title: Synthesis and reactivity of tripodal complexes containing pendant bases.
abstract: The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris(amido)amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt(II) and zinc(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes [Co(L)](-) with dioxygen reversibly generates a small amount of a cobalt(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn[N{CH2CH2NC(O)CH2N(CH2Ph)2}3)](-) ([Zn(TN(Bn))](-)) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes [M(L)](-) results in complete proteolysis and formation of the ligands H3L. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative and acidic conditions.
authors.affiliation: Department of Chemistry, University of Washington , Box 351700, Seattle, Washington 98195-1700, United States.
authors.forename: Johanna M
authors.initials: JM
authors.lastname: Blacquiere
authors.forename: Michael L
authors.initials: ML
authors.lastname: Pegis
authors.forename: Simone
authors.initials: S
authors.lastname: Raugei
authors.forename: Werner
authors.initials: W
authors.lastname: Kaminsky
authors.forename: Amélie
authors.initials: A
authors.lastname: Forget
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Cook
authors.forename: Taketo
authors.initials: T
authors.lastname: Taguchi
authors.forename: James M
authors.initials: JM
authors.lastname: Mayer