Catalytic Synthesis and Cyclometalation of (4-Ferrocenylphenyl)pyridines

Metal-catalyzed [2+2+2] cyclization reactions are powerful
tools
for the preparation of (hetero)aromatic compounds. However, they have
rarely been applied in ferrocene chemistry. We have employed the Co-catalyzed
cyclization of 4-ferrocenylbenzonitrile with α,ω-diynes
to prepare a series of (4-ferrocenylphenyl)pyridines under solvothermal
conditions or using microwave irradiation. The resulting pyridines,
anellated by carbocyclic and heteroatom-containing rings, were reacted
with [(η5-C5Me5)MCl2]2/NaOAc (M = Rh and Ir) to produce the respective cyclometalated
complexes in good yields. All compounds were characterized using spectroscopic
methods (NMR and ESI–MS) and cyclic voltammetry, and the structures
of the representative compounds [(η5-C5Me5)MCl(FcC6H3–C5H2N(CH2)4-κ2C,N)] (M = Rh and Ir; Fc = ferrocenyl)
were determined by single-crystal X-ray diffraction analysis. DFT
calculations performed for the two model compounds confirmed that
the initial electrochemical oxidation occurs at the ferrocene iron
despite the conjugated nature of the compounds and that the highest
occupied molecular orbital is localized predominantly at the (η5-C5Me5)M moiety. An analysis of the
bonding using intrinsic bond orbitals and quantum theory of atoms
in molecules suggested more covalent M–X (X = Cl and N) bonds
for the iridium complex than for its rhodium congener. Notably, the
opposite trend was noted for the M–C bonds, which appear more
ionic in the Ir complex.