Conclusive Insight into the Coordination Complexes of a Flexible Bis(β-diketonato) Ligand and Their Phase-Dependent...

Multichelating ligands with nuclear spin-free donor atoms are of particular interest for creating stable electronic spin qubits based on paramagnetic transition metal ions. We recently focused on the coordinating ability of the bis(β-diketonato) ligand bdhb^2-, featuring two "acac" moieties connected through a 1,3-phenylene bridge (H₂bdhb = 1,3-bis(3,5-dioxo-1-hexyl)benzene). The two crystalline complexes of bdhb^2- so far isolated and structurally characterized, namely [(VO)₂(bdhb)₂] (1) and [Co₂(bdhb)₂(py)₄] (2), are dimeric and contain bridging bdhb^2- ligands; however, they become mononuclear and quasi-macrocyclic in organic solution. To investigate this unique structural isomerism by high-resolution ¹H NMR spectroscopy, we have now synthesized a diamagnetic Zn²⁺ analogue of 1 and 2, namely [Zn₂(bdhb)₂(py)₂] (3). Although both 2 and 3 are dimeric and contain the same ligands, 3 features only one pyridine molecule per metal ion, whose coordination geometry is square pyramidal rather than tetragonally elongated octahedral. The ESI-MS spectra of 3 in THF and CH₂Cl₂ contain peaks from both monomeric and dimeric species. However, molecular weight determinations by DOSY and conformational studies based on J-coupling analysis and DFT calculations conclusively prove the rearrangement of 3 into quasi-macrocyclic monomers in THF-d₈ and CD₂Cl₂ solution at room temperature.