Conclusive Insight into the Coordination Complexes of a Flexible Bis(β‐diketonato) Ligand and Their Phase‐Dependent Structure: A Multi‐Technique Approach

Abstract 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 (H 2 bdhb = 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 [Co 2 (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 1 H NMR spectroscopy, we have now synthesized a diamagnetic Zn 2+ analogue of 1 and 2 , namely [Zn 2 (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 CH 2 Cl 2 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‐ d 8 and CD 2 Cl 2 solution at room temperature.