Organometallic Iridium Complex Containing a Dianionic, Tridentate, Mixed Organic–Inorganic Ligand

A pentamethylcyclopentadienyl–iridium
complex containing
a tricyclic, dianionic, tridentate, scorpionate (facial binding),
mixed organic–inorganic ligand was synthesized and characterized
by single-crystal X-ray crystallography, as well as polynuclear NMR,
UV–vis, and IR spectroscopies. The central cycle of the tridentate
ligand consists of a modified boroxine in which two of the boron centers
are tetrahedral, anionic borates. The complex is stable to hydrolysis
in aqueous solution for >9 weeks at 25 °C but reacts with
a 50
mM solution of sodium periodate within 12 s to form a periodate-driven
oxygen evolution catalyst that has a turnover frquency of >15 s–1. However, the catalyst is almost completely deactivated
within 5 min, achieving an average turnover number of ca. 2500 molecules
of oxygen per atom of iridium. Nanoparticles were not observed on
this time scale but did form within 4 h of catalyst activation under
these experimental conditions. The parent complex was modeled using
density functional theory, which accurately reflected the geometry
of the complex and indicated significant interaction of iridium- and
boracycle-centered orbitals.