Spiro Metalla-aromatics of Pd, Pt, and Rh: Synthesis and Characterization

Since the concept
of aromaticity represents one of the most fundamental
principles in chemistry, the search for unprecedented and exciting
aromatic systems, therefore, continues to drive research in this area.
Herein we report the synthesis and characterization of spiro metalla-aromatics,
in which the transition metal (Pd, Pt, or Rh) is the spiro atom, that
cross-conjugates two aromatic five-membered metallacycles. These spiro
metalla-aromatics tend to take square planar geometries, with the
dihedral angle being influenced by the steric repulsion between the
α-positioned substituents. Rationalized and classified via both
experimental measurements (X-ray structural analysis, NMR spectroscopy,
XPS, etc.) and theoretical analysis (DFT calculation, ISE, AICD, NICS,
and CMOs), all these fundamental observations extend the concept of
aromaticity and organometallic chemistry.