 °C solution of the dioxygen complex 3 with carbon monoxide (CO) results in the displacement of the dioxygen ligand with the formation of the dicopper(I) dicarbonyl complex, 1. This behavior lends further support to the existence of the reversible binding equilibrium, 2 + O2⇔ 3. Carbonyl cycling, where 2 reacts with O2 to produce 3, O2 is displaced by CO to give 1, and 1 is decarbonylated to regenerate 2, can also be followed spectrophotometrically over several cycles. Since (a) manometric measurements indicate that the stoichiometry of the reaction of 2 (and 1a) with dioxygen is Cu:O2 = 2:1, thus formulating 3 as [Cu2(N4PY2)(O2)]2+, and (b) other evidence (e.g. the presence of a d-d band; λmax775 nm (∊ 200 M−1 cm−1)) suggests that 3 possesses Cu(II) ions, 3 is best described as a peroxo-dicopper(II) complex. Crystallographic studies have been completed for both [Cu2(N4PY2)(CH3CN)2](ClO4)2 (1a-(ClO4)2) and [Cu2(N4PY2)](ClO4)2(2-(ClO4)2). Compound 1a-(ClO4)2(C36H46Cl2Cu2N8O8) crystallizes in the monoclinic space group A2/a, with Z = 4 and a = 13.755 (4) Å, b = 20.071 (5) Å, c = 18.017 (4) Å, and β= 121.39 (2)°. Complex 2-(ClO4)2 (C32H40Cl2Cu2N6O8) crystallizes in the monoclinic space group A2/a, with Z = 4, a = 19.437 (4) Å, b = 10.162 (3) Å, c = 23.231 (6) Å, and β = 128.14 (2)°, 1a-(ClO4)2 possesses well-separated (Cu•••Cu = 7.449 Å) equivalent Cu(I)-N4 moieties with pseudotetrahedral ligation to the three nitrogen atoms of the PY2 unit and the acetonitrile molecule. In addition to the PY2 coordination in 2-(ClO4)2, a weak Cu-O(ClO4−) interaction is observed; the bonding parameters for this complex suggest that the bonding is closer to that observed for other similar tricoordinate complexes with the Cu(I)-PY2 unit. The biological relevance of the dioxygen adducts described here and comparisons to oxy-Hc are discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry