title: trans-Methylpyridine cyclen versus cross-bridged trans-methylpyridine cyclen. Synthesis, acid-base and metal complexation studies (metal = Co2+, Cu2+, and Zn2+).
abstract: The synthesis of the cross-bridged cyclen CRpy(2) {4,10-bis((pyridin-2-yl)methyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane}, a constrained analogue of the previously described trans-methylpyridine cyclen Cpy(2) is reported. The additional ethylene bridge confers to CRpy(2) proton-sponge type behaviour which was explored by NMR and potentiometric studies. Transition metal complexes have been synthesized (by complexation of both ligands with Co(2+), Cu(2+) and Zn(2+)) and characterized in solution and in the solid state. The single crystal X-ray structures of [CoCpy(2)](2+), [CuCpy(2)](2+) and [ZnCpy(2)](2+) complexes were determined. Stability constants of the complexes, including those of the cross-bridged derivative, were determined using potentiometric titration data and the kinetic inertness of the [CuCRpy(2)](2+) complex in an acidic medium (half-life values) was evaluated by spectrophotometry. The pre-organized structure of the cross-bridged ligand imposes an additional strain for the complexation leading to complexes with smaller thermodynamic stability in comparison with the related non-bridged ligand. The electrochemical study involving cyclic voltammetry underlines the importance of the ethylene cross-bridge on the redox properties of the transition metal complexes.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Av. da República, 2780-157, Oeiras, Portugal.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Bernier
authors.forename: Judite
authors.initials: J
authors.lastname: Costa
authors.forename: Rita
authors.initials: R
authors.lastname: Delgado
authors.forename: Vítor
authors.initials: V
authors.lastname: Félix
authors.forename: Guy
authors.initials: G
authors.lastname: Royal
authors.forename: Raphaël
authors.initials: R
authors.lastname: Tripier
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D056831:Coordination Complexes / Q000138:chemical synthesis* / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D000080027:Cyclams; D004578:Electron Spin Resonance Spectroscopy; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008968:Molecular Conformation; D011725:Pyridines / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D000080027:Cyclams; D006571:Heterocyclic Compounds; D011725:Pyridines; D003035:Cobalt; D003300:Copper; C038072:cyclen; D015032:Zinc; C023666:pyridine