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