New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces IrI(CO)2(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an
atom-efficient precursor for Ir-based homogeneous oxidation catalysis.
This compound was chosen to simplify analysis of the water oxidation catalyst species formed
by the previously reported Cp*IrIII(pyalc)­OH water oxidation
precatalyst. Here, we present a comparative study on the chemical
and catalytic properties of these two precursors. Previous studies
show that oxidative activation of Cp*Ir-based precursors with NaIO4 results in formation of a blue IrIV species. This
activation is concomitant with the loss of the placeholder Cp* ligand
which oxidatively degrades to form acetic acid, iodate, and other
obligatory byproducts. The activation process requires substantial
amounts of primary oxidant, and the degradation products complicate
analysis of the resulting IrIV species. The species formed
from oxidation of the Ir­(CO)2(pyalc) precursor, on the
other hand, lacks these degradation products (the CO ligands are easily
lost upon oxidation) which allows for more detailed examination of
the resulting Ir­(pyalc) active species both catalytically and spectroscopically,
although complete structural analysis is still elusive. Once Ir­(CO)2(pyalc) is activated, the system requires acetic acid or acetate
to prevent the formation of nanoparticles. Investigation of the activated
bis-carbonyl complex also suggests several Ir­(pyalc) isomers may exist
in solution. By 1H NMR, activated Ir­(CO)2(pyalc)
has fewer isomers than activated Cp*Ir complexes, allowing for advanced
characterization. Future research in this direction is expected to
contribute to a better structural understanding of the active species.
A diol crystallization agent was needed for the structure determination
of 3.