Organometallic Single-Molecule Electronics: Tuning Electron Transport through X(diphosphine)(2)FeC4Fe(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.