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 triple-salophen complexes and
three connecting units. Herein, the convergent synthesis of the triplesalophen
ligand system is described, which differs fom the divergent strategy
for the triplesalen ligand system. The molecular and elecrtronic structures
of the triplesalophen ligands H6baronR (R =
Me, Cl, Br) have 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 H2carlCl 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.