Triferrocenes built on a C3-symmetric ligand platform: entry to redox-active pseudo-triphenylenes via chelation-driven stereoselection of triple Schiff bases.
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.
Lim
Young-Kwan
YK
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, USA.
Wallace
Scott
S
Bollinger
John C
JC
Chen
Xufang
X
Lee
Dongwhan
D
D000480:Alkynes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D005296:Ferrous Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D000075163:Metallocenes; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D012545:Schiff Bases / Q000737:chemistry*; D012680:Sensitivity and Specificity; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013237:Stereoisomerism
D000480:Alkynes; D005296:Ferrous Compounds; D008024:Ligands; D000075163:Metallocenes; D012545:Schiff Bases; C004998:ferrocene