Structural and Electronic Variations of sp/sp² Carbon-Based Bridges in Di- and Trinuclear Redox-Active Iron Complexes Bearing Fe(diphosphine)₂X (X = I, NCS) Moieties

Starting from the mononuclear precursor trans-Fe­(depe)2I2 (depe = 1,2-bis­(diethylphosphino)­ethane), four dinuclear complexes IFe­(depe)2–R–Fe­(depe)2I, with R = 1,4-(−CC–C6H4–CC−) 1, 1,3-(−CC–C6H4–CC−) 2, 4,4′-(−CC–C6H4–C6H4–CC−) 3, and 2,5-(−CC–thiophene–CC−) 4, as well as a trinuclear complex, {I–Fe­(depe)2(CC−)}3(1,3,5-C6H3)} 5, were prepared in a facile way by transmetalation from stannylated precursors. Substitution of the terminal iodides applying an excess of NaSCN yielded the corresponding isothiocyanate complexes 6–10 in very good yields. All complexes 1–10 are intrinsically functional due to the redox-active Fe centers embedded in a structurally rigid and covalent sp/sp2 framework. 1–10 were characterized by NMR, IR, and Raman spectroscopy, as well as elemental analyses. X-ray diffraction studies were carried out for 1, 2, 4, 5, 6, 8, and 9. Cyclic voltammetry was employed to explore the redox behavior of 1–10. The 1,4-(−CC–C6H4–CC−) and the 2,5-(−CC–thiophene–CC−) bridged compounds 1, 4, 6, and 9 exhibit two fully reversible oxidation waves, while the 1,3-(−CC–C6H4–CC−) and 4,4′-(−CC–C6H4–C6H4–CC−) bridged dinuclear complexes and the trinuclear complexes show only one reversible oxidation wave corresponding to 2 e– and 3 e– processes, respectively. Calculations were carried out for truncated model complexes to determine the HOMO/LUMO energies. The DFT results confirmed that by changing the sp/sp2 bridging ligand, tuning of the energies of the molecular orbitals and modifying of the HOMO–LUMO gap ΔE(H‑L) and the chemical hardness are possible.