Probing the Radialene-Character in Triplesalophen Ligands by Spectroscopic and Structural Analysis

The triplesalen ligand system based on three salen-like coordination environments bridged by a common phloroglucinol ring has been designed and successfully applied for the rational synthesis of single-molecule magnets from two trinuclear triplesalen complexes and one hexacyanometallate by supramolecular recognition. In order to optimize this system with respect to magnetic anisotropy, the triplesalophen ligand system has been identified, which should allow for the synthesis of nonanuclear complexes composed of two trinuclear triplesalophen complexes and three connecting units. Herein, the convergent synthesis of the triplesalophen ligand system is described, which differs fom the divergent strategy for the been established by single-crystal X-ray diffraction, NMR, FTIR, and UV-vis spectroscopies. These complementary methods allowed the assignment of the central compartment not to be in the O-protonated tautomer but in the N-protonated tautomer with the prevalence of a keto-enamine resonance description, which resembles a heteroradialene. Furthermore, the comparison with the mononucleating unsymmetrical salophen reference ligand H(2)carl(Cl) and with compounds from the literature provides unique signatures for the appearance of the heteroradialene motif not only in NMR spectra and structural parameters but also in IR and UV-vis spectra. These signatures form the basis for the interpretation and understanding of the electronic structures of transition metal complexes with the triplesalophen ligand system.