Probing Electronic Communications in Heterotrinuclear Fe–Ru–Fe Molecular Wires Formed by Ruthenium(II) Tetraphenylporphyrin and Isocyanoferrocene or 1,1′-Diisocyanoferrocene Ligands

Two new heterotrinuclear Fe–Ru–Fe
complexes of ruthenium­(II) tetraphenylporphyrin axially coordinated
with a pair of isocyanoferrocene ((FcNC)2­RuTPP, 1) or 1,1′-diisocyanoferrocene (([C5H4NC]2­Fe)2­RuTPP, 2) ligands [Fc = ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2−)
anion] were synthesized and characterized by UV–vis, magnetic
circular dichroism, NMR, and FTIR spectroscopies as well as by electrospray
ionization mass spectrometry and single-crystal X-ray diffraction.
Isolation of insoluble polymeric {([C5H4NC]2­Fe)­RuTPP}n molecular
wires (3) was also achieved for the first time. The redox
properties of the new trinuclear complexes 1 and 2 were probed using electrochemical (cyclic voltammetry and
differential pulse voltammetry), spectroelectrochemical, and chemical
oxidation methods and correlated to those of the bis­(tert-butylisocyano)­ruthenium­(II) tetraphenylporphyrin reference compound,
(t-BuNC)2­RuTPP (4).
In all cases, the first oxidation process was attributed to the reversible
oxidation of the RuII center. The second and third reversible
oxidation processes in 1 are separated by ∼100
mV and were assigned to two single-electron FeII/FeIII couples, suggesting a weak long-range iron–iron
coupling in this complex. Electrochemical data acquired for 2 are complicated by the interaction between the axial η1-1,1′-diisocyanoferrocene ligand and the electrode
surface as well as by axial ligand dissociation in solution. Spectroelectrochemical
and chemical oxidation methods were used to elucidate the spectroscopic
signatures of the [1]n+,
[2]n+, and [4]n+ species in solution. DFT and time-dependent
DFT calculations aided in correlating the spectroscopic and redox
properties of complexes 1, 2, and 4 with their electronic structures.