Ferrocenyl-Substituted Triazatruxenes: Synthesis, Electronic Properties, and the Impact of Ferrocenyl Residues on Directional On-Surface Switching on Ag(111)

We report on seven new ferrocenyl-(1, 3)- and ferrocenylethynyl-modified N,N′,N″-triethyltriazatruxenes (EtTATs) 4–7 as well as the dodecyl counterpart 2 of compound 1 and their use as molecular switching units when deposited on a Ag(111) surface. Such functional units may constitute a new approach to molecule-based high-density information storage and processing. Besides the five compounds 1–3, 6, and 7, where the 3-fold rotational symmetry of the triazatruxene (TAT) template is preserved, we also included 2-ethynylferrocenyl-TAT 4 and 2,2′-di(ethynylferrocenyl)-TAT 5, whose mono- and disubstitution patterns break the 3-fold symmetry of the TAT core. Voltammetric studies indicate that the ferrocenyl residues of compounds 1–7 oxidize prior to the oxidation of the TAT core. We have noted strong electrostatic effects on TAT oxidation in the 2,2′,2″-triferrocenyl-TAT derivatives 1 and 2 and the 3,3′,3″-isomer 3. The oxidized complexes feature multiple electronic excitations in the near-infrared and the visible spectra, which are assigned to dδ/δ* transitions of the ferrocenium (Fc+) moieties, as well as TAT → Fc+ charge-transfer transitions. The latter are augmented by intervalence charge-transfer contributions Fc → Fc+ in mixed-valent states, where only a part of the available ferrocenyl residues is oxidized. EtTAT was previously identified as a directional three-level switching unit when deposited on Ag(111) and constitutes a trinary-digit unit for on-surface information storage. The symmetrically trisubstituted compound 6 retains this property, albeit at somewhat reduced switching rates due to the additional interaction between the ferrocenyl residues and the Ag surface. In particular, the high directionality at low bias and the inversion of the preferred sense of the on-surface rocking motion with either a clockwise or counterclockwise switching sense, depending on the identity of the surface enantiomer, are preserved. Unsymmetrical substitution in mono- and diferrocenylated 4 and 5 alters the underlying ratchet potential in a manner such that a two-state switching between the two degenerate surface conformations of 4 or a pronounced suppression of switching (5) is observed.

. Fitting parameters for the simulation of cyclic voltammograms of 1 and 2.

UV/Vis/NIR Spectroscopy and Spectroelectrochemistry
FT-IR spectra were recorded on a Bruker Tensor III instrument in a range between 1000 cm -1 to 11500 cm -1 . UV/Vis/NIR spectra were measured with a TIDAS fiber optic diode array spectrometer, which combines MCS UV/Vis and PGS NIR instruments from J&M. Extinction coefficients were determined in Hellma quartz cuvettes with 0.1 cm and 0.2 cm thickness. Spectroelectrochemical measurements were performed using an OTTLE (optically transparent thin-layer electrochemical) cell according to the design of Hartl et al. 4 The cell is custom-built with CaF2 windows, Pt-mesh working and counter electrodes and a Ag/AgCl pseudo-reference electrode. The measurements were recorded in a dry and degassed NBu4 + [B{C6H3(CF3)2-3,5}4] -/1,2-C2H4Cl2 electrolyte. Potentials were applied using a Wenking Pos 2 potenstiostat by Intelligent Controls GmbH.

DFT-calculations
Quantum chemical calculations on 2-Fc3-TAT were performed using the GAUSSIAN 16 program package. 5 Electronic transitions were obtained by the time-dependent DFT approach (TD-DFT). For iron, the MDF-10 basis set was applied. 6 For all other atoms triple-ζ basis sets (6-31G(d)) 7 were used for structural optimization and construction of the corresponding molecular orbitals. For all calculations the PBE1PBE functional was used. 8 Solvent effects were modelled using the polarizable conductor continuum model (PCCM). 9 Molecular orbitals are depicted in blue and white for positive and negative signs of the wavefunctions.

STM
The STM samples were prepared in situ. The Ag(111) crystal (Surface Preparation Laboratory B. V.) was cleaned by repeated cycles of Ar + sputtering (2kV) and annealing to 600 °C. Solid samples of 2-(Fc-A)n-Et TAT were dissolved in dichloromethane and methanol and subsequently deposited via electrospray deposition (ESD) at room temperature. The used ESD setup is described in ref. 10 . In brief, dissolved molecules are accelerated under ambient pressure by a voltage drop. The solvent with the probe molecules is polarized by applying a high voltage (~kV) to the emitter tip. A jet of charged particles is accelerated into the vacuum and can be manipulated by an atmospheric pressure electrode (APE) and counter flow of N2 gas. The amount of deposited material is monitored via current measurements. Measurements were performed in a two-chamber ultra high vacuum (UHV) system operating at 510 −11 mbar) with an Omicron Cryogenic-STM at 3 to 6 K in constant current mode. For all measurements, grinded and polished PtIr tips (Nanoscore GmbH) were used. 10

Chemical Synthesis and Characterization
All syntheses were carried out under nitrogen atmosphere using common Schlenk techniques. Solvents were dried over appropriate drying agents, deoxygenated by purging with dinitrogen or by three freeze-pump-thaw cycles, and stored under nitrogen. All starting materials were purchased from commercial suppliers and used without further purification. 11 10.00 g of 6-bromoisatin (44.24 mmol, 1.0 eq.) and potassium carbonate (23.44 g, 169.61 mmol, 3.8 eq.) were suspended in 85 mL of dimethylformamide. Subsequently, 20 mL of iodoethane (38.80 g, 248.77 mmol, 5.6 eq.) were added. The reaction mixture was stirred at 70 °C for 6 hours. After cooling to room temperature, 100 mL of H2O were added and the aqueous phase was extracted with CH2Cl2 (3100 mL). The combined organic layers were dried over MgSO4 and the solvent was removed under reduced pressure. 10 11 10.78 g of 6-bromo-N-ethylisatin (42.43 mmol, 1.0 eq.) were suspended in 55 mL of hydrazine monohydrate. The reaction mixture was stirred under reflux conditions for 5 hours. After cooling to room temperature, 100 mL of H2O were added and the aqueous phase was extracted with CH2Cl2 (3100 mL). The combined organic layers were dried over MgSO4 and the solvent was removed under reduced pressure. 9.40 g of 6-bromo-N-ethyloxindole were obtained as a light orange solid (39. 15  2,2',2''-Tribromo-N,N',N''-triethyltriazatruxene (2-Br3-Et TAT) 11 6-Bromo-N-ethyloxindole (9.40 g, 39.15 mmol, 1.0 eq.) was suspended in 50 mL of phosphorus oxychloride. The reaction mixture was stirred under reflux conditions for 4 hours. After allowing to cool to room temperature, the mixture was poured slowly onto 1000 g of crushed ice. The mixture was extracted with CH2Cl2 (4100 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. The black residue was purified via column chromatography with a mixture of CH2Cl2 : petroleum ether (4:1) as the eluent. The fractions containing the desired product were combined, the solvents removed and the residue was washed with hexane (530 mL). 2,2',2''-Tribromo-N,N',N''-triethyltriazatruxene was obtained as a beige solid (2.26 g, 3.39 mmol, 26%  Ferrocenyl boronic acid (0.29 g, 1.26 mmol, 6.0 eq.) and 2Br3-Et TAT (0.14 g, 0.21 mmol, 1 eq.) were placed in a Schlenk tube and dissolved in 12 mL of dimethoxyethane, which had been degassed by flushing with nitrogen for 20 minutes. A solution of 0.1 g NaOH in 2.5 mL of degassed H2O and 15 mg of Pd(dppf)Cl2 was added and the mixture was left stirring for 1 week at 80 °C. After cooling to room temperature, 100 mL of CH2Cl2 and 50 mL of H2O were added. The phases were separated and the aqueous phase was extracted with 350 mL of CH2Cl2. The combined organic phases were dried over Na2SO4 and filtered through a plug of Celite using CH2Cl2 as the eluent. The solvents were removed under reduced pressure and the residue was extracted into diethyl ether and filtered. The solvent was removed under reduced pressure and the orange solid was purified via column chromatography with CH2Cl2 : petroleum ether (1:1) as the eluent. The product was obtained as an orange solid (40 mg, 0.04 mmol, 20%). Other fractions contained small amounts of pure 2,2'-diferrocenyl-N,N',N''triethyltriazatruxene and 2-ferrocenyl-N,N',N''-triethyltriazatruxene. 1 Hz, 6 H, H-12/13), 4.40 (vt, JHH = 1.8 Hz, 6H, H-12/13), 4.14 (s, 15H, H-14), (t, 3 JHH = 7.2 Hz, 9H, H-10). 13 C{ 1 H} NMR (CDCl3, 101 MHz) δ 141.4 (s, C-8), 138.7 (s, C-1), 134.2 (s, C-3), 122.0 (s, C-6), 121.4 (s, C-4), 119.1 (s, C-5), 107.6 (s, C-7), 103.7 (s, C-2), 87.1 (s, C-11), 69.8 (s, C-14),68.9 and 66.8 (each s, C-12, C-13), 41.8 (s, C-9), 15.8 (s, C-10). 11 5-Bromoisatin (10.07 g, 44.55 mmol, 1.0 eq.) and K2CO3 (23.40 g, 169.30 mmol, 3.8 eq.) were suspended in 85 mL of dimethylformamide. 20 mL of iodoethane (38.80 g, 248.77 mmol, 5.6 eq.) were added. The reaction mixture was stirred for 6 hours at 70 °C. After allowing the reaction mixture to cool to room temperature, 400 mL of H2O were added. The mixture was extracted with CH2Cl2 (4100 mL). The combined organic phases were dried over MgSO4 and the solvents were removed under reduced pressure. The product was obtained as a dark red solid in a yield of 96% (10.89 g, 42.86 mmol).  11 10.89 g of 5-bromo-N-ethylisatin (42.86 mmol, 1.0 eq.) were suspended in 55 mL of hydrazine monohydrate. The reaction mixture was stirred under reflux for 5 hours. After cooling to room temperature, 100 mL of H2O were added and the aqueous phase was extracted with CH2Cl2 (3100 mL). The combined organic layers were dried over MgSO4 and the solvent was removed under reduced pressure. 9.40 g of 5-bromo-N-ethyloxindole were obtained as a pale orange solid (39.15 mmol, 91%).   11 6-Bromo-N-ethyloxindole (9.40 g, 39.15 mmol, 1.0 eq.) was suspended in 50 mL of phosphorus oxychloride. The reaction mixture was stirred under reflux conditions for 4 hours. After allowing to cool to room temperature the mixture was poured slowly onto 1000 mL of crushed ice. The mixture was extracted with CH2Cl2 (4100 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. The black residue was purified via column chromatography with CH2Cl2 : petroleum ether (4:1) as the eluent. Fractions that contained the desired product were combined and stripped of the solvents in vacuo. The solid residue was washed with n hexane (530 mL). 2,2',2''-Tribromo-N,N',N''-triethyltriazatruxene was obtained as a light yellow solid (4.60 g, 6.90 mmol, 53%).  In situ formation of the Negishi reagent: 0.34 g of ferrocene (1.80 mmol, 6.0 eq.) and 0.005 g of t BuOK (0.18 mmol, 0.15 eq.) were dissolved in 6 mL of dry THF. The reaction mixture was cooled to -78 °C. 2.40 mL of a 1.9 M solution of t BuLi in n pentane (4.50 mmol, 15.0 eq.) were added dropwise with stirring. After complete addition, stirring was continued for 1 h while the temperature was maintained at -78 °C. A solution of 0.46 g of ZnCl2(TMEDA) (1.83 mmol, 6.0 eq.) in 7 mL of dry THF was then added and the solution was stirred for an additional hour while allowing the reaction mixture to warm to room temperature.

5-Bromo-N-ethyloxindole
Negishi coupling: A solution of 3,3',3''-tribromo-N,N',N''-triethyltriazatruxene (0.20 g, 0.30 mmol, 1.0 eq.) and Pd(dppf)Cl2 (20 mg) in 8 mL of dry THF was added. The reaction mixture was stirred under reflux for 1 week. The solvent was removed under reduced pressure and the residue was dissolved in diethyl ether. The organic phase was washed twice with H2O. The organic phase was dried over Na2SO4 and the solvent was removed under reduced pressure. The residue was dissolved in dry CH2Cl2. For purification, ferrocenium hexafluorophosphate (>3-fold excess) was added to the solution. Crude 3,3',3''-triferrocenyl-N,N',N''triethyltriazatruxene thereby reacts to the ether-insoluble triferrocenium salt. The mixture was sonicated, the solvent was removed and the residue was washed with diethyl ether (440 mL), which removes the formed ferrocene and organic by-products. The solid was redissolved in CH2Cl2 and excess zinc dust was added in order to reduce the 3,3',3''-triferroceniumyl-N,N',N''triethyltriazatruxene back t the neutral form. After sonication and filtration from remaining zinc dust and Zn(PF6)2 the solvent was removed under reduced pressure. The remaining solid was washed with n pentane until the washings remained colorless. The crude product was further purified via column chromatography with CH2Cl2: n pentane (40:60 to 60:40) as the eluent. The product was isolated as an orange solid (0.041 g, 0.042 mmol, 14%).  12 , 11 Isatine (10.0 g, 67.97 mmol, 1.0 eq.) and K2CO3 (31.0 g, 224.29 mmol, 3.3 eq.) were suspended in 85 mL of DMF and 27 mL of iodoethane (53.00 g, 339.83 mmol, 5.0 eq.) were added. The reaction mixture was stirred at 70 °C for 5 hours. After cooling to room temperature, 400 mL of H2O were added and the reaction mixture was extracted with CH2Cl2 (4100 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. N-Ethylisatin was obtained as a red solid. The product was used in the next step without further purification.

N-Ethyloxindole
The crude product was suspended in 55 mL of hydrazine monohydrate. Stirring was maintained under reflux conditions for 5 hours. After allowing the mixture to cool to room temperature 100 mL of H2O were added. The aqueous reaction mixture was extracted with CH2Cl2 (3100 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. 10.12 g of N-ethyloxindole were obtained as a yellow solid, which turns slowly red (62.80 mmol, 92% over two steps). N,N',N''-Triethyltriazatruxene 11 10.0 g of N-ethyloxindole (62.03 mmol, 1.0 eq.) were suspended in 70 mL of phosphorus oxychloride. The mixture was stirred under reflux conditions for 4 hours. After cooling to room temperature, the reaction mixture was poured onto 800 mL of crushed ice. The aqueous phase was extracted with CH2Cl2 (4200 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. The black crude product was purified by column chromatography using a 1:1 mixture of CH2Cl2 and petroleum ether as the eluent. 4.90 g of N,N',N''-triethyltriazatruxene were obtained as a slighty yellow-green powder (11.41 mmol, 56%).  13 6-Bromoisatin (3.0 g, 13.27 mmol, 1.0 eq.) and K2CO3 (3.67 g, 26.54 mmol, 2.0 eq.) were suspended in 39 mL of N,N-dimethylformamide and 3.82 mL of dodecyl bromide (3.97 g, 15.92 mmol, 1.2 eq.) were added. The reaction mixture was heated to 80 °C overnight. After the reaction mixture was cooled to room temperature, 150 mL of H2O were added. The mixture was extracted with CH2Cl2 (350 mL). The combined organic phases were washed with water (250 mL), dried over MgSO4 and the solvents were removed under reduced pressure. The crude product was purified by column chromatography using n hexane : CH2Cl2 (1:1). 3.16 g of 6-bromo-N-dodecyl-isatin were obtained (8.02 mmol, 60% yield) as a red solid.  14 3.16 g of 6-bromo-N-dodecylisatin (8.0 mmol, 1.0 eq.) were dissolved in 20 mL of hydrazine hydrate and the reaction mixture was heated to reflux for 5 hours. After cooling to room temperature, 100 mL of H2O were added. The aqueous phase was extracted with CH2Cl2 (350 mL). The combined organic phases were washed with 50 mL of H2O and dried over MgSO4. The solvents were removed under reduced pressure and the title compound was yielded as a yellow solid (2.56 g, 6.80 mmol, 85%).

6-Bromo-N-dodecyloxindole
2.56 g of 6-bromo-N-dodecyloxindole (6.80 mmol, 1.0 eq.) were dissolved in 15 mL of POCl3 and heated to reflux for 5 hours. After allowing to cool to room temperature, the reaction mixture was poured onto 100 g of crushed ice, stirred for 1 h and extracted with CH2Cl2 (360 mL). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. The crude product was purified by column chromatography using petrol ether : CH2Cl2 (4:1) as the eluent. 0.75 g of the product were isolated as a yellow powder (2.08 mmol, 30.6%).   1.12 g of N,N',N''-triethyltriazatruxene (2.61 mmol, 1.0 eq.) were dissolved in 300 mL of dry CH2Cl2 and cooled to 0 °C. A solution of 0.55 g of N-bromosuccinimide (3.65 mmol, 1.5 eq.) in 20 mL of dry DMF was added over 10 minutes. The reaction was stirred at 0 °C for 2 hours. Then, 200 mL of H2O were added. The phases were separated and the aqueous phase was extracted with CH2Cl2 (3100 mL). The combined organic phases were dried over MgSO4 and the solvents were removed to yield a mixture of mono-and dibrominated Et TAT as a greenish solid.
The greenish solid was used in the following Sonogashira coupling without purification. The crude solid and CuI (0.10 g, 0.53 mmol, 0.18 eq.) were dissolved in 120 mL of dry THF and 50 mL of Et3N, 0.53 mL of 2-methylbut-3-yn-1-ol (0.46 g, 5.22 mmol, 2.1 eq.) and 0.12 g of Pd(dppf)Cl2 were added. The reaction mixture was stirred for 96 h at 60 °C. After cooling to room temperature, 150 mL of H2O were added. The phases were separated and the aqueous phase was extracted with CH2Cl2 (370 mL). The combined organic phases were dried over MgSO4 and the solvents were removed under reduced pressure. The residue was purified via column chromatography using a mixture of ethyl acetate : petroleum ether (2:8 to 4:6) as the eluent.

2-Ethynyl-N,N',N''-triethyltriazatruxene (2-A1-Et TAT)
0.22 g of 2-MebA1-Et TAT (0.43 mmol, 1.0 eq.) were placed in a three-necked round bottom flask. 160 mL of toluene, 6 mL of methanol, 1.6 mL of H2O and 0.5 g of KOH were added and the reaction vessel was equipped with an air cooler. The solution was stirred overnight at 80 °C while purging with nitrogen gas. After cooling to room temperature, 200 mL of H2O were added and the phases were separated. The aqueous phase was extracted with CH2Cl2 (380 mL). The combined organic phases were dried over MgSO4 and the solvents were removed to give 0.14 g of 2-A1-Et TAT as a golden-yellowish solid (0.31 mmol, 72%). Iodoferrocene 15 3.00 g of ferrocene (16.13 mmol, 1.0 eq.) and 0.18 g of t BuOK (1.61 mmol, 0.10 eq.) were dissolved in 150 mL of dry THF. The reaction mixture was cooled to -78 °C. 21.22 mL of a 1.9 M solution of t BuLi in n pentane (40.31 mmol, 2.5 eq.) were added dropwise and the mixture was left stirring at -78 °C for 2 h. 10.23 g of iodine (40.32 mmol, 2.5 eq.) were added. The reaction mixture was stirred for 30 minutes while allowing to warm to room temperature. The organic phase was washed with a saturated solution of Na2S2O3 (50 mL). The solvent was removed under reduced pressure, yielding a black oil. The crude product was dissolved in n hexane and filtered through a plug of silica with n hexane as the eluent. The organic phase was washed with a saturated, aqueous solution of iron tricloride until the aqueous phase did not show a blue-green colour anymore. The organic phase was dried with Na2SO4 and the solvent was removed under reduced pressure. 2.45 g of iodoferrocene were obtained as redbrown crystals (7.85 mmol, 49%). Ferrocene boronic acid 16 1.50 g of ferrocene (8.06 mmol, 1.0 eq.) and 0.14 g of t BuOK (1.21 mmol, 0.15 eq.) were dissolved in 50 mL of dry THF. The reaction mixture was cooled to -78 °C and 10.61 mL of a 1.9 M solution of t BuLi in pentane (20.16 mmol, 2.5 eq.) were added dropwise. Stirring at -78 °C was continued for 2 h. 4.65 mL of triisopropyl borate (20.16 mmol, 2.5 eq.) were added dropwise at -78 °C. The reaction mixture was allowed to warm to room temperature and was stirred overnight. The solvent was removed under reduced pressure and the residue was dissolved in 50 mL of ethyl acetate. The organic phase was extracted with a solution of Na2CO3 and sorbitol (1M, 680 mL). The aqueous phase was washed with 80 mL of a mixture of diethyl ether and pentane (1:1). The aqueous phase was cooled to 0 °C and 250 mL of diethyl ether were added. The mixture was vigorously stirred and acidified with concentrated hydrochloric acid to pH 1. The phases were separated and the aqueous phase was washed with water and brine (80 mL each). The combined organic phases were dried over MgSO4 and the solvent was removed under reduced pressure. 1.02 g of ferrocene boronic acid were obtained as an orange solid (4.45 mmol, 55%).

2,2'-Di(ferrocenylethynyl)-N,N',N''-triethyltriazatruxene (2-(Fc-A)2-Et TAT)
100 mg of 2-A2-Et TAT (0.21 mmol, 1.0 eq.) were dissolved in 15 mL of THF. The solution was cooled to -78 °C and 0.2 mL of a 2.5 M solution of n BuLi in n hexane (0.50 mmol, 2.4 eq.) were added dropwise. The resulting solution was stirred for 1 h. 0.14 mg of ZnCl2(tmeda) (0.54 mmol, 2.6 eq.) were dissolved in 10 mL of THF and added dropwise to the solution. The reaction mixture was allowed to reach room temperature and the solution was stirred for further 90 min. 0.20 g of iodoferrocene (0.63 mmol, 3.0 eq.) and 25 mg of Pd(PPh3)4 (9.5 mol%) were added. The reaction mixture was heated to reflux for one week. After allowing to cool to room temperature, the reaction mixture was filtered over Celite using CH2Cl2 as the eluent. The solvent was removed under reduced pressure. The remaining solid was dissolved in 5 mL of CH2Cl2. 30 mL of n pentane were added. An orange-red solid precipitated and the supernatant was cannulated off. The solid was washed with 40 mL of pentane. The crude product was