Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

The
structural characterization of supported molecular catalysts
is challenging due to the low density of active sites and the presence
of several organic/organometallic surface groups resulting from the
often complex surface chemistry associated with support functionalization.
Here, we provide a complete atomic-scale description of all surface
sites in an N-heterocyclic carbene based on iridium and supported
on silica, at all stages of its synthesis. By combining a suitable
isotope labeling strategy with the implementation of multinuclear
dipolar recoupling DNP-enhanced NMR experiments, the 3D structure
of the Ir-NHC sites, as well as that of the synthesis intermediates
were determined. As a significant fraction of parent surface fragments
does not react during the multistep synthesis, site-selective experiments
were implemented to specifically probe proximities between the organometallic
groups and the solid support. The NMR-derived structure of the iridium
sites points to a well-defined conformation. By interpreting EXAFS
spectroscopy and chemical analysis data augmented by computational
studies, the presence of two coordination geometries is demonstrated:
Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand,
as well as, more surprisingly, a fragment coordinated by two NHC and
two Cl ligands. This study demonstrates a unique methodology to disclose
individual surface structures in complex, multisite environments,
a long-standing challenge in the field of heterogeneous/supported
catalysts, while revealing new, unexpected structural features of
metallo-NHC-supported substrates. It also highlights the potentially
large diversity of surface sites present in functional materials prepared
by surface chemistry, an essential knowledge to design materials with
improved performances.