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

Feldscher B, Stammler A, Bögge H, Glaser T. From Triplesalen to Triplesalalen and Triplesalan - Strengthening the Aromatic Character of the Ligand Backbone in Extended Phloroglucinol Ligands by Prevention of Heteroradialene Formation. European Journal Of Inorganic Chemistry. 2013;2013(3):388-397.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