Synthesis and Reactivity of Tripodal Complexes Containing
Pendant Bases
- Publication date
- Publisher
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 <b>[Co(L)]</b><sup><b>–</b></sup> 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{CH<sub>2</sub>CH<sub>2</sub>NC(O)CH<sub>2</sub>N(CH<sub>2</sub>Ph)<sub>2</sub>}<sub>3</sub>)]<sup>−</sup> (<b>[Zn(TN</b><sup><b>Bn</b></sup><b>)]</b><sup><b>–</b></sup>) 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 <b>[M(L)]</b><sup><b>–</b></sup> results in complete proteolysis and formation of the ligands <b>H</b><sub><b>3</b></sub><b>L</b>. 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 <i>and</i> acidic conditions
