Water Oxidizing Diruthenium Electrocatalysts Immobilized on Carbon Nanotubes: Effects of the Number and Positioning of Pyrene Anchors

The
synthesis of efficient molecular water oxidation catalysts
(WOCs) and their stable anchoring on suitable electron acceptor supports
are crucial, yet challenging, steps for the development of artificial
photosynthesis schemes. Here, a highly active diruthenium complex
based on the bis­(bipyridyl)­pyrazolate (bbp–) ligand
scaffold is anchored on electronically conducting multiwall carbon
nanotubes (MWCNTs) using a pyrene group attached to either the pyrazolate
backbone (2) or to multiple axial ligand positions (1). High-resolution transmission electron microscopy (HRTEM)
and electron energy loss spectroscopy (EELS) show the presence of
>75% sp2 hybridization of the MWCNTs and an increase
of
spectral weight of π–π* transitions upon immobilization
of the pyrene-modified ligand or diruthenium complex, supporting pyrene
anchoring via π–π interactions. Upon electrochemical
oxidation the pyrene groups confined to the MWCNT-modified electrodes
are rapidly converted to redox-active surface-bound quinone species.
The water oxidation performance of the hybrid systems is studied by
cyclic voltammetry and rotating ring disk electrode (RRDE) experiments
under acidic aqueous condition (triflic acid, pH 1). Whereas the complex
anchored at the backbone position shows higher initial catalytic activity,
the complex anchored via four axial ligand positions features a higher
stability. X-ray photoemission (XPS) data before and after electrochemical
measurements reveal that the chemical structure of the immobilized
complex remains intact under catalytic conditions. The results suggest
that anchoring of Ru2 complexes by differently located pyrene groups
on MWCNTs offers good performance for electron transfer, however,
a single pyrene group at the pyrazolate backbone does not provide
sufficiently strong surface attachment. The distinct experimental
results for MWCNT hybrids with anchored 1 and 2 are further discussed in terms of the preferred attachment position
at the pyrazolate-based Ru2 scaffold and the orientation
of the catalyst’s active site with respect to the supporting
surface.