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.