Synthesis and characterization of trinuclear square-planar Ni3 and Cu3 complexes of an extended phloroglucinol ligand: Experimental evidence for the relative contributions of benzene-like and radialene-like resonance structures

The synthesis and characterization of the extended phloroglucinol ligand H₃felddien, its trinuclear Ni^II complex [(felddien)Ni₃](BF₄)₃ and its trinuclear Cu^II complex [(felddien)Cu₃](ClO₄)₃ is presented. Detailed NMR studies provide strong evidence that the ligand H₃felddien has to be described as the N-protonated tautomer and not as the O-protonated tautomer, with strong contribution of a radialene-like keto-enamine resonance structure resulting in a C_s and a C_3h isomer. The trinucleating tris(tetradentate) ligand provides three donor sets comprised of a phenolate, an imine, and two tertiary amine donors. This donor set enables the synthesis of the diamagnetic square-planar coordinated Ni^II complex [(felddien)Ni₃](BF₄)₃ which provides the opportunity to perform detailed NMR spectroscopic characterizations for the evaluation of the electronic structure of the central phloroglucinol unit. In conjunction with a single-crystal X-ray diffraction and a UV-vis absorption spectroscopic analysis, these data indicate that in the coordinated form, the benzene-like phenolate-imine and the radialene-like keto-enamine resonance structures have contributions to the overall resonance hybrid. The weakening of the central π system as a consequence of the contribution of the keto-enamine resonance structure explains the relative small ferromagnetic interactions in the trinuclear Cu^II complex [(felddien)Cu₃](ClO₄)₃. This detailed analysis identifies the strong resonance with unsaturated groups in 2,4,6 position of phloroglucinol as the main source for the low ferromagnetic couplings by the spin-polarization mechanism in all our extended phloroglucinol ligands. A replacement of the unsaturated imine functions by saturated amine functions may be a synthetic opportunity to enhance the ferromagnetic interactions by the spin-polarization mechanism in this ligand system.