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

The methyl-protected triplesalalen ligand Me(3)H(3)talalen(tBu2) 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 Me(3)H(3)talan(tBu2). Deprotection efforts of the methyl-protected Me(3)H(3)talalen(tBu2) ligand with Lewis acids were unsuccessful. Using Cu-II ions as Lewis acid resulted in the formation of the methyl-protected triplesalalen complex [(Me(3)H(3)talalen(tBu2)){Cu-II(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 delta = 3.50 ppm in the H-1 NMR and a resonance typical for an aromatic C-Ar-O group at delta = 160.0 ppm in the C-13 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 [(Me(3)H(3)talalen(tBu2)){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 Cu-II ions.