Synthesis and characterization of trinuclear square-planar NiII3 and CuII3 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 NiII complex [(felddien)Ni3](BF4)3 and its trinuclear CuII 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 Cs and a C3h 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 NiII 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 CuII 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.

CuII complexes of extended phloroglucinol ligands exhibit ferromagnetic but weak coupling by the spin-polarization mechanism. A detailed experimental analysis on the trinuclear NiII and CuII complexes of the ligand H3felddien demonstrate that the radialene-like resonance structure has a significant contribution in the complexes. These significant amounts of resonance in the ligand have been identified as a reason for the relative small coupling.Figure optionsDownload as PowerPoint slideResearch highlights
► Make new ligand H3felddien.
► Make diamagnetic Ni3 complex to determine experimentally the resonance structure contribution.
► Make ferromagnetically coupled Cu3 complete.
► Correlate magnetic interactions to resonance structures.
► Identify optimization strategies.