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From Triplesalen to Triplesalalen and Triplesalan – Strengthening the Aromatic Character of the Ligand Backbone in Extended Phloroglucinol Ligands by Prevention of Heteroradialene Formation

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Abstract

The methyl-protected triplesalalen ligand Me3H3talalenmath image was synthesized from 2,4,6-tris(bromomethyl)-1,3,5-trimethoxybenzene and three equivalents of the secondary amine 4,6-di-tert-butyl-2-{[2-(methylamino)ethylimino]methyl}phenol. A reduction with NaBH4 afforded the methyl-protected triplesalan ligand Me3H3talanmath image. Deprotection efforts of the methyl-protected Me3H3talalenmath image ligand with Lewis acids were unsuccessful. Using CuII ions as Lewis acid resulted in the formation of the methyl-protected triplesalalen complex [(Me3H3talalenmath image){CuII(H2O)}3](ClO4)3, which could be characterized by single-crystal X-ray diffraction. The bond-length analysis reveals the aromatic character of the central backbone without heteroradialene contribution, and this is corroborated by NMR spectra of the ligands, which exhibit a singlet assigned to the benzylic protons at δ = 3.50 ppm in the 1H NMR and a resonance typical for an aromatic CAr–O group at δ = 160.0 ppm in the 13C NMR spectroscopy. FTIR spectra of the ligands and of the complexes exhibit the typical features of a central aromatic unit and do not show the characteristic intense bands for vibrations of the exocyclic C=C and C=O double bonds of a heteroradialene. Additionally, the characteristic strong absorption bands for heteroradialenes in the 26000–35000 cm–1 region are absent in the UV/Vis/NIR spectra. The electrochemistry exhibits the reversible oxidation of the terminal phenolates of [(Me3H3talalenmath image){CuII(H2O)}3](ClO4)3 at +0.72 V vs Fc+/Fc, and the magnetic measurements reveal an uncoupled behavior due to the apical coordination of the central methoxy groups to the CuII ions.

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