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

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 (ΔEox ≈ 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 dxy/dx2‑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.