Hard–Soft Chemistry Design Principles for Predictive Assembly of Single Molecule-Metal Junctions

Abstract

The achievement of atomic control over the organic–inorganic interface is key to engineering electronic and spintronic properties of molecular devices. We leverage insights from inorganic chemistry to create hard–soft acid–base (HSAB) theory-derived design principles for incorporation of single molecules onto metal electrodes. A single molecule circuit is assembled via a bond between an organic backbone and an under-coordinated metal atom of the electrode surface, typically Au. Here, we study molecular composition factors affecting the junction assembly of coordination complexes containing transition metals atoms on Au electrodes. We employ hetero- and homobimetallic lantern complexes and systematically change the coordination environment to vary the character of the intramolecular bonds relative to the electrode-molecule interaction. We observe that trends in the robustness and chemical selectivity of single molecule junctions formed with a range of linkers correlate with HSAB principles, which have traditionally been used to guide atomic arrangements in the synthesis of coordination complexes. We find that this similarity between the intermolecular electrode-molecule bonding in a molecular circuit and the intramolecular bonds within a coordination complex has implications for the design of metal-containing complexes compatible with electrical measurements on metal electrodes. Our results here show that HSAB principles determine which intramolecular interactions can be compromised by inter molecule-electrode coordination; in particular on Au electrodes, soft–soft metal–ligand bonding is vulnerable to competition from soft–soft Au-linker bonding in the junction. Neutral donor–acceptor intramolecular bonds can be tuned by the Lewis acidity of the transition metal ion, suggesting future synthetic routes toward incorporation of transition metal atoms into molecular junctions for increased functionality of single molecule devices.

Formation and Evolution of Metallocene Single-Molecule Circuits with Direct Gold-π Links

“…The height profile of molecular islands, shown in Figure B, is measured to be 3.5 Å, which is in good agreement with the long axis of the molecule and consistent with a submonolayer of 1 adsorbed on the Au(111) surface . We do not observe dissociation of 1 or 2 on the surface under the mild imaging conditions used here. − …”

Electronegativity Equilibration

“… 112 − 114 Since then, HSAB theory has been proven a useful tool in several chemical applications. 116 − 121 What is the physical interpretation of chemical hardness?…”

“…Together with Parr, Pearson later gave a quantitative definition, which became part of the conceptual density functional theory (CDFT) framework . Within CDFT, chemical hardness is defined as the negative of the derivative of electronegativity with respect to electron population. − Since then, HSAB theory has been proven a useful tool in several chemical applications. − What is the physical interpretation of chemical hardness?…”

Experimental assignment of long-range magnetic communication through Pd & Pt metallophilic contacts

“…For nearly a decade, the Doerrer group has been working on developing the science of heterobimetalic lantern complexes incorporating the Pt-metalloligands and a variable hard metal ion of group two and first-row transition metals. [11][12][13][14][15][16][17][18][19] In parallel, the coordination chemistry of both the Pt-and Pd-metalloligands with the lanthanide elements has been developed by us [20][21][22][23][24] and the group of Yamashita. [25][26][27][28][29] In the transition metal chemistry of the Pt-ligands, the complexes are formed by the bridging action of the four thiocarboxylate linking the soft Pt 2+ ion with the hard metal ion.…”