Single-molecule conductance studies on quasi- and metallaaromatic dibenzoylmethane coordination compounds and their aromatic analogs
2023, Mang, André, Rotthowe, Nils, Beltako, Katawoura, Linseis, Michael, Pauly, Fabian, Winter, Rainer F.
The ability to predict the conductive behaviour of molecules, connected to macroscopic electrodes, represents a crucial prerequisite for the design of nanoscale electronic devices. In this work, we investigate whether the notion of a negative relation between conductance and aromaticity (the so-called NRCA rule) also pertains to quasi-aromatic and metallaaromatic chelates derived from dibenzoylmethane (DBM) and Lewis acids (LAs) that either do or do not contribute two extra dπ electrons to the central resonance-stabilised β-ketoenolate binding pocket. We therefore synthesised a family of methylthio-functionalised DBM coordination compounds and subjected them, along with their truly aromatic terphenyl and 4,6-diphenylpyrimidine congeners, to scanning tunneling microscope break-junction (STM-BJ) experiments on gold nanoelectrodes. All molecules share the common motif of three π-conjugated, six-membered, planar rings with a meta-configuration at the central ring. According to our results, their molecular conductances fall within a factor of ca. 9 in an ordering aromatic < metallaaromatic < quasi-aromatic. The experimental trends are rationalised by quantum transport calculations based on density functional theory (DFT).
Synthesis and crystal structures of rhodium acetate paddle‐wheel complexes with anchor group‐functionalized and hydrogen bond‐supported axial ligands
2022-11, Mang, André, Linseis, Michael, Winter, Rainer F.
We report the synthesis and X-ray structures of four Rh 2 (O 2 Ac) 4 (L Ax ) 2 (OAc = acteate, CH 3 COO - ) paddle-wheel complexes ( C1 - C4 ) with methylthio-modified axial ligands L Ax derived from benzamidine ( L1 ), anilinopyrimidine ( L2 ) or isothiourea ( L3 , L4 ) that are capable of forming N‑H×××O hydrogen bonds to the equatorially bridging acetate ligands. This was done with the aim to suppress dissociation of the axial ligands and to make the complexes amenable to single-molecule conductance measurements in a scanning tunneling microscope break-junction (STM-BJ) setup. The characteristic spectroscopic features (NMR, IR, vis), crystal structures, the hydrogen-bonding motifs and a DFT-based screening of their frontier molecular orbitals are presented. Our calculations suggest that hydrogen-bonding stabilizes axial ligand binding by 20kJ/mol to 30 kJ/mol and that the HOMO of the rhodium paddle-wheels is closer to the Au Fermi level than the LUMO, so that the rhodium paddle-wheels are expected to constitute hole conductors.
Organometallic, Nonclassical Surfactant with Gemini Design Comprising π-Conjugated Constituents Ready for Modification
2018-08-31, Bitter, Stefan, Kunkel, Marius, Burkart, Lisa, Mang, André, Winter, Rainer F., Polarz, Sebastian
Surfactants are functional molecules comprising a water-compatible head group and a hydrophobic tail. One of their features is the formation of self-assembled structures in contact with water, for instance, micelles, vesicles, or lyotropic liquid crystals. One way to increase the functionality of surfactants is to implement moieties containing transition-metal species. Ferrocene-based surfactants represent an excellent example because of the distinguished redox features. In most existing ferrocene-based amphiphiles, an alkyl chain is classically used as the hydrophobic tail. We report the synthesis and properties of 1-triisopropylsilylethynyl-1′-trimethylammoniummethylferrocene (FcNMe3TIPS). In FcNMe3TIPS, ferrocene is part of the head group (Gemini design) but is also attached to a (protected) π-conjugated ethynyl group. Although this architecture differs from that of classical amphiphiles and those of other ferrocene-based amphiphiles, the compound shows marked surfactant properties comparable to those of lipids, exhibiting a very low value of critical aggregation concentration in water (cac = 0.03 mM). It forms classical micelles only in a very narrow concentration range, which then convert into monolayer vesicles. Unlike classical surfactants, aggregates already form at a very low concentration, far beneath that required for the formation of a monolayer at the air–water interface. At even higher concentration, FcNMe3TIPS forms lyotropic liquid crystals, not only in contact with water, but also in a variety of organic solvents. As an additional intriguing feature, FcNMe3TIPS is amenable to a range of further modification reactions. The TIPS group is easily cleaved, and the resulting ethynyl function can be used to construct heterobimetallic platinum-ferrocene conjugates with trans-Pt(PEt3)2X (X = Cl, I) complex entities, leading to a heterobimetallic surfactant. We also found that the benzylic α-position of FcNMe3TIPS is rather reactive and that the attached ammonium group can be exchanged by other substituents (e.g., −CN), which offers additional opportunities for further functionalization. Although FcNMe3TIPS is reversibly oxidized in voltammetric and UV–vis spectroelectrochemical experiments, the high reactivity at the α-position is also responsible for the instability of the corresponding ferrocenium ion, leading to a polymerization reaction.