Insight into the structural versatility of the Ln(III)[15-metallacrown-5] platform by comparing analogs with Ni(II), Cu(II), and Zn(II) ring ions

The promise for tuning the molecular recognition and physical properties of coordination driven assemblies through metal ion substitutions has prompted this structural investigation into the structural versatility of Ln(III)[15-metallacrownM(II),L-5]3+ complexes. The structural versatility of this molecular class is expanded through the synthesis of Ln(III)[15-metallacrownZn(II),picHA-5](NO3)3, the first Ln(III)[15-metallacrownM(II),L-5]3+ with Zn(II) metal ions. X-ray crystallography reveals the pentagonal macrocycle has a severely ruffled face, which is attributed to a non-equatorial alignment of the picHA ligands by the square pyramidal Zn(II) ions. ESI-MS and 1H NMR suggest the complex maintains its integrity in pyridine. Comparison of the Eu(III)[15-metallacrownM(II),picHA-5]3+ structures with Ni(II), Cu(II), and Zn(II) reveal elongated M(II)–NpicHA bond lengths and bending of the picHA chelates at the ring metals with Zn(II). Surprisingly, the metallacrowns provide similar coordination environments for the central Eu(III) ion despite the structural differences in the ring. Differences in the visible absorption spectra of the Eu(III)[15-metallacrownM(II),picHA-5]3+ complexes with Ni(II), Cu(II), and Zn(II) demonstrate that the electronics of the metallacrown platform can be varied through ring ion substitutions. The ability of Ln(III)[15-metallacrownM(II),L-5] complexes to accommodate ring ions with disparate coordination geometries is unique for coordination driven assemblies and has implications for the design of modular complexes with tunable magnetic and electronic properties.

The synthesis, solution characterization, and structure of the first Ln(III)[15-metallacrownZn(II),L-5]3+ is reported. Compared to analogous compounds with Cu(II) and Ni(II), the Zn(II) complex is highly ruffled because the square-pyramidal Zn(II) ring ions enforce a non-equatorial alignment of the ligands. The ability of the Ln(III)[15-metallacrownM(II),L-5]3+ platform to accommodate different ring ions is attributed to the stability of the bidentate ligands and its assembly from multiple components, as structural perturbations at one ring ion are cancelled out by minor changes at other ring ions.Figure optionsDownload as PowerPoint slideHighlights
► The synthesis, structure, and solution characterization of the first Ln(III)[15-metallacrown-5] complex with a Zn(II) ring ion.
► Structural analysis reveals the severe ruffling of the Ln(III)[15-metallacrown-5] face and elongated Zn(II)-NpicHA-pyridyl bond distances result from the Zn(II) ion sitting above the ligand mean plane, leading to a non-planar ligand arrangement.
► The structural versatility of the Ln(III)[15-metallacrown-5] is attributed to the bidentate chelating ligands and the relatively large number of metal ions in the ring, which can adapt to different ring ions.