2-Ferrocenylpyridines: Selective Synthesis by [2 + 2 + 2] Cycloaddition, Cyclometalation and Bonding

Cobalt-catalyzed [2 + 2 + 2] cycloaddition of cyanoferrocene with alkynes and diynes was used to selectively prepare a series of five 2-ferrocenylpyridines under conventional, microwave, or photochemical conditions. The pyridines were further converted into cyclometalated Pd(II), Pt(II), and Au(III) complexes featuring five-membered C,N-chelate rings. The pyridines and the cyclometalated compounds were characterized using spectroscopic methods, single-crystal X-ray diffraction analysis and cyclic voltammetry, and the bonding in their molecules was analyzed theoretically using a combination of intrinsic bond orbital and quantum theory of atoms in molecules approaches. The calculations helped explain the peculiar structural distortion of the ferrocene unit resulting from electron density polarization within the substituted cyclopentadienyl ring and rationalize the observed electrochemical behavior.