Reduction of 1,2‐Bis[(2,6‐diisopropylphenyl)imino]acenaphthene (dpp‐bian) with Alkali Metals − A Study of the Solution Behaviour of (dpp‐bian)n−[M+]n (M = Li, Na; n = 1−4) with UV/Vis, ESR and 1H NMR Spectroscopy

An excess of lithium or sodium in diethyl ether consistently reduces 1,2‐bis[(2,6‐diisopropylphenyl)imino]acenaphthene (dpp‐bian) to the mono‐, di‐, tri‐ and tetraanion, (dpp‐bian)n−[M+]n (M = Li, Na; n = 1−4). The process of reduction is easily observed by the change in colour of the solution, on going from the neutral ligand (yellow) to the monoanion (red), the dianion (green for Na or Li in Et2O; dark blue, followed by green, for Li in TMEDA), the trianion (brown) and the tetraanion (red‐brown). In each reduction the maximum of the absorption, caused by electron transition from the HOMO of the respective form (dpp‐bian)n− (n = 1−4), is shifted to the long‐wavelength region in comparison to the previously reduced state. The disproportionation reactions (dpp‐bian)1− ⇄ (dpp‐bian)0 + (dpp‐bian)2− and (dpp‐bian)2− ⇄ (dpp‐bian)1− + (dpp‐bian)3− take place in solution along with the reduction process, thus causing the presence of three reduced forms of the ligand at the same time in the first and second stages of the reduction. Paramagnetic (dpp‐bian)1− and (dpp‐bian)3− are characterised by ESR spectroscopy. For the diamagnetic forms (dpp‐bian)2− and (dpp‐bian)4− 1H NMR spectra were recorded. The complex [Li+(dpp‐bian)(TMEDA)] (1), which formed from the initial stage of the reduction of dpp‐bian with lithium metal in TMEDA, was isolated in a crystalline form and its molecular structure was determined by single‐crystal X‐ray diffraction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)