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

摘要

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 Cu~Ⅱ and V~Ⅳ complexes of these extended phloroglucinol ligands are indeed ferromagnetically coupled but with coupling constants J not exceeding +3 cm~(-1). Moreover, trinuclear Fe~Ⅲ complexes are almost uncoupled and trinuclear Mn~Ⅲ complexes as well as trinuclear Mn~Ⅲ 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 it 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 tau-tomery. 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.