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| The helicene-NHC-Iridium complex. Notice the twisted nature of the helicene and also the chiral-at-iridium attribute. |
I have always been fascinated by the organometallic chemistry of iridium (Ir), and this is a great paper about a new iridium complex from Chem. Comm. [1]. The researchers have reported the first synthesis of a class of helicene- N- heterocyclic carbene-iridium complexes in enantiopure forms. The chirality is due to the helical structure of the helicene (the fused benzene rings) and also the iridium center – so this is what we call a ‘chiral-at-metal’ complex. It is noteworthy that the chiroptical properties of the iridium complex have been investigated by circular dichroism (CD), something very familiar to protein scientists and finding more applications in stereochemistry and supramolecular chemistry in recent years, too.
The complexes were synthesized by a multi-stage approach,
for which the last step involved [Cp*IrCl2]2, a common
starting material for many organoiridium complexes. The complexes were purified
by crystallization, because in many cases the undesirable chlorido complexes
were also formed.
NMR, molecular rotation, circular dichroism, and X-ray
crystallography were among some of the techniques which were used to
characterize these novel iridium complexes. X-ray crystal data were
particularly informative because that suggested the key cyclometallation
occurred at one particular carbon on the helicene (rather than another one) due
to steric considerations, meaning that the helical structure was twisted in a
particular way.
An important insight gained from all the different
experiments was that the formation of the iridium complex led to electronic
coupling between the iridium center and the helicenic-NHC ligand. And the
researchers believed it was the first example that the properties of the
N-heterocyclic carbene have led to impact on the chiroptical properties of the
resulting NHC-transition metal complexes.
All in all, this is great research and I love the beautiful architecture
of the iridium complex!
by Ed Law
29/6/2016
Reference:
1. Electronic and chiroptical properties of chiral cycloiridiated complexes bearing helicenic NHC ligands
Nora Hellou, Claire Jahier-Diallo, Olivier Baslé, Monika Srebro-Hooper, Loïc Toupet, Thierry Roisnel, Elsa Caytan, Christian Roussel, Nicolas Vanthuyne, Jochen Autschbach, Marc Mauduit and Jeanne Crassous
Chem. Commun., 2016, Advance Article
Chem. Commun., 2016, Advance Article
DOI: 10.1039/C6CC04257K
