Spiro Metallaaromatics of Pd, Pt, Rh: Synthesis and Characterization.

Since the concept of aromaticity represents one of the most funda-mental 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 metallaaromatics, in which the transition metal (Pd, Pt, or Rh) is the spiro atom, that cross-conjugates two aromatic five-membered metallacycles. These spiro metallaaromatics 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.