To date, the copper complex with
the tris(2-pyridylmethyl)amine
(
tmpa
) ligand (
Cu
-
tmpa
) catalyzes
the ORR with the highest reported turnover frequency (TOF) for any
molecular copper catalyst. To gain insight into the importance of
the tetradentate nature and high flexibility of the
tmpa
ligand for efficient four-electron ORR catalysis, the redox and
electrocatalytic ORR behavior of the copper complexes of 2,2′:6′,2″-terpyridine
(
terpy
) and bis(2-pyridylmethyl)amine (
bmpa
) (
Cu
-
terpy
and
Cu
-
bmpa
, respectively) were investigated in the present study. With a combination
of cyclic voltammetry and rotating ring disk electrode measurements,
we demonstrate that the presence of the
terpy
and
bmpa
ligands results in a decrease in catalytic ORR activity
and an increase in Faradaic efficiency for H
2
O
2
production. The lower catalytic activity is shown to be the result
of a stabilization of the Cu
I
state of the complex compared
to the earlier reported
Cu
-
tmpa
catalyst.
This stabilization is most likely caused by the lower electron donating
character of the tridentate
terpy
and
bmpa
ligands compared to the tetradentate
tmpa
ligand. The
Laviron plots of the redox behavior of
Cu
-
terpy
and
Cu
-
bmpa
indicated that the formation
of the ORR active catalyst involves relatively slow electron transfer
kinetics which is caused by the inability of
Cu
-
terpy
and
Cu
-
bmpa
to form the preferred
tetrahedral coordination geometry for a Cu
I
complex easily.
Our study illustrates that both the tetradentate nature of the
tmpa
ligand and the ability of
Cu
-
tmpa
to form the preferred tetrahedral coordination geometry for a Cu
I
complex are of utmost importance for ORR catalysis with very
high catalytic rates.
Halogen bonds are shown to possess the same characteristics as hydrogen bonds: charge transfer, resonance assistance and cooperativity. This follows from the computational analyses of the structure and bonding in N-halo-base pairs and quartets. The objective was to achieve an understanding of the nature of resonance-assisted halogen bonds (RAXB): how they resemble or differ from the better understood resonance-assisted hydrogen bonds (RAHB) in DNA. We present an accurate physical model of the RAXB based on the molecular orbital theory, which is derived from the corresponding energy decomposition analyses and study of the charge distribution. We show that the RAXB arise from classical electrostatic interaction and also receive strengthening from donor-acceptor interactions within the σ-electron system. Similar to RAHB, there is also a small stabilization by π-electron delocalization. This resemblance leads to prove cooperativity in N-halo-guanine quartets, which originates from the charge separation that occurs with donor-acceptor orbital interactions in the σ-electron system.
Moving towards a hydrogen economy raises the demand for affordable and efficient catalysts for the oxygen reduction reaction. Cu-bmpa (bmpa = bis(2-picolyl)amine) is shown to have moderate activity, but poor...
The structure of the copper complex
of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH2)4SH) anchored to a gold surface has been investigated.
To enable covalent attachment of the complex to the gold surface,
a heteromolecular self-assembled monolayer (SAM) of butanethiol and
a thiol-substituted tmpa ligand was used. Subsequent formation of
the immobilized copper complex by cyclic voltammetry in the presence
of Cu(OTf)2 resulted in the formation of the anchored Cu-tmpa-O(CH2)4SH system which, according to scanning electron
microscopy and X-ray diffraction, did not contain any accumulated
copper nanoparticles or crystalline copper material. Electrochemical
investigation of the heterogenized system barely showed any redox
activity and lacked the typical CuII/I redox couple in
contrast to the homogeneous complex in solution. The difference between
the heterogenized system and the homogeneous complex was confirmed
by X-ray photoelectron spectroscopy; the XPS spectrum did not show
any satellite features of a CuII species but instead showed
the presence of a CuI ion in a ∼2:3 ratio to nitrogen
and a ∼2:7 ratio to sulfur. The +I oxidation state of the copper
species was confirmed by the edge position in the X-ray absorption
near-edge structure (XANES) region of the X-ray absorption spectrum.
These results show that upon immobilization of Cu-tmpa-O(CH2)4SH, the resulting structure is not identical to the
homogeneous CuII-tmpa complex. Upon anchoring, a novel
CuI species is formed instead. This illustrates the importance
of a thorough characterization of heterogenized molecular systems
before drawing any conclusions regarding the structure–function
relationships.
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