Conclusive Insight into the Coordination Complexes of a Flexible Bis(beta-diketonato) Ligand and their Phase-dependent Structure: a Multi-technique Approach

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(beta-diketonato) ligand bdhb2–, 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 bdhb2– so far isolated and structurally characterized, namely [(VO)2(bdhb)2] (1) and [Co2(bdhb)2(py)4] (2), are dimeric and contain bridging bdhb2– 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.