Triferrocenes Built on a C₃-Symmetric Ligand Platform:  Entry to Redox-Active pseudo-Triphenylenes via Chelation-Driven Stereoselection of Triple Schiff Bases

Abstract

An expedient tandem deprotonation-trapping protocol was employed to prepare a tris(difluoroboronyl) complex of a triferrocenyl ligand that is geometrically analogous to substituted triphenylenes. A triple Schiff base condensation reaction between 1,3,5-triformylphloroglucinol and aminoferrocene afforded the tris(N-salicylideneamine) adducts 5a + 5b in ca. 1:1 ratio. The keto-enamine tautomeric core of this isomeric mixture could be converted to a common enolate-imine intermediate. Subsequent trapping with BF3.Et2O cleanly afforded the tris(difluoroboronyl) adduct 6 in essentially quantitative yield. The electronic and structural properties of this new class of ferrocene compounds were investigated using various methods including UV-vis, cyclic voltammetry (CV), differential pulse voltammetry (DPV), and X-ray crystallography. In CH2Cl2-CH3CN, 6 displayed a reversible three-electron oxidation process at E1/2ox = +210 mV (vs Fc/Fc+). Despite the sharing of a common [pi,pi]/[n,pi]-conjugated core, no significant electronic communication was observed among the three ferrocenyl units in 6 under either CV or DPV conditions. On the other hand, the broad oxidation wave of 5a + 5b at E1/2ox = +60 mV in CH2Cl2-CH3CN was comprised of at least two major components at +20 and +90 mV, which collapsed to become a single peak in DMF electrolyte, despite that the ratios between the two isomers 5a,b remained essentially invariant to the change in solvent.

Triple hydrogen bonding in a circular arrangement: ab initio, DFT and first-principles MD studies of tris-hydroxyaryl enamines

“…However, the calculated ΔE TS values show a large energy barrier (ΔE TS (TS, TS, TS) = 20.62 kcal/mol → ΔE TS (OH, TS, TS) = 19.45 kcal/mol → ΔE TS (OH, OH, TS) = 18.45 kcal/mol (B3LYP/6-31+G(d,p)) and don’t support the observation of the enol-imine tautomeric form under experimental conditions. This conclusion is consistent with the experimental results published by MacLachlan [16, 17], Lee [10, 11], Yelamaggad [12, 13] and Suresh [60], which also confirm the prevalence of the keto-enamine tautomeric form. For the studied compounds a number of conformers was also obtained (VII–XI), all of them possessing a much larger energy (ΔE > 20 kcal/mol) than the energy of the most stable conformer I.…”

“…The combined quantum and classical mechanics calculations of vitamin B6 (pyridoxal 5′-phosphate derivative) have been carried out to elucidate the factors that contribute to the tautomeric equilibrium [8]. The latest investigations [10, 11] of two successfully modelled dimensional fluorescence switches are based on the strengthening of π-electronic conjugation in tris -salicylideneamine by the doubled cyclic intramolecular hydrogen bonding. This approach makes it possible to enhance a fluorescent quantum yield of these compounds which refer to the mechanically coupled biconcave systems [11].…”

“…Though the experimental studies and synthesis of tris -salicylideneanilines are becoming more popular [10–13, 16, 17], quantum–mechanical calculations are very few in the literature [16, 17]. The intramolecular tautomeric equilibrium is an interesting feature of these compounds.…”

Neutral Ni(II) complexes based on keto-enamine salicylideneanilines active for selective dimerization of ethylene

“…The average C-C bond length 1.45 Å of central cyclohexanetrione core is considerably longer than the average bond length of 1.38 Å found in benzene. Conversion of a keto-enamine structure, originated from an imine reaction on triformylphloroglucinol, to an enolate-imine and its subsequent trapping with BF 3 Á Et 2 O had been reported along with its crystal structure for a tris(N-salicylideneaminoferrocene) type complex and this gives an excellent reference molecule for comparison [12]. For this enolate structure, bond length values reported for C2-C3 and C3-C4 in the core aromatic ring and O1-C2 of the enolate are 1.412(3), 1.408(3) and 1.318(3) Å, respectively from the single crystal XRD data.…”

Synthesis and Optical Properties of Salicylaldimine‐Based Diboron Complexes

“…[43][44][45][46] Schiff base boron compounds ( Figure 1, C) are yet another type of four-coordinate boron complex that has gained interest owing to their greater stability than tricoordinate boron compounds. [15,18,24,[47][48][49][50] For example, Ziessel, Ulrich, and co-workers reported the synthesis and optical properties of Boranil complexes. [47] Ziessel and Ulrich are also credited for expanding the scope of the Boranil complexes in a model labeling experiment with bovine serum albumin.…”