Organometallic Single-Molecule Electronics: Tuning Electron Transport through X(diphosphine)$_2$FeC$_4$Fe(diphosphine)$_2$X Building Blocks by Varying the Fe–X–Au Anchoring Scheme from Coordinative to Covalent

A series of X(depe)2FeC≡C–C≡CFe(depe)2X complexes (depe =1,2-bis(diethylphosphino)ethane; X = I 1, NCMe 2, N2 3, C2H 4, C2SnMe3 5, C4SnMe3 6, NCSe 7, NCS 8, CN 9, SH 10, and NO2 11) was designed to study the influence of the anchor group on organometallic molecular transport junctions to achieve high-conductive molecular wires. The FeC4Fe core is electronically functional due to the redox-active Fe centers and sp-bridging ligands allowing a strong electronic delocalization. 1–11 were characterized by elemental analyses, X-ray diffraction, cyclic voltammetry, NMR, IR, and Raman spectroscopy. DFT calculations on model compounds gave the HOMO/LUMO energies. 5–9 were investigated in mechanically controllable break-junctions. For 9, unincisive features at 8.1 × 10–7 G0 indicate that sterical reasons prevent stable junctions to form or that the coordinative binding motif prohibits electron injection. 7 and 8 with the hitherto unexploited coordinatively binding end groups NCSe and NCS yielded currents of 1.3 × 10–9 A (7) and 1.8 × 10–10 A (8) at ±1.0 V. The SnMe3 in 5 and 6 splits off, yielding junctions with covalent C–Au bonds and currents of 6.5 × 10–7 A (Au–5′–Au) or 2.1 × 10–7 A (Au–6′–Au). Despite of a length of almost 2 nm, the Au–5′–Au junction reaches 1% of the maximum current assuming one conductance channel in quantum point contacts. Additionally, the current noise in the transport data is considerably reduced for the covalent C–Au coupling compared to the coordinative anchoring of 7–9, endorsing C–Au coupled organometallic complexes as excellent candidates for low-ohmic molecular wires