Structural and electronic variations of sp/sp(2) carbon-based bridges in di- And trinuclear redox-active iron complexes bearing Fe(diphosphine)X-2 (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. (Chemical Equation Presented).