CO2 Fixation by Cu2+ and Zn2+ Complexes of a Terpyridinophane Aza Receptor. Crystal Structures of Cu2+ Complexes, pH-Metric, Spectroscopic, and Electrochemical Studies

• Published in: Inorganic chemistry Vol. 45; no. 9; pp. 3803 - 3815
• Main Authors: Verdejo, Begoña, Aguilar, Juan, García-España, Enrique, Gaviña, Pablo, Latorre, Julio, Soriano, Conxa, Llinares, José M, Doménech, Antonio
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01-05-2006
• Subjects: Carbon Dioxide - chemistry; Copper - chemistry; Crystallography, X-Ray; Electrochemistry; Hydrogen-Ion Concentration; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Plants - metabolism; Polyamines - chemical synthesis; Polyamines - chemistry; Spectrum Analysis; Zinc - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic060278d

The synthesis of the terpyridinophane-type polyamine 2,6,9,12,16-pentaaza[17]-(5,5‘ ‘)-cyclo-(2,2‘:6‘,2‘ ‘)-terpyridinophane heptahydrobromide tetrahydrate (L·7HBr·4H2O) is described. L presents six protonation constants with values in the range 9.21−3.27 logarithmic units. L interacts with Cu2+ and Zn2+ forming in both cases, neutral, protonated, and hydroxylated mono- and binuclear complexes whose constants have been calculated by potentiometry in 0.15 M NaClO4 at 298.1 K. The crystal structures of the compounds [Cu(HL-carb)(H2O)](ClO4)3·2H2O (1) and [Cu2(H2 L)(CO3)]2(ClO4)8·9H2O (2) have been solved by X-ray diffraction. In 1, the metal center presents square pyramidal geometry. The base of the pyramid is formed by the three nitrogen atoms of pyridine and one oxygen atom of a CO2 group which is forming a carbamate bond with the central nitrogen atom of the polyamine chain. The axial position is occupied by a water molecule. In 2, one Cu2+ is bound by the three pyridine nitrogens and the other one by the three central nitrogen atoms of the polyamine chain. The square planar coordination geometry is completed by a carbonate group taken up from the atmosphere that behaves as a bridging μ,μ‘-ligand between the two centers. The pH-metric titrations on the ternary Cu2+−L−carbonate and Zn2+−L−carbonate systems show the extensive formation of adduct species which above pH 6 are formed quantitatively in solution. The stoichiometries of the main species formed in solution at pH = 6.8 agree with those found in the crystalline compounds. CO2 uptake by the Zn2+ and Cu2+ 1:1 complexes in aqueous solution has also been followed by recording the variations in the band at ca. 300 nm. The formation of the Zn2+ carbamate moiety has been evidenced by 13C NMR and ESI spectroscopy.