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 <i>trans</i>-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−) <b>1</b>, 1,3-(−CC–C₆H₄–CC−) <b>2</b>, 4,4′-(−CC–C₆H₄–C₆H₄–CC−) <b>3</b>, and 2,5-(−CC–thiophene–CC−) <b>4</b>, as well as a trinuclear complex, {I–Fe­(depe)₂(CC−)}₃(1,3,5-C₆H₃)} <b>5</b>, 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 <b>6</b>–<b>10</b> in very good yields. All complexes <b>1</b>–<b>10</b> are intrinsically functional due to the redox-active Fe centers embedded in a structurally rigid and covalent sp/sp² framework. <b>1</b>–<b>10</b> were characterized by NMR, IR, and Raman spectroscopy, as well as elemental analyses. X-ray diffraction studies were carried out for <b>1</b>, <b>2</b>, <b>4</b>, <b>5</b>, <b>6</b>, <b>8</b>, and <b>9</b>. Cyclic voltammetry was employed to explore the redox behavior of <b>1</b>–<b>10</b>. The 1,4-(−CC–C₆H₄–CC−) and the 2,5-(−CC–thiophene–CC−) bridged compounds <b>1</b>, <b>4</b>, <b>6</b>, and <b>9</b> 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 Δ<i>E</i>_(H‑L) and the chemical hardness are possible