Exchange coupling mediated by extended phloroglucinol ligands: Spin-polarization vs. heteroradialene-formation

The meta-phenylene unit is an efficient ferromagnetic coupler in organic chemistry and has been used to develop high spin organic molecules. The ferromagnetic interactions arise from the spin-polarization mechanism. The transfer of this heuristic concept to coordination chemistry has been evaluated using different meta-phenylene bridging ligands by several groups and is not so straightforward. We are interested in using 1,3,5-trihydroxybenzene (phloroglucinol) as an efficient ferromagnetic coupler between 3d transition metal ions. We have attached substituted imine groups in 2,4,6-position to enhance the stability of trinuclear complexes by the chelate effect. Trinuclear CuII and VIV complexes of these extended phloroglucinol ligands are indeed ferromagnetically coupled but with coupling constants J not exceeding +3 cm−1. Moreover, trinuclear FeIII complexes are almost uncoupled and trinuclear MnIII complexes as well as trinuclear MnIII subunits in complexes of higher nuclearity exhibit weak antiferromagnetic coupling. NMR, structural studies, UV–vis, and IR spectroscopy demonstrate that the extended phloroglucinol ligands are not in the expected O-tautomeric form but in the N-tautomeric form with the prevalence of a keto-enamine resonance structure. This electronic structure is reminiscent of [6]radialenes and has been termed heteroradialenes. These heteroradialenes are not delocalized aromatic systems but cross-conjugated alicycles. Thus, the low coupling in the complexes of the extended phloroglucinol ligands can be correlated with this heteroradialene formation. The alicyclic backbone of a heteroradialene has no delocalized π system to transmit the spin-polarization mechanism. The realization of the heteroradialene formation provides us with a handle to optimize the extended phloroglucinol ligands by substituting the imine groups with saturated amine groups, which cannot undergo an enolimine–ketoenamine tautomery. The NMR, UV–vis, and IR spectroscopic features of saturated triamine derivatives clearly establish the delocalized aromatic character of the central phloroglucinol unit without any indication for a heteroradialene formation. These extended phloroglucinol ligands of the second generation are promising candidates as more efficient ferromagnetic couplers in coordination chemistry.

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► Extended phloroglucinol ligands have been developed as ferromagnetic couplers.
► Trinuclear CuII and VIV complexes indeed exhibit weak but ferromagnetic interactions.
► Trinuclear FeIII and MnIII complexes are not ferromagnetically coupled.
► Spectroscopy establishes an N-protonated, non-aromatic heteroradialene formulation.
► This finding allows the optimization of the ligands.