Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization

Abstract

A family of metal dichloride complexes having a bis-ferrocenyl-substituted pyridinediimine ligand was systematically synthesized ((Fc₂PDI)MCl₂, M = Mg, Zn, Fe, and Co) and characterized crystallographically, spectroscopically, electrochemically, and computationally. Electronic coupling between the ligand ferrocene units is switched on upon binding to a MCl₂ fragment, as evidenced by both sequential oxidation of the ferrocenes in cyclic voltammetry (ΔE_ox ≈ 200 mV) and by Inter-Valence Charge Transfer electronic excitations in the near IR. Additionally, UV-vis spectra are used to directly observe orbital mixing between the ferrocenyl units and the imine π system since breaking of the orbital symmetry results in allowed transitions (ϵ = 2800 M^-1cm^-1 vs ϵ ≈ 200 M^-1cm^-1 in free ferrocene) as well as broadening and red-shifting of the ferrocenyl transitions-indicating organic character in formerly pure metal-centered transitions. DFT analysis reveals that interaction between the ferrocenes and the MCl₂ fragment is small and suggests that communication is mediated by better energy matching between the ferrocene and organic π* orbitals upon coordination, allowing superexchange coupling through the LUMO. Furthermore, single crystal diffraction data obtained from oxidation of one and both ferrocenes show distortions, introducing the empty d_xy/d_x2-y2 orbitals into the secondary coordination sphere of the MCl₂ fragment. Such structural rearrangements are infrequent in ferrocenyl mixed-valent compounds, and implications for catalysis as well as electronic communication are discussed.